Acetylene photocycloaddition

Although most of the photocycloaddition reactions have involved olefins, there are several examples of additions to other types of un-saturated systems (e.g., allenes, acetylenes, ketenimines, etc.) which extend their synthetic utility. It is impossible, at this time, to define the limitations of the reaction as applied to other systems however, this will unquestionably be an active and fruitful area of research in the future. 

This is also the case with the oxetenes formed by photocycloaddition to acetylenes. Irradiation of benzaldehyde in the presence of 5-decyne... 

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. 

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. 

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... 

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. 

Al-Jalal and co-workers showed that nitrile groups undergo photocycloaddition to benzene rings to give azocine derivatives (10) [75-77] in a manner analogous to that of acetylene [78-84] (Scheme 7). 

The intermolecular ortho photocycloaddition of acetylenes to benzene has provided routes to cyclooctatetraenes [78-84], Intramolecular photocycloaddition of alkynes to aromatic rings is also investigated. Morrison et al. reported that photolysis of 6-phenyl-2-hexyne in hexane solution using 254-nm light leads to the formation of bicyclo[6.3.0]undecatetraene (251 and/or 252) [284] (Scheme 70). Pirrung prepared various bicyclo[6.3.0]undecatetraenes (254a-e) by the in-... 

Photocycloaddition von Dihalogenmaleinsaure-imiden und -an-hydriden an Olefine und Acetylene. Angew. Chem. 77, 452 (1965). 

In a pioneering study, Zandomeneghi and Cavazza showed that the [2 + 2]-photocycloaddition of racemic 4-acetyloxy-2-cydopentenone can lead to a small, but detectable, enantiomeric excess (ee) of the product if circularly polarized light (CPL) is used as an irradiation source. By preferential excitation of the (—(-antipode of the cyclopentenone with 1-CPL (351-363 nm laser) in the presence of acetylene, an... 

Full details of the effects of acid and solvent polarity on various photocycloaddition reactions of ethylenes and acetylenes to benzene have been published and a mechanistic rationale of the observations has been presented. Thus the largely homopolar concerted meta cycloaddition of ethylenes to benzene, which yields the tricyclo[3.3.0.0 "]oct-3-ene system (36), has little sensitivity to either solvent polarity change or the presence of a proton donor, but the onho process leading to bicyclo[4.2.0]octa-2,4-diene derivatives (37)... 

Irradiation of mixtures of various acetylenes with benzene gives cyclooc-tatetraenes, presumably via an intermediate ortho adduct 74 (Bryce-Smith et al., 1970). For acetylenes with bulky substituents, the bicyclooctatriene intermediate is sufficiently stable for a subsequent intramolecular photocycloaddition to a tetracyclooctene (75) (Tinnemans and Neckers, 1977) ... 

Photoaddition of acetylenes to benzene provides a convenient synthetic route to cyclooctatet-raenes through or//io-photocycloaddition. Although the meta cycloaddition would be a symmetry-allowed concerted process from Sj or Tj excited acetylene and S, benzene, or from Sq acetylene and Si (Bjn) benzene, there are surprisingly few reported cases in which an acetylene underwent weM-photoaddition to a benzene ring to give a semibullvalene. 

The photocycloaddition chemistry of pyridines substituted with electron-donor and electron-acceptor groups at the 2- and 3- positions continues to be exploited. The results of irradiation of such pyridines in the presence of 2-cyanofuran have now been described. The yields of the (47r+47r) cycloadducts (29) and (30), the pyridine dimer (31) and the transposition isomer (32) are dependent on the level of methyl substitution on the heteroarene and are given in Scheme 2. Other photocycloadditions to heteroarenes reported within the year include the reactions of benzodithiophene (33) with butadiyne derivatives and dimethyl acetylene dicarboxylate, giving low yields of (34) and (35) respectively, the latter from photorearrangement of the primary adduct (36). The (271+471) photocycloaddition of indoles (37) to cyclohexa-1,3-dienes (38) is sensitized by the aromatic ketones (39), and yields (14-46%) of the exo and endo isomers of the adduct (40) in ratios which are dependent on the substituents on the addends. 

Bicyclo[5,3,0]dec-l(7)-en-2-one undergoes [2 + 2]photocycloaddition with a variety of olefins and acetylenes to give mixtures of cyclobutane products. ... 

The photocycloaddition of 2tf-azirines to electron-deficient olefins produces d -pyrrolines as primary photoproducts. Some of the dipolarophiles used include acrylic esters, acrylonitriles, fumaric and maleic esters, methyl allenecarboxylate, norbornene, and l,2-dicyanocyclobutene. Similarly, styrenes and vinylpyridines undergo smooth photocycloaddition to 2H-azirines. Addition of acetylene derivatives to the transient nitrile ylide gives 2H-pyrroles (18, 20) which rearrange to pyrroles (19) if the C-2 atom is monosubstituted. 3-Phenyl-2/f-azirines have been found to cycloadd to vinylphosphonium salts and to vinyl sulfones. The initial adducts (22 and 24) undergo ready loss of the phosphorous or sulfur substituent to give 2H-pyrroles (23 and 25). ... 

Other sesquiterpenes whose synthesis are worthy of mention are hibiscone C (38) (also known as gmelofuran), which is found in the heartwood of Hibiscus elatuSy and herbertene (40), isolated from the liverwort Herberta adunca. The synthesis of hibiscone by Koft and Smith" can be commended for its novel use of intramolecular [2 4- 2] photocycloaddition of the acetylenic enone (36) to set up the intermediate (37), whereas Prater s synthesis of herbertene is interesting, to say the least, since it involves a one-step acid-catalysed rearrangement of the alcohol (39) ... 

FIGURE 41.10 Intra-and inter-molecular photocycloaddition of acetylenes to the benzene ring. 

Enantioselective photoreactions induced by CPL have received considerable interest, particularly in regard to answering the question of how enantiomerically enriched material in the universe has been generated. From a synthetic point of view, however, this method has seen limited success. In the few asymmetric fixation reactions examined (in most cases, photocyclizations), only low enantiomeric excesses were achieved. A single example of an asymmetric photocycloaddition has been reported. " " By preferential excitation of the (-)-antipode of racemic cyclopentanone ( )-51 by/-CPL (351- to 363-nm laser) in the presence of acetylene, enantiomerically enriched (1% ee) (-)-52 was obtained (Scheme 17). The unreacted starting material 51 was optically enriched in the (-l-)-enantiomer. This method provided product of sufficient optical purity to examine the chiroptical data. 

Cavazza, M. and Zandomeneghi, M., Photocycloaddition of acetylene and 1-hexyne to 4-acetox-ycyclopent-2-en-l-ones. Intermolecular asymmetric photosyntheses induced by circularly polarized light, Gazz. Chim. Ital, 117,17,1987. 

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