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. 2024 Aug 5;14(16):12385-12391.
doi: 10.1021/acscatal.4c03805. eCollection 2024 Aug 16.

Hydroboration of Terminal Alkynes Catalyzed by a Mn(I) Alkyl PCP Pincer Complex Following Two Diverging Pathways

Daniel P Zobernig  1 , Berthold Stöger  2 , Luis F Veiros  3 , Karl Kirchner  1
Affiliations

Hydroboration of Terminal Alkynes Catalyzed by a Mn(I) Alkyl PCP Pincer Complex Following Two Diverging Pathways

Daniel P Zobernig et al. ACS Catal. .

Abstract

A stereo- and regioselective Mn(I)-catalyzed hydroboration of terminal alkynes with pinacolborane (HBPin) is described. The hydroboration reaction is highly Z-selective in the case of aryl alkynes and E-selective in the case of aliphatic alkynes. The reaction requires no additives or solvents and proceeds with a catalyst loading of 1 mol % at 50-70 °C. The most active precatalyst is the bench-stable alkyl Mn(I) complex cis-[Mn(PCP-iPr)(CO)2(CH2CH2CH3)]. The catalytic process is initiated by the migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate. This species undergoes C-H and B-H bond cleavage of the alkyne (aromatic alkynes) and HBPin (in the case of aliphatic alkynes) forming the active Mn(I) alkynyl and boryl catalysts [Mn(PCP-iPr)(CO)(C≡CR)] and [Mn(PCP-iPr)(CO)(BPin)], respectively. A broad variety of aromatic and aliphatic alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations. The functionalized alkenes can be used for further applications in cross-coupling reactions.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Manganese-Based Catalysts for the Hydroboration of Alkynes (FG = Functional Group)
Scheme 2
Scheme 2. Synthesis of cis-[Mn(PCP-iPr)(CO)2(CH2CH2CH3)] (3) and Structural View of 3 Showing 50% Ellipsoids (Most H Atoms Omitted for Clarity)
Selected bond distances (Å) and angles (°): Mn1–C1 1.807(3), Mn1–C2 1.781(3), Mn1–C3 1.952(3), Mn1–C17 2.214(3), Mn1–P1 2.284(1), Mn1–P2 2.284(1), P1–Mn1–P2 160.52(3), C1–Mn1–C3 173.4(1), C2–Mn2–C17 175.8(1).
Scheme 3
Scheme 3. Synthesis of cis-[Mn(PCP-iPr)(CO)(κ2-H2Bpin)] (4) with Structural View of 4 Showing 50% Ellipsoids (Most H Atoms Omitted for Clarity)
Selected bond distances (Å) and angles (°): Mn1–C1 1.779(2), Mn1–C2 1.935(2), Mn1–H1 1.60(3), Mn1–H2 1.63(1), Mn1–P1 2.2480(8), Mn1–P2 2.2661(8), P1–Mn1–P2 155.94(3).
Scheme 4
Scheme 4. Synthetic Application of the Obtained Vinylboronates
Scheme 5
Scheme 5. Hydroboration of Deuterated Alkynes Catalyzed by 3
Reaction conditions: alkyne (0.25 mmol, 1 equiv), HBpin (0.28 mmol, 1.1 equiv), 2 (1 mol %), 50–70 °C, 24 h, position of deuterium determined by 2H NMR spectroscopy.
Scheme 6
Scheme 6. Simplified Catalytic Cycle for the Hydroboration of Terminal Alkynes (Initiation by C–H Activation)
Scheme 7
Scheme 7. Simplified Catalytic Cycle for the Hydroboration of Terminal Alkynes (Initiation by B–H Activation)
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