Olefin oxidation potentials

In acetonitrile, the valence band of Ti02 lies at about +2.4 V versus the standard calomel electrode and the conduction band lies at -0.8 V. The hole migrates to the surface where it oxidizes adsorbed olefin (oxidation potential = +1.4 V), producing a surface-bound cation radical. The electron also migrates to the surface where it is captured by adsorbed oxygen (reduction potential = -0.75 V), forming adsorbed superoxide. 

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). 

Table 3. Oxidation potentials of allylsilanes and olefins Compound Ep (V vs. Ag/AgCl)...

Electron-poor olefins with higher oxidation potentials may decrease the rate of electron transfer and other processes competing for deactivation of the iminium salt excited states may increase. Alternate reaction pathways involving olefin-arene 2 + 2 cycloaddition may take place in the photochemistry of 133 with electron-poor olefins (equation 62)120,121. 

Tricyclene (8) has been oxidized in acetic acid/Et3N to the Nojigiku alcohol (9) in 11% yield (Eq. 8) [36]. The reaction was also conducted in a 2.25-kg scale to afford pure (9) in 65% yield from crude (8) containing alkenes. The olefins remained unconverted due to their higher oxidation potential. 

The route that is taken depends on the oxidation potential of the nucleophile and the olefin. 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

Disilenes have much lower oxidation potentials than olefins , and consequently they are much more reactive toward 02. Typically, disilenes 93 react in solution with triplet oxygen to give 1,2-disiladioxetanes 94 as the major product, accompanied at room temperature by a smaller amount of disilaoxirane 95 (equation 93) °. ... 

A simple one-electron oxidation process appears to be excluded by the lack of a correlation between the observed reactivities and the half-wave oxidation potentials of the olefins from the literature. What is the role of the sacrificial aldehyde The answer to this question is a good entry point into the mechanism of the transformation. We... 

Burke discussed recently that for easily oxidizable substances, such as alcohols and amines, the redox potentials of Table II might be irrelevant, as incipient formation of oxidic layers at lower oxidation potentials and not the bulk oxide may already cause the observed oxidative conversion (189). Nonetheless, for substances that are less easily oxidized for instance, olefines or arenes, the given redox potentials are certainly relevant with respect to the rate of their mediated oxidation. 

The papers in this volume concern results observed in catalytic systems. They span a broad range of catalytic reactions including hydro-formylation, hydrocarboxylation, hydrogenation, carbonylation, cyana-tion, and olefin oxidation. To some extent, the results provide a significant extension of our understanding of previously studied catalysts and catalytic reactions. However, some of the papers are concerned with newer areas of research and feature results of both scientific and potential industrial importance. 

The Mg2+ ion also shows an acceleration effect on the photoreduction of dimethylfumarate and some other related olefins by 1-benzyl-1,4-dihydro-nicotinamide (BNAH) used as an NADH model compound via photosensitized electron transfer from BNAH to [Ru(bpy)3]2+ (Scheme 16) [160]. In this case, however, the complex formation of BNAH with Mg2+ results in an increase in the one-electron oxidation potential of BNAH [87]... 

Some results are summarized in Table I (27). Figure 1 schematically depicts the reactivity of the olefins examined with their oxidation potentials, and indicates that generally the unreactive olefins are those with higher oxidation potentials, and the reactive olefins are those with lower oxidation potentials. However, among the oxidizable olefins their reactivity is not simply governed by their oxidation potentials as will be discussed later. 

Figure 1. Reactivity of olefins and their oxidation potentials.

Figure 2 shows that the photocurrent measured at 0.5 V increases with decreasing the oxidation potentials of the olefins examined however, benzylideneacetone, etc. with higher oxidation potentials did not show any photocurrent (32). 

