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Catalytic Synthesis of 8-Membered Ring Compounds via Cobalt(III)-Carbene Radicals

preprint
submitted on 19.02.2020 and posted on 20.02.2020 by M. Zhou, J.I. van der Vlugt, Bas de Bruin
The metalloradical activation of o-aryl aldehydes with cobalt(II) porphyrin complexes as catalysts produces cobalt(III)-carbene radical intermediates, providing a novel and powerful strategy for the synthesis of medium-sized ring structures. Herein we make use of the intrinsic radical-type reactivity of cobalt(III)-carbene radical intermediates in the [CoII(TPP)]-catalyzed (TPP = tetraphenylporphyrin) synthesis of two types of 8 membered ring compounds; novel dibenzocyclooctenes and unique monobenzo-cyclooctadienes. The method was successfully applied to a variety of substrates, producing several 8-membered ring compounds in good yields and with excellent substituent tolerance. DFT calculations and experimental results suggest that the reactions proceed via initial hydrogen atom transfer from the bis-allylic/benzallylic C-H bond to the carbene radical moiety, followed by two divergent processes for ring-closure to the two different types of 8-membered ring products. While the dibenzocyclooctenes are formed by dissociation of o quinodimethanes (o-QDMs) from the catalyst that undergo an uncatalyzed ring-closure reaction involving 8-pi-cyclisation, DFT calculations suggest that ring-closure to the monobenzocyclooctadienes involves a radical-rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring-closure reactions to chiral benzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt-porphyrin catalyst.

Funding

China Scholarship Council PhD fellowship (CSC 201806050112)

University of Amsterdam (Research Priority Area Sustainable Chemistry)

Netherlands Organization for Scientific Research (NWO ARC CBBC)

History

Email Address of Submitting Author

b.debruin@uva.nl

Institution

University of Amsterdam

Country

The Netherlands

ORCID For Submitting Author

0000-0002-3482-7669

Declaration of Conflict of Interest

no conflict of interest

Version Notes

This is version 1

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