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Reactive, Stereo- and Regioselectivity of Diels-Alder Reactions Between Five-Membered Heterocycles and Danishefsky’s Diene
1 , 2 , 2 , * 3
1  Laboratorio de Desarrollo Analítico y Quimiometría, Facultad de Bioquímica y Ciencias Biológicas, Universidad Nacional del Litoral-CONICET, S3000ZAA Santa Fe, Argentina
2  Laboratorio G. A. Fester, Facultad de Ingeniería Química, Universidad Nacional del Litoral-CONICET, S3000AOM Santa Fe, Argentina
3  Institute of Chemistry, Karl-Franzens-University Graz, Heinrichstr. 28, A-8010 Graz, Austria

Abstract: Conceptual density functional theory (chemical hardness h, electronic chemical potential µ, global and local electrophilicity index w and wk, Fukui function f+k) is used to predict reactivity, stereo- and regioselectivity of the Diels-Alder (DA) reaction between 2-nitropyrrole and its N-tosylated derivative with Danishefsky's diene to yield 5-hydroxyindoles. In fact, the indexes are not useful for hetero-DA due to the fact that they give very high values at nitrogen or oxygen atoms of the nitro substituent. A detailed mechanistic study of various possible reaction channels is done by DFT [B3LYP/6-31G(d)] calculations. Solvent effects (benzene) are taken into account by the CPCM bulk solvation model. In principle, the nitropyrrole can either act as diene involving the nitro group (hetero-Diels-Alder reaction, HAD) or alternatively as alkene in a "normal" Diels-Alder reaction. While in several reactions of nitroalkenes the dihydrooxazine N-oxides could be observed (i.e. the products of nitroalkenes acting as heterodiene), under the experimental conditions used, N-tosyl-2-nitropyrrole yields solely the aromatic alcohol. This product can be formed either directly by Diels-Alder reaction of the nitropyrrole acting as alkene or by rearrangement of the dihydrooxazine N-oxides. The primary cycloadduct with the nitropyrrole acting as alkene is found to be unstable reacting further by extrusion of nitrous acid to 5-hydroxyindole. Any of the cycloadducts (DA or HDA) could in principle give the ArOH by HNO2 extrusion. Finally, the results are compared with those obtained for the analogous furans, thiophenes, and selenophenes.
Keywords: Cycloaddition reactions, density functional theory, mechanism