A Kinetic Investigation of Carbon Dioxide Insertion Processes Involving Anionic Tungsten-Alkyl and -Aryl Derivatives: Effects of CO2 Pressure, Counterions, and Ancillary Ligands.

Article abstract of DOI:10.1021/ja00311a040

A mechanistic investigation of the 1,2-addition of the carbon dioxide to tungsten-alkyl and -aryl derivatives, namely cis-RW(CO)4L(1-) (R = Me, Et, and Ph; L = CO; PMe3, and P(OMe)3), to provide metallocarboxylates is reported.The reaction is shown to obey second-order kinetics, first order in anionic metal substate and first order in carbon dioxide.A concerted reaction pathway involving an attack of the nucleophilic RW(CO)5(1-) ion at the electrophilic carbon or carbon oxygen center of CO2 is proposed.This proposal is supported by the dependence of the rate of these reactions an carbon monoxide and the nature of the metal center, the activation parameters, and the observed retention of configuration at the α-carbon atom.Replacement of carbon monoxide in the metal's coordination sphere by more electron-donating phosphite or phosphine ligands results in a significant enhancement of the rate of CO2 insertion into the tungsten-carbon bond.For example, the rate of CO2 insertion into cis-CH3W(CO)4PMe3(1-) is 243 times greater than that in CH3W(CO)5(1-).The molecular structure of was determined at - 99 deg C and W-CH3 bond distance of 2.18 (3) Angstroem was found to be somewhat shorter than that seen in the isoelectronic neutral Re-CH3 analogue (2.308 (17) Angstroem).The compound crystallizes in the monoclinic space group Cc with a = 16.538 (8) Angstroem, b = 17.102 (9) Angstroem, c = 16.106 (7) Angstroem, β = 115.26 (3) deg, and V = 4119.7 Angstroem³.Unlike the carbon dioxide insertion reactions which did not exhibit a large rate dependence on the nature of the R group, migratory carbon monoxide insertion reactions involving these anionic RW(CO)5(1-) derivatives display significant retardation with decreasing electron-donating ability of the R substituent.Further mechanistic comparisons of carboxylation processes are provided.