SN P mechanism

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

SCHEME 2. Mechanism of SN(P) reaction for the alkaline hydrolysis of phosphonium salts 

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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. 

In another example710, the phosphonium salt 76 decomposes competitively by the two mechanisms of substitution and elimination (reaction 211). This salt behaves in a manner intermediate between salts 74 and 75 indeed, the SN(P) mechanism is not completely excluded as for the salt 74 but the EHfi mechanism is still favoured with regard to the salt 75 since the alkene formed is stabilized. 

See the lUPAC rules Guthrie, R.D. Pure Appl. Chem., 1989, 61, 23. As given here, the lUPAC designations for BacI and BalI are the same, but Rule A.2 adds further symbols so that they can be distinguished Su-AL for BalI and Su-AC for BacI- por reviews of the mechanisms of ester hydrolysis and formation, see Kirby, A.J. in Bamford Tipper, Ref 193, vol. 10, 1972, p. 57 Euranto, E.K. in Patai, Ref 216, p. 505. This is an Sn 1 mechanism with OR as leaving group, which does not happen. 

In certain cases, the SrnI mechanism has been found (p. 856). When the substrate is a heterocyclic aromatic nitrogen compound, still a different mechanism [the Sn(ANRORC) mechanism], involving opening and reclosing of the aromatic ring, has been shown to take place. 

It is evident from the foregoing sections that simple alkylvinyl halides do not react via an Sn 1 mechanism, if at all, even under extreme solvolytic conditions (146,149). More reactive leaving groups, such as arylsulfonates, were clearly needed to investigate the possible solvolytic behavior of simple alkylvinyl systems, but the preparation of vinyl sulfonates until recently was unknown. Peterson and Indelicato (154) were the first to report the preparation of vinyl arylsulfonates via reaction of the appropriate disulfonate with potassium t-butoxide in refluxing f-butanol. They prepared and investigated the solvolysis of 1-cyclohexenyl tosylate 169 and c/s-2-buten-2-yl tosylate 170 and the corresponding p-bromobenzenesulfonates (brosylates). Reaction... 

The solvent acts as a kinetically significant nucleophile in the overall solvolysis process for many simple secondary substrates, and this appears to be the major cause of the variation in relative rates with changes in solvent (Table 2, p. 11). This conclusion is supported by the quantitative correlations discussed in Section 6. The stereochemical evidence further suggests that, even when the magnitude of nucleophilic solvent assistance is less than a rate factor of 10 at 25°, solvolyses (e.g. of cylcohexyl tosylate in formic acid) can proceed with essentially complete inversion of configuration. These results are consistent with an SN 2 mechanism and the evidence for ion pair intermediates can then be considered in one of two ways. 

Many radical cations derived from cyclopropane (or cyclobutane) systems undergo bond formation with nucleophiles, typically neutralizing the positive charge and generating addition products via free-radical intermediates [140, 147). In one sense, these reactions are akin to the well known nucleophilic capture of carbocations, which is the second step of nucleophilic substitution via an Sn 1 mechanism. The capture of cyclopropane radical cations has the special feature that an sp -hybridized carbon center serves as an (intramolecular) leaving group, which changes the reaction, in essence, to a second-order substitution. Whereas the SnI reaction involves two electrons and an empty p-orbital and the Sn2 reaction occurs with redistribution of four electrons, the related radical cation reaction involves three electrons. 

This seemingly small isotope effect must be compared to the isotope effect measured for the hydrolysis of the dibenzyl ester precursor to the isotopically labeled monoester [phosphate triesters can only hydrolyze by an Sn2(P) mechanism], 1.0070 -I- 0.0038, for proper interpretation. The fact that the hydrolysis of the monoester has a larger isotope effect was interpreted as demonstrating that substantial cleavage of the phosphate ester bond occurs in the transition state this is in accord with the expectation based on an Sn 1(P) mechanism. 

A study of some phosphineplatinum complexes by 2D exchange NMR spectroscopy has shown it to be a useful technique in detecting multiple exchange processes and in elucidating reaction mechanisms/ Where isotopomers are possible (e.g, " Pt- P, Pt- Por "" " Sn- P, " Sn-" P) the method differentiated between intermolecular and intramolecular processes. For example, the 2D exchange P NMR spectrum of complex (1) revealed exchange between P and... 

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