Arene complexes electrophilic substitution

Fig. 12. Li near correlations of the relative reactivity of arenes in electrophilic substitution reactions (log k/kf) with the optical transition energies in the accompanying CT complexes. Data from ref. [62b],...

In arene complexes, there is a charge transfer from the ring to the metal. Therefore, coordinated arenes undergo electrophilic substitutions with greater difficulty than free arenes. Owing to the sensitivity to acids and easy oxidation of bisarene complexes, it has not been possible to perform sulfonation, nitration, and mercuration. However, the acylation reaction of chromium derivatives [Cr(arene)(CO)3] can be carried out ... 

Protonated polymethylbenzenes281 and the chlorohexamethylbenzenium cation,282 intermediates in aromatic electrophilic substitutions known as Wheland intermediates, have been isolated as crystalline salts, allowing investigators to obtain their X-ray crystal structure. Nitrosoarenium a complexes of various arenes were directly observed by transient absorption spectroscopy.283 Kochi presented a method combining appropriate instrumental techniques (X-ray crystallography, NMR, time-resolved UV-vis spectroscopy) for the observation, identification, and structural characterization of reactive intermediates fa and n complexes) in electrophilic aromatic substitution.284... 

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... 

To some degree we have oversimplified electrophilic substitution by neglecting the possible role of the l l charge-transfer complexes that most electrophiles form with arenes (see Section 10-30 for discussion of analogous complexes of alkenes) ... 

Arenes are inert to nucleophilic attack and normally undergo electrophilic substitution. However, arenes coordinate to Cr(CO)6 to form the i/fi-arenechromium tricarbonyl complex 79, and facile nucleophilic attack on the arene generates the anionic jy5-cyclohexadienyl complex 80, from which substituted arene 81, or cyclohexadiene is obtained by oxidative decomplexation. In this reaction, strongly... 

Arenes usually undergo electrophilic substitution, and are inert to nucleophilic attack. However, nucleophile attack on arenes occurs by complex formation. Fast nucleophilic substitution with carbanions with pKa values >22 has been extensively studied [44]. The nucleophiles attack the coordinated benzene ring from the exo side, and the intermediate i/2-cvclohexadienyl anion complex 171 is generated. Three further transformations of this intermediate are possible. When Cr(0) is oxidized with iodine, decomplexation of 171 and elimination of hydride occur to give the substituted benzene 172. Protonation with strong acids, such as trifluoroacetic acid, followed by oxidation of Cr(0) gives rise to the substituted 1,3-cyclohexadiene 173. The 5,6-trans-disubstituted 1,3-cyclohexadiene 174 is formed by the reaction of an electrophile. 

Another instructive scenario may be found when considering the metalation of arenes. There are two distinct mechanisms for the metalation of aromatic C-H bonds - electrophilic substitution and concerted oxidative addition (Box2). The classical arene mercuration, known for more than a century, serves to illustrate the electrophilic pathway whereas the metal hydride-catalyzed deuterium labeling of arenes document the concerted oxidative addition mechanism [8, 17]. These two processes differ both in kinetic behavior and regioselectivity and thus we may appreciate the need to differentiate these two types of process. However, the choice of C-H bond activation to designate only one, the oxidative addition pathway, creates a similar linguistic paradox. Indeed, it is hard to argue that the C-H bond in the cationic cr-complex is not activated. 

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... 

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). 

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... 

It is therefore not surprising that the reactivities of arenes and alkanes in electrophilic substitution reactions are very different, with the former being much more active. At the same time, the mechanism of the interaction (oxidative addition) of both saturated and aromatic hydrocarbons with complexes of metals in a low oxidation state is in principle the same. The reactivities of arenes and alkanes in oxidative addition reactions with respect to low-valent metal complexes therefore usually differ insignificantly. Furthermore, a metal complex via the oxidative addition mechanism can easily cleave the C-H bond in olefin or acetylene. 

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. 

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