Acetylenes, reaction with arynes

The above results show that in reactions with arynes, pyrrolinediones act as synthetic equivalents of azadienes. One therefore wonders how they behave in reactions with other dienophiles. Is it possible to generalize the method described above to obtain a new synthetic route to pyridones and pyridines As a step in this direction, we have found that the pyrrolinedione 29b reacts rapidly with dimethyl acetylene dicarboxylate (DMAD) to give the emetine 42 in 75% yield (Scheme 14)(Ref. 21). 

The major products from the reaction of arynes with thiophene and benzothiophene by addition and insertion are shown in Table 11. Benzyne from phthalic anhydride reacted with thiophene at 690° to give naphthalene and benzothiophene by 1,4-addition and loss of sulfur, and by 1,2-addition and loss of acetylene, respectively, as well as phenyl-thiophene by insertion (Fields and Meyerson, 1966d, 1967e) (Scheme 19). The ratio of naphthalene to benzothiophene was about 9 1, nearly the same preference for 1,4-over 1,2-addition as was inferred from the reaction of benzyne with dichlorobenzenes and pyridine at the same temperature, and again reflects the strong tendency of benzyne to act as a dienophile. 

Aryne complexes of late transition metals are very reactive towards both nucleophiles (amines, alcohols, water) and electrophiles (iodine). They also undergo insertion reactions with CO, alkenes and alkynes,but while the behaviour of ruthenium complexes is somewhat similar to that of titanium or zirconium complexes, the reactivity of nickel complexes is rather different [6,8]. Examples of these reactions that are particularly interesting for the purposes of this chapter are shown in Schemes 8 and 9. Ruthenium complex 33 undergoes insertion of a molecule of benzonitrile,benzaldehyde or di(p-tolyl)acetylene to yield met-allacycles 40,41 and 42, respectively (Scheme 8). Further insertion of a second unsaturated molecule into these metallacycles has not been observed [25,27]. 

Some palladium-catalyzed reactions of organotins, such as carbostannylations, are not related to the Stille cross-coupling. The history of the transition-metal-cata-lyzed carbostarmylation [148] began with alkynylstannylation of alkynes catalyzed by a palladium-iminophosphine complex [149]. Thus, alkynylstannanes added to a carbon-carbon triple bond of various acetylenes, conjugated ynoates and propar-gyl amines and ethers in the presence of a catalytic amount of a palladium-iminophosphine complex [150]. The reaction also proceeded with arynes to afford ortho-substituted arylstannanes, which could further be converted into 1,2-substituted arenes via carbon-carbon bond-forming reactions [151]. 

The very reactive arynes have also been used as dienophiles in Diels-Alder cycloadditions with vinyl heterocycles and are included here, after the acetylenic esters, because of the similarity of their reactive functions. This reaction is of considerable importance because of its application in the synthesis of polycyclic compounds. 

Aromatization processes have also been observed in the reaction of dienamines with acetylene carboxylic esters , ketene" " (Scheme 30) and arynes ". ... 

Photolysis of the mercury compound 286 in benzene leads to 3-phenyl-thiophene (291) and 3-iodothiophene (287). The former could arise from an insertion reaction of the aryne (4) with benzene if it is assumed that the expected but missing 2-phenylthiophene 292 completely isomerizes to the 3-isomer (291) under the reaction conditions. The argument is not convincing, however, since the 3-iodothiophene (287) formed as a coproduct, presumably by homolysis of the C-Hg bond, also yields 3-phenylthiophene (291) on photolysis in benzene, albeit in very low yield. Furthermore, by analogy with the reaction of benzyne the major products should be those of cycloaddition, which, in the case of 4, might lead, after loss of acetylene, to thianaphthene (293). No such products were reported from the photolysis of the mercury compound (286), but they have been observed from other possible sources of the aryne (4) (Section III.3.A.f). 

Although research on the FVT of the thiophene anhydride 229 has been primarily directed at obtaining evidence for the intermediacy of the aryne 4, it has also yielded some information on the chemistry of this species. The most studied reaction of thiophyne (4) thus far is with dienes 146 to give thianaphthenes 293, presumably via the Diels-Alder adducts 390. The ubiquity of this reaction, as discussed above, appears to make it virtually diagnostic for the presence of the aryne (4). Thus the formation of thianaphthene (293) when benzene is the diene has been interpreted to involve a retro-Diels-Alder loss of acetylene from the initially formed thiophyne-ben-zene adduct (390c). Conversely the absence of thianaphthene (293) in the photolysis of the mercury compound 286 in benzene (Section III.3.A.a) strongly suggests that the aryne 4 is not produced in this reaction. 

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