Dinuclear Cobalt Complex-Catalyzed Stereodivergent Semireduction of Alkynes: Switchable Selectivities Controlled by H₂O

Abstract

Catalytic semireduction of internal alkynes to alkenes is very important for organic synthesis. Although great success has been achieved in this area, switchable Z/E stereoselectivity based on a single catalyst for the semireduction of internal alkynes is a longstanding challenge due to the multi-chemo- and stereoselectivity, especially based on less-expensive earth-abundant metals. Herein, we describe a switchable semireduction of alkynes to (Z)- or (E)-alkenes catalyzed by a dinuclear cobalt complex supported by a macrocyclic bis pyridyl diimine (PDI) ligand. It was found that cis-reduction of the alkyne occurs first and the Z–E alkene stereoisomerization process is formally controlled by the amount of H2O, since the concentration of H2O may influence the catalytic activity of the catalyst for isomerization. Therefore, this protocol provides a facile way to switch to either the (Z)- or (E)-olefin isomer in a single transformation by adjusting the amount of water.

Controlled selectivity through reversible inhibition of the catalyst: Stereodivergent semihydrogenation of alkynes

“…Given the above observations, it is likely that (Z)-alkene isomerization occurs at the vacant site of Ru-1, and molecules with higher affinity to the metal center, such as alkynes, are able to block this coordination site and thereby impede the isomerization process. 20 However, the selectivity of the reaction toward (Z)-2a quickly dropped once alkyne conversion reached 50% (see the Z/E curve in Figure 2b), and completely inverted at the end of the reaction. This prompted us to search for an auxiliary additive, which could coordinate to the metal center in a way that would allow the main reaction to take place while impeding the isomerization step, and thereby effectively halting the reaction at intermediate (Z)-2a.…”

“…This translates into a TOF of more than 1000 h -1 , which, to the best of our knowledge, represents the most efficient trans-semihydrogenation of any alkyne reported to date. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] As the reaction progressed, a clear color change from brown-red to yellow was observed, with the latter being that of Ru-1, and this visible change could be used as a reaction indicator. 7 Notably, only negligible over-reduction into alkane 3a was observed (<1 %), possibly due to the very mild conditions employed.…”

“…Nevertheless, the state-of-the-art methodologies to selectively access both (E)-and (Z)-alkenes rely on the transfer-semihydrogenation of alkynes using different catalysts. [10][11][12][13][14][15][16] Some strategies involve the use of additives in order to switch the stereoselectivities of these transformations, [17][18][19][20] but their mechanisms are unclear and they are still limited to transfer-semihydrogenation with H2 surrogates, and also usually require stoichiometric amounts of additives, which inevitably generates waste and is neither atom-economical nor sustainable. Thiol(ate)s are known to have very strong intrinsic affinity to transition metals, and are therefore widely used as ligands, [21][22][23][24] biological inhibitors, [25][26] metal ion probes [27][28][29] and the end groups of self-assembled monolayers.…”

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

“…Catalytic semihydrogenation of internal alkynes is an attractive route to access different alkenes for small-scale laboratory synthesis as well as large-scale industrial processes. − ,− Controlling the stereoselectivity and avoiding the formation of over-reduced alkane products are two major challenges facing the development of such reactions. A popular means of addressing these challenges is the commercially available Lindlar catalyst, which has been used for decades to produce ( Z )-alkenes by cis -semihydrogenation of alkynes .…”

“…Many catalytic (de)­hydrogenative reactions involve in situ generated intermediates, which are usually reactive under the catalytic conditions and are therefore seldom isolated as products. − For example, the trans -selective catalytic semihydrogenation of alkynes typically begins with cis -hydrogenation, but the generated ( Z )-alkene is only a kinetic intermediate, which is then rapidly isomerized into the thermodynamically more stable ( E )-alkene product (Scheme a). − If a strategy can be devised to slow down a specific reaction step, such as the Z -to- E isomerization in the trans -semihydrogenation of alkynes, the reactive intermediates can be stabilized and may even be isolable as end products from the same system, which would be of great interest and is highly advantageous. Nevertheless, the state-of-the-art methodologies to selectively access both ( E )- and ( Z )-alkenes through semihydrogenation of alkynes rely on the utilization of different catalysts. − Some strategies involve the use of additives in order to switch the stereoselectivities of these transformations, − but their mechanisms are unclear and they are usually limited to transfer semihydrogenation with H 2 surrogates and also require stoichiometric amounts of additives, which inevitably generates waste and is neither atom-economical nor sustainable.…”