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

Selective fragmentation of α,β-unsaturated esters into CC, CO, 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(CHCH₂) (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(CR¹CHR²) (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 -CC-C-OR fragment, regardless of the presence or absence of the CO group between the CC and OR fragments, is essential for the bond cleavage reaction.