Controlled Selectivity through Reversible Inhibition of the Catalyst: Stereodivergent Semihydrogenation of Alkynes

Abstract

Catalytic semihydrogenation of internal alkynes using H2 is an attractive atom-economical route to various alkenes, and its stereocontrol has received widespread attention, both in homogeneous and heterogeneous catalyses. Herein, a novel strategy is introduced, whereby a poisoning catalytic thiol is employed as a reversible inhibitor of a ruthenium catalyst, resulting in a controllable H2-based semihydrogenation of internal alkynes. Both (E)- and (Z)-alkenes were obtained efficiently and highly selectively, under very mild conditions, using a single homogeneous acridine-based ruthenium pincer catalyst. Mechanistic studies indicate that the (Z)-alkene is the reaction intermediate leading to the (E)-alkene and that the addition of a catalytic amount of bidentate thiol impedes the Z/E isomerization step by forming stable ruthenium thiol­(ate) complexes, while still allowing the main hydrogenation reaction to proceed. Thus, the absence or presence of catalytic thiol controls the stereoselectivity of this alkyne semihydrogenation, affording either the (E)-isomer as the final product or halting the reaction at the (Z)-intermediate. The developed system, which is also applied to the controllable isomerization of a terminal alkene, demonstrates how metal catalysis with switchable selectivity can be achieved by reversible inhibition of the catalyst with a simple auxiliary additive.

Optically Controlled Recovery and Recycling of Homogeneous Organocatalysts Enabled by Photoswitches

“…Homogeneous catalysts display highly desired properties such as high chemo‐, [1] regio‐, [2] and enantio‐selectivity, [3] yet the recovery and recycling of such catalysts have remained challenges. The most commonly used separation methods, e.g., distillation, [4] chromatography, [5] and extraction, [6] while simple and fast, typically result in the decomposition or loss of catalysts, while the heterogenization of homogeneous catalysts, i.e., placing homogeneous catalysts on a solid support, often suffers from catalyst leaching and the loss of activity [7] .…”

Modulation of metal species as control point for Ni-catalyzed stereodivergent semihydrogenation of alkynes with water

“…1 , bottom). Ideally, several issues should be addressed: (a) independent catalytic cycles should be initiated by simple adjustment of the reaction factor(s) to enable high yield and stereoselectivity 8 – 14 , 21 , 22 ; (b) nonprecious metals and ligands without toxic additives would be more favorable 23 – 27 ; (c) water is the first choice of the hydrogen donor for TH process 10 , 28 – 31 ; (d) alkynes with various substituents should be hydrogenated in high yield and stereoselectivity in mild conditions, and over-reduction to saturated alkanes need to be avoided 32 , 33 . Pioneered by previous Ni-catalyzed alkyne hydrogenation 8 , 34 – 36 , we launched a project with nickel catalysts to address the above challenges.…”

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 .…”