Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes

X-ray Crystallography current in the use of X-ray crystallography and related structural determination methods in various fields. The methods here include single crystal small-molecule X-ray crystallography, macromolecular (protein) single crystal X-ray crystallography, and scattering and spectroscopic complimentary methods. The fields range from simple organic compounds, metal complexes to proteins, and also cover the meta-analyses of the database for weak interactions.


Introduction
"Acylation differs from alkylation in being virtually irreversible" [Olah, 1973], free of rearrangements and isomerizations [Wang, 2009;Norman & Taylor, 1965]. This authoritative exposition of the state of the art of Friedel-Crafts chemistry in 1973 close to the centennial of the invention of the Friedel-Crafts reaction has been long recognized and not without reason. The difference in behavior between Friedel-Crafts acylation and Friedel-Crafts alkylation was attributed to the resonance stabilization existing between the acyl group and the aromatic nucleus [Buehler & Pearson, 1970], which may serve as a barrier against rearrangements and reversible processes. However, if the acyl group is tilted out of the plane of the aromatic nucleus, e.g., by bulky substituents, the resonance stabilization is reduced and the pattern of irreversibility of Friedel-Crafts acylation may be challenged [Buehler & Pearson, 1970;Pearson & Buehler, 1971;Gore, 1974]. Under these conditions deacylations and acyl rearrangements become feasible [Buehler & Pearson, 1970;Pearson & Buehler, 1971;Gore, 1974]. The concept of reversibility in Friedel-Crafts acylations [Gore, 1955[Gore, , 1964 was put forward in 1955 by Gore, who proposed that "the Friedel-Crafts acylation reaction of reactive hydrocarbons is a reversible process" [Gore, 1955]. Gore concluded that "Reversibility is an important factor in acylation reactions" [Gore, 1955]. The reversibility studies have been focused mainly on unusual aspects of selectivity, including deacylations, one-way rearrangements and kinetic versus thermodynamic control [Gore, 1974]. Under classical Friedel-Crafts conditions (e. g., AlCl 3 and a trace of water), the pattern of irreversibility (e. g., in the naphthalene series) has been highlighted [Gore, 1964[Gore, , 1974Andreou et al., 1978;Dowdy et al., 1991]. The incursion of reversibility in Friedel-Crafts acylations was revealed by Agranat, et al. in the benzoylation of naphthalene in polyphosphoric acid (PPA) at elevated temperatures ( Fig. 1) . The kinetically controlled 1-benzoylnaphthalene rearranged to the thermodynamically controlled 2-benzoylnaphthalene (PPA, 140 °C) (vide infra). The reversibility concept was then applied to the synthesis of linearly annelated polycyclic aromatic ketones by intramolecular Friedel-Crafts rearrangements of their angularly annelated constitutional isomers [Agranat & Shih, 1974a;Heaney, 1991]. The Haworth synthesis of PAHs, which previously had allowed access to angularly annelated PAHs could thus be applied to the synthesis of linearly annelated PAHs [Agranat & Shih, 1974b]. Further experimental evidence in support of true reversibility of Friedel-Crafts acylation is limited [Frangopol et al., 1964;Balaban, 1966;Nenitzescu & Balaban, 1964;Effenberger et al., 1973;Levy et al., 2007;Mala'bi et al,. 2009;Titinchi et al., 2008;Adams et al., 1998;Okamoto & Yonezawa, 2009]. Notable cases are the report by Balaban [Frangopol et al., 1964;Balaban, 1966;Nenitzescu & Balaban, 1964] on the reversibility of Friedel-Crafts acetylation of olefins to -chloroketones, the report by Effenberger [Effenberger et al., 1973] of the retro-Fries rearrangement of phenyl benzoates (CF 3 SO 3 H, 170 °C) and the reversible ArS E aroylation of naphthalene derivatives [Okamoto & Yonezawa, 2009]. Additional examples are the acyl rearrangements of acetylphenanthrenes [Levy et al., 2007] and acetylanthracenes [Mala'bi et al., 2009] in PPA, the acetylation of fluorene [Titinchi et al., 2008], the disproportionation of 9acetylanthracene into 1,5-and 1,8-diacetylanthracenes in an ionic liquid systems [Adams et al., 1998]. Complete reversibility of Friedel-Crafts acylation was established in the intramolecular para ortho acyl rearrangements of fluorofluorenones in PPA (Fig. 2) [Agranat et al., 1977]. Friedel-Crafts acyl rearrangement of polycyclic aromatic ketones (PAKs) has been referred to as the Agranat-Gore rearrangement [Levy et al., 2007;Mala'bi et al., 2009]. The Friedel-Crafts acylation can be adjusted to give a kinetically controlled ketone or a thermodynamically controlled ketone [Buehler & Pearson, 1970]. Acyl rearrangements and reversibility in Friedel-Crafts acylations have been associated with thermodynamic control [Pearson & Buehler, 1971;Andreou et al., 1978;Agranat et al., 1977]. The contributions of kinetic control vs. thermodynamic control in Friedel-Crafts acyl rearrangements remain an open question, in spite of the rich chemistry of Friedel-Crafts acylations. We have recently shown that kinetic control wins out over thermodynamic control in the Friedel-Crafts acyl rearrangement of diacetylanthracenes in PPA .

Fig. 2. The Friedel-Crafts intramolecular acyl rearrangements of fluorofluorenones in PPA
A plausible mechanism of the Friedel-Crafts acyl rearrangement of 1-benzoylnaphthalene (1-BzNA) into 2-benzoylnaphthalene (2-BzNA) i n P P A , i s p r e s e n t e d i n F i g . 3 . T h e www.intechopen.com mechanism involves the two dibenzoylnaphthalenes, their O-protonates and theircomplexes. In the kinetically controlled pathway 1 -BzNAH + i s m o r e s t a b l e t h a n 2 -BzNAH + and by virtue of the Hammond-Leffler postulate [Muller, 1994] the transition state leading to 1 -BzNAH + is lower in energy than the transition state leading to 2 -BzNAH + . Thus, 1-BzNA is the kinetically controlled product. By contrast, in the thermodynamically controlled pathway, 1-BzNAH + and 1-BzNA are less stable than 2-BzNAH + and 2-BzNA, respectively. Therefore, 2-BzNA is the thermodynamically controlled product. Under conditions of thermodynamic control, the relative stabilities of the constitutional isomers of a given PAK are detrimental to the products of the Friedel-Crafts acyl rearrangement of the PAK and of the Friedel-Crafts acylation of the corresponding PAH.
