Reactivity of Frustrated Lewis Pair: Carbocation versus Radical Intermediates

Recent reports of radical formation within frustrated Lewis pairs (FLPs) suggested that single-electron transfer (SET) could play an important role in their chemistry especially for C-C coupling. In sharp contrast, our extensive dispersion-corrected DFT calculations show that although reactive benzhydryl radical along with phosphine radical cation species can be kinetically generated from bulky phosphines and benzhydryl cation, direct P-C hetero-coupling may lead to bulky phosphonium cation as reactive carbocation transfer reagents to styrene substrates, which is kinetically much more favorable than the recently proposed radical C-C coupling between benzhydryl radical and styrene. Similarly, meta-stable radical cation Mes3P+& sdot; salt is also kinetically accessible via SET reactions of Mes3P and B(C6F5)3 with 0.5 equivalent of p-O2C6Cl4. Extensive DFT calculations show that both radical HCAr2 & sdot; and carbocation HCAr2+ are thermally accessible from the bulky phosphonium Mes3P-CHAr2+, which are further kinetically favored by HCAr2 & sdot; radical self-dimerization and by nucleophilic alkene trapping, respectively.image