Discovery and Mechanistic Investigation of Pt-Catalyzed Oxidative Homocoupling of Benzene with PhI(OAc)2
We present a Pt-catalyzed direct coupling of benzene to biphenyl. This catalytic reaction employs the cyclometalated platinum(II) complex [PtMe(bhq)(SMe2)] (bhq = benzo[h]quinolate) with PhI(OAc)2 as oxidant and does not require an acid, a co-catalyst or a solvent. Reaction kinetics and characterization of potential catalytic species are reported. The reaction is first-order in Pt and second-order in benzene, which implicates the second C-H activation step as rate-determining. A Pt(II)/Pt(IV) catalytic cycle is suggested. The reaction commences by oxidation of the Pt(II) complex to give the platinum(IV) species [Pt(bhq)(SMe2)(OAc)2](OAc) followed by C-H activation of benzene to afford the intermediate [PtPh(bhq)(SMe2)(OAc)](OAc) concurrently with release of HOAc. A second benzene molecules reacts similarly to give the diphenyl intermediate [PtPh2(bhq)(SMe2)](OAc). C-C bond forming reductive elimination ensues to regenerate Pt(II) and complete catalytic cycle. The proposed mechanism has been examined by DFT computations, which provides support to experimental findings.