ChemInform Abstract: MOLECULAR ORBITAL THEORY OF THE ELECTRONIC STRUCTURE OF MOLECULES. 36. A THEORETICAL STUDY OF SEVERAL α‐SUBSTITUTED VINYL CATIONS

Abstract

The cy-ethynylvinyl (4), n-ethenylvinyl (5), a-cyclopropylvinyl (6), and a-phenylvinyl (7) cations have been investigated by SCF-MO ab initio methods, using both the STO-3G and the 4-31G basis sets. The cations 5,6, and 7 are more stable in perpendicular conformations (5a, 6a, and 7a, respectively) where the interaction between the cationic orbital and the HOMO of the substituent is maximized. The calculated rotation barriers around the C+-substituent bonds are 22.2, 15.8, and 24.7 kcal/mol for 5,6, and 7, respectively, approximateiy half the barrier in the corresponding primary alkyl cations. The efficiency of the n substituent in stabilizing the vinyl cation follows the order CsH5 > c-CsH5 N HC=CHz >> C=CH = CH3 >> H. The ability of the substituents to donate electrons to the empty cationic orbital follows the order C6H5 > CH=CH* > C=CH > c-C~H~ > CH3 > H. No correlation is found between the total charge at the cationic center or the corresponding populations of the formally empty p orbital and the stability of the cation. The cations vinyl (21, a-methylvinyl (3), and 6 have stabilities which are intermediate between those of the corresponding primary and secondary alkyl cations. However, the r-stabilized cations, 4,5, and 7 are of comparable stability to the corresponding primary alkyl cations. Corresponding substituted ethyl cations are 12-17 kcal/mol more stable than the vinyl cations, suggesting that, for the groups examined here, substituent effects are inherently similar for alkenyl and for alkyl cations. The proton affinities of substituted acetylenes and olefins are comparable, with the olefins being 1-5 kcal/mol more basic.