13.5 CT Complexes as Critical Intermediates in Donor/Acceptor Reactions of Arenes 463
Fig. 9. (A) Free-energy dependence of the secondorder rate constant (log k) for electron transfer from hindered and unhindered arene donors to photoactivated quinones. The dashed line represents the best fit of the data points of the hindered donors to the Marcus correlation. (B) Superposition of (A) and the free-energy dependence of
the formation product (KEC) for encounter complexes of unhindered arenes with photoactivated quinones showing coincidence of the maximum of encounter complex formation and the maximum deviation of the ET rate constants of the unhindered donors from the classic Marcus (outer-sphere) behavior. Data from ref. [54].
Scheme 17
As illustrated in Figure 9B by the superposition of the Marcus-type plots and the freeenergy dependence of the formation constant KEC, the greatest deviations (from the Marcus curve) are observed for donor/acceptor couples that form the strongest encounter complexes (as gauged by their formation constants KEC). On the basis of the absorption and emission data, an electronic coupling element of HDA A0:15 eV (1000 cm 1) has been determined for similar complexes (exciplexes) [55]. Even higher values (@0.5 eV) can be estimated for HDA in CT complexes of the same donors and ground state (unexcited) chloranil. In contrast, the CT complexes are not detected for similar ground-state pairs involving hindered donors [54, 56]. Such strong electronic couplings account for the substantial lowering of the innersphere ET barrier (compare Figure 8) and increased rate constants for unhindered donors.
13.5.1.2 Thermal and Photochemical ET in Strongly Coupled CT Complexes
An increase in the electronic coupling interaction results in the disappearance of the ET barrier and complete delocalization of the transferred electron between the donor and the acceptor. Such e ects have been extensively studied for intramolecular ET in bridged intervalence compounds [57]. As regards intermolecular systems, the only spectrally and structurally characterized system has been that of NOþ/arene complexes [28].
The NOþ cation is an e cient oxidant (1.48 V vs. SCE) for various electron-rich arenes [53], and the intermolecular (CT) complexes [ArH, NOþ] with alkylbenzenes have been characterized in detail [27]. However, the recent observation [4b, 28] of structurally and spectroscopically similar complexes with electron-rich arenes has led to a more detailed consideration of the role of CT complexes in NOþ/arene redox processes [28].
464 13 Charge-Transfer Effects on Arene Structure and Reactivity
Chart 4
The reorganization energy for the (cross) ET reactions of NOþ/arene pairs is about 2.5 eV (as evaluated from the self-exchange rates of aromatic donors and the NOþ acceptor) [28].
Scheme 18
Since the measured values of HDA of ca. 1.4–1.8 eV exceed l=2, these CT complexes belong to the Robin–Day Class III category, in which there is complete delocalization of the electron between the donor and the acceptor. As such, there is only one potential minimum on the pathway between the fArH þ NOþg reactants and the fArHþ þ NO g products [28]. The free-energy change along the reaction coordinate for the redox transformation in the (i) endergonic, (ii) isergonic, and (iii) exergonic regions of driving force are qualitatively depicted in Chart 4.
The reversible (temperature-modulated) equilibrium between the CT complex and the ET products is shown in Figure 10. This process can be designated as:
Fig. 10. Temperature modulation of the reversible complexation/electron transfer between aromatic donors and NOþ. (A) Increase of EAþ at (bottom to top): 44 C, 10 C, 3 C, 20 C, and after
argon bubbling (to remove NO). (B) Increase of OMNþ at (bottom to top): 90 C, 30 C,
