Benzophenone photolysis

The Diels-Alder reaction of isopyrazoles 365 with MTAD gives azoalkanes 366. Direct as well as triplet-sensitized (benzophenone) photolysis of these compounds leads to the corresponding housanes (bicyclo[2.1.0]pentanes) 367. Under acidic conditions, the housanes rearrange to the corresponding bicyclic products 368 <1995JOC308,... 

Single-bilayer dioctadecyldimethylammonium chloride (DODAC) (6) vesicles Sonication of surfactant-stabilized magnetite with DODAC Vesicle-incorporated magnetite influenced the outcome of benzophenone photolysis 789... 

More informative is a process described by Roth (1972). The direct and the sensitized (benzophenone) photolysis of diazomethane in toluene yields ethylbenzene, but a CIDNP effect can only be observed from the sensitized reaction. This result leads to the conclusion that the direct photolysis is a concerted CH insertion of a highly reactive species, whereas the sensitized reaction takes place by a hydrogen abstraction, followed by a radical addition (8-24). It is, therefore, likely that it is the singlet that reacts in the first reaction, but the triplet in the second process. 

Figure Bl.16.2. X-band TREPR spectra obtained at 0.1 ps after 308 mn photolysis of a fliiorinated peroxide dimer in Freon 113 at room temperature. Part A is the A/E RPM spectrum obtained upon direct photolysis part B is the E/A RPM spectrum obtained upon triplet sensitization of this reaction using benzophenone.

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

Photolysis in concentrated HCl of 3,5-diphenyl-2,l-benzisoxazole gave 2-amino-3-chloro-5-phenylbenzophenone and 2-amino-5-(p-chlorophenyl)benzophenone via similar intermediates (Scheme 20) (71HCA2111). 

The intermediate diphenylhydroxymethyl radical has been detected after generation by flash photolysis. Photolysis of benzophenone in benzene solution containing potential hydrogen donors results in the formation of two intermediates that are detectable, and their rates of decay have been measured. One intermediate is the PhjCOH radical. It disappears by combination with another radical in a second-order process. A much shorter-lived species disappears with first-order kinetics in the presence of excess amounts of various hydrogen donors. The pseudo-first-order rate constants vary with the structure of the donor with 2,2-diphenylethanol, for example, k = 2 x 10 s . The rate is much less with poorer hydrogen-atom donors. The rapidly reacting intermediate is the triplet excited state of benzophenone. 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

UV absorbers (UVA) act by absorbing UV light hence retarding the photolysis of hydroperoxides. Their activity is also associated with hydrogen bonding between the 2-hydroxy group and the carbonyl chromo-phore [22]. Typical examples are based on 2-hydroxy-benzophenones and 2-hydroxybenztriazoIes (e.g., Table lb, AO 23 and AOs 24-26). 

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

Another photocyclization to a benzo[c]phenanthridine was reported (127). Oppenauer oxidation of ( )-ophiocarpine (92) with potassium fm-butoxide and benzophenone in dioxane effected C-6—N bond cleavage to afford the hydroxyisoquinoline 219 via berberinephenolbetaine (121) (Scheme 39). Although photolysis of 219 gave only the oxepine 221, that of its methyl ether 220 furnished directly norchelerythrine (222) through electrocyclization followed by spontaneous elimination of methanol. 

In Section 3.1 it was shown that the photoreduction of benzophenone can be quenched by addition of small amounts of triplet quenchers such as oxygen or ferric dipivaloylmethide.<60) In fact this was presented as evidence that the benzophenone triplet was involved in the photoreduction. This reaction can also be quenched by naphthalene. In the presence of naphthalene, light is still absorbed by benzophenone and thus benzophenone triplets are produced. However, photoreduction products are decreased. On examining this reaction with flash photolysis, triplet-triplet absorptions were observed but these absorptions corresponded to those of the naphthalene triplet. Thus the triplet excitation energy originally present in the benzophenone triplet must have been transferred to naphthalene and since little of the photoreduction product was observed, this transfer must have been fast in relation... 

