DFT studies of unique stereoelectronic effects of substituents on divergent reaction pathways of methylenecyclobutanone radical cations

The results of DFT investigation suggest that C2–C3 bond cleavage of the 2,2-dianisyl-3,3-dimethyl-4-methylenecyclobutanone radical cation (2bradical dot+) is preferred from both a thermodynamic and a kinetic perspective while C1–C2 bond cleavage is both thermodynamically and kinetically favored in the parent methylenecyclobutanone radical cation (MCBradical dot+) and the 2,2-diphenyl- and 2,2-dianisyl-4-isopropylidenecyclobutanone radical cations (1a-bradical dot+). The DFT calculations also suggest that a bonding character exists in C2–C3 bond of the 2,2-diphenyl-3,3-dimethyl-4-methylenecyclobutanone radical cation (2aradical dot+) but not in that of 2bradical dot+. Consequently, the unique reactivity of 2bradical dot+ can be accounted for by the existence of the steric hindrance between methyl and anisyl substituents, which favors C2–C3 bond cleavage, and the absence of C2–C3 bonding character. Those results support that the previous experimental results of photoinduced electron-transfer reactions of 1 and 2. The combined factors comprise stereoelectronic substituent effects that lead to a drastic change in the reaction pathways followed by 2bradical dot+ relative to that of other methylenecyclobutanone-type radical cations.