Olefin hydrogenation metathesis pathways

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

It is noteworthy that the double-bond isomerization step is faster than the overall elementary steps of alkane metathesis. Formation of lower alkanes is due to the tungsten hydride intermediate, favoring chain walking with double-bond migratirHi followed by fast cross metathesis with coordinated ethylene leading to lower alkenes in turn giving lower alkanes on hydrogenation. This intramolecular reaction pathway, without formation of the free olefin, probably is the difference between alkane and olefin metathesis. 

Starting with -hexane (Cg) metathesis, dehydrogenation should give the corresponding 1-hexene, followed by its homo-metathesis to yield ethylene and decene, which upon hydrogenation, should ideally produce ethane and decane (Cj q products) as the major products (Scheme 2.18, path a). However, this tandem reaction process was not selective since -decane represented <50% of the total primary products of heavy alkanes when the reaction was catalyzed with Ir-2(H2). The authors attributed this unexpected distribution of alkanes to the isomerization of the (x-olefin prior its metathesis, as depicted in pathway b (Scheme 2.18). 

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