Electron photochemical

Pt2(P205H2) - (d8-d8), and Mo6Clft ( )6. Two- electron oxidations of Re2Cl and Pt2(P205H2)it have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl, followed by one-electron oxidation of the Cl -trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported. 

In recent experiments D. G. Nocera has extended this work to include two-electron photochemical oxidation of Re2ClQ. The strategy involved here is to generate Re2Cl by acceptor quenching... 

A. Shafferman and G. Stein, The effect of aromatic amino acids on the photochemistry of a disulfide Energy transfer and reaction with hydrated electrons, Photochem. Photobiol. 20, 399-406 (1974). 

As with cycloadditions, photochemical reactions that look pericyclic may not always be pericyclic, but several do seem to obey the rules. The 4-electron photochemical reaction 4.102 — 4.103 is disrotatory and the 6-electron reaction 4.104 - 4.105 is conrotatory, and in both cases the reaction is cou nter- thermody n amic. 

W.A.Noyes Jr W.E.Vaughan, Chem Revs 7, 239-58 (1930) (Thermal, electronic photochemical decompns of solid substances, such as K chlorate, Ag oxide, oxalic acid and some carbonates) 5a)Mellor 10 (1930), 100 103... 

In summary, for butadienes and cyclobutenes four-electron, thermal, conrotatory four-electron, photochemical, disrotatory. The easiest way to visualize the stereochemical result is to make a fist, use your thumbs to designate substituents at the termini of the 77 system, and rotate your fists to determine the stereochemical result upon disrotatory or conrotatory ring closure or opening. 

Sol 4. (b) The reaction involves Trr-electron photochemical disrotatory electrocyclic reaction. 

Now let us turn to a much more difficult problem the 8 orbitals with 8 electrons photochemical addition of ethylene to benzene (Figure 3.23). At first sight this system would seem intractable for the VB approach. And, in fact, it is not easy to get analytical results such as those obtainable for the 6 orbitals with 6 electrons problem. Accordingly, our strategy, similar to the 6 orbitals with 6 electrons case, is to analyze the computed conical-intersection structures obtained at the ab initio level using the MMVB method and thus deduce the VB analysis. The end result rationalizes a complicated reaction mechanism (more than 10 MECI were located) quite simply as we will now discuss. The details can be found in our recent... 

Next, we must analyze the reaction conditions and determine whether the reaction occurs in a conrotatory or disrotatory fashion. As shown in Table 17.2, fora system with six It electrons, photochemical conditions cause a conrotatory ring closure. This information is used to determine whether the substituents are cis or trans to each other in the product. In this case, the substituents are trans to each other in the product. 

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