Disproportionation Friedel-Crafts reaction

Aromatic hydrocarbons substituted by alkyl groups other than methyl are notorious for their tendency to disproportionate in Friedel-Crafts reactions. This tendency has previously limited the application of the isomerization of para- or ortho-) m ky -benzenes to the corresponding meta compounds. At the lower temperature of the present modification, disproportionation can be minimized. 

Koch Chemical Company is the only U.S. supplier of all PMBs (except hexamethylbenzene). Its process has the flexibility of producing isodurene, prehnitene, and pentamethylbenzene, should a market devdop. Koch s primary process (20) is based on isomerization, alkylation, and disproportionation conducted in the presence of a Friedel-Crafts catalyst. For the synthesis of mesitylene and hemimellitene, pseudocumene is isomerized. If durene, isodurene, or prehnitene and pentamethylbenzene are desired, pseudocumene is alkylated with methyl chloride (see Alkylation Friedel-Crafts reactions). 

In toluene solution, pent-2 ene in the presence of WCl -AlEtCl2 undergoes a Friedel-Crafts reaction with the solvent instead of undergoing the expected disproportionation (see ref. 83) M = Mo or W. 

Transalkylation and Dealkylation. In addition to isomerizations (side-chain rearrangement and positional isomerization), transalkylation (disproportionation) [Eq. (5.56)] and dealkylation [Eq. (5.57)] are side reactions during Friedel-Crafts alkylation however, they can be brought about as significant selective hydrocarbon transformations under appropriate conditions. Transalkylation (disproportionation) is of great practical importance in the manufacture of benzene and xylenes (see Section 5.5.4) ... 

Dihydro-2// -1 -benzothiopyrans (thiochromans) are in almost all their chemistry simply aromatic sulfides. Acylation of the benzene ring under Friedel-Crafts conditions goes, predictably, in position 6, unless blocked, when reaction occurs at position 8 (79MI22506). An even more constrained system (90) may be cyclized efficiently to (91) (equation 35) (76JHC123), though even here a disproportionated product was also formed. 

Further complications arise from the fact that the alkylation reactions sometimes are under equilibrium control rather than kinetic control. Products often isomerize and disproportionate, particularly in the presence of large amounts of catalyst. Thus 1,2- and 1,4-dimethylbenzenes (ortho- and para-xylenes) are converted by large amounts of Friedel-Crafts catalysts into 1,3-dimethyl-benzene (meta-xylene) ... 

Bisarene cations of Cr, Mo, and W are best synthesized by the reaction of aromatic hydrocarbon, metal halide, aluminum trihalide and aluminum metal at elevated temperature. Aromatic hydrocarbons, such as benzene, toluene, mesitylene, and hexamethylbenzene have been employed (Table III). The method is not applicable, however, when the aromatic hydrocarbon is itself reactive under Friedel-Crafts conditions. The bisarene cations are readily reduced to the neutral compounds by aqueous dithionite or, in the case of Mo and W, by alkaline disproportionation, e.g. 152),... 

Nitrobenzene and nitroalkanes are good solvents for Friedel-Crafts acylation reactions. As well as being good solvents they also form addition complexes with Lewis acids such as aluminum chloride. The formation of the complex appears to reduce disproportionation and rearrangement reactions and thus allow acylation to be achieved under mild conditions. The acetylation of toluene in nitrobenzene affords more 4-methylacetophenone than when the reaction is conducted in cafbon disulfide. These results evidently reflect a lower steric demand in the reaction carried out in carbon disulfide. The reaction shown in equation (17) when carried out in nitrobenzene leads to the formation of the products (1) and (2) in good yield. However, when the solvent was changed to nitroethane, an improved yield (82%) was obtained and the ratio of (1) (2) changed from 44 1 to 61 1. ... 

Treatment of the dimolyhdenum quintuply bonded compound 15 with a stoichiometric amount of acyl halide results in the formation of a quadruply bonded dimolyhdenum acyl complex in a reaction analogous to Friedel-Crafts insertion of alkynes [109]. The resulting dimolyhdenum acyl complex shown in Scheme 6.3 can then react with another equivalent of acyl halide, yielding products in which there is a further reduction of the Mo-Mo bond order, and an unusual acyl disproportionation, which yields a metal carbyne complex that also contains a carboxylate ligand. 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

In transalkylation, one of the alkyl groups is transferred from one alkylaromatic molecule to another aromatic molecule. The mechanism of transalkylation was studied extensively in Friedel-Crafts chemistry. Though the reaction conditions are quite different from those of Friedel - Crafts catalysts, it seems quite probable that an essentially same mechanism is operative also in transalkylation with solid-acid catalysts. Thus, Kaeding et al. proposed the following mechanism for disproportionation of toluene over zeolites. ... 

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