Platinum-catalyzed Dehydroalkoxylation-cyclization Cascade Via N-o Bondcleavage

摘要

Carbophilic transition metal catalyzed reactions have attracted much attention to facilitate the construction of highly elaborate molecules in an efficient and atom-economic manner. In general, the first step of these reactions involves a nucleophilic attack to the C-C triple bond, which possesses an enhanced electrophilicity due to its π-co-ordination with a transition metal (Scheme la). In many of these reactions, vinylmetal intermediate A is transformed into metal car-benoid intermediate B driven by the donation of electrons from the metal species. To date, various metal-carbenoid species bearing a net neutral functional group, such as furyl, 1-acyloxyalkenyl, imine, carbonyl, cyclopropyl, and ylide groups, have been employed as key intermediates. Accordingly, we envisioned that iminium-bound metal carbenoid species C would possess enhanced electrophilicity due to the cationic iminium group and may lead to favorable transformations. In this case, C can be generated via nucleophilic attack by an alkoxyamine onto a π-activated triple bond, followed by the elimination of the alkoxy group (Scheme 1b). Herein, we report the platinum-catalyzed dehydroalkoxylation-cyclization cascade of ortho-alkynylphenylureas and -acetamides 1, bearing an alkoxy and aryl group on the nitrogen atom, to afford the corresponding nitrogen-containing tetracyclic compounds 2, via N-O bond cleavage, in good to excellent yields (eq 1).