Cycloaddition, photo olefins

When the thione (17) is photo-excited to its 7r,7r -state, hydrogen-abstraction from alkanes and cycloaddition to olefins takes place, which is in remarkable contrast with the reactivity of (17) in the n, r -state. ... 

This chapter deals with [2 + 2]cycloadditions of various chromophors to an olefinic double bond with formation of a four-membered ring, with reactions proceeding as well in an intermolecular as in an intramolecular pattern. Due to the variety of the starting materials available (ketones, enones, olefins, imines, thioketones, etc.. . .), due to the diversity of products obtained, and last but not least, due to the fact that cyclobutanes and oxetanes are not accessible by such a simple one-step transformation in a non-photo-chemical reaction, the [2+2]photocycloaddition has become equivalent to the (thermal) Diels-Alder reaction in importance as for ring construction in organic synthesis. 

The ketone (265) was prepared via photo[2 + 2]cycloaddition between the corresponding olefin and allene 91). 

Olefins are also susceptible to cycloaddition reactions (Fig 105) (150). In particular, some olefin-containing APIs can dimerize with another molecule of API to form a 2+2 cycloaddition product under photo conditions (151). A classic example of such a 2+2 cycloaddition catalyzed by UV radiation is that of the nucleoside thymidine (Fig. 106) (152,153). These reactions are proposed to go through more than one mechanism concerted, diradical, electron transfer, and radical ion pairs. 

Guo, X.-C. and Chen, Q.-Y. (1999) Photo-induced intramolecular arene-olefin meta-cycloaddition of 5-phenyl-fluorinated-pent-l-enes. Journal of Fluorine Chemistry, 97 (1—2), 149-156. 

Photo sensitized oxygenation of alkenes with singlet oxygen can, in principle, proceed via three competitive reaction pathways [4 + 2]cycloaddition to e do-peroxides, ene reaction of allylic hydroperoxides, and [2 + 2]cycloaddition to 1,2-dioxetanes (see reference [681] for a review). With suitable olefinic substrates, the chemical outcome of such photo-oxygenation reactions can be strongly influenced by the solvent. This is shown in the somewhat simplified Eq. (5-145). 

Yamamura has extensively explored the intramolecular [5 - - 2]-cycloaddition mode. This chemistry bears a remarkable similarity to the so-called arene olefin meta-photocycloaddition [64] reaction (e.g., 267 to 268) that has frequently been used in the synthesis of complex bioactive natural products. While different intermediates are involved, the photo-and electrochemical reactions provide access to many of the same basic ring systems. This is nicely demonstrated by comparing key steps and the outcome of an electrochemical approach to the total synthesis of pentalene (53) [65] and a photochemical pathway that ultimately led to cedrene (compare equations 48 and 49). 

Irradiation of solutions of cycloheptatrieneiron tricarbonyl at low temperatures followed by addition of diphenylacetylene produces the [2 + 6] adduct, 7,8-diphenyl-bicyclo[4,2,l]octa-2,4,7-triene similar results were obtained with cyclo-octatetraene-iron tricarbonyl and related compounds. The mechanism suggested involved photo-induced replacement of one of the olefin bonds or carbon monoxide ligands in the complexes by a solvent molecule, thermal replacement of the solvent by the acetylene, and an intramolecular cycloaddition. ... 

Cycloadditions.—Continuing interest has been shown in the photo-oxygenation of adamantylideneadamantane and in the reverse, thermally initiated, chemiluminescent fragmentation. Photo-oxygenation of the olefin affords the dioxetan (313) and the epoxide (312). With /ncso-tetraphenylporphin as sensitizer, (313) is the major product in all the solvents studied, whereas photo-oxygenation of the olefin in acetone and using a variety of dye sensitizers led to marked changes in the (312) (313) ratio. ... 

By Cycloadditions involving Thiocarbonyl Compounds. The photocatalysed cycloaddition of thiocarbonyl compounds to olefins to form thietans has been reviewed. The photo-addition of thiobenzophenone to cis- and /m j-dichloroethylene and to cw-dicyanoethylene apparently proceeds with retention of configuration in the olefin moiety. Allenes participate in such reactions, thiobenzophenone with tetramethylallene > and methoxy-allene, for example, giving the thietan derivatives (44) and (45). Thiobenzophenone in its triplet excited state apparently attacks the central... 

Photo-triggered ring opening of 2//-azirines is a well-known reaction to produce pyrrolines [8, 71]. Padwa and co-workers showed that photoirradiation of azirines with a mercury arc lamp (450 W) equipped with Vycor filter generated the reactive nitrile ylide intermediate (72), which can be stabilized by the phenyl substituents. The nitrile ylide (72) then reacts with the electron-deficient olefins (73) such as acrylate and acrylonitrile in a cycloaddition reaction to form A -pyrrolines (74) (Scheme 10) [8]. Steenken and co-workers studied reaction kinetics of azirines with dipolarophiles as well as nucleophiles such as alcohols [72]. They showed that the reaction rate depends on the azirine substituents, the nucleophilicity of the reactant and the acidity of the alcohol. 

A series of reports describing cycloaddition reactions of fluorocarbons with coordinated olefins has appeared generally endo attack at a co-ordinated double bond is observed. This contrasts with electrophilic attack at unco-ordinated double bonds as described above where exo attack is preferred. The mechanism of photo-initiated addition of CaF to butadienetricarbonyliron is considered to follow Scheme 5, i.e. photolysis promotes dechelation of the hydrocarbon rather than CO... 

Sakurai, H., Shima, K., and Toki, S., Photo-cycloaddition reaction of carbonyl compounds to olefins, Nippon Kagaku Zasshi, 89, 537,1968. 

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