Hydroboration as an Efficient Tool for the Preparation of Electronically and Structurally Diverse N→B-Heterocycles.

Ladder-type organoboranes featuring intramolecular N→B coordination have been prepared through hydroboration of a 2-(ortho-styryl)pyridine (PhPy) with a series of hydroboranes, including 9H-9-borabicyclo[3.3.1]nonyl (9H-BBN), BH3 ⋅THF, HBCl2 ⋅SMe2 , HB(C6 F5 )2 , and a 9H-9-borafluorene derivative. The hydroboration reaction results in highly regioselective borylation under mild conditions and gives the products in good to excellent yields. The molecular structure and electronic properties of the obtained boranes have been experimentally investigated in detail, and complemented with DFT calculations to further elucidate the origin of differences in optical and electronic properties. The electron affinity of the conjugated system can be controlled through variation of the borane, while the optical properties are likewise directly linked to the type and molecular structure of the substituents on boron. The broad substrate range shows that this preparative approach is widely applicable to introduce chemically diverse boryl groups into conjugated systems.

Generation of an N→B Ladder-type Structure by Regioselective Hydroboration of an Alkenyl-Functionalized Quaterpyridine.

An unusual reactivity of 2-(1-alkenyl)-pyridines towards hydroboration with 9H-borabicyclo[3.3.1]nonane (9H-BBN) has been employed to selectively introduce two borane groups into a conjugated quaterpyridine. Quantitative conversion of the substrate was observed with exclusive regioselectivity. A molecular structure that allows intramolecular N→B coordination was generated. The effect of the ladder formation on the molecular structure and the electronic properties of the conjugated system have been investigated. The synthetic strategy demonstrated herein offers a facile access to N→B ladder-type structures from readily available substrates, and allows to simultaneously introduce several boron centers under mild conditions.