Theoretical prediction of the stability and intramolecular rearrangement reactions of heteroanalogues of cyclopropylcarbene: 2-oxiranyl-, 2-aziridinyl-, and 1-aziridinylcarbene
Ab initio studies have been carried out on the experimentally uncharacterized 2-oxiranyl- (8), fluoro-2-oxiranyl- (15), 2-aziridinyl- (21), and 1-aziridinylcarbene (29) on the B3LYP/6-31G(d) and CCSD(T)/6-31G(d,p)//MP2/6-31G(d,p) levels of theory. Like the parent cyclopropylcarbene (1), the beta-heterosubstituted carbenes 8 and 21-intermediates in the Eschenmoser-Tanabe fragmentation-are predicted to give either ethyne extrusion or ring expansion reactions, depending on the initial conformation. The barriers for these reactions are low (<6 kcal mol(-)(1)), while the interconversion of the carbene conformers requires higher energies. NBO analyses show that the donor-acceptor interactions between the carbene frontier orbitals and sigma-bond orbitals in the oxiranyl and aziridinyl ring destabilize the compounds kinetically. 2-Oxiranyl- and 2-aziridinylcarbenes with a fixed cis (=exo) conformation are predicted to rearrange to the heteroanalogue derivatives of cyclobutene. The fluoro-substituted 2-oxiranylcarbene 15 is predicted to rearrange to the substituted oxete via one equilibrated conformer, due to the low barrier of the C-(CF)-rotation. 1-Aziridinylcarbene 29 is the thermodynamically most stable of the investigated carbenes; however, the fragmentation into HCN and ethene is a facile reaction with a calculated barrier of 15 kcal mol(-)(1), which confirms experiments.
Muck-Lichtenfeld
C
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat, Corrensstrasse 40, D-48149 Munster, Germany.