Phosphorane intermediate

Carbon-phosphorus bond formation involving phosphorane intermediates... 

Strong nucleophilic character of dithizone could assist in the generation of such dipolar intermediates (422, 423, and 424), and there is plenty of general analogy for such intermediates. Particular reference may be made to the closely related cycloaddition reactions of meso-ionic 1,2-diazoles 373 and 382. The transformation 424 - 421 could well involve a pentacovalent phosphorane intermediate as well. 

There have been extensive studies on the reaction of dioxetanes and dioxetanones with nucleophiles (C, N, P, and S) and the reader is directed to CHEC-II(1996) for an exhaustive list up to 1996. The most common reactions of dioxetanes have been with phosphorus nucleophiles. The phosphines and phosphites first insert into the 0-0 bond. The phosphorane intermediate then collapses to afford phosphine oxide or the trialkylphosphate and the corresponding epoxide (Scheme 6). The same is true for the treatment of a-peroxy lactones with phosphines and phosphites except that the so-formed a-lactone intermediate undergoes decarboxylation and ultimately furnishes a ketone <1997JOC1623>. 

In the first report on this famous reaction, Wittig expressed a clear preference for the cyclic phosphorane intermediate 22 such as had already been formulated by Staudinger and Meyer in their earlier work. This view became generally accepted onlv many years later35). In this respect, the Wittig reaction did indeed still reflect some prominent features of the underlying research program on penta-coordinated molecules, from which it now evolved rapidly to a very active life of its own 36). 

This has meant, in general, an application of the BPR mechanism f to the isomerizations of the hypothetical phosphorane intermediates. 

These data and others suggest that permutational isomerizations via intermediates with hexacoordinated phosphorus is a distinct possibility in certain stable phosphoranes. Moreover, this mechanism must be considered when fleeting phosphorane intermediates are postulated in the hydrolysis of phosphate esters, in particular at high pH. 

Intramolecular hydrolysis by Co111-coordinated hydroxide in n s-[Co(0H) OP02(OC6H4NO2) (en)2] has also been demonstrated (Scheme 58a) and a phosphorane intermediate analogous to (163) is suggested. 16 The advantages of metal-coordinated hydroxide over alkoxide in facilitating such cyclizations and hydrolyses are discussed in this report. 

We also examined the reaction rate in different mixtures of D2O and H2O, and we saw that the proton transfers were simultaneous, not sequential (19). Remarkably, the detailed data were almost identical with those observed in the enzyme process, which raised the question of whether the enzyme also used a mechanism proceeding through a five-coordinate phosphorane intermediate, rather than going to the ring-opened product directly. We examined another biomimetic system to explore this question. 

We have proposed that the normal enzymatic hydrolysis process also proceeds through such a phosphorane intermediate, but isomerization does not occur because pseudo-rotation is not permitted in the enzyme (22). Complete agreement does not exist with our proposal (23). In this case, the biomimetic model system was not based only on nature s enzyme, it pointed to possible aspects of the natural enzyme that had not been considered previously. Thus we had learned something from nature, and we returned the favor by learning something that could give insight into nature itself. 

Figure 5 Imidazole buffer catalyzes both the hydrolysis of uridyl-uridine, a simple RNA fragment, and also its isomerization. This finding indicates that a phosphorane intermediate is involved, and it suggests that such a process may also be used by the enzyme ribonuclease itself.

Barton et al. have described a particularly mild method for the conversion of ketoximes into ketimines (aldoximes give nitriles instead). A mixture of tributylphosphine and diphenyl disulfide reacts under anhydrous conditions with ketoximes, probably by way of the phosphorane intermediates shown in Scheme 20, to give ketimines. The reagent is effectively a self-drying one since it reacts irreversibly with water. Chlorodiphenylphosphine is also capable of deoxygenating ketoximes with this reagent... 

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