The Diels–Alder Reaction of 4,6‐Dinitrobenzofuroxan with 1‐Trimethylsilyloxybuta‐1,3‐diene: A Case Example of a Stepwise Cycloaddition

Abstract

The reaction of 4,6-dinitrobenzofuroxan (DNBF) with 1-trimethylsilyloxybuta-1,3-diene (8) is shown to afford a mixture of [2+4] diastereomeric cycloadducts (10, 11) through stepwise addition-cyclization pathways. Zwitterionic intermediate sigma-adduct 9, which is involved in the processes, has been successfully characterized by (1)H and (13)C NMR spectroscopy and UV/visible spectrophotometry in acetonitrile. A kinetic study has been carried out in this solvent that revealed that the rate of formation of 9 nicely fits the three-parameter equation log k=s(E+N) developed by Mayr to describe the feasibility of nucleophile-electrophile combinations. This significantly adds to the NMR spectroscopic evidence that the overall cycloadditions take place through a stepwise mechanism. The reaction has also been studied in dichloromethane and toluene. In these less polar solvents, the stability of 9 is not sufficient to allow direct characterization by spectroscopic methods, but a kinetic investigation supports the view that stepwise processes are still operating. An informative comparison of our reaction with previous interactions firmly identified as prototype stepwise cycloadditions is made on the basis of the global electrophilicity index, omega, defined by Parr within the density functional theory, and highlighted by Domingo et al. as a powerful tool for understanding Diels-Alder reactions.

Investigation of the reaction mechanism of [4 + 2] cyclization of 2,3 dimethylbuta-1,3-diene to methylacrylate using the Michaelis-Menten equation

“…The Diels-Alder reactions may actually take place according to a two-step mechanism [8][9][10][11][12].…”

“…On the other hand, reactions e.g. of 3-nitropyridine with 1-methoxy-3-trimethoxysillylbuta-1,3-diene [9], 4,6-dinitrobenzofuroxan with 1-trimethylsilyloxybuta-1,3-diene [10], aryl-substituted 1,2,4-triazines with 2-cyclopropylidene-1,3-dimethylimidazolidine [11] and 1,1,1-trifluor-3-nitroprop-2-ene derivatives with enamines [12] take place according to a two-step, zwitterionic mechanism.…”

A reexamination of the molecular mechanism of the Diels–Alder reaction between tetrafluoroethene and cyclopentadiene

“…However, the most recent studies undermine this view. In some cases, it was proven that DA reactions may actually take place according to a two-step mechanism [5][6][7][8][9][10]. Such a two-step mechanism with biradical intermediate takes place e.g.…”

“…On the other hand, reactions e.g. of 3-nitropyridine with 1-methoxy-3-trimethoxysillylbuta-1,3-diene [7], 4,6-dinitrobenzofuroxan with 1-trimethylsilyloxybuta-1,3-diene [8], aryl-substituted 1,2,4-triazines with 2-cyclopropylidene-1,3-dimethylimidazolidine [9] and 1,1,1-trifluor-3-nitroprop-2-ene derivatives with enamines [10] take place according to a two-step, zwitterionic mechanism.…”

Experimental and DFT Study of Azo-bis-Cyanuric Chloride Polar Diels-Alder Reaction with a Number of Dienes. Ways of Further Modification of the Obtained Products

“…Its global electrophilicity ω = 4.45 eV makes it one of the most active dienophiles in normal electron demand Diels-Alder reaction, much more active as diethyl azodicarboxylate (DEAD) (2.57 eV) and even more than some Lewis acid complexes, i.e. nitroetylene/BH 3 complex (4.33 eV, [17]). Therefore, comparing global electrophylicity indices of reagents, shown in Table 1 polar or zwitterionic mechanism of this cycloaddition could be suggested.…”