Teaching an old carbocation new tricks: Intermolecular C–H insertion reactions of vinyl cations

• Published in: Science (American Association for the Advancement of Science) Vol. 361; no. 6400; pp. 381 - 387
• Main Authors: Popov, Stasik, Shao, Brian, Bagdasarian, Alex L., Benton, Tyler R., Zou, Luyi, Yang, Zhongyue, Houk, K. N., Nelson, Hosea M.
• Format: Journal Article
• Language: English
• Published: United States The American Association for the Advancement of Science 27.07.2018
• Subjects: Alkanes; Anions; Aromatic compounds; Bifurcations; Carbon; Catalysis; Cations; Chemical synthesis; Chemistry; Computer applications; Friedel-Crafts reaction; Hydrocarbons; Insertion; Intermediates; Organic chemistry; Silicon; Solvolysis; Substitution reactions
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.aat5440

Saturated carbon centers often undergo substitution reactions by initial cleavage of a carbon-halogen or carbon-oxygen bond, which leaves the carbon positively charged. Analogous cations have proven much harder to access from vinylic carbons that are engaged in double bonds. Popov et al. now show that silicon cations paired with noncoordinating anions can pull triflate groups off such vinylic carbons under ambient conditions (see the Perspective by Kennedy and Klumpp). The resultant vinyl cations react with simple alkanes through C–H insertion. Theoretical and mechanistic studies suggest that these reactions proceed through nonclassical pathways that bifurcate after the transition state. Science , this issue p. 381 ; see also p. 331 Silicon cations draw triflate substituents away from olefins to form vinyl cations that react with alkanes via C–H insertion. Vinyl carbocations have been the subject of extensive experimental and theoretical studies over the past five decades. Despite this long history in chemistry, the utility of vinyl cations in chemical synthesis has been limited, with most reactivity studies focusing on solvolysis reactions or intramolecular processes. Here we report synthetic and mechanistic studies of vinyl cations generated through silylium–weakly coordinating anion catalysis. We find that these reactive intermediates undergo mild intermolecular carbon-carbon bond–forming reactions, including carbon-hydrogen (C–H) insertion into unactivated sp 3 C–H bonds and reductive Friedel-Crafts reactions with arenes. Moreover, we conducted computational studies of these alkane C–H functionalization reactions and discovered that they proceed through nonclassical, ambimodal transition structures. This reaction manifold provides a framework for the catalytic functionalization of hydrocarbons using simple ketone derivatives.