Designing N-heterocyclic carbenes: simultaneous enhancement of reactivity and enantioselectivity in the asymmetric hydroacylation of cyclopropenes

Faster, higher, stronger! The N-heterocyclic carbene (NHC) catalyzed diastereo- and enantioselective hydroacylation of cyclopropenes affords structurally valuable acylcyclopropanes. A new family of electron-rich, 2,6-dimethoxyphenyl-substituted NHCs induces excellent reactivity and enantioselectivity. Preliminary kinetic studies unambiguously demonstrated the superiority of this family of catalysts over known NHCs in this challenging transformation.     

Hydrogenation of C?C Multiple Bonds Mediated by [Pd(NHC)(PCy3)] (NHC=N‐Heterocyclic Carbene) under Mild Reaction Conditions

N‐Heterocyclic carbenes do it again! [Pd(SIPr)(PCy₃)] ( 1 ; SIPr=N,N′‐(bis(2,6‐diisopropylphenyl)imidazolidine), which is stable under H₂, was shown to be a highly active catalyst for the hydrogenation of a wide range of alkenes and alkynes. Reactions proceed under mild conditions (relatively low Pd loading, RT, low pressures of H₂) even when sterically hindered alkenes (tri‐ and tetra‐substituted) are used.

     

Carboxylation of alkylboranes by N-heterocyclic carbene copper catalysts: synthesis of carboxylic acids from terminal alkenes and carbon dioxide

Caught in the act: N-Heterocyclic carbene copper(I) complexes (1; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) serve as an excellent catalyst for the carboxylation of alkylboranes (2; R=alkyl) with CO(2) to afford a variety of functionalized carboxylic acids (3) in high yields. A novel copper methoxide/alkylborane adduct (A) and its subsequent CO(2) insertion product (B) have been isolated and shown to be true active catalyst species.     

Hydride‐Rhodium(III)‐N‐Heterocyclic Carbene Catalysts for Vinyl‐Selective H/D Exchange: A Structure–Activity Study

A series of neutral and cationic Rh^III‐hydride and Rh^III‐ethyl complexes bearing a NHC ligand has been synthesized and evaluated as catalyst precursors for H/D exchange of styrene using CD₃OD as a deuterium source. Various ligands have been examined in order to understand how the stereoelectronic properties can modulate the catalytic activity. Most of these complexes proved to be very active and selective in the vinylic H/D exchange, without deuteration at the aromatic positions, displaying very high selectivity toward the β‐positions. In particular, the cationic complex [RhClH(CH₃CN)₃(IPr)]CF₃SO₃ showed excellent catalytic activity, reaching the maximum attainable degree of β‐vinylic deuteration in only 20 min. By modulation of the catalyst structure, we obtained improved α/β selectivity. Thus, the catalyst [RhClH(κ²‐O,N‐C₉H₆NO)(SIPr)], bearing an 8‐quinolinolate ligand and a bulky and strongly electron‐donating SIPr as the NHC, showed total selectivity for the β‐vinylic positions. This systematic study has shown that increased electron density and steric demand at the metal center can improve both the catalytic activity and selectivity. Complexes bearing ligands with very high steric hindrance, however, proved to be inactive.     

Rhodium-catalyzed intramolecular hydroacylation of 1,2-disubstituted alkenes for the synthesis of 2-substituted indanones

The intramolecular hydroacylation of 1,2-disubstituted alkenes was considered to be a challenging task due to the side reactions resulted from the lack of additional substituent at 1-position and the low activity caused by the steric hindrance of substituent at 2-position, and an asymmetric version has not been considered possible due to problems associated with the racemization of the products. We have partially solved these problems. Catalyzed by an activated diphosphine-Rh complex and reacted in a selected dihalogenated solvent, the intramolecular hydroacylation of o-(2-arylvinyl)benzaldehydes provided the corresponding 2-aryl-1-indanones in high yields, and its asymmetric variant using o-(2-alkylvinyl)benzaldehydes afforded chiral 2-alkyl-1-indanones in high yields and with moderate enantioselectivities.     

Direct Allylation of In Situ Generated Aldehyde Acyl Anions by Synergistic NHC and Palladium Catalysis

The direct regioselective allylation of in situ generated aldehyde acyl anions has been achieved by synergistic NHC and Pd catalysis. It provides an efficient access to valuable β,γ‐unsaturated ketones under mild reaction conditions starting from easily accessible allylic carbonates and aldehydes without any preactivation. The synergistic catalysis method demonstrated herein adds a new dimension to the area of metal‐mediated C allylation.