Stereoselectivity and Stereospecificity of Cyclopropanation Reactions with Stable (Phosphanyl)(silyl)carbenes

The stable (phosphanyl)(silyl)carbenes 1a,b react efficiently with various electron-poor alkenes [methyl acrylate, 3,3,4,4,5,5,6,6,6-nonafluorohex-1-ene, styrene, (Z)- and (E)-2-deuteriostyrene, (E)-t-BuOC(O)CHCHC(O)OEt, and (E)-Me₂NCOCHCHCO₂Me] giving the corresponding cyclopropanes in good yields. The stereochemical outcome was such that all monosubstituted alkenes gave exclusively the syn isomer (with respect to the phosphanyl group), and the addition of disubstituted alkenes was totally stereospecific. The high stereoselectivity observed was ascribed to a secondary orbital interaction (LUMO_carbene−HOMO_alkene) similar to that explaining the endo-rule in Diels−Alder reactions.