1.5- Diynes Cope rearrangement

Indeed, cw-l,2-divinylcyclopropanes give this rearrangement so rapidly that they generally cannot be isolated at room temperature,though exceptions are known. When heated, 1,5-diynes are converted to 3,4-dimethylenecyclobu-tenes. A rate-determining Cope rearrangement is followed by a very rapid electro-cyclic (18-27) reaction. The interconversion of 1,3,5-trienes and cyclohexadienes... 

Fig. 16 (a) Comparison of potential energy profile for the formal Cope rearrangement of 3,4-difluorohexa-l,5-diyne-3-ene with that of (Z)-hexa-l,5-diyne-3-ene, (b) Rehybridization in the C(F) bond along the reaction path. EDI = 3,4-difluoro-hex- 3-ene-l,5-diyne ED2 = 1,6-di-fluoro-hex-3-ene-l,5-diyne BZY = difluoro-l,4-didehydrobenzezne TSBC = the transition state for the Bergman cyclization TSRBC = the transition state for the retro Bergman cyclization. 

Radiolysis of the diacetylene hexa-l,5-diyne (50) generates the hexa-1,2,4,5-tetraene radical cation (51 +) via a Cope rearrangement in the Freon matrix. ... 

We first take up the prototype Bergman cyciization, which is the conversion of (3Z)-3-hexene-l,5-diyne (38) into jo-benzyne (40). Computational chemists came to the Bergman cyciization problem at about the same time as they came to understand the computational difficulties of the Cope rearrangement discussed in the previous section. These studies of the Cope rearrangement greatly colored the early choice of methodologies to be used for the Bergman cyciization. 

The thermal rearrangements of meso- and rac-3,4-bistrimethylsilyloxyhexa-l,5-diynes (141) and (142) have been studied. The products are the 1,2-dimethylene-cyclobutene derivatives (143), (144), and (145). The product distributions at moderate temperatures are consistent with initial Cope rearrangement of (141) and (142) to the bisallenes (146) and (147) followed by conrotatory closure of the bisallene to the 1,2-dimethylenecyclobutene. It is of interest that thermolysis of the rac-isomer (142) gives equal amounts of (144) and (145), although (144) would appear to be much moie sterically compressed than (145). 

Terminal Z-enynes (140) are produced by selective hydrogenation of the unsily-lated triple bond of the corresponding diynes (141) prepared by oxidative coupling of an acetylene with trimethylsilylacetylene (Scheme 82).Enynes are also obtained from a-cyclopropylacetylenic carbinols (128), and by Cope rearrangement of dienynols (137). 

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