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

TL;DR

This paper introduces a new photochemical method using manganese catalysts to efficiently perform [2+2+2] cycloaddition reactions under mild conditions.

Contribution

The study presents a novel manganese(I)-catalyzed [2+2+2] cycloaddition method without photoinitiators, enabling efficient cyclization of diverse substrates.

Findings

Manganese(I) complexes with dppm ligands enable efficient [2+2+2] cyclotrimerization of triynes under mild irradiation.

The method is compatible with various functional groups and produces high yields of cyclized products.

Mechanistic studies show the diphosphine ligand stabilizes the catalyst and prevents cluster formation.

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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