30191-83-4Relevant academic research and scientific papers
Oxidative addition of alkyl halides to rhodium(I) and iridium(I) dicarbonyl diiodides: Key reactions in the catalytic carbonylation of alcohols
Ellis, Paul R.,Pearson, Jean M.,Haynes, Anthony,Adams, Harry,Bailey, Neil A.,Maitlis, Peter M.
, p. 3215 - 3226 (1994)
Alkyl iodides (RI) react with [Rh(CO)2I2]- to give acyl species [Rh(CO)(COR)I3]- (R = Et, nPr, iPr) and with [Ir(CO)2]2]- to give alkyl complexes [RIr(CO)2I2]- (R = Et, nPr, iPr, nBu, n-C5H11, n-C6H13). The reactions are analogous to the known reactions of MeI with [Rh(CO)2I2]- and [Ir(CO)2I2]-. The products are characterized spectroscopically and by an X-ray crystal structure determination for Ph4As[(n-C6H13)Ir(CO)2I3] which showed a fac,cis geometry for the anion. [Crystal structure data: monoclinic, a = 9.408(7) ?, b = 19.470(16) ?, c = 19.529(12) ?, β = 94.99(5)°, Z = 4, space group P21/n (a nonstandard setting of P21/c C2h5, No. 14); 2446 independent reflections (of 5197 measured) for which |F|/σ(|F|) > 4.0; R = 0.0966 (Rw = 0.0921, 238 parameters)]. Kinetic data for the reactions of [Rh(CO)2I2] with EtI, nPrI, and iPrI and for [Ir(CO)2I2]- with MeI, EtI, and nPrI show that oxidative addition of RI to [M(CO)2I2]- is first-order in both reactants. For M = Rh, reactions showed clean second-order kinetics below 80°C, though some decomposition occurred at higher temperatures. For M = Ir, clean second-order kinetics were observed with MeI, but reactions with EtI and nPrI showed a more complex kinetic behavior. A competing radical pathway is suggested, which can be quenched by added duroquinone. Second-order rate constants, k2, evaluated over the temperature ranges 70-80°C (M = Rh) and 35-50°C (M = Ir) gave the following activation parameters: (M = Rh) ΔH≠/kJ mol-1 = 50(±1) (R = Me), 56(±10) (R = Et), 51(±10) (R = nPr), 61(±15) (R = iPr); ΔS≠/J mol-1 K-1 = -165(±4) (R = Me), -195(±25) (R = Et), -215(±25) (R = nPr), -180(±30) (R = iPr); (M = Ir) ΔH≠/kJ mol-1 = 54(±1) (R = Me), 66(±5) (R = Et), 66(±3) (R = nPr); ΔS≠/J mol-1 K-1 -113(±4) (R = Me), -123(±15) (R = Et), -132(±11) (R = nPr). Comparisons are made between the reactions of methyl iodide and the higher alkyl iodides with both [Rh(CO)2I2]- (relative rates: Me, 1000, Et, 3; nPr, 1.7) and [Ir(CO)2I2]- (relative rates: Me, 1000; Et, 2.3; nPr, 0.75). The similarity to reactivity trends for organic nucleophiles suggests an SN2 mechanism, but with a competing radical pathway for iridium. Relative rates for the two nucleophiles, kIr/kRh ca. 150 (R = Me), 220 (R = Et), and 140 (R = nPr), are estimated. Alkyl isomerization (iso → n) is observed for both [Rh(CO)(COPr)I3]- and [PrIr(CO)2I3]- and displacement of propene from [Rh(CO)(COPr)I3]- by added ethene gives [Rh(CO)(COEt)I3]- reversibly. A mechanism involving hydridoalkene intermediates is proposed. The data are consistent with the carbonylation of the higher alcohols (ROH) proceeding via rate determining oxidative addition of RI to [Rh(CO)2I2]-, rather than by a route involving a rhodium hydride addition to an olefin derived from the ROH.
