Phosphines aryl group 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 ... 

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

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 . 

The C—P bond cleavage processes responsible for the observed interchange has been described in the literature. Novak and coworkers reported on the reversible formation of tetraarylphosphonium ion, via a reductive elimination pathway, as a possible key intermediate in the exchange of aryl groups on the central palladium with aryl groups on the phosphine moiety (Scheme 4.4) [5]. A subsequent oxidative addition of a different C-P bond in the phosphonium ion to the intermediary palladium(O) complex would then create the interchanged arylpaUadi-um(II) complex also observed by Chernard and coworkers [6]. A similar C-P bond... 

Eijectron spin resonance spectra alkyls and aryls, 7, 173 anion radicals from acylsilanes, 7, 138 metal nitrosyls, 7, 234 phosphine methylenes, 9, 298 Elimination reactions, 7, 166-167 Exchange reactions ferrocenes, 10, 80, 91 organo-Group II compounds, 8, 170-176... 

Intramolecular addition to a carbonyl group (to afford 1-indanols and 1-tetralols) through activation of an o-bromoarene by Pd(OAc)2-phosphine is chemoselectively superior to that relying on Br-Li exchange. However, different reaction patterns are revealed in the following, " in which a-arylation of ketones must be involved. 

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