Zur Chemie von Haftgruppen, IV. Über eine außerordentliche Erhöhung der Reaktivität von Arylboronsäuren durch Nachbargruppen

Abstract

Zur spateren Verwendung als Haftgruppen in Polymeren wurden ortho-substituierte Phenylboronsauren untersucht. Die Einfiihrung verschiedener Aminomethyl-Gruppierungen ergab eine starke Beschleunigung fur die Gleichgewichtseinstellung der Veresterung rnit Diolen sowie der Umesterung solcher Ester. Die Geschwindigkeit der Umesterung wurde an geeigneten Systemen mit Hilfe der dynamischen NMR-Spektroskopie abgeschatzt und war gegeniiber der unsubstituierten Verbindung bis 108fach beschleunigt. Die meisten Verbindungen zeigten eine starke B -N-Koordination, wie im "B-NMR-Spektrum erkennbar war. Jedoch konnte ein einfacher Zusammenhang zwischen B -N-Koordination und katalytischer Wirksamkeit der Nachbargruppe nicht festgestellt werden. Nachbargruppen mit Sauerstoff, Schwefel oder Fluor zeigten weder eine Beschleunigung der Veresterung oder Umesterung noch eine Koordination mit dem Bor.

A Structural Investigation of the N−B Interaction in an o-(N,N-Dialkylaminomethyl)arylboronate System

“…Since the discovery that phenylboronic acid condenses with mannitol and other polyols to form phenylboronates, arylboronic acid functional groups have been incorporated into receptors for polyols and their derivatives. − Target substrates of physiological importance include saccharides, nucleosides, α-hydroxycarboxylates, and catecholamines. Over the past 50 years, sensing applications − (reviews on saccharide sensing, refs −14; examples on α-hydroxycarboxylates sensing, refs −21; examples on catacholamine sensing, refs −26) and chromatographic protocols − that target these molecules on the basis of their reversible covalent associations with boronic acid receptors have been created, including a few near-clinical applications. , Furthermore, some structural and mechanistic understanding of this important reversible covalent association has been gained. − …”

Different donor binding modes of the pincer ligand 2,6-bis[(diethylamino)-methyl]phenyl: intermolecularly chelating in Li[2,6-(NEt2CH2)2C6H3 ] and both mono- and bi-dentate in BCl2{2-[BCl3(NEt2CH2)]-6 -(NEt2CH2)C6H3}

“…2 Intramolecular coordination of the amino group strongly stabilises these compounds. 2 8 which were synthesised from boronic acid and diols, have been reported in the literature. Since base-stabilised arylboron dichloride derivatives would appear to be useful starting materials for the preparation of transition-metal boranediyl complexes, 9 we attempted the preparation of these compounds.…”

Experimental and Theoretical Evidence of an SN2-Type Mechanism for Dissociation of B−N Coordination Bonds in 2,6-Bis((dimethylamino)methyl)phenylborane Derivatives1

“…The organoboron complexes, 2 , 4 , and 5 , were synthesized according to the reactions in Scheme . Compound 3 was prepared in a similar manner as the preparation of 1 . ,10b, In contrast to ligand-free trialkylboranes, which are liable to decomposition, 4 and 5 are stable under ordinary conditions due to the stabilization by the intramolecular coordination.

2

3

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“…Another approach to verify the mechanism is the introduction of one more amine ligand at the remaining o -position in 1 , e.g. using the 2,6-bis((dimethylamino)methyl)phenyl group. − The second NMe 2 ligand can strongly interact with the boron atom from the rear face of the coordinated boron to amplify the S N 2-type contribution. Therefore, we synthesized two series of organoboron compounds carrying the terdentate ligand, the boronate ( 2 ) and borane ( 5 ) complexes, and the rates of the dissociation of the B−N coordination bonds were determined by spectroscopic methods.…”