From triple-bond migration

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. 

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. 

Zelinskil and Levina studied the shift of double bonds when olefins were passed over oxide catalysts (446). Levina also established that in the presence of chromic oxide on alumina at 250° the triple bond in a 1-alkyne is shifted, giving a product one half of which consists of the corresponding 2-alkene and one-half of a 1,3-diene (206). She also reported migration of double bonds from side chains into the ring of naphthenes carrying unsaturated side chains. 

These play an enormous role in organometallic chemistry and will be mentioned frequently. The nature of the M to C bonding is very much dependent on the nature of M and R. At one extreme there are compounds in which M is in a high valence state and the R group(s) not of jr-donor character. In these cases, the bonds are comparable to those just discussed for M=NR and M=N. For these types of compounds, the terms alkylidene (M=CR2) and alkylidyne (M=CR) have been favored. On the other hand, when the metal is in a low valence state and the substituents on carbon are n donors, the M—C bonds are not full double or triple bonds and the systems are rendered stable by the migration of charge from the substituents (such as OR or NR) onto the carbon atom, as shown in resonance terms in (16-VI). For these compounds it is customary to use the terms carbene complex and carbyne complex. The chemistry is qualitatively different for the two classes. 

The prototropic rearrangements of conjugated diynes closely parallel those observed for simple alkynes, the principal differences being in the rates of reaction and the greater complexity of products made possible by the additional unsaturation. Migration of a triple bond from the terminal position toward the centre of the chain is widely observed with simple alkynes, and similar behaviour has been reported for diynes. 

A plausible mechanism for the Lewis acid-catalyzed fran -vinylsilylation is shown in Sch. 74. The coordination of a Lewis acid to the triple bond of 112 would form ji-complex 114 and the a-carbon of the vinylsilane would attack the electron-deficient triple bond from the side opposite to the Lewis acid to produce an aluminum ate complex 115 stereoselectively. The migration of the trimethylsilyl group to the aluminate center would afford 113 and regenerate the Lewis acid catalyst. 

R and R" are cis in the starting compound, they will be trans in the product (2) there is retention of configuration within the migrating group R. " " Since vinylic boranes can be prepared from alkynes (15-16), this is a method for the addition of R and H to a triple bond. If R = H, the product is a (Z)-alkene. The mechanism is believed to involve an iodonium intermediate, such as 92, and attack by iodide on boron. When R is vinylic, the product is a conjugated diene." " ... 

Potassium 3-aminopropylamide (KAPA), prepared by adding potassium hydride " to an excess of 3-aminopropylamine (APA), causes the rapid migration of triple bonds from internal positions to the terminus of the carbon chain at 0 °C within minutes to from acetylide anions. The migration of the triple bond to the terminal position is blocked by an alkyl branch. The corresponding terminal acetylenes are obtained on hydrolytic workup. 

Isomerization of l-alkynes. Migration of the triple bond from a terminal position inwards is catalyzed by a complex prepared from Yb and PhjC=NPh in BMPA/THF. 

Isomerization of alkynes. This difunetional base induees rapid migration of triple bonds from the interior of the chain to the terminus in seconds at 0 ... 

Vinyl bromide 22 (n = 0) gave 61% of acetylene 23 (n = 0), for example, but 1-bromo-l-hexene (22, n = 4) gave only 18% of the corresponding alkyne (23, n = 4).29 Alkynes can also be formed from nonterminal (internal) vinyl halides (such as 3-bromo-3-heptene), but when alkoxide bases are used, the vigorous conditions usually required for elimination of the halogen can cause the triple bond of the product to migrate to the terminal position.30... 

Potassium 3-aminopropylamlde, 6, 476 7, 296. This base can be prepared more conveniently by reaction of potassium amide (prepared from potassium and ammonia) with 1,3-diaminopropane with subsequent evaporation of excess ammonia in vacuo. The Dutch chemists report further examples of use of the reagent for transforming alkynes and acetylenic alcohols into potassium acetylides with migration of the triple bond. 

Isomerization of acetylenic alcohols. In the presence of this base internal triple bonds of acetylenic alcohols migrate to the terminus remote from the —OH group. -Examples ... 

Acetylene derivatives from a, -ethylenehalides with subsequent migration of carbon-carbon triple bonds... 

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