Mechanochemical Influence on the Stereoselectivity of Halide Metathesis: Synthesis of Group 15 Tris(allyl) Complexes

The stereochemical outcomes of reactions
conducted in solution
and those under mechanochemical conditions need not be the same; this
is a well-established observation in organic synthesis, but few examples
are known in organometallic systems. Halide metathesis is now shown
to be a type of mechanochemical reaction that can produce different
ratios of stereoisomers depending on whether the reagents are dissolved
or ball-milled. Trihalides of As (X = I), Sb (X = Cl), and Bi (X =
Cl) react with K­[A′] (A′ = 1,3-(SiMe3)2C3H3) (As, Sb) or [AlA′3] (Bi) to generate the tris­(allyl) complexes [EA′3]. All three complexes are found in two diastereomeric forms of C1 (R,S,S) and C3 (R,R,R) symmetry, and mechanochemical
synthesis increases the C1:C3 ratio relative to that produced in hexanes solution
(up to 3.3× in the case of [AsA′3]). The stereoselectivity
of the metathesis in the solid state can be correlated with the asymmetric
environment found in the group 15 trihalides; mechanochemical induction
provides a new tool for influencing this important class of synthetic
reactions.

溶液中进行的反应与机械化学（mechanochemical）条件下的反应，其立体化学结果未必一致；这是有机合成领域早已确立的观测结论，但在有机金属体系中此类案例仍较为罕见。如今研究证实，卤化物复分解（halide metathesis）属于一类机械化学反应（mechanochemical reaction），其立体异构体的生成比例会因试剂处于溶解状态还是经球磨（ball-milled）处理而有所不同。砷（X=I）、锑（X=Cl）与铋（X=Cl）的三卤化物，分别与K[A′]（A′=1,3-(SiMe3)2C3H3，适用于砷、锑）或[AlA′3]（适用于铋）反应，生成三(烯丙基)配合物[EA′3]。上述三种配合物均存在两种非对映异构体形式，分别为C1（R,S,S）对称型与C3（R,R,R）对称型；且相较于正己烷溶液中合成的产物，机械化学合成可提升C1:C3的比例，其中[AsA′3]的比例最高可达3.3倍。固态下复分解反应的立体选择性，可与第15族三卤化物所具备的不对称环境相关联；机械化学诱导为调控这类重要的合成反应提供了全新的工具。