Technology development for the production of biobased products from biorefinery carbohydrates—the US Department of Energy’s “Top 10” revisited

This Review summarizes the advancements in Pd-catalyzed C(sp3)–H activation via various redox manifolds, including Pd(0)/Pd(II), Pd(II)/Pd(IV), and Pd(II)/Pd(0). While few examples have been reported in the activation of alkane C–H bonds, many C(sp3)–H activation/C–C and C–heteroatom bond forming reactions have been developed by the use of directing group strategies to control regioselectivity and build structural patterns for synthetic chemistry. A number of mono- and bidentate ligands have also proven to be effective for accelerating C(sp3)–H activation directed by weakly coordinating auxiliaries, which provides great opportunities to control reactivity and selectivity (including enantioselectivity) in Pd-catalyzed C–H functionalization reactions.

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Palladium-Catalyzed Transformations of Alkyl C-H Bonds.

Conversion of Biomass into Chemicals over Metal Catalysts

The Golden Age of Transfer Hydrogenation

Research interest in biomass conversion to fuels and chemicals has increased significantly in the last decade as the necessity for a renewable source of carbon has become more evident. Accordingly, many different reactions and processes to convert biomass into high-value products and fuels have been proposed in the literature. Special attention has been given to the conversion of lignocellulosic biomass, which does not compete with food sources and is widely available as a low cost feedstock. In this review, we start with a brief introduction on lignocellulose and the different chemical structures of its components: cellulose, hemicellulose, and lignin. These three components allow for the production of different chemicals after fractionation. After a brief overview of the main reactions involved in biomass conversion, we focus on those where bimetallic catalysts are playing an important role. Although the reactions are similar for cellulose and hemicellulose, which contain C6 and C5 sugars, respectively, different products are obtained, and therefore, they have been reviewed separately. The third major fraction of lignocellulose that we address is lignin, which has significant challenges to overcome, as its structure makes catalytic processing more challenging. Bimetallic catalysts offer the possibility of enabling lignocellulosic processing to become a larger part of the biofuels and renewable chemical industry. This review summarizes recent results published in the literature for biomass upgrading reactions using bimetallic catalysts.

Bimetallic catalysts for upgrading of biomass to fuels and chemicals

C–H arylation via a Pd(II)/Pd(IV) catalytic cycle has been one of the most extensively studied C–H activation reactions since the 1990s. Despite the rapid development of this reaction in the past two decades, an enantioselective version has not been reported to date. Herein, we report a Pd(II)-catalyzed highly enantioselective (up to 99.5% ee) arylation of cyclopropyl C–H bonds with aryl iodides using mono-N-protected amino acid (MPAA) ligands, providing a new route for the preparation of chiral cis-aryl-cyclopropylmethylamines. The enantiocontrol is also shown to override the diastereoselectivity of chiral substrates.

Palladium(II)-catalyzed highly enantioselective C-H arylation of cyclopropylmethylamines.

Hydrogenolysis of glycerol to glycols over ruthenium catalysts: Effect of support and catalyst reduction temperature

Ligand-free Pd(OAc)(2) was found to catalyze very efficiently the direct arylation of imidazo[1,2-a]pyridines at C3 under very low catalyst concentration. The reaction can be performed employing as little as 0.1-0.01 mol % catalyst with electron-deficient and some electron-excessive aryl bromides.

Phosphine-free palladium-catalyzed direct arylation of imidazo[1,2-a]pyridines with aryl bromides at low catalyst loading.

A quinoline-based ligand was shown to promote palladium-catalyzed β-C(sp3)–H fluorination for the first time. A range of unnatural enantiopure fluorinated α-amino acids were obtained through sequential β-C(sp3)–H arylation and subsequent stereoselective fluorination from readily available l-alanine.

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Ligand-Enabled Stereoselective β-C(sp3)–H Fluorination: Synthesis of Unnatural Enantiopure anti-β-Fluoro-α-amino Acids

Pd(II)-catalyzed cross-coupling of C(sp3)–H bonds with organosilicon coupling partners has been achieved for the first time. The use of a newly developed quinoline-based ligand is essential for the cross-coupling reactions to proceed.

Haiyan Fu

Papers

Palladium(II)-catalyzed highly enantioselective C-H arylation of cyclopropylmethylamines.

Hydrogenolysis of glycerol to glycols over ruthenium catalysts: Effect of support and catalyst reduction temperature

Phosphine-free palladium-catalyzed direct arylation of imidazo[1,2-a]pyridines with aryl bromides at low catalyst loading.

Ligand-Enabled Stereoselective β-C(sp3)–H Fluorination: Synthesis of Unnatural Enantiopure anti-β-Fluoro-α-amino Acids

Ligand-Enabled Cross-Coupling of C(sp3)–H Bonds with Arylsilanes