Arene Lewis Acid Activated

In the presence of silicon Lewis acids, aryliodine bis(trifluoroacetate)s mediate efficient oxidative homocouplings of pyrroles and thiophenes to give 2,2 -bispyrroles and 2,2 -bisthiophenes, respectively [158]. The Lewis acids activate the hypervalent iodines (III) for single electron oxidation of the heteroarenes. The cation radicals thus generated add to the unreacted arenes to form the homocoupling products (Scheme 9.67). 

Synthetic utility of stereoselective alkylations in natural product chemistry is exemplified by the preparation of optically active 2-arylglycine esters (38). Chirally specific a-amino acids with methoxyaryl groups attached to the a-carbon were prepared by reaction of the dimethyl ether of a chiral bis-lactam derivative with methoxy arenes. Using SnCl as the Lewis acid, enantioselectivities ranging from 65 to 95% were obtained. 

The reactivity of the closely related system TpMe2PtMeH2 toward electrophiles in arene solvents has also been reported recently (68). The boron-based Lewis acid B(C6F5)3 induced elimination of methane and formation of an aryl(dihydrido) platinum(IV) complex via arene C-H activation (Scheme 17, A -> C). The active acid may be either B(C6F5)3 or alternatively a proton generated from B(C6F5)3 and trace water. It was proposed that the acid coordinates to a pyrazole nitrogen (shown in Scheme 17, B) forming an intermediate five-coordinate platinum(IV) complex, which readily eliminates methane. 

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

Non-activated arenes react only under more severe conditions, - or in the presence of a Lewis acid. In the last case, however, the reaction does not stop at the trichloride step, and is more appropriate for the preparation of diaryltellurium dichlorides (see Section 3.9.2.3). 

This overview impressively demonstrates that Bi(III) salts are not only versatile Lewis acid catalysts for the activation of cr-donors, including benzyl and propargyl alcohols, but also efficient catalysts for the activation of Ji-donors such as styrenes or alkynes. In recent years, various environmentally benign bismuth-catalyzed methods have been developed for the alkylation of arenes, heteroarenes,... 

This Lewis acid-catalyzed electrophilic aromatic substitution allows the synthesis of alkylated products via the reaction of arenes with alkyl halides or alkenes. Since alkyl substituents activate the arene substrate, polyalkylation may occur. A valuable, two-step alternative is Friedel-Crafts Acylation followed by a carbonyl reduction. 

Study of the reactivity of aromatic C-H bonds in the presence of transition metal compounds began in the 1960s despite the quite early discovery of Friedel-Crafts alkylation and acylation reactions with Lewis acid catalysts. In 1967, we reported Pd(II)-mediated coupling of arenes with olefins in acetic acid under reflux [1], The reaction involves the electrophilic substitution of aromatic C-H bonds by a Pd(II) species, as shown in Scheme 2, and this is one of the earliest examples of aromatic C-H bond activation by transition metal compounds. Al-... 

Reaction mechanism It is generally admitted that, over zeolites, acetylation of arenes with AA is catalysed by protonic acid sites. Comparison of the activity of a series of dealuminated HBEA samples allows one to exclude any direct participation of Lewis acid sites in 2-MN acetylation with AA. Indeed, two HBEA samples with similar protonic acidities but with very different concentrations of Lewis acid sites (170 and 16 pmol g ) have practically the same acylating activity.1271 The role of Brpnsted sites is also clearly expressed in Spagnol et a/.131... 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

A spectacular activation of the chiral zirconium-BINOL Lewis acid complex was achieved by the addition of the (achiral ) r-butyl-calix[4]arene. Less than 2% of the catalyst were sufficient in the enantioselective allylation of various aldehydes by allyltributyltin to reach enantiomeric excesses of more than 90%, see Casolari, S. Cozzi, P. G. Orioli, P. Tagliavini, E. Umani-Ronchi, A. Chem. Commun. 1997, 2123-2124. 

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