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σ‐Resonance Stabilization of Aminomethylene by N‐Halogenation.

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Title: σ‐Resonance Stabilization of Aminomethylene by N‐Halogenation.
Authors: Medvedko, Serhii1 (AUTHOR), Bhagat, Virinder2 (AUTHOR), Wagner, J. Philipp1 (AUTHOR) jpw@uni‐bremen.de
Source: Journal of Computational Chemistry. 3/30/2026, Vol. 47 Issue 8, p1-6. 6p.
Subjects: Halogenation, Resonance effect, Carbenes, Computational chemistry, Carbene synthesis, Electronic structure, Pyridine
Abstract: Amino substitution effectively stabilizes carbenes via π‐resonance, enabling their isolation as N‐heterocyclic carbenes (NHCs). Our group has recently demonstrated that N‐halogenation of pyridinylidenes (Hammick intermediates) provides an additional stabilization mode through a rarely utilized σ‐resonance. However, this σ‐resonance interaction might not yet have reached its full potential due to the constriction of the carbene motif within a cyclic structure. Thus, we have studied the impact of N‐halogenation (X = Br, I) on the structure, energetics, and spectroscopic properties of the parent open‐chain aminocarbene, aminomethylene, utilizing fully unconstrained CCSD(T) and NEVPT2 geometry optimizations together with a def2‐TZVPP basis set. We find that the halogenated carbenes prefer a Z configuration of the C–N bond, which is impossible in a cyclic arrangement. Halogenated aminomethylenes are kinetically protected against unimolecular rearrangement by barriers exceeding 35 kcal mol−1. Short C–N bond lengths (1.23 Å) and wide carbene angles (~125°) indicate cooperative π‐ and σ‐resonance stabilization, resulting in carbene stabilization energies (CSEs) increased by ~12–13 kcal mol−1 relative to the parent aminomethylene. The electronic structure is characterized by the σ* orbital of the N–X bond as the lowest unoccupied molecular orbital. [ABSTRACT FROM AUTHOR]
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Database: Engineering Source
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Abstract:Amino substitution effectively stabilizes carbenes via π‐resonance, enabling their isolation as N‐heterocyclic carbenes (NHCs). Our group has recently demonstrated that N‐halogenation of pyridinylidenes (Hammick intermediates) provides an additional stabilization mode through a rarely utilized σ‐resonance. However, this σ‐resonance interaction might not yet have reached its full potential due to the constriction of the carbene motif within a cyclic structure. Thus, we have studied the impact of N‐halogenation (X = Br, I) on the structure, energetics, and spectroscopic properties of the parent open‐chain aminocarbene, aminomethylene, utilizing fully unconstrained CCSD(T) and NEVPT2 geometry optimizations together with a def2‐TZVPP basis set. We find that the halogenated carbenes prefer a Z configuration of the C–N bond, which is impossible in a cyclic arrangement. Halogenated aminomethylenes are kinetically protected against unimolecular rearrangement by barriers exceeding 35 kcal mol−1. Short C–N bond lengths (1.23 Å) and wide carbene angles (~125°) indicate cooperative π‐ and σ‐resonance stabilization, resulting in carbene stabilization energies (CSEs) increased by ~12–13 kcal mol−1 relative to the parent aminomethylene. The electronic structure is characterized by the σ* orbital of the N–X bond as the lowest unoccupied molecular orbital. [ABSTRACT FROM AUTHOR]
ISSN:01928651
DOI:10.1002/jcc.70360