One-Pot Synthesis of Chiral Organometallic Complexes

Currently, organometallic complexes involving ligand oxazolines are typically obtained in two sequential steps, where the free ligand is given firstly from a functionalized nitrile by condensation reaction with an amino alcohol in the presence of a Lewis or Bronsted acid catalyst, followed by a further coordination with metal salts to obtain the corresponding oxazolinyl metal complexes. Usually, the yield of the two-step procedure is relatively low; considering that metal oxazoline complexes often contain Lewis acidic metals, it is possible that the two steps may be telescoped. A series of novel chiral organometallic complexes ( 1–23 ) were assembled in a single step from nitriles, chiral D/L amino alcohols, and a stoichio-metric amount of metal salts (MCl 2 ·nH 2 O/M(OAc) 2 ·nH 2 O), with moderate to high yields (20–95%). All the crystalline compounds were fully characterized by NMR, IR, MS, and X-ray analyses.


Introduction
Chiral oxazolines constitute an important class of "privileged" ligands in asymmetric catalysis [1][2][3][4][5][6][7][8]. Organometallic complexes involving oxazoline ligands are typically obtained in two steps, where the free ligand is given firstly from a functionalized nitrile through condensation reaction with an amino alcohol in the presence of a Lewis or Bronsted acid catalyst, followed by further complexed coordination with metal salts to obtain the corresponding oxazolinyl metal complexes (Figure 1) [9,10]. Usually, the yield of the two-step procedure is relatively low, and certain oxazolinyl organometallic complexes are difficult to obtain due to the poor coordination ability of the imine group from the oxazoline. It is conceivable that the two steps may be telescoped by using the requisite Lewis acid precursor. Herein, through the assembly of three reaction components (a nitrile, an amino alcohol, and metal salts), we first report a simple, one-step procedure for the preparation of N-containing heterocyclic zinc complexes (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15), with the yield of certain products reaching 90% in the presence of a large amount of ZnCl 2 (0.4-2.6 eq.) and certain chiral salicyloxazoline metal complexes (16)(17)(18)(19)(20)(21)(22)(23) with yields ranging from 65 to 95% using 1.0 eq. of copper, cobalt, nickel, manganese, palladium, and platinum salts as the third component. In all the cases, the complexes were isolated, purified, and characterized. All the structures reported in this paper were confirmed by X-ray crystallography.

