PALLADIUM-CATALYZED COUPLING REACTIONS IN SUPERHEATED WATER

A series of Heck palladium-coupling reactions was carried out in superheated (260 degrees C) and supercritical (400 degrees C) water to determine their viability in these media. PdCl2, Pd(OAc)(2), Pd(acac)(2), Pd(dba)(2), 30% Pd/C, (PPh(3))(2)PdCl2, and (dppe)PdCl2 performed indistinguishably as precatalysts for the coupling reaction of iodobenzene with styrene, therefore Pd(OAc)(2) was used for subsequent reactions. Removal of HX generated during the reaction was attempted with 10 bases: NEt(3), (Pr-i)(2)EtN, NaOAc, NaOH, NaHCO3, Na2CO3, NH4OAc, NH4OH, NH4HCO3, and(NH4)(2)CO3. Of these, the NH4HCO3 was found to be the most effective. A series of aromatic compounds were examined in the superheated water system, using styrene as the alkene. The arenes C6H5I, C6H5Br, C6H5Cl, MesI, C6H5CHCHI, C6H5CHCHBr, and C(6)H(5)OTf underwent coupling to styrene, while C(6)H(5)OTs, C6H6, p-CF(3)C(6)H(4)X (X = I, Br, Cl), and aniline derivatives did not. The alkenes C6H5CH=CH2, CH2=C(CH3)CO2CH3, CH2=C(CH3)CO2H, CH2=CHCH2OH, CH2=CHCH2Br, and CH2=CHCH2Cl coupled to iodobenzene, while C4H9CH=CH2, C6H5C(CH3)=CH2, and C3H7CH(OH)CH=CH2 did not. Styrene yielded predominantly stilbene, while the methacrylates yielded methylstyrene. The allyl substrates produced phenylpropanal, with small amounts of methylstyrene. The Heck couplings proceeded to approximately the same degree at 400 degrees C (supercritical) as at 260 degrees C. However, increased hydrogenation and hydrogenolysis side reactions and lower overall recoveries were observed at the higher temperature.