Review on friedel-crafts acylation of benzene derivatives using various catalytic systems

This review covers the Friedel-Craft’s acylation of benzene derivatives using various catalysts such as metal halogen acid, metal oxide, metal triflate, ionic liquids and miscellaneous catalytic systems. The author tries to cover all the literature in this area. Some of the methods described above are useful in organic synthesis, which is of great important synthesis in the preparation of natural products, active pharmaceutical ingredients, agrochemicals, fine chemicals and fragrances.

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Introduction

One of the major goals of modern synthetic organic chemical research is to provide new methodology and reagents that facilitate the synthesis of highly complex carbon-containing molecules, particularly those that might have biological activity or can be incorporated into materials with unique behavior. The invention of new routes to access small molecule substrates for synthesis, and perhaps as targets themselves, is required to satisfy demands for high yields and to maximize molecular diversity. The ability to manipulate or modify substrates containing a variety of functional groups can simplify a synthetic sequence and improve overall product yields. Friedel-Crafts acylation of aromatic compounds is one of the most frequently used reactions in organic synthesis to form C-C bonds, which is of great important synthesis in the preparation of natural products, active pharmaceutical ingredients, agrochemicals, fine chemicals and fragrances [1][2][3][4] . Unfortunately, the traditional Lewis acid-catalyzed Friedel-Crafts acylation has its environmental drawbacks, including requiring more than stoichiometric amounts, strictly anhydrous conditions, and produces corrosive acid wastes. Herein, author summarizes the Friedel craft's acylation of benzene and substituted benzene compounds using various catalytic systems as follows.

Metal halogen acid-Lewis acid Based Catalyst

Electrophilic aromatic substitution allows the synthesis of monoacylated products from the reaction between arenes and acyl chlorides or anhydrides. The products are deactivated, and do not undergo a second substitution. Normally, a stoichiometric amount of the Lewis acid catalyst is required, because both the substrate and the product form complexes. Lewis-acid (LA)-catalyzed reactions are of great interest because of their unique reactivities and selectivities and mild reaction conditions used [5] . Lewis acid catalysis of organic reactions, a metal-based Lewis acid acts as an electron pair acceptor to increase the reactivity of a substrate. Common Lewis acid catalysts are based on main group metals such as aluminum, boron, silicon and tin, as well as many early (titanium, zirconium) and late (iron, copper, zinc) d-block metals. Early developments in Lewis acid reagents focused on easily available compounds such as TiCl 4 , BF 3 , SnCl 4 , and AlCl 3 . Over the years, versatile catalysts bearing ligands designed for specific applications have facilitated improvement in both reactivity and selectivity of Lewis acid-catalyzed reactions. Metal halogen acid catalyzed Friedel craft's acylation reactions of benzene and substituted benzene are described as below. Christophe Le Roux et al, [6] carried out the Friedel-Crafts acylation of activated and polycyclic aromatics is by using the catalyst bismuth (III) chloride which is generated in situ from bismuth (III) oxychloride, a water insensitive and eco-friendly material. Bismuth (III) oxychloride is easily recovered in near quantitative yields after an aqueous work-up. In addition, in this method BiOCl was recycled by in situ transformation into BiCl 3 using the acylating agent, acid chloride, or before the acylation reaction using thionyl chloride. This added advantage of this recovery method makes this method applicable to most of the known BiCl 3 -catalyzed organic reactions. Even though catalytic power of BiCl 3 is limited; this method is a simple, cheap, safe and eco-friendly alternative to the classical processes of the FC acylation and related reactions (Scheme-1). Hossein Ghassemi et al, [7] reported the synthesis of several substituted poly (2,5-benzophenone)s and their copolymers. The monomers were prepared by Friedel-Crafts catalyzed reaction of 2, 5-dichlorobenzoyl chloride and several aromatic compounds.

These polymers were chemically modified for the potential applications. The results further demonstrate the feasibility of synthesizing aryl-substituted poly (2,5-benzophenone)s via Ni(0)-catalyzed coupling polymerization (Scheme-2). Jean-Fran Gois Scuotto., et al, [8]

Metal Triflate Catalyst

Metal trifluoromethane sulfonates (metal triflates) are a class of green homogeneous acidic catalysts, which have been efficiently employed in a number of chemical processes, including valorisation of polysaccharides. These catalysts are able to selectively promote Lewis acid catalysed reactions, or Brønsted acid catalysed processes through Lewis acid-assisted Brønsted acidity [9] . Metal triflates have been used in a wide variety of synthetic organic transformations and offer an efficient, environmentally friendly alternative to traditional Lewis acids. Jianliang Xiao et al, [10] have carried out the aromatic electrophilic substitution reactions such as benzoylation and acetylation catalyzed by metal triflates have been conducted in an ionic liquid, [bmim][BF 4 ](bmim=1-butyl-3-methylimidazolium).

