Triaryl 2- phosphonium salt

This is an extremely useful reaction for the synthesis of alkenes. It involves the addition of a phosphonium ylid, e.g. (136), also known as a phosphorane, to the carbonyl group of an aldehyde or ketone the ylid is indeed a carbanion having an adjacent hetero atom. Such species are generated by the reaction of an alkyl halide, RR CHX (137), on a trialkyl- or triaryl-phosphine (138)—very often Ph3P—to yield a phosphonium salt (139), followed by abstraction of a proton from it by a very strong base, e.g. PhLi ... 

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]. 

Trialkyl- and triaryl-phosphines react with 1,3-benzodithiolylium ions to give a phosphonium salt which is deprotonated by n-butyllithium to give (282) (76TL3695). 

The reaction of trialkyl- and triaryl-phosphines with 1,3-benzodithiolylium salts leads to formation of phosphonium salts which are deprotonated by treatment with n-butyllithium to produce (69). The similar reaction of trialkyl phosphites in the presence of sodium iodide yields dialkyl phosphonates which can be deprotonated to (70). Both (69) and (70) can react further with ketones to give the 2-alkylidene-l,3-dithiole derivatives (71) (80AHC(27)151>. The ylide (72) has also been prepared (80H(l4)27l>. 

Very few reactions of the compounds in this chapter with phosphorus nucleophiles have been reported. The coupling reactions of 1,3-dichalcogenole-2-selenones and -2-thiones with triaryl- and trialkyl-phosphines and with trialkyl phosphites were considered in Section 4.35.5.1.2. 1,3-Thiaselenolylium ions (19b) and (70) react with triphenylphosphine to give phosphonium salts (71) and (58), respectively (80H(14)27l). 

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