The magnitude ot the twist angle of the acetyl group is important. It has been shown that if an acyl group is tilted out of the plane of the aromatic ring of an aromatic ketone by neighboring bulky groups, the resonance stabilization is reduced and the pattern irreversibility of Friedel-Crafts acylation may be challenged, allowing deacylation, transacylation and acyl rearrangments [Buehler & Pearson, 1970;Gore, 1974;Mala'bi, et al., 2011]. Thus, the twist angle may define the ability of diacetylanthracenes to undergo deacylations and rearrangements according to Agranat-Gore rearrangement. Another factor that may influence the tilting of the acetyl group and, as a consequence, the feasibility of acyl rearrangements, is the overcrowding. Its main source is the short contact distances between the carbonyl oxygen and the peri-hydrogen, or between the methyl group and peri-hydrogen. The intramolecular O ... H distances in the crystal structures of the monoand diacetylanthracenes under study are not particularly short, 221-246 pm, for the Z-acetyl groups, which corresponds to 0-9% penetration. There are no close contact distances caused by the E-acetyl groups.

Intermolecular interactions in monoacetylanthracenes and diacetylanthracenes
Aromatic-aromatic interactions are non-covalent intermolecular forces similar to hydrogen bonding [Janiak, 2000]. Aromatic systems may be arranged in three principal configurations:  A stacked (S) configuration, or a π ... π interaction, in which aromatic rings are face-toface aligned, with the interplanar distances of about 3.3-3.8 Å [Janiak, 2000]. This configuration has the maximal overlap but it is rarely observed in real systems containing aromatic rings [Sinnokrot & Sherrill, 2006].  The T-shaped configuration (T), or a C-H ... π interaction, where one aromatic ring points at the center of another ring.  The parallel displaced (D), or offset stacked, configuration; it is reached from the stacked configuration by the parallel shift of one aromatic ring relative to the other [Sinnokrot & Sherrill, 2006], and features both π-π and C-H ... π interactions. The T-and D-type configurations are often observed in small aromatic compounds [Dahl, 1994] and proteins [Hunter et al., 1991]. The crystal structure of the parent anthracene (AN) has been studied [Brock & Dunitz, 1990;Sinclair et al., 1950;Murugan & Jha, 2009]. It crystallizes in the monoclinic space group P2 1 /a. Within the unit cell, the anthracene molecules are packed in a "herringbone" pattern, similar to the parent PAH naphthalene [Desiraju & Gavezzotti, 1989]. In this motif, the C ... C non-bonded interactions are between non-parallel nearest neighbor molecules. The herringbone packing is one of four basic structural types for PAH, which are defined depending on the shortest cell axis and the interplanar angle [Desiraju & Gavezzotti, 1989]. The structures with herringbone packing, "sandwich herringbone" packing and packing obtain crystal stabilization mainly from C ... C interactions, but also from C ... H interactions [Desiraju & Gavezzotti, 1989]. The "graphitic", or , packing characterized by strong C ... C interactions without much contribution from C ... H contacts [Desiraju & Gavezzotti, 1989]. The selected geometric parameters of aromatic interactions in the mono-and diacetylanthracenes under study are presented in Table 4. Cg1 is the centroid for the C 1 -C 2 -C 3 -C 4 -C 4a -C 9a ring, Cg2 is the centroid for the C 4a -C 10 -C 10a -C 8a -C 9 -C 9a ring and Cg3 is the centroid for the C 5 -C 6 -C 7 -C 8 -C 8a -C 10a ring; Cg4-6 are the respective centroids of the second non-equivalent molecule in the unit cell, if it exists. Interplanar angle is the angle between the planes of adjacent molecules. Slippage distance is distance of one centroid from the projection of another centroid. Displacement angle is the angle between the ring normal and the centroid vector. The molecules of 2-AcAN are packed in a "herringbone" pattern, with the interplanar angle of 51.0°. The anthracene moieties in the crystal structure of 2-AcAN adopt the T-configuration with the shortest centroid-centroid separation of 464.7 pm. The shortest distances between the centroids of one molecule and the carbon atoms of the other molecule are Cg3' ... C 4 =343.7 pm, Cg2' ... C 8 =351.2 pm, Cg2' ... C 10 =351.2 pm, Cg3' ... C 9 =357.6 pm and Cg1' ... C 5 =358.2 pm. The respective centroid-hydrogen distances are Cg3' ... H 4 =271.5 pm, Cg2' ... H 8 =283.3 pm, Cg2' ... H 10 =280.8 pm, Cg3' ... H 9 =288.2 pm and Cg1' ... H 5 =287.9 pm. The π ... π interactions in 2-AcAN are very weak despite close lying parallel planes, as reflected in very long distances between the respective centroids (>584 pm). Thus, the aryl C-H ... π interactions dominate in the crystal structure of 2-AcAN. The unit cell of 2-AcAN is shown in Figure 11. The molecules of 1,5-Ac 2 AN are packed in a "herringbone" pattern, with the interplanar angle of 56.2°. The anthracene moieties in the crystal structure of 1,5-Ac 2 AN adopt the Tconfiguration with the shortest centroid-centroid separation of 462.9 and 470.5 pm. The shortest distances between the centroids of one molecule and the carbon atoms of the other molecule are Cg1' ... C 4 =341.9 pm, Cg1' ... C 3 =353.6 pm and Cg2' ... C 4 =376.3 pm. The respective centroid-hydrogen distances are Cg1' ... H 4 =264.3 pm, Cg1' ... H 3 =293.7 pm and Cg2' ... H 4 =342.9 pm. Thus, the aryl C-H ... π interactions dominate in the crystal structure of 1,5-Ac 2 AN, while the π ... π interactions are not possible due to very long distances between molecules lying in the parallel planes (>600 pm). The unit cell of 1,5-Ac 2 AN is shown in Figure 12. The molecules of 1,6-Ac 2 AN are packed by type, forming a layered structure made up of "graphitic" planes with zero interplanar angle. From the point of view of aromatic-aromatic interactions, the anthracene moieties in the crystal structure of 1,6-Ac 2 AN are stacked by the D-type, with the centroid-centroid separation of 359.2 and 385.6 pm. The slippage distances of the centroids are relatively short, 94.0 and 107.1 pm. The shortest contact distances between the aromatic carbons in 1,6-Ac 2 AN are C 5… C 7' =355.1 and C 6… C 8a' =358.5. The unit cell of 1,6-Ac 2 AN is shown in Figure 13. The molecules of 1,7-Ac 2 AN are also packed by type. The anthracene moieties in the crystal structure of 1,7-Ac 2 AN adopt the D-configuration, with the shortest centroidcentroid separation of 370 pm. Despite the longer slippage distance between centroids (154.4-154.8 pm), the contact distances in 1,7-Ac 2 AN are shorter than those in 1,6-Ac 2 AN: C 3… C 8' =333.3, C 4a… C 9' =336.4, C 8… C 9' =337.1 and C 1… C 10' =340.9. In both 1,6-Ac 2 AN and 1,7-Ac 2 AN the aromatic interactions are mainly those of the π ... π type. The unit cell of 1,7-Ac 2 AN is shown in Figure 14. The molecules of 