Benzenes photocycloaddition reactions

The synthetic applications 440) and mechanistic aspects 4411 of intermolecular photocycloaddition reactions of arenes to olefins have been reviewed recently. Intramolecular cycloadditions442a,b) have been studied in the context of the photochemical behaviour of bichromophoric molecules, as to investigate interchromophoric interactions in polyfunctional molecules. Three types of addition products can be formed in the photocycloaddition of benzene to an alkene (4.37)441. ... 

There has been very little study of the photocycloaddition reaction, where the carbonyl compound was excited with light of varying wavelengths. For the simple carbonyl compounds, irradiation absorbed only by the n - it transition has been used with success. Irradiation of the more complex carbonyl compounds, for example, benzophenone, can be such that both the w and the (of the benzene chromo-... 

Clifford and co-workers [190] have performed complete active space self-consistent field (CASSCF) ab initio calculations on the photocycloaddition reactions of benzene and ethene. An eight-electron, eight-orbital active space involving the ir-orbitals of the benzene and ethene moieties was used. The geometries were optimized using the 4-31G basis set, and the energies were recomputed at the 6-31G level. 

Not only carbon-carbon triple bonds but also carbon-nitrogen triple bonds can undergo ortho photocycloaddition to derivatives of benzene. The reaction was discovered by Al-Jalal et al. [86] who found that irradiation (254 nm) of 4-cyanoanisole in acrylonitrile produced three 1 1 adducts in the ratio of 7 1 2. Two of these (ratio 7 1) were formed by the addition of the carbon-carbon double bond of acrylonitrile to positions 1,2 and 3,4, respectively, of 4-cyanoanisole. The third product was an azocine derivative, apparently formed by the addition of the carbon-nitrogen triple bond to positions 2,1 of the arene, followed by ring opening to an azacyclooctatetraene [87] (Scheme 48). 

Another mechanism for the 2n -f 27t ) photocycloaddition of alkenes via electron transfer is the reaction that proceeds via a triplet state which is produced by a back-electron transfer from a radical anion of the electron acceptor to a radical cation of the electron donor. The triplet state alkenes generated by this way can undergo the cyclodimerization (Scheme 22). Farid showed that the DCA-sensitized (2n 3- 2n) photocyclodimerization of 1,2-diphenylcyclo-propene-3-carboxylate occurs via the triplet state of the cyclo-propene in acetonitrile [84]. In this photoreaction, two types of the (An -(- 27t) photocycloaddition reactions take place between DCA and the cyclopropene depending upon solvents. One type of the cycloadduct is produced in benzene via exciplex and the other type of the photocycloadduct is produced in... 

Full details of the effects of acid and solvent polarity on various photocycloaddition reactions of ethylenes and acetylenes to benzene have been published and a mechanistic rationale of the observations has been presented. Thus the largely homopolar concerted meta cycloaddition of ethylenes to benzene, which yields the tricyclo[3.3.0.0 "]oct-3-ene system (36), has little sensitivity to either solvent polarity change or the presence of a proton donor, but the onho process leading to bicyclo[4.2.0]octa-2,4-diene derivatives (37)... 

An approach to the synthesis of homosecoprismanes has also been developed using intramolecular photocycloaddition reactions of the enediones (252) and (253). The irradiation of these in benzene affords the cycloadducts (254) and (255), respectively in 80-90% yield. ... 

Some years ago Cornelisse reported that deuteration of alkyl benzenes results in a deuterium isotope effect upon the quantum yield of the meta photocycloaddition reaction with alkenes. In a new report the same group has published an analysis describing how the observed isotope effect upon the reaction quantvun yield can be ascribed to a kinetic deuterium isotope effect on the excited state reaction and distinguished from an effect upon the unimolecular photophysical modes of decay of the excited state. In addition, it is reported that when the quantum yield of meta photocycloaddition of cyclopentene to alkyl benzenes is measured using a mixture of deuterated and non-deuterated benzenes, the quantum yield is arene concentration dependent.The authors argue that this arises from competition between cycloaddition and the formation of mixed excimers between deuterated and non-deuterated alkyl benzenes which dissociate to yield excited deuterated alkyl benzene and ground state non-deuterated alkyl benzene preferentially. 

Benzene in the Si (n,n ) excited state is no longer aromatic and is capable of undergoing various chemical reactions not observed in the ground state. Three basic types of photocycloaddition reaction of an alkene to the excited benzene can be de-scribed 802,804,809 816 (a) bicyclo[4.2.0]octa-2,4-dienes (e.g. 191) are formed by ortho-photocycloaddition (1,2- or [2 + 2] photocycloaddition), (b) tricyclo[3.3.0.02 8]oct-3-enes (e.g. 192) are obtained by mcia-photocycloaddition (1,3- or [3 + 2]... 

The photocycloaddition mechanism, and consequently the reaction selectivity, may vary considerably depending on the structure of the initial material and reaction conditions. In general, an excited arene and a ground-state alkene may react with initial polarization to form an exciplex.802 In [2 + 2] photocycloaddition reactions, biradical intermediates are often involved (Scheme 6.80a), although excitation of a ground-state charge-transfer (CT) complex (Section 2.2.3) has also been discussed in some cases, such as the [2 + 2] photocycloaddition of benzene with maleic anhydride (Scheme 6.80b).817 Here a zwitterion intermediate 194 collapses to the adduct 195 only in the absence of an acid. 

The photocycloaddition reactions of indole discussed in this section involve the participation of the C2-C3 bond of the pyrrole ring only. No photocycloaddition reactions involving the benzene ring of the indole nucleus have been reported. As will be seen, addition across the C2-C3 positions of the pyrrole ring occurs in a 2 + 2 and 2-1-4 sense the use of this nomenclature throughout this section should not, however, be taken to mean that the cycloaddition reactions are concerted. In fact, they are all stepwise. 

The propellaprismane (156) has been synthesized by the intramolecular (2+2)-photocycloaddition of the diene (157). Interest in the synthesis of cage compounds such as (158) continues. This particular example is produced on irradiation of the triene (159) through quartz in a mixture of acetone and benzene. The reaction is chemically efficient and the product is formed in 80% yield. 

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. 

Photochemical [3 + 2] photocycloaddition of alkenes to benzene ring is a useful tool for construction of 5-membered ring compounds in a one-step. Penkett reported the synthesis of the [5.5.5.5] fenestrane structures (47), (48), (50) by arenyl-diene (either 46 or 49) double [3 + 2] photocycloaddition reactions. Penifulvins (53), natural products, were prepared by use of an intramolecular [3 + 2] photocycloaddition of (51) as a key step." ... 

---