Arenes => substituted benzenes

Arene (e.g. benzene, substituted benzenes) cycloheptatriene, cycloocta-l,3,5-triene Tropylium (cycloheptatrienyl)... 

Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium- l-arylidene compounds (2 2) (Figure 3). The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction is in accord with statistical attack at all available sp3 C-H bonds. 

There is no experimental evidence for 7r-complexation of arenes to gold centers in the condensed phase. Quantum-chemical calculations were carried out on various levels of theory for 1 1 and 2 1 complexes of benzene and substituted benzenes with Au+ in the gas phase. For all model systems investigated, it has been predicted that an -coordination to a single carbon atom is the ground state of the cation (Cy-symmetry for [(C6H6)Au]+ and 6 -symmetry for [(C6H6)2Au] ). Similar results were obtained for hexafluorobenzene, for which a coordina-... 

In order to assess how the bulk of the arene influences the structure of such stacked assemblies, the adducts of 137 with toluene, o-, m-, and -xylenes, and mesitylene have been synthesized and structurally characterized.236 These adducts tend to form extended binary stacks similar to those found in [137-benzene], The substituted benzene molecules adopt an apparently random orientation with respect to the trinuclear core of 137, thus suggesting that the binding might be... 

The complexes Cr(CO)3L, with L = phenanthrene, naphthalene, or anthracene, are more active for diene hydrogenation than with L = substituted benzenes (see also Section VIII), and this is attributed to an easier displacement of the arene by the diene substrate, the phenanthrene type being asymmetrically bonded, having two longer and more readily cleaved chromium-carbon bonds (198, 199). 

The in i /V -generated CpeZ Bu Cl converts the arene into a zirconocene-benzyne complex which undergoes C-C bond formation with a nitrile to form an intermediary azazirconacycle (Equation (14)). The acidic hydrolysis of the latter species provides the corresponding 3-acyl-l-substituted benzene derivatives. 

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

The synthetic potential of transition metal atoms in organometallic chemistry was first demonstrated by the formation of dibenzenechrom-ium (127). Apart from chromium, Ti, V, Nb, Mo, W, Mn, and Fe atoms each form well-defined complexes with arenes on condensation at low temperatures. Interaction has also been observed between arenes and the vapors of Co, Ni, and some lanthanides. Most important, the synthesis of metal-arene complexes from metal vapors has been successful with a wide range of substituted benzenes, providing routes to many compounds inaccessible by conventional reductive preparations of metal-arene compounds. 

Friedel-Crafts alkenylation of arenes with various phenylacetylenes to yield 1,1-diarylalkenes was reported. Alkyl-substituted benzenes and naphthalene react with phenylacetylene in the presence of zeolite HSZ-360 to afford the products in good to excellent yields and selectivities 415... 

The order of reactivity of different alkyl-substituted benzenes is toluene > ethylbenzene > isopropylbenzene, which is opposite to the general reactivity expected from the C—H bond energies. This was explained in terms of an initiating electron transfer equilibrium between Co(OAc)3 and the arene as the rate-determining... 

Pt2(dba)3-catalyzed reaction of a series of substituted benzenes with o-bis(dimethylsilyl)benzene.95 This intriguing process gives high yields of o-(aryldimethylsilyl)(dimethylsilyl)benzenes for a variety of arenes, but appears limited to this particular silane [Eq. (33)]. 

Competition reactions between benzene and various substituted benzenes reveal that electron-withdrawing substituents on the arene increase reaction rates. All ortho, meta, and para forms of the arylsilanes are obtained. The major component of the arylsilane product formed in reactions of arylsilanes with electron-withdrawing groups, such as chlorobenzene, and anisole, is the ortho isomer. In contrast, toluene, which bears a less electronegative substituent, leads to meta isomer as major product. 

The selectivity of chlorination reactions carried on in solution is increased markedly in the presence of benzene or alkyl-substituted benzenes because benzene and other arenes form loose complexes with chlorine atoms. This substantially cuts down chlorine-atom reactivity, thereby making the chlorine atoms behave more like bromine atoms. 

