On the use of mixtures of organotin species for catalytic enantioselective ketone allylation--a detective story

In the presence of enantiopure MTBH(2)(monothiobinaphthol, 2-hydroxy-2prime or minute]mercapto-1,1prime or minute]-binaphthyl; 0.2 eq.) quantitative allylation of ArC(double bond]O)Me takes place with impure Sn(CH(2)CHdouble bond]CH(2))(4)(prepared from allyl chloride, air-oxidised magnesium and SnCl(4)) to yield tert-homoallylic alcohols in 85-92% ee. In the same process highly purified, or commercial, Sn(CH(2)CHdouble bond]CH(2))(4) yields material of only 35-50% ee. The origin of these effects is the presence of small amounts of the compounds, EtSn(CH(2)CHdouble bond]CH(2))(3), ClSn(CH(2)CHdouble bond]CH(2))(3) ClSnEt(CH(2)CHdouble bond]CH(2))(2) in the tetraallyltin sample and the presence of traces of water (which inhibits achiral background reactions). All the triallyl and diallyl species enhance the stereoselectivity in the catalytic allylation reaction, the chlorides more so than the ethyl compound. Hydrolysis of ClSnEt(CH(2)CHdouble bond]CH(2))(2) affords crystallographically characterised Sn(4)(mu(3)-O)(mu(2)-Cl)(2)Cl(2)Et(4)(CH(2)CHdouble bond]CH(2))(4). Reaction of this latter compound with MTBH(2) leads to the most potent catalyst.