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

In the presence of enantiopure MTBH2 (monothiobinaphthol, 2-hydroxy-2' mercapto-1,1'-binaphthyl; 0.2 eq.) quantitative allylation of ArC(=O) Me takes place with impure Sn(CH2CH=CH2)(4) (prepared from allyl chloride, air-oxidised magnesium and SnCl4) to yield tert-homoallylic alcohols in 85-92% ee. In the same process highly purified, or commercial, Sn(CH2CH=CH2)(4) yields material of only 35-50% ee. The origin of these effects is the presence of small amounts of the compounds, EtSn(CH2CH=CH2)(3), ClSn(CH2CH=CH2)(3) ClSnEt(CH2CH=CH2)(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(CH2CH=CH2)(2) affords crystallographically characterised Sn-4(mu(3)-O)(mu(2)-Cl)(2)Cl2Et4(CH2CH=CH2)(4). Reaction of this latter compound with MTBH2 leads to the most potent catalyst.