Reaction Pathways and Redox States in α-Selective Cobalt-Catalyzed Hydroborations of Alkynes

Abstract

Cobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre-catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β-(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re-enter the main catalytic cycle.

Keywords: T-shaped complex; alkenyl boronates; alkynes; cobalt; pincer ligand.

Scheme 1

Left: Synthesis of cobalt(II) alkyl complexes 1. Right: Molecular structure of ^H,iPrboxmiCoCH₂SiMe₃ (1 b) in the solid‐state. Displacement ellipsoids set at 30 % probability level, hydrogen atoms omitted for clarity, only one of two independent molecules shown. Selected bond lengths [Å] and angles [°], values for the second independent molecule are given in square brackets: Co–C23 1.963(7) [1.981(7)], Co–N1 1.905(6) [1.891(6)], Co–N2 1.963(6) [1.947(5)], Co–N3 1.929(6) [1.917(6)], N2‐Co‐C23 134.5(3) [130.7(3)], N1‐Co‐N3 153.5(2) [160.1(2)]. ^[30]

Figure 1

Conversion profile for a cobalt‐catalyzed hydroboration of 1‐ethinyl‐4‐fluorobenzene ([Alkyne]₀/M=0.22, [HBPin]₀/M=0.33, [Co]₀/M 0.005, room temperature, toluene) monitored by in situ ¹⁹F NMR spectroscopy. Relative amounts in reaction mixture given.

Scheme 2

Reaction of precatalyst ^H,RboxmiCoCH₂SiMe₃ (1) with stoichiometric amounts of pinacolborane as well as the internal alkyne 1‐phenyl‐1‐propyne yielding vinyl complex 4 (top), pinacolborane to generate the non‐isolable hydrido key intermediate (middle) as well as an excess of pinacolborane to give the dinuclear Co^I complex 5 (bottom).

Figure 2

Molecular structures of ^H,iPrboxmiCoC(Me)C(H)Ph (4, left) and [^H,iPrboxmiCo]₂ (5, right). Displacement ellipsoids set at 30 % (4) and 50 % (5) probability level, hydrogen atoms omitted for clarity, only one of four independent molecules shown for 4. Selected bond lengths [Å] and angles [°], given as range for all independent molecules in case of 4; 4: Co–C23 1.922(11)…1.956(11), Co–N1 1.921(7)…1.942(7), Co–N2 1.966(8)…1.994(8), Co–N3 1.916(7)…1.934(7); N2‐Co‐C23 162.6(4)…167.3(4), N1‐Co‐N3 171.5(4)…174.8(4). 5: Co1–center(C26‐C27) 2.029(2), Co2–center(C12‐C13) 2.047(2), Co1–N1 2.002(3), Co1–N2 1.976(3), Co1–N3 2.029(3), Co2–N4 2.062(3), Co2–N5 1.987(3), Co2–N6 2.009(3), C12–C13 1.412(4), C26–C27 1.420(4); N2‐Co1‐center(C26‐C27) 104.6(1), N1‐Co1‐N3 133.84(12), N5‐Co2‐center(C12‐C13) 106.0(1), N4‐Co2‐N6 139.60(12). ^[30]

Scheme 3

Mechanistic control experiments. In all schematic illustrations, [Co] denotes the complex fragment ^H,RboxmiCo; a) product distribution given; b) product distribution with respect to α‐3 b given; c) NMR yield given.

Scheme 4

Proposed mechanistic cycle for the cobalt‐catalyzed hydroboration of alkynes.

Scheme 5

Top: Substrate scope of the cobalt‐catalyzed α‐selective hydroboration of terminal alkynes. Reaction conditions: 300 μmol alkyne, 600 μmol HBPin, 0.5 mol % 1 a. [a] α:β‐(E) ratio was determined by ¹H NMR spectroscopy before purification; estimated error: ±2 %. [b] Yield of enriched α‐product. [c] 1 mol % catalyst loading. [d] Yield of isomer mixture. Bottom: Formal synthesis of Bexarotene by cobalt‐catalyzed α‐selective hydroboration.