
Astruc D. - Modern arene chemistry (2002)(en)
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13.4 Charge-Transfer Activation of Coordinated Arenes 453
13.4.1
Carbon–Hydrogen Bond Activation
Arene CaH bond activation by transition metal derivatives has been utilized for a variety of chemical transformations, including H/D exchange and various substitution reactions, such as silylation, carboxylation, phenylimine formation, etc. These reactions may occur by direct coupling of the aryl ligand and the substituent when both are attached to the metal center, or by insertion of functional groups into the metal–carbon bond of the arene ligand [2]. The CaH bond activation in aromatic compounds has been frequently compared with metal-catalyzed activation of hydrogen leading to H/D exchange with metal deuterides (MaD), i.e.:
Scheme 6
However, this simple model does not explain the (kinetically) enhanced reactivity of arenes relative to alkanes, considering the higher (DCaH) bond energy of arenes, i.e. DCaH; benzene ¼ 111 kcal mol 1 vs. DCaH; alkane A95 kcal mol 1. Many more examples of metal-activated cleavage of CaH bonds are known for aromatic compounds than for alkanes [37]. To account for this di erence, arene/metal coordination is proposed [38], although experimental evidence for such intermediate complexes and their structural features is lacking.
Scheme 7
The first instance of metal/arene pðh2Þ-coordination prior to CaH bond activation was observed for the rhodium complex Cp*Rh(PMe3)(H)(C6H5) by 1H NMR spectroscopic measurement of benzene/toluene exchange and reversible interconversion of paraand metatolyl isomers [34]. The observation of an equilibrium between the rhodium h2-complexes with polycyclic arenes and the corresponding (h1) aryl hydrido metal complexes (which can be tuned by variation of the arene, the metal center, and other metal ligands [2]) supports the pre-equilibrium complexation of arene and metal centers as a prerequisite for CaH bond activation. However, a lack of understanding still persists regarding the mechanism of the interconversion h2-coordinated p-complexes and aryl hydrido s-complexes, which formally represents a hydrogen-atom transfer between two tautomeric structures. Insight into this mechanism can be gained using the established CT character of the metal–arene bond. Thus, a two-step process can be conceived involving an initial charge transfer from the p-

45413 Charge-Transfer Effects on Arene Structure and Reactivity
ligated arene to the metal center resulting in the formation of the s-complex with delocalized positive charge on the arene ring, which then readily transfers a proton to the electron-rich metal center.
Scheme 8
An interesting example of CaH bond activation is the ferrocenium (1e-oxidant) catalysis of monomethylaryliridium complexes [39], in which the oxidized (17-electron) iridium complex is coordinated to the arene substrate in a pðh2Þ fashion. Complete charge transfer from the arene to the metal (strong acceptor) takes place upon coordination and results in the formation of the s-arenium complex. The latter can readily transfer a proton to a methyl ligand to yield methane and the oxidized monomethyl complex, which is finally reduced by the original dimethyliridium complex to complete the ET chain process [39]:
Scheme 9
Coordination to transition metals activates benzylic as well as aromatic centers, which become targets for either nucleophilic or electrophilic attack [2]. For example, under acidic conditions, a carbocation is generated, which can then react with a nucleophile, i.e.:
Scheme 10
Coordination to a Cr(CO)3 center results in substantial stabilization of the carbocation, which is commonly explained either in terms of interaction of the filled d-orbitals of chromium with the empty p-orbital of the benzylic carbon or by chromium (h7)-coordination of the entire benzylic moiety (with a delocalized positive charge and partial double-bond character of the exocyclic CaC bond) [2]. Both representations are based on the prediction that

13.4 Charge-Transfer Activation of Coordinated Arenes 455
the Cr(CO)3 group acts as an electron donor in its interaction with a benzylic (carbocationic) acceptor.
