Selectivity of (Trifluoromethyl)chlorocarbene

Article abstract of DOI:10.1021/ja00410a031

(Trifluoromethyl)chlorocarbene (CF3CCl) was generated by photolysis of 3-(trifluoromethyl)-3-chlorodiazirine and added to eight alkenes affording the corresponding cyclopropanes.Additions to cis- and trans-butenes were stereospecific.No allylic CH insertion products were observed with any of the substrates.Substrates, reactivities (relative to Me2C=CH2) = 1.00, and stereoselectivities (syn-Cl/anti-Cl) for the CF3CCl addition reactions follow: Me2C=CMe2, 0.92, none; Me2C=CHMe, 1.17, 1.48; Me2C=CH2, 1.00, none; cis-MeCH=CHMe, 0.88, 1.65; trans-MeCH=CHMe, 0.62, none; EtCH=CH2, 0.48, 1.28; i-PrCH=CH2, 0.25, 1.36; t-BuCH=CH2, 0.11, 1.66.The reactivity data are analyzed and compared to analogous results for CH3CCl and CCl2.The carbene selectivity index, m_CXY, of CF3CCl is ca. 0.19 (m_CCl2 = 1.00), and its steric demand during addition to RCH=CH2 (Taft δ) is 0.41 (δ_CCl2 = 0.88).Relative to CH3CCl, CF3 strongly destabilizes CF3CCl.This conclusion follows both from analysis of the experimental data and from ab initio calculations.CF3CCl is believed to add to alkenes via "early", relatively "open" transition states, in which electronic and steric selectivities are leveled.