C–O and C–C bond cleavage of α,β-unsaturated esters with the assistance of a gallane(pyridyl)iron complex

Abstract

Selective fragmentation of α,β-unsaturated esters into CC, CO, and OR fragments was investigated with the assistance of a gallane(pyridyl)iron complex Cp*(OC)Fe{(η¹-HGaMes₂)(η¹-2-C₅H₄N)} (1, Cp*: η⁵-C₅Me₅, Mes: 2,4,6-Me₃C₆H₂). The reaction of 1 with methyl acrylate in C₇D₈ afforded vinyliron complex Cp*(OC)₂Fe(CHCH₂) (2) and 4-membered Ga₂O₂ cyclic gallane [Mes₂GaOMe]₂ (3). The C–O and C–C bonds in t-butyl acrylate were cleaved by the reaction with complex 1, producing complex 2 and pyridine-coordinated gallane Mes₂(t-BuO)Ga(pyridine) (4). The selective bond cleavage reactions proceeded by the reactions of 1 with methyl methacrylate and methyl 2-butenoate to afford Cp*(OC)₂Fe(CR¹CHR²) (R¹ = Me, R² = H (5), R¹ = H, R² = Me (6)) and 3. In contrast, compound 3 was formed in 11% NMR yield by the reaction of 1 with methyl 3-butenoate (β,γ-unsaturated ester), and complex mixtures of unidentified products were obtained by the reaction of 1 with methyl 4-pentenoate (γ,δ-unsaturated ester) and methyl propionate (saturated ester). The treatment of a cyclic ester, α-methylene-γ-butyrolactone, with 1 afforded a 4-membered Ga₂O₂ cyclic gallane dimer with Cp*(OC)₂FeC(CH₂)CH₂CH₂ substituents on the oxygen atoms, namely [Cp*(OC)₂FeC(CH₂)CH₂CH₂OGaMes₂]₂ (7). In addition, the bond cleavage reaction occurred by the reaction of 1 with allyl methyl ether to afford the (η³-allyl)iron complex Cp*(OC)Fe(η³-CH₂CHCH₂) (8) and 3. These results indicate that the existence of the –CC–C–OR fragment, regardless of the presence or absence of the CO group between the CC and OR fragments, is essential for the bond cleavage reaction.