Synthesis of Substituted Imidazolidines: Base‐Stable Precursors of 4,5‐Dihydro‐1H‐imidazol‐3‐ium Salts and N‐Heterocyclic Carbenes

Abstract

The present work establishes a new synthetic route that leads to substituted azolium salts. The base stable 1‐(4‐bromo‐2,6‐diisopropylphenyl)‐3‐(2,6‐diisopropylphenyl)imidazolidine and 1,3‐bis(4‐bromo‐2,6‐diisopropylphenyl)imidazolidine were synthesized and the 4‐Br substituents converted into various functional groups through Br/Li exchange or Pd‐catalyzed cross‐coupling reactions (Suzuki, Sonogashira, and vinylation). The substituted imidazolidines were oxidized, by using chloranil or N‐bromosuccinimide, to provide the respective azolium salts, which are convenient precursors to N‐heterocyclic carbenes.

A Bicyclic N-Heterocyclic Carbene as a Bulky but Accessible Ligand: Application to the Copper-Catalyzed Borylations of Aryl Halides

“…NH 4 Cl aq., its reproducibility was unacceptable. Thus, the imidazolidine ring was reclosed with paraformaldehyde in the presence of PPTS in an 84% overall yield from cis diamine 6 , and then imidazolinium chloride 11 was obtained through oxidation with NCS in an 89% yield. From imidazolinium chloride 11 , AgCl complex 12 was synthesized in a 90% yield using Wang and Lin’s methodology, and the transmetalation to CuCl smoothly proceeded from this AgCl complex in a 98% yield.…”

Recent Advances in the Schiff Bases and N‐Heterocyclic Carbenes as Ligands in the Cross‐Coupling Reactions: A Comprehensive Review

“…Egert and co-workers [154] established a new synthetic route for the synthesis of base stable substituted azolium salts 1,3-bis(4-bromo-2,6-diisopropylphenyl)imidazolidine) (121) and the 4-Br substituents transformed into several functional groups via Br/Li exchange or Pd-catalyzed crosscoupling reactions (Sonogashira, Suzuki, and vinylation) as shown in Scheme 117. The substituted imidazolidines (122)(123)(124) were oxidized by using N-bromosuccinimide or chloranil to provide the respective azolium salts (L96-L98) in 57-80% yields, which were convenient precursors to NHCs.…”

“…The dehydrogenation of R 1 R 2 PH afforded diphosphines R 2 P-PR 2 (152). The reaction of iPr 2 Im with ArPH 2 led to the synthesis of NHC phosphinidene adducts iPr 2 ImQPAr (153) and cyclic oligophosphines (154), depending on the stoichiometry used. Authors are carrying out experimental and theoretical studies to understand the mechanism and scope of this reaction.…”

A Late-Stage Strategy for the Functionalization of Triazolium-Based NHC Catalysts

“…3). 15 With easy access to the bromo-triazoline 4, we investigated the feasibility of a Suzuki-Miyaura cross-coupling. After finding optimal conditions, we applied this method to elaborate the brominated aminoindanol scaffold (Scheme 3).…”

“…Concurrent with our own work, Plenio and coworkers recently disclosed the same strategy for the late stage modification of imidazolinium-based catalysts (Scheme 2, eqn 3). 15 …”