Phosphonium salts base hydrolysis

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. 

Samaan used reduction at a mercury cathode to convert the phosphonium salt 33 to the phosphine (34) (79PS89). This gave the opposite stereoisomer from that available as the major product (35) from base hydrolysis followed by silane reduction. The stereochemistry of each compound was established by NMR analysis. 

Some Cyclic Phosphonium Salts and Their Relation to the Stereochemical Course of Base Hydrolysis. 

In the base hydrolysis of the unsymmetrical fluoromethylene bisphosphonium salt (35) the regioselectivity of C—bond cleavage can be accounted for by the susceptibility of the phosphonium center to nucleophilic attack by hydroxide ion and not by the relative stability of the newly formed fluoromethylene ylide (Scheme 40). ... 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

Tertiary phosphines, in the absence of special effects 2 ), have relatively high barriers 8) ca. 30-35 kcal/mol) to pyramidal inversion, and may therefore be prepared in otically stable form. Methods for synthesis of optically active phosphines include cathodic reduction or base-catalyzed hydrolysis 3° 31) of optically active phosphonium salts, reduction of optically active phosphine oxides with silane hydrides 32), and kinetic 3 0 or direct 33) resolution. The ready availability of optically pure phosphine oxides of known absolute configuration by the Grignard method (see Sect. 2.1) led to a study 3 ) of a convenient, general, and stereospecific method for their reduction, thus providing a combined methodology for preparation of phosphines of known chirality and of high enantiomeric purity. 

A variation of the Wittig reaction that can overcome problems with the stereochemical outcome is the Homer-Wittig reaction with phosphine oxides. The oxides are obtained by quatemization of triphenylphosphine and hydrolysis of the phosphonium salt, or by reaction of hthiodiphenylphosphide with an alkyl halide or sulfonate and oxidation of the resulting phosphine with hydrogen peroxide. The derived hthio species react with aldehydes or ketones to give p-hydroxy phosphine oxides, which ehminate on treatment with a base such as sodium hydride or potassium hydroxide to form the alkene. In common with the Homer-Wadsworth-Emmons reaction, the phosphorus by-product is water soluble and easily removed from the product. 

For the synthesis of lutein (133), the Wittig olefination of 67 with the Ci5-phosphonium salt 68 was carried out in two phases in the solvent mixture dichloromethane/isopropanol with 50% aqueous KOH as base. Under these reaction conditions, the acetoxy protecting group remained intact. After hydrolysis with aqueous methanolic KOH solution, lutein (133) was obtained in 25% yield [85] (Scheme 18). 

The initial attack of PR3 on the disulphide bond is the rate-limiting step. Hydrolysis of the phosphonium salt is rapid and irreversible. The rate of reaction is enhanced by either dilute acid or dilute base at high pH hydrolysis of the phosphoium salt is rapid but at low pH the thiolate anion becomes protonated, thus suppressing the reverse reaction in the rate-limiting step. 

Mixed Aqueous Solvents Miscellaneous.—Solvolysis. A dissociative mechanism operates for the solvolyses of [CoCl(NH3)5] + in aqueous ethanol and in aqueous DMSO and of cw-[CoCl(OH)(en)2]+ in a wide range of mixed solvents. Kinetics of aquation of [CrI(OH2)6l in aqueous methanol and in aqueous DMSO can be interpreted as indicating a D or, less convincingly, an /a mechanism. Now the establishment of A V values indicates the h mechanism, though a D mechanism for the conjugate base [CrI(OH)(OH2) J+. Some kinetic observations have been made relating to platinblau, the intensely coloured product from [PtCy - in aqueous acetonitrile. Rates of hydrolysis of phosphonium ylides and salts in THF-water mixtures are very sensitive to solvent composition. ... 

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