Stereoselective Ortho Borylation of Pyridylferrocenes

Abstract

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.