Substrates photolysis

The (thermal) decomposition of thiazol-2-yldiazonium salts in a variety of solvents at 0 C in presence of alkali generates thiazol-2-yl radicals (413). The same radicals result from the photolysis in the same solvents of 2-iodothiazole (414). Their electrophilic character is shown by their ability to attack preferentially positions of high rr-electron density of aromatic substrates in which they are generated (Fig. 1-21). The major... 

Because di-/ fZ-alkyl peroxides are less susceptible to radical-induced decompositions, they are safer and more efficient radical generators than primary or secondary dialkyl peroxides. They are the preferred dialkyl peroxides for generating free radicals for commercial appHcations. Without reactive substrates present, di-/ fZ-alkyl peroxides decompose to generate alcohols, ketones, hydrocarbons, and minor amounts of ethers, epoxides, and carbon monoxide. Photolysis of di-/ fZ-butyl peroxide generates / fZ-butoxy radicals at low temperatures (75), whereas thermolysis at high temperatures generates methyl radicals by P-scission (44). 

There are therefore two ways in which lasers may be used to bring about photon-assisted film formation. If the laser emits radiation in the near-ultra-violet or above, photochemical decomposition occurs in the gas phase and some unabsorbed radiation arrives at the substrate, but this latter should be a minor effect in die thin film formation. This procedure is referred to as photolysis. Alternatively, if the laser emits radiation in the infra-red, and tire photons are only feebly absorbed to raise the rotational energy levels of the gaseous... 

Aziridines can be prepared directly from double-bond compounds by photolysis or thermolysis of a mixture of the substrate and an azide. The reaction has been carried out with R = aryl, cyano, EtOOC, and RSO2, as well as other groups. The reaction can take place by at least two pathways. In one, the azide is converted to a nitrene, which adds to the double bond in a manner analogous to that of carbene addition (15-62). Reaction of NsONHC02Et/ CuO [Ns = A(/7-toluenesulfonyl-inimo)] and a conjugated ketone, for example, leads to the A-carboethoxy aziridine derivative.Calcium oxide has also been used to generate the nitrene.Other specialized reagents have also been used." ... 

Pyrazolines (51) can be converted to cyclopropane and N2 on photolysis""" or pyroiysis. The tautomeric 2-pyrazolines (52), which are more stable than 51 also give the reaction, but in this case an acidic or basic catalyst is required, the function of which is to convert 52 to 51." In the absence of such catalysts, 52 do not react/ In a similar manner, triazolines (53) are converted to aziridines." Side reactions are frequent with both 51 and 53, and some substrates do not give the reaction at all. However, the reaction has proved synthetically useful in many cases. In general, photolysis gives better yields and fewer side reactions than pyrolysis with both 51 and 53. S/Z-Pyrazoles" " (54) are stable to heat, but in some cases can be converted to... 

In this reaction, photolysis of the substrate 138 gives the bicyclo[3.1.0]hexenone (143). Though the reaction is formally the same (note the conversion of 133 to 134... 

The photolysis of chlorinated aromatic compounds occurs by several processes which follow predictable routes 13). They frequently undergo photochemical loss of chlorine by dissociation of the excited molecule to free radicals or, alternatively, through a nucleophilic displacement reaction with a solvent or substrate molecule. Either mechanism is plausible, and the operation of one or the other may be influenced by the reaction medium and the presence of other reagents. 

To our knowledge, the first published report of a photocatal-ytic reaction at elevated pressure was W. Strohmeyer1s hydrogenation of 1,3-cyclohexadiene under hydrogen at 10 atm /22/. On photolysis, the iridium complex 8 formed a very active catalyst, probably by dissociation of a phosphine ligand (Equation 17). At 70 C, with hydrogen at 10 atm, and a catalyst/substrate ratio of 1/100,000, the activity was 196 per minute and the turnover number was 96,000 mol of product/mol catalyst. 

Diazepines (150) can be prepared by ultraviolet photolysis of iV-iminopyridinium ylids (151) but N—N bond homolysis in the ylid substrates presents a problem in regard to the synthesis of N-l substituted... 

I,, = 0.7 h on silica gel, tA = 2.2 h on alumina and tA = 44 h on fly ash for different atmospheric particulate substrates determined in the rotary photoreactor (appr. 25 pg/g on substrate) (Behymer Hites 1985) direct photolysis tA = 9.08 h (predicted-QSPR) in atmospheric aerosol (Chen et al. 2001). Photodegradation k = 3 x 10-5 s in surface water during the summertime at mid-latitude (Fasnacht Blough 2002)... 

Photolysis half-lives on different atmospheric particulate substrates (approx. 25 (xg/g on substrate) t,2 = 7.0 h on silica gel, t,/2 = 22 h on alumina and t,/2 = 29 h on fly ash (Behymer Hites 1985) first order daytime photodegradation rate constants for adsorption on wood soot particles in an outdoor Teflon chamber k = 0.0077 min-1 with 1000-2000 ng/mg loading and k = 0.0116 min-1 with 30-350 ng/mg loading (Kamens et al. 1988) ... 

Scheme 3)10. Indeed, independent photolysis of 2,4-cyclohexadien-l-ones 12 and 13 afforded the macrolides 15. These reactions likely proceed via a common intermediate, in this case dienylketene 14, which is trapped intramolecularly by the pendant hydroxyl group. Adjustment of the oxidation level and functional group interconversion then led efficiently to the desired macrolide 17. The sulfonyl group was used for two reasons first, to easily transform lactones 15 into dienyl lactones 16 needed for 17, and secondly, to control the regiochemistry of the Wessely oxidation of phenolic precursor needed to produce the photolysis substrates 12 and 13. 

In one of the earliest applications of this type of process to complex molecule synthesis, Corey and Hortmann, in their synthesis of dihydrocostunolide 38, found that photolysis of 36 afforded a photostationary state of 36 and 37 (Scheme 9)19. Hydrogenation of this mixture then gave 38. A recent modification of this synthesis, which avoids the photostationary equilibrium between eudesmane (36) and germacrane (37) forms, was realized using a modified substrate, 3920. Irradiation of 39 provided a 77% yield of a mixture of diastereomeric ketones 41 these are produced via tautomerization of the intially produced trienol 40. Dienone 41 was then easily converted to 38 via a series of conventional steps (Scheme 9). 

Lactones by ring expansion of lactols. Medium-sized lactones can be obtained in reasonable yield by photolysis of the hypoiodites of catacondensed lactols, which results in regioselective cleavage of the bridging bond. The substrates are available by the general route shown for the 6/6 lactol 1, which is cleaved to the 10-membered lactone 2. This cleavage can be used for preparation of 9-membered... 

Much also depends on how the carbene is generated. Duncan et al showed that carbene generated by the photolysis of ketone adds to the above substrate in a non-stereospecific manner. 

Laser Flash Photolysis at 351 nm of tert-Butyl Peroxide/Benzene Solutions Containing MDI-PUE and Model Compounds. Photolysis of tert-butyl peroxide (TBP) results in a highly efficient production of tert-butoxy radicals. It has recently been shown (15) that tert-butoxy radicals generated by the laser flash photolysis of TBP can rapidly extract hydrogen atoms from appropriate substrates such as aniline and diphenylamine (Scheme III). 

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