The far-ultraviolet photochemistry of alkylcyclopropenes in solution

The photochemistry of 1,3,3-trimethylcyclopropene and 1-tert-butyl-3,3-dimethylcyclopropene has been investigated in hydrocarbon, methanol, and 1-hexene solution with far-ultraviolet (185-228 nm) light. Direct photolysis of the two compounds affords allene, alkyne, and 1,3-diene derivatives, formally as a result of initial bond cleavage to yield vinylcarbene intermediates. Products derived from cleavage of the most substituted (C1C3) cyclopropene bond account for 60-80% of the observed mixture in each case. Results from the photolysis of 1,3,3-trimethylcyclopropene-1- 13 C suggest a second pathway for formation of alkyne products in cyclopropene photochemistry, which occurs in competition with (or by exclusion of) the vinylcarbene [1,2]-hydrogen migration pathway. The data are consistent with the intermediacy of a vinylidene species, formed by [1,2]-hydrogen migration/ring opening, although attempts to chemically trap this intermediate with methanol or alkene were unsuccessful. Keywords: cyclopropene, photolysis, vinylmethylene, propenylidene, far-UV.