Grinding and the anisotropic environment: influences on the diastereoselective formation of Group 15 allyl complexes†

The heavy Group 15 allyls (E = As, Sb, Bi; [A′] = [1,3-(SiMe₃)₂C₃H₃]⁻) can be prepared either in solution or mechanochemically, and exist in two diastereomeric forms of C₁ and C₃ symmetry. For E = As and Sb, their ratio varies with the method of preparation: the C₁ diastereomer is the major form by both methods, but the mechanochemical route increases the C₁ : C₃ ratio compared to synthesis in hexanes solution. The difference in selectivity has previously been identified as a consequence of the layered crystal lattices of the EX₃ reagents, which provide a templating effect through an anisotropic grinding environment. How this selectivity changes with other typical mechanochemical variables is explored here, including the use of different reagents and LAG solvents, pre-grinding the reagents, the use of different milling media (stainless steel, Teflon, etc.) and apparatus (mixer mill, planetary mill), and the number and size of balls. The extent to which the anisotropic environment is either maintained or modified during synthesis (especially by LAG and the choice of metal reagent) affects the diastereomeric ratio.