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(CH₂CHCH₂)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(CH₂CHCH₂)₄ yields material of only 35–50% ee. The origin of these effects is the presence of small amounts of the compounds, EtSn(CH₂CHCH₂)₃, ClSn(CH₂CHCH₂)₃ ClSnEt(CH₂CHCH₂)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(CH₂CHCH₂)₂ affords crystallographically characterised Sn₄(μ₃-O)(μ₂-Cl)₂Cl₂Et₄(CH₂CHCH₂)₄. Reaction of this latter compound with MTBH₂ leads to the most potent catalyst.