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Kuchenbeiser G, Soleilhavoup M, Donnadieu B, Bertrand G: Reactivity of cyclic (alkyl)(amino) carbenes (CAACs) and bis (amino) cyclopropenylidenes (BACs) with heteroallenes: comparisons with their N-heterocyclic carbene (NHCs) counterparts. Chem Asian J. 2009 Nov 2;4(11):1745-50.
Similarly to NHCs, CAAC (a) and BAC (a) react with CO2 to give the corresponding betaines. Based on the carbonyl stretching frequencies of cis-[RhCl (CO) 2 (L)] complexes, the order of electron donor ability was predicted to be CAAC (a) approximately BAC (a)> NHCs. When the betaines nu (asym)(CO2) values are used, the apparent ordering is BAC (a)> NHCs approximately CAAC (a) that indicates a limitation for the use of IR spectroscopy in the ranking of ligand sigma-donating ability. Although all carbenes react with carbon disulfide to give the corresponding betaines, a second equivalent of CS2 reacts with the BAC-CS2 leading to a bicyclic thieno [2,3-diamino]-1,3-dithiole-2-thione, which results from a novel ring expansion process. Surprisingly, in contrast to NHCs, CAAC (a) does not react with carbodiimide, whereas BAC (a) exclusively gives a ring expanded product, analogous to that obtained with CS2. The intermediate amidinate can be trapped, using the lithium tetrafluoroborate adduct of BAC (b) as a carbene surrogate. |
81(1,1,1,1) |