PALLADIUM-CATALYZED COUPLING REACTIONS IN SUPERHEATED WATER

A series of Heck palladium-coupling reactions was carried out in super heated (260 degrees C) and supercritical (400 degrees C) water to dete rmine 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 indi stinguishably 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 bas es: 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 mos t effective. A series of aromatic compounds were examined in the super heated water system, using styrene as the alkene. The arenes C6H5I, C6 H5Br, C6H5Cl, MesI, C6H5CHCHI, C6H5CHCHBr, and C(6)H(5)OTf underwent c oupling 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=CHCH2C l 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 phe nylpropanal, with small amounts of methylstyrene. The Heck couplings p roceeded to approximately the same degree at 400 degrees C (supercriti cal) as at 260 degrees C. However, increased hydrogenation and hydroge nolysis side reactions and lower overall recoveries were observed at t he higher temperature.