Photochemical Generation of Strained Cycloalkynes from Methylenecyclopropanes

The hydrocarbons 1-cyclopentylidene-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene and 1-cyclobutylidene-1a,9b-dihydro-1H-cyclopropa[l]phenanthrene undergo photolysis in solution at ambient temperature to produce cyclohexyne and cyclopentyne, respectively. These strained cycloalkynes, formed via the putative cycloalkylidenecarbenes, were intercepted as Diels-Alder adducts. Calculations at the CCSD(T)/cc-pVTZ//B3LYP/6-31+G* level of theory show that singlet cyclopentylidenecarbene has to overcome a barrier of 9.1 kcal mol^-1 to rearrange into cyclohexyne (with ΔE for ring expansion=-15.1 kcal mol^-1 ). By contrast, cyclobutylidenecarbene only needs to surmount a barrier of 1.6 kcal mol^-1 to rearrange into cyclopentyne (with ΔE for ring expansion=-6.2 kcal mol^-1 ).