Ligand-Controlled, Complementary Stereoselectivity in the Platinum-Catalyzed Intramolecular Silaboration of Alkenes

An intramolecular silaboration of borylsilanyl homoallyl ethers was achieved using a platinum catalyst, giving 1-oxa-2-silacyclopentanes in high yields. The stereoselectivity of the reactions of sec-homoallyl ethers strongly depended on the phosphorus ligand of the platinum catalysts used. The platinum complex bearing the PCyPh2 ligand was found to be the most trans-selective catalyst (trans/cis = 81:19-92:8), whereas a highly cis-selective cyclization was achieved using a platinum catalyst having tris(2,4-di-tert-butylphenyl)phosphite ligand (trans/cis = 8:92-6:94). The synthetic utility of the intramolecular silaboration was demonstrated by the complementary synthesis of a pair of diastereomers of 6-methylheptane-1,3,5-triol.