Scheme 24. Alcoholysis of diisopropylsilane using Rh2 OAc 4 as catalyst

Following our success in finding a suitable silane and catalyst for the first step in the synthesis of unsymmetrical bis-alkoxy si lanes, we set out to evaluate catalysts to achieve the second alcoholysis. We were aware from prior reports in the literature that there were a variety of catalysts capable of performing alcoholysis of silanes.10 17 From prior evaluation of catalysts in the first step, we knew that 10 % Pd/C, Rh2(pfb)4, and CuH all catalyze the reaction of the second step with diethylsilane and Rh2(pfb)4 catalyzes the reaction of the second step (10 %) with diphenylsilane (Table 4). Rli2(OAc)4 was ruled out by our discovery that only one addition of alcohol occurred with diisopropylsilane. 

The mono-alkoxy products from the first addition of both diisopropylsilane and diphenylsilane with (+)-ethyl lactate was evaluated with [PPhjCuHje, Rh2(pfb)4 and 10 % Pd/C. When [PPh3CuH]6 was used as catalyst with the mono-alkoxy product from (+)-ethyl lactate and diisopropylsilane (le), a 30 % yield (isolated by flash chromatography) of the desired silyl ketal product was obtained. The yields rose to up to 70 % for the reaction of the mono-alkoxy diisopropylsilane using [Rh2(pfb)4] after 14 h at room temperature. 

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