Reaction of an Enantiomerically Pure Phosphaalkene-Oxazoline with MeM Nucleophiles (M = Li and MgBr): Stereoselectivity and Noninnocence of the P‑Mesityl Substituent

The addition of alkyl
nucleophiles (MeM, M = Li, MgBr) across the
PC bond of an enantiomerically pure phosphaalkene-oxazoline
followed by protonation of the C anion affords phosphines with three
chirality centers. The formation of palladium­(II) complexes of the
resultant phosphines permitted structural characterization of the
products by X-ray diffraction. The choice of nucleophile has a profound
effect on the product distributions. For instance, the Grignard reagent
adds in a diastereoselective manner to give one major phosphine product
with P- and C-stereocenters. In contrast, addition of methyllithium
has proven not only to be less stereoselective but also affords a
fascinating cyclic phosphine product. Both the Grignard and RLi reactions
involve proton transfer from the o-Me of the P-Mes
substituent even though the products are quite different in each case.

将烷基亲核试剂（alkyl nucleophiles，MeM，其中M=Li、MgBr）加成至对映纯磷烯-恶唑啉（phosphaalkene-oxazoline）的P=C双键上，随后对生成的碳负离子进行质子化，即可得到含有三个手性中心的膦类化合物。将所得膦类产物制备为钯(II)配合物（palladium(II) complexes）后，可借助X射线衍射（X-ray diffraction）技术完成产物的结构表征。亲核试剂的选择对产物分布具有显著影响：例如，格氏试剂（Grignard reagent）以非对映选择性方式加成，主要生成一种同时带有磷和碳立体中心的膦类产物；与之相反，甲基锂（methyllithium）的加成不仅立体选择性更弱，还会得到一种极具特色的环状膦类产物。尽管两类反应的产物差异显著，但格氏试剂反应与烷基锂（RLi）反应均涉及从P-均三甲苯基（P-Mes）取代基的邻位甲基上发生的质子转移过程。