10 C, 0 C, 20 C and after argon bubbling. Data from ref. [28].
13.5 CT Complexes as Critical Intermediates in Donor/Acceptor Reactions of Arenes 465
No activation (energy) barrier separates the donor and the acceptor from the ET products (and vice versa). The electron transfer in Scheme 18 is not a kinetic process, but is dependent on the thermodynamics, whereby electron redistribution is concurrent with complex formation. Accordingly, the rate-limiting activation barrier is simply given by the sum of the energy gain from complex formation and the driving force for electron transfer, i.e.:
DGET0 ¼ DGCT þ DGET :
ð14Þ
Nitrosonium/arene complexes also provide an opportunity for the consideration of photochemical processes in Class III systems. Due to the strong electronic interaction, the chargetransfer complex is characterized by comparable values of the mixing coe cients, b Aa (especially near the isergonic point) [28]. In contrast to the weak CT complexes with b fa, in which the ground state is close to the (diabatic) reactant and excited product states, no appreciable electron redistribution from the donor to the acceptor occurs in Class III complexes upon optical transition. Laser flash-photolysis experiments show high e ciencies for the formation of aromatic cation radicals upon direct photoexcitation of [ArH, NOþ], with quantum yields for the caged radical pair close to unity. Similarly, the quantum yields of free cation radicals are found to be nearly 0.70 in complexes with hexamethylbenzene, 0.60 with pentamethylbenzene, 0.4 with durene, and 0.1 with p-xylene [58]. Such results can be explained in terms of the population of the antibonding orbital of the complex, which results in a sharp decrease in the NO/arene binding. In addition, owing to the sizable extent of CT from the arene to NOþ in the ground state, the nuclear configuration (both in the ground and Franck–Condon excited states) corresponds to that of the arene cation radical and nitric oxide. No nuclear rearrangement of the arene and nitric oxide (nor of the solvent) is needed for the relaxation of the excited state to the solvent-caged radical pair. Such a dissociation proceeds via a practically (barrierless) transition on the vibration timescale of 10 13 –10 14 s. The high e ciency of formation of the radical pair is reflected in microsecond kinetics and results in regeneration of the initial [ArH, NOþ] complex [58].
13.5.2
Electron-Transfer Paradigm for Arene Transformation via CT Complexes
Thermal and/or photochemical electron transfer within the CT (precursor) complex generates the ion pair fDþ; A g as caged or freely di usive ion-radicals. Most important from a synthetic point of view are the processes by which highly reactive ion-radicals undergo further irreversible transformation resulting in new (thermodynamically stable) products. In other words, the formation of the (precursor) CT complex and electron transfer act in tandem as a coupled set of pre-equilibria. The resultant ion-radical pair can undergo a subsequent (irreversible) transformation (with rate constant kf ) or back ET (kBET), which represent the basis for the ET paradigm and drive the coupled equilibria towards the products (P) [4], i.e.:
Scheme 19
466 13 Charge-Transfer Effects on Arene Structure and Reactivity
Chart 5
If kf > kBET, the overall transformation can occur rapidly despite unfavorable driving forces for the electron transfer itself. Only follow-up reactions with high kf can compete with back electron transfer. Di erent kinds of such unimolecular processes can drive the equilibria toward the final product. A representative example is the mesolytic cleavage of the CaSn bond in the radical cation resulting from the oxidation of benzylstannane by photoexcited 9,10-dicyanoanthracene (DCA). This is followed by the addition of the benzyl radical and the tributyltin cation to the reduced acceptor DCA [59]. In the arene/nitrosonium system, [ArH, NOþ] complexes can exist in solution in equilibrium with a low steady-state concentration of the ion-radical pair. However, the facile deprotonation or fragmentation of the arene cation radical in the case of bifunctional donors such as octamethyl(diphenyl)methane and bicumene can result in an e ective (ET) transformation of the arene donor [28, 59]. Another pathway involves collapse of the contact ion pair [Dþ, A ] by rapid formation of a bond between the cation radical and anion radical (which e ectively competes with the back electron transfer), as illustrated by the examples in Chart 5 [59].
Bimolecular reactions of the ion-radical pair can also e ectively compete with the back electron transfer if either component undergoes a rapid reaction with an additive that is present during the ET activation. In NOþ/arene systems, the introduction of oxygen rapidly oxidizes even small amounts of nitric oxide to compete with back ET and thus successfully e ect aromatic nitration [60]. In a related example, the CT complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction. However, if irradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion-radical pair is readily protonated, and the redox equilibrium is driven toward coupling (in competition with the back ET) to yield the photoadduct [59], i.e.:
Scheme 20
13.5 CT Complexes as Critical Intermediates in Donor/Acceptor Reactions of Arenes 467
Chart 6
Generally, highly reactive aromatic ion-radicals can undergo a variety of other uniand bimolecular processes that compete e ectively with back ET and lead to an overall transformation, some examples of which are illustrated in Chart 6 (with Ar representing an aryl group) [59].
