Isolation of a C5-deprotonated Imidazolium, a Crystalline "abnormal" N-heterocyclic Carbene

Overview

Journal Science

Specialty Science

Date 2009 Nov 11

PMID 19900893

Citations 41

Authors

Eugenia Aldeco-Perez

Amos J Rosenthal

Bruno Donnadieu

Pattiyil Parameswaran

Gernot Frenking

Guy Bertrand

Affiliations

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Abstract

The discovery two decades ago of metal-free stable carbenes, especially imidazol-2-ylidenes [N-heterocyclic carbenes (NHCs)], has led to numerous breakthroughs in organic and organometallic catalysis. More recently, a small range of complexes has been prepared in which alternative NHC isomers, namely imidazol-5-ylidenes (also termed abnormal NHCs or aNHCs, because the carbene center is no longer located between the two nitrogens), coordinate to a transition metal. Here we report the synthesis of a metal-free aNHC that is stable at room temperature, both in the solid state and in solution. Calculations show that the aNHC is more basic than its normal NHC isomer. Because the substituent at the carbon next to the carbene center is a nonbulky phenyl group, a variety of substitution patterns should be tolerated without precluding the isolation of the corresponding aNHC.

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