1,8-Ac 2 AN are packed in a "herringbone" pattern, with the interplanar angle of 34.7°. The centroids of the anthracene molecules lying onto the parallel planes are separated by 580-581 pm. These distances together with the slippage distance of 493-494 pm render the aromatic interactions of either S-or D-type impossible. The T-type interactions in 1,8-Ac 2 AN are too weak to be of any importance, due to the long distances between centroids (546-562 pm). However, the plane of the acetyl group (containing C 1 , C 11 , C 13 , O 15 ) of molecule A forms the angle of 4.0° with the aromatic plane of molecule B. Analogously, www.intechopen.com the plane of the acetyl group (containing C 1' , C 11' , C 13' , O 15' ) of molecule B is nearly parallel to the aromatic plane of molecule A, 3.8°. The distances between the anthracene systems and the carbonyl group are sufficiently small to consider the intermolecular π ... π interactions: Cg4' ... O 1 =353.8 pm, Cg4' ... C 11 =384.3 pm, Cg3 ... O 1' =363.3 pm and Cg3 ... C 11' =398.2 pm. Thus, the crystal structure of 1,8-Ac 2 AN features π-π-interactions not between two aromatic systems, but between the aromatic system and the carbonyl π-bond. The unit cell of 1,8-Ac 2 AN is shown in Figure 15. The molecules of 2,7-Ac 2 AN are packed in a "herringbone" pattern. The anthracene moieties in the crystal structure of 2,7-Ac 2 AN adopt the T-configuration, similarly to 1,5-Ac 2 AN. The planes of the adjacent molecules form the angle of 58.1°. The shortest distances between the centroids and the carbon atoms are Cg3 ... C 4 =358.4 pm and Cg1 ... C 5 =375.5 pm on the one side of the anthracene system, and Cg3 ... C 9 =374.0 pm, Cg2 ... C 8 =374.8 pm on the other side. The respective shortest centroid-aryl hydrogen distances are Cg1 ... H 5 =299.3 pm and Cg3 ... H 4 =283.5 pm. The D-type interactions in 2,7-Ac 2 AN are very weak due to the large separation of centroids (420-433 pm) and large slippage distances (226-242 pm). The unit cell of 2,7-Ac 2 AN is shown in Figure 16. The anthracene moieties in the crystal structure of 9,10-Ac 2 AN adopt the T-configuration, similarly to 1,5-Ac 2 AN and 2,7-Ac 2 AN. The planes of the adjacent molecules form the angle of 73.6°. The shortest distances between the centroids and the carbon atoms are Cg2 ... C 7 =356.1 pm, Cg2 ... C 8 =379.6 pm and Cg1 ... C 8 =384.7 pm. The respective shortest centroid-aryl hydrogen distances are Cg2 ... H 7 =287.9 pm, Cg2 ... H 8 =329.4 pm and Cg1 ... H 8 =294.4 pm. The D-type interactions in 9,10-Ac 2 AN are non-existent. The molecules lying in the parallel planes are separated by >720 pm, probably due to the considerable twist angles of the acetyl group in 9,10-Ac 2 AN (-85.0° and 87.0°), making the tighter arrangement impossible. The unit cell of 9,10-Ac 2 AN is shown in Figure 17. Symmetry codes: a 0.5-x, 0.5+y, 1.5-z; b 0.5-x, -0.5+y, 1.5-z; c 1.5-x, 0.5+y, 1.5-z; d 1.5-x, -0.5+y, 1.5-z; e -1+x, y, z; f x, 0.5-y, 0.5-z; g 1-x, 0.5+y, 0.5-z; h x, 1+y, z; i 1-x, 1-y, 1-z; j 1-x, -y, -z; k 1.5-x, 1+y, 1.5-z; l 1-x, -y, 1-z; m 0.5+x, -y, -0.5+z; n 0.5-x, 0.5-y, 0.5-z; o -x, 0.5+y, 0.5-z; p -0.5+x, 1.5-y, z; q 0.5+x, 0.5-y, z; r x, -1+y, z. Thus, the monoacetylanthracenes and diacetylanthracenes under study may be divided into two groups, based on the aromatic-aromatic interactions in their crystal structures. The anthracene units in 1,6-Ac 2 AN and 1,7-Ac 2 AN are offset stacked (the D-type arrangement) and feature aromatic-aromatic π ... π interactions. The anthracene molecules in ketones 2-AcAN, 1,5-Ac 2 AN, 2,7-Ac 2 AN and 9,10-Ac 2 AN adopt the T-type arrangement, and feature aryl C-H ... π interactions. The analysis of the literature crystal structures of 1-AcAN and 9-AcAN shows that these ketones also adopt the T-type arrangement. In 1-AcAN, 9-AcAN, 1,5-Ac 2 AN and 9,10-Ac 2 AN the considerable twist angles of the acetyl groups prevents the molecules from being arranged in close lying parallel planes. The exception is the crystal structure of 1,8-Ac 2 AN, which features π … π-interactions between the aromatic system and the carbonyl π-bond. Most likely the methyl groups are the reason for the lack of more examples of slipped-stacking and also in some cases the competing ketone-π system as well.
It should be noted, however, that the centroid-centroid analysis can be misleading, and its limitations should not be overlooked.

NMR Study of monoacetylanthracenes and diacetylanthracenes
The structure of a compound in crystal is not necessarily the same as that in solution. More often, in the case of substances that are not conformationally homogeneous, e.g. diacetylanthracenes, the crystal has a unique conformation and the conformational heterogeneity appears in fluid phases [Eliel & Wilen, 1994]. An insight into the conformations of mono-and diacetylanthracenes in solution may be gained from the chemical shifts of the aromatic protons adjacent to the carbonyl groups. The magnetic shielding (or deshielding) effect on the chemical shifts of protons that lie in or near the plane of the carbonyl group is well known. The McConnell equation [McConnel, 1957] predicts shielding for protons lying above the center of a carbon-oxygen double bond and deshielding for protons located within a cone aligned with the carbon-oxygen bond axis. The McConnell model, however, takes into account only the effect of magnetic anisotropy.

DFT computational study of monoacetylanthracenes and diacetylanthracenes
DFT methods are capable of generating a variety of isolated molecular properties quite accurately, especially via the hybrid functional, and in a cost-effective way [deProft & Geerlings, 2001, Koch & Holthausen, 2000]. The B3LYP hybrid functional was successfully employed to treat overcrowded BAEs [Biedermann et al., 2001, Pogodin et al., 2006 and overcrowded naphthologues of BAEs-1, i.e. mono-bridged tetraarylethylenes [Assadi et al., 2009]. The monoacetylanthracenes and diacetylanthracenes under study were subjected to a systematic computational DFT study of their conformational spaces and of their relative stabilities. The B3LYP/6-31G(d) relative energies of the global minima conformations of certain diacetylanthracenes have been previously reported . The total and relative B3LYP/6-31G(d) energies (E Tot and ΔE Tot ) and Gibbs free energies (ΔG 298 and ΔΔG 298 ) of the acetylanthracenes are presented in Table 6. Selected calculated geometrical parameters of the acetylanthracenes are also given in Table 6. The following geometrical parameters were considered: the twist angles 1 , 2 and 9 and the respective twist angles around the anthracenyl-carbonyl bond; the dihedral angle θ between the least-square planes of the carbonyl group and the anthracene system; the dihedral angle between the leastsquare planes of two side rings of the anthracene system; the pyramidalization angles at C 1 , C 2 and C 9 .