One-step multicomponent synthesis of chiral oxazolinyl-zinc complexes
The one-pot procedure was initially tested from the reaction of different 1-piperidine propionitrile derivatives with 2-3 eq. of amino alcohol refluxed in chlorobenzene for 72 h in the presence of 1-2.6 eq. of ZnCl 2 . After cooling to room temperature, the solvent was removed under reduced pressure, and the residue was dissolved in H2O and extracted with CH 2 Cl 2 . The combined organic extracts were evaporated to give a crude red oil, which was purified by column chromatography (petroleum ether/CH 2 Cl 2 , 4/1) to afford the title compound as crystals. During the preliminary work, it quickly became apparent that the reaction results are controlled by the amount of ZnCl 2 used (Figure 2); for example, employing 1.1 eq. or 2.6 eq. of ZnCl 2 , the desired crystal structures of the aminooxazolidinyl zinc complex 1 and bis-oxazolidinyl zinc complex 2 containing two monodentate ligands can be obtained from the reaction of L-leucinol or L-valinol with 3-piperidin-1-yl-propionitrile, respectively, followed by evaporation of different ratios of petroleum and dichloromethane from the mixture after column separation but with only a low yield (25%) for complex 1 and moderate yield (65%) for complex 2.
The nature of the side chain (R 1 ) influenced the reaction outcome. Using L-phenylalaninol with 1.6 eq. of ZnCl 2 or 1.5 eq. of ZnCl 2 with 1-morpholinepropionitrile (X = O) and D-phenylglycinol, both led to the cleavage of the propionitrile, providing asymmetric diamine complexes 3 and 4 at very good yields of 86% and 90%, respectively. Interestingly, using 1-(2-cyanoethyl)-4-methylpiperazine (Z = NMe) as a precursor with 2.5 eq. of ZnCl 2 led only to the formation of the zwitterionic piperazine complex 5, irrespective of the amino alcohol used. The results again prove the effects of different amounts of metal salts on the reaction.
From the crystal structures of complexes 2-5, we conclude that the propionitrile precursors are unstable and decompose into acetonitrile or the parent cyclic amines to afford complexes 2 and 3-5, respectively, in the presence of a large amount of zinc chloride. For this reason, a number of nitrile precursors with additional N-donor were selected to be more robust against degradation under the reaction conditions. Consequently, a number of aromatic nitrile precursors containing additional N-donors were applied widely in these three-component reactions. In the process of selecting these reactions, the appropriate amount of ZnCl 2 was carefully optimized to ensure specific results. Complex 6, listed in Figure 3, contains two monodentate ligands coordinated via the oxazoline nitrogen and was afforded from the use of 3-aminobenzonitrile and D-leucinol in the presence of 0.44 eq.  of ZnCl 2 . Similarly, the bis-chelated complex 7 and the mono-chelated complex 8 were afforded to the corresponding yields of 80 and 78% from the use of 2-cyanopyridine with L-phenylalaninol and D-valinol, respectively, in the presence of 1.2 eq. of ZnCl 2 .
The formation of complexes 9 and 10 were studied using different amino alcohols, as seen in Figure 4. C2-symmetrical bis-oxazolines formed seven-membered chelate rings which derived from 1,2-dicyanobenzene, affording a 1:1 adduct with zinc dichloride. Indeed, the addition of isophthalonitrile with D-phenylglycinol (0.56 eq.) provided the predicted mono-chelated complex 9 [11] at a good yield (68%). However, a combination of a slight excess of L-valinol (0.72 eq.) caused the addition of three amino alcohols to give complex 10 with a yield of 66%.
Surprisingly, the combination of L-leucinol and L-phenylglycinol with tetracyanoethylene in the presence of 0.42 eq. of ZnCl 2 , respectively, provided neutral bis[bis(oxazoline)]zinc (II) complexes 11 and 12 with corresponding yields of 88 and 86% (Figure 5). The crystal structures of these methylene-bis(oxazoline) indicate that the tricyanomethane was formed as an intermediate from a disproportionation-rearrangement of the tetracyanoethylene precursor, although the precise mechanism of this pathway is unclear. In 2016, Kögel et al. reported the synthesis of complex 12 by a different route [12]. Interestingly, complex 12 was reported to exhibit an intense cotton effect as a result of exciton coupling. Indeed, the X-ray crystal structures of complexes 11 and 12 have been proven that due to the their coordination environments, isobutyl-substituted complex 11 has shown a fairly symmetrical tetrahedral comformation, while complex 12 is in highly distorted. This maybe the result of the favorable intramolecular π-interaction between one of the phenyl groups with the semicorrin structure of the adjacent ligand within 3.5 Å, effectively leading to the two chiral chromophores' close proximity to convenience exciton coupling [13].
In further study, 2-hydroxy-6-methylnicotinonitrile was employed as a precursor to test the applicability of the one-pot methodology in assembling complex multinuclear structures. In the presence of different amounts of ZnCl 2 (1.72, 1.31,    and 1.54 eq.), the corresponding condensation products with valinol, leucinol, or phenylalaninol furnished the binuclear zwitterionic complex 13 and highly symmetrical tetramers 14 and 15 at yields of 86, 80, and 82%, accordingly (Figure 6). Presumably, the formation of higher aggregates is prevented by the sterically demanding isopropyl substituents. A six-membered N,O-chelate ligand is complexed at each zinc metal center and connected to another metal center with a bridging donor ligand from the pendant pyridine. With each zinc atom located at a corner of a square grid, the planar N,O,N-ligands are oriented perpendicularly to one another with diagonal Zn···Zn distances of ca. 6 Å.    The crystal structures of all the complexes (Figures 7-21) are determined and reported by X-ray diffraction, elemental analysis, and IR. In all the cases, a distorted tetrahedral geometry is found at zinc(II), and the C〓N double-bond character of the oxazolinyl ligand is largely retained in the metal complexes ( Table 1).

