Carbon dioxide pressure induced heterogeneous and homogeneous Heck and Sonogashira coupling reactions using fluorinated palladium complex catalysts

The Heck reaction of iodobenzene and methyl acrylate was investigated with CO2-philic Pd complex catalysts having fluorous ponytails and the organic base triethylamine (Et3N) in the presence of CO2 under solventless conditions at 80 °C. The catalysts are not soluble in the organic phase in the absence of CO2 and the reaction occurs in a solid–liquid biphasic system. When the organic liquid mixture is pressurized by CO2, CO2 is dissolved into the organic phase and this promotes the dissolution of the Pd complex catalysts. As a result, the Heck reaction occurs homogeneously in the organic phase, which enhances the rate of reaction. This positive effect of CO2 pressurization competes with the negative effect that the reacting species are diluted by an increasing amount of CO2 molecules dissolved. Thus, the maximum conversion appears at a CO2 pressure of around 4 MPa under the present reaction conditions. The catalysts are separated in the solid granules by depressurization and are recyclable without loss of activity after washing with n-hexane and/or water. When the washing is made with hexane alone, the catalytic activity tends to increase on the repeated Heck reactions, probably due to the accumulation of such a base adduct as Et3NHI on the catalysts. When the washing is further made with water, however, the base adduct is taken off from the catalysts and they show similar activity levels in the repeated runs. The potential of CO2 pressure tunable heterogeneous/homogeneous reaction system has also been investigated for Sonogashira reactions of iodobenzene and phenylacetylene under similar conditions.

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