Issue 14, 1992

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.

Article information

Article type
Paper

J. Chem. Soc., Dalton Trans., 1992, 2193-2200

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

G. J. Sunley, I. M. Saez and P. M. Maitlis, J. Chem. Soc., Dalton Trans., 1992, 2193 DOI: 10.1039/DT9920002193

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