Intramolecular photoreactions, carbonyl compounds

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. 

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-... 

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. 

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. 

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. 

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,... 

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... 

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