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 in situ1H NMR spectroscopic investigation of the relative rates of
cleavage of model substrates containing two different Si–O
substructures, namely alkoxydisiloxanes [GRO–Si(Me2)–O–SiMe3] and carbodisiloxanes [GR–Si(Me2)–O–SiMe3]. 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., Me2NR–Si(Me2)–O–SiMe3 + HOSiEt3 ⇋
Me2NR–Si(Me2)–O–SiEt3 + HOSiMe3] facilitated by the pendant dimethylamino
group, a process that was also probed by computation.