Acyclic Organotin Enolates

Organotin enolates (vinyloxystannanes) (14-18), containing an Sn-0 bond, are in metal-lotropic equilibrium with the corresponding Sn-C bonded a-stannylcarbonyl compounds (14-19), providing a further example of the equivalent behaviour of hydrogen (prototropy) and metals (metallotropy) (Section 3.1.1.7). 

The position of the equilibrium has been shown by IR, and H 37 101 and 119Sn NMR spectroscopy102 to depend sensitively on the nature of the substituents, particularly their size, bulky groups favouring the less sterically congested enol form. Some typical results are shown in Table 14-6. 

The reversible dependence of the keto/enol composition on temperature (e.g. equation 14-52) establishes that the two forms are in equilibrium. This structural mobility renders the reactions independent of the proportion of enol and keto form, and it is convenient to treat the compounds here together with the alkoxides, irrespective of the precise composition of the isomeric mixture. 

In the presence of a good ligand (e.g. HMPT, BU3PO, or BiuNBr), the 5-coordinate enolate complex is formed the equilibrium is shifted in favour of the enolate (equation 14-53)103 and the enolate nucleophilicity is enhanced. 

A number of methods are now available for preparing the tin enolates, which are often not isolated, but are generated and caused to react further in situ. Tributyltin methoxide reacts exothermically with enol acetates without a solvent, to give the corresponding stannyl enolate, and methyl acetate which can be removed under reduced pressure or by slow distillation (b.p. 57 °C) (e.g. equation I4-54).37-104-105 

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