Stereoselective intramolecular bis-silylation of alkenes promoted by a palladium-isocyanide catalyst leading to polyol synthesis

Article abstract of DOI:10.1021/ja00068a003

Details of a study on the intramolecular bis-silylation of terminal alkenes promoted by a palladium-tert-alkyl isocyanide catalyst are described. With a disilanyl ether derived from a homoallylic alcohol, intramolecular regioselective addition of the Si-Si linkage to the C=C bond took place to furnish an exo-ring closure product, i.e., 1,2-oxasilolane. The bis-silylation of alkenes having substituents α to the C=C bond gave trans-3,4-disubstituted oxasilolanes, while substitution β to the C=C bond favored cis-3,5-disubstituted oxasilolanes. The stereoselectivity trends are formulated as arising from a preference for a chairlike transition state over a boatlike one. A substituent, either a or β to the C=C bond, prefers the equatorial position in a chairlike transition state. The 1,2-oxasilolanes thus produced stereoselectively were oxidatively converted to the corresponding 1,2,4-triols. The present methodology for the synthesis of 1,2,4-triols was successfully extended to the stereoselective synthesis of 1,2,4,5,7- and 1,2,4,6,7-pentaols through a sequence of intramolecular bis-silylations. The bis-silylation was also performed with alkenes linked to disilanyl groups through a three-carbon chain and through an amide linkage. Stereoselections analogous to those of the ether substrates were observed. Alkenes tethered to disilanyl groups through chains of two atoms underwent similar intramolecular bissilylation. In conclusion, the intramolecular bis-silylation of C=C bonds followed by oxidation constitutes a new synthetic transformation equivalent to the stereoselective dihydroxylation of olefins.