Reduction of enamines and derivatives

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The use of a highly rigid chiral ferrocenylphosphine-spiro phosphonamidite ligand enables a highly efficient Rh-catalyzed hydrogenation of a wide range of α-dehydroamino acid esters and α-enamides with excellent enantiocontrol.
Y. Chen, X. Yi, Y. Cheng, A. Huang, Z. Yang, X. Zhao, F. Ling, W. Zhong, J. Org. Chem., 2022, 87, 7864-7874.

Various carbon-carbon double bonds in olefins and α,β-unsaturated ketones were effectively reduced to the corresponding alkanes and saturated ketones, using ammonium formate as a hydrogen transfer agent in the presence of Pd/C as catalyst in refluxing methanol.
Z. Paryzek, H. Koenig, B. Tabacka, Synthesis, 2003, 2023-2026.

Rhodium complexes with chiral bisphospholanes are highly enantioselective catalysts for the asymmetric hydrogenation of functionalized olefins such as dehydroamino acid derivatives, itaconic acid derivatives, and enamides. The use of the hydroxyl phospholane system enables hydrogenation of some substrates in water with >99% ee and 100% conversion (e.g., itaconic acid).
W. Li, Z. Zhang, D. Xiao, X. Zhang, J. Org. Chem., 2000, 65, 3489-3496.

A Rh/Cu co-catalytic system enables an asymmetric reduction of aromatic α-dehydroamino acid esters with water as the hydrogen source to provide chiral α-amino acid esters. The reaction tolerates various functional groups and provides access to chiral deuterated α-amino esters by using D₂O.
Y. Dai, J. Chen, Z. Wang, T. Wang, L. Wang, Y. Yang, X. Qiao, B. Fan, J. Org. Chem., 2021, 86, 7141-7147.

(R,R)-binaphane possesses both binaphthyl chirality and phospholane functionality. Excellent enantioselectivities have been observed in hydrogenation of an isomeric mixture of (E)- and (Z)-β-substituted-α-arylenamides by using a Rh-binaphane catalyst.
D. Xiao, Z. Zhang, X. Zhang, Org. Lett., 1999, 1, 1679-1681.

A highly enantioselective hydrogenation of enamides is catalyzed by a dual chiral-achiral acid system. By employing a substoichiometric amount of a chiral phosphoric acid and acetic acid, low catalyst loadings of the chiral catalyst were sufficient to provide excellent yield and enantioselectivity of the reduction product.
G. Li, J. C. Antilla, Org. Lett., 2009, 11, 1075-1078.

Conjugate reduction of dihydropyridones have generally been carried out using expensive L or K-Selectrides, as catalytic hydrogenation often leads to over reduction. The use of zinc/acetic acid enables a simple, inexpensive, and mild reduction of N-acyl-2,3-dihydro-4-pyridones to various racemic or enantiopure 4-piperidones or indolizidinones.
D. L. Comins, C. A. Brooks, C. L. Ingalls, J. Org. Chem., 2001, 66, 2181-2182.

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A highly efficient and highly enantioselective Hantzsch ester mediated conjugate transfer hydrogenation of β,β-disubstituted nitroolefins is catalyzed by a Jacobsen-type thiourea catalyst.
N. J. A. Martin, L. Ozores, B. List, J. Am. Chem. Soc., 2007, 129, 8976-8977.

A highly efficient and enantioselective Hantzsch ester mediated conjugate reduction of β-nitroacrylates is catalyzed by a Jacobsen thiourea catalyst. The reaction is a key step in a new route to optically active β²-amino acids.
N. J. A. Martin, X. Chen, B. List, J. Am. Chem. Soc., 2008, 130, 13862-13863.

A chiral bisphosphine-thiourea ligand was applied in the highly enantioselective hydrogenation of β,β-disubstituted nitroalkenes. The thiourea group of the ligand takes on an important role in this catalytic system as a H-bond donor.
Q. Zhao, S. Li, K. Huang, R. Wang, X. Zhang, Org. Lett., 2013, 15, 4014-4017.

A mild catalytic asymmetric transfer hydrogenation of β,β-disubstituted nitroalkenes using formic acid as reductant in combination with an Ir catalyst is conducted in water at low pH and open to air to give products in good yield and selectivity.
O. Soltani, M. A. Ariger, E. M. Carreira, Org. Lett., 2009, 11, 4196-4198.

An inert, rigid chiral-at-metal iridium(III) complex enables a highly efficient catalytic asymmetric transfer hydrogenation of β,β′-disubstituted nitroalkenes. The catalysis does not involve any direct metal coordination but operates exclusively through weak interactions with functional groups properly arranged in the ligand sphere of the iridium complex.
L-A. Chen, W. Xu, B. Huang, J. Ma, L. Wang, J. Xi, K. Harms, L. Gong, E. Meggers, J. Am. Chem. Soc., 2013, 135, 10598-10601.