Conjugated dienes 4+4 photocycloaddition

With conjugated dienes, photocycloaddition of carbonyl compounds occurs at one of the double bonds to give vinyloxetanes. An interesting example is the reaction of acetone with 2-methyl-l,3-butadiene, which gave the two oxetanes (60) and (61) in a ratio of 3 1 and a total yield of about 20% (72JA8761). Other alkenes which have been used for photosynthesis of oxetanes include enol ethers, ketene acetals, enamines, allenes and diketene, with the reaction of the last compound with benzaldehyde illustrated in equation (105) (75CPB365). 

One of the exciting areas that has gained importance over the recent decade is the photochemical cyclization of non-conjugated dienes in the presence of species that can act as templates. One such species that has been used is copper(I) salts. The earliest example of the use of copper salts in the intramolecular photocycloaddition of non-conjugated dienes is that described for cycloocta-1,5-diene. When this is irradiated in the presence... 

One of the limitations of the photocycloaddition reaction is that the unsaturated system may itself act as a quencher. Conjugated dienes fall within this category since they quench the n,ir triplet of some carbonyl compounds. For the photoreduction of benzophenone in benzhydrol, the ratio, kqlka, for m-piperylene is 750 (Table II), which indicates that this diene is indeed an efficient quencher for the reaction. 

This catalyzed photocycloaddition of a conjugated diene to a double bond provides a useful route to bicyclic vinylcyclobutanes.-Examples ... 

Table 1 Photocycloadditions of Conjugated Dienes widi Tiicarbonyl[l-6-i)-(8,8-dimethyIheptafuIvene)]chromium(0]f ...

K. Avatschi, S. R. Raychaunduri, and R. G. Salomon, Copper(I) catalysis of olefin photoreactions. 13. Synthesis of bicyclic vinylcyclobutanes via copper(I)-catalyzed intramolecular 2.pi. + 2.pi. photocycloadditions of conjugated dienes to alkenes, J. Org. Chem. 49,4322-4324 (1984). 

Avasthi, K., Raychaudhuri, S. R., Salomon, R. G., Copper Catalysis of Olefin Photoreactions. 13. Synthesis of Bicyclic Vinylcyclobutanes via Copper catalyzed Intramolecular 2ji + 2ji Photocycloadditions of Conjugated Dienes to Alkenes, J. Org. Chem. 1984, 49, 4322 4324. 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

Photoinduced [2 + 2] cycloaddition (Section 4.9) of alkenes (alkynes) to form cyclobutane (cyclobutene) derivatives is one of the best studied reactions in photochemistry.680 682 According to the Woodward Hoffmann orbital symmetry rules,336 the cycloaddition of one singlet excited (Si) and one ground-state alkene is allowed by a suprafacial suprafacial concerted stereospecific pathway (Scheme 6.45) 695 699 700 Rare concerted [4 + 2] and [4 + 4] photocycloadditions of conjugated singlet excited dienes must occur in a suprafacial antarafacial and suprafacial suprafacial manner, respectively.690 Since the suprafacial antarafacial reactant approach is geometrically difficult to achieve, [4 + 2] reactions usually proceed stepwise (involving biradical intermediates). [2 + 2] or [4 + 4] photocycloadditions can occur in either a concerted or stepwise fashion. 

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