Synthesis of ferrocenes with ene-terminus via water-promoted Barbier-like carbonyl allylation using bimetallic copper(II)/tin(II) reagent

Barbier-type γ-regiospecific allylation of formylferrocene (1) with allyl bromides in the presence of stannous chloride dihydrate and catalytic cupric chloride in dichloromethane–water (1:1) afforded corresponding ferrocenyl dienes FcCHC(R1)C(R2)CH2 (3–6). On the other hand, similar reactions of 1,1′-bis-formylferrocene (2) yielded oxa-bridged [3]-ferrocenophanes having allyl pendants Fc[CH2C(R2)CH(R1)CH-μ(O)-CHCH(R1)C(R2)CH2] (8–11). The latter appear to result from the dehydration of intermediate homoallylic alcohols. Dehydration could be arrested in case of reaction of 1 and 2 with 1-bromo-3-methyl-but-2-ene, which results in the formation of homoallylic alcohols FcCH(OH)C(Me2)CHCH2 (7) and Fc[CH(OH)C(Me2)CHCH2]2 (12), respectively. All the reactions completely fail in absence of water.