Me
Ketone 1-AcAN adopts a C s -Z conformation as its global minimum. The planar (excluding the methyl hydrogens) C s -1Z-AcAN is overcrowded due to the short O 13... H 9 contact distance (the O 13... H 9 distance is 215 pm, 14% penetration, based on the sum of the wan-der-Vaals www.intechopen.com radii of oxygen and hydrogen, 244 pm [Zefirov, 1997]). The non-planar C 1 -1E-AcAN conformation (the twist angle 1 (C 9a -C 1 -C 11 -O 13 )=150.8°) is higher in energy by 13.0 kJ/mol. The energy barrier for the E,Z-diastereomerization C s -1Z-AcAN→C 1 -1E-AcAN by the rotation of the acetyl group via a nearly orthogonal transition state is 19.5 kJ/mol. As mentioned above, 1-AcAN [Langer1993] crystallizes as the Z-diastereomer, which is correctly described by the calculated structure of C s -1Z-AcAN. However, the carbonyl group in the crystal structure of 1-AcAN is considerably twisted out of the plane of the anthracene ring system, 1 =27.1°. As a result, the calculated C s -1Z-AcAN structure is more overcrowded than the X-ray structure (in the latter the O 13... H 9 distance is 223 pm). Ketone 2-AcAN adopts a C s -E conformation as its global minimum. Its local minimum C s -2Z-AcAN conformation is 2.2 kJ/mol higher in energy. Both conformations are not overcrowded, lacking any peri-interactions. The energy barrier for the E,Zdiastereomerization C s -2E-AcAN→C s -2Z-AcAN by the rotation of the acetyl group via a nearly orthogonal transition state is 31.5 kJ/mol. The calculated C s -2E-AcAN conformation corresponds well to the E-conformation of the crystal structure. The latter, however, features a small twist angle of 2 (C 1 -C 2 -C 11 -O 13 )=173.1°, in contrast to the planar (excluding the methyl hydrogens) calculated structure.
In the global minimum conformation of 9-AcAN the twist angle 9 (C 9a -C 9 -C 11 -O 13 ) is -67.0°. This conformations cannot be defined as either E or Z, and no other minimum conformation was located. Comparing the calculated structure of 9-AcAN with the crystal structure of 9-AcAN reported in the literature [Zouev2011], the carbonyl group in the latter is almost orthogonal to the plane of the anthracene ring system: the twist angle 9 (C 9a -C 9 -C 11 -O 13 )=87.9° is considerably larger than the twist angle predicted by the DFT calculations. The energy barrier for the enantiomerization of 9-AcAN via the orthogonal [C s -9-AcAN] transition state is only 3.6 kJ/mol. The low enantiomerization barrier as compared to the diastereomerization barriers in 1-AcAN and 2-AcAN is due to an already high twist angle in 9-AcAN. Ketone 1,5-Ac 2 AN adopt a C 2h -1Z,5Z conformation as its global minimum. The geometry optimizations under C 2 or C i symmetry constraints converged to the C 2h symmetry structure. C 2h -1Z,5Z-Ac 2 AN is considerably overcrowded due to the short O 15... H 9 /O 16... H 10 contact distances (14% penetration). The C 2h -1Z,5Z-Ac 2 AN conformation corresponds to the Z,Z Xray structure of 1,5-Ac 2 AN. However, the calculated structure is planar (excluding the methyl hydrogens), while the X-ray structure has the twist angle 1 (C 9a -C 1 -C 11 -O 15 )=20.0° and the dihedral angle θ=22.7°, and, as a result, is less overcrowded. In addition to the global minimum, there are three local minima conformations of 1,5-Ac 2 AN: C 1 -1Z,5E-Ac 2 AN, C i -1E,5E-anti-Ac 2 AN and C 2 -1E,5E-syn-Ac 2 AN. The four conformations of 1,5-Ac 2 AN undergo diastereomerizations by the rotation of one of the acetyl groups via "nearly orthogonal" transition states, in which the rotating acetyl group has the twist angle of =85-97°, and the other acetyl group retains its E-or Z-conformation. The rotation of an acetyl group of C 2h -1Z,5Z-Ac 2 AN via [C 1 -1Z,90-Ac 2 AN] leads to the C 1 -1Z,5E-Ac 2 AN conformation, which is 12.8 kJ/mol higher in energy than the global minimum. The Eorientation of the acetyl group at the 5-position and the peri-interactions of its methyl hydrogens with H 10 force the acetyl group out of the aromatic plane, thus decreasing the conjugation. Due to the twist angle 1 (C 10a -C 5 -C 13 -O 16 )=152.4° which differs from either 0° or 180°, rotation of the 1Z-acetyl group of C 1 -1Z,5E-Ac 2 AN may be realized in either anti-(via [C 1 -90,5E-anti-Ac 2 AN]) or in syn-direction (via [C 1 -90,5E-syn-Ac 2 AN]) relative to the 5Eacetyl group. These processes lead to the different local minima C i -1E,5E-anti-Ac 2 AN and C 2 -1E,5E-syn-Ac 2 AN conformations, respectively, which are 27.4 and 28.0 kJ/mol higher in energy than C 2h -1Z,5Z-Ac 2 AN, due to both acetyl groups being forced out of the aromatic plane: 1 (C 9a -C 1 -C 11 -O 15 )=150.6° and 151.9°, respectively. In addition, the C i -1E,5E-anti-Ac 2 AN and C 2 -1E,5E-syn-Ac 2 AN conformations may undergo syn,anti-diastereomerization via the [C 1 -1E,5E 180 -Ac 2 AN] transition state. It is a "nearly planar" transition state of a different type than the "nearly orthogonal" ones; the twist angle of the rotating acetyl group is close to zero, and the other acetyl group retains its E-or Z-conformation. The [C 1 -1E,5E 180 -Ac 2 AN] transition state is considerably strained due to the short O 16... H 10 distance (205.3 pm) and the distorted sp 2 angles C 13 -C 5 -C 10a (127.9°) and C 13 -C 5 -C 6 (113.4°). The diastereomerization processes in 1,5-Ac 2 AN are shown in Fig. 19. Ketone 1,6-Ac 2 AN adopts a C s -1Z,6E conformation as its global minimum. Like C 2h -1Z,5Z-Ac 2 AN, it is overcrowded due to the short O 15... H 9 contact distance (14% penetration). The