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

Abstract

Herein, a palladium-catalyzed semi-hydrogenation of alkynes to E- and Z-alkenes employing EtOH as hydrogenating agents was reported. The selectivity of the reaction system was effectively controlled by ligand/additive and solvent regulation. The use of sodium acetate/triethanolamine (NaOAc/TEOA), THF and (1R,2R)-bis[(2-methoxypheny)phenyl- phosphino]ethane ((R,R)-DIPAMP), CH3CN was critical for the stereoselective semihydrogenation of alkynes. The general applicability of this procedure was highlighted by the synthesis of more than 36 alkenes, with good yields and high stereoselectivities.

H2O as the Hydrogen Donor: Stereo-Selective Synthesis of E- and Z-Alkenes by Palladium-Catalyzed Semihydrogenation of Alkynes

“…Also, the reaction is shut down in the absence of H 2 O. Deuterated product 2a′ / 3a′ was obtained in 87/80% yield when D 2 O was used. According to these observations and reported literature studies, , we proposed the following catalytic cycle including cis -hydrogenation and trans -isomerization, as shown in Scheme . Initially, an active Pd­(II) catalyst A is generated by coordination of TEOA/NaOAc (L1/L2) to Pd­(OAc) 2 .…”

“…Fortunately, A 98% yield of E -alkene with E / Z 98:2 selectivity was obtained when t -Bu 2 PCl (di- tert -butylphosphinous chloride) was used (Table , entry 26 ), which may be due to the large steric hindrance of t -Bu 2 PCl. According to our previous research, triethanolamine (TEOA) and NaOAc play important roles in catalytic transfer hydrogenation . TEOA and NaOAc were used as ligands/additives to study the effect on the reaction.…”

“…According to our previous research, triethanolamine (TEOA) and NaOAc play important roles in catalytic transfer hydrogenation. 52 TEOA and NaOAc were used as ligands/additives to study the effect on the reaction. The use of TEOA provided a 100% conversion rate of alkyne with E / Z 17:83 selectivity ( Table 1 , entry 28).…”

“…Recently, various reaction systems have been developed to provide switchable selectivity for either the Z - or E -alkene isomer. − Moreover, these reaction methods usually require the use of flammable, explosive, corrosive, or expensive hydrogen sources, for example, hydrogen, − borane, ,− formic acid, ,− amine, and alcohol, − to obtain good yields (Scheme a). Also, these methods may have disadvantages, such as poor stereo-selectivity, ,, their relatively high prices, and poor functional group tolerance .…”

Multicomponent and Metal‐Free Diels–Alder/Aromatization Approach to the Stereospecific Synthesis of E‐(Hetero)Stilbenes and Diarylacetylenes

“…The synthetic strategies towards the stereoselective assembly of this scaffold can be broadly divided in five main different approaches: i . reduction of the corresponding diaryacetylenes [16–19] or elimination from functionalized 1,2‐diarylethanes [20,21] (Figure 2, Path A); ii . dimerization of activated benzylic derivatives [22–27] (Path B); Alongside the abovementioned strategies, the more popular and widely adopted approaches are: iii .…”