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The Advantages of Cyclic Over Acyclic Carbenes To Access Isolable Capto‐Dative C‐Centered Radicals
Author(s) -
Mahoney Janell K.,
Jazzar Rodolphe,
Royal Guy,
Martin David,
Bertrand Guy
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201700144
Subject(s) - dative case , radical , chemistry , computational chemistry , stereochemistry , organic chemistry , philosophy , linguistics
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 + cyc and 2 + cyc derived from cyclic carbenes affords the corresponding radicals 1 cyc and 2 cyc that are stable at room temperature. In contrast, radicals 1 acy and 2 acy , derived from acyclic carbenes, cannot be isolated. It is shown that 1 acy is as thermodynamically stabilized as its cyclic counterpart 1 cyc , 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 2 acy is thermodynamically unstable, and undergoes disproportionation into the corresponding iminium 2 + acy and 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.