| Abstract
| - Photolysis of exo-2-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)propan-2-ol in benzene-d6afforded phenanthrene and the β-hydroxycarbene intermediate 2-hydroxy-2-methylpropylidene. Thecarbene showed an overwhelming preference for 1,2-methyl migration as evident from the formationof 2-butanone as the major product via the enol 2-hydroxy-2-butene. Also produced, albeit in smalleramounts, were 1-methylcyclopropanol and 2,2-dimethyloxirane from intramolecular insertion intothe C−H and O−H bonds, respectively. These results stand in sharp contrast to the intramolecularreactions of simple alkylcarbenes which usually prefer insertion into C−H bonds over 1,2-alkylmigrations. Calculations at the B3LYP/6-311+G**//B3LYP/6-31G* level of theory give a loweractivation barrier for 1,2-methyl migration leading to the eventual formation of 2-butanone thanfor the other two pathways. The lower activation energy for methyl migration, relative to C−Hand O−H insertions, strongly supports the observed experimental product distribution of thecarbene. The parent carbene exists in three distinct conformations, each with stabilizing interactionsbetween 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 isinvolved in a hyperconjugative interaction with the empty p orbital and the O−H bond issimultaneously interacting with the sp2 lone pair of the carbene.
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