Cobalt makes it to the front line of Asymmetric Hydrogenation

Yesterday, a Co-catalyzed enantioselective hydrogenation of α,β- unsaturated carboxylic acid has been published in nature communication. This is important because although the metal catalyzed hydrogenations are well-established (Noyori hydrogenation won the Nobel in 2001), most of them uses rare earth metals such as Rh, Ir and Ru. These metals, while having found great uses in catalysis, are not so great in terms of economy and green chemistry. Whereas Cobalt is earth abundant and less toxic.

The optimum ligand used is (S,S)-Ph-BPE, not some complicated system. and the reaction uses 40 atm pressure of hydrogen gas in methanol solvent with Zn additive. Zn additive presumably helps in activating the pre-catalyst and suppresses the deactivation of the active catalyst.

The excellence lies in the high yield and high ee achieved by this reaction. All yields are >80% and all but one ee are  >90% with most being >96% as reported in the substrate scope.
The importance of this work has been shown in terms of synthesis of some well-known drugs and leads such as ibuprofen, naproxen and artemisinin.
It is interesting to note that another Co-catalyzed hydrogenations of α,β- unsaturated carboxylic acids had been reported in JACS earlier this year, although the authors in the current paper has claimed to have submitted their report prior to the JACS paper got published.