Recent advances in neutral and anionic N-heterocyclic carbene – betaine interconversions. Synthesis, characterization, and applications

Andreas Schmidt,Sascha Wiechmann,Tyll Freese,Viktor V Zhdankin

doi:10.3998/ark.5550190.p008.251

Abstract

Some mesoionic compounds, i.e. five-membered representatives of the class of conjugated mesomeric betaines (CMB), are in equilibrium with their tautomeric normal N-heterocyclic carbenes (nNHC). In addition, anionic N-heterocyclic carbenes, generated by deprotonation of mesoionic compounds, have been described. The first examples of conversions of crossconjugated mesomeric betaines (CCMB), 6-oxopyrimidinium-4-olates, into normal Nheterocyclic carbenes have been reported as well. CCMB such as imidazolium-4-carboxylate and pyrazolium-4-carboxylate can decarboxylate to form abnormal (aNHC) or remote N-heterocyclic carbenes (rNHC). Most conversions of betaines into N-heterocyclic carbenes start from pseudocross-conjugated mesomeric betaines (PCCMB) which can be regarded as heterocumulene adducts of nNHC. Thus, decarboxylations of imidazolium-2-carboxylates, 1,2,4-triazole-3carboxylates, pyrazolium-3-carboxylates or indazolium-3-carboxylates yield N-heterocyclic carbenes which have been used in catalysis, complex chemistry, heterocyclic synthesis, and organocatalysis.

Highlights

The first mesoionic compound, tetrazoliumthiolate 1, was unknowingly prepared by Fischer as early as 1882 (Scheme 1).[1]
The chemistry of heterocyclic mesomeric betaines and of N-heterocyclic carbenes have an interesting area of overlap
N-Heterocyclic carbene formations from mesomeric betaines have been performed so far by tautomerism of type-A mesoionic compounds which are members of the class of conjugated mesomeric betaines (1), by tautomerism of ylides (2), by extrusion of heterocumulenes as leaving groups from iminium-2-heterocumulene adducts i.e. from pseudo-cross-conjugated mesomeric betaines (3), or from X=Y=Z adducts in position 2 of prop-2-en-1-iminium partial structures (4) which belong to cross-conjugated mesomeric betaines (X=Y=Z: CO2, SO2) (Figure 10)

Summary

Introduction

The first mesoionic compound, tetrazoliumthiolate 1, was unknowingly prepared by Fischer as early as 1882 (Scheme 1).[1]. Mesomeric betaines have been defined as compounds which can exclusively be represented by dipolar canonical forms and which delocalize the positive as well as negative charge within a common π-electron system. Boron adducts are best represented by these dipolar structures with a positive charge on carbon or nitrogen, which are entirely consistent with the representation of mesomeric betaines such as mesoionic compounds or heterocumulene adducts of NHCs. Representations of complexes of NHCs with other metals, include resonance forms with C=M double bonds. In order to avoid the formulation of tetrapolar or tripolar structures with two formal negative and one or two formal positive charges within the same molecule, representation III or – where appropiate – a selected dipolar structure taken from the resonance forms I is used throughout this review. This interesting area of overlap will serve as strong impetus for additional research in that field

Interconversion reactions

Findings

Conclusions