Phosphonium salts hydrolysis

A one-pot synthesis of alkyl perfluoroalkyl ketones has been developed. Phosphoranes, generated in situ, are acylated with a perfluoroacyl anhydnde, and the resultmg phosphonium salts are hydrolyzed with alkali [4S (equation 48) Hydrolysis of a carbon-sulfur bond in 2-chloro-2,4,4-trifluoro-1,3-dithietane-S-trioxide, which can be obtained from 2,2,4,4-tetrachloro-l,3-dithietane by fluor-mation with antimony trifluoride followed by selective oxidations, opens the nng to produce 2-chloro-1,1,2-trifluorodimethyl sulfone [49] (equation 49)... 

A. Alkaline Hydrolysis. -The low kinetic isotope effect observed in the protonation of carbanions formed in phosphonium salt hydrolysis leads to the idea that there is little breaking of the phosphorus-carbon bond and correspondingly little transfer of a proton to the incipient carbanion in the transition state (87) of the rate-determining step. ... 

Steric crowding of intermediates, preventing groups from attaining correct conformations for maximum stabilization of departing anions, may be responsible for the observation that, in the alkaline hydrolysis of sterically crowded phosphonium salts, the group lost need not be that which is most stable as the anion. 

The rates of hydrolysis of a variety of cyclic and acyclic phosphonium salts do not correlate with their P chemical shifts. ... 

Trippett and Stewart have shown that the phosphonium salts (11) derived from the reaction of phenyl di-t-butylphosphinite (12) with alkyl halides are highly resistant to hydrolysis and they suggest that this is due to the reluctance of phosphorus to accommodate two t-butyl groups in a trigonal-bipyramidal intermediate. 

Phosphoniosilylotion. This combination reacts with acyclic or cyclic enones to give phosphonium salts, formed by addition of P(C6H,)3 to the p-position of the enone and silylation of the carbonyl group. The products can be converted into p-substituted enones by deprotonation (BuLi), a Wittig reaction, and hydrolysis. 

Reactions.—Alkaline Hydrolysis. The first total resolution of a heterocyclic phosphonium salt containing an asymmetric phosphorus atom (128) has been reported, providing ready access to optically active phospholan derivatives of value for studies of the stereochemistry of nucleophilic displacement at phosphorus.124 Alkaline hydrolysis of (128) proceeds with retention of configuration at phosphorus to form the oxide (129). Stereochemical studies in the phospholan series have also been facilitated by the X-ray investigation125 of an isomer of l-iodomethyl-l-phenyl-3-methylphospholanium iodide, which is shown to have the structure (130). 

The mechanism of hydrolysis of benzylidenetriphenylphosphorane is similar to that of the corresponding phosphonium salt.52 It is proposed that the low polarities of the solutions in which ylides are usually hydrolysed increase the equilibrium... 

In a study which is relevant to the mechanism of hydrolysis of phosphonium salts, Glaser and Streitwieser297 studied the ions H4PO- and H3PFO- and their derivatives with Li +, NH4 and HF at the 6-31G level augmented by diffuse functions. They found that the structures of the anions are those of a hydride or fluoride ion solvated by or complexed with phosphine oxide, rather than phosphoranes297. A very important point is that earlier studies with diffuse functions yielded the pentacoordinated phosphoranes which they judged297 to be computational artifacts of the small basis set. 

Alkaline hydrolysis of phosphonium salts is one of the oldest575 and of the best investigated reactions in organophosphorus chemistry and several excellent reviews are... 

The different evolution possibilities for the alkaline hydrolysis of phosphonium salts are shown together in the general Scheme 1. Two major parts can be distinguished on the one hand, SN(P), SN(P)mig and Ep reactions, which result from the initial attack on the phosphorus atom by hydroxide anion acting as a nucleophile and on the other, EH(X and EHp reactions (and also bearing in mind the formation of phosphonium ylides), which result from the initial attack of the hydroxide anion on the hydrogen in the a- or / -position to the phosphorus. 

SCHEME 1. Different mechanisms in the alkaline hydrolysis of phosphonium salts... 

Main reaction in the hydrolysis of phosphonium salts SN(P) mechanism... 

The mechanism of the hydrolysis of phosphonium salts was formulated by McEwen and coworkers583 584, after an initial proposal as early as 1929 by Fenton and Ingold585 (Scheme 2). This SN(P) mechanism accounts for most results, even though in some cases584 the results can also be explained by variations of the classical mechanism. 

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. 

TABLE 15. Stereochemistry of alkaline hydrolysis of phosphonium salts with a good leaving group... 

Preparation, properties and reactions of phosphonium salts TABLE 19. Stereochemistry of the alkaline hydrolysis of cyclic phosphoniums salts... 

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