Photocycloaddition reactions of alkenes

Bach, T. and Bergmann, H. (2000) Enantioselective intermolecular [2 + 2]-photocycloaddition reactions of alkenes... 

A review has highlighted the photocycloaddition reactions of alkenes with aromatic esters and nitriles. Cycloadditions occur by a (3+2)-mode and provides a path to medium size ring systems. When the cinnamic acid derivative (9) is irradiated at 359 nm in ethanol with added Ti02 the product (10) is formed in 30% yield. Analogous products are formed from other straight chain alcohols such as (11) from propan-l-ol. Benzonitrile can be photochemically hydrated in the presence of oxophosphorus porphyrins. " ... 

Langer, K., Mattay, J., Copper(I) catalyzed Intra and Intermolecular Photocycloaddition Reactions of Alkenes. In Horspool, W. M., Song, P. S. (eds), CRC Handbook of Organic Photochemistry and Photobiology, CRC Press, Boca Raton, FL, 1995, pp. 84 104. 

Copper(I)-Catalyzed Inter- and Intramolecular [2 + 2]-Photocycloaddition Reactions of Alkenes... 

Photocycloaddition Reactions of Carbonyl Compounds and Alkenes. Photocycloaddition of ketones and aldehydes with alkenes can result in formation of four-membered cyclic ethers (oxetanes), a process often referred to as the Paterno-Buchi reaction.196... 

Scheme 6.11. Photocycloaddition Reactions of Carbonyl Compounds and Alkenes...

The synthetic applications 440) and mechanistic aspects 4411 of intermolecular photocycloaddition reactions of arenes to olefins have been reviewed recently. Intramolecular cycloadditions442a,b) have been studied in the context of the photochemical behaviour of bichromophoric molecules, as to investigate interchromophoric interactions in polyfunctional molecules. Three types of addition products can be formed in the photocycloaddition of benzene to an alkene (4.37)441. ... 

Demonstration of the unique synthetic utility of the [2 + 2] photocycloaddition reaction of enones to alkenes and the success in controlling the stereoselectivity, to some extent, in the intermolecular additions (discussed above) prompted further studies and development of new synthetic applications in the intramolecular photoadditions during the last decade. In most cases that have been studied, the alkene was tethered to the cyclic enone by three carbon units or two carbons and one heteroatom. 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

The Patemo-Biichi reaction is a photocycloaddition reaction of a n,ji carbonyl compound to an alkene in the ground state from either the Si or the rl i state. The reaction can occur through the initial C O bond formation or through a previous formation of the C—C bond. A frontier orbitals approach can be used to explain the formation of oxetanes. We can observe the HSOMO-LUMO interaction in which the half-occupied ji carbonyl orbital interacts with the unoccupied ji molecular orbital of an electron-deficient alkene, and a C,0-biradical is formed. The LSOMO-HOMO interaction in which the half-occupied n orbital of the carbonyl O atom interacts with the ji orbital of an electron-rich alkene, and a C,C-biradical is formed [13, 14]. 

In contrast to photocycloaddition reactions of carbonyl compounds to electron-rich alkenes (which proceed with a low degree of stereoselectivity in the case of triplet excited carbonyls), reactions with electron-deficient alkenes, such as cyanoalkenes, are, although rather inefficient, but highly stereoselective [101]. Kinetic analysis showed that these reactions involve the... 

Photocycloaddition reactions of alkyl and aryl 2-thioxo-3//-benzoxazole-3-carboxylates 142 to alkenes afforded stable isolable spirocyclic aminothietanes 143 <02HCA2383> similar reactions with both electron-poor and electron-rich alkenes were also performed on 2-methyloxazolo[5,4-h]pyridine <02EJO4211>. 

There are few reports of photocycloaddition reactions of di- and poly-ynes which involve more than one of the alkyne groups, but the production of toluene or other alkylbenzenes in the irradiation of butadiyne with propene or other terminal alkenes (equation 60) is one such process. ... 

The photocycloaddition reactions of 3-aryl-1,2-benzisothiazole with alkenes is regio- and stereospecific and yields 2,3-dihydro-1,4-benzothiazepines (Scheme 10) <8IT3377>. When electron-rich alkynes are used, then substituted 3,4-benzo-2,6-thiazabicyclo[3.2.3]hepta-3,6-dienes are the major product (Scheme 10) <83JOCi275>. 

