Competitive 1,2- and 1,5-hydrogen shifts following 2-vinylbiphenyl photocyclization.
[reaction: see text] The photocyclization of 2-vinylbiphenyl and its derivatives has been proposed to occur via a two-step mechanism: photocyclization to form an unstable 8a,9-dihydro-phenanthrene intermediate, followed by exothermic unimolecular isomerization to a 9,10-dihydrophenanthrene. The mechanism of the hydrogen shift process has been investigated using deuterated derivatives of 2-isopropenylbiphenyl and 2,6-diphenylstyrene. 1H NMR analysis of the photoproducts indicates that the thermally allowed 1,5-hydrogen or deuterium shift is a minor product-forming pathway and that an unusual double 1,2-hydrogen or deuterium shift is the major product-forming pathway. The potential energy surface for photocyclization and hydrogen shift processes has been explored computationally. The calculated barrier for the 1,5-shift is predicted to be significantly lower than that for the 1,2-shift. Alternative mechanisms for the occurrence of 1,2-hydrogen or deuterium migration are presented.
Lewis
Frederick D
FD
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA. lewis@chem.northwestern.edu
Sajimon
Meledathu C
MC
Zuo
Xiaobing
X
Rubin
Michael
M
Gevorgyan
Vladimir
V