Aryl group exchange

Scheme8.16. Aryl group exchange between phosphines and palladium[ll, 131].

Rapid exchange of the aryl groups was observed spectroscopically in both benzene and diethyl ether. The addition of NNN N -tetraethylethylenediamine (TEED) considerably reduces the rate of aryl group exchange. This observation was attributed to strong coordination of TEED to the monomeric complexes, thereby inhibiting formation of the associated intermediate 8 [1]. The donicity of TEED exceeds 55 kcal mol [19]. 

An equilibrium reaction involving aryl group exchange, as depicted in Scheme 9, is responsible for the product pattern, and donor-substituted derivatives underwent particularly facile aryl-aryl exchange. 

The phosphine ligands suffer from P-C-bond cleavage, which result in the corporation of the phosphine aryl groups into an unwanted side product. This is due to the facile exchange of Ph and Ph Y between the Pd" centres and co-ordinated phosphines on an intermediates of type tranj-Pd(PPh,i)2(aryl)X ... 

Arylphosphines in rhodium catalyzed hydroformylation reactions exchange an aryl group for an alkyl, principally linear alkyl, corresponding to the alkene being hydro-formylated to give an alkyldiarylphosphine [22](see Equation 2.5). 

More recently a variation of this mechanism was reported by Novak [37], The mechanism involves nucleophilic attack at co-ordinated phosphines and it explains the exchange of aryl groups at the phosphine centres with the intermediacy of metal aryl moieties. After the nucleophilic attack the phosphine may dissociate from the metal as a phosphonium salt. To obtain a catalytic cycle the phosphonium salt adds oxidatively to the zerovalent palladium complex (Figure 2.38). Note where the electrons go . 

Anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxides (4) and anhydro-5-thiolo-l,2,3,4-oxatri-azolium hydroxides (5) do not act as 1,3-dipoles <79JCS(P1)732>. However, anhydro-5-phenylamino-1,2,3,4-oxatriazolium hydroxides (16) react with aryl isocyanates with exchange of the exocyclic aryl group <79JCS(P1)736>. This process can be explained in terms of a 1,3-dipolar cycloaddition to give (18) which then eliminates phenyl isocyanate producing (19) (Scheme 3). 

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