Semireduction of Alkynes

Recent Literature

The reaction of PdCl₂ with K₂CO₃ and HCO₂H in dioxane provides a black precipitate, which is an effective catalyst for the semireduction of alkynes to alkenes in the presence of formic acid as the reductant.
R. Iwasaki, E. Tanaka, T. Ichihashi, Y. Idemoto, K. Endo, J. Org. Chem., 2018, 83, 13574-13579.

An unprecedented reduction of alkynes with formic acid can selectively produce cis-, trans-alkenes and alkanes by slightly tuning the reaction conditions via the generation of an alkenylpalladium intermediate and subsequent transformation of this complex in a variety of reactions catalyzed by a combination of Brønsted acid and Pd(0) complex.
R. Shen, T. Chen, Y. Zhao, R. Qiu, Y. Zhou, S. Yin, X. Wang, M. Goto, L.-B. Han, J. Am. Chem. Soc., 2011, 133, 17037-17044.

The use of Mn/water enables a mild and highly selective reduction of alkenes and alkynes. The highly controlled generation of H₂ allows a selective reduction in the presence of labile functional groups under mild and environmentally acceptable conditions.
J. Rosales, T. Jiménez, R. Chahboun, M. A. Huertos, A. Millán, J. Justicia, Org. Lett., 2024, 26, 2147-2151.

A B₂Pin₂-assisted copper-catalyzed semihydrogenation of alkynes provides various alkenes in good to excellent yields with Z-selectivity under mild reaction conditions. The present protocol enabled convenient synthesis of deuterium-substituted Z-alkenes using readily available ethanol-d₁ as the deuterium source.
H. Bao, B. Zhou, H. Jin, Y. Liu, J. Org. Chem., 2019, 84, 3579-3589.

The use of unsupported nanoporous gold (AuNPore) as a heterogeneous catalyst enables a facile, highly chemo- and stereoselective transfer semihydrogenation of alkynes to Z-olefins in the presence of formic acid as a hydrogen donor. Various alkynes were reduced to the corresponding alkenes in high chemical yields with good functional-group tolerance. The catalyst is robust enough to be reused without leaching.
Y. S. Wagh, N. Asao, J. Org. Chem., 2015, 80, 847-851.

A bisacylphosphine oxide photoinitiator was used for a very convenient light mediated preparation of palladium nanoparticles (PdNPs) with a small diameter of 2.8 nm. The PdNP-hybrid material was applied as catalyst for the semihydrogenation of various internal alkynes to provide the corresponding alkenes in excellent yields and Z-selectivities.
F. Mäsing, H. Nüsse, J. Klingauf, A. Studer, Org. Lett., 2017, 19, 2658-2661.

(Tetraphenylporphyrin)palladium can be used as a catalyst for the chemoselective and stereoselective hydrogenation of alkynes to cis-alkenes in good to excellent yields via syn-addition of hydrogen. Alkynes containing various functional groups were tolerated.
R. Nishibayashi, T. Kurahashi, S. Matsubara, Synlett, 2014, 25, 1287-1290.

A bench-stable cationic bis(σ-B-H) aminoborane iron complex efficiently catalyzes the semihydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes. The catalytic reaction takes place under mild conditions in 1 h, and alkenes were obtained with high Z-selectivity for a broad scope of substrates.
N. Gorgas, J. Brünig, B. Stöger, S. Vanicek, M. Tilset, L. F. Veiros, K. Kirchner, J. Am. Chem. Soc., 2019, 141, 17322-17330.

Copper-catalyzed semihydrogenation of internal alkynes proceeds under an atmosphere of hydrogen (5 atm) at 100 °C in the presence of a readily available catalyst to give various Z-alkenes stereoselectively.
K. Semba, R. Kameyama, Y. Nakao, Synlett, 2015, 26, 318-322.

The selective synthesis of Z-alkenes in alkyne semihydrogenation relies on the reactivity difference of the catalysts toward the starting materials and the products. In an Ir(III)-catalyzed semihydrogenation with EtOH as hydrogen source, an amine as additive is crucial to promote the alcoholysis. This catalytic system exhibits an unprecedented level of substrate scope, generality, and compatibility.
Z. Huang, Y. Wang, X. Leng, Z. Huang, J. Am. Chem. Soc., 2021, 143, 4809-4843.

