Effect of Benzylic Methyl Groups on Kinetic Basicity of Amine Ligand in o-Boron Substituted N,N-Dimethylbenzylamines

Abstract

Rates of the dissociation of the intramolecular B–N coordination bond in two series of phenylborane derivatives, the boronate and diethylborane complexes, with –CHMeNMe2 or –CMe2NMe2 group at the o-position were determined by the NMR lineshape analysis or saturation transfer method. The new organoboron compounds were synthesized from the corresponding organolithium compounds with appropriate boron reagents. Comparison of the kinetic data with those of the –CH2NMe2 compounds reveals that the barrier height to the dissociation, namely the kinetic basicity of the amine ligand, is increased as the molecule possesses more methyl groups at the benzylic position for both of the series of boron compounds. The X-ray structure of one of the boronate complexes and the NMR titration measurements of model amines indicate that the basicity of the amine ligand is not affected much by the methyl substitution in the coordinated form. Therefore, the substituent effect on the kinetic basicity is mainly ascribed to the destabilization of the transition state by the geminal dimethyl groups rather than to any inductive or steric effects at the initial state, especially for the –CMe2NMe2 compounds.

An illusive chiral aminoalkylferroceneboronic acid. Structural assignment of a strong 1∶1 sorbitol complex and new insight into boronate–polyol interactions

“…However, if one considers a B-N interaction, and therefore the processes in Scheme 1, a ∆G ‡ c = 61.3 kJ mol Ϫ1 for B-N bond breaking is found in the water-MeOH buffer [k c for this process would be k c (obs)/2]. 27 The calculated value is more than 20 kJ mol Ϫ1 higher than that found by Wulff et al 27 and others 34, 35 for the ortho-substituted benzene analogue.…”

Design, synthesis and structure of new potential electrochemically active boronic acid-based glucose sensorsThis work was supported by the Danish National Technical Research Council (grant 9900690).

“…Chemical shifts are reported in ppm. The spectra are referenced as follows: CD 3 OD, 13 C referenced internally to CD 3 OD = 49.0 ppm and 1 H to CHD 2 OD = 3.30 ppm. Evaporations were performed in vacuo on a rotary evaporator at 40-50 ЊC.…”

“…1 H NMR gave only limited information due to overlap of the carbohydrate signals and absorptions in the ferroceneboronic acids. On the other hand 13 C NMR on samples with ferroceneboronic acid and uniformly 13 C labelled glucose gave the spectra as depicted in Fig. 2 only showing signals from the sugar part.…”

“…In all three structures there are intramolecular B-N bonds. In boroxin 13 the three B-N bonds are 1.829(4), 1.842(3) and 13 except for the two of 1.829(4) and 1.842(3) Å, which are somewhat longer.…”

“…The reversible B-N bond formation, which fails for ferroceneboronic acid 16, is well documented both in solution [11][12][13] and in the solid phase 14, 15 when the structural motif as in II (Scheme 1) is attached to a 6-membered benzene system. This motif, originally developed by Wulff for other purposes, 16 has successfully been used by Shinkai's group in the design of a glucose selective fluorescent sensor molecule.…”