The dicarbonylation of diiodomethane to malonate esters catalysed by triethylphosphine complexes of rhodium

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Rhodium complexes, in the presence or absence of PEt3, catalyse the carbonylation of CH2I2 to dialkylmalonates in the presence of alcohols (ROH, R = Me, Et, Pr-i, Bu) with side products from reactions in EtOH being CH2(OEt)(2), EtI and traces of EtCO2Et and EtOAc. The active species when using PEt3 is shown to be [RhI(CO)(PEt3)(2)], formed via [Rh(OAc)(CO)(PEt3)(2)] from [Rh-2(OAc)(4). 2MeOH] and PEt3. Mechanistic studies show that the first step of the catalytic cycle is oxidative addition of CH2I2 to give [Rh(CH2I)I-2(CO)(PEt3)(2)], but that insertion of CO into the Rh-CH2I bond gives an iodoacyl complex which is unstable. The analogous [Rh(COCH2X)X-2(CO)(PEt3)(2)], (X = Cl or Br) have been synthesised by oxidative addition of XCH2COX to [RhX(CO)(PEt3)(2)] and fully characterised (by X-ray crystallography, for X = Cl). [Rh(COCH2Br)Br-2(CO)(PEt3)(2)] has also been formed from reaction of [Rh(COCH2Cl)Cl-2(CO)(PEt3)(2)] with excess NaBr. However, the analogous reaction with NaI does not give the iodoethanoyl complex, but rather [RhI3(CO)(PEt3)(2)] and its decomposition products. It is proposed that [Rh(COCH2I)I-2(CO)(PEt3)(2)] is unstable towards loss of I- to form the ketene complex, [RhI2(CH2 = C = O)(CO)(PEt3)(2)]I, which is transformed into [Rh(COCH2CO2Et)I-2(CO)(PEt3)] by nucleophilic attack of ethanol at the central C atom, followed by CO insertion into the Rh-C bond. An analogue, [Rh(COCH2CO2Et)Cl-2(CO)(PEt3)(2)], has been isolated by oxidative addition of EtO2CCH2COCl across [RhCl(CO)(PEt3)(2)], and characterised both spectroscopically and crystallographically. In refluxing ethanol, [Rh(COCH2CO2Et)Cl-2(CO)(PEt3)(2)] produces diethylmalonate and [RhCl(CO)(PEt3)(2)], thus completing the catalytic cycle. Possible pathways of deactivation of the catalyst to give [RhI3(CO)(PEt3)(2)] are discussed. One involves the reaction of ketene with ethanol to give EtOAc, whilst the others involve protonation of the Rh-Z bond in [RhZI(2)(CO)(PEt3)(2)] (where Z = CH2I, CH2CO2Et or H) by HI. The isolation of CH2DCO2Et, when carrying out the reaction in EtOD, is consistent with all of these deactivation pathways except protonation of [RhHI2(CO)(PEt3)(2)]. (C) 1998 Elsevier Science S.A. All rights reserved.

Original languageEnglish
Pages (from-to)99-117
Number of pages19
JournalInorganica Chimica Acta
Publication statusPublished - 15 Oct 1998


  • catalysis
  • carbonylation
  • rhodium complexes
  • phosphine complexes


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