Alkyls, photochemistry

The predominant, if not exclusive, formation of 5/7-fused hydroxy ketones was observed in the case of 4-alkylated dienones [(204) (205) (R = CH3) 6 1 from (201) (R = CH3)] ° and of prednisone 21-acetate [(206)-> (207)]. It appears therefore likely that intermediates which represent the conjugate acids of the postulated zwitterionic intermediates in the dienone photoisomerizations [c/. (202), (203)] participate both in the acid-catalyzed transformations of (200) and in the dienone photochemistry in protic solvents. 

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

Two examples from ketone photochemistry that has been recently analyzed within the context of solid-to-solid transformations are the Norrish type and Nor-rish-Yang type Ip44,i45 tactions. In general terms, the type I reaction consists of a homolytic cleavage of bond a-to the carbonyl to generate an acyl-alkyl radical pair (RP-A) or an acyl-alkyl biradical (BR-A) when the ketone is cyclic (Scheme 7.15). 

Polymer Matrix Effects. In order to approximate the environment experienced by the arylcarbamate moieties in coatings based on aromatic diisocyanates, we chose to study the photochemistry of alkyl N-arylcarbamates in polymethacrylate (PMMA) and polypropyl-methacrylate (PPMA) films. First, however, 2a and 3a were irradiated in ethyl propionate (a model solvent for PMMA and PPMA) to determine the effect of the solvent polarity (dielectric) on the photolysis of the carbamates. Upon excitation at 280 nm, where the solvent absorbance was negligible, is 0.006 for 2a and 0j) is 0.005 for 3a. These values are significantly smaller... 

It is generally accepted that photolysis of R3SnX (R = alkyl group X = halogen) results in the homolytic cleavage of the Sn—X bond. However, under certain conditions, for example in polar solvents such as EtOH which can act as a Lewis base, the photochemistry can switch to heterolytic cleavage of the Sn—X bond followed by formation of solvent adducts such as R3(X)Sn(Sol) (Sol = solvent)17. 

Substituent effects on the -(aminoethyl)cyclohexenone photochemistry were carried out to study the relative kinetic acidities of the tertiary aminium radical47. The ease of the methylene hydrogen to be removed as H+ increased in the order of X = alkyl < Si(CH3)3 < C=CH (equation 13). 

Dauben and coworkers reached similar conclusions on the basis of their extensive investigations of the photochemistry of 3-alkyl-6,6,9,9-tetramethyl-A3 5(10 -hexalins (122)217. Direct irradiation of these compounds leads to the formation of 129 and 130 due to secondary irradiation of the initially formed triene 128, which exists as an equilibrium mixture of conformers cZc-128 and cZz-128 (equation 48). Their results showed that as the steric bulk of the 3-substituent increases, the rate of disappearance of starting material decreases and the cyclobutene (129) bicyclo[3.1.0]hex-2-ene (130) product ratio... 

Alkyl nitrates Oxidation capacity Photochemistry -30 Gg Significant... 

Photoexcited nitrobenzene may be used for benzylic hydroxylation (at C-9) of 17/3-acetoxy-3-methoxyoestra-l,3,5(10)-triene. The photochemistry of the 17/3-nitro-steroid (217) is markedly solvent dependent, the major products being in ether the 17-desnitro-compound (218), in propan-2-ol the hydroxylamine (219), and in EtOH-NaOEt the hydroxamic acid (220) and the cyclopropane (221). The hydroxamic acid (220) is probably formed through the oxaziridine (Scheme 7). Although there are analogies to this in the photochemistry of nitrones and oximes, the photoreduction of a nitroalkane in propan-2-ol to an alkyl-hydroxylamine appears to have no precedent. Further studies of photochemistry of conjugated... 

The absorption cross sections and photochemistry of alkyl nitrates are discussed in Chapter 4.J, where the... 

The photochemistry of ferf-butylazide in a nitrogen matrix at 12 K was followed by IR spectroscopy. Only one product, trimethyhmine, was observed. Thus, the removal of alpha hydrogen from the alkyl azide does not suppress 1,2-migration reactions. 

The solution-phase photochemistry of nine alkylazides was studied by Kyba and Abramovitch." ° Photolysis of tertiary alkyl azides leads cleanly to 1,2 alkyl migration. 

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