Since benzylic hydrogens in (arene)Cr(CO)3 complexes are more acidic than those in the corresponding free arene, the Cr(CO)3 group is frequently considered to be an electron acceptor capable of stabilizing carbanions [2] (which can then be trapped by various electrophiles). Such an e ect is commonly explained as the stabilizing e ect induced by the delocalization of the negative charge onto the chromium moiety [2], i.e. the Cr(CO)3 group acts as an electron acceptor, and complete CT generates an exocyclic double bond and a negatively charged h5-coordinated chromium center. Enhanced rates of deprotonation of benzylic hydrogen have also been found in h6-coordinated Mn(CO)3þ and FeCpþ complexes, as well as in h2-coordinated OsII complexes. Based on the enhanced acidity of benzylic centers in aromatic cation radicals [40], a CT scheme can be formulated [2]. In the extreme case, this considers a complete electron transfer from the arene to the metal acceptor, resulting in the formation of a cation-radical ligand, which is readily deprotonated at the benzylic position
Scheme 11
This reaction scheme is applicable to CaH bond activation by strong electron acceptors. For example, the formation of benzyl acetate from toluene using CoIII as the oxidant has been shown to occur via the toluene cation radical [41]:
Scheme 12
The ET mechanism is proposed on the basis of the high value of r ¼ 2:4 obtained from the Hammett plot, as well as the observation of benzyl chloride and chlorotoluene as the main products when the oxidation is carried out in the presence of high concentrations of LiCl [41]. Similarly, activation of benzylic CaH bonds by other strong oxidants such as MnIII or PbIV has been suggested to occur through an initial electron transfer, especially in the case of aromatic substrates with low ionization potentials [42]. However, there have been no reports of complex formation between the metal and the arene prior to ET, and no such reactive complex has been isolated and characterized by X-ray crystallography.
13.4.2
Nucleophilic/Electrophilic Umpolung
The majority of arene functionalizations are based on electrophilic aromatic substitution, i.e. the electron-rich p-system of the arene acts as the Lewis base and is readily attacked by elec-

45613 Charge-Transfer Effects on Arene Structure and Reactivity
trophiles (or Lewis acids) such as the cationic nitronium and halonium acceptors [43]. Owing to the electron-rich character of arenes, nucleophilic substitution is generally not favored, but it can be achieved by attaching both electron-withdrawing substituents (such as nitro, cyano, etc.) and good leaving groups (such as chloro, sulfonato, etc.) to the arene substrate [44]. The simultaneous presence of both types of groups promotes nucleophilic substitution according to an addition/elimination mechanism, whereby the electron-withdrawing substituents induce electron deficiency in the aromatic ring to such a degree that addition of the nucleophile becomes feasible.
Activation of an aromatic substrate to nucleophilic addition can also be achieved by coordination of the arene ring to an electron-withdrawing metal center. Moreover, arene/metal complexes can be more e ective because: (i) the donor/acceptor properties of the metal complexes can be varied to a much greater extent than those of organic substituents, and (ii) nucleophilic substitution may be carried out with a wide variety of arenes and is not limited to just those bearing electron-withdrawing substituents. Analysis of the reactivity data of coordinated arenes towards nucleophiles in a variety of organometallic complexes indicates that the nucleophile/electrophile umpolung of the metal-ligated arenes can be readily explained in terms of a strong CT interaction between the arene and the metal. This is best demonstrated in the extreme case of complete electron transfer from the arene to the metal center, which leads to an aromatic cation radical that readily reacts with variety of nucleophiles [2], e.g.:
Scheme 13
Such a charge transfer from the ligated arene can lead to: (a) nucleophilic addition or substitution, (b) electron transfer, and (c) proton elimination/transfer, thus revealing the close relationship between all of these processes. The reactivity of the arene ligands towards nucleophiles in (arene)MLn complexes depends on the electrophilicity of the metal fragments [MLn], this increasing in the order [Cr(CO)3] < [Mo(CO)3] f[FeCp]þ < [Mn(CO)3]þ [2]. For example, in (arene)FeCpþ, which is widely used for synthetic purposes, a chloro or nitro substituent on the arene is readily substituted by such nucleophiles as amides, enolates, thiolates, alkoxides, and carbanions [45].
It is generally accepted that the addition of the nucleophile occurs initially at a position ortho to the leaving group. In fact, in the reaction of [(C6H5Cl)FeCp]þ with relatively stable carbanions such as CH(COR)2 , the ortho adduct can be isolated [46]. Subsequent migration of the nucleophile can result in formation of the ipso adduct, which then loses chloride [46c]. Analogous reaction mechanisms have been formulated for substitution reactions involving (arene)Cr(CO)3 and (arene)Mn(CO)3þ complexes. In such a way, [(C6H5X)Cr(CO)3] (X ¼ Cl, F) reacts with carbanions, amines, and thiolates [2, 47]. The use of organometallic nucleophiles such as carbonylmetallates M(CO)n (M ¼ Fe, W, etc.) leads to bimetallic complexes [2].

