Organocatalytic transfer hydrogenation

Transfer hydrogenation

Transfer hydrogenation is the addition of hydrogen (H2; dihydrogen in inorganic and organometallic chemistry) to a molecule from a source other than gaseous H2. It is applied in industry and in organic synthesis, in part because of the inconvenience and expense of using gaseous H2. One large scale application of transfer hydrogenation is coal liquifaction using "donor solvents" such as tetralin.

In the area of organic synthesis, a useful family of hydrogen-transfer catalysts have been developed based on ruthenium and rhodium diamine and phosphine complexes. These catalysts are mainly employed for the organic reduction of ketones and imines to alcohols and amines, respectively. The hydrogen-donor (transfer agent) is typically isopropanol, which coverts to acetone upon donation of hydrogen. Transfer hydrogenations can proceed with high enantioselectivities when the starting material is chiral:

RR'C=O + Me2CHOH → RR'C*H-OH + Me2C=O
where RR'C*H-OH is a chiral product. A typical catalyst is (arene)Ru(R,R-HNCHPhCHPhNTs), where Ts = SO2C6H4Me and R,R refers to the absolute configuration of the two chiral carbon centers. This work was recognized with the 2001 Nobel Prize in Chemistry to Ryoji Noyori. Another family of hydrogen-transfer agents are those based on aluminium alkoxides, such as Aluminium isopropoxide.

Proton donors

A historically prominent transfer hydrogenation agent is diimide, which becomes oxidized to N2. Half of the hydrogen in the hydrazine is transferred to the substrate.

The reaction produces the very stable molecule N2 molecule. Another similar example of the use of transfer hydrogenation where the product is an alkane is when the hydrogen supplier is cyclohexene or cyclohexadiene. In this case an alkane is formed along with the formation of benzene. The driving force of the reaction being the gain of aromatic stabilization energy when benzene is formed. Pd can be used as a catalyst and a temperature of 100 °C is employed. One limitation of using transfer hydrogenation for the production of alkane is that it cannot be used to prepare methane as no unsaturated hydrocarbon contain only one carbon. More exotic transfer hydrogenations have been reported, including this intramolecular one:

Many reactions exist with as proton donor a simple alcohol. Examples are the sodium metal mediated Birch reduction (arenes) and the Bouveault-Blanc reduction (esters). Magnesium-methanol is another combination and used in alkene reductions for instance in this one en route to asenapine :

Organocatalytic transfer hydrogenation

Organocatalytic transfer hydrogenation has been described by the group of List in 2004 in a system with a Hantzsch ester as proton donor and an amine catalyst :

In this particular reaction the substrate is an α,β-unsaturated carbonyl compound. The proton donor is oxidized to the pyridine form and resembles the biochemically relevant coenzyme NADH. In the catalytic cycle for this reaction the amine and the aldehyde first form an iminium ion, then proton transfer is followed by hydrolysis of the iminium bond regenerating the catalyst. By adopting a chiral imidazolidinone MacMillan organocatalyst an enantioselectivity of 81% ee was obtained:

The group of MacMillan independently published a very similar asymmetric reaction in 2005 :

In an interesting case of stereoconvergence, both the E-isomer and the Z-isomer in this reaction yield the (S)-enantiomer.

Extending the scope of this reaction towards ketones or rather enones requires fine tuning of the catalyst (add a benzyl group and replace the t-butyl group by a furan) and of the Hantzsch ester (add more bulky t-butyl groups) :

With a different organocatalyst altogether, hydrogenation can also be accomplished for imines. In one particular reaction the catalysts is a BINOL based phosphoric acid, the substrate a quinoline and the product a chiral tetradehydroquinoline in a 1,4-addition, isomerization and 1,2-addition cascade reaction :
The first step in this reaction is protonation of the quinoline nitrogen atom by the phosphoric acid forming a transient chiral iminium ion. It is noted that with most traditional metal based catalysts, hydrogenation of aromatic or heteroaromatic substrates tend to fail.


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