Acetylene derivs benzene ring

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. 

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 most thorough analysis of the relative contributions of the three competing pathways (see Scheme 3) is due to Zimmermann and co-workers, who have addressed this problem by studying the thermocyclization of various substituted and D-labeled dienynes, in which, inter alia, one of the double bonds was part of a benzene ring. As summarized in Scheme 8 for the two acetylenes 27 and 30, which cycloaromatize to naphthalene (31) and its two mono-deuterio derivatives 28 and 29 on flash vacuum pyrolysis, the radical route (pathway (c)) plays a significant role in the formation of all these aromatization products [9, 16]. 

As found a few years ago [71], acetylenes that can be traced back to the diethynylbenzenes 45 and 47 are useful starting materials for the preparation of cyclophanes that possess triple bonds and triple-bond derived functionalities in their molecular bridges. Compared to cyclophanes bearing functional groups in benzene ring positions, relatively little is known about bridge-functionalized cyclophanes. That these - and especially acetylenic derivatives -have considerable potential in synthetic aromatic chemistry will be shown below. 

It is best to speak at this time of hydrocarbons derived from benzene by the substitution in the ring of unsaiurated open chain radicals. Just as the homologues of benzene are prepared by substitution of a saturated paraffin radical, methyl, ethyl, propyl, etc., in the benzene ring, so other series of hydrocarbons have been obtained by substituting radicals of the unsaturated open chain hydrocarbons of the ethylene and acetylene series. These new compounds will be poorer in hydrogen than the benzene series by two and four atoms respectively. 

In contrast to furan, the benzene ring in benzo[ ]furan (because of its large resonance energy) is dominant to such an extent that [4+2] cycloadditions are not possible. On the other hand, photochemical [2+2] cycloaddition occurs readily on the C-2/C-3 double bond leading, e.g. with an acetylene di-carboxylic ester, to the cyclobutene derivative 1 ... 

Synthesis with cobalt complex compounds Benzene ring from acetylene derivatives... 

Dicarbonylbis(triphenylphosphine)nickel Benzene ring from acetylene derivs. by trimerization... 

Hitherto unknown 4,5-dihydrobenz[g]indole is obtained from 1-tetralone oxime and acetylene in the system KOH/DMSO in one stage (Scheme 1.30) [199]. 4,5-Dihydrobenz[g]indoles and their N-vinyl derivatives substituted at the benzene ring are isolated from the reaction of the corresponding 1-tetralone oximes and acetylene in the systems MOH/DMSO (Scheme 1.30) [200]. 

Hydrazides of vicinal acetylene-substituted derivatives of benzoic and azole carboxylic acids are important intermediate compounds because they can be used for cyclization via both a- and /3-carbon atoms of a multiple bond involving both amine and amide nitrogen atoms (Scheme 131). Besides, the hydrazides of aromatic and heteroaromatic acids are convenient substrates for testing the proposed easy formation of a five-membered ring condensed with a benzene nucleus and the six-membered one condensed with five-membered azoles. 

The investigations on acetylenic compounds in the thiophene series added to the evidence that in most cases thiophene resembles benzene (78). It has been shown in our laboratories that reactions with sodium amide and sodium acetylide in liquid ammonia could be readily utilized in this series. The introduction of a triple bond in the side chain of a thiophene ring was possible only by employing halogenatcd thiophene derivatives such as ... 

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