Unexpected Selectivity in Cyclotetrasiloxane Formation by the Hydrolytic Condensation Reaction of Trichloro(phenyl)silane

The hydrolytic condensation reaction of PhSiCl3 in aqueous solution forming cyclotetrasiloxane [C6H5SiO(OH)](4) was studied in detail. The reaction, originally reported by J. F. Brown, was believed to provide only one cyclotetrasiloxane diastereomer, cis,cis,cis-isomer 1. However, our detailed study clearly showed not only the formation of 1 (55.5% yield) but also the formation of two other diastereomers, the cis,cis,trans isomer (11.9% yield) and the cis,trans,cis isomer (4.3% yield), without the formation of the last conceivable trans,trans,trans isomer (<0.1% yield). A possible mechanism for the selective formation of the three diastereomers was proposed, in which it was assumed that the cyclotetrasiloxanes were formed by cyclization exclusively through the meso diastereomer of the linear tetrasiloxanes but not through the dl diastereomer. This selective cyclization was reasonably explained by considering the stable conformations of the linear tetrasiloxanes in a highly polar aqueous medium.