Arylcyclopropane Photochemistry. Substituent Effects on the Photochemical 1,3-Hydrogen Migration of 1,1-Dimethyl-2-phenylcyclopropane

Article abstract of DOI:10.1021/jo00247a009

Irradiation of 1,1-dimethyl-2-arylcyclopropanes 1a-g (Ar = p-CF3C6H4 (1a), m-CF3C6H4 (1b), p-CH3C6H4 (1c), m-CH3C6H4 (1d), C6H5 (1e), m-OCH3C6H4 (1f), p-OCH3C6H4 (1g) gave in every case a 2-methyl-4-aryl-1-butene (2a-g) via a 1,3-hydrogen migration, accompanied by lesser amounts of a 3-methyl-1-aryl-2-butene (3a-g).Rearrangement is a singlet-state process.Rate constants for rearrangement were determined from reactant fluorescence lifetimes and product quantum yields.The rates for rearrangament of 1 to 2 were as follows:1a, 25*10⁶s^-1; 1b,15*10⁶s^-1; 1c, 7.9*10⁶s^-1; 1d, 6.1*10⁶s^-1; 1e, 2.3*10⁶s^-1; 1f, 1.9*10⁶s^-1; 1g, 0.87*10⁶s^-1.It isconcluded that the energetics of cyclopropane ring opening are important in the rate-determining step of the reaction.