Alkenes photoaddition reactions

Photolysis of 2-oxetanones gives decarboxylative cleavage to alkenes, similar to pyrolysis, but subsequent photoaddition reactions of the alkene product may lead to complex reaction mixtures. A very useful example of 2-oxetanone photolysis is that of 5-oxabicyclo[2.2.0]oct-2-en-6-one, the photoisomer of a-pyrone when it was irradiated in a argon matrix at 80 K, carbon dioxide and cyclobutadiene were formed (equation 7) (73JA1337). 

Bcnzofuran-4,7-dioncs have been synthesized regioselectively by [3 + 2] photoaddition of 2-hydroxy-1,4-benzoquinones with a range of alkenes (equation 185)664. The reaction occurs in 30-60% yield and is a useful method for the synthesis of the benzofuran ring system, which is important in natural products like acamelin665. Substituted naphthoquinones may also be used in this reaction666,667 and this has lead to a very simple two-step synthesis of maturinone. In a similar reaction, a [3 + 2] photoaddition reaction of 2-amino-1,4-naphthoquinones with electron-rich alkenes gave 13-82% yields of 2,3-dihydro-177-bcn/ /]indole-4,9-diones in a single-step process which involved photolysis followed by oxidation (equation 186)668,669. 

The possibility of the intermediacy of the triplet state of benzene itself has been discussed by Atkins et al. [108], Photoaddition of alkenes to arenes is often accompanied by the formation of dimers of the alkene, a reaction sensitized by triplet benzene. With methyl acrylate and methyl vinyl ketone, however, it was found that the ratio of ortho cycloadducts to alkene dimers increased with the concentration of benzene. Because the yield of T, benzene increases with benzene concentration, these results might indicate that ortho photocycloaddition of aery-... 

It is widely believed that enone-alkene photoadditions proceed through an exciplex (excited complex). For cyclopentenone and 7, the exciplex forms from the photoexcited enone in its triplet state and the glycal in its ground-state. The regiochemistry of addition probably reflects a preferred alignment of the addends in the exciplex. Because a photoexcited enone probably has considerable diradical character, it is reasonable to assume that the more electrophilic a-keto radical would prefer to bind to the more nucleophilic portion of the alkene, since this would maximise attractive interactions within the complex. The reaction of 7 with cyclopentenone would thus favour the formation of diradical 22, which would then ring-close to cyclobutane 14 (Scheme 6.7). 

Photoaddition reactions involving alkynes have been studied much less than those of alkenes. This can be ascribed not only to the lower reactivity of electron-poor... 

In aqueous acetonitrile 2-cyanopyridine undergoes an interesting photoaddition reaction with alkenes as shown in Scheme 30 to give [133] (Saito,... 

In the photoaddition reactions of some alkyne/alkene pairs an alternative 1 2 alkyne alkene product arises which contains a bicyclopropyl unit (equation 50, and see equation 43 above). The formation of the alternative product is dependent... 

Several reviews have appeared during the year. The photoaddition reactions of aromatic compounds have been discussed with special attention to the mechanistic and structural features governing the outcome of the interaction of photoexcited arenes with alkenes and other arenes. Morrison has published an account of the photoactivation of distal functional groups in polyfunctionai... 

Open-chain Systems - Chow and his coworkers have carried out further studies on the cycloaddition reactions of dibenzoyl(methanato)boron difluoride (1) to alkenes and dienes. This present work has examined the mechanistic details of the process. Previously they have reported the addition of the same reactant to enol ethers. The influence of the chiral centre in the alkene (2) on the cycloaddition of a,P-unsaturated nitriles such as acrylonitrile has been evaluated. The products obtained from these (2+2)-photoaddition reactions are the azabicyclooctanes (3)." ... 

Topics of relevance to the content of this chapter which have been reviewed during the year include photoactive [2]rotaxanes and [2]catenanes, photochemical synthesis of macrocycles, phototransformations of phthalimido amino acids, photoaddition reactions of amines with aryl alkenes and arenes, photoreactions between arenenitriles and benzylic donors, photostability of drugs, polycyclic heterocycles from aryl- and heteroaryl-2-propenoic acids, photoreactions of pyrroles, photoamination reactions in heterocyclic synthesis, switching of chirality by light, photochromic diarylethenes for molecular photoionics and solid state bimolecular photoreactions. 

Photoaddition reactions have also been described with (84) as the substrate.(2+2)-Photocycloaddition of simple alkenes to these enones results in the formation of the adducts (85) and phenylacetylene undergoes similar addition to this substrate. A substituent effect has been detected in these... 

Photoaddition reactions of azo compounds and photochemical para additions to benzenes are both rare so formation of the adduct (28) from N-methyltriazolinedione and benzene is an unusual process on bodi counts (Hamrock and Sheridan). In the meta-cvcloaddition of alkenes to benzenes, there are both theoretical and experimental grounds for believing that the Si benzene undergoes... 

A novel 1,8-photoaddition of dimethyl 1,4-naphthalenedicarboxylate and 1,4-dicyanonaphthalene to alkenes, a formal [3-1-2] cycloaddition, was reported by Kubo and co-workers. Although yields of the desired products were low, Cu(OAc)2 showed a profound impact on the reaction by increasing the yields considerably [85]. 

---