Novel α-spirocyclic (alkyl)(amino)carbenes at the theoretical crossroad of flexibility and rigidity

Abstract

Spirocyclic (alkyl)(amino)carbenes (CAACs) with reasonable conformations of cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl substituents in the position α to the carbenic center are investigated at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G*. The singlet–triplet energy separations (∆E s−t), HOMO–LUMO energy gaps (∆E HOMO−LUMO), hydrogenation energies (∆E H), heats of methylene formation (∆E) as well as philicity indices (N and ω) of these carbenes are compared and contrasted. The highest ∆E s−t is encountered for the cyclopropyl substituted CAAC (54.2 kcal/mol), while the other ones lay in a narrow range of 48.2–51.8 kcal/mol. Assignment of the barriers for ring inversions is carried out in order to probe the “flexible steric bulk” of cyclic substituents, showing negligible differences with those of the unsubstituted rings. The calculated N and ω indices indicate that in contrast to the popular belief, CAACs appear less electrophilic than N-heteorcyclic carbenes (NHCs).