Electron-deficient arenes

In the absence of transition metal catalysts, InBr3 was found by Peppe et al. to catalyze the dimerization of vinylarenes [281]. A variety of arenes, electron deficient to electron rich, was studied. The extent of reaction of the proposed benzylic carbocation, and hence the dominant product, depended on reaction temperature and time (Figure 8.127). 

The intra- and intermolecular ring expansion of arenes by an electron-deficient nitrene is a popular and versatile synthetic route to 1H- and 3//-azepines (cf. Houben-Weyl, Vol. 5/2 b p544ff). 

Nucleophilic substitutions of halogen by the addition-elimination pathway in electron-deficient six-membered hetarenes by sulfinate anions under formation of sulfones have been described earlier120. The corresponding electron-poor arenes behave similarly121 (equation 30). A special type of this reaction represents the inverse Smiles rearrangement in equation 31122. 

The well-defined copper complexes 94 and 95 (Fig. 2.16) have been used as catalysts for the intermolecular hydroamination of electron-deficient alkenes [Michael acceptors, X=CN, C(=0)Me, C(=0)(0Me)] and vinyl arenes substituted... 

Hydrothiolations (addition of H-SR across the CC multiple bond) of alkynes, electron-deficient aUcenes and electron-deficient vinyl arenes have been catalysed by NHC complexes of Ni and Cu, respectively [Scheme 2.17a-c],... 

Supramolecular chirality can be introduced in columns of discotic molecules by using specific interactions, apart from the arene-arene interactions. To achieve this, a study with chiral dopants has been performed.73 Hexa-n-hexyloxytriphenylenes were mixed in dodecane solutions with a variety of chiral electron-deficient dopants and the resulting charge transfer complexes... 

Yields of the arenes from the hydrazines are variable and generally low (<40%) for simple aryl compounds, although acceptable yields (50-90%) are obtained for the 7t-electron-deficient heteroaryl derivatives. In this respect, the procedure has some utility, as the more usual reductive conversion of haloheteroarenes into the parent... 

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. 

Some Schrock-type carbene complexes, i.e. high-valent, electron-deficient, nucleophilic complexes of early transition metals, can undergo C-H insertion reactions with simple alkanes or arenes. This reaction corresponds to the reversal of the formation of these carbene complexes by elimination of an alkane (Figure 3.36). 

The ring expansion of arenes by electron-deficient singlet nitrenes is by far the most versatile synthetic route to 1H -azepines. The first l//-azepines were prepared independently in 1963 by Hafner, and by Lwowski, and their coworkers. They found that ethoxycarbonyl-nitrene (Scheme 26, path a R=C02Et), generated by photolysis of ethyl azidoformate, adds to benzene to give initially the unstable azanorcaradiene (227), electrocyclic ring... 

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. 

Arene Substitution via Nucleophilic Addition to Electron Deficient Arenes... 

The first arene-transition metal complexes were prepared in the 1950s1314 and it was immediately recognized that the added polarizibility or electron deficiency would promote addition of nucleophiles to the arene ligand. A number of cyclohexadienyl complexes were characterized following nucleophilic addition, but the question of inducing the ipso hydrogen to depart was not answered (equation 1). 

The relative rates of each step depend critically on the nature of M and of the nucleophile. More reactive nucleophiles and more reactive complexes disfavor equilibration, (k k-1) and the process can stop with formation of the first cyclohexadienyl intermediate (9). Equilibration leads through (10) to the substitution product. The overall order of reactivity for electron deficient arenes is 2,47 [(arene)Mn(CO)3]+ > 2,4-(N02)C6H3Cl > [(arene)FeCp]+ 4-(N02)C6H4Cl > [(arene)Cr(CO)3]. 

The Meerwein arylation is at least formally related to the atom transfer method because a net introduction of an aromatic ring and a chlorine across a double bond is accomplished (Scheme 62). Facile elimination of HC1 provides an efficient route to the kinds of substituted styrenes that are frequently prepared by Heck arylations. Standard protocol calls for the generation of an arene diazonium chloride in situ, followed by addition of an alkene (often electron deficient because aryl radicals are nucleophilic) and a catalytic quantity of copper(II) chloride. It is usually suggested that the copper salt operates in a catalytic redox cycle, reducing the diazonium salt to the aryl radical as Cu1 and trapping the adduct radical as Cu11. 

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