A cobalt-catalyzed stereodivergent transfer hydrogenation of alkynes provides either Z- or E-alkenes based on a rational catalyst design. Substrates bearing a wide range of functional groups can be hydrogenated in good yields using catalyst loadings as low as 0.2 mol %.
S. Fu, N.-Y. Chen, X. Liu, Z. Shao, S.-P. Luo, Q. Liu, J. Am. Chem. Soc., 2016, 138, 8588-8594.

Ni catalysis enables a transfer hydrogenative alkyne semireduction protocol that can be applied to both internal and terminal alkynes in the presence of formic acid and Zn as the terminal reductants. Both (E)- and (Z)-isomers can be accessed selectively under similar reaction conditions.
E. Richmond, J. Moran, J. Org. Chem., 2015, 80, 6922-6929.

Using small amounts of a copper catalyst, an efficient semireduction of alkynes can be accomplished with a wide range of substrates, including both internal and terminal alkynes without over-reduction. The new method has excellent chemoselectivity and tolerates nitro and aryl iodo groups. Finally, commercial availability of a catalyst precursor adds to the appeal of the new catalytic system.
A. M Whittaker, G. Lalic, Org. Lett., 2013, 15, 1112-1115.

Solid amidophosphine boranes were synthesized to replace more commonly used borane reagents. These compounds demonstrated excellent reactivity and functional group tolerance toward a wide variety of nitriles, alkynes, and carboxylic acids, affording the corresponding ammonium salts, alkenes, and alcohols in good yield.
R. Kumar, R. K. Meher, J. Sharma, A. Sau, T. K. Panda, Org. Lett., 2023, 25, 7923-7927.

Commercial first and second generation Hoveyda-Grubbs catalysts enable a selective transfer semihydrogenation of alkynes to yield alkenes in the presence of formic acid as a hydrogen donor. This catalytic system tolerates many functional groups (halogens, cyano, nitro, sulfide, alkenes).
R. Kusy, K. Grela, Org. Lett., 2016, 18, 6196-6199.

(Ph₃P)₃RuCl₂ is an inexpensive catalyst, that enables a chemoselective reduction of alkyne, ketones, or nitro groups in the presence of Zn/water as a stoichiometric reductant. Depending on the nature of the additive and the temperature, chemoselective reduction of a nitro group in the presence of a ketone or an alkyne was possible.
T. Schabel, C. Belger, B. Plietker, Org. Lett., 2013, 15, 2858-2861.

A simple ruthenium catalyst achives a semihydrogenation of diaryl alkynes to the corresponding E-alkenes in very good yields using alcohols as the hydrogen source. Best results were obtained using benzyl alcohol as the hydrogen donor, although biorenewable alcohols such as furfuryl alcohol could also be applied.
A. Ekebergh, R. Begon, N. Kann, J. Org. Chem., 2020, 85, 2966-2975.

The use of commercially available reagents (Cl₂Pd(PPh₃)₂, Zn⁰, and ZnI₂) enables an efficient E-selective semihydrogenation of internal alkynes under low dihydrogen pressure and low reaction temperature. The transformation involves syn-hydrogenation followed by isomerization.
R. Maazaoui, R. Abderrahim, F. Chemla, F. Ferreira, A. Perez-Luna, O. Jackowski, Org. Lett., 2018, 20, 7544-7549.

H₂Se (or HSe^-) produced in situ from Se/DMF/H₂O is an active reducing species. This reduction system with water as an inexpensive, safe, and environmentally friendly hydrogen donor displayed high selectivity and good activity in the reduction of α,β-unsaturated ketones and alkynes.
C. An, G. Wu, G.-X. Li, X.-B. Huang, W.X. Gao, J.-C. Ding, Y.-B. Zhou, M.-C. Liu, H.-Y. Wu, Org. Lett., 2018, 20, 5573-5577.

Green water can be used as hydrogen donor for a highly stereoselective and efficient transition-metal-free semihydrogenation of various internal diarylalkynes to E-alkenes. The reactions are conducted under convenient conditions and provide products in good to excellent yields, with broad substrate scope.
Z. Chen, M. Luo, Y. Wen, G. Luo, L. Liu, Org. Lett., 2014, 16, 3020-3023.

An air and moisture stable manganese pincer complex catalyzes the semireduction of various alkynes to the corresponding (Z)-olefins in high yields. The reaction is stereo- and chemoselective and scalable.
J. Sklyaruk, V. Zubar, J. C. Borghs, M. Rueping, Org. Lett., 2020, 22, 6067-6071.

