Methanol as the Hydrogen Source in the Selective Transfer Hydrogenation of Alkynes Enabled by a Manganese Pincer Complex

Abstract

The first base metal catalyzed transfer hydrogenation of alkynes with methanol is described. An air and moisture stable manganese pincer complex catalyzes the reduction of a variety of different alkynes to the corresponding (Z)-olefins in high yields. The reaction is stereo-and chemoselective and scalable.

Palladium-Catalyzed Semihydrogenation of Alkynes with EtOH: Highly Stereoselective Synthesis of E- and Z-Alkenes

“…Both THF and CH3CN can provide good conversion rate of alkyne, and the use of CH3CN solvent seems to prefer the product to obtain the E-alkene isomer (Z/E 29:71 ) (Table 1, entry 14). To improve the reaction selectivity, we further screened different ligand/addition to identify a suitable reaction ligand/addition (Table 1, entries [17][18][19][20][21][22][23][24]. GC analysis showed a 87% conversion rate of alkyne (Z/E 76:11) was obtained with NaOAc, and a 93% conversion rate of alkyne (Z/E 70:23) when TEOA was used.…”

“…(Z)-1-chloro-3-styrylbenzene(2d) 18 Colorless liquid (20.4 mg, 95% yield). 3, 136.8, 134.2, 131.7, 129.6, 129.0, 128.9, 128.5, 127.6, 127.3, 127.2.…”

“…(Z)-1-(2-methoxystyryl)benzene(2k) 18 Colorless liquid (18.3 mg, 87% yield). 3, 137.5, 130.4, 130.2, 129.0, 128.7, 128.2, 127.0, 126.4, 125.9, 120.4, 110.8, 55.6.…”

Manganese—New prominent actor in transfer hydrogenation catalysis

“…reported that methanol could also act as an hydrogen donor for Z-selective transfer semi-hydrogenation of alkynes using 2 mol% of Mn(I) pre-catalyst bearing PNP ligand. [33] In this case, the addition of 2 equivalents of weak base Cs2CO3 was required and the reaction proceeds under rather harsh conditions at 130-150 °C. However, a variety of aromatic and aliphatic (Z)-alkenes was obtained by this procedure including substrates containing various nitrogen and sulfur heterocycles.…”

The Rise of Manganese-Catalyzed Reduction Reactions

“…[136] The group of Rueping continued the work on Z-alkene formation by using a Mn(I) catalyst ( 47) and methanol as the only sacrificial hydrogen source. [137] The group of Lacy also managed to reduce C-C -bonds on chalcones by using an amino acid-derived bidentate ligand (68) and [MnBr(CO)5]. [138] Scheme 21: Manganese-based transfer hydrogenation catalyst for the reduction of C-C -bonds.…”