P—C bond cleavage

Carbonylation reactions encompass a diverse set of transformations used to synthesise many important high-value fine chemicals, synthetic intermediates and materials such as polycarbonates [36]. Palladium catalysts modified with PRj ligands facilitate these reactions. However, carbonylation often requires harsh conditions, especially for less reactive C-X bonds, thereby promoting catalyst degradation via P-C bond cleavage. The strength of the NHC bond may demonstrate the utility of... 

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 ... 

Hence, P-C bond-cleavage followed by isomerization is responsible for the formation of side products. Furthermore, due to destabilization of the catalyst complex, deactivation occurs and palladium black is formed, which is a notorious disadvantage of Pd-phosphine catalysts in general. Catalyst decomposition and the formation of side products causes additional separation and catalyst recovery problems. These problems have been solved by the discovery of novel catalyst complexes, which are active and stable at temperatures of over 250 °C (Cornils and Herrmann, 1996). 

Reaction of hexafluorobenzene with MeIr(PEt3)3 results in C—F and P—C bond cleavage and P—F bond formation, according to reaction Scheme 28.504 An X-ray crystallographic study of (310) reveals a distorted square-planar arrangement of Ir. 

This result has been corroborated by numerous other examples43,593,597,601,608-614 and fits well with the classical mechanism described in Scheme 2. Only a few examples of second-order reactions are known they concern in particular the alkaline hydrolysis of phosphonium salts 29s 86,615 and 30616. In both cases, the second-order kinetics are very likely the result of a direct substitution induced by the very high stability of the carbanion resulting from the P—C bond cleavage. 

An unusual feature of this complex was that one of the phosphorus-naphthyl bonds in the BINAP ligand was ruptured. Based on the mechanisms elucidated for die P-C bond cleavage on Rh-phosphine complexes (14), we speculated a possible path for the formation of 3 via oxidative addition of the phosphorus-naphthyl bond to the Ru center (Figure 3). 

A less general route to metal phosphides, but one that is desirable due to the increased availability of tertiary phosphines is a reductive P—C bond cleavage... 

The thermolysis of Pt(PPh3)3 and related compounds gives di- and trinuclear phosphido complexes, evidently via oxidative P—C bond cleavage. 

A prolonged reaction time at room temperature is preferred to a short reaction time at reflux, since P—C bond cleavage by LiAIH4 under more forcing conditions has been observed. ... 

Detailed kinetic studies revealed an intrinsic deactivation process of the standard phosphine-palladium catalyst [50, 51] both the system Pd(OAc)2/P(C6H5)3 and isolated Pd[P(C6Hs)3]4, employed in the reference reaction of eq. (15), suffer from P-C-bond cleavage, the extent of which seems to increase with temperature. 

Scheme 1. Formation of side products via isomerization of arylpalladium(II) intermediates through P-C-bond cleavage (taken from Ref. [50, 51]).

It is P-C-bond cleavage and subsequent isomerizations that are responsible for the deactivation in the case of aryl chlorides and not a missing reactivity for oxidative addition as previously suggested Furthermore, the nature of the anion seems to dominate the subsequent steps in the catalytic cycle. Recently, these problems have been solved by the application of defined catalyst systems such as pallacycles. 

The ditertiary phosphine l,2-bis(diphenylphosphinomethyl)ethene (88) was conveniently prepared in 68% yield from Ph2PNa and 3-chloro-2-chloromethylprop-l-ene,190 whilst (89) (6(P) —23 ppm m.p. 137-138 °C) was synthesized from the bismesylate and Ph2PLi.191 The synthesis of the methoxy-functionalized ditertiary phosphine (90) has recently been described and involves P—C bond cleavage of P(o-OMeC6H4)3 with sodium in ammonia and quenching with 1,2-dichloroethane.192... 

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