Intramolecular photoreactions, carbonyl

The course of the intramolecular photoreaction of carbonyl compounds with electron-rich alkenyl- or aryl-substituents in the side-chain is dictated essentially by the thermodynamics of the electron-transfer step. This relationship has been intensively studied for phthalimides. When AG°et is positive, [n 2 + 2] cycloaddition reactions were observed with alkenyl substituents and classical Norrish II chemistry for aryl-substituted substrates. When AG°et was negative, electron transfer prod-... 

Intramolecular Photoreactions of Carbonyl Compounds and Related Processes. The intramolecular photoreactivity of aromatic ketones is subjected to major changes when reactions are carried out in the presence of CD. The formation of inclusion complexes was detected by NMR spectroscopy and X-ray powder diffractometry. 

The solid-state photoreaction of O-methyl iV-(/3,7-unsaturated carbonyl)-iV-phenylthiocarbamate resulted in intramolecular [2+2]-thietane 23 formation, followed by rearrangement to 7-thiolactone 24 (Scheme 1) <1998CC2315>. 

Sakamoto et al. also demonstrated the absolute oxetane synthesis in the solid-state photolysis of N-(a, 3-unsaturated carbonyl)benzoylformamides 39 (Scheme 18) [25]. These compounds undergo intramolecular [2 + 2] cyclization to give bicyclic oxetanes 40 in homogeneous solution. The x-ray analysis of N-isopropyl substituted imide 39a revealed that the crystal system was monoclinic and the space group P2 (Table 7). Crystals of 39a were powdered and photolyzed at 0°C. The imide undergoes the [2 + 2] cycloaddition to afford the bicyclic oxetane 40a, which is a mixture of diastereomers, namely, syn- and anti isomers at the C-7 position. In this reaction optically active syn-oxetane 40a with 37% ee (84% chemical yield) and racemic anti-40a were obtained (Table 8). The solid-state photoreaction proceeded even at — 78°C, and optically active syn-40a, which showed ee value as high as > 95% ee (conv 100%, chemical yield 89%), was... 

In Schemes 30 to 35, products of the irradiation of several systems containing carbonyl chromophores (oxoamides, adamantyl phenyl ketones, a-mesitylaceto-phenones, and a-benzonorbomyl aceotophenones) are presented [282,301-303], Primary photoreaction in every one of these cases is intramolecular 7- or 8-hydrogen abstraction. In the majority of these cases the final products of interest are cyclobutanols (Scheme 36). Of the forty compounds examined, the de of the cyclobutanols or cyclopentanols in solution is less than 15%. On the other hand,... 

Norrish type II photoreaction Intramolecular abstraction of a y-hydrogen by an excited carbonyl compound to produce a 1,4-hiradical as a primary photoproduct e-g ... 

A recent synthesis of 3-substituted furan derivatives illustrates an important application of the furan-carbonyl photocycloaddition. Zamojski has reported the rearomatization of oxetane (115) in the presence of p-toluenesulfonic acid to 3-furylmethanol derivative (116). Synthesis of (117), itself a substrate for the intramolecular photocycloaddition reaction (Section 2.4.6), involved a similar rearomatization process (PPTS/CHjClj) and capitalized upon the chemoselectivity observed in the ketone-furan photocycloaddition. Similarly, a synthesis of perillaketone (118) by Kawanisi involved irradiation of a carbonyl compound and furan. A complication in the rearomatization is that acid also catalyzes the reversion of the photoadduct to starting materials to circumvent this problem the photoreaction was run in the presence of acid, so that rearomatization would occur in situ and the products of competitive reversion would promptly recombine. 

One of the earliest photoreactions to be studied was the photoreduction of benzophenone (Ciamician and Silber, 1900)—that is, the conversion of a carbonyl compound into an alcohol by an intermolecular hydrogen abstraction reaction. Intramolecular hydrogen abstraction by the carbonyl group, usually from the y site, is referred to as a Norrish type II reaction. Hydrogen abstraction by olefins and heterocycles has also been observed. 

Hydrogen abstraction by an excited carbonyl group is the most typical photoreaction in both solution and solid states. A number of intramolecular Norrish type II hydrogen abstraction reactions in the crystalline state are already known, and the geometric requirements have been precisely discussed by Scheffer [58]. Solid-state asymmetric induction in the Norrish type II photocyclization of carbonyl compounds using supramolecular approaches has been also intensively studied. 

In contrast to the preferred meta mode of intramolecular photoaddition of 5-phenyl-l-pentenes, where the alkene and benzene groups are separated by three atoms, irradiation of the styrene (64) yields a single stereoisomer of the ortho adduct (65). In (64), not only are the reacting units separated by 4 atoms, but also it is the styrene rather than the benzene which is excited. Comparable photoreactivity is seen for phenanthrene-styrene systems such as (66) which yield 2+2 adducts (67) along with products derived from competing Paterno-Btichi reaction of the ester carbonyl with the alkene side chain. The photochemical cycloaddition also proceeds in an intermolecular fashion between the ester of 9-phenanthrene carboxylic acid and para-methoxy-0-methylstyrene. The mechanism of this reaction is shown to involve addition of the styrene to the singlet excited state of the phenanthrene derivative. °... 