Cu(OTf) 2 (OTf = trifluoromethanesulfonate) was found to be the most efficient catalyst used, affording quantitative conversion of benzoyl chloride and anisole to methoxybenzophenone within 1 h and with an ortho/para product ratio (o/p) of 4/96. The same reaction performed in the molecular solvents CH 3 CN and CH 2 ClCH 2 Cl gave lower 1 h conversions of 64 and 73%, respectively, and a reduced o/p regioselectivity of 7/93 in both cases. The scope and limitations of the reaction and recyclability of the ionic liquid/catalyst system are presented (Scheme-4). Shu Kobayashi et al, [11] reported the Friedel-Crafts acylation of benzene and unactivated benzenes such as chlorobenzene, and fluorobenzene in the presence of catalytic amounts of hafnium(IV)triflate (Hf(OTf) 4 ) and trifluoromethanesulfonic acid (TfOH). They found that combination of Hf(OTf) 4 and TfOH accelerated the catalytic Friedel-Crafts acylation of benzene dramatically. Both aromatic and aliphatic carboxylic acid chlorides reacted smoothly under the conditions to afford the corresponding aromatic ketones in good yields (Scheme-5).

Scheme 5: Friedel-Crafts acylation of benzene and unactivated benzenes such as chlorobenzene, and fluorobenzene in the presence of catalytic amounts of hafnium (IV) triflate (Hf(OTf)4) and trifluoromethanesulfonic acid (TfOH).

Atsushi Kawada et al, [12] reported the Lanthanide triflate catalysed Friedel-Crafts acylation of substituted benzenes proceeds using a catalytic amount of a lanthanide trifluoromethanesulfonate [Ln(OTf) 3 ], which is easily recovered from the reaction mixture and reused without a decrease in catalytic activity. Among all the Lanthanide triflate catalyst used 0.2 eq.Yb(OTf) 3 gives excellent yield of 93% (Scheme-6). Shu Kobayashi et al, [13] performed the catalytic Friedel-Crafts acylation reactions of substituted benzenes with acid anhydrides were successfully carried out by using Hf(OTf) 4 as a catalyst. Among the various catalytic systems used in the reaction, they found that the turnover of Lewis acid catalysts in the Friedel-Crafts acylation is 10-20 times improved in lithium perchloratenitromethane (LiCl0 4 -MeNO 2 ) (Scheme-7). Tobin J. Marks et al, [14] investigated Lanthanide trifluoromethane sulfonates, Ln(OTf) Yoh-ichi Matsushita et al, [15] synthesized aromatic ketones by solvent less Friedel-Crafts acylation of anisole, mesitylene, xylenes, and toluene with acetic acid and benzoic acid in the presence of trifluoroacetic anhydride by use of bismuth or scandium triflate at 30 0 C.Toluene, benzene, and even deactivated substrate such as chlorobenzene were benzoylated by the combined use of heptafluorobutyric anhydride and bismuth triflate at 75-100 0 C. The catalyst could be easily recovered and reused repeatedly for the reaction (Scheme-9).

Metal oxide catalyst

Metal oxides represent one of the most important and widely employed categories of solid catalysts, either as active phases or as supports. Metal oxides are utilized both for their acid-base and redox properties and constitute the largest family of catalysts in heterogeneous catalysis [16] . Metals and metal oxides form the bulk of catalysts employed for many synthetic conversions. Transition and noble group metals are frequently used as catalysts and their activity has been attributed to the outer electron configuration [17] . Metal oxide catalyzed Friedel craft's acylation of benzene and substituted benzene are given below. Hashem Sharghi et al, [18] reported a rapid Friedel Crafts acylation of a range of activated and unactivated aromatic compounds such as anisole and chlorobenzene with acid chlorides in solvent-free conditions at room temperature using zinc oxide (ZnO). The ZnO powder can be reused up to three times after simple washing with dichloromethane (Scheme-10). Reactionson a Solid Surface. A Simple, Economical and Efficient Friedel-Crafts Acylation Reaction over Zinc Oxide (ZnO) as a New Catalyst. Kartik Kumar Nandi et al, [19] reported a simple, efficient, and green procedure developed for mono-acylation of arenes with acid chlorides/dichloroacetylchloride over zinc oxide in 2:1:2 molar ratios at room temperature. Selective monoacylations in mild reaction conditions and recycling of recovered catalyst are notable green features and advantages of this method (Scheme-11).

