Bench-Stable Stock Solutions of Silicon Grignard Reagents: Application to Iron- and Cobalt-Catalyzed Radical C(sp3 )-Si Cross-Coupling Reactions.

A robust method for the preparation of silicon-based magnesium reagents is reported. The MgBr2 used in the lithium-to-magnesium transmetalation step is generated in situ from 1,2-dibromoethane and elemental magnesium in hot THF. No precipitation of MgBr2 occurs in the heat, and transmetalation at elevated temperature leads to homogeneous stock solutions of the silicon Grignard reagents that are stable and storable in the fridge. This method avoids the preparation of silicon pronucleophiles such as Si-Si and Si-B reagents. The new Grignard reagents were applied to unprecedented iron- and cobalt-catalyzed cross-coupling reactions of unactivated alkyl bromides. The functional-group tolerance of these magnesium reagents is excellent.