Ethene photocycloaddition with benzene

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. 

The photocycloaddition of ethene and benzene has been studied by CAS-SCF computation using the 6-3IG basis set for energies and 4-3IG orbitals for structural minimization. The structure of the Cl is shown in Figure 12.24. The ortho and meta cycloaddition processes proceed through alternate electron-pairing schemes from a single Cl without barriers. For unsubstituted alkenes, the meta Cl is lower in energy than the ortho, whereas the ortho Cl is stabilized by alkenes with ERG and EWG substituents. Product compositions tend to reflect these differences. ... 

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. 

The 2-I-2-cycloaddition of l,l-difluoro-2,2-bis(dimethylamino)ethene with ethyl propiolate yields a cyclobutene which readily undergoes electrocyclic ring opening to form a diene. The 1,4-diacetal-bridged cycloadduct (7), produced in four steps from the 2 -I- 2-photocycloaddition of DMAD with l,4-dichlorobut-2-ene, reacts with DMAD to provide a synthesis of the Dewar-benzene structure (8) (Scheme 3). ... 

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