Betaine–N‐Heterocyclic Carbene Interconversions of Quinazolin‐4‐One Imidazolium Mesomeric Betaines. Sulfur, Selenium, and Borane Adduct Formation

Sergey Deev,Andreas Schmidt,Tatyana S Shestakova,Mikhail A Kiskin,Oleg Chupakhin,Alexander S Paramonov,Sviatoslav Batsyts,Ekaterina Sheina,Valery Charushin,Igor Khalimbadzha,Zakhar O Shenkarev,Jan C Namyslo

doi:10.1002/ejoc.201901622

Abstract

Reaction of N‐alkylated imidazoles with 2‐chloro‐4‐quinazolinone gave mesomeric betaines, 2‐(1‐alkyl‐1H‐imidazolium‐3‐yl)quinazolin‐4‐olates, for which three tautomeric forms of N‐heterocyclic carbenes (NHCs) can be formulated, in addition to an anionic NHC after deprotonation. The NHC tautomers were trapped with sulfur, selenium, triethylborane, and triphenylborane as thiones, selenones and borane adducts, respectively. We obtained two isomers of the cyclic borane adducts, diazaboroloquinazolinones with [1,5‐a] and [5,1‐b]‐type fusion between the quinazolinone and the diazaborole rings. They correspond to two different NHC tautomers and to the anionic NHC derived thereof. The third NHC tautomer was trapped as a non‐cyclic adduct with tris(pentafluorophenyl)borane by coordination to the quinazoline oxygen atom. 2D 1H‐15N HMBC experiments of 15N‐labeled quinazolinone fragments, quantitative measurements of long‐range 1H‐15N coupling constants (JHN), and five X‐ray single crystal analyses have been carried out for the structure elucidations and to gain insight into the NMR spectroscopic properties of these compounds.

Highlights

Since the first isolation of a stable N-heterocyclic carbene (NHC) by Arduengo in 1991,[1] this class of compounds has developed considerably and nowadays N-heterocyclic carbenes (NHCs) are ubiquitous in organic as well as inorganic chemistry
Imidazole 15a was dissolved in 1,2-dichlorobenzene (1,2-DCB), whereas all liquid azoles 15b–d reacted without additional solvent. In the latter cases the mesomeric betaines 13bb–d were formed in one step, whereas 15a gave the salt 16 which was converted into the betaine 13ba by the anion exchange resin Amberlite IRA-400 in its hydroxide form
Imidazole-2-thiones and imidazole-2-selenones were obtained on reaction of sulfur and selenium, respectively, with the betaines, which behave as N-heterocyclic carbenes under these conditions

Summary

Introduction

Since the first isolation of a stable N-heterocyclic carbene (NHC) by Arduengo in 1991,[1] this class of compounds has developed considerably and nowadays NHCs are ubiquitous in organic as well as inorganic chemistry. Kurnakov Institute of General and Inorganic Chemistry, RAS, 31 Leninsky Av., 119991 Moscow, Russia and review articles,[4] the latest of which reflect the broad applicability of this class of compounds They deal with main group element adducts,[5] NHCs in materials chemistry,[6] NHCs as ligands in cross-couplings[7] and in rhodium-catalyzed reactions[8] as well as copper, nickel, and cobalt complexes.[9] The enhancement of the σ-donating properties of N-heterocyclic carbenes was one of the initial goals of the design of a broad variety of interesting structures, resulting in strong donors such as cyclic alkyl amino carbenes (CAACs)[10] and others. The carbene properties depend significantly on the type of conjugation in the NHC framework.[12] For example, the negative charge of the anionic N-heterocyclic carbene 2[13] (Scheme 1) derived from zwitterion 1 is not delocalized in terms of resonance It can be classified as isolated anionic N-heterocyclic carbene.

Methods

Results

Conclusion