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.邻近的锚定官能团（G，G为叔酰胺或胺类）的存在，可显著影响Si-O键的断裂速率。本文报道了一项基于原位¹H核磁共振波谱的研究，旨在考察两类含不同Si-O亚结构的模型底物的Si-O键断裂相对速率，这两类底物分别为烷氧基二硅氧烷[GRO–Si(Me₂)–O–SiMe₃]与碳基二硅氧烷[GR–Si(Me₂)–O–SiMe₃]。我们通过计算手段探究了烷氧基二硅氧烷水解反应的相对速率变化趋势（该趋势随链长增加而减慢，仅存在一处显著例外），计算结果与实验数据吻合度良好。与烷氧基二硅氧烷的水解反应不同，碳基二硅氧烷并未发生完全水解，而是生成了由初始不对称二硅氧烷、两种硅醇及一种新型对称二硅氧烷组成的平衡混合物。我们还发现了一种由侧挂二甲氨基基团促进的、进攻性硅醇与碳基二硅氧烷底物间的易进行硅氧基复分解反应[例如：Me₂NR–Si(Me₂)–O–SiMe₃ + HOSiEt₃ ⇌ Me₂NR–Si(Me₂)–O–SiEt₃ + HOSiMe₃]，该过程同样通过计算手段进行了探究。