Phase-Transfer Catalysis in Supercritical Carbon Dioxide:  Kinetic and Mechanistic Investigations of Cyanide Displacement on Benzyl Chloride

Abstract

This work reports the first studies designed to examine the detailed mechanism of a phase-transfer catalyzed reaction between a supercritical CO2 phase and a solid salt phase. The nucleophilic displacement of benzyl chloride with potassium cyanide to form phenylacetonitrile and potassium chloride was carried out with a tetraheptylammonium salt as the phase-transfer catalyst. The effects of various reaction variables on the kinetics were investigated, including the amount of catalyst, the amount of potassium cyanide, the presence of acetone cosolvent, and temperature. The kinetic data, along with catalyst solubility measurements, indicate that the operating reaction mechanism is a three-phase system consisting of a supercritical CO2 phase, a catalyst-rich phase, and a solid salt phase and that the reaction actually occurs in the catalyst-rich phase. Further, the reaction mechanism was investigated with two additional phase-transfer catalysts, 18-crown-6 and poly(ethylene glycol), and these results are consistent with the postulated three-phase mechanism.

Hydroformylation reactions in supercritical carbon dioxide using insoluble metal complexes

“…The reaction could be also occurring at the phase boundary between the supercritical fluid and the insoluble catalytic phase, in a manner similar to that which has been observed when using solid phase transfer catalysts for halide exchange or cyanation reactions in scCO 2 . 15, 16 We conclude that these systems provide a method for the hydroformylation of alkenes which show high activity and selectivity (much higher selectivity towards the straight chain product than under comparable conditions in conventional organic solvents), but especially that they provide a facile method for the separation of the reaction products from the catalyst and from the solvent. The decanal produced is sufficiently pure that it crystallises directly from the product mixture.…”

Phase Transfer and Ammonium Salt Catalyzed Reactions

“…The phase transfer-catalyzed displacement reactions [19][20][21] that have been performed in supercritical fluids to date have been reactions between insoluble solid salts and soluble organic liquids, in the presence of solid catalysts. A diagram of the apparatus for such an experiment is shown in Figure 4.10-3.…”

“…The reaction between benzyl chloride 1 and potassium cyanide in scC02 in the presence of tetraheptylammonium chloride (THAC) to yield benzyl cyanide 3 (Scheme 4.10-2) showed interesting results [21]. Similar first-order kinetics were found; however, the authors propose that the reaction occurred in a highly reactive third phase, the omega phase.…”

Recent advances in the application of supercritical fluids for carbon–carbon bond formation in organic synthesis

“…The first PTC reaction in SCF was the displacement reaction of benzyl chloride 105 with potassium bromide in supercritical carbon dioxide 125,126 with 5 mol% acetone, in the presence of tetraheptylammonium bromide (THAB), to yield benzyl bromide 106 (Scheme 37). Chandler et al 127 investigated the reaction between benzyl chloride 105 and potassium cyanide in scCO 2 in the presence of tetraheptylammonium chloride (THAC) to yield benzyl cyanide 107.…”