A mild and general iridium-catalyzed, highly enantioselective hydrogenation of sterically hindered N-arylimines with a new H 8 -BINOL-derived phosphineÀphosphoramidite ligand has been developed. The present catalytic system features high turnover numbers (up to 100000) and good to perfect enantioselectivities (up to 99% ee) for the hydrogenation of a variety of sterically hindered N-arylimines.Chiral amines are important synthetic intermediates in the preparation of many physiologically active compounds. As a result, significant efforts have been devoted to the asymmetric synthesis of these compounds via catalytic methods.1 For its inherent efficiency and atom economy, catalytic asymmetric hydrogenation of prochiral imines has emerged as one of the most direct and convenient approaches to chiral amines and their derivatives. To date, a variety of imine frameworks including cyclic and acyclic imines have proven to be suitable substrates.3 However, asymmetric hydrogenation of sterically hindered imines remains scarely explored, 4 despite the fact that the corresponding chiral amines are important building blocks in organic synthesis and agrochemistry. Except for a relevant example on the industrial production of the key intermediate for chiral herbicide (S)-metolachlor, 5 the most impressive example in this area was described by Zhang et al., 6 in which high enantioselectivities were observed in the hydrogenation of some sterically hindered N-arylalkylarylimines employing an Ir catalyst with a ferrocenyldiphosphine ligand, (R,R)-f-binaphane. However, incomplete conversion was observed by using 2 mol % of a IrÀf-binaphane complex and under a H 2 pressure of 1000 psi. In addition, this catalytic system failed in the hydrogenation of sterically hindered N-aryldialkylimines. Therefore, the development of a highly efficient catalytic system that successfully addressed the challenges of low reactivity, narrow substrate scope, and harsh hydrogenation conditions encountered in the hydrogenation of † Dalian Institute of Chemical Physics. ‡ Dalian Polytechnic University.

Reductive catalytic fractionation (RCF) of lignocellulosic biomass has emerged as a leading biorefinery strategy. Herein, we present a copper-based catalyst (CuO/C) derived from a metal–organic backbone (HKUST-1) with enhanced catalytic performance for RCF of woody sawdust, which affords high yields of propyl and propanol end-chain monomeric phenols (15.5 wt % in softwoods and 46.7 wt % in hardwoods) via the C–O bond scission. A series of conifer β-O-4 models and their deuterated analogues revealed that the synergistic action of CuO/C and hydrogen could effectively cleave aryl ether linkages. It was deduced that lignin alcoholysis led to partial α-OH etherification of the β-O-4′ units, which promoted the C–O bond breakage of Cα-OMe and Cβ-O, thus giving propanol phenolic compounds through the hydrogenation of coniferyl alcohol over the CuO/C catalyst. When both α- and γ-OH of β-O-4′ motifs were meoxylated, the para-propyl phenolics were obtained through the scission of C–O linkages (Cα-OMe and Cβ-O), followed by the Cγ-OMe cleavage of propenyl ethers and hydrogenation of propenyl phenols. We envision that this work may pave the way for the development of non-noble catalysts with high reactivity and selectively for lignin valorization.

A new class of chiral ferrocenyl diphosphine ligands with an imidazole ring, (R(c),S(Fc))-ImiFerroPhos, has been prepared from acylferrocenes through a five-step transformation and successfully applied in the Rh-catalyzed asymmetric hydrogenation of various 3-aryl-substituted 2-phosphonomethylpropenoates, in which a series of chiral 3-phosphono-2-arylmethylpropanoic acid derivatives were achieved in ee values of up to 98%.

The first highly diastereo- and enantioselective propargylic alkylation of acyclic ketone enamines to form vicinal tertiary stereocenters has been reported by employing copper catalysis in combination with a bulky and structurally rigid tridentate ketimine P,N,N-ligand.

The first copper-catalyzed intermolecular enantioselective decarboxylative propargylic alkylation of propargylic esters with b-keto acids as surrogates of ketones has been successfully developed by using a ketimine P,N,N-ligand. High yields and excellent enantioselectivities (up to 98% ee) have been achieved under the mild reaction conditions. The catalytic asymmetric propargylic substitution of propargylic esters with various nucleophiles has been developed in the past decade. [1] However, the use of carbanions, especially unstabilized ketone enolates as nucleophiles, remains very limited. [2] Since the asymmetric alkylation of unstabilized ketone enolates constitutes one of the most powerful tools for constructing chiral centers via C À C bond forming reactions, [3] the development of a new strategy for the seteroselective propargylic alkylation of propargylic esters with unstabilized ketone enolates is particularly appealing.Quite recently, we reported a strategy for the enantioselective propargylic alkylation of ketone enolates by a copper-catalyzed intramolecular decarboxylative alkylation of propargylic b-keto esters. The reaction works through the loss of CO 2 and does not need preformed ketone enloates. [4,5] The mechanistic study suggested that the reaction proceeded with a copper allenylidene complex enolate ion pair as the key intermediate (Scheme 1). We therefore envisioned that an intermolecular decarboxylative propargylic alkylation could also be possible since a similar ion pair could be generated when a propargylic ester and a b-keto acid were subjected to a copper catalyst under the appropriate reaction conditions (Scheme 1). [6] Although some examples have shown that b-keto acids are capable of undergoing the decarboxylative carboncarbon bond-forming reaction as surrogates of ketones with various carbon electrophiles, [7] the decarboxylative alkylation with the use of a propargylic ester as the carbon electrophile remains unexplored. Herein, we report the first copper-catalyzed intermolecular asymmetric decarboxylative propargylic alkylation of propargylic esters with b-keto acids, in which excellent performance has been achieved. Importantly, this process provides an efficient way to obviate employing preformed propargylic b-keto esters for the intramolecular decarboxylation through the transesterification between the corresponding propargylic alcohols and b-keto esters, which generally requires long reaction time (up to 3 days) and gives unsatisfactory yields. Scheme 1. Copper-catalyzed decarboxylative propargylic alkylation.

No abstract

A new class of sterically hindered chiral ferrocenyl P,N,N-ligands have been prepared through a two-step transformation from (S,R)-PPFNH, in which a new (R)-stereogenic center at the pyridinylmethyl position was generated in high diastereoselectivity. With these newly developed P,N,N-ligands, Ir-catalyzed asymmetric hydrogenation of various α-alkyl-substituted β-aryl-β-ketoesters via dynamic kinetic resolution has been realized in high diastereo- and enantioselectivities for the first time, which led to a variety of optically active anti-β-hydroxyesters in up to 99% ee. The study indicated that the additional stereocenter at the pyridinylmethyl position of these ligands is crucial to realize this hydrogenation.

The Ir-catalyzed asymmetric hydrogenation of b-keto esters with chiral ferrocenyl P,N,N-ligands has been developed. Under the optimized conditions, a wide range of b-keto esters were hydrogenated to afford the corresponding b-hydroxy esters in good to excellent enantioselectivities (up to 95% ee).