Zwitterionic allenolate

As shown in Scheme 1, aliphatic phosphines such as P(n-Bu)3 catalyze the addition of alcohols (2) to methyl propiolate (3) [35]. The mechanism is believed to involve an initial addition of the phosphine to the C = C moiety to give a zwitterionic allenolate (I), which then deprotonates the alcohol, yielding a vinyl phosphonium salt (II). An alkoxide addition to give an enolate (III), followed by phosphine elimination gives the product 4 and regenerates the catalyst. Several experiments suggest that when alcohols are used in excess, the catalyst rests as the original phosphine [34]. 

Garcia-Tellado and his coworkers have reported an organocatalyzed 1,3-dipolar cycloaddition of alkynoate 231 and nitrones 230 in the presence of triphenylphos-phane, quinine or quinuclidine on water [91], affording 2,3-dihydroxazoles 232, Scheme 3.74 [92], Initial addition of the organocatalyst (e.g., quinine, quinuclidine or Ph P) to alkynoate 231 generated the zwitterionic allenolate. Regioselective... 

Phosphine-catalysed annulation between aldehydes (RCHO) and ethyl allenolate (H2C=C=CHC02Et) gives 6-substituted 2-pyrones (52), proceeding via a zwitterionic (g) enolate.201 The product is derived from the -intermediate, which is favoured by the use of sterically demanding trialkylphosphines, such as tri(cyclopentyl). However, overdoing the phosphine bulk with, for example, the tri(r-butyl) derivative gives no yield. 

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