The head-to-head reductive coupling of homoallylic alcohols promoted by titanium(II)-olefin complexes

Reaction of homoallylic alcohols 1a-e with i-PrMgBr in the presence of Ti(i-PrO)(4) leads to the unbranched saturated diols 2a-e as the main products in moderate to good yields. The head-to-head regioselectivity in reductive coupling of 4-penten-1-ol and 5-hexen-2-ol was also observed. Coupling of 2-methyl-5-hexen-2-ol, as well as unsaturated alcohols in which vinyl and hydroxyl groups are more distant from one another, proceeded with head-to-tail or tail-to-tail regioselectivity. It is supposed, that the unusual head-to-head regioselectivity in reductive coupling of homoallylic alcohols la-e is due to the formation of the key titanacyclopentane intermediates F and G having two fused oxatitanacyclopentane rings.