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.