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Chiral Organoborane Lewis Pairs Derived from Pyridylferrocene
Author(s) -
Chen Jiawei,
Lalancette Roger A.,
Jäkle Frieder
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201400057
Subject(s) - ferrocene , chemistry , borane , steric effects , borylation , boranes , pyridine , metathesis , nuclear magnetic resonance spectroscopy , lewis acids and bases , planar chirality , medicinal chemistry , crystallography , stereochemistry , boron , polymerization , organic chemistry , catalysis , enantioselective synthesis , aryl , alkyl , polymer , electrode , electrochemistry
Abstract In an effort to develop a new class of redox‐active chiral Lewis pairs, pyridine and borane moieties with different steric and electronic properties were introduced onto a planar chiral 1,2‐disubstituted ferrocene framework. Metathesis of lithiated, stannylated, or mercuriated pyridylferrocenes with boron halides afforded (p R )‐2‐[bis(pentafluorophenyl)boryl]‐1‐(3,5‐dimethylpyrid‐2‐yl)ferrocene ( 4‐Pf ), (p R )‐2‐[dimesitylboryl]‐1‐(3,5‐dimethylpyrid‐2‐yl)ferrocene ( 4‐Mes ), (p S )‐2‐(bis(pentafluorophenyl)boryl)‐1‐(2‐trimethylsilylpyrid‐6‐yl)ferrocene ( 5‐Pf ), or (p S )‐2‐[dimesitylboryl]‐1‐(2‐trimethylsilylpyrid‐6‐yl)ferrocene ( 5‐Mes ). The borylated products were analyzed by multinuclear NMR spectroscopy, HRMS, and single‐crystal X‐ray diffraction. Chiral HPLC and optical‐rotation measurements were employed to assess the stereoselectivity of the borylation process and to establish the correct stereochemical assignments. The strength of the B–N interactions were investigated in solution and in the solid state. Compounds 4‐Pf and 4‐Mes formed robust ‘closed’ BN heterocyclic systems that proved to be perfectly stable to air and moisture, whereas 5‐Pf established a dynamic equilibrium, in which the BN heterocycle was observed exclusively at room temperature, but opened up at high temperature according to 19 F NMR exchange spectroscopy data. As a consequence, 5‐Pf reacted readily with a molecule of water to generate a ring‐opened pyridinium borate. The combination of bulky borane and bulky pyridyl groups in 5‐Mes led to a completely ‘open’ frustrated Lewis pair system with uncomplexed pyridine and borane groups, even at room temperature. Electrochemical studies were performed and the effect of preparative ferrocene oxidation on the structural features was also explored.