Abbreviations arene, i/6-benzene or substituted benzene derivative bipy, 2,2 -bipyridyl Bu, Bu", Bu, iso-, n-, or rerf-butyl COD, 1,5-cyclo-octadiene Cp, /5-C5H5 DAD, dimethyl-acetylene dicarboxylate dam, 1,2-bis(diphenylarsino)methane DBA, dibenzylideneacetone DMF, A. A -dimethylformamide dpe, l,2-bis(diphenylphosphino)ethane dpen, os-l,2-bis(di-phenylphosphino)ethylene dpm, 1,2-bis(diphenylphosphino)methane ESR, electron spin resonance F6-acac, hexafluoroacetylacetone FN, fumaronitrile MA, maleic anhydride Me, methyl MLCT, metal ligand charge transfer phen, 1,10-phenanthroline Pr , Pr", iso- or n-propyl py, pyridine RT, room temperature TCNE, tetracyanoethylene tetraphos, (Ph2PCH2CH2)jP THF, tctrahydrofuran Xylyl, 2,6-Me2C6H3. 

Detailed study of the mechanism of solvolysis of a number of non-K-region arene oxides like 86,90 alkyl substituted benzene oxides,91 45,47,88 and 4888 has been carried out. They present a simple and consistent picture (Fig. 3). Below pH 6 all of them show general acid catalysis, and above pH 6 the rate remains constant with an increase in pH. The pH dependence of aromatiza-tion of 45 is described in terms of two independent reactions taking place... 

The 1,2 and 3,4 regioselectivity of the ortho addition of an alkene to an arene substituted with a strong donor substituent is, according to Houk, dominated by interaction of the benzene S orbital with the alkene -tt HOMO, because the A -tt interaction would result in a 2,3 ortho adduct. For an arene substituted with a strong donor, the donor substituent, rather than the approaching alkene, will determine the symmetry of the orbitals [188],... 

Arenes usually undergo electrophilic substitution, and are inert to nucleophilic attack. However, nucleophile attack on arenes occurs by complex formation. Fast nucleophilic substitution with carbanions with pKa values >22 has been extensively studied [44]. The nucleophiles attack the coordinated benzene ring from the exo side, and the intermediate i/2-cvclohexadienyl anion complex 171 is generated. Three further transformations of this intermediate are possible. When Cr(0) is oxidized with iodine, decomplexation of 171 and elimination of hydride occur to give the substituted benzene 172. Protonation with strong acids, such as trifluoroacetic acid, followed by oxidation of Cr(0) gives rise to the substituted 1,3-cyclohexadiene 173. The 5,6-trans-disubstituted 1,3-cyclohexadiene 174 is formed by the reaction of an electrophile. 

Anodic treatment of 1,2- or 1,4-dihydroxy-substituted benzenes to form the corresponding quinones or masked congeners is well known, since they represent valuable synthetic intermediates [64]. Benzoquinone ketals of electron rich arenes like 18 can be challenging since the oxidative aryl-aryl coupling reaction usually competes. When using BDD anodes the benzoquinone ketal 19 is obtained in an almost quantitative manner, demonstrating the superior properties of this electrode material. Despite the basic conditions, no deblocking of the silyl-protected phenol moiety is observed [65] (Scheme 9). 

Due to the relatively slow addition of aryl radicals to substituted benzenes [7], inter-molecular Gomberg-Bachmann type arylations often require favorable reaction conditions in which the substrate arene is used as solvent to be efficient. To achieve good control and selectivity in the addition step, radical biphenyl synthesis can alternatively be conducted in an intramolecular fashion by taking advantage of a removable tether. In the first examples, sulfonamides, sulfonates [130], and azetidinones [131] were employed as linkers between the two aryl moieties. Recently, phosphinates [132] and siloxanes [133] have been used for this purpose (Scheme 25). 

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