Benzene, photocycloaddition with olefins

Benzene undergoes photocycloaddition with simple olefins to produce 1,3, 1,2 and 1,4 adducts as shown below for tetramethylethylene ... 

For photocycloaddition, to benzene the following conclusions were drawn from this empirical correlation [124], Olefins with poor electron-donor or poor electron-acceptor abilities yield mainly meta adducts with benzene (i.e., if AG > 1.4-1.6 eV, all other olefins yield mainly ortho adducts). Even ethene, which had seemed to behave exceptionally, fits into this correlation provided that it acts as the acceptor. The transition area from ortho to meta cycloaddition (i.e., the AG region where ortho meta = 1 1) is relatively large ( 0.2 eV). This is considered not to be surprising because the AG correlation is based on many different types of olefins. When only AG values for derivatives of 1,3-dioxole and for 1,4-dioxene were used, the transition area was narrowed to 0.03 eV. Not only ethene but also vinylene carbonate now fit into the correlation. According to the ionization potential rule, this compound should give only ortho photocycloaddition with benzene. Mattay s empirical rule predicts mainly meta addition, which is indeed found experimentally. 

Figure 7.32. Orbital energy diagram for the photocycloaddition of excited benzene to an olefin a) with an electron-donating substituent X and b) with an electron-withdrawing substituent Z (adapted from Houk, 1982).

Meta photocycloaddition was discovered simultaneously and independently by two groups in 1966. Wilzbach and Kaplan [4] found that the adducts from m-but-2-ene, cyclopentene, and 2,3-dimethylbut-2-ene with benzene are substituted tricyclo 3.3.0.02X]oct-3-enes. The adducts were formed by irradiation of solutions (-10%) of the olefins in benzene, at room temperature under nitrogen, with 2537-A light. Bryce-Smith et al. [5] subjected an equimolar mixture of m-cyclooctene at room temperature or in the solid phase at 60°C to ultraviolet radiation of wavelength 235-285 nm. A mixture of 1 1 adducts was obtained from which the main component (-85%) was readily obtained pure by treatment of the mixture with methanolic mercuric acetate. This 1 1 adduct proved to be a meta photocycloadduct (Scheme 2). The minor nonaromatic adduct (10-15%) could, at that time, not yet be obtained completely free from the meta photocycloadduct the structure of a rearranged ortho adduct was provisionally assigned to this isomer. 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

The simple diastereoselectivity of the photocycloaddition of electronically excited carbonyl compounds with electron rich olefins was studied as a function of the substituent size—at identical starting conditions ignoring the electronic state involved in the reaction mechanism [123], The [2+2] photocycloaddition of 2,3-dihydrofuran with different aldehydes in the nonpolar solvent benzene resulted in oxetanes 118 with high regioselectivity and suprising simple diastereoselectivites the addition to acetaldehyde resulted in 45 55 mixture of endo and exo diastereoisomer, with increasing the size of the ot-carbonyl substituent (Me, Et, i-Bu, t-Bu), the simple diastereoselectivity increased with preferential formation of the endo stereoisomer (Sch. 37). 

Aromatic compoimds also undergo photocycloaddition reactions with alkenes, leading to 1,2-, 1,3-, and (less often) 1,4-adducts, as shown for the reaction of benzene with ethene in equation 12.70. Olefins with strongly electron-withdrawing or electron-donating substituents tend to give 1,2-photoaddition products, while olefins with alkyl substituents tend to give mostly 1,3-photoaddition. 

Ortho-cycloaddition takes place with a olefin which has low ionization potential in comparison to benzene where polar nature of reaction overpowers the symmetry imposed barrier to this reaction. Polar nature of ortho-cyclo-addition is supported by the fact that in case of doner substituted ethylenes, reaction is promoted by polar solvent, but in meta-addition no solvent effect is there, o-and p-photocycloadditions are disallowed to occur as concerted addition between of benzene and Sq of alkene until mixing of charge-transfer states occurs. 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

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