Fluoride-Promoted Aryl and Allyl Migration from Boron to Tin in 1-Stannyl-2-borylferrocenes

Transmetalation reactions where organotin reagents are used for the transfer of organic groups to boron halides are among the most selective and hence synthetically useful methods for the preparation of organoboranes. We describe here a rare example of the reverse reaction, where migration of the organic group from boron to tin is promoted by addition of fluoride. The bidentate Lewis acids Fc(BMeR)(SnMe2Cl) (Fc = 1,2-ferrocenediyl; R = phenyl (Ph), thienyl (Th), allyl (All)) react with KF at 45 °C to give the rearranged fluoroboranes Fc(BMeF)(SnMe2R) as oily liquids. With an excess of KF, the fluoroborate complexes K[Fc(BMeF2)(SnMe2R)] are obtained, which are readily isolated as light yellow solids in good to high yields. For the phenyl derivative it was demonstrated that the borate salt can be converted back to the fluoroborane by treatment with Me3SiOTf or addition of pyridine, which gives the adduct Fc(BMeF)(SnMe2Ph)·Py as a crystalline solid. The fluoride and pyridine complexes have been characterized by multinuclear NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction (for R = Ph). Preliminary mechanistic studies suggest an intramolecular process and indicate that Lewis acid induced Sn−F bond activation may play an important role.