Experimental and theoretical push-pull Chemo- and regioselectivity in 1,3-Dipolar cycloaddition reactions: the case of benzotriazepin-5-one with mesitylnitrile oxide

A novel heterocyclic compound 3-mesityl-5-methyl-4,5,11,11a-tetrahydro-6H-[1,2,4]oxadiazolo [5,4-b][1,3,4]benzotriazepin-6-one 4 has been synthesised by a 1,3 dipolar cycloaddition (13DC) reaction of 1,3,4-benzotriazepin-5-one 1 with mesitylnitrile oxide 3. The reaction, beside its synthetic interest, has shown to be completely chemo- and regioselective. The structure of the compound was determined by X-ray crystallography and analysed by spectral methods (NMR and mass spectrometry). The molecular mechanism for the reaction has been studied using quantum mechanical calculations at the B3LYP/6-31G* theory level. Two mechanisms are possible for the formation of the cycloadduct 4. The first one involves a 13DC reaction between 1, as dipolarophile and 3, as dipole. Analysis of the results indicates that it takes place along asynchronous concerted bond-formation process with a very low polar character. The regioselectivity obtained from the calculations are in complete agreement with the unique formation of the cycloadduct 4. The second mechanism is initiated by the nucleophilic attack of the N3 nitrogen of the tautomer form of 2, to the C5 carbon of the nitrile oxide 3 to yield an amidoxime. However, the large energy involved in this addition prevents this mechanism. The large energy difference between the tautomers 1 and 2, makes that only the CN site of benzotriazepin-5-one 1 could act as a dipolarophile site. This fact makes the 13DC reaction to be chemoselective. The analysis of global electrophilicity of the reagents allows explaining the low polar character of these 13DC reactions. Copyright © 2007 John Wiley & Sons, Ltd.