Initiator electron transfer

This symmetry is important in bringing the two chlorophyll molecules of the "special pair" into close contact, giving them their unique function in initiating electron transfer. They are bound in a hydrophobic pocket close to the symmetry axis between the D and E transmembrane a helices of both... 

In triethylamine instead of benzene the reaction products are completely different, and are indicative of a homolytic process involving an initial electron transfer from triethylamine followed by a hydrogen atom transfer. Scheme 10-68 gives the major products, namely 1,3,5-tri-tert-butylbenzene (10.36, 20%), the oxime 10.39 (18%), formed from the nitroso compound 10.38, and the acetanilide 10.37 (40%). ESR and CIDNP data are consistent with Scheme 10-68. In their paper the authors discuss further products which were found in smaller yields. 

The decarboxylation of the caesium salt of 9-methylanthracene-10-acetic acid occurs at an even lower potential (0.7 V) and affords the dimer as well as the methyl ether (Eq. 40) [342], The low oxidation potentials for the decarboxylation of 54 (0.13 to 0.77 V) [306a] and 55 (—0.17 V) [306b] indicate too, that the initial electron transfer occurs from the amino or aryl group rather than from the carboxylate. 

Rate laws and coefficients were determined for the oxidation of all intermediate compounds by MnOJ and were compatible with the scheme as presented, i.e. including a route for direct oxidation to pyruvaldehyde. An estimate of k2 ( 5x10 l.mole sec ) suggests an initial electron-transfer to give CH3 COCH2 . This can then be rapidly oxidised in two ways... 

SRNl substitution include ketone enolates,183 ester enolates,184 amide enolates,185 2,4-pentanedione dianion,186 pentadienyl and indenyl carbanions,187 phenolates,188 diethyl phosphite anion,189 phosphides,190 and thiolates.191 The reactions are frequently initiated by light, which promotes the initiating electron transfer. As for other radical chain processes, the reaction is sensitive to substances that can intercept the propagation intermediates. 

For convenience of discussion, a schematic diagram of bacterial photosynthetic RC is shown in Fig. 1 [29]. Conventionally, P is used to represent the special pair, which consists of two bacterial chlorophylls separated by 3 A, and B and H are used to denote the bacteriochlorophyll and bacteriopheophytin, respectively. The RC is embedded in a protein environment that comprise L and M branches. The initial electron transfer (ET) usually occurs from P to Hl along the L branch in 1—4 picoseconds (ps) and exhibits the inverse temperature dependence that is, the lower the temperature, the faster the ET. It should be noted that the distance between P and Hl is about 15 A [53-55]. 

Both thermodynamic and kinetic factors are involved in the competition between concerted and stepwise mechanisms. The passage from the stepwise to the concerted situation is expected to arise when the ion radical cleavage becomes faster and faster. Under these conditions, the rate-determining step of the stepwise process tends to become the initial electron transfer. Then thermodynamics will favor one or the other mechanism according to equation (18). AG )eav is also the standard free energy of cleavage of the ion radical. 

Mattay et al.5i suggested from the photoreaction of biacetyl with highly electron-rich olefins that an initial electron transfer from an electron-rich olefin to photoexcited ketone is the key step in the oxetane formation via the ion-radical pair (equation 26). 

In type A reactions one electron is removed from one of the two double bonds to form a cation radical, and allylic substitution and oxidative addition take place as the following reactions. On the other hand, in type B reactions the initial electron transfer from the double bond is accompanied by a transannular reaction between the two double bonds. 

It is suggested46 that the reaction mechanism involves initial electron transfer from Ph3.Sn to the arene on irradiation to form the Pl Sn- radical and Ar. Disubstitution is also observed on irradiation of dihaloarenes, with a mechanism as outlined in Scheme 9. 

There are two mechanisms that could explain the catalytic effects of nitrite an inner-sphere mechanism in which nitrite acts as a nucleophile toward the FeIINO+ moiety (Scheme 3, pathway A) and an outer-sphere path in which nitrite is oxidized to N02 which then reacts with excess NO to form N203 (Scheme 3, pathway B). Although the initial electron transfer step in pathway B is thermodynamically uphill (AE = — 0.3 V) (41,70), one cannot rule out pathway B since N203 is rapidly hydrolyzed, once formed (71). 

This scheme implies that dioxygen is activated only in a secondary step by an intermediate formed in the initial electron transfer reaction between the metal ion and a co-substrate, Si- The reduced form of the metal ion is re-oxidized to its original oxidation state by O2, but such a reaction with a secondary intermediate cannot be excluded. If the rate determining step is Eq. (9), the overall reaction is again zeroth order with respect to dioxygen. 

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

The initial electron transfer to form the anion radical species seems to be reversible. For example, Allred et al. investigated the ac polarography of bis(trimethylsilyl)benzene and its derivatives which showed two waves in di-methylformamide solutions [71] the first one is a reversible one-electron wave, and the second one corresponds to a two-electron reduction. Anion radicals generated by electrochemical reduction of arylsilanes have been detected by ESR. The cathodic reduction of phenylsilane derivatives in THF or DME at — 16° C gives ESR signals due to the corresponding anion radicals [5] (See Sect. 2.2.1). 

Reaction (I) involves the initial electron transfer from the metal to the monomer, which leads to the formation of a radical anion which could then participate in the reactions shown in (II) and (III), i.e., by coupling of two radical ions or by transfer of another electron from the metal, respectively, both processes leading to a dianionic species. 

We start with the case where the initial electron transfer reaction is fast enough not to interfere kinetically in the electrochemical response.1 Under these conditions, the follow-up reaction is the only possible rate-limiting factor other than diffusion. The electrochemical response is a function of two parameters, the first-order (or pseudo-first-order) equilibrium constant, K, and a dimensionless kinetic parameter, 2, that measures the competition between chemical reaction and diffusion. In cyclic voltammetry,... 

The mechanism for the photoreaction between 133 and cyclohexene can be summarized as in Scheme 8. The initiating electron transfer fluorescence quenching of 133 by cyclohexene resulted in the formation of an w-amino radical-radical cation pair 136. Proton transfer from the 2-position of the cyclohexene radical cation to the nitrogen atom of the a-amino radical leads to another radical cation-radical pair 137. Recombination of 137 at the radical site affords the adduct 134, while nucleophilic attack at the cation radical of 136 leads to another radical pair 138 which is the precursor for the adduct 135. 

The efficient photoaddition reactions between pyrrolinium perchlorate (133) and ben-zyltrimethylsilane (equation 67) or the allylsilane (equation 68) are examples of the initial electron transfer induced desilylation processes. 

Hence, despite the presence of the central atom Co ", an initial-electron transfer is directed not to this center, but to thep-nitrophenyl fragment. Obviously, this fragment, and not Co ", provides an orbital, which is symmetrically appropriate and low-lying. A similar situation occurs in a reductive cleavage of arylsulfonic salts (see Chapter 3). 

The radical >C(Ar)-C < is oxidized by ligand transfer as Jenkins and Kochi (1972) indicated. If the cation-radical [>C=C<]+ obtained as a result of the initial electron transfer is not fully consumed in the reaction, it is reduced by Cu(I) and returns in the form of its geometrical isomer. In the olefin cation-radical state, cis—>trans conversion has to take place, and it indeed takes place in the systems considered (Obushak et al. 2002). 

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