Benzene photocycloaddition with alkenes

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-... 

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-... 

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],... 

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],... 

A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

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. 

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. 

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. 

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. 

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... 

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. 

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 second product (21) in the reaction of the diester (equation 70) results from 1,3-addition to the aromatic ring. Such addition is the major mode of reaction for benzene/alkene photocycloadditions, but with alkynes it is less common. One of the few reported examples is the addition of diphenylacetylene to esters of trimesic acid (equation 71),... 

The meta photocycloaddition of ( )-l,2-dichloroethene with benzonitrile, toluonitriles, benzene, phenol, cresols, fluorobenzene, chlorobenzene, trifluoromethylbenzene and 3-trifluoro-methylbenzonitrile shows not only the influence of the aromatic substituents on the direction of the alkene attack, but also a control of the cyclopropane closure by the vinyl chlorine to give exclusively the exo-(>, ent/o-7-dichloro adduct isomer 5 which was, in the most favorable cases, isolated in gram quantities. ... 

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 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. ... 

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