Migratory Hydrogenation of Terminal Alkynes by Base/Cobalt Relay Catalysis

Abstract

Migratory functionalization of alkenes has emerged as a powerful strategy to achieve functionalization at a distal position to the original reactive site on a hydrocarbon chain. However, an analogous protocol for alkyne substrates is yet to be developed. Herein, a base and cobalt relay catalytic process for the selective synthesis of (Z)‐2‐alkenes and conjugated E alkenes by migratory hydrogenation of terminal alkynes is disclosed. Mechanistic studies support a relay catalytic process involving a sequential base‐catalyzed isomerization of terminal alkynes and cobalt‐catalyzed hydrogenation of either 2‐alkynes or conjugated diene intermediates. Notably, this practical non‐noble metal catalytic system enables efficient control of the chemo‐, regio‐, and stereoselectivity of this transformation.

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

“…Young nuclear magnetic resonance (NMR) tube pressurized with 5 bar of hydrogen gas (2 equiv; Figure a). 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. − ,− 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 . Notably, only negligible over-reduction into alkane 3a was observed (<1%), possibly due to the very mild conditions employed.…”

“…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 .…”

Catalytic Hydrogenation of Alkenes and Alkynes by a Cobalt Pincer Complex: Evidence of Roles for Both Co(I) and Co(II)

“…1 H NMR spectra were recorded on a Bruker spectrometer operating at 500 MHz ( 1 H) in benzene-d 6 unless otherwise noted and referenced to the residual protium resonance of the solvent (δ 7.16 ppm). 31 P NMR spectra were recorded at 202 MHz and referenced automatically using the 2 H lock frequency. Elemental analyses were performed by the CENTC facility at the University of Rochester.…”

“…In the area of cobalt chemistry, reports from several groups have described the catalytic hydrogenation activity of well-defined cobalt catalysts featuring pincer ligands. − These catalysts have been applied successfully for the hydrogenation of olefins and semi-hydrogenation of alkynes, with several systems displaying impressive stereoselectivities. Despite this success, no unified mechanistic picture has arisen comparable to the better understood features of H 2 activation and olefin insertion at play with heavier Group 9 metals.…”

“…Surprisingly, the 1 H NMR spectrum of the resulting reaction mixture demonstrated the formation of a new diamagnetic cobalt species with a broad resonance apparent in the hydride region near −18 ppm. In addition, the 31 The solid-state structure of 5 can be found in the Supporting Information and resembles that of 3. In agreement with the three-component reaction described above, direct treatment of 5 with H 2 also generated dihydride 4 as judged by 1 H and 31 P NMR spectroscopy.…”