Alkaline Hydrolysis of Phosphonium Salts

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... 

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... 

Kinetic studies have shown that the alkaline hydrolysis of phosphonium salts (and phosphine oxides) is unaffected by the addition of the supernucleophilic hydroperoxide ion. ... 

The alkaline hydrolysis of phosphonium salts gives phosphine oxides and hydrocarbons resulting from the most stable carbanions in each case. O Benzyl tris(2-pyridyl)phosphonium bromide (17) undergoes rapid alkaline hydrolysis at room temperature to give good yields of benzylbis(2-pyridyl) phosphine oxide and pyridine. ... 

Although alkaline hydrolysis of phosphonium salts usually results in inversion by the process (13.64), the presence of bulky substituent groups can effect a retention of configuration (e.g. MePhBuP+Bz Br ). 

Inversion usually occurs in the alkaline hydrolysis of phosphonium salts when they contain a benzyl or allyl substituent group (13.132). Benzyl is much more apicophilic than the other substituent groups but when the apicophilicities of the leaving groups are more similar, varying degrees of inversion or retention can occur. 

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