Alkylation, of benzene derivatives

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... 

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. 

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

Friedel-Crafts Alkylation Reactions. The activation of glyoxylate esters,trifluoromethyl pyruvate esters, and unsaturated a-ketoesters by catalyst 2 converts these materials into effective electrophiles for asymmetric Friedel-Crafts alkylation reactions with activated arenes (eqs 16 and 17). In fact, bis(triflate) (2) is far superior to tbe bis(hexafluoroantimonate) complex at catalyzing the enantioselective alkylation of benzene derivatives. Aniline and anisole derivatives both give the reaction, as do heterocyclic aromatic compounds such as indole and furan. 

Cations formed by alkylation of benzene derivatives have also been characterized. 

Reversible nature of Friedel-Crafts alkylation of benzene derivatives has been applied to remove ortho-tert-huty group, which is an essential substituent to isolate an atropisomeric anilide derivative as a stable compound at an ambient temperature. Simpkins and coworkers reported that during the AICI3 catalyzed trans-tert-butylation reaction of the particular enamide compound (12) in benzene, enamide function undergoes Friedel-Crafts alkylation to give phenylated product (13) in good yield (Scheme 6.12) [14]. Thus, in this case, Friedel-Crafts alkylation and dealkylation occur at the same time. 

Immobilized ionic liquids were shown to be used as acid catalysts for the alkylation of benzene derivatives with alkene. For example, H older ich and co workers demonstrated that the ionic liquid prepared by mixing 1 -butyl-3-methylimidazolium chloride ([bmim]Cl) and AICI3, AL-IL effectively catalyzed the alkylation of benzene with dodecene at 80 °C to give monoalky la ted product with 98% selectivity at 99% conversion of dodecene (Scheme 6.13) [15]. 

Rate constants for homolytic alkylation of several protonated heterocyclic bases with alkyl radicals have been measured and the values obtained (10 — 10 lmor s ) contrasted with those from alkylation of benzene derivatives (10 1 mol This great reactivity of protonated heteroaromatic bases towards alkyl radicals has been put to preparative use in the reactions of l-acetyl-2-pyrrolidinyl and l-formyl-2-pyrrolidinyl radicals with pyridazine, giving nicotine analogues (Scheme 1). ... 

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. 

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. 

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. 

In contrast to earlier known imines, those imines derived from a-(methoxymethyl)benzene-ethanamine, which allow formation of a rigid chelate by additional coordination of the lithium with the methoxy group, enabled the preparation of a-alkylated cyclic ketones in very high enantiomeric excesses (90-99% ee)7,8. However, alkylations of imines derived from medium ring ketones were accomplished in 30-82% ee9. The alkylation of acyclic ketones was performed with enantiomeric excesses of more than 75 % and, in the case of the imine derived from 4-heptanone, proceeded with complete asymmetric induction10. 

Dewar,120 as well as Brown and Olah, respectively, raised the importance of initial n complexing of aromatics in alkylations.109121 Relevant information was derived from both substrate selectivities (usually determined in competitive alkylations of benzene and toluene, or other alkylbenzenes), and from positional selectivities in the alkylation of substituted benzenes. Olah realized that with reactive alkylating agents substrate and positional selectivities are determined in two separate steps. 

Stereoselective Friedel-Crafts alkylation. 4 Alkylation of benzene with methyl (S)-2-(mesyloxy)propionate, derived from (S)-lactic acid, under Friedel-Crafts conditions (2 equiv. of A1C13) affords methyl (S)-phenylpropionate in 50-80% chemical yield and as high as 97% optical yield. Unfortunately extension to other aromatics results in mixtures of isomeric products. 

Since alkylation greatly reduces the ionization potential of benzene derivatives it was hoped that alkylated benzene cations could be prepared by oxidation in sulphuric acid (Bolton and Carrington, 1961b Hulme, unpublished results). However, although oxidation of p-xylene... 

Ortho photocycloadditions of benzene derivatives to maleic anhydride have been tabulated in Table 1. Only the structures of the primary ortho adducts are given, but these are not the isolated adducts They always undergo endo [2 + 4] cycloaddition with maleic anhydride, yielding 1 2 adducts. An interesting feature to be seen from Table 1 is that substituents on the benzene (alkyl, phenyl, or halogen) always turn up at the position most remote from the site of addition. In view of the different nature of these substituents, it seems that steric rather than electronic factors are responsible for this regioselectivity. 

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