Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, USA.
Nat Chem. 2013 Sep;5(9):768-74. doi: 10.1038/nchem.1708. Epub 2013 Jul 28.
Catalytic enantioselective monosilylations of diols and polyols furnish valuable alcohol-containing molecules in high enantiomeric purity. These transformations, however, require high catalyst loadings (20-30 mol%) and long reaction times (2-5 days). Here, we report that a counterintuitive strategy involving the use of an achiral co-catalyst structurally similar to the chiral catalyst provides an effective solution to this problem. A combination of seemingly competitive Lewis basic molecules can function in concert such that one serves as an achiral nucleophilic promoter and the other performs as a chiral Brønsted base. On the addition of 7.5-20 mol% of a commercially available N-heterocycle (5-ethylthiotetrazole), reactions typically proceed within one hour, and deliver the desired products in high yields and enantiomeric ratios. In some instances, there is no reaction in the absence of the achiral base, yet the presence of the achiral co-catalyst gives rise to facile formation of products in high enantiomeric purity.
手性催化剂对二醇和多醇的催化对映选择性单硅烷化反应可以高对映体纯度提供有价值的含醇分子。然而，这些转化需要高催化剂负载量（20-30mol%）和长反应时间（2-5 天）。在这里，我们报告了一种反直觉的策略，涉及使用与手性催化剂结构相似的非手性共催化剂，为解决这一问题提供了有效的解决方案。似乎竞争性的路易斯碱分子的组合可以协同作用，使得一种分子作为非手性亲核促进剂，另一种分子作为手性布朗斯台德碱。在添加 7.5-20mol%的商业可得的 N-杂环（5-乙基噻唑四唑）时，反应通常在一小时内进行，并以高产率和对映体比得到所需产物。在某些情况下，如果没有非手性碱，就没有反应，但非手性共催化剂的存在会导致产物以高对映体纯度轻松形成。
