Transfer hydrogenation

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In

dihydroanthracene, dehydrogenating them to CO2, acetone, or anthracene respectively.[1] Often, the donor molecules also function as solvents for the reaction. A large scale application of transfer hydrogenation is coal liquefaction using "donor solvents" such as tetralin.[2][3]

Organometallic catalysts

In the area of

enantioselectivities when the starting material is prochiral
:

where RR'C*H−OH is a chiral product. A typical catalyst is (cymene)R,R-HNCHPhCHPhNTs, where Ts refers to a tosyl group (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 Ryōji Noyori.[5]

Another family of hydrogen-transfer agents are those based on aluminium alkoxides, such as aluminium isopropoxide in the MPV reduction; however their activities are relatively low by comparison with the transition metal-based systems.

Transfer hydrogenation catalyzed by transition metal complexes proceeds by an "outer sphere mechanism."

The catalytic asymmetric hydrogenation of

ketones was demonstrated with ruthenium-based complexes of BINAP.[6][7]

Even though the BINAP-Ru dihalide catalyst could reduce functionalized ketones, the hydrogenation of simple

ketones remained unsolved. This challenge was solved with precatalysts of the type RuCl2(diphosphane)(diamine).[8]
These catalysts preferentially reduce ketones and aldehydes, leaving olefins and many other substituents unaffected.

Complementing traditional diphosphine-based Noyori catalysts are arene-Ru catalysts, which operate similarly.[9] The stoichiometric asymmetric reduction of ketones has long been known, e.g., using chiral borones.[10]

Metal-free routes

See also: reductions with diimide and quinone

Prior to the development of catalytic hydrogenation, many methods were developed for the hydrogenation of unsaturated substrates. Many of these methods are only of historical and pedagogical interest. One prominent transfer hydrogenation agent is diimide or (NH)2, also called diazene. This becomes oxidized to the very stable N2:

Transfer hydrogenation
Transfer hydrogenation

The diimide can be generated from hydrazine or certain other organic precursors.

Two hydrocarbons that can serve as hydrogen donors are cyclohexene or cyclohexadiene. In this case, an alkane is formed, along with a benzene. The gain of aromatic stabilization energy when the benzene is formed is the driving force of the reaction. Pd can be used as a catalyst and a temperature of 100 °C is employed. More exotic transfer hydrogenations have been reported, including this intramolecular one:

Transfer hydrogenation
Transfer hydrogenation

Many reactions exist with alcohol or

aromatic hydrocarbons). Less important presently is the Bouveault–Blanc reduction of esters. The combination of magnesium and methanol is used in alkene reductions, e.g. the synthesis of asenapine:[11]

Magnesium methanol reduction in asenapine synthesis
Magnesium methanol reduction in asenapine synthesis

Organocatalytic transfer hydrogenation

Organocatalytic transfer hydrogenation has been described by the group of List in 2004 in a system with a Hantzsch ester as hydride donor and an amine catalyst:[12]

Organocatalytic Transfer Hydrogenation Yang 2004

In this particular reaction the substrate is an

enantioselectivity of 81% ee
was obtained:

Asymmetric Organocatalytic Transfer Hydrogenation Yang 2004

[13]

MacMillan Asymmetric Organocatalytic Transfer Hydrogenation

In a case of

Z-isomer in this reaction yield the (S)-enantiomer
.

Extending the scope of this reaction towards

t-butyl group by a furan) and of the Hantzsch ester (add more bulky t-butyl groups):[14]

Organocatalytic Transfer Hydrogenation Enones Tuttle 2006

With another organocatalyst altogether, hydrogenation can also be accomplished for imines. One cascade reaction is catalyzed by a chiral phosphoric acid:[15]

Transfer hydrogenation Imine Reduction Rueping 2006

The reaction proceeds via a chiral

heteroaromatic
substrates tend to fail.

See also

References

  1. ISSN 0009-2665
    .
  2. ISBN 0-8247-1915-8
    .
  3. PMID 16187395
    .
  4. ^ T. Ikariya, K. Murata, R. Noyori "Bifunctional Transition Metal-Based Molecular Catalysts for Asymmetric Syntheses" Org. Biomol. Chem., 2006, volume 4, 393-406.
  5. doi:10.1021/ar700101x
  6. doi:10.1021/jo00090a026
  7. doi:10.1021/ja00210a070
  8. PMID 11169691
  9. S2CID 106394152
    .
  10. doi:10.1021/cr00097a010
    .
  11. doi:10.1021/op700240c
    .
  12. PMID 15540245
    .
  13. PMID 15631434
    .
  14. S2CID 12456921
    .
  15. PMID 16639754
    .
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