摘要：

IntroductionPhosphorus-stabilised carbanions are very important reactive intermediates in carbon–carbon bond formation reactions.1The excellent regio- and stereoselectivity achieved in the addition of electrophiles to, for example, metalated phosphine calchogenides,2phosphonates 3and phosphoramides 4has led to a large variety of synthetic applications. Lithium phosphazenes are isosteric to the corresponding phosphine oxides. However, their use in organic synthesis has been much less exploited.5The delocalisation of the negative charge of the CLi bond through the PN linkage suggests the possibility of acting as bidentate nucleophiles. This chemical behaviour can be correlated with the ability of phosphazenes to participate in silylotropic 6and prototropic 7rearrangements. These last processes are phosphazene–aminoylide tautomer equilibria, which can be influenced by the substituents on the α-carbon and on the nitrogen of the P&z.dbd6;N group. Thus, in the case of diisopropoxyphosphazenes (Scheme 1),8the aminoylide tautomer is the unique species detected when X = Y = CO2Me and R = Ph. Changing one CO2Me for a phenyl group displaces the equilibrium to the phosphazene tautomer.In the last few years crystal structures of α-lithium phosphazenes have been reported showing in all cases a common structural characteristic: the formation of a four-membered ring by coordination of the lithium cation to the carbon and nitrogen atoms of the phosphazene moiety.9Even though, all the known reactions of lithium phosphazenes with electrophiles afford exclusively the products arising from theC-regioselective attack of the anion.10In an attempt to explore the bidentate behaviour of metalated phosphazenes we described in a previous paper,11the formation of cyclopentenones by reaction of the salt of lithiumP,P-diphenyl-P-(alkyl)(N-phenyl)phosphazenes with DMAD or dimethyl maleate (Scheme 2). The mechanism we proposed was based on atandemreaction with the key steps being the addition of the metalated carbon α to the PN to one carbonyl group of the DMAD and an intramolecular Michael-type addition through the nitrogen of the P&z.dbd6;N group (see below).This was the first example of a lithiated phosphazene acting as a bidentate nucleophile. The reaction mechanism proposed involved the formation of several intermediate compounds and an olefination reaction that represented the participation for the first time of a phosphazene in a process similar to the Wittig and Horner carbon–carbon double bond synthesis. Here we give full details of the reaction of lithiumP,P-diphenyl-P-(alkyl)(N-phenyl)phosphazenes with DMAD. In order to obtain some insights to support the reaction mechanism previously proposed and to extend the scope of the reaction we will include the results obtained with other activated double and triple bonds related to DMAD. The electrophiles selected for the mechanistic studies were methyl phenylpropiolate, methyl cinnamate and dimethyl oxalate, affording in all cases the correspondingC-addition products. On the other hand, the use of methyl benzoylpropiolate as electrophile yielded unusual β-aminobutenolides with a quaternary γ-carbon atom in reasonable yields and high stereoselectivity. The formation of these new compounds is explained again in terms of the bidentate behaviour of metalated phosphazenes.

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