Enantioselective silyl protection of alcohols promoted by a combination of chiral and achiral Lewis basic catalysts
作者:Nathan Manville、Hekla Alite、Fredrik Haeffner、Amir H. Hoveyda、Marc L. Snapper
DOI:10.1038/nchem.1708
日期:2013.9
Catalytic enantioselective monosilylations of diols and polyols furnish valuable alcohol-containing molecules in high enantiomeric purity. These transformations, however, require high catalyst loadings (20â30 mol%) and long reaction times (2â5 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 7.5â20 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. A counter-intuitive strategy that combines a chiral Lewis base catalyst with an achiral Lewis base co-catalyst results in an exceptionally large increase in the facility of catalytic enantioselective silylation of polyols. The catalytic ensemble drives such reactions to completion within a few hours, rather than the usual twoâfive days, without loss of enantioselectivity.
二元醇和多元醇的催化对映体选择性单硅烷化反应可以提供高对映体纯度的有价值含醇分子。然而,这些转化需要较高的催化剂负载量(20-30 摩尔%)和较长的反应时间(2-5 天)。在此,我们报告了一种反直觉策略,即使用结构上与手性催化剂相似的非手性辅助催化剂,从而有效地解决了这一问题。看似相互竞争的路易斯碱分子组合可以协同发挥作用,其中一个作为非手性亲核促进剂,另一个作为手性布氏碱。加入7.5â20Â mol%的市售N-杂环(5-乙基噻四唑)后,反应通常会在一小时内进行,并以较高的产率和对映体比率得到所需的产物。在某些情况下,如果没有非手性碱,则不会发生反应,但如果有非手性辅助催化剂存在,则很容易形成对映体纯度很高的产物。将手性路易斯碱催化剂与非手性路易斯碱助催化剂结合在一起的反直觉策略可显著提高多元醇的对映选择性硅烷化催化效率。这种催化组合可在几小时内完成此类反应,而不是通常的两天到五天,而且不会丧失对映选择性。