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

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.