The N-Bromo-Hammick Intermediate.

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Hammick intermediates are transient species in organic reactions.
• Understanding carbene electronic structures is crucial for predicting reactivity.
• Low-temperature isolation allows characterization of reactive intermediates.

Purpose of the Study:

• To characterize the N-bromo-Hammick intermediate, bromopyridin-2-ylidene.
• To elucidate the electronic structure and stability of this singlet carbene.
• To investigate its isomerization pathway and reactivity with molecular hydrogen.

Main Methods:

• Isolation of bromopyridin-2-ylidene in solid neon at 4.4 K.
• Computational studies using B2PLYP and NEVPT2 methods.
• Infrared (IR) and UV/vis spectroscopy for characterization.

Main Results:

• Bromopyridin-2-ylidene was identified as a singlet carbene with a vacant σ*-type frontier orbital (σ²σ*⁰ electronic structure).
• The carbene isomerizes to a more stable pyridine tautomer via a planar transition state.
• A minor but prohibitive energy barrier (8.4 kcal mol⁻¹) was observed for its reaction with molecular hydrogen at experimental temperatures.

Conclusions:

• The characterized N-bromo-Hammick intermediate exhibits unusual electronic properties.
• Its planar isomerization pathway distinguishes it from other singlet carbenes.
• Low temperatures prevent reaction with molecular hydrogen due to a small energy barrier.