Arenes Friedel-Crafts acylation

One of the most useful reac tions of acyl chlorides was presented in Section 12 7 Friedel-Crafts acylation of aromatic rings takes place when arenes are treated with acyl chlorides in the presence of aluminum chloride... 

What combination of acyl chloride or acid anhydride and arene would you choose to prepare each of the following compounds by Friedel-Crafts acylation ... 

Chromium carbene complexes having electron-rich arenes tethered to the car-bene oxygen or carbon underwent photodriven intramolecular Friedel-Crafts acylation in the presence of zinc chloride (Eqs. 32 and 33) [118]. The process was highly regioselective, undergoing acylation exclusively para to the activating group. 

On the basis of these initial results, various rare earth metal triflates, including Sc(OTf)3, Hf(OTf)4 and Yb(OTf)3 were applied as catalysts [27-29]. Recently Beller and coworkers developed efficient Friedel-Crafts alkylations with catalytic amounts of Rh, W, Pd, Pt and Ir complexes [30] or FeCl3 [31-34] as Lewis acid catalysts. However, in the latter cases high catalyst loadings had to be applied. To overcome these major drawbacks, we decided to develop a Bi(III)-catalyzed Friedel-Crafts alkylation of arenes with benzyl alcohols. Although bismuth-catalyzed Friedel-Crafts acylations were well known at this time, Friedel-Crafts alkylations using benzyl alcohols had not been reported. 

Friedel-Crafts acylations of arenes with RCOCl or anhydrides (RC—O—CR) in the presence of AICI3 give good yields of ketones. 

In the case of the tricarbonylarene metals, enhancement of nucleophilic substitution relative to the free arene is reported 106), In contrast to earlier reports 106) Friedel-Crafts acylation of tricarbonylbenzene chromium occurs under mild conditions 18), Molecular-orbital calculations of the 7r-electron activation energies for these reactions 63) confirm enhanced nucleophilic reactivity and suggest electrophilic activity similar to that of the free arene. The nucleophilic displacement of halide by methoxide ion... 

Cross-linked polystyrene can be acylated with aliphatic and aromatic acyl halides in the presence of A1C13 (Friedel-Crafts acylation, Table 12.1). This reaction has mainly been used for the functionalization of polystyrene-based supports, and only rarely for the modification of support-bound substrates. Electron-rich arenes (Entry 3, Table 12.1) or heteroarenes, such as indoles (Entry 5, Table 15.7), undergo smooth Friedel-Crafts acylation without severe deterioration of the support. Suitable solvents for Friedel-Crafts acylations of cross-linked polystyrene are tetrachloroethene [1], DCE [2], CS2 [3,4], nitrobenzene [5,6], and CC14 [7]. As in the bromination of polystyrene, Friedel-Crafts acylations at high temperatures (e.g. DCE, 83 °C, 15 min [2]) can lead to partial dealkylation of phenyl groups and yield a soluble polymer. 

In most instances these catalysts were not satisfactory in terms of yield, range of substrates and turnover numbers of the catalyst. Additionally, they were mostly efficient only in the case of activated aromatics. In order to develop also Friedel-Crafts acylations of less activated or even deactivated arenes, a new generation of catalysts had to be developed. Here, particularly bismuth(III) triflate [33] and hafnium(IV) triflate in the presence of lithium perchlorate [34] or triflic acid [35] are effective, alternative catalysts actually acylating benzene, toluene and halobenzenes. 

In 2000, Dubac s group reported the microwave-assisted Friedel-Crafts acylation of slightly activated and deactivated arenes under solvent-free conditions with FeCl3 as catalyst. Here, for the acylation of toluene a 90% product yield is obtained after 5 min of irradiation and an overall reaction time of 30 min in the presence of only 5 mol% of FeCl3. A sequential MW irradiation at 300 W afforded the acylation of fluorobenzene with 2 -chlorobenzoyl chloride, with a surprisingly high yield of 92% of 2-chloro-4 -fluorobenzophenone (Scheme 6.11). 

Although these reactions operate under solvent-free conditions (with an excess of the arene), many Friedel-Crafts acylations utilize volatile and hazardous halogenated solvents. Here, their replacement by ionic liquids can considerably lower the environmental risks and provide a greener chemistry . Ionic liquids with their unique miscibility properties, high thermal stability and miniscule vapor pressure are valuable alternatives for the wide range of traditional solvents available. 

The reaction that occurs with arenes and acid anhydrides in the presence of aluminum chloride is Friedel-Crafts acylation. The methoxy group is the more powerful activating substituent, so acylation occurs para to it. 

