D-A complex

The generality of such intermolecular [D, A] complexes is shown in Fig. lc by the exposure of a colorless solution of a-tetralone enol silyl ether to different... 

Having shown that the enol silyl ethers are effective electron donors for the [D, A] complex formation with various electron acceptors, let us now examine the electron-transfer activation (thermal and photochemical) of the donor/ acceptor complexes of tetranitromethane and quinones with enol silyl ethers for nitration and oxidative addition, respectively, via ion radicals as critical reactive intermediates. 

Comments on the thermal nitration of enol silyl ethers with TNM. The strikingly similar color changes that accompany the photochemical and thermal nitration of various enol silyl ethers in Table 2 indicates that the preequilibrium [D, A] complex in equation (15) is common to both processes. Moreover, the formation of the same a-nitroketones from the thermal and photochemical nitrations suggests that intermediates leading to thermal nitration are similar to those derived from photochemical nitration. Accordingly, the differences in the qualitative rates of thermal nitrations are best reconciled on the basis of the donor strengths of various ESEs toward TNM as a weak oxidant in the rate-limiting dissociative thermal electron transfer (kET), as described in Scheme 4.40... 

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

Thermal or photochemical activation of the [D, A] pair leads to the contact-ion pair D+, A-, the fate of which is critical to the overall efficiency of donor/acceptor reactivity as described by the electron-transfer paradigm in Scheme 1 (equation 8). In photochemical reactions, the contact ion pair D+, A- is generated either via direct excitation of the ground-state [D, A] complex (i.e., CT path via irradiation of the charge-transfer (CT) absorption band in Scheme 13) or by diffusional collision of either the locally excited acceptor with the donor (A path) or the locally excited donor with the acceptor (D path). 

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

Productive bimolecular reactions of the ion radicals in the contact ion pair can effectively compete with the back electron transfer if either the cation radical or the anion radical undergoes a rapid reaction with an additive that is present during electron-transfer activation. For example, the [D, A] complex of an arene donor with nitrosonium cation exists in the equilibrium with a low steady-state concentration of the radical pair, which persists indefinitely. However, the introduction of oxygen rapidly oxidizes even small amounts of nitric oxide to compete with back electron transfer and thus successfully effects aromatic nitration80 (Scheme 16). 

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

The D/A complexation in equation (41) is further substantiated by infrared and NMR studies. These observations suggest that an initial thermal electron transfer within the D/A charge-transfer complex generates an ion-radical pair, and a rapid methyl transfer subsequently completes the 1,4-addition (equation 42). 

Unfortunately, the fast rates of 0s04 addition to most alkenes preclude the observation of D/A complexes, and they are not readily characterized. However, a variety of aromatic electron donors form similar (colored) D/A complexes with Os04 that are more persistent and the observation of ArH/ 0s04 complexes forms the basis for examining the electron-transfer paradigm in osmylation reactions. 

Similar vivid colorations are observed when other aromatic donors (such as methylbenzenes, naphthalenes and anthracenes) are exposed to 0s04.218 The quantitative effect of such dramatic colorations is illustrated in Fig. 13 by the systematic spectral shift in the new electronic absorption bands that parallels the decrease in the arene ionization potentials in the order benzene 9.23 eV, naphthalene 8.12 eV, anthracene 7.55 eV. The progressive bathochromic shift in the charge-transfer transitions (hvct) in Fig. 13 is in accord with the Mulliken theory for a related series of [D, A] complexes. 

Ideas very similar to those presented here were developed by Litt and Wellinghoff [19] with regard to the formation of D—A complexes and the involvement of competition of donors (D) and solvents in the solvation shell of the acceptors (A) and of the complexes. Their theory was used successfully to account for deviations from the Benesi-Hildebrand equation [20]. 

The propagation by both types of modified ester probably involves a 6-centred cyclic transition state. The same applies to a third type of /-cat propagation, in which a polymeric ester reacts with a D-A complex formed from M + I2. 

Once again the active species is an associative D-A complex =CH2-SR2 of donor-acceptor type, but - in contrast to the positive modifiers - the negative modifiers are the donors, and the acceptor (the acidic CH2) is part of the polymer end-group. 

Esters, such as alkyl halides, which are too inert to propagate, require a positive modifier (acceptor), e.g., a metal halide, to activate them by forming a D-A complex. 

Esters which are unstable and/or too reactive, such as those of CF3S03H, require a negative modifier (donor), e.g., a dialkyl sulphide, to give living systems such modifiers form a D-A complex with an acceptor centre, e.g., the acidic protons, at the growing end. 

The living polymerisations by an alkyl halide and I2 are a third category of living cationoid polymerisations they are propagated by a polymer iodide reacting with a D-A complex between the monomer and I2. 

If use is made of the time-dependence of both yield and DP, the resulting kinetic equations make possible the calculation of propagation rate-constants and D-A complexation constants. Some examples of such calculations are given. 

Wachtershauser [11] has produced a classification of elementary variational motifs describing evolution at the level of biochemical phenotypes (Scheme 1). Presumably because wachtershauser was interested in autotrophs, he did not consider the fundamental pathway operation of retro-evolution, discovered by Horowitz [140], which we include here as an additional fundamental operation i). Horowitz assumed that D, a complex organic molecule was present in the soup that heterotrophs first used, but that it ran out. Heterotrophs then evolved to use D s precursor C to synthesise D, and so on for B and A. Horowitz s mechanism is important if the end product D is indeed available at an early stage, and if C, B and A are available in excess in the environment also. This is only likely where autotrophs produce these compo-... 

The cr-radical (III) and 02 are considered to be formed by electron transfer from the skatole anion to oxygen in the ternary complex (II), which is composed of a strong electron donor and a weak electron acceptor through Fe(II)-porphyrin (see Reaction 4). In such a complex (D-Fe(II)P-A type complex, D = electron donor, A = electron acceptor), electron transfer from the donor to the acceptor should occur more easily than the direct electron transfer in D-A complex, because in the former the cooperative interaction of the three components should decrease the energy barrier of the electron transfer. 

Most of the studies of linked transition metal D/A complexes have employed bridging ligands that have relatively low-energy rr-type LUMOs and metals in which the electron-transfer process involves cirr-orbitals. There are fewer studies of purely a analogs, partly for reasons of complex instability. Some general features are characteristic of any type of bridging ligand. 

Case 3. The nuclear coordinates within the bridging ligand of the D/A complex may be different from those in the complexes with the metals in identical oxidation states. 

Figure 4. Qualitative illustration of the effects of configurational mixing in an A D-A complex, a) Diabatic states with degenerate acceptors b) Effect of mixing between the three states it is assumed that only MLCTi mixes with the ground state.

Li, Y. Dubin, P.L. Spindler, R. Tomalia, D.A. Complex formation between poly(dimethyldiallylammonium chloride) and carboxylated starburst dendrimers. Macromolecules 1995, 28, 8426-8428. 

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