Chiral Organoborane Lewis Pairs Derived from Pyridylferrocene

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 (pR)-2-[bis(pentafluorophenyl)boryl]-1-(3,5-dimethylpyrid-2-yl)ferrocene (4-Pf), (pR)-2-[dimesitylboryl]-1-(3,5-dimethylpyrid-2-yl)ferrocene (4-Mes), (pS)-2-(bis(pentafluorophenyl)boryl)-1-(2-trimethylsilylpyrid-6-yl)ferrocene (5-Pf), or (pS)-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' B-N heterocyclic systems that proved to be perfectly stable to air and moisture, whereas 5-Pf established a dynamic equilibrium, in which the B-N 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.

Ambiphilic Frustrated Lewis Pair Exhibiting High Robustness and Reversible Water Activation: Towards the Metal-Free Hydrogenation of Carbon Dioxide

“…The signals of the aliphatic carbons directly linked to the heteroatoms are also consistent with the formation of a zwitterion; the 13 C{ 1 H} NMR signal for C11 and C12, directly linked to the nitrogen, shifts to lower field by 7.6 ppm when those of C16 and C17, directly linked to the boron, shift to higher field by 5.1 ppm. Finally, a resonance is observed in the 1 H-NMR spectrum at very low field (δ = 17.1) for the splitting of water, which is quite similar to the equivalent resonance, observed by Jäkle, for a pyridylferrocene derivative [50] and suggests hydrogen bonding between O and the proton linked to N. It was found that the water adduct can be reverted back to 1 after a solution of 2 in benzene- d 6 was stored over 4 Å molecular sieves at room temperature for ca. 12 h. However, when placed in presence of excess water (10 equiv.)…”

Naphthylaminoborane: from structural switches to frustrated Lewis pair reactivity

“…We hypothesize that this reaction mechanism preferentially occurs via hydrolysis of the C-B bonds, as it has been reported by Hoefelmeyer for 8-(dimesitylboryl)quinolone, Jäkle for (dimesitylboryl)pyridinyl-ferrocene, and Wang for (dimesitylboryl)ferrocenylbenzimidazole. [35][36][37] Mechanistic evidence has been obtained through isolation of the borinic acid 10 corresponding to partial hydrolysis of the precursor, as well as the fully hydrolyzed boronic acid analog 11, which confirm this reaction path. Moreover, 6 can be prepared by an alternative route starting from 10.…”

Recent Advances in π-Conjugated N^C-Chelate Organoboron Materials

“…In addition to the above-mentioned applications, other applications have been reported too. This includes the activation of small molecules [91,92], switchable chiral anions [93], NLO materials [94,95], among others. In a recent example, Wang et al [96] reported blue fluorescent polymers exhibiting thermally reversible photochromism.…”