Photoinduced controlled radical

TAS 10a] Tasdelen M.A., Uygun M., Yagci Y., Photoinduced controlled radical polymerization in methanol , Macromolecular Chemistry and Physics, vol. 211, pp. 2271-2275, 2010. 

Tasdelen MA, Uygun M, Yagci Y (2011) Photoinduced controlled radical polymerization. Macromol Rapid Commun 32(l) 58-62... 

Mattay et al. examined the regioselective and stereoselective cyclization of unsaturated silyl enol ethers by photoinduced electron transfer using DCA and DCN as sensitizers. Thereby the regiochemistry (6-endo versus 5-exo) of the cyclization could be controlled because in the absence of a nucleophile, like an alcohol, the cyclization of the siloxy radical cation is dominant, whereas the presence of a nucleophile favors the reaction pathway via the corresponding a-keto radical. The resulting stereoselective cis ring juncture is due to a favored reactive chair like conformer with the substituents pseudoaxial arranged (Scheme 27) [36,37]. 

While solid matrices have been employed successfully, they may be less than ideal for controlled mechanistic studies. A more appropriate technique for controlled doublet photochemistry appears to be two-photon excitation in solution. In this experiment, the first photon is used to initiate radical ion formation, whereas the second photon, appropriately delayed to coincide with the maximum concentration of the radical cation so generated and tuned to its absorption maximum, serves to excite these intermediates. However, we hasten to add that the benefits of this technique have yet to be demonstrated. The photoinduced rearrangement of radical cations very likely will benefit substantially from a mismatch between (quartet vs. doublet) potential surfaces, much as triplet sensitized isomer-izations can be ascribed to mismatches between triplet and ground state surfaces. 

A kinetic scheme can be written, Scheme 1, in which the rates of back electron transfer from the contact, solvent-separated, and free ions must each be individually considered. Clearly, the greater the tendency of a given solvent to separate the ions, the less significant will be the electrostatic interaction between them, and the greater tendency to suppress back electron transfer for a period sufficiently long to allow radical ion chemistry to ensue. A major concern of this article will be in defining the efficacy of supramolecular assemblies which create inhomogeneous arrays to controll the secondary chemistry of radical ions formed via photoinduced electron transfer. 

Radical ion pairs play an important role in photoinduced electron transfer (PET) processes. Several types of them may exist and it will be shown in tins article that contact ion pairs (CIPs) and solvent-separated ion pairs (SSIPs) can be differentiated by various experimental methods. Moreover, their controlled formation facilitates the control of chemical reactions. Although this review is by no means exhaustive, it is hoped that interest in the fundamental mechanistic aspects of PET processes in homogeneous media as well as in their synthetic applications will arise. 

As discussed earlier, deprotonation of a-carbon forms a major reaction pathway for the disappearance of the amine radical cation. Studies of photoinduced electron-transfer reactions of tertiary amines by Lewis [7, 11] and by Mariano [5, 10] have contributed significantly towards our understanding of the factors that control this process. Lewis and coworkers used product-distribution ratios of stilbene-amine adducts to elucidate the stereoelectronic effects involved in the deprotonation process [5, 10, 121, 122]. In non-polar solvents, the singlet excited state of tran -stilbene forms non-reactive but fluorescent exciplexes with simple trialkylamines. Increasing solvent polarity brings about a decrease in the fluorescence intensity and an increase in adduct formation. For non-symmetrically substituted tertiary amines two types of stilbene-amine adduct can be formed, as is shown in Scheme 9, depending on whether the aminoalkyl radical adding to the stilbene radical is formed by de-... 

Irradiation of a system consisting of 2-methyl-1,4-dihydroxynaphthalene and benzophenone leads to formation of benzophenone ketyl radical and 2-methyl-naphthosemiquinone, and 1 a,7a-dihydro-1 a-methyl-1,1 -diphenyl-1 H-cyclopropa-[b]naphthalene-2,7-dione derivatives (4) in the presence of Mg(C104)2 give indenonaphthoquinones (5) (Scheme 1), by photoinduced intramolecular electron transfer in almost quantitative yield. In agreement with Rehm-Weller predictions, the rate constants for electron transfer between the triplet states of various substituted naphthoquinones and N,N-dimethylaniline (DMA) in acetonitrile solution were found to be diffusion controlled. " Using this information, it has been shown that the efficiencies of formation of naphthalene " and DMA are unity. 

Few things are known in the control of photopolymerization reactions. In a light-induced reaction, a photoiniferter can be used. Both the initiation and the reversible termination are photoinduced. The mechanism of a classical living radical photopolymerization process is recalled in e21. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

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