An efficient and recyclable iron(III)-containing imidazolium salt catalyst for cross-coupling of aryl Grignard reagents with alkyl halides

Abstract

1,3-Bis(2,6-diisopropylphenyl)imidazolium chloride, [DIPrim]Cl, was used to produce a novel iron(III)-containing imidazolium salt [DIPrim] [FeCl 4 ], which included a N,N-diarylimidazolium cation (R = 2,6-diisopropylphenyl), [DIPrim] + , and tetrachloroferrate(III) anion, [FeCl 4 ]  . This compound was an effective and easy-to-use catalyst for the cross-coupling of aryl Grignard reagents with primary and secondary alkyl halides bearing -hydrogens. After simply decanting the cross-coupling product in the ether layer, [DIPrim] [FeCl 4 ] could be reused in at least four successive runs without significant loss of catalytic activity. The transition metal-catalyzed Grignard cross-coupling reaction is a powerful and widely used method for the construction of carbon-carbon bonds [1]. Although a variety of palladium or nickel-based catalysts are particularly effective for the cross-coupling of alkenyl or aryl halides, recent reports on the use of alkyl halides, especially those with -hydrogen atoms, as substrates suggests iron catalysis has potential in this field [2,3]. In comparison with established palladium or nickel-based systems, iron-based catalysts offer a cost efficient, safe and more environmentally benign approach, and the capacity to suppress undesirable -hydride eliminations.To date, a variety of iron-based catalysts have been developed for the cross-coupling of an aryl Grignard reagent with primary or secondary alky halides bearing -hydrogens. For example, well-defined iron complexes, such as the iron (II) tetrakisferrate complex [4], iron(III) salen complexes [5], iron(II) or iron(III) imine complexes [6] and iron(III)amine-bis(phenolate) complexes [7], have shown good crosscoupling activity. In addition, although Fe(acac) 3 (acac = acetylacetonate) can catalyze this cross-coupling reaction [8], amine additives are necessary to achieve higher yields of the cross-coupled product [9]. Notably, FeCl 3 can be used effectively in the presence of appropriate additives, such as amines [9][10][11], phosphines, phosphites, arsines and carbene precursors [12]. Although the direct use of FeCl 3 generally suffers from limitations such as high hygroscopicity of FeCl 3 and the variable yield according to the purity and commercial source of FeCl 3 [13], this method appears particularly attractive because of the simplicity and low cost of the metal salt combined with the high efficiency of the catalytic system. Consequently, this method represents a very powerful approach, if these limitations can be overcome [9,11].Two reports describing the use of FeCl 3 have appeared in the literature. Cahiez et al. [9] discovered that hygroscopic FeCl 3 could be easily modified into a hydrophobic and airstable iron(III) complex [(FeCl 3 ) 2 (tmeda) 3 ] (tmeda = N,N,N′,N′-tetramethylethylenediamine) by direct reaction of FeCl 3 with tmeda in a 2 : 3 molar ratio. Moreover, this complex showed significantly better activity than a stoichiometric mixture of FeCl 3 and tmeda [10,11]. Of particular note is a

HSO3-functionalized Brønsted acidic ionic liquids promote esterification of aromatic acid

“…Room temperature ionic liquids (ILs) have been the subject of considerable current interest as environmentally benign reaction media in organic synthesis [5][6][7][8][9][10]. Their advantages include wide liquid range, good thermal and chemical stability, high ionic conductivity, negligible vapor pressure, and the potential for recycling.…”

Synthesis, structure of iron(III)-containing imidazolium salts and their catalytic activity in the alkylation of aryl Grignard reagents

“…The target imidazolium salt of iron(III) [DIPrim][FeCl 4 ] (1) was easily synthesized according to a published procedure [24]. As shown in Scheme 1, similar reactions with other imidazolium halides provide the corresponding iron(III)-containing imidazolium salts 2-6 in nearly quantitative yields.…”

“…The reaction outlined in Equation 1 was chosen as a model system, as it represents a prototype cross-coupling of aryl Grignard reagents with alkyl halides bearing -hydrogens [4,6,7,24]. The results obtained are summarized in Table 3.…”

“…Very recently, we developed a 1,3-bis(2,6-diisopropylphenyl)imidazolium-modified iron(III)-containing complex [DIPrim] [FeCl 4 ] (IPr = 2,6-diisopropylphenyl), which was obtained as yellow-green crystals and could be used as an efficient and recyclable catalyst for cross-coupling of aryl Grignard reagents with alkyl halides [24]. As a continuation of our work on the development of iron-based catalysts for carbon-carbon bond formation [24,25], herein we reported the synthesis and structural characterization of a series of iron(III)-containing imidazolium salts (1)(2)(3)(4)(5)(6) and their use in the catalytic cross-coupling of aryl Grignard reagents with alky halides bearing -hydrogens.…”

“…As a continuation of our work on the development of iron-based catalysts for carbon-carbon bond formation [24,25], herein we reported the synthesis and structural characterization of a series of iron(III)-containing imidazolium salts (1)(2)(3)(4)(5)(6) and their use in the catalytic cross-coupling of aryl Grignard reagents with alky halides bearing -hydrogens.…”