Photocycloaddition ortho addition

Section V describes ortho photocycloadditions of mostly simple alkenes that do not form ground-state complexes with arenes, do not absorb light in the presence of the arenes, and add to the arene in its singlet excited state. Mechanistic investigations, including the search for intermediates (ground-state complexes, excited states, exciplexes, zwitterions), the formulation of empirical rules, and theoretical descriptions of the reaction, have mostly been concerned with this type of ortho addition. Therefore, this section is divided into a number of subsections, each describing a particular aspect of ortho photocycloaddition. 

In one of the earliest reports on ortho photocycloaddition, in which the reaction of benzonitrile with 2-methylbut-2-ene is described, a diradical (triplet) intermediate was proposed [73], The structure of the product corresponds to the most stable of the four possible diradical intermediates. When benzophenone was added as a sensitizer in an attempt to increase the yield of the photoadduct, only 0.05% of ortho adduct was isolated along with 54% of an oxetane formed by the addition of benzophenone to 2-methylbut-2-ene. In the absence of benzophenone, the ortho adduct was isolated in 63% yield. It is, however, thermally as well as photochemically unstable and reverts to starting materials, supposedly also via a biradical. The authors propose that benzophenone catalyzes bond cleavage of the adduct more efficiently than ortho addition and this would account for the low yield of photoadduct in the presence of benzophenone. From these experiments, no conclusion about the identity of the reactive excited state can be drawn. 

On the basis of these observations, Bryce-Smith et al. [115] introduced a rule stating that for addition to benzene, Pmeta when 9.6 eV < IP (alkene) <8.65 eV. They concluded that if this rule is correct, ortho addition of ethylenes to Si benzene necessarily involves an element of charge transfer to or from the ethylene. Indeed, a marked effect of polar solvents (methanol or acetonitrile) in promoting the ortho addition of benzene to ethyl vinyl ether and tetramethylethene was observed (portho increased by 20-50%, whereas cpmeta was unaffected. One exception to this rule was found by Heine and Hartmann [10], who discovered that vinylene carbonate (IP = 10.08 eV) undergoes mainly meta photocycloaddition to benzene, accompanied by some para addition. Bryce-Smith and Gilbert [46] commented that their rule referred to quantum yields and not chemical yields, whereas no quantum yields were given for the vinylene carbonate additions. Moreover, quantum yield measurements should be made at low conversions because most ortho cycloadducts are photolabile. 

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. 

Hoffmann and Pete [170] have also investigated the photochemical behavior of derivatives of methyl benzoate bearing a hydroxy or alkoxy substituent at one meta position and an alkenyloxy side chain at the other meta position. In contrast to most other intramolecular ortho photocycloadditions, the addition in these molecules takes place at the positions ortho and meta with respect to the side chain, instead of ipso and ortho (Scheme 58). 

Prinzbach and co-workers studied the photocycloaddition of a rigid compound in which naphthalene and alkene were placed [294], Distances between naphthyl (3-carbons and olefinic carbons of 272 are 2.911 and 2.944 A. Direct or acetone-sensitized photoexcitation induced an efficient meta addition to the naphthalene ring to give 273, the ortho addition did not occur, although it is not clear whether the ortho process occurred (Scheme 76). 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

Intermolecular photocycloaddition also occurs between alkenes and simple phenols. The swing from meta addition illustrated above in the [3 + 2]-mode to ortho-addition is a result of charge-transfer interactions between the alkene and the phenol and a greater charge transfer favours the ortto-addition mode. These aspects have been the subjects of reviews . This reaction mode is exemplified by the addition of acrylonitrile... 

As in the case of the ortho photocycloaddition, the first examples of the meta addition were followed by hundreds of others. A comprehensive review summarizing the state of the art up to 1993 has appeared [6],... 

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. 

The carbon-nitrogen triple bond can also undergo ortho photocycloaddition to derivatives of benzene. Al-Jalal et al. [86] found that irradiation of 4-cyanoanisole in acrylonitrile produced three 1 1 adducts. Two of these 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 aza-cyclooctatetraene, apparently formed by the addition of the carbon-nitrogen triple bond to the arene, followed by ring opening [87],... 

The following subsections are devoted to various mechanistic aspects of the ortho photocycloaddition. The possible role of ground-state complexes will be discussed and, subsequently, the intermediate species that are formed or may be formed upon photoexcitation will be treated the reactive excited state, exciplexes, and zwitterions, biradicals, and ion pairs. Empirical rules, aimed at predicting under what circumstances ortho photocycloaddition (or other modes of addition) may occur, will be discussed next and, finally, the results of theoretical considerations and calculations will be reviewed. 

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. 

From 1987 onward, many examples of rearrangements of ortho photocycloadducts to derivatives of cyclooctatriene and subsequent reactions of these compounds have been reported. The same year marks the start of a series of investigations on intramolecular ortho photocycloadditions. This may not be a coincidence, because ortho adducts formed by intramolecular photoaddition seem especially prone to undergo rearrangement, although it has also frequently been observed in adducts arising from intermolecular addition. 

Almost all triplet-state photocycloadditions studied by Wagner s group [93-103] yield ortho adducts that undergo thermal ring opening to a cyclooctatriene followed by photochemical 4-rr ring closure. Many examples of these reactions are shown in the section devoted to additions via the triplet state. Scheme 51 schematically illustrates the basic reactions for intermolecular as well as intramolecular cycloadducts. A substituted benzene has been chosen for the intermolecular reaction in order to demonstrate the two modes of ring closure. 

The photocycloaddition of alkenes to benzene rings can be classified into three types (2 + 2) (ortho), (3 + 2) (meta), and (4 + 2) (para) photocycloaddition, depending on the substituents, the reaction media, and the additives, as shown in Scheme 3. In general, para addition is very limited, and ortho and para adducts are more labile thermally and photochemically than the meta adducts. The (2 + 2) (ortho) photocycloadducts, bicyclo[4.2.0]octa-2,4-dienes 1, give thermally the ring-opened 1,3,5-cyclooctatrienes 4 and their following photochemical ring clo-... 

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],... 

To clarify the effects of linking chains, Gilbert et al. investigated the photocycloaddition of 163-180, which produce various types of adducts [231,234-237] (Scheme 53). In some cases, not only the meta addition but also ortho and para addition and ene reaction took place. The observed regiochemistry was rationalized in terms of the stabilization of zwitterionic intermediates and the steric constraints. 

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... 

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. 

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