Ionic liquid catalyst

Ionic liquids are attracting a great deal of attention as possible replacement for conventional molecular solvents for catalytic and organic reactions. They complete the use of environmentally friendly water, supercritical fluids or perfluorinated solvents. Features that make ionic liquids attractive include their lack of vapor pressure and the great versatility of their chemical and physical properties. By a judicious combination of cations and anions, it is possible to adjust the solvent properties to the requirement of the reactions, thus creating an almost indefinitely set of "designer solvents". Besides the possibility of recycling the catalytic system, one main potential interest in using ionic liquids results in the unique interactions of these media with the active species and in the possibility to modify the reaction activity and selectivity. Their successful use as solvents has been demonstrated for a wide range of organic reactions including acid catalyzed reactions and transition metal catalyzed transformations [20] . Christopher Hardacre et al, [21] reported, the comparison of three ionic liquid-mediated catalytic processes for the benzoylation of anisole with benzoic anhydride is presented. The predominant activity is associated with the in situ formation of a homogeneous acid catalyst, the low cost and facile separation of the zeolitecatalysed process leads to this route being the most economically viable overall option. The results of a continuous flow miniplant based on the zeolite catalyst are also presented and compared with the reaction using a small plug flow reactor (Scheme-12). Michel Vaultier et al, [22] studied the activity of four bismuth (III) derivatives when employed as Friedel-Crafts catalysts for the acylation of aromatics was found to increase dramatically when dissolved in ionic liquids. Sanjay V. Malhotra et al, [23] investigated the Friedel-Crafts acylations of aromatic compounds with acetic anhydride in pyridinium based ionic liquids (ILs). The effect of factors such as reactant composition, catalyst-IL composition, catalyst dosage and reaction temperature were studied. The reactions were found to proceed under relatively mild conditions with excellent conversions; and a simple product isolation procedure was achieved. ILs could also be recycled and reused effectively, thus rendering green characteristic to this reaction (Scheme-14). Phuong Hoang Tran et al, [24] investigated an efficient and facile bismuth trifluoromethanesulfonate catalyzed benzoylation of aromatic compounds using benzoic anhydride under solvent-free microwave irradiation has been developed. The microwave-assisted Friedel-Crafts benzoylation results in good yields within short reaction times. Bismuth triflate is easily recovered and reused for several times without significant loss of catalytic activity (Scheme-15). W.F. Holderich et al, [25] reported and discussed use of different ionic liquids catalysts for Friedel-Crafts acylation reactions. Supported chloroferrate ionic liquids were tested in liquid and in gas phase reactions. The catalysts, consisting of the ionic liquid and charcoal as a carrier, are easy to prepare and show interesting catalytic properties. Comparisons between different ionic liquids in the liquid phase are presented, as well as reactions in the gas phase (Scheme-16).

Scheme 16: Friedel-Crafts acylation of Benzene derivatives with acetic anhydride or acetyl chloride using ionic liquid catalyst.

Martyn J. Earle, et al, [26] described the Chloroindate (III) ionic liquids are versatile reaction media for Friedel-Crafts acylation reactions; the system is catalytic and totally recyclable, using an aqueous workup, with no leaching of the indium into the product phase (Scheme-17).

Miscellaneous catalytic systems

Friedel craft's acylation of benzene and substituted benzene carried out using various miscellaneous (uncategorized) catalysts are given as below.

