Benzene derivatives alkylation

Benzene is alkylated with ethylene to produce ethylbenzene, which is then dehydrogenated to styrene, the most important chemical iatermediate derived from benzene. Styrene is a raw material for the production of polystyrene and styrene copolymers such as ABS and SAN. Ethylbenzene accounted for nearly 52% of benzene consumption ia 1988. 

Fig. 6. Key intermediates derived from benzene. The alkylation reaction shown employs ethylene oxide. Hydrazine condenses with acetoacetic acid to form...

The reduction of the C— Br and C—1 group moments from 1.10 and 0.90 in bromo- and iodo-benzene to about 0.80 and 0.50 in 2-bromo- and 2-iodo-thiophene has been ascribed to the larger weight of resonance forms such as (8) and (9) in the thiophene series. The chlorine, nuclear, quadrupole, resonance frequencies of chloro-substituted thiophenes are much higher than those of the corresponding benzene derivatives. This has been ascribed to a relayed inductive effect originating in the polarity of the C—S o-bond in thiophenes. The refractive indices, densities, and surface tension of thiophene, alkyl- and halo-thiophenes, and of some other derivatives have been... 

Nametkin and co-workers hrst reported the alkylation of benzene derivatives with allylchlorosilanes in the presence of aluminum chloride as catalyst. " 2-(Aryl)propylsilanes were obtained from the alkylation of substituted benzenes (Ph—X X = H, CL Br) with allylsilanes such as allyldichlorosilane and allyltrichlo-rosilane.The yields ranged from 34 to 66% depending upon the substituents on the benzene ring, but information concerning reaction rates and product isomer distribution was not reported. 

Substituent effects of the benzene ring in the alkylation of benzene derivatives with I were studied by comparison with benzene it.self. These results are summarized in Table HI. 

Vinylchlorosilanes react with aromatic compounds in the presence of Lewis acid to give the alkylation products 2-(chlorosilyl)ethylarenes. In the Friedel-Crafts alkylation of aromatic compounds, the reactivity of vinylchlorosilanes is slightly lower than that of allylchlorosilanes.Friedel-Crafts alkylation of benzene derivatives with vinylsilanes to give 2-(chlorosilyl)ethylarenes was first reported by the Andrianov group (Eq. (5))." The reactivity of vinylsilanes in the... 

Vinyldialkylsilanes and vinyltrimethylsilane having no chlorine atoms do not undergo alkylation with benzene derivatives in the presenee of aluminum chloride but vinylchlorosilanes react with benzene to give the alkylation products. The reaetivities of vinylchlorosilanes decrease in the following order vinyl(methyl)di-chlorosilane > vinyltrichlorosilane > vinyl(dimethyl)chlorosilane. 

The alkylation of benzene derivatives with methyl(vinyl)dichlorosilane (3) will be described in detail. Alkylation of monosubstituted benzenes such as toluene, chlorobenzene, and biphenyl at 75-80 C for 2 h afforded the corresponding alkylated products in 50-63% yields." ... 

In the alkylation of benzene with (dichloroalkyl)chlorosilanes in the presence of aluminum chloride catalyst, the reactivity of (dichloroalkyl)silanes increases as the spacer length between the C—Cl and silicon and as the number of chloro-groups on the silicon of (dichloroalkyl)chlorosilanes decreases as similarly observed in the alkylation with (cD-chloroalkyl)silanes. The alkylation of benzene derivatives with other (dichloroalkyl)chlorosilanes in the presence of aluminum chloride gave the corresponding diphenylated products in moderate yields.Those synthetic data are summarized in Table XI. 

Hexa(oligophenyl)benzenes (e. g. 31 or 33) present one possible approach to the realization of this aim. Two efficient synthetic routes have been elaborated for the preparation of hexa(terphenyl)- and hexa(quaterphenyl)benzene. The first, involving palladium-catalyzed trimerization of diarylacetylenes [54] as the key step, was demonstrated by the synthesis of a hexakis-alkylated hexa(terphenyl)benzene derivative 31 from the corresponding bis(terphenyl) acetylene (32). The peripheral tert-alkyl substituents serve to solubilize the molecule. 

The second synthetic route consists of the coupling of hexa(4-iodophenyl)ben-zene (34) with an alkylated oligophenylboronic acid to produce a hexa(oligo-phenyl)benzene by extending the aromatic chain [52]. This route is illustrated by the reaction of hexa(4-iodophenyl)benzene (34) with an alkylated terphenyl boronic acid with formation of the hexa(quaterphenyl)benzene derivative 33. Once again, the aliphatic substituents serve to guarantee sufficient solubility. 

Bismuth tra-tri lluoromcthancsulfonate, Bi(OTf)3, and BiCh were found to be effective catalysts for the Friedel-Crafts acylation of both activated and deactivated benzene derivatives such as fluorobenzene.19 Ga(III) triflate is also effective for Friedel-Crafts alkylation and acylation in alcohols and can tolerate water.20 This catalyst is water-stable... 

Sandmeyer s synthesis of aromatic nitriles is far more elegant than the removal of water from the ammonium salts of carboxylic acids, which latter reaction is also applicable to benzene derivatives. In particular, the former synthesis permits of the preparation of carboxylic acids via the nitriles, and so provides a complete substitute for Kolbe s synthesis (alkyl halide and potassium cyanide), which is inapplicable to aromatic compounds. The simplest example is the conversion of aniline into benzoic add. The converse transformation is Hofmann s degradation (benzamide aniline, see p. 152). 

A large number of papers has been devoted to the influence of substituents upon the reactivity of benzene nucleus. Extensive studies concerning various benzene derivatives and catalysts from the platinum group metals have been published by H. A. Smith and his co-workers (for a summary see 36). The most consistent sets of data on alkylbenzenes are available from him and other groups of authors. Table VI summarizes the influence of the structure of a single alkyl group Table VII (94, 95, 97-103) summarizes the influence of the number and position of the methyl groups. Both series show very similar behavior on all metal catalysts, a decrease in rate with the size... 

Allylchlorosilanes undergo Friedel-Crafts alkylation with aromatic compounds such as benzene derivatives and ferrocene to give [p-(chlorosilyl)alkyl]arene compounds in the presence of Lewis acid catalyst. Allylsilanes containing two or more chlorine atoms on silicon react smoothly with benzene under mild conditions to give alkylation products in good yields [Eq. (15)]. In alkylations of benzene, the reactivity of the allylsilanes increases as the number of chlorine atoms on the silicon increases, but decreases as the number of methyl groups increases. Because the reactivity of allylsilanes is sensitive to the electronic nature of the substituents on the silicon atom, allylsilane selection is an important factor for alkylation reactions. 

The effects of ring constituents in the alkylation of benzene derivatives (Ph-R) with la are summarized in Table I. 

Many chromophores are suitable for use in the exciton chirality method. One of the features required for such a chromophore is its planarity or near-planarity. Nonplanar (inherently dissymmetric) chromophores would contribute to the CD spectra by other mechanisms. The other limiting factor is the position of the transition in the spectral region studied. For example, the 1B transition in the alkyl-substituted benzene chromophore appears near the short-wavelength recording limit around 200 nm, making its use in the exciton chirality method less attractive. Furthermore, the direction of polarization of the lB transition in alkyl-substituted benzene derivatives is not readily determined. In such cases calculation of the rotatory strength is more reliable than qualitative analysis. 

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