Although the absolute amount of the photocurrents is governed by various factors such as the oxidation potentials of olefins and the extent of adsorption of olefins on the electrode, the above findings show that the reactive olefins in the photocatalytic oxygenation exhibit photocurrents and the olefins which do not exhibit photocurrents are unreactive in the photocatalytic oxygenation. On the other hand, the olefins which exhibit photocurrents are not always reactive. For example, stilbene shows a higher photocurrent than DPE, but is not so reactive as DPE. The electron transfer to the excited semiconductor takes place more efficiently from stilbene than from DPE due to the lower oxidation potential of the former, but in the subsequent free radical reactions, stilbene is less reactive than DPE (33). 

In view of the above results, in photocatalytic oxidation of a series of 4-substituted diphenylethylenes, an increase in reactivity with decreasing Hammett s sigma constants (31) seems to arise not only from the lowering of the oxidation potentials of the olefins in this sequence but also from the general trend of the increase in the reactivity of olefins toward peroxyl radicals with increasing the electron donating ability of olefins (33). 

The oxidation potentials 170 ——- 777 of a large number of aromatic hydrocarbons, amines, phenols,heterocycles and olefins are tabulated I0,10a>25-48 65,525-528) an(j nee(j not repeated here. Such potentials have been successfully correlated with HMO-parameters 525 530>538) ie in oxidations with the energy of the highest filled MO (HFMO).Adams 25) and Peover 65) have discussed some precaution to which attention should be paid in such correlations, e.g., shifts in potentials due to the irreversibility of the electrode process or due to fast follow-up reactions. 

Further studies have shown that, to obtain an efficient amination, it is necessary to take into account both the difference in oxidation potential between the photocatalysts and the substrates and the positive charge distribution in the cation radicals of the olefin. The synthetic utility of the method was proven by the successful preparation of an aminotetraline, itself an intermediate for the synthesis of a compound with biological activities such as 2-aminoindan (26, Scheme 3.16b) [39]. As with the last synthesis, the redox-photosensitized amination occurred with no need for acids or bases, as usually required when using general protocols. 

Mn(III) oxidation of olefins in the allylic position (Gilmore and Mellor, 1971) and saturated hydrocarbons (Jones and Mellor, 1977) is considered to take place by attack on a C—H bond, indeed very likely in view of the high oxidation potentials of such compounds. 

However, as we will see later on, other modes of evolution of the primary intermediate radical ions can be suggested to explain some oxidation reactions mediated by electron-transfer processes. In fact, several exceptions to the Foote s BQ-controlled photooxygenation procedure have been reported during the last years on several electron-rich substrates. Thus, the involvement of superoxide ion, as an oxygen active species, in all of the DCA-sensitized photooxygenations, remains questionable [96,105,112,115,128]. Schaap and co-workers [98] recorded under inert atmosphere the characteristic ESR spectrum of the (DCA ) radical anion. On the other hand, the involvement of aryl-olefin radical cations and their reactions with superoxide ion was easily observed by quenching experiments with compounds exhibiting lower oxidation potentials than those of aryl-olefins [85, 95, 98],... 

The authors, on the basis of their experimental results, pointed out that olefin radical cations-allylically methylated and/or stabilized by heteroatoms or n conjugation [136] do not react with an active oxygen species, suggesting, in spite of their low oxidation potentials, that several factors may contribute to determine this behavior. Thus, one of the important factors is the presence of an independent 7t-system in which odd-electron density is not delocalized, whereas, a second and a third factor can be the insufficient steric hindrance to block the attack of oxygen... 

An intriguing electrochemical aziridination is based on the selective anodic oxidation of A -aminophthalimide (550, oxidation potential +1.60 V) in the presence of olefins. Thus, /ra t-hex-4-en-3-one 551 is converted to the corresponding aziridine 552 in acetonitrile solution using a platinum electrode at a constant potential of +1.80V (Scheme 135). The reaction mixture is buffered using triethylammonium acetate, since the cathodic process reduces proton to hydrogen gas. The use of platinum at the anode is critical, as graphite electrodes yielded no aziridination products <2004PAC603>. 

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