Photocycloaddition meta addition

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],... 

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. 

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). 

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. 

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). 

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... 

The effect of substituents on the regiochemistry of the intramolecular meta photocycloaddition of the l-(3-butenyl)indanes (49)-(54) has been examined by Keese and coworkers.In the methoxy-substituted compounds (49)-(51) meta addition of the side chain alkene occurs across the 1,5-positions of the benzene ring while for (52)-(54) addition occurs across the 1,3-positions. 

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. 

Wender and Dreyer have demonstrate that meta-areiw photocycloaddition chemistry can lead expediently to modhephene A( tate 7S>5, a photoproduct derived from indane and vinyl acetate was converted to tetracyclic ketone 797 (Scheme XCVI). Because the enolate of 797 partakes of the dynamic behavior of semibuUvalenes, it proved possible to trimethylate the substance to produce 798. A fourth methyl group was introduced by conjugate addition and the carbonyl was simultaneously converted to an olefinic center. Selective hydrogenation of799 provided modhephene in seven steps. 

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. 

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-... 

In general, intramolecular photocycloaddition between simple benzene and simple alkenes, in which any electron-releasing or electron-withdrawing groups are absent, proceeds at 1,3 (meta) positions [23], The products obtained by the me la addition is most commonly divided into three types l,3-addition-2,4-clo-... 

In recent investigations about the intramolecular meto-addition processes, the effects of heteroatoms in the linking chains are obvious. Blakemore and Gilbert reported the photoreaction of 185 linked by the amide group [240], Pho-toinduced intramolecular electron transfer can be inhibited by /V-acetyl or N-car-bomethoxy groups. Fluorescence is then observed from the arene chromophore and intramolecular meta photocycloaddition occurs through high selectivity. 

Blakemore and Gilbert found that the SiMe2 unit induces high selectivity in the meta photocycloaddition [241], Irradiation (254 nm) of the silicon-tethered bichromophore 186 gives only one photoisomer 187 in 60% yield (Scheme 56). The ability of silicon to stabilize a (3-positive center acts to stabilize the developing positive charge in the ipso carbon hence, the photocycloaddition directs to the 2,6-position of the benzene ring rather than the 1,3-addition. 

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... 

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... 

Among the variety of terpenes which were synthesized with an intramolecular meta photocycloaddition as key step, triquinane derivatives were particularly well studied. Two isomers of these compounds are readily accessible via intramolecular meta cycloaddition in position 1,3 (Sch. 9). The following cyclopropanation reaction of intermediate O controls which of the angular or linear isomers is formed (Sch. 17) [67]. As this step is almost always unselective, both isomers are concomitantly formed. However, one of the isomers can be obtained predominantly via an additional photochemical equilibration step (compare Sch. 11). The triquinane frame is obtained by rupture of the distant C-C bond of the cyclopropane in connection to the 10-membered ring moiety. 

The photochemical transformations of vinylcyclopropanes are of mechanistic interest, but because of the high energy of the reaction intermediates, especially in the triplet manifold, practical applications are scarce. The regio- and stereo-selective aspects are, however, similar to those of the thermal processes. Additional methods that provide for the cyclopentene rearrangement involve the degenerate photochemical rearrangements observed by Wender during meta-photocycloaddition of arenes and alkenes and utilized extensively in the synthesis of triquinane sesquiterpenes. -" -" (See Section 8.1.9 or ref. 41 for a recent summary.)... 

The meta photocycloaddition of cis-cyclooctene to benzene was one of the first examples of this reaction reported, and the addition of this cycloalkene to low molecular weight polystyrene and mono- and di-aromatic model-compounds has now been described. The results were analysed by mass spectroscopy, and it was reptorted that for polymers having up to six styrene units, more than one phenyl group in each chain had reacted. In the 2 1... 

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. 

The intramolecular meta photocycloaddition of 5-phenyl-l-pentene derivatives has been applied very successfully by Wender and his group to the synthesis of a number of natural products. Some of their recent work in this area has been summarised in a paper based upon a lecture given at the Xlllth lUPAC Photochemistry Symposium. In addition, a communication has described the synthesis of the natural product subergorgic acid, (44), by a route which commences with intramolecular meta photocycloaddition of the phenylpentene... 

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