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. 2025 Mar 24;15(7):5718-5730.
doi: 10.1021/acscatal.5c00349. eCollection 2025 Apr 4.

Photochemical Manganese-Catalyzed [2 + 2 + 2] Cycloaddition Reactions

Benedikt N Baumann  1 , Phong Dam  2 , Jabor Rabeah  2 , Christoph Kubis  2 , Angelika Brückner  2 , Haijun Jiao  2 , Marko Hapke  1   2
Affiliations

Photochemical Manganese-Catalyzed [2 + 2 + 2] Cycloaddition Reactions

Benedikt N Baumann et al. ACS Catal. .

Abstract

We report the cyclotrimerization reactions of triynes using Mn(I) complexes derived from MnBr(CO)5 and phosphine ligands, such as 1,1-bis(diphenylphosphino)methane (dppm). These reactions are driven by irradiation under mild conditions (30-80 °C) without the need of additional photoinitiators. Our catalytic screening revealed that counteranions and ligands significantly influence the process. This method accommodates a broad range of functionalities in the substrates, including alkyl, aryl, Bpin, SiMe3, GeEt3, PPh2, pyridyl, and thienyl moieties, without notable interference in the transformation. Additionally, this method enables reactions with oligoalkynes-like (un)substituted hexaynes, producing 2-fold cyclization products in very good yields. Under stoichiometric conditions, the cyclization of diynes with phosphaalkynes results in the unique photochemical synthesis of phosphinines. Experimental and theoretical mechanistic studies indicate that the dissociation of the diphosphine ligand precedes the involvement of the Mn carbonyl species in the catalytic cycle. The ligand plays a crucial role in stabilizing the catalyst during the catalytic transformation and preventing the formation of unreactive cluster species.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Known and Novel Manganese–Carbonyl-Catalyzed Formal Cyclotrimerizations of Alkynes toward Arene and Heteroarene Products
Chart 1
Chart 1. Investigation of Triynes Comprising Different Alkyne-Bridging Units
Chart 2
Chart 2. Substrate Screening for the Photocatalytic Mn(I)-Catalyzed [2 + 2 + 2] Cycloaddition Reaction Using a Variety of Functionalized Triynes (Isolated Yields)
Chart 3
Chart 3. Showcase of Complex Substrates for Successful Cyclotrimerization Reactions Using Precatalyst Mn7 (Isolated Yields)
Chart 4
Chart 4. [2 + 2 + 2] Cycloaddition of Diynes and Phosphaalkynes Using Mn7 in Stochiometric Amounts (Isolated Yields)
Chart 5
Chart 5. Follow-up Chemistry with Compound 39: Synthesis of Highly Substituted Benzenes by Suzuki–Miyaura Reactions with Aryl Bromides (Isolated Yields)
Scheme 2
Scheme 2. (a) Light On–Off Experiment Using Mn7 and Triyne 4, (b) Time–Yield Plot for the Cyclization Reaction Triyne 4 Using Mn7, (c) Filtration Test, (d) Control Experiments to Exclude Thermal Background Reactions, and (e) Attempted Cyclization under CO Atmosphere
Scheme 3
Scheme 3. Double Integration of EPR Spectra of a Mixture of (a) Mn1 (1 mM) and Different Ligands (1 mM); and (b) in Addition of Triyne 4 (10 mM)
Scheme 4
Scheme 4. Presumed Reaction Mechanism for the Manganese-Catalyzed Cyclization of Triyne 17 from Theoretical Calculations
Scheme 5
Scheme 5. Tentative Catalytic Mechanism of the Manganese-Catalyzed [2 + 2 + 2] Cycloaddition Reaction
See this image and copyright information in PMC

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