1,3-Diynes, rearrangement

The reaction of diacetylene and its asymmetric homologs (penta-l,3-diyne, hexa-1,3-diyne) with semicarbazide (72ZOR2605) affords the amides of 3-methyl-pyrazole- 1-carboxylic acid (27) (80°C, EtONa, EtOH, 40 h). Amide 26 undergoes irreversible rearrangement to amide 27 at 80°C (EtONa, EtOH). 

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. 

In the nearly 1970s, Bergman and co-workers48,49 postulated that cis-hex-2-ene, 1,5-diyne (A) upon thermolysis would undergo a thermal rearrangement to the benzene 1,4-diradical intermediate (or, 1,4-dehydrobenzene, B), which could revert to starting material or collapse to the rearrangement product (C) [Eq. (8)]. 

A variety of palladium-catalyzed dimerizations of conjugated enynes and their additions to diynes and triynes gave rise to styrene and phenylacetylene derivatives, respectively. Inter alia, 1,2,4-cyclohexatrienes have been invoked as intermediates in these reactions [134], 5,6-Diphenyl-l,2,4-cyclohexatriene has been proposed as an intermediate in the rearrangement of 4,4-diphenylcyclohexa-2,5-dienylidene to o-ter-phenyl and its possible existence was supported by quantum-chemical calculations [135],... 

Flash vacuum pyrolysis of deuterium-labeled [l,2-bis(ethynyl)cyclobutadiene]CoCp 262a affords the rearranged product 262b and recovered starting material (Scheme 68)236. None of the dideuteriated product 262c or any of the potential [l,3-bis(ethynyl)cyclobuta-diene] CoCp isomers were observed. These results are difficult to reconcile with a mechanism involving a bis(diyne)CoCp intermediate (263) and are most consistent with the intermediacy of either cyclooctadiendiyne complex 264 or cyclooctahexaene complex 265. 

The reaction of two equivalents of W(C=CC=CH)(CO)3Cp with Ru3(CO)io (NCMe)2 gives the RU3W cluster 149 (Scheme 30), which is also obtained from 135 and W(C=CC=CH)(CO)3Cp. The extended organic ligand is formed by coupling of two molecules of the diynyl complex with two of CO, to form a cyclopen-tadienone attached by a carbenic interaction to the cluster W atom, and featuring formylethynyl and C=CW(CO)3Cp substituents. " One of the elementary steps in the reaction mechanism may involve formal rearrangement of the diyne to a dicarbyne. 

There are many examples of reactions of diynes with metal species that give mono or binuclear products containing ligands other than simple this section and there are obvious connections with that presented earlier as many of the products are derived from isomerization, rearrangement, and bond-forming reactions of initially formed diyne complexes. 

The reaction conditions (80 °C) used for the addition of 33 and 34 to the o-quinodimethane 32 are incompatible with the presence of 1,2-bromochlorocy-clopropene (27), thus the potential of this approach was for quite a while not further exploited. However o-quinodimethanes may be synthesized under much milder conditions, and trapped as reactive intermediates with 27. Thus base-induced isomerization of cw-oct-4-ene-2,7-diyne (38) at -78 °C leads to bis-allene (39). Upon warming of 39, rearrangement to o-quinodimethane (40) occurs between -20 and -10 °C this adds smoothly to 27 and furnishes the adduct 41. Conversion of... 

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. ... 

Evidence for the generation of a silylene was obtained in the presence of triethylsilane, which afforded 2,3-diphenyltetrasilane (82) in 79% yield. The reactions of some 1,3-diynes, R C=CC=CR (83), with silylenes afforded the silylene adduct. The course of this reaction strongly depends on the nature of the substituents R and R . When R is an alkyl group, the bis(silirene) (84) is formed initially but undergoes rearrangement to a bicyclohexadiene derivative upon longer photolysis (Eq. 9)... 

Scheme 16.13. 9,10-Didehydroanthracene (33) was generated from various precursors, but rapidly rearranges to cyclic diyne 35.

Photoexciting 1,4-dibromobenzene in a molecular beam with 307-nm pump pulse photons leads to sequential loss of both bromine atoms within 100 fs the excited dibromide has a lifetime of 50 fs and the monobromide of 80 fs. The p-benzyne generated decays relatively slowly, showing a lifetime of 400 ps. It presumably leads to (Z)-3-hexene-l,5-diyne through a valence isomerization of the sort associated with the Bergman rearrangement. 

Ring expansion of vinylcyclopro-penes and cyclobutenes 8-34 Ring expansion of vinylcycloal-kanes cyclization of diynes 8-39 Metathesis of dienes 8-40 Metal-ion-catalyzed o-bond rearrangements... 

The hydration of conjugated diynes readily affords 1,3-diketones. The hydration may be acid- or mercuric sulfate-catalyzed. The Rupe rearrangement is observed under general acid catalysis (equation 204).310... 

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