Chapter 19
Imidazol-2-ylidenes and Their Reactions with Small Reagents Taramatee Ramnial and Jason A. C. Clyburne* Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, V5A 1S6 Canada
The reactivity of N-heterocyclic carbenes (NHCs) with small reagents has been explored and the focus of this review is mainly on those reactions involving some type of redox process. Results from our studies include the formation of neutral radicals derived from hydrogen atom addition to the carbeneic center (1), electrochemical and chemical reduction of imidazolium ions to produce NHCs (2), formation of radical cations upon treatment of NHCs with one-electron oxidizing agents (3) and identification of unconventional [C- +••• --B] dihydrogen bonds (4). Hδ
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Hδ
© 2006 American Chemical Society
267 Carbenes are two-coordinate carbon compounds that have two non-bonding electrons and no formal charge on the carbon. The formal divalent state exhibited by the carbon site is accountable for the high reactivity of these molecules, and in turn, is partly responsible for their elusive nature. In 1988, Bertrand reported the first stable singlet carbene, [èw(diisopropylamino)phosphino]-trimethylsilylcarbene] (5), which was soon followed by the synthesis and characterization of the first imidazol-2-ylidenes (aka JV-heterocyclic carbenes, NHCs) by Arduengo in 1991 (6). Recognizing the impact that the isolation of these highly reactive molecules has had on synthetic chemistry, reviews have covered the quest for isolable carbenes (7), main group element carbene complexes (