13.4 Charge-Transfer Activation of Coordinated Arenes 457
Synthetically useful reactions of (arene)Mn(CO)3þ complexes with such nucleophiles as methoxide, benzenethiolate, azide, various amines, and anilines are also noteworthy. These complexes have also been extensively studied with regard to nucleophilic addition reactions resulting in thermally stable cyclohexadienyl complexes [2].
Scheme 14
Grignard reagents and enolates are especially suitable for such nucleophilic additions. The resulting products are of synthetic interest since oxidation leads to rapid release of the metal group, and the free, functionalized arenes are readily obtained:
Scheme 15
Here, either X ¼ (H2C(O)-R0) and ox ¼ Jones’ reagent or X ¼ CN and ox ¼ CeIV. It has been emphasized that the addition of nucleophiles to (arene)Mn(CO)3þ complexes does not occur through an initial ET from the nucleophile to the metal center [2]. This represents an additional advantage since such redox reactions frequently lead to the decomposition of the metal complex, a typical example being the reductive deligation of bis(arene)Fe2þ complexes [48]. On the other hand, intramolecular charge transfer from the arene to the metal not only induces an electron deficiency in the arene ring (which is critical for e ective attack of the nucleophile), but it also results in an attenuation of the electrophilicity of the metal center so as to avoid undesired ET reactions of the metal with the nucleophile.
13.4.3
Modification of the Donor/Acceptor Properties of Coordinated Arene Ligands
Charge redistribution between the arene and the acceptor moiety upon complexation results in significant changes in their donor/acceptor properties. For example, charge transfer from an arene to an acceptor converts the aromatic compound from an electron-rich donor to an electron-poor acceptor. Furthermore, the complete charge transfer from hexamethylbenzene to the bromonium ion upon formation of the bromoarenium s-complex leads to a delocalized positive charge on the arene ring. As a result, such positively charged s-complexes readily react with a variety of nucleophiles and can be stabilized and crystallized only in the presence of such anions as SbCl6 and PF6 or similar (very weakly nucleophilic) counter-

45813 Charge-Transfer Effects on Arene Structure and Reactivity
anions. However, X-ray crystallographic studies clearly reveal that even such weak nucleophiles exhibit strong interactions with the positively charged arenium s-complex, as evidenced by unusually short intermolecular distances. In crystalline complexes of bromohexamethylbenzenium hexafluoroantimonate, there is a close contact between a fluorine and
the carbon atom in the position para to the bromine atom, with a shortened FaC distance of 3.066 A˚ , as compared to the sum of the van der Waals radii of 3.17 A˚ [2]. In these crystals, the fluorine atoms of hexafluoroantimonate act as electron donors that bond to a positively charged hexamethylbenzene moiety.
Similar structural e ects are known with organometallic complexes. Thus, upon coordination to a strong acceptor, the donor ability of hexamethylbenzene is attenuated, and it will then be capable of forming CT complexes with other (even weaker) donors. For example, the hexafluorophosphate salt of [(HMB)2, FeII] forms complexes with durene, as established by the observation of new CT absorption bands [2]. The crystal structure of the complex shows alternating stacks of durene with [(HMB)2, FeII], in which the interplanar distance between durene and hexamethylbenzene is d ¼ 3:65 A˚ . This close separation and the absorption spectrum of the complex point to substantial CT interactions between durene as the donor and hexamethylbenzene, which acts as an acceptor owing to its high electron deficiency induced by coordination to the electron-poor iron(II) center.
Complete electron transfer from arenes results in aromatic cation radicals, which di er dramatically in their donor/acceptor properties. As such, cation radicals can interact with the parent donor. The resulting (CT) interaction accounts for the self-association (p- dimerization) of aromatic donors with their corresponding cation radicals and results in the formation of charge-resonance (CR) complexes or p-mers [49].
Scheme 16
Analogous processes also occur between arenes and their anion radicals.