As with 2-cyclopentenone, the ratio (70) (71) varies with the molar concentration of the enone, the head-to-head dimer (71) becoming increasingly important at higher concentrations/133 This reaction is efficiently sensitized by acetophenone, benzophenone, thioxanthone, and naphthalene. The same enone concentration effect was observed in the sensitized photo-dimerization as in the direct photolysis. Similarly, quenching of the dimerization by piperylene was not accompanied by a change in dimer ratio. Systematic... 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

In the presence of benzophenone, (8) was again the major product (>95°/0) and only trace amounts of the cyclohexane products were produced. These results suggest the intermediacy of a singlet 1,6-hexylene biradical in the direct photolysis and a longer lived triplet 1,6-diradical in the sensitized photolysis. In the triplet biradical more time is available for 1,6-hydrogen transfer to occur prior to spin inversion and hence more olefin (8) is produced. Similar results were reported for the direct and photosensitized photolysis of the 3,8-dimethyl derivative of (7). 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

On flash photolysis of benzophenone in benzene two transients are observed/9-11 One of these, the shorter lived, is assumed to be the excited triplet of benzophenone, and the longer lived transient would appear to be the ketyl radical by comparison of its spectrum with that of authentic ketyl radical formed in solvents that are good hydrogen donors. This, however, may be an entirely new species which fortuitously absorbs in the same region as the ketyl radical. 

This reaction was first discovered by Paterno and Chieffi in 1909.(90) These workers isolated a compound from the photolysis of benzophenone in... 

Isopropanol forms 2 1 adducts with acetylenedicarboxylic acid upon photolysis in the presence of benzophenone.<77)... 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

Though the triplet sensitized photolysis of isoprene (159) does, as noted above, produce a complex mixture of products, one of these adducts has been used in the context of complex molecule synthesis (equation 5)71. Cyclobutane 160, which was formed in ca 20% yield by the benzophenone sensitized photolysis of 159, could be easily transformed into fragrantolol, 161, an isomer of grandisol isolated from the roots of the Artemisia fragrans, by simple hydroboration/oxidation of the less hindered double bond. 

In an attempt to sensitize the thiosulfate bond cleavage, benzophenone (10% by weight) was incorporated into the polymer film. Upon photolysis at 366 nm, the 639 cm 1 thiosulfate band was reduced (Figure 10) as in the case of direct photolysis at 254 nm and 280 nm. Since benzophenone is a known triplet sensitizer it is likely that the S-S bond cleavage in the thiosulfate group occurs from a triplet excited state in the sensitized reaction. Incidentally photolysis of a PATE film at 366 nm in the absence of benzophenone resulted in no loss of the 639 cm 1 IR peak. Unfortunately due to the film thickness, we were unable to obtain accurate quantum yields for either the direct or sensitized photolysis. Finally it should be noted that no chemical evidence has been presented to confirm disulfide formation. Results from the photolysis of a PATE-type model compound will be offered to substantiate the claim of disulfide formation as well as quantitate the primary photolysis step. But first, we consider photolysis of a PASE polymer film. 

Preparative photolysis of AETSAPPE (0.25 M aqueous solution) at 254 nm (Rayonet reactor) resulted in the formation of the disulfide product 2-amino(2-hydroxy-3-(phenyl ether) propyl) ether disulfide (AHPEPED) as the primary photoproduct Photolysis of AETSAPPE at 254 nm (isolated line of medium pressure mercury lamp) resulted in rapid initial loss of starting material accompanied by formation (analyzed by HPLC) of AHPEPED (Figure 12a and 12b) (Scheme IV). Similar results were obtained for photolysis- at 280 nm. Quantum yields for disappearance of AETSAPPE and formation of AHPEPED at 254 nm and 280 nm are given in Table I. The photolytic decomposition of AETSAPPE in water was also accomplished by sensitization ( x =366 nm) with (4-benzoylbenzyl) trimethylammonium chloride (BTC), a water soluble benzophenone type triplet sensitizer. The quantum yield for the sensitized disappearance (Table I) is comparable to the results for direct photolysis (unfortunately, due to experimental complications we did not measure the quantum yield for AHPEPED formation). These results indicate that direct photolysis of AETSAPPE probably proceeds from a triplet state. 

Use of less than one equivalent of benzophenone increases the photolysis... 

Fig. 14 (a) Epr fine structure exited of 5 = 4 at a high field, (b) An X-band epr spectrum observed after photolysis of a mixed crystal of benzophenone with [15a m = 4] at 4.2 K in an esr cavity. The external magnetic field is along the direction 26° from the a axis in the ab plane of the benzophenone crystal lattice. The microwave frequency is 9550.6 MHz. The central line is due to Cr(iii) in the MgO powder used as a reference. 

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