Neighboring Group Effects on the Rates of Cleavage of Si-O-Si-Containing Compounds.

The presence of a nearby tethered functional group (G, G = tertiary amide or amine) can significantly impact the rate of cleavage of an Si-O bond. We report here an H NMR spectroscopic investigation of the relative rates of cleavage of model substrates containing two different Si-O substructures, namely alkoxydisiloxanes [GRO-Si(Me)-O-SiMe] and carbodisiloxanes [GR-Si(Me)-O-SiMe]. The trends in the relative rates (which slowed with increasing chain length, with a notable exception) of alkoxydisiloxane hydrolyses were probed via computation. The results correlated well with the experimental data. In contrast to the hydrolysis of the alkoxydisiloxanes, the carbodisiloxanes were not fully hydrolyzed, but rather formed an equilibrium mixture of starting asymmetric disiloxane, two silanols, and a new symmetrical disiloxane. We also uncovered a facile siloxy-metathesis reaction of an incoming silanol with the carbodisiloxane substrate [e.g., MeNR-Si(Me)-O-SiMe + HOSiEt ⇋ MeNR-Si(Me)-O-SiEt + HOSiMe] facilitated by the pendant dimethylamino group, a process that was also probed by computation.