Aqueous conditions homocoupling

The homocoupling of aryl halide to diaryl compounds, known as Ull-mann coupling, is a synthetically useful reaction and has wide applications in material research. Such couplings have been studied in aqueous conditions. In 1970, arylsulfinic acids were coupled with Pd(II) in aqueous solvents to biaryls (Eq. 6.25).53 However, the reaction required the use of a stoichiometric amount of palladium. In the presence of hydrogen gas, aryl halides homocoupled to give biaryl compounds in moderate yields (30-50%) in an aqueous/organic microemulsion (Eq. 6.26).54... 

Though the hydrogenation of the double bond is one of the basic catalytic reactions in Pd chemistry, httle has been published on aqueous variants of this process. Hydrogenation of o ,j8-unsaturated aldehydes in recyclable biphasic systems in the presence of TPPTS complex of palladium has been studied. - Also, a biphasic hydrogenation of nitrocompounds in the presence of PdCla and TPPTS under mild conditions has been reported. Room temperature homocoupling of aryl iodides was achieved in neat water or water-acetone mixture in the presence of Pd/C or Pd(OAc)2, and Zn dust (Scheme 87). The addition of 18-crown-6 was required for the reaction in neat water. The reactions are run under aerobic conditions. The effect of phosphine is negative. - ... 

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