C s -1Z,6E-Ac 2 AN conformation corresponds to the Z,E X-ray structure of 1,6-Ac 2 AN. As in the case of 1,5-Ac 2 AN, the DFT calculations predict a planar structure for 1,6-Ac 2 AN, while the X-ray geometry features the twisted 1Z-acetyl group: the twist angle 1 (C 9a -C 1 -C 11 -O 15 )=30.0° and the dihedral angle θ=32.2°. The 6E-acetyl group remains in the aromatic plane in both calculated and the X-ray geometries. The rotation of the 1Z-acetyl group leads from C s -1Z,6E-Ac 2 AN via [C 1 -90,6E-Ac 2 AN] to the local minimum C 1 -1E,6E-Ac 2 AN, which is 13.6 kJ/mol higher in energy. The 6E-acetyl group, in contrast to the 1E-acetyl group, lies in the aromatic plane: 1 (C 9a -C 1 -C 11 -O 15 )=150.6° and 2 (C 5 -C 6 -C 13 -O 16 )=179.9°. The rotation of the 6E-acetyl group of C 1 -1E,6E-Ac 2 AN may be realized either via [C 1 -1E,90-syn-Ac 2 AN] or via [C 1 -1E,90-anti-Ac 2 AN] transition states; both pathways lead to C 1 -1E,6Z-anti-Ac 2 AN, which is 15.4 kJ/mol higher in energy than the global minimum. The rotation of the 6Eacetyl group in C s -1Z,6E-Ac 2 AN via [C 1 -1Z,90-Ac 2 AN] leads to the local minimum C s -1Z,6Z-Ac 2 AN, which is only 1.7 kJ/mol higher in energy than the global minimum. The rotation of the 1E-acetyl group in C 1 -1E,6Z-Ac 2 AN via [C 1 -90,6Z-Ac 2 AN] also leads to C s -1Z,6Z-Ac 2 AN. The diastereomerization processes in 1,6-Ac 2 AN are shown in Fig. 20. Ketone 1,7-Ac 2 AN, similarly to 1,6-Ac 2 AN, adopts a C s -1Z,7E conformation as its global minimum. It is overcrowded due to the short O 15... H 9 contact distance (15% penetration). The C s -1Z,7E-Ac 2 AN conformation corresponds to the Z,E X-ray structure of 1,7-Ac 2 AN. The differences between the geometries of the planar DFT calculated structure of C s -(1Z,7E)-Ac 2 AN and the twisted X-ray structure of 1,7-Ac 2 AN are smaller than in 1,5-Ac 2 AN and 1,6-Ac 2 AN. In the X-ray structure of 1,7-Ac 2 AN the twist angles are 1 (C 9a -C 1 -C 11 -O 15 )=-15.2° and 2 (C 8 -C 7 -C 13 -O 16 )=-176.6. The relative stabilities of the conformations of 1,7-Ac 2 AN and its conformational space are very similar to those of 1,6-Ac 2 AN, both being ,diacetylanthracenes. The local minima conformations C s -1Z,7E-Ac 2 AN, C 1 -1E,7E-Ac 2 AN and C 1 -1E,7Z-anti-Ac 2 AN are higher in energy than the global minimum by 3.5, 14.5, and 16.6 kJ/mol, respectively. The diastereomerization processes in 1,7-Ac 2 AN are shown in Fig. 21.
Although the conformational space of 1,8-Ac 2 AN resembles that of another ,diacetylanthracene, 1,5-Ac 2 AN, it is more complicated. There are three local minima conformations of 1,8-Ac 2 AN: C 1 -1Z,8E-Ac 2 AN, C s -1E,8E-syn-Ac 2 AN and C 2 -1E,8E-anti-Ac 2 AN. Rotation of an acetyl group of C 2 -1Z,8Z-Ac 2 AN via [C 1 -1Z,90-Ac 2 AN] leads to the C 1 -1Z,8E-Ac 2 AN conformation, which is only 0.4 kJ/mol higher in energy. The tilting of the 8E-acetyl group ( 2 (C 8a -C 8 -C 13 -O 16 )=150.4°) allows the 1Z-acetyl group to align itself with the aromatic plane ( 1 (C 9a -C 1 -C 11 -O 15 )=1.5°), restoring the conjugation and thus stabilizing this conformation. The rotation of the 1Z-acetyl group of C 1 -1Z,8E-Ac 2 AN may be realized www.intechopen.com in either syn-(via [C 1 -90,8E-syn-Ac 2 AN]) or in anti-direction (via [C 1 -90,8E-anti-Ac 2 AN]) relative to the 8E-acetyl group. These pathways lead to the local minima C s -1E,8E-syn-Ac 2 AN and C 2 -1E,8E-anti-Ac 2 AN conformations, respectively, which are 17.6 and 17.7 kJ/mol higher in energy than the global minimum. These two conformations undergo interconversion via the [C 1 -1E,8E 180 -Ac 2 AN] transition state. The diastereomerization processes in 1,8-Ac 2 AN are shown in Fig. 22. Ketone 2,7-Ac 2 AN adopts a C s -2E,7Z conformation as its global minimum. It is not overcrowded, lacking peri-interactions. The C s -(2E,7Z)-Ac 2 AN conformation corresponds well to the E,Z X-ray structure of 2,7-Ac 2 AN. The differences between the geometries of the planar DFT calculated structure of C s -2E,7Z-Ac 2 AN and the twisted X-ray structure of 2,7-Ac 2 AN are not large: in the latter structure the twist angles are 2 (C 1 -C 2 -C 11 -O 15 )=171.9° and The conformational space of 2,6-Ac 2 AN is similar to that of 2,7-Ac 2 AN. The global minimum is the C 2h -2E,6E-Ac 2 AN conformation. Rotation of the 6E-acetyl group leads to C s -2E,6Z-Ac 2 AN conformation, which is only 0.4 kJ/mol higher in energy than the global minimum. The rotation of the 2E-acetyl group in C s -2E,6Z-Ac 2 AN leads to C 2h -2Z,6Z-Ac 2 AN conformation, which is 3.8 kJ/mol higher in energy than the global minimum. The diastereomerization processes in 2,6-Ac 2 AN are shown in Fig. 24. Ketone 9,10-Ac 2 AN stands out of the other diacetylanthracenes by virtue of its acetyl groups being each flanked by two peri-hydrogens. In order to avoid short non-contact distances to H 1 /H 4 /H 5 /H 8 , the acetyl groups in all the conformations of 9,10-Ac 2 AN are considerably twisted. Another mode for the relief of the steric strain in 9,10-Ac 2 AN is elongation of the C 11 -C 9 and C 12 -C 10 carbonyl bonds, 151.6 pm, as compared to 149.7 pm in planar C s -(2E,7Z)-Ac 2 AN and 149.8 pm in C s -(2E,6E)-Ac 2 AN. The global minimum of 9,10-Ac 2 AN is a C i -E conformation, with the twist angles 9 (C 9a -C 9 -C 11 -O 15 )=-72.6°, 9 (C 10a -C 10 -C 13 -O 16 )=72.6° and the dihedral angle θ=74.7°. It corresponds well to the X-ray structure, which features even higher twist angles 9 =-85.0°, 87.0° and the dihedral angles θ=86.7°, 