Charge-transfer CT complexes

Scheme 10.5 Tentative mechanism for cytochrome P450-cata-lyzed epoxidation of a double bond. The reactive iron-oxo species VII (see Scheme 10.4) reacts with the olefin to give a charge transfer (CT) complex. This complex then resolves into the epoxide either through a radical or through a cationic intermediate.

A. Nitropyridinium cations. The spontaneous formation of vividly colored charge-transfer (CT) complexes occurs upon exposure of jV-nitropyridinium (PyNO ) cation to various aromatic donors,235 i.e.,... 

B. Tetranitromethane. Tetranitromethane forms colored charge-transfer (CT) complexes with a variety of organic donors such as substituted benzenes, naphthalenes, anthracenes, enol silyl ethers, olefins, etc. For example, an orange solution is instantaneously obtained upon exposure of a colorless solution of methoxytoluene (MT) to tetranitromethane (TNM),237 i.e.,... 

The key distinguishing feature of charge-transfer (CT) complexation is that the partial admixture of ionic D+A character in the resonance hybrid (5.72) confers a tendency toward association that is absent in the purely covalent D- A limit, i.e., in the absence of covalent-ionic resonance. The mechanism of CT binding is... 

Wagnerova Class I intrazeolite photooxygenation, 233-253 of alkanes, 234—235 of alkenes, 235-243 regiochemistry, 236, 237, 243, 244 steric confinement effects, 237/ 246-247 Wagnerova Class II intrazeolite photooxygenation, 253-261 of alkanes, 256/ 258-259 of alkenes, 253-257, 253/ 256/ charge-transfer (CT) complexes in, 253-254, 255, 257... 

The first objection needs to be discussed in terms of the formation of a charge-transfer (CT) complex MSv between an alkene monomer M and the solvent Sv, with the formation constant Km. Since... 

Photochemical ET reactions can be classified in at least three categories (which can co-exist), namely (i) simple homolysis of bonds of neutral molecules to give radicals of low redox reactivity (ii) excitation of a species D to produce an excited state D which initiates a second-order ET reaction involving another component of acceptor type, A, with formation of the radical pair D + A (iii) direct excitation of a charge transfer (CT) complex formed between two reaction components D and A to form the same radical pair D + A -. The first case is obviously an ideal situation if it can be realized, but this is seldom the case. The incursion or predominance of situations (ii) and/or (iii) in almost any system is possible, and precautions must be taken to avoid these complications. Much can be done by controlling the wavelength of the light source, but it is also possible to affect the chemistry in a predictable manner. 

Abiotic Model Syatema. Possible abiotic model systems are listed in Table I. We have recently studied steric en charge transfer (ct) complex formation (11). Both Vand At/were considered as steric parameters, t is defined by the expression... 

The term charge tranter refers to a succession of interactions between two molecules, ranging from very weak donor-acceptor dipolar interactions to interactions that result in the formation of an ion pair, depending on the extent of electron delocalization. Charge transfer (CT) complexes are formed between electron-rich donor molecules and electron-deficient acceptors. Typically, donor molecules are p-electron-rich heterocycles (e.g., furan, pyrrole, thiophene), aromatics with electron-donating substiments, or compounds... 

The first resolution of [6]-helicene was achieved by Newman and Lednicer 78) by crystallization with the aid of a chiral complexing agent, 2-(2,4,5,7-tetranitro-9-fluorenylidene aminooxy)propionic acid (TAPA), which was especially designed for this purpose. (R)- and (S)-TAPA (79 a) form diastereomeric charge-transfer (CT) complexes with the enantiomers of hexahelicene. Several other helicenes could also be resolved using this reagent. 

Figure 1.40 Structures of charge-transfer (CT) complexes (a) TMPD-TCNB and (b) MiP-TCNQ.

If the charge transfer (CT) complex is sufficiently strong, the ion-radical pair would dissociate to induce ionic and/or radical reactions. The mechanism of this photoexcitation is different from the n — n or n — n excitations. The later process is the excitation of isolated molecules whereas the CT excitation requires two molecules in contact. Surprisingly, rather limited attention has been directed to this field of photosensitized CT process from the viewpoint of organic reactions. 

When two or more reactants first approach one another, non-covalent recognition and attraction of the molecules or of parts of the molecules takes place and permits system aggregation, which may be of importance in obtaining the reaction products. For instance, the approach of a carbocation to a n system of olefins starts by forming a charge-transfer (CT) complex which precedes the formation of the C—C bond6. 

An investigation with the electron donor 4-methoxybenzo[b]thiophene (35) and electron acceptor p-chloroacetophenone (36) and with the bichro-mophores 37 and 38, where the above donor and acceptor moieties are connected by an olefinic (unsaturated as well as saturated) spacer, was performed (02JPP(A)(152)41). The absorption spectra of the donor 35 in the presence of the electron acceptor 36 were measured in n-heptane and highly polar acetonitrile solutions. In both nonpolar and highly polar media, it was found that the spectra of the mixture of 35 and 36 are just the superposition of the absorption bands of the individual components. This observation excludes the possibility of formation of any ground state charge transfer (CT) complex. 

C. Photoelectron Spectra and UV Spectra of Charge Transfer (CT) Complexes... 

The electronic absorption spectroscopy of charge transfer (CT) complexes of donor molecules of n-, n- and cr-type (DX) with jt- and rr-acceptors (A = TCNE, I2 etc.) allows one to study the influence of the X substituents bonded to a donor centre, D, on the energies of charge transfer bands, hvcr129- The hvcr and Ip parameters are connected by a linear dependence given in equation 19. 

Photoisomerization of charge-transfer (CT) complexes of (E)- and (Z)-l,2-fe-(l-methyl-4-pyridinio)ethenes 99 with iodine can be effected by irradiating at the CT band in acetonitrile (Sch. 41). The CT excitation of the... 

Donor/acceptor interactions in charge-transfer (CT) complexes are determined primarily by the symmetry and the energetics of the frontier orbitals (HOMO and LUMO) [20]. A... 

Charge transfer (CT) complexes are kept together by rather weak forces, and it is not to be expected that such forces should influence their electrochemical behaviour significantly. Thus, the CT complex between tetracyanoethylene and hexamethylbenzene has its halfwave potential for reduction shifted 0-039 V towards a more negative potential as compared to tetracyanoethylene itself (Peover, 1967) as is predictable from theoretical considerations of the formation of the CT complexes. 

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