Issue 69, 2020

Preferential N–H⋯:C[double bond splayed right] hydrogen bonding involving ditopic NH-containing systems and N-heterocyclic carbenes

Abstract

Hydrogen bonding plays a critical role in maintaining order and structure in complex biological and synthetic systems. N-heterocyclic carbenes (NHCs) represent one of the most versatile tools in the synthetic chemistry toolbox, yet their potential as neutral carbon hydrogen bond acceptors remains underexplored. This report investigates this capability in a strategic manner, wherein carbene-based hydrogen bonding can be assessed by use of ditopic NH-containing molecules. N–H bonds are unique as there are three established reaction modes with carbenes: non-traditional hydrogen bonding adducts (X–H⋯:C[double bond splayed right]), salts arising from proton transfer ([H–C[double bond splayed right]]+[X]), or amines from insertion of the carbene into the N–H bond. Yet, there are no established rules to predict product distributions or the strength of these associations. Here we seek to correlate the hydrogen bond strength of symmetric and asymmetric ditopic secondary amines with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr, a representative NHC). In symmetric and asymmetric ditopic amine adducts both the solid-state (hydrogen bond lengths, NHC interior angles) and solution-state (1H Δδ of NH signals, 13C signals of carbenic carbon) can be related to the pKa of the parent amine.

Graphical abstract: Preferential N–H⋯:C [[double bond splayed right]]  hydrogen bonding involving ditopic NH-containing systems and N-heterocyclic carbenes

Supplementary files

Article information

Article type
Paper
Submitted
05 Okt 2020
Accepted
05 Nov 2020
First published
20 Nov 2020
This article is Open Access
Creative Commons BY-NC license

RSC Adv., 2020,10, 42164-42171

Preferential N–H⋯:C[double bond splayed right] hydrogen bonding involving ditopic NH-containing systems and N-heterocyclic carbenes

Z. J. Kinney, A. L. Rheingold and J. D. Protasiewicz, RSC Adv., 2020, 10, 42164 DOI: 10.1039/D0RA08490E

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