Phosphine palladium© halide dimers

The essential factor which differentiates the monomeric and dimeric carbonylations seems to be the presence or absence of halide ion coordinated to the palladium. The dimerization-carbonylation proceeds satisfactorily with halide-free palladium phosphine complexes. Most conveniently, Pd(OAc)2 is used with PPh3. PdCl2(PPh3)2 can be used as a catalyst with addition of an excess of bases. The reaction is carried out at 1I0°C under 50 atm of carbon monoxide pressure in alcohol. Higher... 

The carbonylation was explained by the following mechanism. Formation of dimeric 7r-allylic complex 20 from two moles of butadiene and the halide-free palladium species is followed by carbon monoxide insertion at the allylic position to give an acyl palladium complex which then collapses to give 3,8-nonadienoate by the attack of alcohol with regeneration of the zero-valent palladium phosphine complex. When halide ion is coordinated to palladium, the formation of the above dimeric 7r-allylic complex 20 is not possible, and only monomeric 7r-allylic complex 74 is formed. Carbon monoxide insertion then gives 3-pentenoate (72). 

Vilar and coworkers reported the isolation of two new dinuclear Pd(I) dimers containing P(Bu-f)2Bph (Bph = biphenyl) (equation 48) as ligand168. The complexes were prepared by the same conproportionation used to prepare the palladium dimers with trialkylphosphines. The complexes contain a single phosphine, and both metals are coordinated to the terminal phenyl group of the biphenyl unit in a p2 — rf -iy fashion. These complexes were tested for the animation of aryl halides with arylamines. The reaction of 4-chlorotoluene with p-tolylamine catalyzed by 52a or 52b formed di-p-tolylamine in 78 to 81% yield within 3 h at room temperature, whereas the same reaction conducted with a mixture of Pd2(dba)3 and P(Bu-f)2Bph required 80 °C to obtain 90% yield. 

Palladium-phosphine complexes catalyse the carbonylation of butadiene with carbon monoxide and alcohols, when nona-3,8-dienoic acid esters are formed when the catalyst has halide co-ordinated to palladium, dimerization is suppressed, and pent-3-enoic acid esters are obtained (Scheme 33). 

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