Parallel hydrogenation cycles

Diene)Cr(CO)4 complexes serve as catalysts for the addition of hydrogen to 1,3-dienes to give 2Z-alkenes (equation 21)78a. Alternatively, Cr(CO)3(MeCN)3 may also be used as a catalyst for this reduction156. Use of deuterium instead of hydrogen affords the 1,4-dideuterio-2Z-alkene. The rate of reduction for uncomplexed acyclic dienes decreases in the order E, E— > E, Z— > Z, Z-dienes. This order parallels the ease of formation of the corresponding (diene)Cr(CO)4 complexes. These results implicate the formation of a 16 valence electron [VE] (diene)Cr(CO)3 intermediate as part of the catalytic cycle. 

Thus, additional experimental and computational studies will be needed to draw definitive conclusions regarding the mechanism of Ir-catalyzed asymmetric hydrogenation. The Ir(I)-Ir(III) and Ir(III)-Ir(V) cycles seem to be similar in energy, so it may well be that depending on the catalyst, substrate, and the hydrogenation conditions, one or the other pathway will be preferred or both cycles could operate in parallel. 

If a reaction can be accomplished by two or more paths, the paths are called parallel paths and the reaction is called a parallel reaction. The overall reaction rate is the sum of the rates of all the reaction paths. The fastest reaction path is the rate-determining path. For nuclear hydrogen burning, the PP I chain is one path, the PP II chain is another path, and the CNO cycle is yet another path. 

Here n is 2 or 3 with the two hydrogenation cycles running in parallel. In this cycle the oxidative addition of hydrogen is the most probable rate determining step which suggests a rate expression ... 

The abundant chemistry of Ni(CO>4 under reductive reaction conditions leading to the formation of dinuclear nickel complexes or even to nickel clusters suggests the involvement of higher aggregates, however. An overview of the reactivity of nickel complexes, and of Ni(CO)4 in particular, is given in a series of excellent reviews by Jolly [13]. There seems to be evidence of an autocatalytic cycle for the formation of the active catalyst [14]. Parallel to this, the water-gas shift reaction (eq. (5)) occurs, resulting in the formation of carbon dioxide and hydrogen, which is known to form metal hydrides in the presence of metal carbonyls [15]. 

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