Synthesis of alkan-2-ones by dirhodium-mediated four-carbon coupling

Abstract

Butan-2-one and pentan-2-one are obtained by C–C–C–C coupling of the four ligands in the dirhodium complexes [{(C₅Me₅)Rh}₂(µ-CH₂)₂R(CO)]⁺1b, (R = CH₃, ¹³CH₃ or CD₃) or 2b(R = Et), which are readily generated from the acetonitrile complexes [{(C₅Me₅)Rh}₂(µ-CH₂)₂R(MeCN)]⁺ with CO. Complete decomposition of 1b or 2b occurred readily under mild conditions (CO, 1–12 atm, MeCN solvent, 25 °C, 12–48 h) to give RCH₂COMe and [(C₅Me₅)Rh(CO)₂]. Complexes [{(C₅Me₅)Rh}₂(µ-CH₂)₂R₂]3(R = Me) or 4(R = Et) reacted similarly with CO in MeCN (6 atm, 25 °C, 20 h) in the presence of [Fe₄(OH)₂(SO₄)₅] or slowly, in its absence, under more forcing conditions (6 atm, 90 °C, 20 h). Analogous reaction occurred in methanol (or CD₃OD), but with the co-production of methyl acetate. When the complex 3c(R =¹³CH₃) was decomposed in the presence of [Fe₄(OH)₂(SO₄)₅] in CD₃OD, labelling showed that the majority of the methyl acetate contained ¹³CH₃CO₂CD₃. A chief function of the iron salt is to cleave off R as RH; thus complex 3 yielded substantial amounts of methane and propene, in addition to butan-2-one. Analysis of RCH₂COMe from [{(C₅Me₅)Rh}₂(µ-CH₂)₂R(MeCN)]⁺1a(R =¹³CH₃ or CD₃) and from 3(R =¹³CH₃ or CD₃) or 4 show that the R is derived from the rhodium alkyl, while both the CH₂ and the CH₃ are from the µ-CH₂ ligands. A crossover experiment in which equimolar amounts of [{(C₅Me₅)Rh}₂(µ-CH₂)₂Et₂]4 and [{(C₅Me₅)Rh}₂(µ-¹³CH₂)₂(¹³CH₃)₂](92%¹³C) were decomposed together gave labelled butan-2-one (largely ¹³C₂H₅¹²CO¹³CH₃) and only natural-abundance ¹³C levels in the pentan-2-one; this shows the reaction to be intramolecular. Carbonylation of [{(C₅Me₅)Rh}₂(µ-CH₂)₂Me(CD₃CN)]⁺ in the presence of CD₃CN–D₂O gave CH₃CH₂COCH₂D; ¹³CH₃CH₂COCH₂D was obtained from the carbonylation of 3c(92%¹³C) in CD₃OD in the presence of [Fe₄(OH)₂(SO₄)₅]. A mechanism is proposed for the C–C–C–C coupling leading to ketone; it involves µ-ketene, -oxaallyl, and -enolate intermediates and terminates with a step which is formally a hydrolysis. The role of the dinuclear framework on which reaction occurs is to hold the reactants in place and to supply the electrons needed for the C–C–C–C coupling.