Theoretical study of the electrocyclization product of butadiyne: Structure, stability and possible formations

Abstract

The four-membered cyclic C 4H 2 resulting from electrocyclization of butadiyne has been found 75.2 kcal/mol above the latter species. In its ground state, it is a singlet non-planar biscarbene of C 2 symmetry. The electronic structures of the singlet and triplet have been studied using ELF analysis. The opening into butadiyne needs ca. 20 kcal/mol activation and follows a pathway which does not preserve the C 2 symmetry. Breaking the carbene-carbene bond results in a bicyclo-biscarbene, a degenerate isomerization which offers a reaction path for the scrambling of C 2C 3 atoms in butadiyne. Breaking the HC-C bond results in a sigmatropic shift yielding cyclopropylidene. © 2004 Elsevier B.V. All rights reserved.