Unusual rearrangements in di-.pi.-methane systems: Mechanistic and exploratory organic photochemistry

Abstract

The triplet photochemistry of 1,1-dicyano-5,5-diphenyl-3,3-diisopropyl-1,4-pentadiene was investigated to determine if the high reactivity of di-{pi}-methane systems with central diphenyl substitution derived from electronic or from steric effects. Sensitized photolysis of the dicyanopentadiene afforded 1,1-dicyano-2-(2,2-diphenyl-vinyl)-3,3-diisopropylcyclopropane as the singlet photoproduct with an efficiency of {phi} = 0.041. This is intermediate between the inefficiently reacting dienes with central methyl substitution and the highly reactive dienes having central phenyl groups. Direct irradiation of the dicyanopentadiene led to four primary photoproducts. The first was the di-{pi}-methane product observed in the triplet photochemistry. The second was cis-1,1-dicyano-5,5-diphenyl-3-isopropyl-6-methyl-1,3-heptadiene, which is related to the reactant by a 1,3-shift of an isopropyl group. The third was trans-3,3-dicyano-7,7-diphenyl-5-isopropyl-2-methyl-4,6-heptadiene, also related to the reactant by 1,3-displacement of an isopropyl group. The last was 1,1-dicyano-5,5-diphenyl-3-isopropyl-1,4-pentadiene, a product in which an isopropyl group has been lost. The photochemistry of the two isopropyl-shifted dienes was also investigated. The 1,3-heptadiene underwent further di-{pi}-methane rearrangement, while the 4,6-heptadiene gave rise to three photochemical reactions affording naphthalene derivatives.