Transfer hydrogenation with Hantzsch esters and related organic hydride donors

Abstract

In recent years, Hantzsch esters and their related organic hydride donors have been widely utilized in biomimetic approaches of asymmetric transfer hydrogenation (ATH) reactions. Various compounds containing C=C, C=N and C=O unsaturated functionalities could be reduced in the presence of organocatalysts or transition metal complexes, affording versatile chiral building blocks in high yields and excellent enantioselectivities under mild conditions. In this critical review, recent advances in this area are summarized and classified according to unsaturated functional groups being reduced and catalytic systems employed (91 references).

‘‘Hydrogen-Free’’ Hydrogenation of Nitrobenzene Over Cu/SiO2 Via Coupling with 2-Butanol Dehydrogenation

“…Issues of sustainable production and safe handling are now drivers for alternative sources of hydrogen and/or hydrogen donors. Several hydrogen carriers including hydrazine [3], ammonia-borane complexes [4], esters [5], alcohols [6] and acids [7] have been employed in homogeneous and heterogeneous transfer hydrogenation but application is hampered due to the required separation steps for catalyst reuse. A continuous heterogeneous catalytic system that combines dehydrogenation with hydrogen release that is utilised in hydrogenation represents an alternative approach that circumvents the requirement for an external hydrogen supply.…”

Radical‐Based C−C Bond‐Forming Processes Enabled by the Photoexcitation of 4‐Alkyl‐1,4‐dihydropyridines

“…For example, we showed that electron‐rich enamines I , which act as nucleophiles in the ground state, could become strong reductants upon light excitation and trigger the formation of radicals through single‐electron transfer (SET) reduction of electron‐poor organic halides (Figure 1 a) 4a,4b. In the same vein, 1,4‐dihydropyridine derivatives II (H‐DHP, Figure 1 b) are primarily understood as hydride (H − ) sources in their ground state 5. Other researchers recently demonstrated that exciting them with visible light affords strong photoreductants that can productively engage in SET manifolds 6…”

Thiourea catalysed reduction of α-keto substituted acrylate compounds using Hantzsch ester as a reducing agent in water

“…28,29 We reported the rst example of catalyst free transfer hydrogenation of C]C with Hantzsch ester in water. 30 A series of alkenes conjugated with strong electronwithdrawing groups such as nitrile, ester and nitro could be reduced to the corresponding products in water with excellent yields.…”