New facets in the photochemistry and thermal reaction of 2,2-diphenylmethylenecyclopropane
Photochemistry and thermal reaction of 2,2-diphenylmethylenecyclopropane 1a have been reinvestigated to make their mechanistic refinement. A new finding in the photochemistry of 1a is the detection and isolation of cyclobutene 7. While fragmentation and 1,3-carbon (C) shift is responsible for previously reported photoproducts, 1,1-diphenylethylene 3 and diphenylmethylenecyclopropane 2a, respectively, a new pathway is required to explain the formation of cyclobutene 7. A mechanism involving the 1,2-C shift (Scheme 3, arrow b) followed by 1,2-hydrogen (H) shift is proposed. In the thermal reaction of 1a, on the other hand, a trimethylenemethane type of species is shown to be a common intermediate for degeneracy, rearrangement from 1a to 2a, and formation of indene 5. In the photochemistry of 1a, intervention of cyclobutylidene 8 is strongly supported by circumstantial evidences provided by steady-state and laser flash photolytic investigations. Further experiments designed to independently generate cyclobutylidene 8 showed the methylenecyclopropane/cyclobutene branching ratio (1a/7) to be in the range between 0 and 1.7, which is much lower than the value of 4.0 for parent cyclobutylidene. Nevertheless, the relative efficiency of 1,2-shift pathway to generate 8 is shown to be as high as 17–47% compared to 1,3-C shift to give 2a at the early photochemical stage of 1a.
Journal: Journal of Photochemistry and Photobiology A: Chemistry - Volume 185, Issues 2–3, 25 January 2007, Pages 253–262