Anion Controlled Stereodivergent Semi‐Hydrogenation of Alkynes using Water as Hydrogen Source

Abstract

An anion controlled stereodivergent semi‐hydrogenation of alkynes is reported. The reactions are catalyzed by the low‐cost and stable nickel(II) salts, using water as environmental friendly hydrogen source with zinc powder as metal reductant. Interestingly, the stereoselectivity of the reaction is well controlled by the anion of the nickel(II) salt. The present methodology allowed a low‐cost and convenient preparation of trans‐ and cis‐alkenes under mild conditions. The reaction mechanism is proposed with water as the hydrogen donor on the basis of the control experiments.

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

“…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

“…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.…”

Highly Functional Group Tolerant, (E)-Selective Transfer Semihydrogenation of Alkynes Catalysed by Iridium Complex Bearing Unsymmetrical Ferrocene-Based Phosphine Ligand

“…One of them is a lack of complete stereoselectivity control leading to a mixture of isomers and thus affecting the yield and purification process of products. [19][20][21][22] An illustration of this may be work by Sun et al regarding iridium-based catalytic system using ethanol as a hydrogen donor. 23 Another limitation is the over-reduction of the triple C-C bond resulting in the formation of undesired alkane.…”

“…Furthermore, the majority of published semihydrogenation methods suffer from insufficient chemoselectivity. In particular, easily reducible functional groups such as formyl 25 , keto 20,26 , nitro 19-20, 22, 25, 27, 28-29 , cyano 30 , or aryl halides 21,26 are not tolerated. In many cases, explicit infor-mation concerning the compatibility of the above functional groups was not provided, which may obscure the actual efficiency of a given catalytic system.…”