Polysilsesquioxane materials containing a dienic unit: unexpected reactivity of the organic unit
作者:Geneviève Cerveau、Robert J. P. Corriu、Bertrand Dabiens
DOI:10.1039/a909356g
日期:——
The hydrolytic sol–gel polymerisation of molecular precursors (RO)3Si-(CH2)n-(CHCH)2-(CH2)n-Si(OR)3, 1 (R = Et, n = 1), 2 (R = Et, n = 2), and 3 (R = iPr, n = 2) was investigated under various conditions. In the case of the xerogels obtained from 1, a partial cleavage of the allylic Si–C bond occurred during the hydrolytic process, whereas starting from 2 and 3 no Si–C breaking was observed. The storage conditions of the solids appeared to be of great importance on the stability of the various xerogels. Under inert conditions (sealed tubes under vacuum) all the xerogels were very stable. In contrast, under atmospheric humidity after several months the solids undergo a total chemical oxidation of the CC bonds leading to the cleavage of CC bonds with formation of acid and allenic functionalities. This oxidation is followed by a drastic decrease of the specific surface area. It has been shown that H2O vapour and oxygen are both necessary for oxidation. Each of these reagents alone does not lead to degradation. Gels obtained from 1 exhibited extensive cleavage of the allylic Si–C bond and an oxidative degradation of the organic fragment. The xerogels derived from 2 exhibited the same degradation of the organic unit, however only a few Si–C bonds were cleaved. It has been shown that this unexpected behaviour occurred only when the dienic moiety was included and bound to the silica network, suggesting a catalytic effect of the oxide matrix since the total oxidative cleavage of CC bonds in organic chemistry needs either powerful reagents like O3 or catalytic conditions.
研究了分子前体 (RO)3Si-(CH2)n-(CHCH)2-(CH2)n-Si(OR)3、1(R = Et,n = 1)、2(R = Et,n = 2)和 3(R = iPr,n = 2)在不同条件下的水解溶胶-凝胶聚合反应。由 1 生成的异凝胶在水解过程中出现了烯丙基 Si-C 键的部分断裂,而由 2 和 3 生成的异凝胶则没有出现 Si-C 键断裂。固体的储存条件似乎对各种异凝胶的稳定性有重要影响。在惰性条件下(真空密封管),所有的异凝胶都非常稳定。相反,在大气湿度下,经过几个月后,固体中的 CC 键会发生彻底的化学氧化,导致 CC 键裂解,形成酸和异戊烯官能团。这种氧化作用会导致比表面积急剧下降。研究表明,氧化需要 H2O 蒸汽和氧气。单独使用这两种试剂不会导致降解。由 1 生成的凝胶显示出烯丙基 Si-C 键的广泛裂解和有机片段的氧化降解。由 2 制得的异凝胶也表现出有机单元的降解,但只有少数 Si-C 键被裂解。研究表明,只有当二烯基被包含并结合到二氧化硅网络中时,才会出现这种意想不到的行为,这表明氧化物基质具有催化作用,因为有机化学中 CC 键的完全氧化裂解需要强力试剂(如 O3)或催化条件。