Alternatives to allylstannanes for remote stereocontrol
Allylstannanes with remote heteroatom substituents are transmetallated by tin(IV) halides to give allyltin trihalides which react with aldehydes with useful levels of remote stereocontrol; the use of this chemistry is exemplified by syntheses of several natural products. However, organotin starting materials are toxic and lead to residues which are difficult to remove and to dispose of. Studies are therefore underway to find procedures to carry out these and analogous reactions without the use of organotin reagents. Following mechanistic studies into allylstannane transmetallation, the use of allylsilicon and germanium reagents was investigated, with the allylgermanium reagents being viable alternatives to allylstannanes. In addition, investigations of reactions of allylstannanes mediated by bismuth(III) iodide led to the development of reactions of allyl bromides with aldehydes promoted by the low-valency species formed by treatment of bismuth(III) iodide with zinc powder. These reactions proceed with useful complementary 1,5-stereocontrol.