Phosphonium salts alkaline hydrolysis

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. 

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). 

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... 

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... 

In the alkaline hydrolysis of phosphonium salts, the action of the hydroxide ion can sometimes be paralleled705 to each of the other nucleophiles, but it can also be different as is shown by the decomposition of the salt 73 (equation 207). We shall mention only... 

The alkaline hydrolysis of phosphonium salts has been particularly studied from a theoretical point of view as a model for nucleophilic substitution reactions on tetracoor-dinated phosphorus. However, it also has interesting applications, since it corresponds... 

Another important use of the susceptibility of phosphonium salts to undergo alkaline hydrolysis concerns their use as liquid-liquid phase-transfer catalysts. Phosphonium salts can be decomposed much more easily than ammonium salts, under alkaline conditions726-729, and they have to be used under much milder conditions [i.e. low temperature (< 25 °C) and moderate aqueous base concentration (< 15%)] in all cases reaction conditions should be used which prevent the extraction of OH - into the organic phase or minimize its reactivity730. 

Further examples in which pentacoordinate phosphorus intermediates are implicated are the reactions of phosphonium salts. The cyclic phosphonium salt (161) shows a rate enhancement of 1300 1 in alkaline hydrolysis relative to either its six-membered or acyclic analogues (65ACS931). Ring strain relief in going from (163) to (164) seems to account for the higher rate of formation of the favoured intermediate (163 see Scheme 19). 

We have simulated the steric course of the alkaline hydrolysis of chiral five- (12) and six- (13) membered cyclic phosphonium salts, whose reaction kinetics and product stereochemistries had been studied previously by Marsi and coworkers (14,15). For this purpose, we determined the absolute configuration of the phospholan-ium iodide (12), and the x-ray structures of the related phos-thorinanium salts, 4 and (13). 

The procedures to break the C—bond are rather limited. The main methods are alkaline hydrolysis and LAH reduction of phosphonium salts. Electrochemical reductions can also be used for this purpose. 

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