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.
- late transition metals
- renewable chemicals
- fine chemicals
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].
1.1 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 . 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.
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 , as resonance stabilized ylide form. After a few years, the first stable NHC was reported by Arduengo et al. as an imidazolium ring . 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 (
1.3 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.
1.4 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.
1.5 Transmetalation route
Even though, transmetalation method has operational simplicity but lacks atom economy. Hence, it is applied, in general, in scalable industrial processes.
2. Applications of late transition metal NHC’s
2.1 CO2 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.
2.1.1 Formylation of amines
The use of CO2 for procuring C1-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 CO2 and hydrosilanes (Figure 4) .
2.1.2 Carboxylation of terminal alkynes
2.1.3 Methylation of amines
2.1.4 Insertion of CO2 into terminal alkynes
2.2.1 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 .
2.2.2 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 .
2.2.3 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) .
2.3.1 Cp*IrCl2(NHC) in hydrogen transfer initiated dehydration (HTID)
A recyclable Cp*IrCl2(NHC) (Cp* = pentamethylcyclopentadienyl) complex in ionic liquid could covert glycerol into 1,3-propanediol and subsequently to propionaldehyde by hydrogen transfer initiated dehydration (HTID) in excellent yields in the presence of air (Figure 11) [34, 35].
2.3.2 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 .
2.4.1 Hydrogenolysis of aryl ethers using Ni-NHC
Ni-NHC complex in the presence of a suitable base (NaOtBu) could effectively convert C-O bonds in lignin to various useful scaffolds useful in biomass conversion . Hartwig et al. mechanically investigated the reduction of diaryl ethers to corresponding phenols (Figure 12) .
2.4.2 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) .
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:
The present research is directed towards the conversion of methanol to H2 and CO2 using LTM-NHC catalysis.
We do hope this compilation on very important LTM-NHC applications would help wide readers among synthetic organic chemists.
Dr. RV is thankful to Dr. Ch.V. Rajasekhar, Scrips Pharma, and Dr. P.G. Kiran, Swastha BioSciences for their continued support.
Conflict of interest
The authors declare no conflict of interest.
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