Johnsson Richard, Olsson Dan, Ellervik Ulf
Organic Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden.
J Org Chem. 2008 Jul 18;73(14):5226-32. doi: 10.1021/jo800396g. Epub 2008 Apr 30.
The mechanisms of regioselective reductive openings of acetals were investigated in several model systems by a combination of Hammett plots, kinetic experiments, density functional calculations, and (11)B NMR. The regioselectivity of borane reductions of cyclic acetals can be controlled by the choice of borane. Lewis acid activation of BH3 x NMe3 increases the reaction rate and renders the borane the most electrophilic species, which associates to the more electron-rich oxygen of the acetal. In contrary, without activation, the regioselectivity is instead directed by the Lewis acid, as exemplified by the reaction with BH3 x THF.
通过哈米特图、动力学实验、密度泛函计算和硼-11核磁共振等方法，在多个模型体系中研究了缩醛区域选择性还原开环的机理。环缩醛硼氢化反应的区域选择性可通过硼烷的选择来控制。BH₃·NMe₃的路易斯酸活化提高了反应速率，使硼烷成为最亲电的物种，它与缩醛中电子云密度更高的氧原子结合。相反，在没有活化的情况下，区域选择性则由路易斯酸决定，如与BH₃·THF的反应所示。
