Synlett 2019; 30(15): 1765-1775
DOI: 10.1055/s-0037-1611887
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© Georg Thieme Verlag Stuttgart · New York

Stable Carbon-Centered Radicals Based on N-Heterocyclic Carbenes

Anorganische Molekülchemie und Katalyse, Anorganische Chemie und Strukturchemie, Centrum für Molekulare Materialien, Fakultät für Chemie, Universität Bielefeld, Universitätsstraße 25, 33615 Bielefeld, Germany   Email: rghadwal@uni-bielefeld.de
› Author Affiliations
Funding from the Deutsche Forschungsgemeinschaft (GH 129/4) is gratefully acknowledged.
Further Information

Publication History

Received: 25 May 2019

Accepted after revision: 23 June 2019

Publication Date:
12 July 2019 (eFirst)

Abstract

Carbon-centered radicals and diradicaloids based on classical N-heterocyclic carbene (NHC) scaffolds are readily accessible as crystalline solids. The presence of an aryl (Ar) substituent at the C2-position is the key to the remarkable stability of these open-shell species as it provides appropriate room for the spin-density delocalization. Two catalytic as well as high-yielding protocols have been developed to install a suitable aryl group at the C2-position of NHCs. The spin-density in mono-radicals (NHCAr) is mostly located on the parent carbene carbon (C2) atom. The bridging of two NHCs through a phenylene spacer (C6H4) n enables the isolation of various p-quinodimethane (p-QDM) derivatives, which may be considered as open-shell Kekulé diradicaloids. The diradical character of these NHC-analogues of Thiele (n = 1), Chichibabin (n = 2), and Müller (n = 3) hydrocarbons [(NHC)(C6H4) n (NHC)] can be tuned by a rational choice of the size and/or the topology of spacers. In this account, the synthesis, structure, and properties of this new class of radical hydrocarbons is presented.

1 Introduction

2 NHC-Monoradicals

3 NHC-Diradicaloids

4 Conclusion

 
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