Open-chain remote stereocontrol using allylgermanes

Alk-2-enyl(trialkyl- and -triaryl)germanes are transmetallated by tin(IV) halides at −78 °C with 15 min being sufficient for the transmetallation to proceed effectively to completion. Alk-2-enyl(trialkyl)germanes are transmetallated by both tin(IV) chloride and tin(IV) bromide under these conditions, but the corresponding triphenylgermanes are only transmetallated using tin(IV) chloride. These observations led to procedures for reactions of 5-substituted-pent-2-enyl- and -hex-2-enyl-germanes with aldehydes that proceed with useful levels of 1,5- and 1,6-stereocontrol in favour of the (Z)-1,5-anti- and (Z)-1,6-syn-alkenols, respectively. Slightly improved stereoselectivities were obtained using (Z)-alk-2-enylgermanes. The reaction of (Z)-6-hydroxyhept-2-enyl(triethyl)germane with benzaldehyde when promoted using tin(IV) bromide gave a 3-ethenyl-6-methyl-2-phenyltetrahydropyran rather than the expected homoallylic alcohol.

Useful levels of 1,5- and 1,6-stereocontrol are observed in tin(IV) halide promoted reactions of alk-2-enylgermanes with aldehydes.