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Issue 14, 1992
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Synthesis of alkan-2-ones by dirhodium-mediated four-carbon coupling


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.

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Article information

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

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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