Division of Chemistry and Biological Chemistry, Nanyang Technological University , 21 Nanyang Link, Singapore 637371.
Division of Science and Math, Singapore University of Technology and Design , 8 Somapah Road, Singapore 487372.
J Am Chem Soc. 2018 Feb 7;140(5):1952-1955. doi: 10.1021/jacs.7b13056. Epub 2018 Jan 27.
Highly enantioselective bisguanidinium-catalyzed tandem rearrangements of acylsilanes are reported. The acylsilanes were activated via an addition of fluoride on the silicon to form a penta-coordinate anionic silicate intermediate. The silicate then underwent alkyl or aryl group migration from the silicon atom to the neighboring carbonyl carbon atom (1,2-anionotropic rearrangement), followed by [1,2]-Brook rearrangement to provide the secondary alcohols in high yields with excellent enantioselectivities (up to 95% ee). The isolation of an α-silylcarbinol intermediate as well as DFT calculations revealed that the 1,2-anionotropic rearrangement occurred via a bisguanidinium silicate ion pair, which is the stereodetermining step. The chiral center formed is then retained without inversion through the subsequent [1,2]-Brook rearrangement. Crotyl acylsilanes were smoothly transformed into homoallylic linear crotyl alcohols with retention of E/Z geometry, and no branched alcohols were detected. This clearly suggested that the 1,2-anionotropic rearrangement occurred through a three-membered instead of a five-membered transition state.
报道了一种高对映选择性的双胍催化酰基硅烷的串联重排反应。酰基硅烷通过在硅原子上加成氟来活化，形成五配位阴离子硅酸盐中间体。然后，硅原子上的硅烷基或芳基迁移到相邻的羰基碳原子上（1,2-阴离子重排），接着发生[1,2]-Brook 重排，以高收率和优异的对映选择性（高达 95%ee）得到仲醇。α-硅基醇中间体的分离和 DFT 计算表明，1,2-阴离子重排是通过双胍硅酸盐离子对进行的，这是立体决定步骤。随后的[1,2]-Brook 重排过程中，形成的手性中心未经反转保留下来。烯丙基酰基硅烷可顺利转化为保留 E/Z 几何构型的同型烯丙基线性烯丙基醇，而没有检测到支链醇。这清楚地表明，1,2-阴离子重排是通过三元过渡态而不是五元过渡态进行的。
