Tuning the nucleophilicity in cyclopropenylidenes.

Cyclopropenylidenes are Hückel aromatic pi systems in which one of the ring atoms is a carbene center. Quantum chemical calculations at the density functional level of theory, supplemented by coupled-cluster calculations, indicate that there is a sizeable energy separation between the lowest-energy singlet and triplet states of these species. Amino groups considerably increase the energy difference between these two states, whereas electron-withdrawing substituents decrease it. The 1,1-dimerization products of cyclopropenylidenes, namely, triafulvalenes, have been investigated. The calculations show that, without steric hindrance and considerable electronic stabilization, cyclopropenylidenes are kinetically unstable and dimerize. Different substituents (alkyl, silyl, terphenyl, amino, and phosphoraneiminato) were probed to tune the frontier orbital energies of cyclopropenylidenes. Accordingly, it is predicted that by a suitable choice of substituents at the olefinic positions, cyclopropenylidenes can be more nucleophilic than their five-membered ring congeners, namely, imidazol-2-ylidenes.