Photochemical Generation of Strained Cycloalkynes from Methylenecyclopropanes

The hydrocarbons 1‐cyclopentylidene‐1a,9b‐dihydro‐1H ‐cyclopropal ]phenanthrene and 1‐cyclobutylidene‐1a,9b‐dihydro‐1H ‐cyclopropal ]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).