Late Transition Metal (LTM)-NHC Catalyzed Transformations of Renewable Chemicals to Fine Chemicals, Fuels, and Intermediates

This title of the book chapter deals with the late transition metal-NHC (N-heterocyclic carbene) catalyzed transformations of renewable chemicals, i.e., bio-mass resources (carbohydrates/vegetable oils/natural products) into useful chemicals via oxidation, hydrogenation, dehydration, polymerization, hydrolysis, etc. along with brief introductory notes on late transition metals, carbenes, and renewable chemicals for better understanding to the reader.


Introduction
Organocatalysis plays a pivotal role in the field of synthetic organic chemistry as well as the pharmaceutical industry through diversifying activation strategies owing to meeting the principles of green chemistry [1][2][3][4] in terms of cost-effectiveness, longevity, and less toxic compared to conventional transition metal catalysis [5][6][7][8]. In this regard, N-heterocyclic carbene (NHC) plays a major role in diversified organic transformations [9][10][11].

Renewable chemicals
Renewable chemicals or "bio-based chemicals" are chemicals obtained from renewable sources, such as agricultural feedstock, agricultural waste, organic waste products, biomass, and microorganisms [12]. In general, in chemical industries, processes include the utilization of fossil resources. As the need for energy consumption and population increasing, limited availability of fossil resources has become a risky task in the low or underdeveloped nations to perform trade. Henceforth, alternative renewable resources such as lignin, hemicellulose, cellulose, starch, and protein have become more focus of utility.

Carbene
The term "Carbene" refers to the presence of neutral bivalent carbon with six valence electrons in N-heterocyclic compounds (Figure 1). The first reported carbene (I) was by Bartrand et al. in 1988 [13], as resonance stabilized ylide form. After a few years, the first stable NHC was reported by Arduengo et al. as an imidazolium ring [14]. In NHC, the singlet state of carbene is more thermodynamically favorable than triplet carbene. Because nitrogen is present near to carbon of carbene, it lowers the energy of the highest occupied molecular orbital (HOMO) while it increases the energy of the lowest unoccupied molecular orbital's. The nucleophilicity of carbene also increases (A) not only above energy character but also presence of inductive effect, mesomeric and lone pair to vacant p-orbital favors singlet carbene. Most NHCs are based on imidazolium, triazolium, or thiazolium ring-containing molecules. NHCs dimerize reversible in the form of the Wanzlick equilibrium (B) [15][16].
Since the discovery of metal carbenes in 1964 by Fisher et al. [17], fascinating applications in both catalysis and synthesis are being observed [18].

Late transition metals
Late transition metals are on the right side of the d-block, from group 8 to 11 (and 12 if it is counted as transition metals) as shown in Figure 2.

Free carbine route
The general synthesis of carbene complexes involves the utilization of strong bases and harsh reaction conditions which involves high cost and more time.

Transmetalation route
It involves the transfer of the carbine fragment from a suitable metal center [generally Ag(I) or Cu(I)] to a precursor of the metal center of interest [19][20][21][22] as shown in Figure 3.
Even though, transmetalation method has operational simplicity but lacks atom economy. Hence, it is applied, in general, in scalable industrial processes.

Applications of late transition metal NHC's 2.1 CO 2 as building blocks
The exploitation of carbon dioxide as a renewable green source of carbon in organic synthesis is of continued interest. In this regard, late transition metal NHCs play a major role for the specified purpose.

Formylation of amines
The use of CO 2 for procuring C 1 -containing molecules is an evolved methodology exploiting N-heterocyclic carbenes (NHCs) as efficient catalysts [23][24]. NHCs promoted the formylation of a wide scope of N-H bonds, with CO 2 and hydrosilanes (Figure 4) [25].

Carboxylation of terminal alkynes
Yu and Zhang [26] developed a Cu-NHC catalyzed conversion of CO 2 to carboxylic acids in good to excellent yields under ambient conditions with wide substrate/ functional group tolerance (Figure 5).

Methylation of amines
Olivier et al. have designed using CO 2 as a C1 − building block for the catalytic methylation of amines using simple zinc salts and ligands (Figure 6) [27].

Insertion of CO 2 into terminal alkynes via copper bis-NHC
Silver bis-NHC has exhibited better performance than Copper bis-NHC towards the carboxylation of terminal alkynes using Cs 2 CO 3 as an additive (Figure 7) [28].

Dehydrogenative oxidation of alcohols
Ir-NHC complexes were synthesized in aqueous media for the oxidation of secondary alcohols to ketones. In addition, primary alcohols were transformed to carboxylic acids in the absence of a base [30].

Oxidation of bio-polyols to lactic acid
Lactic acid has prominent applications in bio-plastics manufacturing. A recyclable NHC-iridium coordination polymer with a porous structure can oxidize a wide range of bio-polyols such as sorbitol to prepare lactic acid with superior selectivity and reactivity [31].

Dehydrogenative catalysis using alcohols
Huang et al. reported LTM-NHCs for the conversion of alcohols into aldehydes or ketones through acceptors alcohol dehydrogenation (AAD). In addition, they  successfully demonstrated oxidative coupling of alcohols to form C-O, C-C, and C-N/C=N bond formations (Figure 9) [32].

Dehydrogenation of sugar alcohols
Manas and Campos et al. [33] reported Ir-NHC catalyzed oxidative protocol for the selective conversion of sorbitol, xylitol, and other polyols into lactic acid (Figure 10).

Fructose to 5-hydroxymethylfurfural (HMF)
A new heterogeneous and recyclable Fe-NHCs immobilized on mesoporous expanded starch and Starbon™ 350 could be utilized successfully for the effective dehydration of fructose to HMF [36].

Hydrogenolysis of aryl ethers using Ni-NHC
Ni-NHC complex in the presence of a suitable base (NaO t Bu) could effectively convert C-O bonds in lignin to various useful scaffolds useful in biomass conversion [37]. Hartwig et al. mechanically investigated the reduction of diaryl ethers to corresponding phenols (Figure 12) [38].

Transfer hydrogenation using Ir-NHC
Using water soluble Ir-NHCs proved that glycerol can be exploited as a hydrogen donor to convert a biomass-derived phytochemical, levulinic acid, to selectively produce γ-hydroxyvaleric acid (GHV) and lactic acid (LA) [39].    (Figure 13).

Sugars to heterocycles
Zhang and Yong developed a synthetic protocol employing Cr-NHC along with ionic liquid for the selective production of 5-hydroxymethylfurfural from glucose and fructose (Figure 14) [41].

Conclusion
In this book chapter, authors tried to emphasize the applications of "Late Transition Metal" (LTM)-NHC catalyzed organic transformations as given in a nutshell below: Oxidation of carbohydrates: To develop carbohydrate oxidation products as a useful alternative to those derived from petrochemical sources.
Hydrogenation of carbohydrates/fatty acids: This objective concerns the development of LTM-NHC catalysts for the hydrogenation of carbohydrates and unsaturated vegetable oils.
Dehydration/hydrolysis of carbohydrates/fatty acids: Development of dehydration/ hydrolysis of carbohydrates/fatty acids with LTM-NHC catalysts to obtain fine chemicals and fuel intermediates.
Polymerization with renewable resources: This objective deals with the application of LTM-NHC catalysts in the polymerization of natural monomers of renewable chemicals or monomers derived from renewable resources to synthetic polymers (polymerization of lactic acid, glucose, glycerol, terpenes, etc.).
The present research is directed towards the conversion of methanol to H 2 and CO 2 using LTM-NHC catalysis.
We do hope this compilation on very important LTM-NHC applications would help wide readers among synthetic organic chemists.