In a Friedel-Crafts acylation reaction an acyl chloride or acid anhydride reacts with an arene to yield an aryl ketone. 

Remember from Section 12.16 that it is the more activating substituent that determines the regioselectivity of electrophilic aromatic substitution when an arene bears two different substituents. Methoxy is a strongly activating substituent fluorine is slightly deactivating. Friedel-Crafts acylation takes place at the position para to the methoxy group. 

This Lewis acid-catalyzed electrophilic aromatic substitution allows the synthesis of alkylated products via the reaction of arenes with alkyl halides or alkenes. Since alkyl substituents activate the arene substrate, polyalkylation may occur. A valuable, two-step alternative is Friedel-Crafts Acylation followed by a carbonyl reduction. 

Formaldehyde, acetaldehyde, and acetone are important commercial chemicals, synthesized by special methods. In the laboratory, aldehydes and ketones are most commonly prepared by oxidizing alcohols, but they can also be prepared by hydrating alkynes and by Friedel-Crafts acylation of arenes. Aldehydes and ketones occur widely in nature (see Figure 9.1). 

These catalytic reactions provide a unique pathway for addition of aromatic C-H bonds across C=C bonds. In contrast with Friedel-Crafts catalysts for olefin hydroarylation, the Ru-catalyzed hydrophenylation reactions of a-olefins selectively produce linear alkyl arenes rather than branched products. Although the selectivity is mild, the formation of anti-Markovnikov products is a unique feature of the Ru(II) and Ir(III) catalysts discussed herein. Typically, the preferred route for incorporation of long-chain linear alkyl groups into aromatic substrates is Friedel-Crafts acylation then Clemmensen reduction, and the catalysts described herein provide a more direct route to linear alkyl arenes. 

Under similar oxidative conditions, with activation of the aromatic C-H bond, some arenes could be used directly as aryl sources [41]. Unfortunately, by analogy with the Friedel-Crafts acylation, this reaction is regioselective for very few substrates only. High regioselectivity was, however, obtained if coordinating substituents on the arenes facilitate an orthopalladation reaction by a Pd(II) species [42]. After carbometallation and reductive elimination, Pd(0) is released, which has to be converted into the initial Pd(II) species in an extra oxidation step. Usually, quinines are used for this purpose, but in combination with certain heteropolyacids as cocatalysts even molecular oxygen can be employed as the oxidant. 

Photolysis of chromium alkoxycarbene complexes 1193 leads to isochroman-4-ones 1194 via an intramolecular Friedel-Crafts acylation of the arene. The reaction yield is highly dependent upon the amount of ZnCl2 present, which varies considerably for specific substrates (Equation 458, Table 56) <1998JOC1462>. 

Friedel-Crafts Acylation ofanArene Section 17.8 Friedel-Crafts Alkylation of an Arene Section 17.7 Figure 17.4... 

A complex of SbCls and benzyltriethylammonium chloride, which is insensitive to air and moisture, catalyzes the Friedel-Crafts acylation of arenes with acyl chlorides and sulfonyl chlorides (Equation (36)).69 The Diels-Alder reaction of toluquinone with 1,3-dienes is catalyzed by SbCl5 with high regioselectivity (Equation (37)).70... 

Isatins fail to yield Knoevenagel condensation products with malonic acid419. However, malonic acid can be condensed with isatin in a mixture of ethanol and pyridine, in which the initial condensation product suffers decarboxylation, furnishing an acetic acid derivative. This can be converted to the acid chloride and submitted to a Friedel-Crafts acylation reaction, yielding acetophenone derivatives420. Alternatively the oxoindolinylidene acetic acid derivative can be treated with an arene in the presence of AICI3 to yield. sy />o[indoline-3,3 -indan]-2,l-dione derivatives. ... 

Friedel-Crafts acylation of arenes (c/. Volume 2, Chapter 3.1) can be achieved with 5-aryl or 5-n-alkyl chlorothioformates in good to moderate yields (equation 30). ... 

New C—C bonds to arenes can be made by Friedel-Crafts reactions. Friedel-Crafts alkylations are traditionally executed with an alkyl chloride and catalytic AICI3 or an alkene and a strong Brpnsted or Lewis acid the key electrophilic species is a carbocation. Friedel-Crafts acylations are usually executed with an acyl chloride and an excess of AICI3 the key electrophilic species is an acylium ion (RC=0+). In the Bischler-Napieralski reaction, intramolecular attack on a nitrilium ion (RC=NR) occurs. 

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