PREPARATION OF PHOSPHONIUM SALTS

Phosphonium salts are produced in a variety of ways, and they generally show a much wider range of reactions than the corresponding tetra-aUcyl ammonium salts R4N+X. This is probably due to the larger size of the P atom compared to N, and the participation of d orbitals in the bonding to phosphorus. 

Alkyl and aryl phosphonium halides can be made by quatemisation of tertiary phosphines with the appropriate halogen or organic halide under anhydrous conditions (6.38,6.360-6.363,6.84,6.87). 

PPh3 + Br2 PhjPBr+Br-PPh3 + Mel — PhjPMe+C PPhj -H Br(CH2) Br Ph3F(CH2) Br Bi-PCI3 + RK + AICI3 RPCI5AICI4 

Tertiary phosphines are more readily quaternised than the corresponding amines (e.g. PhPEt2 reacts with EtI some 500 times faster than does PhNEt2). Although reaction (6.361) proceeds easily, a corresponding reaction with PhjN has not been observed. 

Tertiary phosphines react with acyl halides (6.364), unsaturated aldehydes (6.365) and esters (6.98-6.100), and with carbon tetrachloride (6.92) to produce phosphonium salts. Primary and secondary phosphines can be sources of phosphonium salts (6.93,6.94) as can tertiary phosphines. 

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Preparation of phosphonium salts as ionic liquids [PiBu3Et][tosylate] Gytec Technology Gorp., USA 2001 24... 

Nonstabilized Ylides. These ylides are devoid of electron-v ithdrawing substituents at the anionic center they carry hydrogen or electron-donating alkyl groups on the ylide carbon and phenyl groups on the phosphorus. Nonstabilized ylides are very nucleophilic and react with CO2, O2, and H2O hence they must be handled in an inert atmosphere. Primary as well as secondary alkyl halides may be used for the preparation of phosphonium salts and subsequently ylides. 

Phosphonium Salts The preparation of phosphonium salts is based on the reaction of chloromethylated or bromomethylated aryl compounds with the corresponding phosphines [32-36]. As illustrated in Chart 11.7, phosphonium salts with absorptions acceptable for direct photolysis have been synthesized. 

Interest continues in the preparation of phosphonium salts which involve unusual anions. The tetraphenylphosphonium arenethiolate (142) and the selenium analogue have been prepared by metathesis, and shown to be ionic in the solid state and also in... 

Hilarius, V Heider, U. Schmidt, M. Ionic liquids. Fur. Pat. Appl. 1160249, 2001. Robertson, A. J. Preparation of phosphonium salts as ionic liquids. PCT Int. Appl. 0187900, 2001. 

Quaternization is the most basic method for the preparation of phosphonium salts. Alkyl-tris(2-pyridyl)phosphonium salts were synthesized from tris(2-pyridyl)phosphine and alkyl chlorides. The phosphonium salts obtained from butyl bromide and benzyl chloride were found to decompose rapidly in hot acetone to 2,2 -bipyridinium bromide and (2-Py)Bn(C(0H)Me2)P(0), respectively (Scheme 51) ... 

The quaternisation of phosphines with electrophiles or Bronsted acids is doubtlessly the most typical and simple reaction for the preparation of phosphonium salts. The preparation of most phosphonium salts reported in the period surveyed by this review followed this approach. The structures of these compounds are summarised in Fig. 1. 

Quaternisation of the corresponding phosphine by reaction with an electrophile or a Bronsted acid is the most typical and simple procedure for the preparation of phosphonium salts. Besides this classic approach, new interesting synthetic pathways have been described in the period... 

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