Phosphonium salts substituent effect

Other Onium Salt Catalysts. We have examined several quaternary ammonium and phosphonium salts in this process. And while they vary in their effectiveness, most seem to have at least some activity. In the case of symmetrical onium salts, substituents having four or more carbon atoms are preferred (Table III). This likely originates from a more favorable partitioning into the organic phase as the size of the substitutents increase. Herriott has reported that there is a... 

The reported 2 e reductive cleavage of phenylsulphonylacetonitrile (PhSO CH CN) and the observation that in protic media the products were PhSO and CH CN, suggested that this reaction could be a useful source of CHjCN. However, careful re-examination showed that in acetonitrile solution the reaction is pseudo one-electron, analogous to the phosphonium and sulphonium salts (Scheme 8), and that phenylsulphonylacetonitrile is sufficiently acidic rapidly to protonate "CHjCN assuming additivity of substituent effects an estimate of pK 14-16 was made for PhSOjCHjCN, cf. pK 31 for CH,CN... 

Most of the pKa values for the phosphonium salts have been determined by potentio-metric measurements in the case of / -ketophosphonium salts562 (Table 13). The acidity of these phosphonium salts depends not only on the R and R groups of the carbonyl chain, but also on the other substituents linked to the phosphorus562 564. Particularly for the cyclic phosphonium salts, the pKa value can vary significantly depending on the different steric and electronic (even transannular) effects on the phosphorus atom564 ... 

With bulky substituents attached to the phosphorus, none of these special measures are necessary because complex formation is precluded through steric effects (74). Therefore ethylidene triethylphosphorane may be obtained in high yield without complications from the tetraethyl-phosphonium salt and an alkyllithium reagent (74) ... 

The reaction of ylides with saturated aliphatic alkyl halides (like methyl iodide, ethyl iodide etc.) usually stops at the stage of the alkylated salt because the +/ effect of the aliphatic substituent causes the resulting salt to be a weaker acid than the conjugated salt of the original ylide (which would result in the course of a transylidation reaction). However since partial transylidation also occurs between al-kylidenephosphoranes and phosphonium salts with equal or not very different base and acid strength, mixtures may result from Ae reaction with saturated aliphatic alkyl halides. At this point it should be mentioned that the synthesis of dialkylated ylides via the salt method is also difficult since the preparation of the necessary phosphonium salt is accompanied by -elimination. The successful synthesis of dialkylated ylides may be achieved by fluoride ion induced desilylation of a-trimethylsilylphosphonium salts (see equation 18). There is no doubt about the course of ylide alkylation in cases where the inductive effect of the new substituent leads to complete transylidation (e.g. equation 54). ... 

Salt, solvent, and substituent effects in the alkaline hydrolysis (and alkoxide-promoted decomposition) of phosphonium compounds have been reviewed. The rate of alkaline hydrolysis of tetraphenylphosphonium chloride in DMSO-water mixtures increases by as much as 10 -fold as the DMSO content is increased, due to desolvation of the reactant ions. ... 

The alternative direction ((b) on page 124) is suitable for 1,4-diaryl butadienes (28). These were needed with a variety of substituents for a systematic study of electronic effects on the Diels-Alder reaction. Disconnection to give benzyl phosphonium salts (30) and easily made cinnamaldehydes (29) (see Chapter 18) is best. 

Preparation. - A range of 1,3-dithianylphosphonium salts (218) has been prepared in the course of further studies of sulfur lone pair anomeric effects in these systems.Conventional quatemization reactions have been used in the synthesis of the salt (219) and a range of polymer-supported phosphonium salts (220). A new efficient route to salts of the type (221) has been developed. The of>-azolylalkylphosphonium salts (222) are readily accessible from the reactions of the corresponding a>-bromoalkylphosphonium salts and azoles. Routes to vinylphosphonium salts, e.g., (223), continue to be explored, and their reactivity utilised in the synthesis of phosphonium salts bearing heterocyclic substituents, e.g., (224). The betaine (225) has been... 

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 ). 

The often superior properties of diphenylphosphine oxides, compared with the corresponding phosphonates and phosphonium salts, has led to the development of AT-morpholinomethyldiphenylphosphine oxide (42), a new formyl anion equivalent. This new reagent converts aromatic and aliphatic aldehydes into their homologous enamines by a Horner-Wittig reaction (Scheme 44). To effect this reaction satisfactorily with ketones a less strongly basic amino substituent is necessary (e.g. AT-methylaniline). 

A closely related solvent effect can be achieved by use of phase transfer catalysis. These catalysts are salts in which one of the ions has large nonpolar substituent groups which confer good solubility in nonpolar solvents. The most familiar examples are quaternary ammonium and quaternary phosphonium salts with several long-chain alkyl substituents. In a two-phase solvent system consisting of water and a nonpolar organic solvent containing a salt, there will be virtually no ions present... 

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