Alkenes photocycloadditions

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... 

The isomeric cage products 248-250 are formed from 247 via intramolecular benzene-alkene photocycloadditions between chloranil and 1,1-diarylethenes, induced by the triplet excited enedione moiety [283] (Scheme 69). 

Griesbeck, A.G., Fiege, M., Bondock, S., and Gudipati, M.S. (2000) Spin-directed stereoselectivity of carbonyl-alkene photocycloadditions. Organic Letters, 2 (23), 3623-2625. 

Scheme 5 Mechanistic pathway for triplet alkene-alkene photocycloaddition.

Alkyl tethered alcohols such as D-mannitol and L-erythritol [20], have been used to bring cinnamyl units together for photocycloadditions. However, this probably involves cycloaddition between an excited enone moiety and the tethered alkene. Other examples of carbon linked alkenes include perhaps the earliest example of alkene+alkene photocycloaddition. Liu and Hammond [21] reported the formation of cyclobutane from the triplet irradiation of myrcene (see Sch. 11). 

Perhaps the most common tethering is the use of alkyl linked styrenes that result in cyclophanes upon irradiation [24]. Okada et al. have reported several systems that undergo efficient alkene+alkene photocycloaddition (see Sch. 13) [25]. A common explanation for the effectiveness of the cycloaddition between these tethered styryl groups is that 7i-stacking orients the alkenes in close proximity so that the excited state alkene can readily interact with the adjacent ground-state alkene. 

Cubane-type compounds have been synthesized by a variety of routes. Most take advantage of the tethered 2+2 photocycloaddition. The first synthesis was reported by Eaton [33], although his synthetic route employed an enone+alkene photocycloaddition. An alkene+alkene cycloaddition was used to synthesize the propellacubane shown in Sch. 21 [34]. A more recent report decribed the synthesis of permethylated cubane using the alkene+ alkene photochemical approach [35]. That photocycloaddition is shown in Sch. 22. Although this particular reaction is inefficient, the product was made in sufficient quantity to allow for its complete characterization. 

Alkene+alkene photocycloaddition in the presence of electron transfer sensitizers involves a similar path. For example, see (a) Asaoka S, Ooi M, Jiang P, Wada T, Inoue Y. J Chem Soc, Perkin 2 2000 73 (b) Goez M, Frisch I. J Am Chem Soc. 1995 117 10486. 

An early synthetic application of such intermolecular enone + alkene photocycloadditions was presented by de Mayo in 1964. 5,5-Dimethyl-3-hydroxycyclohex-2-enone (1), i.e., the enol of dimedone, adds to e.g., cyclohexene, and the primarily formed cyclobutane 2 undergoes a... 

In contrast to these intermolecular enone + alkene photocycloadditions, the regioselectivity in the corresponding intramolecular cycloisomerizations of alkenylcycloalkenones is controlled primarily by... 

Enantioselectivity in intermolecular enone+ alkene photocycloadditions is quite difficult to accomplish, due to the fact that stereochemical information inherent in the starting materials can easily be lost on the stage of the triplet 1,4-biradical intermediate. Nevertheless highly enantioselective photocycloadditions of quinolones to alkenes mediated by a chiral lactam... 

Three typical procedures will be presented (Sch. 19). The first represents one of the (only) two enone + alkene photocycloaddition reactions which have published as a procedure in Organic Synthesis [75], the second one is of... 

A complete overview of syntheses of nonnatural cyclobutanes via enone + alkene photocycloadditions would be far beyond the scope of this chapter, recent reviews on this topic having been published recently [33,83]. 

The second product (21) in the reaction of the diester (equation 70) results from 1,3-addition to the aromatic ring. Such addition is the major mode of reaction for benzene/alkene photocycloadditions, but with alkynes it is less common. One of the few reported examples is the addition of diphenylacetylene to esters of trimesic acid (equation 71),... 

Furthermore, the occurrence of an exciplex intermediate is assumed an empirical correlation based on the AG t values calculated from the Weller relation. Equation (5.28), has been established which allows the prediction of mode selectivity for a wide range of arene-alkene photocycloadditions (Mattay, 1987). For negative or very small values of AGet. addition is preferred over cycloaddition. This is the case for the reactions of certain electron-rich alkenes with excited arenes. For positive values of AGet. cycload-... 

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