Catalytic cycle and enantioselective step

Depending on the nature of the catalyst and the substrate, equilibria can be shifted differently allowing detection of one or another intermediate. Thus, although it is hardly possible to observe all of these intermediates in one catalytic system, there is little doubt that either of them can exist as a low concentrated species under the catalytic conditions. 

Furthermore, reversibility of the early steps of the catalytic cycle implies that different pathways are converging in this system. Indeed, the solvate complex A can first react either with the substrate yielding B or with dihydrogen affording F. However, this is not a bifurcation point separating two different mechanisms. An addition of dihydrogen to B or of a substrate to molecule F would produce the common intermediate D that connects numerous interconverting intermediates of the two pathways 

In view of the above considerations, the handedness of the product must be decided by the mode of coordination of the prochiral double bond taking place in nonchelating Rh(III) octahedral complex I. This coordination should be followed by facile migratory insertion and reductive elimination steps, since otherwise the reversibility of all steps before reductive elimination would bring the intermediate I back. 

On the other hand, the oxidative addition to a catalyst substrate complex could be more facile than hydrogenation of the solvate, and 

in all described cases, the handedness of the product will be determined by the mode of the double bond coordination in the nonchelating octahedral Rh(III) complex T (or its diastereomer, e.g., I, vide infra) and relative easiness of the following migratory insertion and reductive elimination steps. 

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