Synthesis of Phosphinines from CoII-Catalyzed [2+2+2] Cycloaddition Reactions

Tim Gläsel,Marko Hapke,Haijun Jiao

doi:10.1021/acscatal.1c03483

Tim Gläsel, Marko Hapke + Show 1 more

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https://doi.org/10.1021/acscatal.1c03483

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Abstract

The CoII-catalyzed [2+2+2] cycloaddition of functionalized diynes and phosphaalkynes resulting in the formation of an array of structurally interesting phosphinines is reported. A combination of 1,2-bis(diphenylphosphino)benzene (dppbenz) and CoI2 turned out to be the most suitable and active catalyst system under microwave reaction conditions. Chemoselective cyclizations of phosphaalkynes in the presence of nitriles have been observed, and a number of carbonyl functionalities are tolerated as well. The reaction can successfully be conducted with a 1:1 ratio of phosphaalkyne/diyne substrate. Model reactions with benzonitrile and diyne demonstrated that CoII catalysts in the presence of bisphosphines prefer carbocyclization, while reduction to lower oxidation states promotes heterocyclization to the corresponding pyridine. A reaction mechanism comprising a CoII oxidation state for the cyclotrimerization process is proposed and rationalized based on control experiments and theoretical calculations.

Highlights

We report on the efficient synthesis of functionalized phosphinines by CoII-catalyzed [2+2+2] cycloaddition from phosphaalkynes and functionalized diynes, allowing a high atom-economical 1:1 ratio of the substrate and yielding the products with up to 91% yield
The reaction tolerates functional groups such as esters, ketones, lactone, sulfone, and even nitriles to be present in the phosphinine products
The cyclization catalyst consists of a cobalt(II)− bisphosphine complex without a reducing agent, and high reaction temperatures are required for successful and fast catalysis

Summary

■ INTRODUCTION

Catalytic [2+2+2] cycloadditions of alkynes and heterocumulenes, leading to a wealth of functionalized aromatic and heteroaromatic molecules, have become an interesting synthetic strategy for the assembly of complex molecules from structurally simpler precursors since the onset of the first systematic studies. The development includes the introduction of various catalysts from the whole range of transition metals. The large variety of metal complexes available for cyclotrimerizations is manifested in the diversity of reaction mechanisms. Cobalt compounds have always taken a front seat in the application of late transitionmetal catalysts in cyclotrimerization reactions, especially by cyclopentadienyl (Cp)CoI-(neutral ligand) complexes like CpCo(CO) or modified congeners. A number of in situ generated CoI complexes from the reduction of CoII halides containing bisphosphine or NHC (N-heterocyclic carbene) ligands have been utilized for the transformation of triynes to furnish highly substituted benzenes. the isolable complex CoCl(PPh3) was identified as a useful molecularly defined precatalyst for this transformation under mild conditions. The reaction scope has been extended to asymmetric cyclizations by an in situ generated chiral CoI-(P,N ligand) catalyst, affording atropochiral biaryls and triaryls.. While pyridines belong to the core area of heteroaromatic chemistry, phosphinines ( coined phosphabenzenes) have a much shorter history.. While pyridines belong to the core area of heteroaromatic chemistry, phosphinines ( coined phosphabenzenes) have a much shorter history.10 Their molecular properties, e.g., in comparison to pyridines, in homogeneous catalysis and coordination chemistry, have stirred significant work over recent decades toward their synthesis and functionalization.. De novo synthesis of phosphinines by transition-metalcatalyzed reactions besides traditional approaches like the [4+2] cycloaddition of phosphaalkynes with 2-pyrones is still rare (Scheme 1, top). The only known successful transition-metalcatalyzed cyclization of phosphaalkynes was reported by Nishibayashi and co-workers, applying FeI2 without additional ligands, proposing an FeII/FeIV catalytic reaction cycle (Scheme 1, middle).. While CpCoI complexes are established catalysts for pyridine ring assembly under a variety of conditions, a large tendency toward dimerization of the phosphaalkynes using

■ RESULTS AND DISCUSSION

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES