Stannyl ethers coordination

The chemical shifts that one sees in the H Sn NMR spectrum indicate that these are not simple stannyl ethers (that is, four coordinate Sn) as the chemical shifts observed indicate at least hexacoordinate Sn. Moreover, we do not know... 

Diketones. The diketene-tin oxide adduct is a useful Michael donor that reacts with enones to give 1,5-diketones. Note that simple stannyl enol ethers do not undergo the same reaction thus the coordination of the 0-stannyl group by the ester carbonyl must be important. 

In the presence of a nucleophile like a bromide ion, stannyl enol ethers are converted into anionic pentacoordinate stannates, which nucleophilically react with organic halides to give or-alkylated carbonyl compounds. In contrast, such penta-coordinated tin species hardly react with carbonyl electrophiles. The higher coordinated species is also generated by treatment of tin(IV) enol ethers with not only Bu4NBr but an aprotic polar solvent like Such pentacoordinate stannates add in a similar... 

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