Rh(II)-Catalyzed Synthesis of 1,3-Diols via 5-endo-trig Cyclization of Silyl Radicals

Abstract 1,3-Diols, which are a frequent motif in biologically active molecules, can be prepared from readily available allylic alcohols via formal anti-Markovnikov hydration. The commonly employed hydroboration–oxidation sequence for the synthesis of terminal alcohols is challenging for allylic alcohols, and O-protection of the alcohol can be necessary. To increase atom economy, we explored the use of silane protecting groups that can be engaged in intramolecular hydrosilylation. Oxidative cleavage of the cyclized product yields the desired 1,3-diol and obviates the need for super-stoichiometric borane reagents. Based on a detailed study of O-silylation conditions, a protocol is presented that furnishes quantitative yields of a wide range of O-silylated alcohols which contain Si–H bonds for further functionalization. We show that a MOF-based Rh(II) porphyrin can furnish efficient intramolecular hydrosilylation, while the corresponding homogeneous analogue proved unreactive. Radical trapping studies suggest that silyl radicals constitute key intermediates in Rh(II)-catalyzed intramolecular hydrosilylation. Preferential 5-endo-trig versus 6-exo-trig cyclization and 5-exo-trig versus 6-endo-trig cyclization of the silyl radical intermediates led to chemoselective 1,3-diol formation for substrates containing multiple olefins.