Anti-lock and key mechanism

An important commercial application of these types of catalysts is in the production of L-dopa for the treatment of Parkinson s disease. The key to this application is the stereoselectivity shown by the phosphine chelate, (25,35)-bis(diphenylphosphino)butane, called chiraphos. In a benchmark study, Halpem and co-workers found that the Rh system is unusual in that the most stable olefin adduct does not lead to the major or desired product. This mechanistic pathway has been termed the anti-lock-and-key mechanism to contrast it with the lock-and-key mechanism often proposed for enzyme catalysis. In the latter, it is assumed that the best fit of substrate and enzyme will give the most effective catalysis. 

These calculations were able to reproduce the anti lock-and-key mechanism observed experimentally the most stable pro-5 intermediate displays the highest reaction barrier, while the pro-/ complex, higher in energy, has the lowest barrier to... 

Experimental studies by the groups of Halpern and Brown [69, 70] on the hydrogenation of prochiral alkenes by a chelating-diphosphine catalyst provided a detailed, and unexpected, picture of the overall mechanism of hydrogenation. The reaction of the catalyst with a prochiral alkene can produce two diastereomeric catalyst-alkene complexes. The most stable (and most abundant) diastereomeric complex is relatively unreactive, whereas the small amount of the complex in the less populated diasteromeric state gives the productive catalytic cycle. Landis has coined the phrase anti-lock-and-key motif to describe this mechanistic model. 

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