Hexadiyne isomerization

Ethynylallenes [35], the didehydro analogs of vinylallenes, have also been known for some time, the parent system 7 having been obtained in 1960 by reduction of 3-chloro-l,4-pentadiyne (101) with zinc-copper couple in butanol [36] and the 5-methyl derivative 103 by base-catalyzed isomerization of 1,4-hexadiyne (102) with sodium ethoxide in ethanol (Scheme 5.13) [9]. 

The best known example is the thermal isomerization of 1,5-hexadiyne, which involves a [3.3]-sigmatropic shift to 1,2,4,5-hexatetraene (12) followed by an electrocydic ring closure to 3,4-bismethylencydobutene (324) for reviews, see (a) W. D. Huntsman, in The Chemistry of Ketenes, AUenes and Related Compounds, S. Patai (ed.), Wiley Chichester, 1980, Vol. 2, Chapter 15, pp. 521-667 (b) A. Viola, J. J. Collins, N. Fillip, Tetrahedron 1981, 37, 3765—3811 for quantum chemical calculations on this and related isomerizations, see K. A. Bleck,... 

Tetraacetylenes such as 115 and 116 contain the 1,5-hexadiyne group as a bridging element. Since the base-catalyzed isomerization of this unit to hexa-l,3-dien-5-yne (6) constitutes the basic reaction of Sondheimer s annulene chemistry [75], it appeared attractive to attempt to apply this classic reaction of planar aromatic chemistry to a layered precursor and create three-dimensional relatives of Sondheimer s dehydroannulenes. Indeed, both 115 and 116 could be isomerized to their fully conjugated isomers 129 and 130, respectively, by treatment with potassium tert-butoxide in tert-butanol, the original Sondheimer conditions (Scheme 28). From the X-ray structure obtained for 130, it was concluded that both hydro-... 

Conjugated diynes with a terminal triple bond give products in which the triple bonds remain conjugated as illustrated by the isomerization of 1,3-hexadiyne to... 

Diynes require somewhat more vigorous conditions for isomerization than 1,4-diynes. Thus, 1,5-hexadiyne is stable to sodium ethoxide in ethanol or 0 03m potassium /-butoxide in /-butyl alcohol at room temperature, but isomerization is effected at higher temperatures or by the use of a higher concentration of potassium /-butoxide- . The reaction is complex, but many of the intricacies have been unravelled, and the relationships are depicted in Scheme... 

CIS- and /rfl j-l,3-Hexadien-5-yne (277 and 278) arc formed as major products, along with small amounts of 2,4-hexadiyne (283), when 275 is heated with potassium /-butoxide in /-butyl alcohol or ethanolic sodium ethoxide. In addition a small amount of 1,2,4,5-hexatetraene (280) was present in the reaction mixture involving sodium ethoxide. l,2-Hexadien-5-yne (276) the product expected from the first isomerization step was not detected, but this is understandable because it was demonstrated in separate experiments that 276 isomerizes much faster than the starting diyne. When 276 is treated with /-BuOK//-BuOH at room temperature, reaction is immediate, producing a mixture of the dienynes 277 and 278 on the other hand, when NaOEt is used, 280 is the exclusive product . ... 

Wolovsky describes the oxidative coupling of 1,5-hexadiyne, followed by prototropic rearrangement by heating with potassium /-butoxide in f-butanol-benzene at 100°. The resulting mixture of unsaturated hydrocarbons was easily separated by elution chromatography on a column of alumina impregnated with 20% silver nitrate into the tetradehydro[18]annulene (111) and the two isomeric tridehydro[18]annulenes (1) and (11). 

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