Remote Back Strain: A Strategy for Modulating the Reactivity of Triarylboranes

Abstract

A strategy for modulating the Lewis acidity of triarylboranes is proposed based on the concept of ‘remote’ back strain. Steric repulsion and non-covalent interactions, both generated between the aryl meta-substituents of triarylboranes, are found to be critical for determining the strength of the remote back strain. Applying this concept, we synthesized B(2,6-F2-3,5-TMS2-C6H)3 and the liquid B(2,6-F2-3,5-allyl2-C6H)3 and demonstrated their superior catalytic activity for the hydrogenation of quinoline relative to B(C6F5)3 and B(2,6-F2-C6H3)3. Moreover, we established the first example of the catalytic hydrogenation of quinoline using B(2,6-F2-3,5-allyl2-C6H)3 in the presence of a gaseous mixture of H2/CO/CO2 (1/1/1 molar ratio).

In-silico-assisted derivatization of triarylboranes for the catalytic reductive functionalization of aniline-derived amino acids and peptides with H2

“…We further conducted extensive in-silico derivatization of B 2a and B 3a , given that these boranes exhibit far superior catalytic activity than B 1a , B 6a , and B(C 6 F 5 ) 3 in the hydrogenation of N-heteroaromatics using a gaseous mixture of H 2 /CO/CO 2 /CH 4 45 . In these cases, we envisioned that the modulation of the intrinsic Lewis acidity of the triarylboranes, i.e., the energy levels of the LUMO, which includes the p orbital on the boron center 48 , as well as the remote back-strain 49 that would influence the stability of the four-coordinated tetrahedral Lewis base-borane adducts. The introduction of 2,6-Br 2 -C 6 H 3 (j) and its derivatives (k-r) into the B 2 core was also explored, as we expected an increase in the front strain that influences the accessibility of the Lewis bases to the boron centers 37 .…”

“…We also aimed to investigate relatively unexplored theoretical parameters for the construction of a regression-based model for the prediction of the catalytic activity of triarylboranes. In this context, we explored the deformation energy (E DEF ) [kcal mol -1 ] that can be used to evaluate the degree of remote back-strain 49 , where E DEF represents the energetic penalty associated with the change in the conformation at the boron center from trigonal planar to tetrahedral upon the formation of adducts between Lewis bases (LBs) and triarylboranes 48,53,54 . We calculated the E DEF (LB) values for 18 boranes shown in Fig.…”