Silicon-, Germanium- and Tin-Oxygen Rings

There is an interesting analogy between cyclosiloxanes and the cyclic metasilicates [Si 03 ] n = 3-6), e.g. (Me2SiO)3 (10.42) and [SisOg] (10.43) the methyl groups in the dimethylsiloxanes are replaced by the formally iso-electronic in the silicates. Cyclic metasilicates occur naturally in certain minerals. For example, the trimer (n = 3) is found in CasSisOg (a-wollastonite), whereas the hexamer (n = 6) is a constituent of Be3Al2Si60ig (beryl). 

A wide range of p-block elements and transition metals have been incorporated into silicon-oxygen ring systems (heterocyclosiloxanes), primarily with a view to their use as precursors to Si-O polymers incorporating another element. The most common synthetic approaches to six-membered heterocyclosiloxanes containing another p-block element involve cyclocondensation reactions between 1,3-dichloro- or 1,3-dihydroxytetraalkyl/aryldisiloxane 

The six-membered Sn303 ring in the three derivatives (R2SnO)3 (R = Bu 

6-Et2C6H3 ) is essentially planar. These tin-oxygen heterocycles are prepared by hydrolysis of the corresponding diorganotin dihalides, which, in the case of the aryl derivative, is formed by the reaction of (2,4-pentane-dionato)tin dichloride with the aryl Grignard reagent. 

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