1-Decene, double bond migration

Further investigation on the metathesis of 1-decene (an expected olefin during the alkane metathesis of -decane) and the hydrogenation of the products formed clearly demonstrated that the distribution resulting from alkane and olefin metathesis completely differs with the same catalyst. If there is no double-bond migration, 9-octadecene and ethylene are expected to be the major primary products. Indeed, these primary products are observed, as the temperature reaches 150°C. However, after just 15 min, Cj to C12 and C13 to C20 olefins are also observed, clearly indicating that some isomerization of double bond occurs leading to several competitive metatheses (Fig. 5). 

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 high conversion polymerisation of a-decene gave a monomer fraction which contained 60% a-decene, 35% internal alkenes and 5% methylnonenes. The conclusion was that, in addition to methyl group migration, isomerisation of the double bond could also take place, even with the monomer. As polyalphaolefins derived from internal olefins usually give products with inferior temperature/viscosity properties, double bond isomerisation can be a problem. 

Upon El the exo and endo isomers of the 8,9-disub-stituted-tricyclo[5.2.1.0 > ] decenes [1] and [2], respectively, fragment essentially differently. The ionized endo isomers [2] eliminate CsHs via a retro-Diels- Alder reaction after migration of the hydrogen atoms from positions 8 and 9 to the double bond. This reaction is not observed for the exo isomers [1] where the distance between the hydrogen atoms of positions 8 and 9 and the double bond is too large for the hydrogen migration required to effect the retro-Diels- Alder reaction (Scheme 1). 

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