Benzylation of arenes

Friedel-Crafts reactions. The reaction of osmium-complexed anisoles with electrophiles such as enones is catalyzed by TfOH. Benzylation of arenes by a reductive alkylation with arenecarbaldehyde acetals involves an intramolecular redox process (hydride shift) after protonation of the benzylic ether intermediates. 

The benzylation of arenes with benzyl formate is catalyzed by Ru3(CO)I2 Benzene is shown to give diphenylmethane according to... 

Silica-supported polytrifluoromethanesulfosiloxane (Si02-Si-SCF3) has been prepared by Jiang et al., and has been used to catalyze the Friedel-Crafts benzylation of arene with benzyl alcohol [124]. The selectivity of the reaction was influenced by catalyst concentration, reaction time and arene/benzyl alcohol molar ratios. In the range 30 1 to 10 1 of toluene/benzyl alcohol molar ratios, the yield of alkylated product was quantitative, but with a molar ratio of 5 1, 37% the intramolecular condensed product, dibenzyl ether, was formed (Scheme 5.23). 

Friedel-Crafts benzylation. The benzylation of arenes proceeds in good yields with benzyl chloride as reagent when ZnCh is activated by the addition of a ketone, a primary alcohol, or water. ... 

The same catalytic system was applied to the directed benzylation of arenes with benzyl chlorides (Scheme 19.30) [50]. Arenes with an oxazohne directing... 

Trimethylaluminum exhaustively methylates O-functions and thus converts tert. alcohols to alkanes, ketones to gem-dimethyl compounds, and carboxylic acids to terUhuiyX derivatives . AgC104-SnCl4 gives high yields of diarylmethanes by benzylation of arenes, e. g. with benzyl alcohol . 

Peris and co-workers reported the benzylation of arenes using complex 26b as catalyst (0.1 or 1 mol%). Different benzylating agents such as benzyl alcohols, benzyl ethers or styrene derivatives were used in combination with toluene, xylenes, phenol and anisole (Equation (8.16)). Catalyst 26b displayed high activities, except for a few cases, but poor selectivities for both the ortho... 

Section 11 10 Chemical reactions of arenes can take place on the ring itself or on a side chain Reactions that take place on the side chain are strongly influ enced by the stability of benzylic radicals and benzylic carbocations... 

Intramolecular acylnitrene-induced ring expansions of arenes are of great interest for the synthesis of novel 1//-azepines. Benzyl azidoformates, when subjected to spray-vacuum pyrolysis (SVP),152 yield l//,3//-oxazolo[3,4-a]azepin-3-ones which dimerize spontaneously 153,154 however, 2,6-dichlorobenzyl azidoformate yields the thermally stable (< 100 C) 5,9-dichloro-1 //,3//-oxazolo[3,4-a]azepin-3-one (8).154... 

SIDE-CHAIN BROMINATION OF AROMATIC COMPOUNDS Benzylic bromination of arenes... 

Although deprotonation at the benzylic position of arenes coordinated to ruthenium and chromium was reported,27 in the case of the coordinated oxo-ri5-dienyl unit, nucleophilic attack at one terminus of the complexed r 5-dienyl ligand, rather than deprotonation, was expected.28 The reason for the successful deprotonation (even at relatively hindered isopropyl sites) is, according to the authors, the cationic nature of the Cp M fragment. In addition, the transition state for the deprotonation might involve stabilization by the metal (Scheme 3.15). 

A series of asym DAMs were synthesized by the condensation reaction of arenes with benzyl alcohols. Benzene, p-xylene, mesitylene, durene, and isodurene were used as arenes because they give only one condensation product. For example, pure 2,5,3 -TrMeDPM is easily prepared by a distillation of the crude product obtained by the following reaction ... 

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. 

Beside the Friedel-Crafts-type alkylation of arenes, the direct functionalization of 2,4-pentanediones is of great interest in Lewis acid catalysis. Although Pd-catalyzed Tsuji-Trost type allylations of 1,3-diketones are known, direct benzylation procedures catalyzed by Lewis acids are less explored [40-43]. Based on the previously described Friedel-Crafts alkylation of arenes and heteroarenes, the Rueping group developed a Bi(OTf)3-catalyzed benzylation of 2,4-pentanediones. Alcohols such as benzyl, allyl or cinnamyl alcohols were used as the electrophilic component to yield important 2-alkylated 1,3-dicarbonyl compounds. Initially, different Bi(III) salts were screened. In contrast... 

This overview impressively demonstrates that Bi(III) salts are not only versatile Lewis acid catalysts for the activation of cr-donors, including benzyl and propargyl alcohols, but also efficient catalysts for the activation of Ji-donors such as styrenes or alkynes. In recent years, various environmentally benign bismuth-catalyzed methods have been developed for the alkylation of arenes, heteroarenes,... 

Oxidation to Quinones. Direct oxidation of arenes to quinones can be accom-plished by a number of reagents. Very little is known, however, about the mechanism of these oxidations. Benzene exhibits very low reactivity, and its alkyl-substituted derivatives undergo benzylic oxidation. Electrochemical methods appear to be promising in the production of p-benzoquinone.797 In contrast, polynuclear aromatic compounds are readily converted to the corresponding quinones. 