Habib Firouzabadi et al, [27] reported the use of hetropolyacids aluminum dodecatungstophosphate (AlPW12O40) as a stable and effective heterogeneous catalyst (3 mol%) for the efficient acylation of aromatic compounds using carboxylic acids in the presence of trifluoroacetic anhydride (TFAA) under mild reaction conditions in the absence of solvent (Scheme-18). K. M. Lokanatha Rai et al, [28] investigated the Friedel-Crafts acylation of benzene derivatives such as anisole, toluene, and xylene has been successively carried out using silver nitrate as the catalyst in the presence of an eco friendly solvent (ethyl alcohol). Both benzoyl chloride and acetyl chloride reacted smoothly under the conditions to afford the corresponding ketones in good yield (Scheme-19). Babasaheb P. Bandgar et al, [29] carried out the Friedel-Crafts acylation reactions of substituted benzenes with acid chloride or anhydrides in the presence of a catalytic amount of novel solidsupported reagents. The reaction proceeds smoothly with these catalysts and theses catalysts are reusable and environmentally friendly (Scheme-20). Iliyan Ivanov et al, [30] have been synthesized number of aminobenzophenones by acylation of benzene and its derivatives with different 2-, 3-, 4-aminobenzoic and 4-aminophenylacetic acids in polyphosphoric acid via Friedel-Crafts reaction as compounds with expected antitumor activity (Scheme-21).

Scheme 21: Synthesis of number of aminobenzophenones by acylation of benzene and its derivatives with different 2, 3, 4-aminobenzoic and 4-aminophenylacetic acids in polyphosphoric acid.

Daniel J. Swartling et al, [31] have attempted to make more environmentally friendly chemical synthesis procedures. They synthesized isobutyrophenone, which is synthesized through a

Friedel-Crafts acylation of benzene using the solar reflector (Scheme-22). Arumugam Sudalai et al, [32] reported the liquid-phase acylation of activated aromatic substrates with aromatic and aliphatic carboxylic acids as acylating source giving aromatic ketones (45-93%) in a regioselective manner using P 2 O 5 catalyst, has been found to be a highly efficient and environmental friendly catalyst.

Both. The process is particularly demonstrated at 100 g scale in the case of anisole and acetic acid to produce 4methoxyacetophenone (Scheme-23). Chung-Yuan Mou et al, [33] demonstrated the Friedel-Crafts acylation of aromatics. High conversion (>90%) and high selectivity (100%) using Ultrastable acidic mesoporous aluminosilicates (MCM-48-S, MCM-48-SH, MCM-41-S) with mesostructures, strong acidity, and high hydrothermal stability was synthesized from assembly of cetyltriammonium bromide (CTAB) and pre-formed zeolite Beta seed (Scheme-24).

+ HCl Aluminosilicates

Scheme 24: Acylation of aromatics such as toluene with octylchloride using mesoporous aluminosilicates catalyst.

Mark C. Wilkinson., et al, [34] reported a novel methodology for FriedelCrafts acylation reactions, which allows the preparation of various aryl ketones with minimal waste containing no halogenated or metallic components, clearly differentiating it from other methodologies. The reactions precedes on the parent carboxylic acids via 1.3 equiv of a activating agent (methanesulfonic anhydride) and as little as 1.3 equiv of an aromaric nucleophile, with in general no additional solvent (Scheme-25). Rajive Gupta et al, [35] have described a novel and highly efficient microwave-induced solvent-free protocol for Friedel-Crafts acylation of aromatic compounds using non-toxic and inexpensive Zn powder. The Zn powder can be re-used up to six times after simple washing with diethyl ether and dilute HCl. The advantages of this environmentally benign and safe protocol include a simple reaction set-up not requiring specialized equipment, high product yields, short reaction times, and the elimination of solvents or solid support. Moreover, the activated aromatic substrates give the para-isomer as the major product (Scheme-26). P. Hoang Tran et al, [36] given a simple, facile and efficient method has been developed for the Friedel-Crafts benzoylation of arenes using magnetic γ-Fe 2 O 3 nanoparticles (NPS) under solvent-free sonication. Theγ-Fe 2 O 3 nanoparticles were used as an efficient and magnetically recoverable catalyst for thesynthesis aromatic ketones in good to excellent yields at room temperature under solvent-free.The reaction occurred with high regioselectivity under mild condition. The magnetic γ-Fe 2 O 3 nanoparticles are economically synthesized in large-scale, easily separated from the reaction mixture by an external magnet and able to be reused several times without significant loss of thecatalytic performance, which make them easy application to industrial processes (Scheme-27).

Conclusion

The author made a sincere effort to his best to describe the Friedel crafts acylation of benzene and substituted benzene done using various catalytic systems. Recent progress in this area replaces the water intolerable Lewis acids by the water tolerable catalyst. This article may be useful for the development of new catalyst for the Friedel crafts acylation reaction.

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