Arenes benzylation

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). 

The cavity of these calixarenes is usually too small to accommodate fullerenes, and is confirmed by solution, unless they have extended cavity walls as in calix[4]-naphthalenes. Nevertheless, some calix[4]arenes were shown to form stable crystalline complexes with C60 with the fullerene eno- to the calixarene cavity, and they include p-Ph-calix[4]arene, which has a toluene molecule in the cavity, the overall structure dominated by fullerene-fullerene and eso-calixarene fullerene interactions p-Br-calix[4]arene propyl ether, the structure showing very close interfullerene contacts in a columnar structure (Fig. 2d), which most likely results in opposing induced dipoles from the unidirectionally aligned calix-arenes " and /7-I-calix[4]arene benzyl ether, where the fullerenes are ordered without appreciable interfullerene interactions. There is also a calix[4]resorcinarene, R = CH2CH2Ph, 3, which has a molecular capsule derived from head-to-head hydrogen bonding of two resorcinar-enes and propan-2-01 molecules, with the fullerenes also... 

When the benzene ring is named as a substituent the word phenyl stands for CgH5— Similarly an arene named as a substituent is called an aryl group A benzyl group IS CgHjCH,—... 

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... 

Electrostatic potential map—cont d ammonia, 145 aniline, 925 anilinium ion, 925 anisole, 111 18]annulene, 535 arene, 74 azulene, 541 benzaldehyde, 565, 704 benzene, 44, 521, 565 benzenediazonium ion, 945 benzoquinone, 631 benzyl carbocation, 377 benzyne, 576 borane, 223... 

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... 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

CpFe(arene)+ Mono-cations Benzylic Deprotonation.63... 

When the iron sandwich complex bears an arene substituent with at least one benzylic hydrogen, the acidity of the latter is enhanced by the 7t-complexation to the 12e fragment FeCp+. The pKa of the conjugate acid of superoxide radical... 

Similarly, when both the Cp and arene ligands are permethylated, the reaction of 02 with the Fe1 complex leads to C-H activation of the more acidic benzyl bond [57]. When no benzylic hydrogen is present, superoxide reacts as a nucleophile and adds onto the benzene ligand of the FeCp(arene)+ cation to give a peroxocyclohexadienyl radical which couples with a Fe Cp(arene) radical. A symmetrical bridging peroxo complex [(Fe"Cp)2(r 5-C6H60)2] is obtained. The C-H activation reactions of the 19e Fe1 radicals BH can be summarized as follows... 

The acidity of benzylic protons of aromatics complexed to transition-metal groups was first disclosed by Trakanosky and Card with (indane)Cr(CO)3 [61]. Other cases are known with Cr(CO)3 [62], Mn(CO)3 [63], FeCp+ [64, 65], and Fe(arene)2+ [31, 66] but none reported the isolation of deprotonated methyl-substituted complexes. We found that deprotonation of the toluene complex gives an unstable red complex which could be characterized by 13C NMR ( Ch2 = 4.86 ppm vs TMS in CD5CD3) and alkylated by CH3I [58] Eq. (13) ... 

When the nucleophile is a stabilized carbanion such as the enolate of acetylacetone, 1-benzoylacetophenone, diethylmalonate, or ethyl acetatoacetone, the reaction proceeds similarly. The monosubstituted complex is isolated as long as it contains an acidic hydrogen in the benzylic position. In addition, for the case of diketones CH2(COR)2 (R = Me, Ph, OEt), a deacetylation is observed in an acidic medium [92,93]. These features are the same as described above in the case of the substitution of Cl by stabilized carbanions in monochloroaromatics (the second chlorine being an inert arene substituent [99] Scheme XVII, Eq. (31) and Tables 10 and 11. 

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

Avramoff et al. have already reported that the reaction of hydrocarbons such as toluene with tetramethylammonium tribromide (TMA Br3) in benzene, in the presence of benzoyl peroxide at room temperature gave benzylic bromination products (ref. 21). However, TMA Br3 is not easy to handle in comparison with the stable BTMA Br3 because of its hydroscopic character. Furthermore, as shown in their literature, a large excess of TMA Br3 is necessary to brominate arenes. 

The oxidation by strains of Pseudomonas putida of the methyl group in arenes containing a hydroxyl group in the para position is, however, carried out by a different mechanism. The initial step is dehydrogenation to a quinone methide followed by hydration (hydroxylation) to the benzyl alcohol (Hopper 1976) (Figure 3.7). The reaction with 4-ethylphenol is partially stereospecific (Mclntire et al. 1984), and the enzymes that catalyze the first two steps are flavocytochromes (Mclntire et al. 1985). The role of formal hydroxylation in the degradation of azaarenes is discussed in the section on oxidoreductases (hydroxylases). 

Dehydration reactions catalysed by NHC complexes have been reported where a new C-C bond is formed. Peris has used [Ir(OTf)2Cp (NHC)] complexes including compound 35 to benzylate arenes with alcohols and other reagents [14]. For example, the dehydrative C-C coupling of benzyl alcohol 8 with toluene 33 is catalysed by 0.1 mol% of 35 to give a mixture of benzylated products 34 (Scheme 11.8). 

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). 

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