Platinum(II)-Catalyzed Cyclopropanation of Vinyl Arenes and 1,3-Aryl Dienes Employing Aryl Diazirines as Carbene Precursors

PtCl₂(NCC₆F₅)₂ catalyzes the reaction of electron-rich vinyl arenes with aryl diazirines in chloroform at 45 °C for 12 h to form 1,2-diarylcyclopropanes in >75% yield with cis/trans selectivity ranging from 3.0 to 10:1. The method is similarly applicable to the cyclopropanation of 1-aryl-1,3-butadienes, albeit with diminished diastereoselectivity (cis/trans = 0.5-2.6). Kinetic and deuterium-labeling experiments, <i>in situ</i> analysis of platinum coordination complexes, and DFT calculations support a mechanism for diazirine to alkene carbene transfer initiated by ligand displacement of a platinum(II) <i>trans</i>-bis(π-vinyl arene) complex with aryl diazirine to form a four coordinate <i>trans</i>-platinum π-vinyl arene σ-diazirine complex. The turnover-limiting oxidative addition of coordinated diazirine forms a five-coordinate Pt(IV) diazometallacyclobutane complex that undergoes facile Pt-N bond cleavage followed by expulsion of dinitrogen from the resulting C-bound aryldiazomethane complex to form a <i>trans</i>-platinum benzylidene complex. The concerted, outer-sphere attack of vinyl arene on the carbene carbon atom of the platinum benzylidene complex though a polarized transition state releases cyclopropane.