86.5°. The local minima conformations of 9,10-Ac 2 AN are C s -Z (0.1 kJ/mol), C 2 -E (1.0 kJ/mol) and C 2 -Z (2.1 kJ/mol). They all have high twist angles, ±71.8°, 75.4° and -71.9°, respectively. The similarity of the energies and the geometries of the four conformations of 9,10-Ac 2 AN stems from the fact that in 9,10-Ac 2 AN, each of the Z and E conformations is defined relative to the other acetyl group, and not by the twist angles of the carbonyl groups relative to the anthracene system, which are very similar for all four conformations of 9,10-Ac 2 AN. The C i -E global minimum undergoes diastereomerization to the C 2 -E conformation via [C 1 -(9E,10E 180 )] transition state, in which one of the carbonyl groups lies in the aromatic plane. The C s -Z and C 2 -Z conformations interconvert via the analogous [C 1 -(9Z,10Z 0 )] transition state. The C i -E conformation diastereomerizes to the C 2 -Z conformation and the C 2 -E conformation diastereomerizes to the C s -Z conformation via the pair of transition states [C 1 -90-syn] and [C 1 -90-anti], in which one of the carbonyl groups is orthogonal to the aromatic plane. The diastereomerization processes in 9,10-Ac 2 AN are shown in Fig. 25. Ketone 1,9-Ac 2 AN has never been isolated. Recently 1,9-Ac 2 AN has been claimed to be a putative intermediate in the Friedel-Crafts acyl rearrangements of 1,5-Ac 2 AN, 1,8-Ac 2 AN and 9,10-Ac 2 AN in PPA to give 3-methylbenz[de]anthracen-1-one . Ketone 1,9-Ac 2 AN adopts a C 1 -1Z,9Z-anti conformation as its global minimum. Both acetyl groups are considerably twisted because of their mutual peri-positions: 1 (C 9a -C 1 -C 11 -O 15 )=-50.9°, 9 (C 9a -C 9 -C 13 -O 16 )=-59.6°. The local minimum conformation C 1 -1E,9Z-syn-Ac 2 AN is considerably higher in energy than the global minimum, 25.9 kJ/mol. Potentially, two more conformations may exist due to the twist angles 1 and 9 being different from 0° or 180°, i.e. C 1 -1Z,9Z-syn-Ac 2 AN and C 1 -1E,9Z-anti-Ac 2 AN. However, the search after these conformations has not resulted in any additional stationary points. The C 1 -1Z,9E-Ac 2 AN and C 1 -1E,9E-Ac 2 AN conformations have also not been found in the conformational space of 1,9-Ac 2 AN, probably due to the considerable steric strain caused by the peri-interactions between the methyl of the 9E-acetyl group and the 1-acetyl group. Ketone 1,10-Ac 2 AN (which has never been synthesized ) adopts a C 1 -1Z,10E conformation as its global minimum. Contrary to 1,9-Ac 2 AN, its acetyl groups do not affect directly each other. Hence, their twist angles, 1 (C 9a -C 1 -C 11 -O 15 )=0.2°, 9 (C 4a -C 10 -C 13 -O 16 )=-108.0°, are very close to the twist angles of the lone acetyl groups in C s -1Z-AcAN (0.0°) and C 1 -9-AcAN (-67.0°), respectively. Another consequence of the non-interacting acetyl groups in 1,10-Ac 2 AN is the abundance of conformations -six minima conformations have been identified. The local minimum C 1 -1Z,10Z-Ac 2 AN conformation is only 1.0 kJ/mol less stable than the global minimum, and differs from it in the twist angle 9 (C 4a -C 10 -C 13 -O 16 )=-65.9°. There are four 1E conformations of 1,10-Ac 2 AN, which have the twist angles 1 (C 9a -C 1 -C 11 -O 15 ) of 148-150° and the relative energy of 13.9-15.3 kJ/mol. The conformational behavior of 1,10-Ac 2 AN is complicated. Depending on the rotational direction of the 1Z-acetyl group, the C 1 -1Z,10E-Ac 2 AN conformation may undergo diastereomerization to either C 1 -1E,10E-anti-Ac 2 AN or C 1 -1E,10E-syn-Ac 2 AN via the respective "nearly orthogonal" transition states. Analogously, C 1 -1Z,10Z-Ac 2 AN may undergo diastereomerization to either C 1 -1E,10Z-anti-Ac 2 AN or C 1 -1E,10Z-syn-Ac 2 AN. The C 1 -1E,10E-anti-Ac 2 AN and C 1 -1E,10Zanti-Ac 2 AN conformations are interconnected via the [C 1 -1E,90-anti-Ac 2 AN] transition state, while C 1 -1E,10E-syn-Ac 2 AN and C 1 -1E,10Z-syn-Ac 2 AN are interconnected via the [C 1 -1E,90syn-Ac 2 AN] transition state. Finally, C 1 -1E,10E-anti-Ac 2 AN is interconnected with C 1 -1E,10Esyn-Ac 2 AN and C 1 -1E,10Z-anti-Ac 2 AN is interconnected with C 1 -1E,10Z-syn-Ac 2 AN, via the "nearly planar" transition states [C 1 -1E,10E 180 -Ac 2 AN] and [C 1 -1E,10Z 0 -Ac 2 AN], respectively. The diastereomerization processes in 1,10-Ac 2 AN are shown in Fig. 26. Ketone 2,9-Ac 2 AN (which has never been synthesized) adopts a C 1 -2E,9E conformation as its global minimum. The acetyl groups in 2,9-Ac 2 AN do not affect directly one another, and twist angles are similar to the respective twist angles in 2-AcAN and 9-AcAN: 2 (C 1 -C 2 -C 11 -O 15 )=-178.9° and 9 (C 4a -C 10 -C 13 -O 16 )=-106.9°. There are two local minima conformations of 2,9-Ac 2 AN, C 1 -2E,9Z-Ac 2 AN (3.6 kJ/mol above the global minimum) and C 1 -2Z,9E-Ac 2 AN (5.1 kJ/mol). Surprisingly, the search after the C 1 -2Z,9Z-Ac 2 AN conformation was not successful. The acetyl groups in the putative C 1 -2Z,9Z-Ac 2 AN conformation are not expected to cause a steric hindrance more severe than in the C 1 -1Z,9Z-anti-Ac 2 AN conformation. Nevertheless, the latter conformation exists and even was found to be a global minimum, while the former does not seem to exist. The global minimum C 1 -2E,9E-Ac 2 AN conformation may diastereomerize either to the C 1 -2E,9Z-Ac 2 AN conformation via the [C 1 -2E,90-Ac 2 AN] transition state, or to the C 1 -2Z,9E-Ac 2 AN conformation via the [C 1 -90,9E-Ac 2 AN] transition state. The diastereomerization processes in 2,9-Ac 2 AN are shown in Fig. 27.
