Rearrangement pathways of 2-hydroxy-2-methylpropylidene: an experimental and computational study

Abstract

Photolysis of exo-2-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)propan-2-ol in benzene-d₆ afforded phenanthrene and the β-hydroxycarbene intermediate 2-hydroxy-2-methylpropylidene. The carbene showed an overwhelming preference for 1,2-methyl migration as evident from the formation of 2-butanone as the major product via the enol 2-hydroxy-2-butene. Also produced, albeit in smaller amounts, were 1-methylcyclopropanol and 2,2-dimethyloxirane from intramolecular insertion into the C−H and O−H bonds, respectively. These results stand in sharp contrast to the intramolecular reactions of simple alkylcarbenes which usually prefer insertion into C−H bonds over 1,2-alkyl migrations. Calculations at the B3LYP/6-311+G**//B3LYP/6-31G* level of theory give a lower activation barrier for 1,2-methyl migration leading to the eventual formation of 2-butanone than for the other two pathways. The lower activation energy for methyl migration, relative to C−H and O−H insertions, strongly supports the observed experimental product distribution of the carbene. The parent carbene exists in three distinct conformations, each with stabilizing interactions between the adjacent bonds and the empty p orbital and the filled sp² orbital of the carbene center. The most stable conformer is perfectly poised for a 1,2-methyl migration as the C−CH₃ group is involved in a hyperconjugative interaction with the empty p orbital and the O−H bond is simultaneously interacting with the sp² lone pair of the carbene.