DMF/KOH is an efficient hydrogen source in the Pd(OAc)₂-catalyzed transfer semihydrogenation of various functionalized internal alkynes to afford cis-alkenes in good to high yields with excellent chemo- and stereoselectivity. This catalytic process was also applied to the synthesis of analogues of combretastatin A-4.
J. Li, R. Hua, T. Liu, J. Org. Chem., 2010, 75, 2966-2970.

The combination of a (PCN)Ir complex as the precatalyst and tBuNH₂ as the cocatalyst mediates an efficient trans-semihydrogenation of 1,3-enynes with ethanol as the hydrogen source. The reaction provides an atom-economical access to unsymmetrical (E,E)-1,4-diarylbutadienes with high yields and stereoselectivities.
F. Huang, Z. Huang, G. Liu, Z. Huang, Org. Lett., 2022, 24, 5486-5490.

A transition-metal-free semireduction of 3-substituted primary and secondary propiolamides with pinacolborane and catalytic potassium tert-butoxide provides 3-substituted acrylamide derivatives in very good yield with excellent E selectivity. Mechanistic studies suggest that an activated Lewis acid-base complex transfers a hydride to the α-carbon followed by rapid protonation in a trans fashion.
R. J. Grams, C. J. Garcia, C. Szwetkowski, W. L. Santos, Org. Lett., 2020, 22, 7013-7018.

γ-Hydroxy-α,β-acetylenic esters are used as precursors for the preparation of γ-hydroxy-α,β-alkenoic esters by means of trans-selective additions of two hydrogen atoms or one hydrogen atom and one iodine atom across the triple bonds. These methods allow the preparation of β-substituted and α,β-disubstituted alkenoic esters in highly stereoselective manners.
C. T. Meta, K. Koide, Org. Lett., 2004, 6, 1785-1787.

The hydrosilylation of alkynes using the ruthenium catalyst [Cp*Ru(MeCN)₃]PF₆ gives only (Z)-trans addition products. Subsequent protodesilylation of the crude vinylsilane products by the action of cuprous iodide and TBAF provides a general trans-alkyne reduction, which is compatible with many sensitive functional groups.
B. M. Trost, Z. T. Ball, T. Joege, J. Am. Chem. Soc., 2002, 124, 7922-7923.

Indium hydride generated from readily available Et₃SiH and InCl₃ offers mild conditions and low toxicity, and is therefore a promising alternative to Bu₃SnH.
N. Hayashi, I. Shibata, A. Baba, Org. Lett., 2004, 6, 4981-4983.

A number of alkynyl pinacolboronates were stereoselectively reduced to the cis-alkenyl pinacolboronates via hydroboration with dicyclohexylborane followed by chemoselective protodeboronation using acetic acid. Treatment with potassium hydrogen fluoride smoothly converted these to the corresponding potassium organotrifluoroborates.
G. A. Molander, N. M. Ellis, J. Org. Chem., 2008, 73, 6841-6844.

The use of indium metal in aqueous ethanol enables a highly selective reduction of aryl propargyl ethers, amines, and esters in high yields. This method avoids over-reduction of the double bond formed and tolerates several easily reducible functionalities.
B. C. Ranu, J. Dutta, S. K. Guchhait, J. Org. Chem., 2001, 66, 5413-5418.

Indium hydride (Cl₂InH) was generated by the transmetalation of InCl₃ with Et₃SiH. In the previously reported system (NaBH₄-InCl₃), the coexistent borane can cause side reactions. The use of Et₃SiH instead of NaBH₄ affords effective hydroindation of alkynes.
N. Hayashi, I. Shibata, A. Baba, Org. Lett., 2004, 6, 4981-4983.

A heterogeneous Au/TiO₂ catalyzed stereoselective hydrogenation of ynamides provides Z-enamides in a highly stereoselective manner in the presence of inexpensive ammonium formate as the hydrogen source. The commercially available gold nanoparticle catalyst could be recycled multiple times without a significant loss of activity.
L. Lin, X. Zeng, B. Xu, J. Org. Chem., 2019, 84, 11240-11246.

A base-mediated semihydrogenation of ynamides using p-toluenesulfonyl hydrazide as an inexpensive and easy-to-handle hydrogen donor provides the thermodynamically unfavorable Z-enamides exclusively without overhydrogenation and reduction of other functional groups.
Z. Zhao, Q. Tian, Y. Chen, S. Wen, Y. Zhang, G. Cheng, J. Org. Chem., 2021, 86, 10407-10413.