Photochemistry intramolecular -cycloadditions

Aryl substituents also activate benzyHc C-H bonds adjacent to a nitrene center. An interesting study on this topic has dealt with the atropoisomeric 3,5-dimethyl-2-(9-fluorenyl)phenylazides 49 and 52. ° The low-temperature photochemistry (77 K) of these rotameric azides proved to be fundamentally different Photolysis of 49 resulted in the formation of nitrene 50 in addition to azanorcaradiene 51, while photolysis of 52 gave nitrene 53 in addition to iminoquinone methide 54. This subsequently underwent intramolecular cycloaddition to yield dihydroindenocridine 55. Laser flash photolysis showed that the formation of iminoquinone methide 54 from azide 52 occurred in less than 10 ns. Deuteration of the 9-position of the fluorene chromophore resulted in a significantly diminished yield of 55. LFP of the rotameric azide 49 yielded the rate constant for the formation of azanorcaradiene 51 which was determined as k = 7.1 x 10 ... 

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. 

Xue, J., Xu, J.W, Yang, L., and Xu, J.H., Photoinduced reactions of chloranil with 1,1-diarylethenes and product photochemistry-intramolecular [2+2] (ortfeo-)cycloadditions of excited enedione s C=C double bond with substituted benzene ring, /. Org Chem., 65, 30, 2000. 

The present volume contains 13 chapters written by experts from 11 countries, and treats topics that were not covered, or that are complementary to topics covered in Volume 1. They include chapters on mass spectra and NMR, two chapters on photochemistry complementing an earlier chapter on synthetic application of the photochemistry of dienes and polyenes. Two chapters deal with intermolecular cyclization and with cycloadditions, and complement a chapter in Volume 1 on intramolecular cyclization, while the chapter on reactions of dienes in water and hydrogen-bonding environments deals partially with cycloaddition in unusual media and complements the earlier chapter on reactions under pressure. The chapters on nucleophiliic and electrophilic additions complements the earlier chapter on radical addition. The chapter on reduction complements the earlier ones on oxidation. Chapters on organometallic complexes, synthetic applications and rearrangement of dienes and polyenes are additional topics discussed. 

The photochemistry of phthalimide systems was thoroughly investigated by many groups over the last two decades. This chromophore shows a broad spectrum of reactivity leading mainly to cycloaddition and photoreduction products by either intermolecular or intramolecular processes. In the presence of electron donors, the electronically excited phthalimide could also undergo electron transfer and act as an electron acceptor. 

De Keukeleire, D., Bako, P., and Van der Eycken, E. (2000) Intramolecular ortho and meta photocycloadditions of 4-phenoxybut-l-enes substituted in the arene residue with carbomethoxy, carbomethoxymethyl, and 2-carbo-methoxyethyl groups. Journal of Photochemistry and Photobiology A Chemistry, 133, 135-146 (c) Wender, P.A. and Dore, T.M. (1995) Intra and inter-molecular cycloadditions of benzene derivatives, in CRC Handbook of Organic Photochemistry and Photobiology (eds W.M. Horspool and P-.S. Song), CRC Press, Boca Raton, pp. 280-290. 

The reaction is highly stereospecific, as was shown by Dauben and Ritscher in their study of the photochemistry of the isomeric 3-ethylidenecyclooctenes 20 (equation 25 and 26) . The fact that two isomeric bicyclobutanes (73 and 74) which are both epimeric at a single stereocenter are produced from ,Z-20 is incompatible with an intramolecular [7t2s 7r2i]-cycloaddition mechanism for the process. This and other considerations led... 

Although little has previously been known of the photochemistry of indoles, five reports of cycloadditions to indole have appeared this year (Giesler et al., inter alia). Intramolecular photoaddition of an alkene to an N-acylindole has been used in synthesis of the alkaloid vindorosine (36) (Winkler et al.). Irradiation of mesitylene... 

Intramolecular [2+2] cycloaddition reactions of enones D. Becker, N. Haddad, Organic Photochemistry 1989,10, 1- 162. 

The 1980s witnessed a revival of interest in synthetic photochemistry, largely based on the successful application of intramolecular arene-olefin and enone-olefin photochemical cycloaddition reactions to the synthesis of polycyclic natural and unnatural products. The potential application of intramolecular arene-amine addition to the synthesis of alkaloids was suggested in 1973 by Btyce-Smith et al.," based on their observation of intramolecular photochemical 1,4-addition reaction of the (N,A/-dimethylaminoalkyl)benzenes I and 2 to yield 3 and 4, respectively. Intramolecular addition reactions have subsequently been observed for a large number of (aminoal-... 

The following additional examples offer a hint of the utility of organic photochemistry in s)mthesis. A [2 + 2] photochemical cycloaddition was a key step in the synthesis of cubane. S)mthesis of 2-bromocyclopenta-2,4-dienone resulted in a spontaneous Diels-Alder reaction, which produced 132. The monoketal of the dimer (133) underwent intramolecular photocycloaddition to produce 134 (equation 12.85). The Favorskii reaction of 134 gave 135, which was decarboxylated to 136 (equation 12.86). Hydrolysis of the ketal gave 137. Subsequent Favorskii reaction to 138 and then decarboxylation produced cubane, 139. Similarly, de Meijere and co-workers reported that the intramolecular [2 + 2] photochemical cycloaddition of 140 produced octacyclopropylcubane (141, equation 12.87) Meder and coworkers utilized the reaction sequence in equation 12.88 to convert the diazo compound 142 into tetra-f-butylcyclobutadiene (143) and tetra-f-butyltetra-hedrane (144). " ... 

The photochemistry of aromatic compounds is classified into the same categories adopted in the previous reviews in the series. The photoisomerization of arylalkenes, photoaddition and cycloaddition to aromatic rings, photosubstitution, photorearrangement reactions have less appeared in the period (2010-2011) considered. On the other hand, the photo-chromism including photoisomerization of azobenzenes and intramolecular photocyclization and cycloreversion of 1,2-diarylethenes, and the photodimerization have been widely developed. Supramolecular Photochemistry as a series of Molecular and Supramolecular Photochemistry was edited by Ramamurthy and Inoue in the period. In addition, it should be noteworthy that so many photochemical reactions in solid and/or crystalline states have appeared and developed. 

As for the synthesis of acylcyclobutanes by cycloaddition, this is often a very eflhcient process and is one of the most largely used synthetic procedures in organic photochemistry. An example is the addition of uracil to alkenes (see Scheme 2.14) [25]. The intramolecular version is most often used and gives excellent results (see Scheme 2.15) [26, 27]. 

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