Diorganopalladium complexes

The postulated steps that constitute the Suzuki coupling process are shown in Scheme 25. After oxidative addition of the organic halide to the palladium(o) catalyst, it is presumed that a metathetical displacement of the halide substituent in the palladium(ii) complex A by ethoxide ion (or hydroxide ion) takes place to give an alkoxo-palladium(ff) complex B. The latter complex then reacts with the alkenylborane, generating the diorganopalladium complex C. Finally, reductive elimination of C furnishes the cross-coupling product (D) and regenerates the palladium(o) catalyst. 

Owing to the geometrical requirement for reductive elimination, trans-MR(R )L2 complexes must be isomerized to their cis isomers prior to reductive elimination. The trans to cis isomerization of MX2L2 type complexes bearing halide ligands (X) is known to proceed by either stepwise ligand displacement or Berry s pseudo rotation of a hve-coordinate intermediate (Scheme 9.11) [37], On the other hand, diorganopalladium complexes have been shown to isomerize via a transmetallation process [38,39],... 

Shown in a simplified form in Figure 10.5, the mechanism of the Suzuki-Miyaura reaction involves initial reaction of the aromatic halide with the palladium catalyst to form an organopalladium intermediate, followed by reaction of that intermediate with the aromatic boronic acid. The resultant diorganopalladium complex then decomposes to the coupled biaryl product plus regenerated catalyst. 

The syntheses of trans and cis isomers of unsymmetrical diorganopalladium(II) complexes have been reported [104]. For example, the reaction of arylpalladium(II) halides, which have been prepared either by the oxidative addition of aryl halides to palladium(O) phosphine complexes or by the treatment of diarylpalladium(II) with HCl, with 1 equivalent of methyllithium provides /ra x-PdAr(Me)L2 complexes (eq (75)). On the other hand, treatment of arylpalladium(II) halides with excess methyllithium followed by methanolysis of the resulting dimethyl(aryl)palladate intermediates leads to c/.v-PdAr(Me)L2 complexes (eq (76)). 

It seems that fast C—C bond formation occurs via a concerted mechanism from the cw-diorganopalladium(II) complex whereas the C—O and C—bond formations are more sluggish and require an extra reagent, which may be the nncleophile present in large excess in the catalytic reaction. 

Diorganopalladium(iv) dichalcogenides have been synthesized by oxidative addition of (PhE)2 (E = S, Se) to [PdR2(bpy)] and similar compounds, providing 169-171. The sulfur compounds were unstable and were observed by NMR. Complex 171 was in equilibrium with [PdMe(C6H40Me)(bpy)] and (SeC6H4Cl)2 it is the major compound at low temperatures and can be isolated. An X-ray structure of 170 (E = Se) has been obtained. 

Under stoichiometric conditions, the postulated diorganopalladium(II) c -ArPdNu(L-L) complexes are formed with hard nucleophiles Grignard reagents, alkoxides, - and amides. cw-ArPdNu(L-L) complexes in which the nucleophile is a carbanion (L-L = dppp, dppf) have been characterized in situ by P NMR spectroscopy performed at low temperatures but have not been isolated due to fast reductive elimination. The stability of cis-ArPdNu(L-L) (L-L = BINAP, Tol-BINAP, dppf) with alkoxides as nucleophiles strongly depends on the ligand and the aryl group for a given alkoxide. - Complexes in which the nucleophile is an amide (with L-L = dppf) are more stable and have been isolated. 

The coordination and subsequent assimilation of the nucleophilic coupling partner to the discrete oxidative addition complex generating a new diorganopalladium(ll) species represent the next step in the catalytic cycle. Depending on the type of the catalytic process and the incoming nucleophile two major pathways are typically considered transmetallation or carbopalladation. In the following a short presentation and description of the two distinct reactions will be given. 

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