The calculated acidity of benzocyclopropene and its relevance to the ease of base cleavage of 1-trimethylsilybenzocyclopropene
1980; Elsevier BV; Volume: 192; Issue: 2 Linguagem: Inglês
10.1016/s0022-328x(00)94418-8
ISSN1872-8561
Autores Tópico(s)N-Heterocyclic Carbenes in Organic and Inorganic Chemistry
ResumoSTO-3G calculations indicate that benzocyclopropene is substantially more acidic than toluene, in line with the previously reported ease of base cleavage of 1-trimethylsilylbenzocyclopropene and ease of metallation of benzocyclopropene; the calculated pKa value of 33 compares with the value of 36 derived from the cleavage data. The corresponding calculations for cyclopropene indicate a very low acidity, comparable with that of ethane. The higher acidity of benzocyclopropene than of toluene appears to be associated with a high polarizability of the CC bonds between the carbanionic centre and the adjacent carbon atoms of the aromatic ring; the large σ-charge thus placed on the latter carbon atoms in the carbanion is compensated by delocalization of π-charge away from these centres. The corresponding CC bonds of the cyclopropenyl anion are similarly polarized, but in this case there can be no compensating delocalization of π-charge.
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