Electrophilic substitution of arenes

Aiyl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or Iron(III) chloride. 

Silver in Pd(II)-Promoted Electrophilic Substitution of Arenes (C-H Activation)... 

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... 

El and E2 mechanisms, 130 Electron-dot structures, 2 Electronegativity, 21 Electrophiles, 35, 44, 47 Electrophilic substitution of arenes, 205 in napthalene, 213 Elimination, 34 1,2-Elimination, 91 Enantiomers, 69 conditions for, 80 conformational, 80 Enamines, 315, 378 Enolsilanes, 396 Enthalpy, 36 diagrams, 40, 49 Entropy, 36... 

The only electrophilic substitution of arene chromium tricarbonyl complexes so far achieved is Friedel-Crafts acetylation. Benzene and substituted benzene chromium tricarbonyls undergo this reaction under mild conditions giving the corresponding acetyl-substituted complexes 109, 176, 218, 233, 234, 355). Substituent and conformational effects play an important role in directing the position of acetylation in arene chromium tricarbonyl complexes 176, 218, 233, 234). 

The mechanism of Friedel-Crafts acylation is believed to involve ratedetermining exo attack of the acylating species generating an intermediate 11, analogous to the Wheland intermediates generated during electrophilic substitution of arenes. Rapid loss of a proton from 11 results in formation of the neutral product 7. ... 

Electrophilic Substitutions of Arenes with Ruthenium(II) and C-H Bond Activation. .. 151... 

Other metals capable of electrophilic substitution of C-H bonds are salts of palladium and, environmentally unattractive, mercury. Methane conversion to methanol esters have been reported for both of them [29], Electrophilic attack at arenes followed by C-H activation is more facile, for all three metals. The method for making mercury-aryl involves reaction of mercury diacetate and arenes at high temperatures and long reaction times to give aryl-mercury(II) acetate as the product it was described as an electrophilic aromatic substitution rather than a C-H activation [30],... 

These compounds are more stable than the alkyl derivatives. They do not undergo thermal isomerization, and hydrolysis requires very severe conditions. The aryl cyanurates react smoothly with amines to yield 2,4,6-triamino-l,3,5-triazines. It is possible to effect electrophilic substitution of the aryl rings <59HC(l3)l,p.l7). Aryl cyanurates are hydrogenated to form cyanuric acid and the arene (equation 22) (74RTC204). 

The electrophilic substitution of P-diketonate complexes appears to occur as for arenes, and a process involving initial coordination of the electrophile, followed by an intramolecular group transfer, has not been observed, although it has been postulated for the reaction of copper(II) acetylacetonate with thioacetals (equation 14).31... 

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-... 

The second mechanism which cannot always be strictly differentiated from the first one 41) should be considered as a typical electrophilic substitution of the arene by the iiigh valent metal compound. The latter is still more electrophilic with tri-fluoroacetate than with acetate anions. The organometallic arene intermediates were isolable in some cases 43). In the second step, the metallic residue is substituted by the acetate or trifluoroacetate ion. 

Within the year a wide range of photoreactions in which an aromatic residue undergoes change in substitution has been published. As previously, the diversity of the various processes makes any classification of the reactions unrealistic, and so their order of presentation here is somewhat arbitrary. Aromatic photosubstitution reactions have been reviewed by Parkanyi although the treatment is not extensive, the processes of free radical, electrophilic, and nucleophilic photoinduced substitutions of arenes are well covered.Arene photoreactions initiated by electron transfer with electron donors or acceptors are the subject of a review by Pac and Sakurai. The requirements for the efficient photogeneration of the ion radicals are considered and the synthetic utility of the photoreactions, which include reduction, cyanation, and amination, is discussed. 

The synthesis of various azido compounds with arene sulfonyl azides has, at least in a formal sense, to be considered as an electrophilic substitution of hydrogen by an azide function.By this route a number of amines, cyclic derivatives and bridgehead compounds included, after deprotonation with NaH or Grignard reagents, could be transformed into azides in high yield on treatment with arene sulfonyl azides." Details cannot be given here. Enolates can also be converted into azido derivatives in a very similar way, via triazene intermediates. Evans et alP did recently investigate the influence of the countercation, the structure of the transfer azide and work-up conditions on this process. 

Polycyclic arene(tricarbonyl)chromium complexes.h These complexes arc best prepared by treatment of polycyclic arcnes with (NHj),CT(CO)37 and BFj ethcratc. As in complcxation with Cr(CO), the terminal or most aromatic ring is complexed selectively. However, the lower temperatures used in the newer method are advantageous with thermally labile polycyclic arcnes. These complexes are useful for substitution reactions at positions that arc not available by electrophilic substitution of the arenc directly. One such reaction is hydroxylation effected by simultaneous reaction with a base (BuLi or I. DA) and tributoxyborane (excess) followed by H2O2/HOAC workup. Rcgiosclectivc silylation is effected by reaction of the complex with LiTMP and (CHj SiCI with... 

In this chapter, we will consider the reactions of C-H compounds, such as alkanes, arenes as well as some others, with platinum complexes containing mainly chloride ligands. The reactions of alkanes with platinum(II) complexes have been the first examples of true homogeneous activation of saturated hydrocarbons in solution. Complexes of Pt(II) exhibit both nucleophilic and electrophilic properties, they do not react with alkanes via a typical oxidative addition mechanism nor can they be regarded as typical oxidants. Due to this, it is reasonable to discuss their reactions in a special chapter which is a bridge between previous chapters (devoted to the low-valent complexes) and further sections of the book that consider mainly complexes in a high oxidation state. Chloride cortplexes of platinum(IV) are oxidants and electrophiles and they will constitute the first subjects in our discussion of processes of electrophilic substitution in arenes and alkanes as well as their oxidation. 

The electrophilic substitution of hydrogen in arenes is known both for nontransition - Hg(ll), Tl(lll), Pb(IV) - and transition metals, such as Au(lll), Pd(II), Pt(IV), Rh(III). All these reactions apparently proceed with the intermediate formation of Wheland complexes. Some parameters for these reactions are summarized in Table Vlll. 1 [2]. 

It was demonstrated by Heck in the late 1960s that arylpaUadium salts, prepared by trans-metallation of organomercury compounds, constitute useful reactants in various vinylic substitution reactions.f t Independently, Moritani, Fujiwara, and colleagues conducted similar vinylic substitutions, but generated the organopalladium intermediates by direct electrophilic palladation of arenes.t ° In these reactions the palladium(II) salt employed is reduced to paUadium(O) (Scheme ). ... 

The highly impressive and remarkably stereospecific cobalt-catalysed diyne cooligomerization reaction, previously used to synthesize tricyclic systems, has now been applied to steroidal structures (Scheme 27), producing the shortest approach to the steroid nucleus, from an acyclic precursor, to date. Electrophilic substitution of the o-bis(trimethylsilyl)arene unit and the introduction of heteroatoms into the precursor (118) should expand the range of steroidal products considerably. 

In addition to benzylations, palladium-catalyzed C-H allylation reactions have also been described, but they usually involve the electrophilic substitution of an electron-rich (hetero)arene with a 7t-allyl palladium complex and therefore deviate from the scope of this chapter [41, 42]. In contrast, the palladium/copper-catalyzed allylation of polyfluoroarenes with allyl carbonates (Scheme 19.27) has been reported to occur through a different mechanism [43]. Thus, base-induced cupration of the arene would give rise to intermediate 15 that was previously characterized by X-ray crystallography [44]. Attack of the in sitw-generated JT-allyl... 

---