Introducing Alkene Moieties via Iterative Carbenoid Insertions: Vinylene Homologation of Organoboronates

The Matteson homologation of organoboronates has been an attractive approach for constructing aliphatic carbon chains via iterative insertion of carbenoids. However, the corresponding homologation that can introduce alkene moieties to molecular backbones remains elusive. Here we report the development of a stereoselective vinylene homologation of various alkyl and aryl boronates. The reaction is enabled by diastereoselective consecutive insertion of a silyl- and an alkoxy-substituted carbenoid, followed by a Peterson-type elimination. Diverse alkenyl boronates can be obtained in good yield and good to excellent trans selectivity. Density functional theory (DFT) calculations revealed the origin of diastereoselectivity in carbenoid insertion and how Lewis acids with different sulfide binding affinities affect the competing SN2- and SN1-type 1,2-boronate migration pathways with distinct levels of stereospecificity. This protocol has been successfully applied to programmable synthesis of piperamide-family natural products by merging with the methylene homologation. Guided by the mechanistic understanding, preliminary success has been achieved with the cis-selective vinylene homologation enabled by oxyphilic Lewis acids.