Hydroxylamines as oxidation catalysts: thermochemical and kinetic studies

Area of Science:

• Organic Chemistry
• Physical Chemistry
• Catalysis

Background:

• Hydroxylamines are versatile compounds with applications in catalysis.
• Understanding the O-H bond strength is crucial for predicting reactivity.
• The influence of nitrogen substituents on hydroxylamine O-H BDE is not fully elucidated.

Purpose of the Study:

• To determine the bond dissociation enthalpies (BDE) of various substituted hydroxylamines.
• To investigate the effect of alkyl, aryl, vinyl, and carbonyl substituents on the O-H bond strength.
• To rationalize the catalytic activity of N-hydroxyphthalimide (NHPI) and its derived radical (PINO).

Main Methods:

• Electron Paramagnetic Resonance (EPR) radical equilibration technique was employed.
• Kinetic studies were performed to elucidate reaction mechanisms.
• Computational methods may be used for further validation (though not explicitly stated in abstract).

Main Results:

• Dialkyl hydroxylamines exhibit O-H BDEs around 70 kcal/mol.
• Acyl substituents, particularly two acyl groups in N-hydroxyphthalimide (NHPI), significantly increase O-H BDE to ~88 kcal/mol.
• Kinetic data supports hydrogen atom abstraction by cumylperoxyl radicals as the rate-determining step in NHPI-catalyzed oxidation.

Conclusions:

• Acyl substitution dramatically enhances the O-H bond strength of hydroxylamines.
• The high O-H BDE of NHPI supports its proposed role as an efficient oxidation catalyst.
• The findings provide insights into the mechanism of NHPI-catalyzed aerobic oxidation reactions.