N-Heterocyclic carbenes derived from benzimidazolium salts are effective catalysts for generating homoenolate species from alpha,beta-unsaturated aldehydes. These nucleophilic intermediates can be protonated, and the resulting activated carbonyl unit is trapped with an alcohol nucleophile, thereby promoting a highly efficient conversion of an alpha,beta-unsaturated aldehyde into a saturated ester. A kinetic resolution of secondary alcohols can be achieved using chiral imidazoylidene catalysts. [reaction: see text]
N-heterocyclic carbenes catalyze the oxidation of allylic, propargylic, and benzylic alcohols to esters with manganese(IV) oxide in excellent yields. A variety of ester derivatives can be synthesized, including protected carboxylates. This one-pot tandem oxidation represents the first organocatalytic oxidation of alcohols to esters. Saturated esters can also be accessed from aldehydes using this method. Through the utilization of a chiral catalyst, the acyl-heteroazolium intermediate becomes a chiral acylating agent, which can desymmetrize meso-1,2-diols. [reaction: see text].
N-heterocyclic carbenes derived from triazolium salts are effective catalysts between 10 and 15 mol % for the hydroacylation of activated ketones. The reducing equivalent is generated via the interaction of a nucleophilic carbene species and an aromatic aldehyde. The subsequent alcohol product can undergo an acylation event with the resulting acyl heteroazolium intermediate formed in situ between the NHC and the aldehyde. This unprecedented multiple bond-forming reaction can accommodate aromatic aldehydes as the hydride source and various electron-deficient ketones. Preliminary mechanistic evidence indicates that the reduction and acylation steps are sequential operations. The intramolecular variant of this organocatalytic reaction affords benzofuranones in good yield.
Metal‐less Michael: A highly diastereo‐ and enantioselective intramolecular Michael addition of α,β‐unsaturated aldehydes to enones catalyzed by an N‐heterocyclic carbene (NHC) has been developed. The reaction is tolerant of alkyl and aromatic substituents, as well as saturated and unsaturated tethers between the enal and conjugate acceptor (see scheme).
Intermolecular cycloadditions are powerful methods for the convergent synthesis of cyclic compounds from simple precursors. 1 While major advances have been made in the area of metal-catalyzed cycloadditions over the past decade, 2 there is great potential for these reactions using organic molecules as catalysts. 3 In 1968, Dorn and coworkers demonstrated that 3-oxopyrazolidin-1-ium-2-ides such as 2 are stable and easily handled compounds. 4,5 Fu, Hayashi and Suga have separately shown that these compounds are efficient substrates in metalcatalyzed cycloadditions to furnish five and six-membered heterocycles. 6 We have been interested in developing new reactions catalyzed by N-heterocyclic carbenes (NHCs) derived from azolium salts. 7 Our recent studies, along with those of Glorius, Bode, and Nair, have shown that the combination of NHCs and α,β-unsaturated aldehydes generate unique homoenolate species. 8,9 The use of an organic molecule to catalyze a formal [3 + 3] cycloaddition of azomethine imines has yet to be realized. 10 Herein, we report the direct synthesis of pyridazinones (3) by the NHC-catalyzed reaction of aldehyde (1) and azomethine imines (2, eq 1).
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