The aforementioned examples reveal that a wide variety of aromatic transformations can proceed from the CT complex. If the (CT/ET) pre-equilibria are fast relative to the follow-up process, the overall second-order rate constant k2 ¼ KCTKETkf . In this kinetic regime (especially in the strongly endergonic ET range), the ion-radical pair may not be experimentally observed in a thermally activated adiabatic process, and the applicability of the ET paradigm is di cult to establish. However, photochemical (laser) activation by the deliberate irradiation of the CT absorption (hnCT) will lead to the spontaneous generation of the ion-radical pair, which is experimentally observable when the time resolution of the laser pulse exceeds that of the follow-up processes (kf and kBET). The relationship between the thermal and photochemical generation of the ion-radical pair is illustrated in Figure 11, and this provides a basis for the experimental demonstration of the viability of the electron-transfer paradigm through comparison of the thermal/photochemical reactions with (thermal) rate constant/ CT band energy correlations.
468 13 Charge-Transfer Effects on Arene Structure and Reactivity
Fig. 11. Graphical relationship between the photochemical and thermal pathways for the CT transformation of the precursor complex (PC) to the successor complexes (SC).
The close relationship between the thermal and photochemical (ET) pathways via CT complexes can also be seen from a consideration of oxidative CaC cleavage in benzopinacol donors, which readily form vividly colored CT complexes with various acceptors such as chloranil (CA), DDQ, and NOþ [59]. Thus, benzopinacol/CA solutions are stable (at 23 C) in the dark. However, irradiation of the CT band leads to a slow bleaching of the color with concomitant formation of the retropinacol product. With a stronger acceptor, such as DDQ (E ored ¼ 0:6 V vs. SCE), the cleavage of the benzopinacols can also be achieved by thermal means. For example, the CT complex formed upon mixing tetraanisylpinacol and DDQ bleaches within minutes (in the dark) to a ord the retropinacol products in quantitative yield [59].
Identical pre-equilibrium CT complexes of various substituted pyridinium salts with organoborates such as BMe4 as electron donors are observed in both the thermal and photochemical methyl transformations. Both processes proceed through the radical pair, in which rapid scission of one Me–B bond in the oxidized BMe4 generates a methyl radical and BMe3. This is followed by the coupling of pyridine and methyl radicals within the solvent cage. Moreover, the spectral characteristics show: (i) a red shift of the CT bands with increasing Ered o of the pyridinium acceptor and a blue shift with increasing Eox o of the organoborates, and (ii) that the process with N-methylisoquinolinium/BMe4is slow, but that the methyl-transfer reaction between BMe4 and 3-cyano-N-methylpyridinium (with the highest Ered 0) occurs instantaneously. On the other hand, the yellow mixture of 4- phenylpyridinium cation (with relatively low Ered 0) and BMe4 persists for prolonged periods without reaction [59].
The same features are observed in the osmylation of arene donors. Thus, osmium tetraoxide spontaneously forms complexes with arenes, and the systematic spectral shift in the CT bands parallels the decrease in the arene IP [59]. The same osmylated adducts are obtained thermally on leaving mixtures to stand in the dark or upon irradiation of the CT bands at low temperature. Time-resolved spectroscopy establishes that irradiation of the CT band of the anthracene/osmium tetraoxide complex leads directly to the radical-ion pair fANTþ; OsO4 g, which then collapses to the osmium adduct (with a rate constant k @109 s 1) in competition with back ET [59].
