Photochemical Generation of Strained Cycloalkynes from Methylenecyclopropanes

Daniel P Maurer; Rui Fan; Dasan M Thamattoor

doi:10.1002/anie.201701275

Photochemical Generation of Strained Cycloalkynes from Methylenecyclopropanes

Angew Chem Int Ed Engl. 2017 Apr 10;56(16):4499-4501. doi: 10.1002/anie.201701275. Epub 2017 Mar 21.

Authors

Daniel P Maurer¹, Rui Fan¹, Dasan M Thamattoor¹

Affiliation

¹ Department of Chemistry, Colby College, 5765 Mayflower Hill, Waterville, ME, 04901, USA.

PMID: 28323378
DOI: 10.1002/anie.201701275

Abstract

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 ).

Keywords: carbenes; cycloaddition; cycloalkynes; rearrangement; strained molecules.

Publication types

Research Support, U.S. Gov't, Non-P.H.S.