Stereoselectivity and stereospecificity of cyclopropanation reactions withstable (phosphanyl)(silyl)carbenes

The stable (phosphanyl)(silyl)carbenes la,b react efficiently with various electron-poor alkenes [methyl acrylate, 3,3,4,4,5,5,6,6,6-nonafluorohex-1-e ne, styrene, (Z)- and (E)-2-deuteriostyrene, (E)-t-BuOC(O)CH=CHC(O)OEt, and (E)-Me2NCOCH=CHCO2Me] giving the corresponding cyclopropanes in good yield s. The stereochemical outcome was such that all monosubstituted alkenes gav e exclusively the syn isomer (with respect to the phosphanyl group), and th e addition of disubstituted alkenes was totally stereospecific. The high st ereoselectivity observed was ascribed to a secondary orbital interaction (L UMOcarbene-HOMOalkene) similar to that explaining the endo-rule in Diels-Al der reactions.