13.5 CT Complexes as Critical Intermediates in Donor/Acceptor Reactions of Arenes 469
Chart 7
Such examples indicate the applicability of the CT/ET paradigm in describing a variety of aromatic transformations. In the case of a (highly) endergonic process, the involvement of the electron-transfer step in the reaction pathway can be ambiguous. The objection against such a step is often based on the high ET reaction barrier due to Marcus reorganization energy and/or an unfavorable driving force. However, the development of Marcus–Hush theory in the strong interaction range has indicated that the donor/acceptor electronic coupling can result in: (i) a substantial lowering of the barrier to electron transfer, and (ii) a stabilization (lowering in energy) of the adiabatic (CT) precursor and successor complexes relative to the diabatic (non-interacting) reactant and product states, as illustrated in Chart 7 [61].
Because the electronic distribution and nuclear configuration of the donor and the acceptor in the (Class II) successor complexes are similar to those of the free donor/acceptor product (i.e. radical pair), it is reasonable to suggest that products can originate directly from the successor complex (pathway P1). Such a reaction, which includes an electron-transfer step, does not necessarily proceed via a pair of free ion radicals, and the e ective activation energy can be even lower than that required by pathway P2. When the follow-up reaction involves the coupling of radicals, the reaction directly proceeding from the (ET) successor complex state can be kinetically favorable (since it excludes di usional processes).
13.5.3
Electron-Transfer Activation of Electrophilic Aromatic Substitution
Electrophilic aromatic substitution represents one of the most important applications of the transformation of an arene via a CT (precursor) complex. The process is considered to proceed via an (encounter) p-complex between the electrophile (Eþ) and the aromatic substrate (ArH), which collapses in a single rate-limiting step to the Wheland intermediate or s- complex [43, 44], i.e.:
Scheme 21
The focal point in this mechanism is the activation process that leads to the wellestablished Wheland intermediate. In order to address the mechanism of the activation of p-complexes, we first recognize that most electrophiles (such as NOþ and NO2þ, various nitrating agents, halogens, carbocations, diazonium cations, sulfur trioxide, lead(IV), mercu-
470 13 Charge-Transfer Effects on Arene Structure and Reactivity
Fig. 12. Linear correlations of the relative reactivity of arenes in electrophilic substitution reactions (log k=k0) with the optical transition energies in the accompanying CT complexes. Data from ref. [62b].
ry(II), and thallium(III) salts, etc.) are excellent electron acceptors, as judged by their Ered o, as well as by their ability to oxidize a variety of donors to the corresponding cation radicals [4]. Furthermore, the various arenes are electron donors, as reflected by their relatively easy oxidation in solution. As a consequence, the p-complex in Chart 7 is more accurately described as an electron donor/acceptor or CT complex between the electrophile and the aromatic nucleophile. The CT absorption bands are visible indications of their strong MO interaction that precedes the transformation [4]. The application of the ET paradigm to electrophilic aromatic substitution is based on (i) the observation of CT interactions between electrophiles and aromatic donors, (ii) the coincidence of the products of the CT photochemical and thermal reactions, and (iii) the quantitative correlations of the second-order rate constants with the CT transition energies for halogenation, mercuriation, thallation, etc. (Figure 12) [62]. Accordingly, let us take halogenation, nitration, and nitrosation with various electrophilic agents as typical examples to elucidate the importance of CT complexes and ET activation as the fundamental steps leading to the Wheland intermediate.
13.5.4
Structural Pre-organization of the Reactants in CT Complexes
Aromatic hydrocarbons are known to spontaneously form weak 1:1 molecular CT complexes with halogens [30, 62]. The recent (low-temperature) refinement of their structure reveals the highly structured pre-equilibrium complex shown in Figure 13A [32], in which the axially symmetric Br2 is poised specifically over a single (CaC) center of benzene. Its pre-reactive character is revealed by the spontaneous transformation of the crystalline [C6H6, Br2] complex into an equimolar mixture of bromobenzene and hydrogen bromide. Further (structural) delineation of electrophilic aromatic bromination has been provided by the isolation and X-ray crystallographic analysis of the s-adduct or Wheland intermediate with hexamethylbenzene as the arene under similar reaction conditions (see Figure 13B) [36]. (Note that the highly unstable s-adduct of benzene is unlikely to be isolable owing to its facile a-proton loss.) Such structural studies taken together provide an unequivocal pathway for electrophilic aromatic bromination, in which the key steps are:
13.5 CT Complexes as Critical Intermediates in Donor/Acceptor Reactions of Arenes 471
Fig. 13. (A) ORTEP diagram of the pre-equilibrium complex of benzene and bromine showing the pertinent charge-transfer p-bonding. (B) Molecular structure of the s-adduct (or Wheland inter-
Scheme 22
mediate) derived from the bromine interaction with hexamethylbenzene to emphasize the least-motion transformation from the p-complex in A. Data from ref. [32].
