Towards supramolecular fixation of NOX gases: encapsulated reagents for nitrosation.
The use of simple calix[4]arenes for chemical conversion of NO2/N2O4 gases is demonstrated in solution and in the solid state. Upon reacting with these gases, calixarenes 1 encapsulate nitrosonium (NO+) cations within their cavities with the formation of stable calixarene-NO+ complexes 2. These complexes act as encapsulated nitrosating reagents; cavity effects control their reactivity and selectivity. Complexes 2 were effectively used for nitrosation of secondary amides 5, including chiral derivatives. Unique size-shape selectivity was observed, allowing for exclusive nitrosation of less crowded N-Me amides 5 a-e (up to 95 % yields). Bulkier N-Alk (Alk>Me) substrates 5 did not react due to the hindered approach to the encapsulated NO+ reagents. Robust, silica gel based calixarene material 3 was prepared, which reversibly traps NO2/N2O4 with the formation of NO+-storing silica gel 4. With material 4, similar size-shape selectivity was observed for nitrosation. The N-Me-N-nitroso derivatives 6 d,e were obtained with approximately 30 % yields, while bulkier amides were nitrosated with much lower yields (<8 %). Enantiomerically pure encapsulating reagent 2 d was tested for nitrosation of racemic amide 5 t, showing modest but reproducible stereoselectivity and approximately 15 % ee. Given high affinity to NO+ species, which can be generated by a number of NOX gases, these supramolecular reagents and materials may be useful for NOX entrapment and separation in the environment and biomedical areas.
Kang
Yanlong
Y
Department of Chemistry & Biochemistry, University of Texas at Arlington, Arlington, TX 76019-0065, USA.
Zyryanov
Grigory V
GV
Rudkevich
Dmitry M
DM
D000577:Amides / Q000737:chemistry*; D047250:Calixarenes / Q000737:chemistry*; D005740:Gases / Q000737:chemistry; D007202:Indicators and Reagents; D008968:Molecular Conformation; D009589:Nitrogen Oxides / Q000737:chemistry*; D015538:Nitrosation; D013237:Stereoisomerism
D000577:Amides; D005740:Gases; D007202:Indicators and Reagents; D009589:Nitrogen Oxides; D047250:Calixarenes