Isopropanol electron transfer

The mechanism of an oxidation-reduction reaction can be simple, as illustrated by the ferrocene-ferricenium self-exchange in equation (1) where only electron transfer need occur.1 In other cases the mechanistic demands imposed by the net reaction are far greater. An example is shown in reaction (2) where, in the net sense, two protons and two electrons must be transferred from isopropanol, which is the reducing agent, to the RuIV oxidizing agent.2... 

In general, any of several possibile mechanisms may be operative for complex reactions. For example, in the oxidation of isopropanol by RuIV the key redox step could involve initial outer-sphere electron transfer, initial H-atom transfer, or even two-electron hydride transfer. The hydride mechanism, which has been proposed to be the actual low-energy pathway in water at 25 °C, is illustrated in reaction (3).2... 

A recently reported regioselective photoreduction of benzoates by photosensitized electron transfer reaction was applied to nucleosides [85]. In presence of jV-methyl-carbazole as the electron donor sensitizer and in an isopropanol, water solution, w-trifluoromethylbenzoates of adenosine 83 or benzoates of uridine 84 give deoxygenated products in good yields (73 %). 

Bimolecular quenching of the excited states of metal complexes generally involves electron transfer or energy transfer processes ( 1). Recently, however, Pt2(pop)4 " has been found to undergo a photochemical reaction involving atom abstraction as a primary photoprocess (.26). The reaction involves the catalytic conversion of isopropanol to acetone ... 

Photolysis of ring-substituted phenacyl bromides gives ketyl radicals which in the presence of alcohols leads to the corresponding carbonyl compounds.Two different chain mechanisms seem to operate and in the presence of methanol, oxidation predominantly occurs by hydrogen transfer whereas in the presence of isopropanol, acetone is formed mainly by an electron transfer process. 

Transfer hydrogenation of aldehydes with isopropanol without addition of external base has been achieved using the electronically and coordinatively unsaturated Os complex 43 as catalyst. High turnover frequencies have been observed with aldehyde substrates, however the catalyst was very poor for the hydrogenation of ketones. The stoichiometric conversion of 43 to the spectroscopically identifiable in solution ketone complex 45, via the non-isolable complex 44 (Scheme 2.4), provides evidence for two steps of the operating mechanism (alkoxide exchange, p-hydride elimination to form ketone hydride complex) of the transfer hydrogenation reaction [43]. 

Ketimines were hydrogenated faster than aldimines, and electron-donating groups accelerated the rate of hydrogenation. The OH and RuH bonds are regenerated by hydrogen transfer to the unsaturated 16-electron Ru complex from isopropanol, generating acetone (Scheme 7.13). 

The photochemist is rather familiar with the photoexcited triplet states and the associated intersystem crossing processes. It is well documented that the photoexcited triplet state plays an important role in organic photochemistry. It is thus conceivable that the electron spin polarization of the photoexcited triplet can be further transferred to a radical pair formed by the reactions of the triplet with a suitable substrate. Such a photoexcited triplet mechanism was first proposed by Wong and Wan in 1972 (135) to account for the "initial polarization" observed in the naphthosemiquinone radical formed in the photoreduction of the parent quinone in isopropanol. It was further considered that the triplet mechanism might also lead to CIDNP if such initially polarized radicals react rapidly to give products with nuclear spin polarization induced via the Overhauser mechanism. 

Polarization Transfers and Reaction Mechanisms. Polarization transfers include the previously mentioned electron-nuclear Over-hauser effect and the nuclear-nuclear Overhauser effect. In this section we will discuss only electron-electron polarization transfer via a secondary chemical reaction involving a primary polarized radical. Again we shall use the photoreduction of quinone (t-butyl-p-benzoquinone) as an example. In solvent containing isopropanol, reaction of triplet quinone by phenols leads to two structural isomers, radicals I and II ... 

Other examples of electron polarization transfer in photochemical systems include the reaction of polarized isopropanol radicals with a ground-state quinone (reaction 55) and the polarization transfer from the primary amine radical to biacetyl (97). These examples serve to emphasize how CIDEP and polarization transfer can be used to follow complex photochemical reaction mechanisms. 

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