Some years ago Cornelisse reported that deuteration of alkyl benzenes results in a deuterium isotope effect upon the quantum yield of the meta photocycloaddition reaction with alkenes. In a new report the same group has published an analysis describing how the observed isotope effect upon the reaction quantvun yield can be ascribed to a kinetic deuterium isotope effect on the excited state reaction and distinguished from an effect upon the unimolecular photophysical modes of decay of the excited state. In addition, it is reported that when the quantum yield of meta photocycloaddition of cyclopentene to alkyl benzenes is measured using a mixture of deuterated and non-deuterated benzenes, the quantum yield is arene concentration dependent.The authors argue that this arises from competition between cycloaddition and the formation of mixed excimers between deuterated and non-deuterated alkyl benzenes which dissociate to yield excited deuterated alkyl benzene and ground state non-deuterated alkyl benzene preferentially. 

Benzene in the Si (n,n ) excited state is no longer aromatic and is capable of undergoing various chemical reactions not observed in the ground state. Three basic types of photocycloaddition reaction of an alkene to the excited benzene can be de-scribed 802,804,809 816 (a) bicyclo[4.2.0]octa-2,4-dienes (e.g. 191) are formed by ortho-photocycloaddition (1,2- or [2 + 2] photocycloaddition), (b) tricyclo[3.3.0.02 8]oct-3-enes (e.g. 192) are obtained by mcia-photocycloaddition (1,3- or [3 + 2]... 

Photodecomposition of 1,2-benzisothiazole (17) affords the ring-opened disulfide 7.40 If the 3-position is blocked by substitution, then more interesting products may result. Photocycloaddition reactions of 3-phenyl-1,2-benzisothiazole (23) with alkenes afford excellent yields of 2,3-dihydro-1,4-benzothiazepins such as 24.41 With electron-rich alkynes, thiazabicyclohep-tadienes (25) are produced, but no reaction occurs with 2-butyne, 1-phenylpropyne, or dimethyl acetylenedicarboxylate.42 An analogous S—N bond fission occurs with 2-aryl-1,2-benzisothiazolinones.43... 

The photocycloaddition reaction of a cyclic enone to a cyclic alkene affords a tricyclic ring system (see II). 

As noted in the introduction to this section, under UV light irradiation, carbonyl groups [42] and alkenes [43] will add photochemically in a 2 + 2 sense across the 2- and 3-positions of indole, but only if an acyl or aroyl group is present on the indole nitrogen atom. The photocycloaddition reaction of carbonyl compounds with acylindoles was first reported by Julian and Tringham in 1973 [42], They found that irradiation of N-acetylindole or N-para-chlorobenzoylindole with benzophenone, benzoyl-formamide, or methyl benzoylformate gave oxetane products, as shown in Scheme 12. The stereochemistry of the products was not reported. No products were obtained when the indoles were irradiated with acetophenone, benzaldehyde, acetone, or propionaldehyde. This observation... 

A wide variety of N-acyl substituents has been found to activate the indole ring toward photocycloaddition reaction with alkenes without themselves undergoing any photochemistry [54]. These are summarized in Scheme 22. Selection of the appropriate substituent allows removal of the activating group from the cycloadducts using conditions that will not disturb any other functional groups that might be present. For example, the... 

A detailed account of the (2 + 2)-photocycloaddition reactions of homo-quinones has been published. The addition reactions afford regio and exo.endo isomers of tricyclic diones. The major product is thought to arise from the more stable triplet 1,4-biradical intermediate.A review has reported on the addition of alkenes and alkynes to the enone system of homobenzoquinones. Dibromo-9-(3-oxo-3-phenylprop-l-ynyl)-[9 -fluoren-9-yl-3-fluoren-9-ylidene-1-phenylpropenone is photochromic in the crystalline state. 

The intramolecular variant of the [2+2] photocycloaddition of indoles has also been reported. In an elegant formal synthesis of vindorosine (44, Scheme 9), Winkler and coworkers employed the intramolecular photocycloaddition reaction of a vinylo-gous amide carbon-carbon double bond to the alkene of an indole in a substrate... 

Andrew D, Hastings DJ et al (1992) Triplet 1,4-biradical intermediates in the photocycloaddition reactions of enones and N-acylindoles with alkenes. Pure Appl Chem 64 1327-1334... 

Aromatic compoimds also undergo photocycloaddition reactions with alkenes, leading to 1,2-, 1,3-, and (less often) 1,4-adducts, as shown for the reaction of benzene with ethene in equation 12.70. Olefins with strongly electron-withdrawing or electron-donating substituents tend to give 1,2-photoaddition products, while olefins with alkyl substituents tend to give mostly 1,3-photoaddition. 

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