Palladium N-Heterocyclic Carbene Complexes: Synthesis from Benzimidazolium Salts and Catalytic Activity in Carbon-carbon Bond-forming Reactions

Summary

Detailed and generalized protocols are presented for the synthesis and subsequent purification of four palladium N-heterocyclic carbene complexes from benzimidazolium salts. The complexes were tested for catalytic activity in arylation and Suzuki-Miyaura reactions. For each reaction investigated, at least one of the four complexes successfully catalyzed the reaction.

Abstract

Detailed and generalized protocols are presented for the synthesis and subsequent purification of four palladium N-heterocyclic carbene complexes from benzimidazolium salts. Detailed and generalized protocols are also presented for testing the catalytic activity of such complexes in arylation and Suzuki-Miyaura cross-coupling reactions. Representative results are shown for the catalytic activity of the four complexes in arylation and Suzuki-Miyaura type reactions. For each of the reactions investigated, at least one of the four complexes successfully catalyzed the reaction, qualifying them as promising candidates for catalysis of many carbon-carbon bond-forming reactions. The protocols presented are general enough to be adapted for the synthesis, purification and catalytic activity testing of new palladium N-heterocyclic carbene complexes.

Introduction

N-heterocyclic carbenes (NHCs) have attracted much attention, particularly for their ability to catalyze various important reactions such as metathesis, creation of furan, polymerization, hydrosilylation, hydrogenation, arylation, Suzuki-Miyaura cross-coupling and Mizoroki-Heck cross-coupling^{1,2,3,4,5,6,7,8,9,10,}

Protocol

Caution: Many volatile solvents are used as part of the protocols detailed below so carry out all experiments in a working fume hood. Wear appropriate personal protective equipment throughout and consult the MSDS of each reagent before use; herein, brief information has been provided about the hazardous reagents and steps.

1. Synthesis and purification of benzimidazolium salts (compounds 1-4)

1. Clamp a 100 mL Schlenk tube upright and put a stirrer bar, 1 mmol of benzimidazole, 1 mmol of potassium hydroxide and 60 mL of ethyl alcohol as the solvent into it.
   Caution: Potassium hydroxide can b....

Results

Benzimidazolium salts (1-4) (Figure 1) were synthesized in anhydrous DMF using N-alkylbenzimidazoles and various alkyl halides, then purified and characterized as reported before^18,24. They were white or cream-colored solids and had yields ranging from 62% to 97%. Palladium NHC complexes (5-8) (Figure 2

Discussion

The protocols for the synthesis and purification of four benzimidazolium salts and subsequently their palladium NHC complexes were deliberately presented in utmost detail to help young scientists or those new to the field master them. With this same goal in mind, the protocols for testing the catalytic activity of the four complexes in arylation and Suzuki-Miyaura reactions were also presented in utmost detail. Furthermore, we have attempted to present the protocols in as general a form as possible to allow others to eas.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
1-chloro-4-nitrobenzene	Sigma-Aldrich (Interlab A.S., USA)
2,5-dimethoxyphenylboronic acid	Sigma-Aldrich (Interlab A.S., USA)
2-n-butylfuran	Sigma-Aldrich (Interlab A.S., USA)
2-n-butylthiophene	Sigma-Aldrich (Interlab A.S., USA)
3-chloropyridine	Merck (Darmstadt, Germany)
4-bromoacetophenone	Merck (Darmstadt, Germany)
4-bromoanisole	Sigma-Aldrich (Interlab A.S., USA)
4-chlorotoluene	Sigma-Aldrich (Interlab A.S., USA)
4-methoxy-1-chlorobenzene	Merck (Darmstadt, Germany)
4-tert-butylphenylboronic acid	Sigma-Aldrich (Interlab A.S., USA)
Benzimidazole	Merck (Darmstadt, Germany)
Bromobenzene	Merck (Darmstadt, Germany)
Celite	Merck (Darmstadt, Germany)
Dichloromethane	Merck (Darmstadt, Germany)
Diethyl ether	Sigma-Aldrich (Interlab A.S., USA)
Ethyl acetate	Sigma-Aldrich (Interlab A.S., USA)
Ethyl alcohol	Merck (Darmstadt, Germany)
Hexane	Merck (Darmstadt, Germany)
Magnesium sulfate	Scharlau (Barcelona, Spain)
N,N-dimethylacetamide	Merck (Darmstadt, Germany)
N,N-dimethylformamide	Merck (Darmstadt, Germany)
Palladium chloride	Merck (Darmstadt, Germany)
Phenylboronic acid	Sigma-Aldrich (Interlab A.S., USA)
Potassium acetate	Merck (Darmstadt, Germany)
Potassium carbonate	Scharlau (Barcelona, Spain)
Potassium hydroxide	Merck (Darmstadt, Germany)
Silica gel	Merck (Darmstadt, Germany)
Sodium tert-butoxide	Merck (Darmstadt, Germany)
Thianaphthene-2-boronic acid	Sigma-Aldrich (Interlab A.S., USA)