Photoelimination Reactions

The immediate impact of this research will be a clearer understanding of ligand motions during photoelimination reactions. In particular, comparative studies of molecular motions in the gas phase (using ultrafast electron diffraction) and in the liquid phase should become a source of very detailed understanding of the influence of solvation on chemical processes. Such combined studies in collaboration with Peter Weber, Dept, of Chemistry, Brown University are planned. 

Photoelimination reactions. In these reactions one of the fragments is a small, stable, closed-shell molecule such as N2, C02, CO, HX (X = halogen), etc. The name photo-extrusion is also used for these reactions. Since the reactant molecule is of closed-shell type in the ground state, the second fragment must be either another closed-shell molecule or a biradical, that is a species with two unpaired electrons held in distinct orbitals. 

Figure 4.34 Photoelimination reactions of nitrogen, (a) Formation of a carbene through triplet state sensitization, and addition of molecular oxygen, (b) Formation of nitrenes through photodissociation of azo compounds and azides...

In addition to photosubstitution and photoelimination reactions, in the cases of some Ni(II) complexes, photoexcitation of square-planar complexes Ni(TP) and formation of the photoassociative ligand-field (LF) excited state 3Blg can lead to photoaddition reactions yielding hexacoordinate complexes Ni(TP)L2 [65, 66, 75-77], Such processes differ from the second step of photosubstitutions since an excited complex participates in them and the addition is conditioned by the electronic structure of the complex in its excited state (see Table 3). 

The number of photoelimination reactions that occur in a concerted manner is very small. One of the best-characterized examples does not lead to the formation of a C—C bond between the two a-carbons. Rather, it involves the stereospecific photoelimination of CO from 3,5-cydoheptadienone 15 to 1,3,5-octatriene 16 (Scheme 2.5) [18]. The reaction occurs in a stereospecific manner with the relative stereochemistry of the two a-carbons determining the configuration of the resulting 1,5-double bonds. Another interesting and more relevant example involves the photodecarboxylation of enantiomerically pure aromatic esters derived from (+ )-or (—)-2-methylbutyric acid and 2,4,6-trimethylphenol 17, which occurs with 100% retention of configuration in the product 18. [19] While this is a promising lead that... 

Whenever a carbonyl is in or adjacent to the backbone of a polymer chain, the main cause of molecular weight breakdown will be the Norrish Type II photoelimination reaction. TTie quantum efficiency of the reaction in polymers appears to be nearly independent of temperature and physical state (i.e., solid or liquid). 

The photoelimination reactions seem to present the cleanest progress the products mostly do not absorb at the irradiation wavelength. These reactions are the prime candidates for verification of the kinetic relations [12,40]. In many other cases, photoracemization, side reactions or, if the primary products absorb at the irradiation wavelength, secondary photolysis occur and obscure the kinetics of CD development. The isomerization of E-cyclooctene 42 also is a well-defined reaction. The a vs. time plots show a maximum [103], but a photostationary state will be reached at long times as the Z — E isomerization is also active under irradiation. 

Photoelimination reactions of 2-butylpyrazine and 2- -hydroxyethylpyrazine to give 2-methylpyrazine have been examined (722). 

The reactions of nitrenes produced in the photoelimination reactions of aryl azides. 

A detailed study of the photoelimination reactions of the racemic tri-fluoroacetates (59, 60) has been reported by Gano and Chien. A kinetic analysis... 

R = Me, R + R = OCH2O, R = were prepared by this method. The last two compounds were easily converted into (+ )-sinactine and (+)-cavidine. A number of other substituents (160 X = OAc, Cl, or Br) were also shown to be useful for the photoelimination reaction.The dealkylation of N-substituted protoberberinium salts on a semi-industrial scale has been described.( )-Mecambridine has been synthesized along conventional lines.New protoberberine derivatives of the type (162) have been synthesized (see also Vol. 3). 

Aryl-substituted cyclic sulfites undergo an interesting photoelimination reaction [72JOC2589 82JCR(S)175 84MI1]. For example, 4,4,5,5-tetraphenyl-l,3,2-dioxathiolane 2-oxide, when photolyzed in methanol, furnished a mixture of benzophenone, 9,10-diphenylphenanthrene, and benz-hydryl methyl ether. In contrast, photolysis in acetone gave only olefin along with phenanthrene (Scheme 15). 

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