Reactions s. a. Photoreactions

SRay irradiation s. y-Irradiation ays s. Irradiation Reaction, transannolar - control by chelation 24,735 Reactions (s. a. Photoreactions) -, biodhemlcal s. Enzymes, Microorganisms -, mnlticomponent s. Multi-component reactions nncleophilic s. Nucleophilic reactions... 

Reactions (s. a. Photoreactions, Solid-phase reactions) control, charge and orbital 17,865s31... 

The contribution of fluorescence to the deactivation of the excited singlet state of the phytochromobilin chromophores in Pr and Pfr (see Section II.D) is negligible in quantitative terms. The total fluorescence quantum yield of the photochromic P and P3 components amounts only to from Tables 1 and 4). This means that deactivation proceeds predominantly through nonradiative channels, i.e., via internal conversion back to the electronic ground state of Pr and via primary photoreaction(s). Nevertheless, the fluorescence efficiency suffices to serve as a sensitive tool to monitor certain aspects of the competing primary reaction(s) of P (see Sections III.A and III.C). 

Photochemical asymmetric fixation, a technique by which the asymmetric element is fixed through a photoreaction induced by CPL irradiation, has not been discussed in the recent literature. However, for the sake of completeness, we will briefly describe the process here. This technique normally requires that the thermal racemization process is fast enough to keep the substrate racemic during the photolysis and that the excited state racemizes much more slowly than the relaxation to the product. It is, of course, essential that there be an appreciable g factor at the irradiation wavelength. Under these conditions, the product s op is governed simply by the ratio of the extinction coefficients at the irradiation wavelength. Only a few reactions have been reported to satisfy these criteria, and none of these more recently than the mid 1970s [11, and refs, therein]. 

The first and only report on a photoreaction of a cyclic thioanhydride describes the insertion of alkenes into the C(0)-S bond of naphthalene-dicarboxylic thioanhydrides [87]. Thus irradiation of 1,2-naphthalenedi-carboxylic thioanhydride or its 2,3-isomer with alkenes in acetonitrile gives naphthothiepindiones. The singlet excited molecules undergo S-acyl cleavage, either directly or via a zwitterionic intermediate. Reactions with E- or Z-but-2-ene proceed in a stereospecific manner (Scheme 38). 

Simultaneous with Mariano et al. s [65] first publication on the desilylation reaction from a-trimethylsilylamine radical cation, we reported [70] photocyclization of silylated methylamine of type 69 by sensitized photoreaction using DCN as an electron acceptor in i-PrOH to produce iV-heterocycles... 

Catalyzed Diels-Alder Reactions. The uncatalyzed thermal intramolecular Diels-Alder reaction of 5,5 -oxyhis[( )-l,3-penta-diene] nonstereoselectively generates four isomeric 4-vinylcyclo-hexenes (eq 37). The major product has a trails ring fusion, in contrast to the single cis ring-fused isomer generated in the cop-per(I) triflate-catalyzed photoreaction of the same tetraene (eq 25). Copper(I) triflate also catalyzes a thermal Diels-Alder reaction of 5,5 -oxybis[( )-l,3-pentadiene] that proceeds under milder conditions than the uncatalyzed reaction. The stereoselectivity is remarkably enhanced, generating mainly the major isomer of the uncatalyzed thermal reaction and a single c/s-fused isomer (eq 37) that is different than the one favored in the photochemical reaction (eq 25). 

For reviews dealing with previous work on asymmetric induction in organic photochemical reactions, see (a) Rau, H., Asymmetric photochemistry in solution, Chem. Rev., 83, 535, 1983 (b) Inoue, Y., Asymmetric photochemical reactions in solution, Chem. Rev., 92, 741,1992 (c) Pete, J. R, Asymmetric photoreactions of conjugated enones and esters, Ativ. Photochem., 21, 135, 1996 (d) Everitt, S. R. L. and Inoue, Y., Asymmetric photochemical reactions in solution, in Molecular and Supramolecular Photochemistry, Ramamurthy, V. and Schanze, K. S., Eds., Marcel Dekker, New York, 1999, chap. 2. 

In order to rationalize such characteristic kinetic behaviour of the topochemical photoreaction, a reaction model has been proposed for constant photoirradiation conditions (Hasegawa and Shiba, 1982). In such conditions the reaction rate is assumed to be dependent solely on the thermal motion of the molecules and to be determined by the potential deviation of two olefin bonds from the optimal positions for the reaction. The distribution of the potential deviation of two olefin bonds from the most stable positions in the crystal at OK is assumed to follow a normal distribution. The reaction probability, which is assumed to be proportional to the rate constant, of a unidimensional model is illustrated as the area under the curve for temperature Tj between 8 and S -I- W in Fig. 7. 

We have already discussed one of the earliest photoreactions to be studied, that is, the (4w + 4w) photodimerization of anthracene. That the singlet state was involved in this reaction was conclusively shown in the period 1955-1957. The first reaction in which the triplet state of the molecule was shown to be involved was the photoreduction of benzophenone by Hammond and co-workersa) and Backstrom and co-workers<2) 1959-1961. This was the first in a series of many papers from Hammond s laboratory... 

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