Nitration of aromatic compounds using a zeolite catalyst and a combination of dinitrogen tetroxide and air in a sealed system leads to high yields and para-selectivities in a clean, solvent-free process.
Calix[4]arenes substituted by four urea functions are self-complementary molecules that spontaneously combine in apolar solvents in the presence of an ammonium salt to form dimeric capsules held together by a belt of hydrogen bonds. In the presence of tetraethylammonium salts, the Et4N+ cation is included as a guest. The sorting between dimeric capsules formed in a mixture of calix[4]arenes directly depends on the steric crowding of the substituents grafted on the urea groups whether aromatic derivatives or aliphatic chains linking urea functions in mono-, di-, or tetraloop structures. Simple rules allow one to anticipate which capsules will be exclusively formed when calix[4]arenes are mixed in different proportions. The stabilization of the dimeric structures by hydrogen bonds is thwarted by the overlaps of aliphatic loops and/or by bulky groups that cannot pass through these loops. Despite the structural similarity of the calixarenes, the exclusive formation of dimers of well-defined compositions and clear titration breaks are observed by electrospray mass spectrometry. This technique yields reliable information on stoichiometries and composition despite measurements in the gas phase rather than in solution and it does not suffer from excessive peak overlaps in contrast with NMR.
Nitration of aromatic compounds using a zeolite as a solid inorganic catalyst and a combination of dinitrogen tetroxide and oxygen or air as the nitrating reagent, in open or sealed systems, leads to high yields and significant para selectivities for halogenobenzenes and other aromatics in a relatively clean process. The material used as catalyst has a strong influence on the reaction.
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