Stereoselective Ortho Borylation of Pyridylferrocenes

In an effort to develop new routes
to planar chiral Lewis acids
and Lewis pairs, the borylation of the pyridylferrocenes (pS)-1-stannyl-2-(3,5-dimethylpyrid-2-yl)­ferrocene and (pS)-1-mercurio-2-(3,5-dimethylpyrid-2-yl)­ferrocene with PhBCl2 was investigated. In both cases, clean conversion to 2-(chlorophenylboryl)-1-(3,5-dimethylpyrid-2-yl)­ferrocene,
a novel boron–nitrogen heterocycle, was observed. The structures
of the products were confirmed by multinuclear and 2D NOESY NMR, high-resolution
MS, and single-crystal X-ray diffraction analysis. While all NMR data
proved to be identical, the single-crystal X-ray structures revealed
retention of stereochemistry for the mercury precursor but inversion
of stereochemistry for the organotin precursor. The opposite chirality
of the products was further confirmed by chiral HPLC analyses and
optical rotation measurements. These results imply that the reaction
of PhBCl2 with 1-mercurio-2-(3,5-dimethylpyrid-2-yl)­ferrocene
proceeded as expected via an ipso-borodemercuration process, whereas
a stereoselective rearrangement occurred in the reaction with 1-stannyl-2-(3,5-dimethylpyrid-2-yl)­ferrocene.
A mechanism is proposed in which the Lewis basic pyridyl group in
the stannylferrocene precursor acts as an ortho-directing group that
facilitates highly selective attack of a borenium intermediate at
the adjacent C–H position. Subsequent proton migration and
elimination of Me3SnCl result in a single isomer of the
product with inverted stereochemistry. Reactivity studies were performed
with Brønsted and Lewis acids to support the proposed mechanism.