An STO-3G study of acyclic 1,2-ylides derived from iminium ions

Abstract

An STO-3G study of acyclic mono- and di-substituted 1,2-ylides derived from iminium ions has been carried out and these species are compared with the pyridinium and thiazolium ylides involved in the Hammick reaction and in processes involving the coenzyme thiamine pyrophosphate respectively. Calculated bond lengths, π-orbital populations and charges, indicate a variation of ylide and carbene character with substituent(s). For C-substituted iminium ylides, strong electron-attracting substituents promote the ylide form in preference to the carbene, whereas in N- substituted species, strong electron donors furnish the same effect. The thiazolium ylide, however, requires the inclusion of d orbitals or a 3-21G basis set at sulphur in the calculation to produce an acceptable geometry and electronic configuration. The lowest energy configuration of the acyclic carbene was found to be the singlet carbene. Barriers to rotation and inversion of an acyclic ylide were calculated and found to be comparable.