Principles elucidated by Halpern in early studies of organotransition metal catalysts include: on-cycle species often have fleeting existence whereas off-cycle species accumulate and are more readily detectable, the distribution of observable species may bear little relationship to the product distribution, catalysis is wholly a kinetic phenomenon for which mechanistic insight requires kinetic analysis, determination of catalytic reaction rate laws are necessary but are not sufficient to distinguish which paths are productive, and determination of the true catalytic pathways requires detailed kinetic studies of the component steps of a putative mechanism.  One common summary of Halpern’s rule regarding catalyst speciation is “if you can see it, it is probably not the catalyst.” Hydroformylation of alkenes as catalyzed by bisdiazaphospholane complexes of rhodium proceeds with high rates, regioselectivity, and enantioselectivity for a variety of alkenes. But the effect of reaction conditions, especially CO partial pressure, on reaction rates and selectivity are complex.  For example, the reaction rate increases as the pressure is increased from 20 psia CO to 115 psia CO and then decreases as the pressure is increased further to 200 psia.  Operando NMR studies of both the catalytic reaction and critical elementary steps reveal the fundamental kinetic thermodynamic principles that control rate and selectivity in hydroformylation.

Operando methods, off-cycle species, and rate-controlling pathways in catalytic, enantioselective hydroformylation

256th National Meeting (2018)

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