Selective Manganese-Catalyzed Dimerization and Cross-Coupling of Terminal Alkynes.

Stefan Weber,Karl Kirchner,Luis F Veiros

doi:10.1021/acscatal.1c01137

Abstract

Herein, efficient manganese-catalyzed dimerization of terminal alkynes to afford 1,3-enynes is described. This reaction is atom economic, implementing an inexpensive, earth-abundant nonprecious metal catalyst. The precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)3(CH2CH2CH3)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn–alkyl bond to yield an acyl intermediate that undergoes rapid C–H bond cleavage of alkyne, forming an active Mn(I) acetylide catalyst [Mn(dippe)(CO)2(C≡CPh)(η2-HC≡CPh)] together with liberated butanal. A range of aromatic and aliphatic terminal alkynes were efficiently and selectively converted into head-to-head Z-1,3-enynes and head-to-tail gem-1,3-enynes, respectively, in good to excellent yields. Moreover, cross-coupling of aromatic and aliphatic alkynes selectively yields head-to-tail gem-1,3-enynes. In all cases, the reactions were performed at 70 °C with a catalyst loading of 1–2 mol %. A mechanism based on density functional theory (DFT) calculations is presented.

Highlights

CO ligands in Mn(I) alkyl carbonyl complexes are known to undergo migratory insertions to form highly reactive coordinatively unsaturated acyl complexes, which can be trapped in the presence of strong field ligands such as CO
The initiation step involves C−H bond activation of the terminal alkyne forming an active Mn(I) acetylide catalyst together with free butanal (Scheme 1). This is the first example of manganese-catalyzed dimerization and cross-coupling of terminal alkynes,[8,9] which proceeds without any additives under mild conditions
[Mn(dippe)(CO)[3] (CH2CH2CH3)] (1), which is air-stable for several weeks in the solid state

Summary

■ INTRODUCTION

CO ligands in Mn(I) alkyl carbonyl complexes are known to undergo migratory insertions to form highly reactive coordinatively unsaturated acyl complexes, which can be trapped in the presence of strong field ligands such as CO or tertiary phosphines a well-known textbook reaction.[1]. Dihydrogen is able to react with transition metal−acyl complexes to afford aldehydes and metal hydride complexes. The initiation step involves C−H bond activation of the terminal alkyne forming an active Mn(I) acetylide catalyst together with free butanal (Scheme 1). This is the first example of manganese-catalyzed dimerization and cross-coupling of terminal alkynes,[8,9] which proceeds without any additives under mild conditions

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES