Stilbenes photocycloaddition

Photocycloaddition of aromatic carbonyl compoimds with p,p-dimethylketene silylacetals gave 2-alkoxy-4-aryloxetanes <98JCS(P1)3253>. A similar photoaddition of derivatives of stilbene to chloranil gave the isomeric spiro oxetanes 16 in very high total yields <99JOC2250>. 

Cyclobutane formation via light-induced [2 + 2] cycloaddition is probably one of the best studied photochemical reactions and has been reviewed thoroughly up to 1972 (Houben-Weyl, Vols. 4/5 a and 4/5 b). The most important types of C —C double-bond chromophores undergoing such reactions arc alkenes, 1,3-dienes, styrenes, stilbenes, arenes, hetarenes, cycloalk-2-enones, cyclohexa-2,4(and 2,5)-dienones, 1,4-benzoquinones, and heteroanalogs of these cyclic unsaturated carbonyl compounds. For p notocyciodimerizations see Houben-Weyl, Vol. 4/5 a, p 278 and for mixed [2 + 2] photocycloadditions of these same chromophores to alkenes see Section 1.3.2.3. 

That the cycloaddition occurs via the less highly polar exciplex is also supported by Kaupp s studies of photocycloaddition between trans-stilbene and cyclic unsaturated ethers (57). 

Addition to six-membered oxygen heterocycles is also common. The photocycloaddition of 5,7-dimethoxycoumarin to tetramethylethylene has been described,269 and 4-hydroxycoumarin (326) undergoes facile addition to cyclohexene on direct irradiation to give the cyclobutane (327)270 analogous additions to a variety of other alkenes have been reported, and the cycloaddition of 4-methoxycoumarin to 2-methylpropene has been employed in a synthesis of l,2-dihydrocyclobuta[c]coumarin.271 Photoaddition of the 1,2-bisenol lactone (328) to tran.s-stilbene yields propellane (329),272 and [ 2 + 2] cycloaddition is observed along with other competing photoreactions on irradiation of chromone in the presence of alkenes.273... 

Prior, to our work on (2 + 2)-photocycloaddition reactions to carbon-nitrogen double bonds, there were only three relevant reports, two of which have subsequently been shown to be in error. Irradiation of N-(4-dimethylaminobenzylidene)aniline (2) was reported to give c/s-4,4 -bis-(dimethylamino)stilbene (2) and azobenzene (5) via an unstable 1,2-diazetidine intermediate (4)4. Ohta and Tokumaru, however, have recently shown that this mode of reactivity is not reproducible5. The only... 

As mentioned above, in photoreactions, the intermediates are formed through the excitation of one partner or the excitation of CT complex. In the former case, the excited molecule interacts with the partner to form the exdplex, while the latter produces excited EDA complex or in short, excited complex . Actually, some studies have shown that exciplexes and excited complexes are identical species [32-35] 1) They have the same spectral character. For example, when 0.12 M Pms-stilbene and 0.43 M fumaronitrile are excited in the charge-transfer absorption band (360 nm), both the spectral distribution and lifetime of this emission are identical with those obtained for their exciplex [36]. 2) They undergo the same follow-up reactions. Lewis [35] reported a similar cycloaddition quantum yield of the two above processes in the photocycloaddition of stilbene with dimethyl fumarate. Accordingly, here we will use them interchangeably. 

Control of absolute asymmetry is a relatively untouched area for [2 + 2] photochemical cycloaddition reactions despite the recent advances in the field of asymmetric synthesis. The first example of the use of a removable chiral auxiliary was reported by Tolbert, who obtained impressive enantioselectivity in the photocycloaddition of bomyl fumarate to stilbenes (equation 37). More recently, Lange has shown that menthyl cyclohexenonecarboxylates are useful in control of absolute stereochemistry (equation 38). Baldwin and Meyers have also obtained excellent facial selectivity in systems where the stereogenic center which controls the diastereoselectivity can be excised to afford products of high enantiomeric purity (equations 39,40). 

Wenz et al. have demonstrated that photoreactions are effective for the construction of polyrotaxanes (Scheme 18) [114]. In the knowledge that stil-benes undergo [2+2]photocycloaddition to yield cyclobutane derivatives by UV irradiation, they prepared a quaternary polymeric inclusion complex from /1-cyclodextrin, y-cyclodextrin, (E)-4,4-bis(dimethylaminomethyl)stil-bene (B), and (li)-stilbene polymer (A). Upon irradiation at 312 nm, the (E)-stilbene units of A underwent [2+2]photocycloaddition with B by catalysis of y-cyclodextrin to form the tetraphenylcyclobutane group, which acted as blocking group for /1-cyclodextrin. Wenz et al. claimed that this was the... 

Intramolecular Cyclization Reactions This Section is concerned with photo-induced cyclization reactions of aromatic compounds in which the final product is also an arene. Examples of intramolecular cyclization in which the product is a stable aliphatic system (e.g. intramolecular meta photocycloaddition of ethylenes to benzenes) are considered in Section 3 of this Chapter. The present type of process occurs for a wide variety of systems and may involve arene-arene and arene-ethylene photo-oxidative cyclization or reactions proceeding by loss of halogen acid. For several years now one of the major interests in these processes has been in their use as synthetic procedures and again within the review period there have appeared a number of accounts which illustrate this feature. Stilbene-... 

Photocycloadditions of ethylene with chiral lactam 1.92 (R1 = R" = Me, R = /-Pr) take place selectively on the exo (convex) face of the a,(3-unsaturated system. After acid treatment and epimerization of the liberated methyl ketone, Meyers and coworkers [327] obtained ketone 9.19 with an excellent enantiomeric excess (Figure 9.8). This compound is a precursor of optically active grandisol. Photocycloaddition of chiral fiimarate 9.20 with frans-stilbene is also highly stereoselective [1533] (Figure 9.8). 

Scheme 12. Different photochemical reaction pathways of czs-stilbene (1) (i) to phenanthrene (2) by conrotatory [n6] electrocyclization, and (ii) to tetraphenylcyclobutane (33) by a [2+2] photocycloaddition [67a-d, 68]...

Another side route is photocycloaddition of stilbene to olefins as studied by Chapman and Lura [467], A quantum yield of 0.54 (313 nm in n-hexane) has been reported for 4 M olefin (e.g., tetramethylethylene). Cycloaddition may occur by reaction of the excited trons-stilbene with the olefin or from an excited ground state complex with electron-deficient alkenes (e.g., fumaronitrile) [468,469]. 

The first successful example of an enantioselective and intermolecular 12 -I- 21 photocycloaddition involved the reaction of chiral fumarates 165 with trans-stilbene. After removal of the chiral auxiliaries, the ee of the dimethyl-5-truxinate (166) and /i-truxinate (167) produced depends strongly on the nature of the chiral appendages of the starting maleates (Scheme 27) [106]. 

The reactions of polynuclear aromatics can also be rationalized by simple models such as those shown for benzene in Figures 12.48 and 12.49. Two notable reactions are (i) the photodimerizations of anthracenes (equation 12.71) and (ii) the photoisomerization of frans-stilbene (99) to the cis isomer (100), followed by intramolecular photocycloaddition to a dihydrophenanthrene (101), which can then be oxidized to phenanthrene (102) ... 

The rra 5-azetidinone (71) and stilbene dimers were formed by the photocycloaddition of either cis- or /ra j-stilbene to phenyl isocyanate. The... 

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