Photocycloaddition ortho

Ortho Photocycloaddition of Alkenes and Alkynes to the Benzene Ring... 

Cycloaddition of alkenes to the benzene ring does not occur when both molecules are in their ground electronic states. The reaction can only be brought about by photoexcitation of either of the two addends. Three types of photochemical cycloaddition of alkenes to benzene and its derivatives are presently known. Ortho photocycloaddition, also referred to as 1,2-photocycloaddition or [2 + 2] photocycloaddition, leads to bicyclo[4.2.0]octa-2,4-dienes. Meta photocycloaddition, also referred to as 1,3-photocycloaddition or [2 + 3] photocycloaddition, gives triyclo[3.3.0.02 8]oct-3-enes, also named l,2,2a,2b,4a,4b-hexahydrocyclo-propa[crf]pcn(alcnes. Para photocycloaddition, also referred to as 1,4-photocy-cloaddition or [2 + 4] photocycloaddition, results in bicyclo[2.2.2]octa-2,5-... 

Scheme 1 Ortho, meta, and para photocycloaddition of ethene and benzene.

Ortho photocycloaddition was first reported in a U.S. patent [1] dated September 3, 1957. Irradiation of benzonitrile in the presence of various alkenes resulted in the formation of derivatives of l-cyanobicyclo[4.2.0]octa-2,4-diene. The first ortho photocycloaddition to benzene was reported in 1959 by Angus and Bryce-Smith [2], who discovered that benzene and maleic anhydride react to form a stable adduct at 60°C under the influence of ultraviolet radiation. This 1 2 adduct was formed from one molecule of benzene and two molecules of maleic anhydride. Two years later, Bryce-Smith and Lodge [3] found that acetylenes could also be photoadded to benzene. The isolated products were cyclooctatetraenes, formed by ring opening of the primarily formed bicyclo[4.2.0]octa-2,4,7-trienes. Since those early years, hundreds of examples of ortho photocycloadditions of alkenes to the benzene ring and many mechanistic investigations have been reported and they will be discussed in this chapter. 

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. 

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],... 

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. 

Most of the ortho photocycloadditions of alkenes and alkynes to the benzene ring that have been reported during the past 40 years have been tabulated in the 4 major sections of this chapter. Many ortho photocycloadducts are unstable under the conditions of irradiation. In some cases, they undergo spontaneous photochemical or thermal rearrangements in other cases, the investigators add a reagent that turns the unstable ortho adduct into a more stable product that can be isolated and identified. Tables 1-7, for the sake of comparison, list the primary ortho adducts, even if they have never been isolated or detected. In Section VI, the many types of secondary reactions that ortho photocycloadducts can undergo are discussed. 

The tables also contain concise information concerning the conditions of irradiation (solvent, concentration, wavelength), product yields and quantum yields, the formation of other products, and types of secondary reactions. Again for the sake of comparison, a few cases have been included in which ortho photocycloaddition was attempted but failed to take place. 

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. 

II. ORTHO PHOTOCYCLOADDITION VIA EXCITATION OF A GROUND-STATE CHARGE-TRANSFER COMPLEX BETWEEN ARENE AND ALKENE... 

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... 

From the stereochemical structure of the adduct it can be inferred that the initial ortho photocycloaddition occurs with exo stereochemistry, whereas the subsequent Diels-Alder reaction of the ortho adduct with a second molecule of maleic anhydride proceeds with endo stereochemistry. 

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. 

III. ORTHO PHOTOCYCLOADDITION VIA EXCITATION OF THE ALKENE OR ALKYNE FOLLOWED BY REACTION WITH GROUND-STATE ARENE... 

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. 

Table 1 Ortho Photocycloaddition of Arenes to Maleic Anhydrides...

Ortho photocycloaddition of acetylenes to hexafluorobenzene has also been found by Bryce-Smith et al. [67], Irradiation at 20°C of hexafluorobenzene saturated with propyne gave one major product that could be identified as... 

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). 

Scheme 15 Ortho photocycloaddition of benzene and dichlorovinylene carbonate.

Scheme 16 Ortho photocycloaddition of /V-benzylstyry I acetamides. (From Ref. 85.)...

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],... 

Intermolecular and intramolecular ortho photocycloadditions of derivatives of benzene to alkynes are collected in Table 2. Nitriles that give ortho cycloadducts are listed at the end of this table, although mechanistically, as explained, they fall in another category. Table 3 contains the ortho photocycloaddi-... 

Ortho photocycloaddition reactions proceeding via excitation of the arene followed by intersystem crossing to the triplet state and reactions with the alkene have been reported by Wagner [93-103], Gilbert [104,105], Pete [106], and their co-workers. 

Wagner and McMahon [101] have irradiated compounds bearing chiral auxiliaries and observed that high diastereoselectivity can be achieved with photoinduced intramolecular ortho cycloaddition (Scheme 26). Carbon atom 8 is the only permanent new stereocenter created by the ortho photocycloaddition. WhenX = (2R,5R)-( )-2,5-dimethylpyrrolidine or(7/ )-H Fcamphorsultam, di-astereomeric excesses as high as 90% are obtained. The (85)-cyclooctatriene-sul-tam derivative could be isolated as a single diastereomer after one recrystallization. 

Scheme 29 Intramolecular ortho photocycloaddition of cyano- and methoxycarbonyl-substituted 4-phenoxybut-l-enes.

Scheme 30 Intramolecular ortho photocycloaddition of ketones, nitriles, and esters.

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-... 

Scheme 32 Ortho photocycloaddition of p- and o-methoxyacetophenone to ethyl vinyl ether and vinyl acetate.

A tabular survey of ortho photocycloadditions proceeding via the triplet excited state of the arene is given in Table 4. As in Tables 1-3, the primary ortho adducts are listed, even if they are not isolated. 

V. ORTHO PHOTOCYCLOADDITION OF SINGLET-EXCITED ARENES TO ALKENES... 

In Table 5, the ortho adducts are collected, which have been reported to be formed by intermolecular photocycloaddition of benzene and derivatives of benzene to simple alkenes. Ortho photocycloadditions of alkenes to perfluorinated benzenes are separately collected in Table 6. Intramolecular ortho photocycloadditions have been grouped in Table 7. (The many intramolecular ortho photocycloadditions proceeding via the triplet state of the arene moiety are found in Table 4.) All adducts shown in the tables are the primary ortho adducts. Many of these bicyclo[4.2.0]octa-2,4-diene derivatives are, however, unstable and not all of... 

Table 6 Intermolecular Ortho Photocycloaddition of Perfluorinated Arenes to Alkenes... 

Therefore, if ratios are given, these must be considered with great care, especially if irradiation times were long. Ortho adducts formed by intramolecular photocycloaddition are even less stable than those from intermolecular reactions. Only from pentafluorophenyl prop-2-enyl ether [111], 2-methyl-6-(4-fluo-rophenyl)hex-2-ene [112], and l-(2-methoxybenzyloxy)-3-methylbut-2-ene [113] have primary ortho adducts reportedly been isolated. The many secondary... 

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. 

---