Phosphonium salts, conversion

Kinetics are slow and many hours are requited for a 95% conversion of the reactants. In the case of the subject compound, there is evidence that the reaction is autocatalytic but only when approximately 30% conversion to the product has occurred (19). Reaction kinetics are heavily dependent on the species of halogen ia the alkyl haHde and decrease ia the order I >Br >C1. Tetrabutylphosphonium chloride exhibits a high solubiHty ia a variety of solvents, for example, >80% ia water, >70% ia 2-propanol, and >50% ia toluene at 25°C. Its analogues show similar properties. One of the latest appHcations for this phosphonium salt is the manufacture of readily dyeable polyester yams (20,21). 

Phase-tiansfei catalysis (PTC) is a technique by which leactions between substances located in diffeient phases aie biought about oi accelerated. Typically, one OI more of the reactants are organic Hquids or soHds dissolved in a nonpolar organic solvent and the coreactants are salts or alkah metal hydroxides in aqueous solution. Without a catalyst such reactions are often slow or do not occur at ah the phase-transfer catalyst, however, makes such conversions fast and efficient. Catalysts used most extensively are quaternary ammonium or phosphonium salts, and crown ethers and cryptates. Although isolated examples of PTC can be found in the early Hterature, it is only since the middle of the 1960s that the method has developed extensively. 

The formation of the heterocycle 1 from the xylylene-bis-phosphonium salt 2 and PCI3 proceeds via a detectable intermediate 3 in a cascade of condensation reactions that is terminated by spontaneous heterolysis of the last remaining P-Cl bond in a cyclic bis-ylide-substituted chlorophosphine formed (Scheme 1) [15]. The reaction scheme is applicable to an arsenic analogue of 1 [15] and to bis-phosphonio-benzophospholides with different triaryl-, aryl-alkyl- and aryl-vinyl-phosphonio groups [16, 18, 19], but failed for trialkylphosphonio-substituted cations here, insufficient acidity prohibited obviously quantitative deprotonation of the phosphonium salts, and only mixtures of products with unreacted starting materials were obtained [19]. The cations were isolated as chloride or bromide salts, but conversion of the anions by complexation with Lewis-acids or metathesis was easily feasible [16, 18, 19] and even salts with organometallic anions ([Co(CO)4] , [CpM(CO)3] (M=Mo, W) were accessible [20]. 

Cleavage reactions are best carried out in aqueous solution. In aprotic solvents, electrogenerated bases lead to the conversion of onium salts to the ylids which are not reducible [49]. The sequence of reactions shown in Scheme 5.2 shows that the bond cleavage process for phosphonium salts proceeds with retention of configuration around the phosphorus atom [50]. Retention of configuration at arsenic is also observed [51]. This electrochemical process is a route to asymmetric trisub-stituted phosphorus and arsenic centres. 

Conversion of phosphines via phosphonium salts into phosphonium ylides induces a deshielding of the a alkyl and aryl carbons (Table 4.49). Considerable shielding of the ylide sp2 — C = P carbon nucleus reflects the presence of a large electron density, thus indicating a significant contribution of the ylide dipole to the actual state of the molecule. 

An elegant strategy for the synthesis of fused cyclopentanoids has been reported by Marino and coworkers (Scheme 21).38 Reaction of (104) with the phosphonium salt (105) generated the bicyclic system (106). Further conversion of (106) to (107) enabled the annulation sequence to be repeated to form tri-quinane derivatives such as (108). 

A Wittig style polymerization, shown in Scheme 33, is the result of condensation of dialdehyde monomers with bis(phosphonium) salts containing aromatic cores, and was reported for the first time in 1960 [134]. Unfortunately, due to low reactivity and conversion, the Wittig polymerization typically only affords materials with a DP of 10. Despite its limitation to forming low molecular... 

Adopting an alternative convergent strategy, the two key halves of (446) were readily assembled as shown. Catalytic reduction of (219) with phenylhydrazine produced an intermediate phenylhydrazone which smoothly exchanged with /i-nitrobenzaldehyde to give (444). Conversion of /i-toluoyl chloride to the phosphonium salt (445) was straightforward. Wittigcondensation of (445) with (444) followed by catalytic reduction and saponification afforded (446). 

To delineate further the role of N-10 in the action and catalytic mechanism of TS, Samantham and Broom have also begun the synthesis of (686) in which N-10 has been replaced by a methylene group (Scheme 3.151) [279], In this approach, 6-bromomethyl-8-deazapterin (164) [280] was converted into the phosphonium salt (683) and then treated successively with sodium methoxide and (252) to produce (684a) and (684b) as an 81 19 E Z mixture, which was reduced uneventfully to (685). Alkylation of (685) with (331) and subsequent conversion to (686) has been reported to be in progress. 

Very likely the ammonium fluorides are the proton sources and therefore the reason for incomplete conversions, since potassium fluoride in acetonitrile gives high yields in a very elegant [3 + 2]-annuIation process 87). It combines a Michael addition to a vinyl phosphonium salt with an intramolecular Wittig reaction and proceeds only in the presence of 18-crown-6 with satisfying yield. This cyclopentene synthesis has been executed in a repetitive manner to prepare linear triquinanes as illustrated in Scheme 6. Unfortunately, the sequence is non-stereoselective with regard to the ethoxycarbonyl functions. 

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