The ready formation of benzylic hydroperoxides is used in industrial oxidations, as in the synthesis of propylene oxide and phenol (see Sections 9.5.2 and 9.5.4, respectively). In contrast with autoxidation of alkenes, where various secondary processes may follow, autoxidation of arenes is less complicated. Chain termination of 99 may lead to an alcohol and aldehyde [Eq. (9.151)], and the rapid autoxidation of the latter may produce the corresponding carboxylic acid [Eq. (9.152)] ... 

Aromatic hydrocarbons are mainly hydroxylated to phenolic products. Complex (12) hydroxylated benzene in MeCN at 20 °C into phenol in ca. 55% yield, and no isotope effect was found for this reaction. Hydroxylation of toluene mainly occurs at the ring positions, with minor amounts of benzylic oxidation products. Hydroxylation of 4-deuterotoluene by (12) occurred with 70% retention and migration of deuterium in the formation of p-cresol. This high NIH shift value is in the same range as that found for liver microsome cytochrome P-450 hydroxylase, and suggests the transient formation of arene oxide intermediates. 

The first example of syn stereoselective expoxidation of arene dihydrodiols was reported in 1981.11 The trans-dihydrodiols 11 and 12, when treated with a 10-fold excess of MCPBA in tetrahydrofuran (THF) at room temperature, gave stereoselectively the syn-diol epoxides 13 and 14, respectively. This stereoselectivity has been ascribed to the operation of steric control by the axial benzylic hydroxy group the equatorial hydroxy group does not exert such control. The isomeric 9,10-epoxides of 7,8-dihydroxy-7,8-dihydro-benzo[a]pyrene can be prepared by the same method.12... 

Sarca and Laali199 have used triflic acid in butylmethylimidazolium hexafluor-ophosphate BMIM][PF6 ionic liquid for the benzylation of various arenes with benzyl alcohol [Eq. (5.76)]. When compared with Yb(OTf)3, triflic acid proved to be a better catalyst showing higher selectivity (less dibenzyl ether byproduct) by exhibiting similar activity (typically complete conversion). Of the isomeric products, para isomers dominate. Experimental observations indicate that dibenzyl ether originates from less complete protonation of benzyl alcohol and, consequently, serves as a competing nucleophile. Both substrate selectivity (kT/kB) and positional selectivity (ortho/para ratio) found in competitive benzylation with a benzene-toluene mixture (1 1 molar ratio) are similar to those determined in earlier studies, indicating that the nature of the electrophile is not affected in the ionic liquid. 

Fukuzawa et al.245 used 2-phenyl- 1,3-dioxane to benzylate a variety of arenes [Eq. (5.91)]. Similar observations were made when substituted benzaldehydes were treated in the presence of 1,3-propanediol under identical conditions. Although 2-phenyl-1,3-dioxane gave similar results, benzaldehyde dialkylacetals in general were unreactive under similar conditions. Mechanistic studies including reaction of a labeled dioxane indicate the involvement of the alkylated intermediate 65 and product formation was interpreted via an 1,3-hydride shift. 

Scheme 6.13 Benzylation of different arenes with 1-phenylethyl acetate CCyield (regioselectivity). Reaction conditions 0.5 mmol 1-phenylethyl acetate, 10mol% FeCI3, 2.0mmol arene, 5mL CH2CI2, 50°C, 20 h (a) 5 mL arene, no solvent (b) 5 mL MeN02, 120°C (c) 5 mL MeNQ2.

Again FeCl3-6H20 is the catalyst of choice for the benzylation of o-xylene by 4-chlorostyrene (Table 6.2). This iron-catalyzed arylation reaction of styrenes allows the synthesis of a wide variety of 1,1-diarylalkanes employing various styrenes and diverse arenes. Scheme 6.15 illustrates selected examples of successfully synthesized 1,1-diarylalkanes and Table 6.3 shows the diversity of different substituted styrenes employed as benzylating agents. 

Until now, three main F-C transformations have been used for catalytic stereoselective formation of benzylic carbon stereocenters - 1,2-addition of arenes to carbonyl (C=X, X O, NR) moieties, 1,4-addition of arenes to electron-deficient C-C double bonds, and ring-opening reaction of epoxides. 

In the presence of a ruthenium catalyst, 3-diazochroman-2,4-dione 716 undergoes insertion into the O-H bond of alcohols to yield 3-alkyloxy-4-hydroxycoumarins 717 (Equation 285) <2002TL3637>. In the presence of a rhodium catalyst, 3-diazochroman-2,4-dione 716 can undergo insertion into the C-H bond of arenes to yield 3-aryl-4-hydroxy-coumarins (Equation 286) <2005SL927>. In the presence of [Rh(OAc)2]2, 3-diazochroman-2,4-dione 716 can react with acyl or benzyl halides to afford to 3-halo-4-substituted coumarins (Equation 287) <2003T9333> and also with terminal alkynes to give a mixture of 477-furo[3,2-f]chromen-4-ones and 4/7-furo[2,3-3]chromen-4-ones (Equation 288) <2001S735>. 

As mentioned in Sec. 15.2.3, benzylic radicals are obtained also from the cleavage of a nucleofugal group from the radical anion. This may lead again to benzylation, and it has been shown that irradiation of 1,4-dimethoxynaphthalene in the presence of substituted benzyl halides leads to benzylated naphthalenes (mainly in position 2) via benzyl radical/arene radical cation combination, which is analogous to the benzyl radical/radical... 

Moreover, it is difficult to visualize a simple mechanism for the proposed rearrangement of a o-aryl to a-benzyl species. More studies are clearly needed before any definite conclusions can be drawn about the mechanisms of Pd(II) oxidations of arenes. 

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