Stable four-π-electron, four-membered heterocyclic cations and carbenes
- University of California–Centre National de la Recherche Scientifique Joint Research Chemistry Laboratory (Unité Mixte Internationale 2957), Department of Chemistry, University of California, Riverside, CA 92521-0403
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Edited by Ronald Breslow, Columbia University, New York, NY, and approved July 24, 2006 (received for review June 7, 2006)
Abstract
Replacement of halide from N-(haloboryl)formamidines for the weakly coordinating anion [B(C6F5)4]−, using [Et3Si(toluene)]+[B(C6F5)4]−, induces a ring closure leading to the cationic four-π-electron four-membered heterocycles [HC(N-2,6-diisopropylphenyl)2BR]+ [R = N(i-Pr)2, Ph]. Subsequent deprotonation of the B-amino derivative affords the corresponding N-heterocyclic carbene, which has been isolated as yellow crystals. According to single-crystal x-ray diffraction studies, both the carbene and its conjugate acid precursor have a planar structure, with no pyramidalization of the ring atoms. The structural parameters indicate that the four π electrons are mostly distributed over the N-C-N fragment with little involvement of the boron center, and therefore both types of heterocycles escape from antiaromaticity. However, considering the ring strain and the presence of the Lewis acid center, the thermal stability of the carbene (mp 98°C without decomposition) is rather surprising. These results clearly suggest that the backbone of N-heterocyclic carbenes can be modified at will, without preventing their isolation.
Footnotes
- *To whom correspondence should be addressed. E-mail: gbertran{at}mail.ucr.edu
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Author contributions: G.B. designed research; Y.I. performed research; B.D. contributed new reagents/analytic tools; Y.I. and B.D. analyzed data; and Y.I. and G.B. wrote the paper.
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Conflict of interest statement: No conflicts declared.
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This paper was submitted directly (Track II) to the PNAS office.
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Data deposition: The atomic coordinates have been deposited in the Cambridge Structural Database, Cambridge Crystallographic Data Centre, Cambridge CB2 1EZ, United Kingdom (CSD reference nos. 610084–610086).
- Abbreviations:
- Dipp,
- 2,6-diisopropylphenyl;
- NHC,
- N-heterocyclic carbene.
- © 2006 by The National Academy of Sciences of the USA
