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-1to 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-1to rearrange into cyclopentyne (with ΔE for ring expansion=-6.2 kcal mol-1).
Department of Chemistry, Colby College, 5765 Mayflower Hill, Waterville, ME, 04901, USA.
Daniel P
0000-0003-2074-5416
DP
Maurer
Department of Chemistry, Colby College, 5765 Mayflower Hill, Waterville, ME, 04901, USA.
Rui
0000-0002-1485-2808
R
Fan
Department of Chemistry, Colby College, 5765 Mayflower Hill, Waterville, ME, 04901, USA.
Dasan M
0000-0002-6619-4392
DM
Thamattoor
carbenes; cycloaddition; cycloalkynes; rearrangement; strained molecules