Arenes as Nucleophiles

Accordingly, the pyrylium ion 7 reacts with CH-acidic compounds and activated arenes as nucleophiles to give new pyrylium systems with dehydrogenation. 

The ring opening of aziridine using arenes as nucleophiles was first reported by Yadav et a. [89]. Reaction is catalyzed by In(OTf)3 and regioselectivity is highly favored toward attack on the benzylic carbon of the aziridine substrate (Figure 8.46). 

Both methodologies described in this chapter impressively illustrate the versatility of Bi(OTf)3 as a mild Lewis acid catalyst. Most notably, in addition to arenes, other nucleophiles such as amines, amides, alcohols and thiols could as well be used in this transformation. 

The concept of a diastereoselective Friedel-Crafts alkylation of a-chiral benzyl alcohols was first examined by Bach and coworkers [62, 63]. The initial protocol required stoichiometric amounts of strong Brpnsted acids like HBF4 and was followed by a more valuable methodology in which catalytic amounts of AuC L were employed for the diastereoselective functionalization of chiral benzyl alcohols [64], Beside arenes, allyl silanes, 2,4-pentanediones and silyl enol ethers have been used as nucleophiles. Depending on the diastereodiscriminating group and on the catalyst (Brpnsted or Lewis acid), the authors observed either the syn or the anti diastereoisomer as the major product. 

On the basis of previous results, the Rueping group started to study alkylations with activated alkenes and nucleophiles other than arenes. Recently, Lewis- and Brpnsted-acid-catalyzed hydroalkylation procedures with 1,3-dicarbonyl compounds as nucleophiles have been developed [69, 70]. These reactions primarily utilized Pd... 

Enol or Arene Double Bonds as Nucleophiles Enolate alkylation has been a powerful... 

The chemistry of all fullerenes is dominated by their ability to react as poorly conjugated and electron-deficient 2ir alkenes they show very few properties typical of dienes or arenes (5). In addition, because of the high cage stability, they never undergo substitutions. C60 shows behavior similar to that of a monosubstituted alkene such as vinyl chloride or acrylate. All fullerenes readily add to electron-rich species such as nucleophiles, bases, radicals, or reducing agents. They are, for example, perfect dienophiles for Dieles-Alder reactions. The types of reactions undergone by fullerenes are illustrated in Scheme 1. 

Very recently, the reaction of styrene 3,4-epoxide (245) with ethyl mercaptan has been reported.148 A mixture of 2-, 3-, and 4-ethylthiostyrenes is formed in the ratio of 1 9 7 along with small amounts (18%) of 4-vinylphenol. These results can be explained as nucleophilic attack on the intact arene oxide and the reaction of the zwitterion formed by the spontaneous reaction. 

This reaction is a powerful tool and represents an alternative for the synthesis of substituted arenes difficult to prepare via classical electrophilic or nucleophilic aromatic substitution. Using bi- or polyfunctional arenes as starting materials, this reaction affords novel organoiron polymers [76] (Scheme 1.35). 

Dicationic bis(arene)iron complexes are prepared from Fe(II) salts using the corresponding arene as solvent in the presence of aluminum trichloride at elevated temperatures. Because of the double positive charge, they easily add two nucleophiles at one arene ring. 

Benzylic electrophiles bearing electron-withdrawing groups at the arene do not always yield the expected products of nucleophilic substitution on treatment with a nucleophile. One important side reaction is the dimerization of these compounds to yield 1,2-diarylethenes (stilbenes). This dimerization does not require such highly activated systems as the example sketched in Scheme 4.28, but can even occur with, for example, 2- or 4-nitrobenzyl chloride [120, 121]. The latter compounds are converted into the corresponding stilbenes by treatment with KOH in ethanol [120]. Di-arylmethyl halides behave similarly and can yield tetraarylethenes on treatment with a base. These reactions presumably proceed via the mechanism sketched in Scheme 4.27, in which the amphiphilic character of the nitro group plays a decisive role (metalated nitroalkanes or 4-nitrobenzyl derivatives can act as nucleophiles and as electrophiles). 

The metal-bound carbon atom in organopalladium(II) complexes can formally react either as an electrophile or as a nucleophile. Treatment of arylpalladium(II) complexes with alkyl halides, for example, yields products of homo- or cross-coupling, possibly via intermediate formation of hexacoordinated Pd(IV) complexes [31,33] (Scheme8.1). Treatment of the same type of complex with alkyl Grignard reagents or other carbon nucleophiles, on the other hand, also yields the corresponding alkyl arenes via nucleophilic displacement of a ligand followed by reductive elimination (Scheme 8.1). 

Of particular interest is the observation that in certain cases products such as 24 resulting from domino processes are obtained After the formation of the furan, evidently a double Michael-type addition of these intermediates to the remaining starting material 23 can take place at the unsubstituted 5-position. Preliminary experiments to investigate scope and limitations of such addition reactions in the presence of gold salts also confirm the applicability to the functionalization of other electron-rich arenes (Scheme 6) Besides furans, azulene 28 and di- and trialkoxybenzene are suitable as nucleophiles for the reaction with unsaturated carbonyl compounds [14]. For instance, 2-methylfuran (25) reacts at the reactive 5-position with methyl vinyl ketone 26 to give the addition product 27, and with azulene 28 a twofold... 

As might be expected for a polyene metal cation, the arene ligand in complexes (395) is unreactive toward normal Friedel-Crafts substitution. In turn, the FeCp+ adjunct acts as a good electron-withdrawing group and facilitates attack by hydride and carbanion nucleophiles on the benzene ligand to form ( -cyclohexadienyl)FeCp complexes. For LiAlH4 as nucleophile and at low... 

When benzylic alcohols are the substrates, arenes behave as nucleophiles, as shown in equation (123) and for the analogs in the 1,4-addition.These reactions have been performed with alcohols that are activated by an arene substituent and another group that could be an olefin, an alkyne, or other arene (equation 140). In the case of dimerization reported by Hashmi (equation 141), primary benzylic alcohols produce dimers of the type in equation (141) that arise from the activation of this moiety, together with the expected cyclization products. In a very recently reported example, aryl-alkyl ethers are obtained by reaction with NaAuCU. ... 

The scope of this reaction is limited to electron-rich arenes and heteroarenes such as thiophenes, pyrroles, furans, indoles, and alkoxybenzenes as nucleophilic partners, corresponding to a Mayr ir-nucleophilicily parameter N>-1 [75-78], Electron-neutral to electron-deficient iodo(hetero)arenes are suitable electrophilic partners. Aryl halides or pseudohalides that are less reactive towards oxidative addition (Br, Cl, OTf) are not sufficiently reactive partners in this reaction. The reactivity of sterically hindered and/or ortho substituted iodoarenes has not been demonstrated. However biaryls bearing one ortho substituent of relatively small steric demand (e.g., from methoxybenzene or /V-mcthylindole) have been prepared. 

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