[C 1 -(90,9E)] 0.0 3.6 5.1 Fig. 27. The interconversion of conformations of 2,9-Ac 2 AN and their relative Gibbs free energies (ΔG 298 , kJ/mol) Ketone 2,10-Ac 2 AN (which has never been synthesized) adopts a C 1 -2E,10E conformation as its global minimum. The twist angles are 2 (C 1 -C 2 -C 11 -O 15 )=179.9° and 9 (C 4a -C 10 -C 13 -O 16 )=-113.9°. The global minimum C 1 -2E,10E-Ac 2 AN conformation may diastereomerize either to the C 1 -2Z,10E-Ac 2 AN conformation (the relative energy of 2.1 kJ/mol) via the [C 1 -90,10E-Ac 2 AN] transition state, or to the C 1 -2E,10Z-Ac 2 AN conformation (0.3 kJ/mol) via the [C 1 -2E,90-Ac 2 AN] transition state. Both these local minima configurations undergo diastereomerization to the C 1 -2Z,10Z-Ac 2 AN conformation (3.9 kJ/mol) via the [C 1 -2Z,90-Ac 2 AN] and [C 1 -90,10Z-Ac 2 AN] transition states, respectively. The diastereomerization processes in 2,10-Ac 2 AN are shown in Fig. 28. The comparison between the X-ray structures of mono-and diacetylanthracenes and their respective calculated geometries deserves a brief discussion. The absolute values of the twist angles of the B3LYP/6-31G(d) calculated conformations (including the local minima conformations) of mono-and diacetylanthracenes may be summarized as follows: | 1 |=0-17.3°2 for the 1Z-acetyl groups, | 1 |=141.2-152.4° for the 1E-acetyl groups, | 2 |=0.0-1.8° for the 2Z-acetyl groups, | 2 |=178.9-180.0° for the 2E-acetyl groups, and | 9 |=44.8-75.4° (180-| 9 | values were taken for | 9 |>90°). The respective twist angles derived from the X-ray structures are | 1 |=15.2-34.0° for the 1Z-acetyl groups, | 2 |=0.9° for the 2Z-acetyl group, | 2 |=177.9-178.6° for the 2E-acetyl groups, and | 9 |=85.0-87.9°. There is no X-ray structure of acetylanthracenes featuring a 1E-acetyl group, and such conformations are always found to be local minima by the DFT calculations. The B3LYP/6-31G(d) calculations seem to underestimate the twist angles of the 1Z-and 9-acetyl groups in mono-and diacetylanthracenes. In a number of cases it leads to predicting planar and more overcrowded conformations than the respective twisted X-ray geometries. There is a limited number of reports in the literature that DFT methods, including B3LYP, could overstabilize planar conformations of biphenyl and related heteroaromatic compounds [Viruela et al., 1997;Karpfen et al., 1997]. As in the X-ray structures, the acetyl groups and the anthracene systems in the mono-and diacetylanthracenes under study are essentially planar. Thus, B3LYP/6-31G(d) satisfactorily predicts the overall conformations of mono-and diacetylanthracenes under study, i.e. the Z-conformation of 1-AcAN, the E-conformation of 2-AcAN, the twisted conformation of 9-AcAN, the Z,Z conformations of 1,5-Ac2AN and 1,8-Ac 2 AN, the Z,E conformations of 1,6-Ac 2 AN, 1,7-Ac 2 AN and 2,7-Ac 2 AN, and the E,E conformation of 9,10-Ac 2 AN. It has not escaped our mind, however, that the packing interactions in crystals can readily dominate and suppress any preference for one conformation or another, especially in the cases of low diastereomerization barriers and low energy differences. We also note the limitations of the DFT calculations in the gas phase and in the comparison of the computational results with the crystal structures. The relative free Gibbs energies of the diacetylanthracenes under study are given in Table 6. It should be noted that 1,5-Ac 2 AN is 11.2 kJ/mol more stable than its constitutional isomer 1,8-Ac 2 AN. The acetyl groups of 1,5-Ac 2 AN are attached to a starred and an unstarred aromatic carbons of alternant anthracene, while the acetyl groups of 1,8-Ac 2 AN are both attached to starred aromatic carbons. Simple resonance considerations would favor the stabilization of 1,8-Ac 2 AN over 1,5-Ac 2 AN, due to the better delocalization of the partial positive charge in the dipolar Kekulé structures. However, the twist angle of the acetyl groups in 1,8-Ac 2 AN are notably larger than that in 1,5-Ac 2 AN, in both the crystal structures (32.4°/34° vs. 20.0°) and the DFT calculated geometries (17.3° vs. 0.0°). This increased twist angle decreases the conjugation between the acetyl group and the aromatic system, thus destabilizing 1,8-Ac 2 AN relative to 1,5-Ac 2 AN.

