D5h [PhSiO1.5]10 synthesis via F- catalyzed rearrangement of [PhSiO1.5]n. An experimental/computational analysis of likely reaction pathways

We describe here the synthesis and analysis of the reaction pathways leading to formation of the rare D-5h decaphenylsilsesquioxane (SQ) [PhSiO1.5](10) via F- catalyzed rearrangement of [PhSiO1.5](n) n = 8, 12, and oligomers initially synthesized from PhSi(OEt)(3). Isolated yields of similar to 50% [PhSiO1.5](10) are obtained via rearrangement of all starting materials. The recovered starting materials can be re-equilibrated using catalytic F- to generate similar yields in second batches. These yields arise because [PhSiO1.5](10) exhibits higher solubility and better energy stabilization (10 kcal mol(-1) theory) in CH2Cl2 compared to [PhSiO1.5](8) or [PhSiO1.5](12). Reaction intermediates were identified using time dependent F-19 NMR and MALDI-ToF mass spectrometry eventually equilibrating to form the 8 : 10 : 12 cages in a 1 : 3 : 1.3 equilibrium in CH2Cl2. Experimental results coupled with modeling using the Gamess computational package provide multiple reasonable pathways for SQ rearrangements to [RSiO1.5](10), starting from [RSiO1.5](8). Heats of reaction for interconversion of the model intermediates [HSiO1.5](x) determined computationally, were used to select the most reasonable reaction pathways. The findings support a mechanism involving activation and cleavage of a T-8 cage corner by F- attachment, followed by the corners stepwise removal as [i. e. RSi(OH)(3)], followed thereafter by reinsertion forming [RSiO1.5](9)-OH followed by, insertion of another corner to form [RSiO1.5](10)-(OH)(2) and finally condensation to give [RSiO1.5](10). The most enthalpically favorable path (-24 kcal mol(-1)) involves a hybrid mechanism.