Arenes charge-transfer complexes

Structural Features of Arene Charge-Transfer Complexes... 

Quantitative Evaluation of Arenes as Electron Donors 437 Spectral (UV/vis) Probe for the Formation of CT Complexes 438 IR Spectroscopic Studies of Charge-Transfer Complexation 442 Thermodynamics of Charge-Transfer Complexation 443 Structural Features of Arene Charge-Transfer Complexes 445 Bonding Distance of the Donor/Acceptor Dyad in Arene Complexes 446 Relationship Between Hapticity and Charge Transfer in Arene Complexes 447 Effect of Charge Transfer on the Structural Features of Coordinated Arenes 448... 

Electron-transfer activation. Time-resolved spectroscopy establishes that irradiation of the charge-transfer band (hvCj) of various arene/0s04 complexes directly leads to the contact ion pair. For example, 25-ps laser excitation of the [anthracene, 0s04] charge-transfer complex results in the ion-radical pair instantaneously, as shown in Fig. 14218 (equation 76). 

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... 

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

E.K. Kim u. J.K. Kochi, Oxidative Aromatic Nitration with Charge-Transfer Complexes of Arenes and Nitrosonium Salts, J. Org. Chem. 54, 1692 (1989). 

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) ... 

Bis(trifluoroacetoxy)iodo]benzene (14, BTIB) can be utilized in hexafluoro-2-propanol for the installation of nucleophiles at the ortho-position of para-sub-stituted alkoxyarenes [59-63]. Such reactions have been employed for the construction of carbon-carbon and carbon-heteroatom (N,0,S) bonds, trimethylsi-lyl compounds serving as useful progenitors of the heteroatom nucleophiles (Scheme 20). Oxidative substitutions of this type appear to proceed through arene radical-cations, generated by single electron-transfer within BTIB/sub-strate charge-transfer complexes. 

Excitation of a ground-state charge-transfer complex between arene and alkene (Sec. II)... 

II. ORTHO PHOTOCYCLOADDITION VIA EXCITATION OF A GROUND-STATE CHARGE-TRANSFER COMPLEX BETWEEN ARENE AND ALKENE... 

Coulombic forces will determine the regioselectivity of the ortho addition [189], In the charge-transfer complexes of monosubstituted benzenes with alkenes, the charge (positive or negative) on the arene is largely located at the carbon atoms ipso and (to a lesser extent) para to the substituent. The carbon atoms of the alkene double bond will preferentially be located in the neighborhood of either the ipso carbon or (to a lesser extent) the para carbon atom of the monosubstituted benzene. This would explain the 1,2 and 3,4 selectivity in the ortho photocycloaddition. 

If the alkene, from which the ortho photocycloadduct is derived, is a powerful dienophile, a thermal Diels-Alder reaction between adduct and alkene may take place during the irradiation so that a 2 1 alkeneiarene adduct is formed. Such adducts are quite stable and can easily be purified. Their structure contains all the regiochemical and stereochemical information of the primary 1 1 ortho adduct. This Diels-Alder reaction occurs spontaneously with the alkenes maleic anhydride, maleimide, and their derivatives, as discussed in the sections of this chapter that deal with photoreactions of ground-state charge-transfer complexes (maleic anhydrides) and reactions of excited alkenes with ground-state arenes (maleimides). Ortho adducts formed from other alkenes have often been identified via their reaction with a good dienophile. 

Further functionalizations are obtained via the electron transfer— radical cation fragmentation pathway a typical example is side-chain nitration by irradiation of methyaromatics with tetranitromethane. Aromatics form charge-transfer complexes with C(N02)4 irradiation leads to electron transfer and fragmentation of the C(N02)4 radical anion to yield the triad [Ar + C(NO)J N02], followed by combination between the arene radical cation and the trinitromethanide anion. Thus, cyclohexadienes are formed that generally eliminate and rearomatize at room temperature yielding ring-functionalized products [234] (Sch. 21). 

While the meta photocycloaddition occurs at the n,n singlet state of the arene [4], different cases must be distinguished for the ortho cycloaddition depending on the structure of the substrates [5] (a) excitation of a ground state charge-transfer complex (b) excitation of the alkene (or alkyne) reaction partner (c) excitation of the arene partner and reaction at the singlet state and (d) excitation of the arene followed by intersystem crossing and reaction with the alkene at the triplet state. 

A. Charge-Transfer Complexes of Bis(arene)iron(II) and Ferrocene ... 

Kochi and co-workers have recently identified and characterized the weak charge transfer complexes between tropylium ion and a series of substituted arenes in acetonitrile solution [74], Photoexcitation of these electron donor acceptor (EDA) complexes leads to an electron transfer from the arene donors to the tropylium ion in accord with Mulliken s theory [75]. Time resolved spectroscopic observation of the arene radical cations (formation within the 30 ps laser pulse) has confirmed their intermediacy. The subsequent decay of the photogenerated radical cation and the concomitant regeneration of the ground state EDA complex occurs with a rate constant, kBET > 4 x 1010 s 1 (Scheme 11). This fast back electron transfer... 

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