Migration triple bonds

Later, Dale used the extensive triple-bond migration, which takes place easily in macrocyclic dialkynes (C12-C20) when they are treated with KO-t-Bu in DMSO, to confirm his predictions for the relative stability of cyclic diyne isomers [4 c, 62]. These predictions were based on conformational considerations. Thus, 1,7-cyclotetradecadiyne (119) is almost completely rearranged to the 1,8-isomer 120 because in the latter compound both angle and terminal strain are absent, i.e., an ideal conformation is possible (Scheme 8-11). Dale s predictions, which were made long before the routine use of X-ray crystallography, were summarized in a seminal paper [63] and later confirmed by various X-ray investigations. The triple bond migration in cyclic diynes proceeds via allenic intermediates, which, however, are found only in small quantities in the reaction mixture. 

A primary allylic hydrogen at the ene 1 is especially reactive a secondary hydrogen migrates less facile, and a tertiary one is even less reactive. The enophile unit should be of an electron-poor nature it can consist of a carbon-carbon double or triple bond, a carbonyl group or an azo group. Mixtures of regioisomeric products may be obtained with substituted enophiles. The acrylic ester 6 reacts with... 

Triple bonds can also migrate in the presence of bases, but through the allene intermediate ... 

As in the case of the base-catalyzed reaction, the thermodynamically most stable alkene is the one predominantly formed. However, the acid-catalyzed reaction is much less synthetically useful because carbocations give rise to many side products. If the substrate has several possible locations for a double bond, mixtures of all possible isomers are usually obtained. Isomerization of 1-decene, for example, gives a mixture that contains not only 1-decene and cis- and franj-2-decene but also the cis and trans isomers of 3-, 4-, and 5-decene as well as branched alkenes resulting from rearrangement of carbocations. It is true that the most stable alkenes predominate, but many of them have stabilities that are close together. Acid-catalyzed migration of triple bonds (with allene intermediates) can be accomplished if very strong acids (e.g., HF—PF5) are used. If the mechanism is the same as that for double bonds, vinyl cations are intermediates. 

Similarly, a-hydroxy triple-bond compounds have given a,p-unsaturated ketones. See 15-1 for related reactions in which double bonds migrate or isomerize. 

The mechanism involves electrophilic attack by iodine at the triple bond, which induces migration of an alkyl group from boron. This is followed by elimination of dialkyliodoboron. 

Other types of nucleophiles can also react with the electrophically activated triple bond. An O-acyl group has been shown to be an excellent participating group. Interestingly, this process, which is reminiscent of the well-known palladium(ll)-mediated 1,3-migration of allylic acetates327 represents a highly efficient means for the formation of... 

By using hydrazine, 3-alkyl-5-aminopyrazoles 584 were formed in excellent yields via conjugate addition, C=C bond migration and intramolecular animation of the C-N triple bond [268]. 

Isomerization is blocked by alkyl or hydroxyl branches the triple bond then migrates to the free terminus. 

Metathesis of enynes, which have alkene and alkyne moieties in the molecule, is also a very interesting reaction. In this reaction, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds, and the cleaved alkylidene part migrates onto the alkyne carbon to produce a cyclic compound having a diene... 

In 1975 Brown and Yamashita 2 reported that a triple bond in any position of a straight chain hydrocarbon or acetylenic alcohol, when treated with a sufficiently strong base, could be isomerized exclusively to the free terminus of the chain. The "zipper reaction thus provides a general solution to the problem of remote functionalization of a long hydrocarbon chain. Isomer-lzations along chains of thirty carbon atoms have been achieved. Isomerization is blocked by alkyl or hydroxyl branches, the triple bond then migrates to the free terminus. 

Migration of double and triple bonds 2-40 Decarboxylation of unsaturated acids... 

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