One-pot synthesis of zinc complexes (1-15).
In 2017, our research group first reported a one-pot multicomponent synthesis of chiral oxazolinyl-zinc complexes [44] in the presence of a large amount of ZnCl 2 (0.4-2.6 eq.); the yields of certain products reached 90%. Herein, the chiral salicyloxazoline metal complexes 16-23 can be obtained by using 1.0 eq. of metal salts such as copper, cobalt, nickel, manganese, palladium, and platinum salts as the third component. The structures of these complexes were characterized by X-ray crystallography. The results prove that organometallic complexes can be assembled with two reactants and different amounts of metal salts. Chiral bis(oxazoline) copper complex 16, nickel complex 17, cobalt complex 18, and palladium complex 19 were generated as crystals with the chemical formula ML 2 (L = 2-(4-R 1 -4,5-dihydrooxazol-2-yl)phenol, R 1 : d-Ph, M: Cu, Ni, Co; R 1 : l-CH 2 Ph; M: Pd). The syntheses of these complexes can be summarized as follows: A mixture of 2-hydroxybenzonitrile and d-phenylglycinol or l-phenylalaninol in 50 mL of chlorobenzene was refluxed for 72 h with 1.0 eq. of each of the above appropriate metal salts. After removal of the chlorobenzene, single crystals of chiral bis(oxazolinyl) metal complexes 16-19 were present after natural evaporation of the recrystallization or chromatographic solvent with petroleum and dichloromethane (Figure 22).
In Figures 23 and 24, refluxing a mixture of 2-cyanophenol and d-phenylglycinol in chlorobenzene for 72 h with 1.0 eq. of cobalt chloride hexahydrate or 1.0 eq. of cobalt acetate tetrahydrate, respectively, afforded complexes 20 and 21. Further, through slow evaporation from a 1:1 mixture of ethanol and chloroform, crystals of complex 20 were obtained. However, the crystals of complex 21 were present after column separation with a 4:1 solution of petroleum ether and dichloromethane, followed by evaporation of the volatile components.
Notably, the product complexes 18 and 20 were obtained using CoCl 2 ·6H 2 O as a reagent with different solvents in the workup procedure. As seen in Figure 23, when a nonpolar solvent, such as petroleum ether or n-hexane, was used in the recrystallization medium, crystals of complex 18 were obtained. However, if the recrystallization was carried out with a mixture of two polar solvents, such as ethanol and chloroform, crystals of complex 20 were obtained.
Similarly, in the synthesis of chiral oxazoline manganese complex 22 by the title method, 2-hydroxybenzonitrile and d-phenylglycinol were refluxed with 1.0 eq. of manganese acetate tetrahydrate in chlorobenzene for 60 h (Figure 24). After removal of the chlorobenzene and slow evaporation with a mixture of absolute ethanol and chloroform, crystals of complex 22 were obtained.
Interestingly, in Figure 25, when employed by 1.0 eq. of PtCl2 in the reaction of 2-hydroxybenzonitrile with D-phenylglycinol in chlorobenzene, the crystal structure of the resulting Pt complex was different from the aforementioned complexes 16-22 (Figures 26-32); a complex containing              one unit of (R)-2-(4-phenyl-4,5-dihydrooxazol-2-yl)phenol and one unit of D-phenylglycinol was obtained after column chromatography with petroleum ether and dichloromethane (4:1) followed by crystallization via slow evaporation (Figures 25 and 33).
The proposed mechanism shows that excess metal salts can activate the reaction of 2-hydroxybenzonitrile with D-phenylglycinol in chlorobenzene to form ligand intermediates and then directly furnish the corresponding organometallic complexes via a one-step procedure ( Table 2).

Conclusions
One-pot synthesis of oxazolinyl-zinc(II) complexes 1-23 at yields 25-95% was firstly demonstrated by assembling three-component reactions between metal salts, amino alcohols, and a variety of nitrile precursors. From the crystal structures of the complexes 1-23, the reaction product is highly dependent on the presence of ligands, the amount of metal salts, and the nature of the substituent at the stereogenic center, giving a variety of coordination modes, such as mono-and bis-chelate complexes, mononuclear and mutinuclear complexes, etc.
Investigations into other oxazolinyl organometallic complexes and the catalytic properties of these complexes as chiral ligands are currently ongoing. These complexes exhibit bioactivities as anticancer reagents, and the future use of these complexes in medical fields is currently being developed.