to afford, after 2-4 h, the corresponding olefins in 89-99% yield. In each case the trans isomer prevails, cis/trans ratios ranging from 33:67 to 4:96. The remarkable cis-stereoselectivity of Wittig reactions using triphenylphosphonium alkylides is now attributed to a very specific and almost rigid orientation of the Ph groups around the P atom, which sterically hinders approach to the aldehyde in
在 25 度的 THF 中,Et3P:CHMe 与
庚醛、Me3CCHO、PhCHO 和 p-ClC6H4CHO 反应,在 2-4 小时后以 89-99% 的产率得到相应的烯烃。在每种情况下,反式异构体占优势,顺式/反式比例范围为 33:67 至 4:96。使用三苯基
鏻烷基化物的 Wittig 反应的显着顺式立体选择性现在归因于 P 原子周围 Ph 基团的非常特殊且几乎刚性的取向,这在空间上阻碍了接近醛的方式,从而导致形成反式-取代的氧代膦烷。[在 SciFinder (R) 上]