Cobalt Catalyzed Z-Selective Hydroboration of Terminal Alkynes and Elucidation of the Origin of Selectivity

Abstract

A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.

Cobalt-Catalyzed 1,1-Diboration of Terminal Alkynes: Scope, Mechanism, and Synthetic Applications

“…Cobalt acetylide complexes 20a and 20b , derived from phenylacetylene and 1-octyne, respectively, were prepared by a previously described procedure. 21 As shown in Scheme 9, treatment of cobalt acetylide complex 20a with 1 equiv of B 2 Pin 2 in benzene- d 6 cleanly generated a single new species within 60 h, as judged by 1 H and 13 C NMR spectroscopy. In contrast, the reaction of 20b with B 2 Pin 2 to give vinylcobalt 21b proceeded to 95% conversion within 4 h. Recrystallization of vinylcobalt complex 21b from pentane at −35 °C yielded single crystals suitable for X-ray diffraction analysis.…”

“…With aliphatic alkynes such as 1-octyne, the protonation is likely rate-determining, as was observed in the previously reported Z -selective hydroboration. 21 In contrast, with more acidic alkynes such as phenylacetylene, the protonation step is comparatively fast, such that formation of the vinylcobalt complex is likely rate-determining.…”

Site-Selective and Stereoselective trans-Hydroboration of 1,3-Enynes Catalyzed by 1,4-Azaborine-Based Phosphine–Pd Complex

“…10 In 2012, Leitner showed that a Ru/PNP pincer complex can produce Z -vinylboronates via trans -hydroboration of alkynes with H-Bpin, 11 and Chirik introduced a Co-based system in 2015. 12 What these systems have in common is that they require the presence of the terminal alkyne proton due to the operating reaction mechanisms. Metal vinylidene species have been proposed for the Miyaura 10 and Leitner 11 catalysts whereas the Chirik system involves a Co-alkynyl 12 intermediate.…”

“…12 What these systems have in common is that they require the presence of the terminal alkyne proton due to the operating reaction mechanisms. Metal vinylidene species have been proposed for the Miyaura 10 and Leitner 11 catalysts whereas the Chirik system involves a Co-alkynyl 12 intermediate. Thus, internal enynes would not be suitable substrates for these systems.…”

Deviation from the anti-Markovnikov rule: a computational study of the regio- and stereoselectivity of diene hydroboration reactions

“…Hydroboration of alkenes has great importance because of the wide variety of uses of the resulting organoboranes, such as in Suzuki cross-coupling reactions and aldol condensation reactions [6][7][8]. Compared with other intermediate compounds, organoboranes are readily prepared from alkenes and alkynes with various structures, giving them their great applicability in organic synthesis [6,[9][10][11]. Furthermore, the highly stereo-and regioselective nature of hydroboration products has lent themselves to applications in pharmaceuticals [12], as well in the reduction of environmental CO 2 [13] and in the polymerization of propylene into polypropylene [14].…”