Alkenes photocydoadditions

The reactions with a.p-unsaturated carbonyl compounds also lead to cyclobutenes (2.102), and there is evidence that, in some cases at least, the mechanism is non-concerted and goes through biradical intermediates that can be trapped by a second molecule of the conjugated alkene (2.103). Intramolecular photocydoadditions offer routes to polycyclic structures, and the cyclobutene unit in the product provides a basis for subsequent chemical transformations such as oxidation 12.104). 

A second major mode of photocydoaddition involves 1.2-addition to the aromatic ring, and this predominates if there is a large difference in electron-donor/acceptor capacity between the aromatic compound and the alkene. It is therefore the major reaction pathway when benzene reacts with an electron-rich alkene such as 1,1-dimethoxyethylene (3.43) or with an electron-deficient alkene such as acrylonitrile (3.441. When substituted benzenes are involved, such as anisole with acrylonitrile (3.45), or benzonitrile with vinyl acetate (3.46), reaction can be quite efficient and regioselective to give products in which the two substituents are on adjacent carbon atoms. 

One of the first reported photocydoaddition reactions of benzene was with the highly electron-deficient alkene, maleic anhydride. 

Many examples of the intramolecular [2 + 2] photocydoaddition of alkenes to benzene derivatives have been reported. The acetophenone derivative 41 undergoes an efficient [2 + 2] photocydoaddition, leading to the cyclobutane derivative 42 (Scheme 5.9, reaction 18) [46, 47]. It was shown that, in this case, a nn triplet state is involved. The presence of a nitrile group in compound 43 induces a [2 + 2] cycloaddition at position 1,2 of the aromatic moiety, leading to intermediate VIII (reaction 19) [48]. Following tautomerization, the final product 44 is formed. 

Frequently, the intramolecular [2 + 2] photocydoaddition of an alkene to a benzene ring is followed by further pericyclic reactions. Such transformations yield... 

Photochemical reactions provide a classical access to four-membered ring compounds that generate major interest in organic synthesis, notably as intermediates in multistep syntheses. The [2 + 2] photocycloaddition of a,(3-unsaturated carbonyl and carboxyl compounds with alkenes and [2 + 2] photocydoaddition of ketones with alkenes (the Paterno-Buchi reaction) are discussed in Chapters 6 and 7, respectively. Yet, aside from these transformations, a variety of further reactions provides a systematic access to four-membered rings that possess a wide structural variation. Four-membered ring compounds may also be created via less-systematic photochemical transformations, many of which can be carried out without additional chemical activation. As a consequence, such transformations are rendered not only very convenient but also extremely interesting within the context of green chemistry. ... 

Cornelisse, J. andde Haan, R. (2001) Ortho photocydoaddition of alkenes and alkynes to the benzene ring, in Understanding el Manipulating Excited State Processes, Molecular and Supramolecular Photochemistry, Vol. 8 (eds V. Ramamurthy and K.S. Schanze). Marcel Dekker, New York,... 

The formation of exo-products is also the predominant pathway if acyclic alkenes are employed as reaction partners. The reaction of cyclopentenone and ethyl vinyl ether serves as an instructive example (Scheme 6.4) for two reasons. First, it exemplifies the regiochemical outcome (r.r. = regioisomeric ratio) of the [2 + 2]-photocydoaddition with HT-products 7 being predominantly formed (versus HH-products 8). Second, it illustrates the exo-preference with compound 7a prevailing over 7b (d.r. = diastereomeric ratio) [21]. However, it is also clear from this... 

The regioselectivity of intramolecular [2 + 2]-photocydoaddition reactions is predictable if five-membered ring formation is possible in the formation of biradicals of type C or C (rule of five, vide supra). If five-membered ring formation is not feasible, then six-membered rings are most readily formed. The facial diastereo-selectivity is efficiently controlled by a stereogenic center in the cyclopentenone if the intramolecular alkene is attached via a tether to this stereogenic center. The key step 16 —> 17 in the stereoselective synthesis of (—)-incarvilline (18) illustrates the point (Scheme 6.7) [28]. The side chain attached to C-4 in the cyclopentenone 16 carries the terminal alkene, which reacts intramolecularly with perfect regio- and diastereoselectivity to cyclobutane 17. 

Maleic anhydride (117) belongs to the first compounds, the [2 + 2]-photocycloaddi-tion reactions of which were extensively explored [112]. It is preferably converted to the corresponding cyclobutanes by irradiation in the presence of a sensitizer, for example, benzophenone, allowing the addition of a plethora of alkenes (Scheme 6.42). In a recent application the photocydoaddition product 118 of maleic anhydride and l,4-dichloro-2-butene was converted into the marine alkaloid ( )-sceptrin (119) [113]. 

Margaretha, P. (2005) Photocydoaddition of Cycloalk-2-enones to Alkenes, in Organic Photochemistry (Molecular and Supramolecular Photochemistry), Vol. 12 (eds A.G. Griesbeck and J. Mattay), Marcel Dekker, New York, pp. 211-237. 

Broeker, J.L., Eksterowicz, J.E., Belk, A.J., and Houk, K.N. (1995) On the regio-selectivity of photocydoadditions of triplet cyclohexenones to alkenes. Journal of the American Chemical Society, 117, 1847-1848. 

Schwebel, D. and Margaretha, P. (2000) Photocydoaddition of 2H-l-benzopyran-3-carbonitriles and 2H-l-benzothiopyran-3-carbonitriles to alkenes and alkenynes. Helvetica Chimica Acta, 83, 1168-1174. 

Photocydoadditions of enaminones 415 with alkenes that have either electron-withdrawing or electron-donating substituents give regioselec-tively the 2-hydroxy-l,2,3,4-tetrahydropyridines 416, which are dehydrated to give 1,4-dihydropyridines 417, Scheme 115 (79AGE540 82AGE539 ... 

Scheme 6.10. Photocydoaddition of Carbonyl Compounds with Alkenes... 

---