The thioformamidyl group as an α-substituent in carbocations

Abstract

Abinitio: molecular orbital calculations at MP2(FULL)/6-311G(d,p) or MP2(FULL)/6-31G(d,p) are reported for carbocations RR′CCHO+, RR′CCHS+, RR′CCONH2+, and RR′CCSNH2+where R and R′ are H, CH3, C-C3H5, and C6H5. Primary (R=R′=H), secondary (R=H, R′=alkyl or phenyl), and tertiary (R′=R′=CH3) ions prefer the cyclic oxiranyl or thiiranyl structure 9, with the open structure 8 being a transition structure for ring opening. Tertiary carbocations with R=R′=phenyl or cyclopropyl and the 9-thioformamidyl-9-fluorenyl cation have the open structure. Isodesmic reactions show CONH2 to be weakly stabilizing in the methyl cation, and CSNH2 has a larger stabilizing effect, roughly equivalent to that of a methyl group. An α-thioamide substituent is less stabilizing in the ethyl cation and even less stabilizing in the isopropyl cation. In C6H5CHCSNH2+ the CSNH2 group is slightly destabilizing and, by extrapolation, is more destabilizing in Ar2CCSNH2+. The rearrangement of the α-thioformamidyl-benzyl cation to benzothiophene is calculated to have a low barrier, 15.4 kcal/mol at HF/6-31G(d,p). Keywords: molecular orbitals, destabilized carbocations, rearrangement.