Towards Supramolecular Fixation of NO_X Gases:Encapsulated Reagents for Nitrosation

摘要

The use of simple calix[4]ar-enes for chemical conversion of NO_2/ N_2O_4 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 5a-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 NO_2/ N_2O_4 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 6d,e were obtained with -30% yields,while bulkier amides were nitro-sated with much lower yields (<8%).Enantiomerically pure encapsulating reagent 2d was tested for nitrosation of racemic amide 5t,showing modest but reproducible stereoselectivity and -15% ee.Given high affinity to NO + species,which can be generated by a number of NO_x gases,these supra-molecular reagents and materials may be useful for NO_x entrapment and separation in the environment and bio-medical areas.