Recent work on the role of solvated electrons in intra-DOM reduction processes has demonstrated the importance of trapped e in reactions with species adsorbed on the DOM matrix [98-100]. Modeling of DOM mediated photoreactions indicated the importance of sorption of molecules to DOM for reaction to occur [98, 99]. This is consistent with the lifetime of e" precluding escape from the aqueous DOM matrix into bulk solution. Since many important reactions with environmental implications involve binding or adsorption to DOM - see, for example, [3,101,102] - the role of matrix effects and the caged electron could be very significant. Some workers have suggested that since e remains primarily trapped within the DOM matrix, Oj must be formed by direct electron transfer from the excited triplet state of DOM to O2 [14]. However, it is equally if not more plausible that Oj may be produced by the reduction of Oj by radicals or radical ions produced by intramolecular electron transfer reactions from irradiated DOM [25]. The participation of radicals in the production of carbonyl sulfide and carbon monoxide from irradiated DOM in South Florida coastal waters was recently demonstrated by Zika and co-workers [81-83] and potential pathways for the formation of free radicals from irradiated DOM were discussed. Clearly, the relative contribution of e q and associated transients to the photochemistry of DOM has not been unequivocally resolved in the literature. 

In addition to alkenes and acetylenes, carbonyl-containing compounds also participate in [2+2] cycloaddition reactions with indoles. The first example of oxetane formation via the photoreaction of indoles with a carbonyl group (a type of Patemo-Biichi reaction) was reported by Machida and coworkers [34, 35] (Scheme 16), who carried out the intramolecular cyclization of iV-acetylindole with phthalimide tethered with appropriate linkers 67a-d. Interestingly, only the... 

A stereogenic center on the tether (44a), composed of a relatively small hydroxyl substituent, results in the four possible product isomers shown in Figure 16. The designations anti and syn isomers refer to the orientation of the tether substituent and the adjacent carbonyl group in the product. Alcohol 44a yields all four possible products, and trans is preferred over cis by a factor of 2.5 or 4.2, depending on the solvent. The primary difference in the product ratios comes from the solvent-dependent synlanti ratio. In the protic solvent methanol, the anti isomers comprise 85% of the product whereas in the aprotic methylene chloride the syn isomers are 56% of the mixture. The syn selectivity may result from an intramolecular hydrogen bond of the alcohol to the nearby carbonyl (see pro-trans-syn conformation). Steric enhancement of the alcohol as a /-butyldimethylsilyl ether (44b) results in a solvent independent anti-selective photoreaction. The amount of syn isomers produced with a t-butyldimethylsilyloxy substituent is less than 1%. ... 

Photolysis of a carbonyl compound having y-hydrogen commonly gives a shorter chain carbonyl compound, an alkene, and a cyclobutanol, which are formed from a 1,4-biradicaP produced via y-hydrogen abstraction by the excited carbonyl. The photoreactions derived from the 1,4-biradical are termed the Norrish type 11 reaction, and those involving intramolecular hydrogen abstraction from y- and other positions are generically referred to as the type II family. The chemical behavior of the 1,4-biradical determines the type 11 reactivity of carbonyl compounds. In this chapter, the influence of environment on the formation and behavior of the 1,4-biradical from simple alkyl aryl ketones will be described. Environmental effects on the type 11 reaction have been reviewed. ... 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

The photochemistry of carbonyl compounds has been one of the main areas of research in organic photochemistry for many years. Among aU the different types of carbonyl compounds, P,y-unsaturated ketones have been the subject of extensive studies. The results obtained from these efforts, conducted over a 30-year period, show that, in general, direct irradiation of P,y-unsaturated ketones yields products resulting from 1,3-acyl migration, while triplet-sensitized reactions of these compounds affords cyclopropyl ketones by oxa-di-n-methane (ODPM) rearrangement pathways. Alternative reaction routes, such as decarbonylation, ketene formation, epimerization, [2+2]-intramolecular cycloadditions, Norrish Type I and Norrish Type II reactions, cis-trans isomerizations, and reductions of the C-C double bond, have also been described in some instances, depending on some particular structural features present in the P,y-unsaturated ketone. However, the photoreactivity of these compounds is dominated by the two main processes mentioned above. 

The studies carried out on di-Jt-methane for many years involved the direct or triplet-sensitized irradiation of 1,4-unsaturated systems. The results obtained show that, in general terms, acyclic 1,4-dienes undergo the DPM reaction in the singlet excited state while cyclic and polycyclic dienes are reactive as triplets. > > f " >" The ODPM, 1-ADPM, and 2-ADPM counterparts only occur in the triplet excited state of P,y-unsaturated carbonyl compounds, 8 1-aza-1,4-dienes,and 2-aza-l,4-dienes, > > respectively. An attempt to observe di-7i-methane reactions promoted by SET sensitization was unsuccessful. Thus, Zimmerman and Hoffacker carried out a study on the photoreactivity of 1,4-dienes 21 using 9,10-dicyanoanthrathene (DCA) or 1,4-dicyanonaphthalene (DCN) as electron-acceptor sensitizers. The results obtained show that these compounds undergo intramolecular cychzation to yield dihydronaphthalenes 22 (Scheme 5). The corresponding DPM products are not produced in these photoreactions. 

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