The formation of ion-radical dimers (p-mers) in solution can be identified spectroscopically by a new broad (CR) absorption band in the NIR region that is absent in the spectrum of either the neutral arene or its ion-radical component (Table 4) [49]. For di erent aromatic
Tab. 4. Electronic spectra of various (monomer and dimer) cation radicalsa
Arene donor (ArH) |
Absorption band, l (nm) |
Arene donor (ArH) |
Absorption band, l (nm) |
||
|
|
|
|
|
|
|
monomer |
dimerb |
|
monomer |
dimerb |
|
ArHB: |
(ArH)2B: |
|
ArHB: |
(ArH)2B: |
BEN |
555 |
920 |
NAP |
575 |
1050 |
HMB |
476,508 |
1351 |
ANT |
680 |
>900 |
DTPc |
460 |
>800 |
PYR |
500 |
1400 |
a From ref. [49] and references therein.
b Additional CR band that is absent in the monomer. c 1,3-p-Tolylpropane.

13.4 Charge-Transfer Activation of Coordinated Arenes 459
Fig. 7. Mulliken plots for CT complexes of hexamethylbenzene (HMB), octamethylbiphenylene (OMB), and naphthalene (NAP) with various acceptors. Points on the ordinate at E0ox E0red ¼ 0 represent the band energies of the charge-resonance (CR) complexes between these donors and their cation radicals. Data from ref. [49].
compounds, this spectral band can be satisfactorily included into the Mulliken plot to describe the interchangeability of the CT and CR energies of a parent arene donor with a series of di erent types of electron-deficient acceptors (Figure 7). The successful isolation and X- ray crystallographic characterization of such ion-radical dimers provides the basis for the estimation of structural changes during dimerization, in comparison with those occurring during CT formation. For example, the octamethylbiphenylene system, in which both the monomeric cation radical OMBþ and the dimeric cation radical (OMB2)þ are well characterized (as are the neutral OMB and its CT complexes with various acceptors), represents the best basis for such a comparison. The average CaC bond length, dav, within the aromatic rings of the neutral OMB donor is 1.405 A˚ , while in the cation radical it is 1.415 A˚ . In the CT complexes of OMB with TCNE and TCNQ, the value of dav is 1.408 A˚ , while in (OMB2)þ it is 1.410 A˚ . Thus, dav is 0.3 pm longer in CT complexes than in the neutral OMB donor, and in (OMB2)þ the average bond length is intermediate between that in the neutral OMB and that in its cation radical [49]. The infinite stacks contain dimeric units of (OMB)2þ with an interplanar separation of 3.41 A˚ , which corresponds to a tight van der Waals contact between a pair of identical octamethylbiphenylene moieties.
If the substituents on an aromatic compounds prevent the close approach of the cation radical to its neutral counterpart (due to steric hindrance), p-mer formation is inhibited. Such e ects can clearly be seen on comparing 2,3,6,7-tetramethylnaphthalene (TMN) and its hindered analogue OMN. Spectrophotometric studies show that the tetramethylnaphthalene forms the dimeric (TMN)2þ cation radical, as characterized by a very broad absorption band at 1150 nm and the formation constant Kdimer ¼ 490 m 1 at 10 C. In contrast, dimer formation is not observed with the hindered cation radical OMNþ. The fact that relatively few ion-radical dimers are known is associated with the opposing requirements for their formation. On the one hand, the cation radical should be relatively stable, which is usually the case with encumbered donors (for which steric hindrance prevents close contact between the radicals), while on the other hand they should be able to approach each other in order for electronic interaction to be appreciable.

460 13 Charge-Transfer Effects on Arene Structure and Reactivity
Chart 2
The enhanced acceptor properties of aromatic cation radicals (ArHþ ) suggest that they should form CT complexes with the parent arene (ArH) as well as with di erent arene donors (ArH0). However, in mixed ArHþ /ArH0 systems, the (partial) reduction of the cation radical leads to the formation of homomolecular dimers. Moreover, the same symmetry and energy of the interacting orbitals favors the ion-radical interaction with the neutral parent, better stabilizing the (homomolecular) dimer and promoting the redox process ArHþ þ ArH0 ! ArH þ ArH0þ , even if the electron transfer is formally endergonic. We thus conclude that heteromolecular complex formation takes place only when such an ET process is precluded. Indeed, the only known example of a heteronuclear cation-radical/neutral donor complex hitherto found (and characterized spectroscopically and by X-ray crystallography) is that of the hindered OMNþ cation radical, which is not prone to forming homomolecular p- mers [50]. The qualitative MO diagrams describing CR complex formation and their relationship with the CT complexes are presented in Chart 2. In all cases, the driving force for complex formation derives from the resonance interaction between various HOMO/LUMO, SOMO/LUMO, and HOMO/SOMO combinations of donor/acceptor pairs. It should be noted, however, that CR complexes are symmetrical (isergonic) systems. The position of the absorption band is determined either by the reorganization energy or by the orbital interaction (see Figure 2). ESR measurements indicate complete delocalization of the unpaired electron in the p-mers: (OMB2)þ and (TMN)2þ . The average bond length found in (OMB2)þ (which is half the sum of those extant in the neutral and isolated cation-radical) supports such a conclusion.