Activation barriers
As noted above, monoacetylanthracenes and diacetylanthracenes may undergo E,Zdiastereomerizations and syn,anti-diastereomerizations by rotation of their acetyl groups. The diastereomerization of the first type connects an E-diastereomer with a Z-diastereomer and proceeds via a "nearly orthogonal" transition state, in which the acetyl group, rotating around the C arom -C carb bond, has the twist angle of =85-97° (the other acetyl group in diacetylanthracenes retains its E-or Z-orientation). The diastereomerization of the second type occurs only in diacetylanthracenes and connects either an E-syn-diastereomer with an E-anti-diastereomer, or a Z-syn-diastereomer with a Z-anti-diastereomer. It proceeds via a "nearly planar" transition states, in which the twist angle of the rotating acetyl group is close to either 180° (E-diastereomer) or zero (Z-diastereomer), and the other acetyl group retains its E-or Z-orientation. Fig. 29 and Fig. 30 show the E,Z-diastereomerization and syn,anti-diastereomerization on the example of 1,8-Ac 2 AN. Table 7 gives the energy barriers for the E,Z-diastereomerization and syn,antidiastereomerization in the monoacetylanthracenes and diacetylanthracenes under study by rotation of the acetyl groups via the respective nearly orthogonal or nearly planar transition states. The E,Z-diastereomerization energy barriers may be divided into three groups, depending on the position of the acetyl group that undergoes rotation and on its conformation. E-Acetyl groups at positions 1, 5 and 8 and acetyl groups at positions 9 and 10 have the lowest energy barriers, 3.6-9.5 kJ/mol, due to their already significant twist angles ( =141-152° for E-acetyl groups at positions 1, 5 and 8 and =67-73° for acetyl groups at positions 9 and 10). Z-Acetyl groups at the same positions 1, 5 and 8 have medium energy barriers, 19.5-23.5 kJ/mol. The twist angles of these acetyl groups are smaller ( =0-17°), but the E,Z-diastereomerization in this case is facilitated by the steric strain due to repulsive peri-interactions between the carbonyl oxygen and aromatic H 9 /H 10 hydrogens. Finally, both E-and Z-acetyl groups at positions 2, 6 and 7 have the highest energy barriers for diastereomerization, 27.3-31.6 kJ/mol, due to the lack of steric strain and negligible twist angles (less than 1°). For comparison, the experimental rotational energy barrier for methyl 1-(2-methylnaphthyl) ketone is 33.9 kJ/mol (-110 °C, [Wolf, 2008]). Table 8 gives the relative Gibbs free energies of the global minima and the most stable local minima of the acetylanthracenes whose crystal structures have been determined in this study or reported in the literature, i.e. 1-AcAN, 2-AcAN, 9-AcAN, 1,5-Ac 2 AN, 1,6-Ac 2 AN, 1,7-Ac 2 AN, 1,8-Ac 2 AN, 2,7-Ac 2 AN and 9,10-Ac 2 AN, as well the energy barriers for their E,Z- Fig. 29. E,Z-Diastereomerization of C 2 -1Z,8Z-Ac 2 AN to C 2 -1Z,8E-Ac 2 AN via [1Z,90-Ac 2 AN] transition state Fig. 30. syn,anti-Diastereomerization of C 2 -1E,8E-anti-Ac 2 AN to C s -1E,8E-syn-Ac 2 AN via [1E,8E 180 -Ac 2 AN] transition state diastereomerizations. The energy barriers are in the range of 20-32 kJ/mol (relative to the respective global minima) for the rotation of the acetyl groups at 1, 2, 5, 6 and 7 positions. The lower energy barrier in 1,8-Ac 2 AN (9.8) may be rationalized by destabilization of the global minimum due to the larger twist of the acetyl groups. This effect is even more pronounced in the case of 9-AcAN and 9,10-Ac 2 AN, which have large twist values (67° and 73°, respectively) and remarkably low E,Z-diastereomerization barriers (less than 4 kJ/mol). All these barriers are sufficiently low to allow a swift E,Z-diastereomerizations on the NMR time scale (at room temperature), in accordance with the results of the NMR experiments (vide supra). The differences in the relative energies of the global minimum and the most stable local minimum of these acetylanthracenes are relatively small, 0.06-3.5 kJ/mol (with the exception of 1-AcAN and 1,5-Ac 2 AN), which suggests the presence of both E-and Zdiastereomers in equilibrium mixture at ambient temperature. 1E,8E-anti-Ac 2 AN→1E,8E-syn-Ac 2 AN 9.42 C i -E-9,10-Ac 2 AN→C 2 -Z-9,10-Ac 2 AN 3.73 [C 1 -90-anti-9,10-Ac 2 AN] -844.79633239 C 2 -Z-9,10-Ac 2 AN→C i -E-9,10-Ac 2 AN 3.67 C 2 -E-9,10-Ac 2 AN→C s -Z-9,10-Ac 2 AN 3.44 [C 1 -90-syn-9,10-Ac 2 AN] -844.79605863 C s -Z-9,10-Ac 2 AN→C 2 -E-9,10-Ac 2 AN 4.36

Conclusions
The monoacetylanthracenes and diacetylanthracenes under study adopt non-planar conformations in their crystal structures. The twist angles are maximal for the 9-acetyl groups (| 9 |=85.0-87.9°) and significant for the 1Z-acetyl groups (| 1 |=15.2-34.0°), but very small for 2-acetyl groups. The conformations in solution are in agreement with the X-ray crystal structure conformations, according to the NMR data. The crystal structures are stabilized by intermolecular interactions: aromatic-aromatic π-π interactions (1,6-Ac 2 AN and 1,7-Ac 2 AN), C ... H-π interactions (2-AcAN, 1,5-Ac 2 AN, 2,7-Ac 2 AN and 9,10-Ac 2 AN), or π-π interactions between the anthracene unit and the carbonyl bond (1,8-Ac 2 AN). The B3LYP/6-31G(d) calculated conformations of the monoacetylanthracenes and diacetylanthracenes are in good agreement with the X-ray crystal structures. The acetyl groups in the crystal structures and the B3LYP/6-31G(d) calculated global minima of the monoacetylanthracenes and diacetylanthracenes preferentially adopts 1Z and 2E conformations. The order of stabilities of the diacetylanthracenes under study is 2,7-Ac 2 AN>1,7-Ac 2 AN≈1,6-Ac 2 AN>1,5-Ac 2 AN>1,8-Ac 2 AN>9,10-Ac 2 AN. The acetyl groups at positions 1, 5 and 8 destabilize the diacetylanthracenes because of the repulsive interactions between the carbonyl oxygen/methyl group and the aromatic peri-hydrogens, and because of the decreased resonance stabilization. This effect is even more pronounced for the acetyl groups at positions 9 and 10. The B3LYP/6-31G(d) calculated energy barriers for the E,Zdiastereomerizations show that the E,Z-diastereomerizations is swift on the NMR time scale (at room temperature), in accordance with the results of the NMR experiments. The present results of the crystallographic and theoretical study of monoacetylanthracenes and diacetylanthracenes contribute to our understanding of the motifs of reversibility and thermodynamic control in the Friedel-Crafts acyl rearrangements of these representative PAKs. Table 9 summarizes the applied methods of preparation of the monoacetylanthracenes and diacetylanthracenes. Melting points are uncorrected. All NMR spectra were recorded with Bruker DRX 500 MHz spectrometer. 1 H-NMR spectra were recorded at 500.13 MHz using CDCl 3 as solvent and as internal standard, (CDCl 3 )=7.263 ppm. 13 C-NMR spectra were recorded at 125.75 MHz using CDCl 3 as a solvent with internal standard, (CDCl 3 )=77.008 ppm. Complete assignments were made through 2-dimensional correlation spectroscopy (COSY, HSQC, HBMC and NOESY). Anthracene and nitrobenzene were obtained from Sigma-Aldrich; acetyl chloride and aluminum chloride were obtained from Acros. All the solvents were AR grade. Chloroform and dichloromethane were distilled before use. Single crystal X-ray diffraction was carried out on a Bruker SMART APEX CCD X-ray diffractometer, equipped with graphite monochromator and using MoK radiation (λ=0.71073 Å). Low temperature was maintained with a Bruker KRYOFLEX nitrogen cryostat. The diffractometer was controlled by a Pentium-based PC running the SMART software package [Bruker AXS GmbH, 2002a]. Immediately after collection, the raw data frames were transferred to a second PC computer for integration and reduction by the SAINT program package [Bruker AXS GmbH, 2002b]. The structures were solved and refined by the SHELXTL software package [Bruker AXS GmbH, 2002c].  Table 9. Summary of methods of preparation of monoacetylanthracenes and diacetylanthracenes.