Palladium complexes with sulfur dioxide

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

Palladium alkoxide complexes are thought to be formed in the reactions of alcohols catalyzed by palladium(II) chloride. These reactions include the oxidation of alcohols, yielding acetals or ketones,137,138 and their carbonylation, yielding esters.139 Alkoxide intermediates are also thought to be involved in the reaction of sulfur dioxide with [PdCl2] suspended in alcohol (equation 15).140,141... 

Balch ei al. (306) have given a detailed report on novel A-frame-type methylene bridged palladium complexes of composition Pd2(dpm)2( p-CHR)X2 (X = I, Br, Cl R = H, CH3) and [Pd2(dpm)2(joi-CH2)L2]2+ (L = pyridine, methylisocyanide). It was demonstrated that the Pd—C (methylene) bond resists insertion of carbon monoxide, isocyanides, or sulfur dioxide. Protonation of Pd2(dpm)2(/x-CH2)I2 with fluoroboric acid yields the compound [Pd2(dpm)2(ju.-I)(CH3)I]BF4 in which the bridging methylene group of the precursor has been converted into a terminal methyl group, a process that has also been encountered in a previous example (52). 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

Sodium arylsulfinates and palladium salts react to form arylpaUadium salts and sulfur dioxide. When carried out in the presence of some dienes, stable organopalladium complexes have been obtained. For example, 1,5-cyclooctadiene and sodium p-toluenesulfinate with palladium chloride yields the a,it-complex shown in equation (48).109... 

In some cases the insertion reactions are reversible. This is the case with the palladium complexes for carbon monoxide and sulfur dioxide. With acetylenes, preliminary studies indicate that the insertion products can be made to eliminate the acetylene on photolysis. The remaining A-frames shown in Figure 5.16 are formed irreversibly. 

The reaction of sulfur dioxide with ethylene, catalyzed by palladium complexes, affords rra s-2-butenyl ethyl sulphone [equation (4.180)]. 

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