title: The Advantages of Cyclic Over Acyclic Carbenes To Access Isolable Capto-Dative C-Centered Radicals.
abstract: A cyclic and an acyclic di(amino)carbene as well as a cyclic and an acyclic (alkyl)(amino)carbene cleanly react with benzoyl chloride to give the corresponding adducts 1+cyc, 1+acy, 2+cyc, and 2+acy, respectively. The reduction of 1+cycand 2+cycderived from cyclic carbenes affords the corresponding radicals 1cycand 2cycthat are stable at room temperature. In contrast, radicals 1acyand 2acy, derived from acyclic carbenes, cannot be isolated. It is shown that 1acyis as thermodynamically stabilized as its cyclic counterpart 1cyc, but its instability is the result of β-hydrogens of the nitrogen substituent, along with the enhanced flexibility around C-N bonds, which allow for a H.-migration-elimination process. Radical 2acyis thermodynamically unstable, and undergoes disproportionation into the corresponding iminium 2+acyand enolate 2-acy. This is due to the excessive steric hindrance, which prevents electron-delocalization on the NCCO fragment, and thus, the capto-dative stabilization. This work suggests general guidelines for the design of highly persistent (amino)(carboxy)radicals, especially by emphasizing the key advantage of cyclic patterns.
authors.affiliation: UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, 92093-0358, USA.
authors.forename: Janell K
authors.initials: JK
authors.lastname: Mahoney
authors.affiliation: UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, 92093-0358, USA.
authors.forename: Rodolphe
authors.initials: R
authors.lastname: Jazzar
authors.affiliation: Département de Chimie Moléculaire (UMR 5250), Université Grenoble-Alpes, CNRS, CS 40700, 38058, Grenoble Cedex 9, France.
authors.forename: Guy
authors.initials: G
authors.lastname: Royal
authors.affiliation: Département de Chimie Moléculaire (UMR 5250), Université Grenoble-Alpes, CNRS, CS 40700, 38058, Grenoble Cedex 9, France.
authors.forename: David
authors.initials: D
authors.lastname: Martin
authors.affiliation: UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, 92093-0358, USA.
authors.forename: Guy
authors.identifier: http://orcid.org/0000-0003-2623-2363
authors.initials: G
authors.lastname: Bertrand
keywords: EPR spectroscopy; capto-dative effect; carbenes; radicals