Half-hydrogenated intermediates

One may inquire whether the evidence that 77-allyl complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. 

Surface intermediates may be hydrogenated to form half-hydrogenated intermediates (4), and they may desorb to form aldehydes [Eq. (3.15)] and alcohols [Eq. (3.16)] ... 

Alkene and alkane formation was suggested to take place through p cleavage and subsequent hydrogenation [Eq. (3.17)] chain branching involves the reaction of 1 with a half-hydrogenated intermediate [Eq. (3.18)] ... 

When deuterium instead of hydrogen is used, reversal of the half-hydrogenated intermediate accounts for deuterium exchange in alkenes and the formation of saturated hydrocarbons with more than two deuterium atoms in the molecule18,52 (Scheme 11.1). 

Study of the addition of gas-phase D atoms to 1-alkene monolayers adsorbed on Cu(100) suggested that addition to the terminal carbon predominates to generate the corresponding secondary alkyl group (half-hydrogenated intermediate).353... 

The stereochemistry of hydrogenation, the long recognized predominantly cis addition103-105 (see Section IV.A.3) is also consistent with the stepwise addition of the two H atoms via the half-hydrogenated intermediate. H-D exchange reaction of alkanes is also interpreted with the involvement of the surface alkyl intermediate106,107. 

The competing processes of double-bond isomerization and hydrogenation (or deuteration) are best discussed in terms of a half-hydrogenated intermediate which may undergo a number of different fates (Scheme 10.3). 

The first step in the reaction sequence (i) is the formation of a jr bond between the catalyst and the double bond. Reaction with catalyst-bound hydrogen (ii) then gives the half-hydrogenated intermediate attached to the catalyst by a a bond. This may react with a second catalyst-bound hydrogen atom (iii) to furnish the hydrogenation... 

For but-l-ene isomerization, it was proposed that the isomerization reaction takes place by at least two different mechanisms an intramolecular hydrogen shift and an associative mechanism with the formation of a half-hydrogenated intermediate. Both pathways were... 

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