13.5
CT Complexes as Critical Intermediates in Donor/Acceptor Reactions of Arenes
The basic principles describing the e ects of CT complexes on the energy profile along the reaction coordinate stem from the theory of electron transfer. Redox processes may occur: (i) as ground-state thermal reactions, (ii) by direct irradiation of the CT band, and (iii) upon photoexcitation of one of the redox partners followed by di usional complex formation [4, 24], as depicted in Chart 3.
Combination of Mulliken’s formalism with the Marcus quadratic representation [10] of the initial and final (diabatic) states allows the energy profile of the ET reaction coordinate to be constructed. As illustrated in Figure 8, an increase in HAB results in (i) a lowering of the ET barrier, (ii) a stabilization of the precursor and successor (CT) complexes, and (iii) a shift of their positions along the reaction coordinate (i.e. the charge is partially transferred from the


462 13 Charge-Transfer Effects on Arene Structure and Reactivity
If ligands are involved in the formation of discrete intermediates or if metal ions become ligand-bridged, the process is designated as inner-sphere (IS) electron transfer [52]. In these cases, the electronic interaction between the redox centers is increased substantially, and leads to a lowering of the activation barrier (and hence to increased rates) for the ET reaction [13, 15, 53].
Since such a chemically based di erentiation between OS/IS has often proved to be ambiguous in organic reactions [53], we prefer a more general definition of inner/outer-sphere ET based primarily on the degree of the donor/acceptor electronic coupling [24]. According to this classification, outer-sphere ET implies weak coupling ðHAB flÞ, and it is described theoretically by the classical Marcus formulation [10], in which the activation barrier is determined by the reorganization energy l and the free energy change DGET, i.e.:
DG0 ¼ lð1 þ DGET=lÞ2=4 |
ð13Þ |
which predicts the characteristic (quadratic) dependence of the rate constant on the driving force DGET. As the donor/acceptor interaction becomes appreciable relative to l (usually when HAB > 200 cm 1), a substantial change in the ET reaction profile occurs. For example, for the isergonic process DG0 ¼ ðl 2HABÞ2=4l, and such an inner-sphere ET process is accompanied by significant deviations from Marcus behavior.
This definition implies the possibility of a continuous transition between outer-sphere and inner-sphere ET processes, and the kinetics can be directly related to the experimental properties of the precursor (CT) complexes as measured by X-ray crystallography, UV/vis/NIR spectroscopy, etc. Arene donors are especially well suited for the demonstration of such a changeover, since (i) the oxidation and reduction potentials (which are important for the free-energy correlation) can be easily tuned over a wide range by varying the substituents, without significant changes in the size and orientation of redox centers, and (ii) their steric encumbrance can be readily modulated by the introduction of bulky substituents at the aromatic ring to allow control of the electronic interaction without a ecting the driving force.
Electron transfer (ET) to photoactivated quinone acceptors from a series of unhindered, partially hindered, and heavily hindered aromatic donors (with matched Eox o values) can be examined kinetically by laser flash photolysis [54]. The second-order rate constants for electron transfer from hindered donors such as hexaethylbenzene depend strongly on the temperature, solvent polarity, and salt e ect, and they follow the free-energy correlation predicted by Marcus theory (Figure 9A). Moreover, no spectroscopic or kinetic evidence for the formation of the encounter complexes (exciplexes) with photoactivated quinones prior to electron transfer is observed. In contrast, electron transfer from non-hindered (or partially hindered) donors such as hexamethylbenzene, mesitylene, and di-tert-butyltoluene, is associated with temperature-independent rate constants that are up to 102 times greater than those predicted by Marcus theory and are poorly correlated with the accompanying freeenergy changes (Figure 9).
Such a rate constant behavior is ascribed to an inner-sphere ET process. Most importantly, there is unambiguous spectroscopic and kinetic evidence for the formation of encounter complexes [ArH, Q*] between the arene and the photoexcited (quinone) acceptor prior to electron transfer [54].