Phosphoranes conductivity

Monocyclic Phosphoranes. - Further studies on the mechanism and stereochemistry of the Wittig reaction have been conducted by a combination of 1H, 13C and 3 P n.m.r.2k 25. The results show that at -18°C both ois and trans diastereomeric oxaphosphetans (e.g. 17 and 18) may be observed and their decomposition to alkenes monitored by n.m.r. Evidence was presented to suggest that during this process oxaphosphetan equilibration involving the siphoning of (17) into (18) occurred in competition with alkene formation. 

Catalyzed Wittig reactions. Wittig reactions of cyclopropylidenetriphenyl-phosphorane with carbonyl compounds proceed in low yield under standard conditions. Reactions conducted at 62° in THF with 1 (10 mole %) as the phase-transfer catalyst result in alkylidenecyclopropanes in 60-95% yield. ... 

The carbonyl of trifluoroacetates is reactive enough to react with phosphoranes and to yield trifluoromethyl enol ethers (the reaction must be conducted without lithium salts i.e., the phosphorane must be generated without a nonlithiated base). In the same way, trifluoroacetamides and trifluorothioacetates afford, respectively, trifluoromethyl enamines and vinyl thioethers (Figure 2.44). " ... 

TABLE 4. Conductances of phosphoranes. Reproduced with permission from Reference 29. Copyright 1985, Pergamon Press PLC... 

Wittig methylenation4 Potassium f-butoxide has seen limited use for generation of stabilized ylides. Actually, it is a superior base for generation of meth-ylenetriphenylphosphorane. For methylenation of moderately hindered ketones, the phosphorane is generated and used in ether. For reactions of strongly hindered ketones, the ylide is generated in benzene and the reaction is conducted at 90-120° in benzene. 

The products of the reaction between alkyldiphenylphosphines and hexachloro-ethane in the presence of a tertiary amine are the ylide (88) and the a-chloro-alkylphosphine (89), which are interchangeable, constitutional isomers that interconvert via an intramolecular reversible 1,2-chlorine shift from phosphorus to carbon. Application of the triphenylphosphine-hexachloroethane reagent for the self-condensation polymerization of / -aminobenzoic acid has been studied in detail. Related reactions between dicarboxylic acids and diamines, conducted in the presence of pyridine, involve the cationic phosphorane (90) as a key intermediate. Replacement of hexachloroethane by hexabromoethane or... 

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