Chemoselective Tin–Boron Exchange Aided by the Use of Dummy Ligands at the Tin Atom

C­(sp2)–Sn bonds
generally react more quickly
than C­(sp3)–Sn bonds in tin–boron exchange
processes, and discrimination between (hetero)­aryl groups to be transferred
from tin to boron and those to remain at the tin atom (usually methyl
groups) is reliable. Conversely, transmetalation of benzylic carbon–tin
bonds is disfavored, and the chemoselectivity against other C­(sp3)–Sn bonds at the tin atom is poor. That inherent problem
is overcome by the installation of one or more Me3SiCH2 (TMSM) groups at the tin atom as nontransferable substituents.
With these groups, the tin–boron metathesis of benzylic C­(sp3)–Sn bonds occurs cleanly and rapidly. The efficiency
of the new method is demonstrated in the demanding synthesis of C6F5-substituted dihydroborepines with binaphthyl
backbones.