Potential N‐Heterocyclic Carbene Precursors in the Palladium‐Catalyzed Heck Reaction

Abstract

A novel 1‐(cyclobutylmethyl)‐substi‐tuted imidazolidinium/benzimidazolium salts as N‐heterocyclic carbene (NHC) precursors were successfully synthesized and characterized by 1H NMR, 13C NMR, IR, and elemental analysis techniques. These compounds were easily prepared from the reaction of N‐alkyl imidazoline/N‐alkyl benzimidazole with bromomethylcyclobutane in high yields. The in situ formed catalytic system derived from the NHC precursor and Pd(OAc)2 was used in the Heck reaction between aryl halides and styrene with potassium hydroxide in water. The corresponding Heck products were obtained in good yields. © 2012 Wiley Periodicals, Inc. Heteroatom Chem 24:77–83, 2013; View this article online at http://wileyonlinelibrary.com. DOI 10.1002/hc.21065

Microwave-Assisted Synthesis of Novel Perimidinium Salts as N-Heterocyclic Carbene Precursors: Involvement in Palladium-Catalyzed Cross-Coupling Reactions

“…These signals are consistent with general acidic proton shift of the related imidazolium and benzimidazolium salts. 17,18 IR spectra of compounds 2a-e support the proposed structure, signals at 1656-1667 cm -1 indicate the presence of -N=C-N-group in the structure.…”

“…13 Strong electron-donating ability of NHC ligands give their metal complexes stability to heat, air, and moisture. [14][15][16][17][18][19] Since the early 90s, various NHC ligands have been reported.…”

“…[16][17][18] In the Heck and Suzuki reactions, five perimidinium salts 2 were used as NHC precursors. The azinium salts 2 that we used as carbene precursors were stable against air and moisture.…”

“…As NHC precursor, imidazolium, 14,15 imidazolinium, 16,17 and benzimidazolium 18,19 scaffolds were used intensively, and very active catalytic systems were prepared with not only palladium but also with various transition metals. In spite of the rapid development of the synthesis of NHC complexes and their application in catalytic reactions, perimidine-based NHC's were not studied sufficiently due to difficulties of producing of perimidine derivatives compared with five-membered analogs.…”

Cross coupling reactions catalyzed by (NHC)Pd(II) complexes

“…The in situ formed catalytic system derived from the cyclobutylmethyl functionalized imidazolinium/benzimidazolium salts (L2-L3), and Pd(OAc) 2 was used in the Heck reaction between aryl halides and styrene with KOH in water ( Table 7, entries 5-11). 96 The carbene ligand L2a is much more active than those carbene ligands. The in situ-generated Pd(OAc) 2 /L4 (1:5) complex represented an efficient and reusable catalyst system for Heck coupling reactions (Table 7, entries 12-17).…”