Most relevant is the structural information provided by the pre-equilibrium p-complex in its transformation to the critical s-adduct. Thus, the composite of the ORTEP structures in Figures 13 A and B represents a close to ideal least-motion study, i.e.:
Scheme 23
Even more striking is the regioselective transformation of the highly structured toluene p- complex, with bromine situated specifically over the ortho and para positions to a ord the same isomeric (product) mixture of o- and p-bromotoluenes as that obtained in solution [63]. As close as these pre-equilibrium intermediates are structurally akin to the (ordered) transition states for electrophilic bromination, it is important to emphasize that they are formed essentially upon bimolecular collision with no activation energy, and the donor/acceptor binding is in accord with the Mulliken formulation.
The continuous transition from p- to s-coordination in the complexes with di erent acceptors, as presented in Figure 6, implies the ready transformation of one into the other. The
47213 Charge-Transfer Effects on Arene Structure and Reactivity
formation of the s-complex can be presented as an ET process in which the CT complex leads to the reduced acceptor Br2 , i.e.:
Scheme 24
However, the driving force for the ET step (DGET ¼ 60 kcal mol 1 is taken as the di erence between the redox potentials of benzene and Br2) is clearly too endergonic to be consistent with the facile bromination of benzene. However, such an evaluation of the driving force could be valid if electron transfer leads to the ion-pair state. Furthermore, the quantitative measurement of the CT absorption leading to the formation of the CT complex [C6H6, Br2] indicates the donor/acceptor electronic coupling interaction to be HDA @0:5 eV. As such, the ion radicals C6H6þ and Br2 are born as a contact (inner-sphere) ion pair, in which the energy is substantially lowered due to the radical-ion interaction, so that the driving force is much more favorable than that simply calculated from the values of E oox and
E ored.
13.5.5
CT Complexes in Aromatic Nitration and Nitrosation
Due to very fast rates of aromatic nitration with the NO2þ cation, an independent experimental study of the reactive intermediates pertaining to the ET paradigm is not readily forthcoming. However, such a kinetic restriction is overcome by the use of nitronium ion carriers (NO2Y) such as nitric acid, acetyl nitrate, N2O5, N-nitropyridinium, tetranitromethane, etc., as milder nitrating agents [59]. The latter form colored complexes with arenes, in which the energy of the CT bands corresponds to the predictions of Mulliken theory [9]. Thermal and photochemical reactions of these complexes result in identical isomeric mixtures of aromatic nitration products. Moreover, the thermal nitration of various arenes shows a strong rate dependence on the donor/acceptor strength of the nitrating agent and ArH. These observations support ET activation as a viable mechanistic basis for aromatic nitration. Furthermore, analysis of the products from toluene nitration yields isomeric compositions of o-, m-, and p-nitrotoluene that are singularly invariant over a wide range of substrate selectivities (k=ko based on the benzene reference [64]) with di erent nitrating agents (ranging from the very reactive NO2þ to the relatively unreactive p-MeOPyNO2þ). In other words, there is a complete decoupling of the product-forming step from the ratelimiting activation of electrophilic aromatic nitration. Such a clear violation of the reactivity/ selectivity principle can only arise when there is at least one reactive intermediate such as the (successor) ion-radical pair, as predicted by the ET paradigm.
In order to combine the CT/ET pathways with essentially di usion-controlled rates [64] of nitration by NO2þ, and the understanding of the dramatically lower reactivity of the struc-
