BIPHASIC SYNTHESIS OF 2-PHENYLPROPIONIC ACID AND ESTER BY INTERFACIALCARBONYLATION OF ALPHA-METHYLBENZYL BROMIDE

An interfacial synthesis technique has been successfully extended to t he carbonylation of alpha-methylbenzyl bromide in an organic-aqueous s odium hydroxide mixture at 35-60 degrees C and 1 atm using surface-act ive palladium - (4-dimethylaminophenyl)diphenylphosphine complex as th e catalyst and dodecyl sodium sulfate as the emulsifier, Depending on the reaction conditions, 2-phenylpropionate in the form of sodium salt and an ester was obtained in 0-83% yield, along with varying amounts of side products that included alpha-methylbenzyl alcohol, 2,3 dipheny lbutane, di(alpha-methylbenzyl)ether, and an asymmetric ether derived from the substrate and an alcoholic medium. When 2-methyl-1-butanol or 2-ethyl-1-hexanol was used as the organic phase, 2-phenylpropionate e ater and sodium salt were obtained in 40-83% yield, with a maximum yie ld obtained at an optimal aqueous base concentration of about 5 M, At a lower aqueous base concentration, more of alpha-methylbenzyl alcohol was formed, whereas at a higher aqueous base concentration, more of 2 ,3-diphenylbutane and asymmetric ether were formed. When toluene was u sed as the organic phase, 2-phenylpropionate salt was obtained in less than 13% yield, and the major side product was a-methylbenzyl alcohol at a low aqueous base concentration and 2,3-diphenylbutane at a high aqueous base concentration. In all cases, the formation of 2,3-dipheny lbutane was accompanied by a stoichiometric formation of carbonate. Th e latter implicates the involvement of an oxidative intermediate-tenta tively identified as hypobromous acid-that could deactivate the cataly st complex through ligand degradation. Along with the carbonylation re action, carbon monoxide also underwent a slow, base-induced hydrolysis reaction to form formic acid. With 2-ethyl-1-hexanol as the organic p hase, the carbonylation of alpha-methylbenzyl bromide showed an appare nt temperature-dependent activation energy, a first-order dependence e ach on the substrate, catalyst, and ligand concentrations up to the ca talyst concentration of 0.0020 M and a ligand:catalyst ratio of 3:1, a nd a variable-order dependence on the carbon monoxide pressure that sw itched from first to zeroth order as the carbon monoxide pressure was increased above 450 mmHg, A reaction mechanism is proposed which yield s model rate and yield expressions in accord with the experimental fin dings. Results of control experiments with alpha,alpha-dibromotoluene in a toluene-aqueous sodium hydroxide mixture indicate that replacemen t of the alpha-methyl group in alpha-methylbenzyl bromide by a second bromo group suppressed the formation of substituted benzyl alcohol and coupled product. They suggest that the broad product distribution in the carbonylation of alpha-methylbenzyl bromide relative to the carbon ylation of benzyl chloride and alpha,alpha-dibromotoluene is attributa ble to the electron-releasing alpha-methyl group making the substrate susceptible to hydrolysis and coupling reactions.