Benzene photocycloaddition with

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

The possibility of the intermediacy of the triplet state of benzene itself has been discussed by Atkins et al. [108], Photoaddition of alkenes to arenes is often accompanied by the formation of dimers of the alkene, a reaction sensitized by triplet benzene. With methyl acrylate and methyl vinyl ketone, however, it was found that the ratio of ortho cycloadducts to alkene dimers increased with the concentration of benzene. Because the yield of T, benzene increases with benzene concentration, these results might indicate that ortho photocycloaddition of aery-... 

In 1977, Scharf and Mattay [123] found that benzene undergoes ortho as well as meta photocycloaddition with 2,2-dimethyl-1,3-dioxole and, subsequently, Leismann et al. [179,180] reported that they had observed exciplex fluorescence from solutions in acetonitrile of benzene with 2,2-dimethyl-l,3-dioxole, 2-methyl-l,3-dioxole, 1,3-dioxole, 1,4-dioxene, and (Z)-2,2,7,7-tetram-ethyl-3,6-dioxa-2,7-disilaoct-4-ene. The wavelength of maximum emission was around 390 nm. In cyclohexane, no exciplex emission could be detected. No obvious correlation could be found among the ionization potentials of the alkenes, the Stern-Volmer constants of quenching of benzene fluorescence, and the fluorescence emission energies of the exciplexes. Therefore, the observed exciplexes were characterized as weak exciplexes with dipole-dipole rather than charge-transfer stabilization. Such exciplexes have been designated as mixed excimers by Weller [181],... 

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. 

Gilbert also reported that the photocycloaddition of furan to benzonitrile gave only the 2,5-2, 6 (meta) adduct (9) as a sole product, although the donor-acceptor interaction between addends is relatively high [71,72] (Scheme 6). Unsubstituted benzene reacts with furan to give a mixture of (4 + 4), (4 + 3), (3 + 2), and (2 + 2) adducts by 254-nm irradiation [73,74],... 

Although intermolecular photocycloaddition of two benzene rings in the condensed phase has not been observed, this reaction is common for polycyclic aromatic hydrocar-bons. For example, anthracene-9-carbonitrile (231) in acetonitrile undergoes efficient [4 + 4] photocycloaddition with anthracene to give an adduct 232 in 94% chemical yield (Scheme 6.90).833 This process is thermally reversible. 

Perfluorinated benzene and perfluorinated benzene derivatives can also imdergo photochemical [2 -t- 2] photocycloaddition with cycloalkenes. Hexa-fluorobenzene reacts with cycloalkenes when irradiated to afford two products which react further to form three other products [128]. The anti addition products predominate, with the selectivity decreasing with increasing ring size of the cycloalkenes (see Scheme 51). When the alkene is indene or dihydro-naphthalane, the syn product is the sole product, probably because of rt-stack-ing. 

The strained undecacyclic pagodane framework was obtained in a series of 14 one-pot operations with an overall yield up to 24% from commercial isodrin. The key steps are (i) a benzene-benzene [6 -I- 6]photocycloaddition, and (ii) a domino Diels-Alder reaction. 

Whereas the cycloaddition of arylazirines with simple alkenes produces A -pyrrolines, a rearranged isomer can be formed when the alkene and the azirine moieties are suitably arranged in the same molecule. This type of intramolecular photocycloaddition was first detected using 2-vinyl-substituted azirines (75JA4682). Irradiation of azirine (54) in benzene afforded a 2,3-disubstituted pyrrole (55), while thermolysis gave a 2,5-disubstituted pyrrole (56). Photolysis of azirine (57) proceeded similarly and gave 1,2-diphenylimidazole (58) as the exclusive photoproduct. This stands in marked contrast to the thermal reaction of (57) which afforded 1,3-diphenylpyrazole (59) as the only product. 

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-... 

Furans are able to undergo photocycloaddition of the [W2S+ 2S] and the [W4S+ 4S] type to suitable substrates. With benzene (80JCS(P1)2174) five 1 1 products are obtained. The relative proportions of these products are highly variable and depend on the relative concentration of the reactants, the irradiation time, the light intensity and the temperature of the solution. For the shortest irradiation time with a low-pressure mercury lamp at 15 °C, the relative proportions are 1 1 10 40 2. The major product is the 2,5 l, 4 -adduct (301) and the next most prolific is the 2,3 l, 2 -adduct (302). Adduct (301) is unreactive to dienophiles but gives adduct (302) by Cope reaction at 60-70 °C. This reaction can also be achieved by irradiation of a cyclohexane solution of (301). Adduct (302) reacts readily with dienophiles in ethereal solution to form Diels-Alder adducts. The minor adducts possess structures (303), (304) and (305). The reaction is thought to involve the first excited triplet of benzene or an excited state complex. A [ .4s+ .4g] photoadduct (306) is formed... 

In certain cases benzene will undergo photocycloaddition to oxygen and sulfur heterocycles. The two major photoproducts of irradiation of a mixture of furan and benzene are adducts 356 and 357, arising, respectively, by [ 2 + 2] and [ 4 + 4] cycloaddition processes.306 Irradiation of benzene and 2,2-dimethyl-l,3-dioxol (358) similarly affords adducts 359 and 360, together with dimer 361.307 Thiochromone 1,1-dioxide also undergoes photoaddition to benzene.308... 

Aoyama et al. in relation to their studies on photochemical synthesis of (3-lactams [91] reported the synthesis of 4-spirocyclopropylazetidin-2-one [92] via photocycloaddition of 4-thioxoazetidin-2-one to alkenes followed by subsequent desulfurization. A solution of 1-isopropyl-3-phenyl-4-thioxoazetidin-2-one 70 and 1,1-diphenylethylene in benzene on irradiation with a high pressure mercury lamp afforded a [2 + 2] adduct 72 (R = Ph), in 67% yield which, on desulfurization with Raney-nickel [93] in anhydrous ethanol gave two isomeric... 

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. 

Para photocycloaddition of arenes to the benzene ring was first reported in 1971 by Wilzbach and Kaplan [7] as a minor process accompanying ortho and meta photocycloaddition. Since that time, relatively few cases of para photocycloaddition have been described. Para adducts were found as minor products from benzene with cyclobutene [8], ra- 3,4 - dimethyIcycIobu(ene [9], vinylene carbonate [10], 2,3-dihydropyran [11,12], and 1,3-dioxole [13,14] and from a,a,a-tri-fluorotoluene with vinylene carbonate [15], Intermolecular para photocycloadducts were major products from the irradiations of benzene and allene [16,17], benzene and cyclonona-1,2-diene [16,17], and from fluorobenzene and cyclopentene [18], Intramolecular para photocycloadducts were found as major products from the irradiations of phenethyl vinyl ether [19-21] (Scheme 3) and 2,3-dimethyl-6-phenylhex-2-ene [22], No detailed mechanistic investigations have been published. 

In this chapter, we will be concerned with the ortho photocycloaddition of arenes to the benzene ring. This implies that photocycloadditions to larger aromatic systems and to heterocyclic aromatic molecules will not be discussed. The photoadditions of alkynes to benzene and derivatives of benzene, however, are included in this review. The material is organized in sections, according to the pathway that is followed from the ground state of the addends to the ortho photocycloadducts. 

Ortho photocycloadditions proceeding via excitation of a ground-state charge-transfer complex have been reported for the combination of benzene and alkyl-benzenes with maleic anhydride. The reaction was discovered by Angus and... 

Ortho photocycloadditions of benzene derivatives to maleic anhydride have been tabulated in Table 1. Only the structures of the primary ortho adducts are given, but these are not the isolated adducts They always undergo endo [2 + 4] cycloaddition with maleic anhydride, yielding 1 2 adducts. An interesting feature to be seen from Table 1 is that substituents on the benzene (alkyl, phenyl, or halogen) always turn up at the position most remote from the site of addition. In view of the different nature of these substituents, it seems that steric rather than electronic factors are responsible for this regioselectivity. 

Ortho photocycloaddition to benzene of derivatives of acetylene and maleimide proceeds via excitation of the alkyne or the maleimide. A few other alkenes follow the same route to ortho photocycloadducts among those are dichlorovinylene carbonate and some alkenes in which the double bond is conjugated with a cyano, carbonyl, or phenyl group, which makes it possible to excite them in the presence of the arene. 

The photocycloaddition of cyclooctyne to benzene [72], producing bicy-clo[6.6.0]tetradeca-l,3,5,7-tetraene can be sensitized (with acetone) and quenched (with piperylene). The unsensitized reaction occurs with very high efficiency (56% yield at 66% conversion). Because transfer of triplet energy from acetone to benzene is improbable, the authors consider the possibility that the acetylene triplet may be the reactive species in the cycloaddition. 

Another reaction in which the excited alkene is the reactive species in ortho photocycloaddition is that of dichlorovinylene carbonate with benzene [82-84] (Scheme 15). 

The ortho photocycloaddition of 2,3-dihydropyran is a very efficient process [11,12]. The quantum yield, measured with a solution of 1.1 M benzene and 3.5 M 2,3-dihydropyran, is 0.7 in iso-octane and 0.78 in acetonitrile. The higher quantum yield in the more polar solvent supports the proposal that ortho photocycloaddition of alkenes to benzene involves a polar intermediate. It is useful to compare these quantum yields of product formation with the quantum yield of in-... 

Scheme 42 Photocycloaddition of benzene to 1,3-dioxole. The quantum yields [13,122] were measured at 254 nm in dioxane at 20°C with 1.1 M benzene and a dioxole concentration at which the quantum yield reached the maximum value.

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). 

Coulombic forces will determine the regioselectivity of the ortho addition [189], In the charge-transfer complexes of monosubstituted benzenes with alkenes, the charge (positive or negative) on the arene is largely located at the carbon atoms ipso and (to a lesser extent) para to the substituent. The carbon atoms of the alkene double bond will preferentially be located in the neighborhood of either the ipso carbon or (to a lesser extent) the para carbon atom of the monosubstituted benzene. This would explain the 1,2 and 3,4 selectivity in the ortho photocycloaddition. 

Several reviews have been appeared about the synthetic utility and mechanistic details of photocycloadditions to aromatic rings [18,20-23], In this subsection, we will deal with mainly the examples of last two decades. Photocycloaddition of alkenes to benzene rings proceeds at ortho, meta, and para positions as described earlier. It has been suggested that alkenes and arenes having either electron-releasing or electron-accepting substituents favor the ortho process, whereas relatively simple alkenes and arenes undergo meta addition [24,44],... 

Farid et al. reported the formation of two types of (4 + 2) photocycloadduct of 9,10-dicyanoanthracene (DCA) with 3-carbomethoxy-l,2-diphenylpropene [192,193], The product ratio depends on the solvent polarity. In benzene, exo-125 is selectively obtained via exciplex (Scheme 38). In acetonitrile, endo-125 is obtained as a sole product via the radical ion pair. Photochemical reactions of DCA with 1,2-diarylcyclopropanes gave (4 + 3) cycloadducts [194,195], In degassed acetonitrile solution, (4 + 3) photocycloaddition occurred to give cis and trans cycloadducts in a 3 1 ratio in good chemical yields, although the quantum yields... 

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