Wang Yueh

Mechanistic aspects of aminium salt‐catalyzed Diels‐Alder reactions: The substrate ionization step

Substituent effects in the aminium salt catalyzed Diels-Alder reactions of 2,3-dimethyl-1,3-butadiene with a series of meta and para substituted β-methylstyrenes are used to probe detailed mechanistic aspects of these reactions. Kinetic studies were carried out using two different aminium salt catalysts and also electrochemically, using anodic potentials corresponding to the oxidation potentials of the aminium salts. Substituent effects in the equilibrium oxidations of the styrene substrates to the corresponding cation radicals were also studied, via oxidation potential measurements. The results indicate rate determining one electron oxidation of the sytrenes to their cation radicals via an outer sphere electron transfer.

Characterization of cation radical reactions. Aminium salt-catalysed Diels–Alder reactions

Kinetic studies of the Diels–Alder cycloadditions of a series of trans-stilbenes (2) to 2,3-dimethylbuta-1,3-diene (3), catalysed by tris(4-bromophenyl)aminium hexachloroantimonate (1˙+), reveal a curved Hammett–Brown plot, consistent with a mechanistic transition from reversible to rate determining ionization of 2 to 2˙+ as the substituents become progressively more electron donating. Further, substituent effects in mono- and di-substituted stilbenes are essentially multiplicative, describing a distribution of positive charge which is symmetrical with respect to both aryl rings. The symmetry of the charge development is inconsistent with an electrophilic addition to the substrates, proceeding via a carbocation. The combination of the magnitude and symmetry of the positive charge distribution provides strong evidence for a cation radical mechanism.

CRITERIA FOR THE CATION RADICAL VS. ELECTROPHILIC MECHANISTIC DISTINCTION:THE AMINIUM SALT-CATALYSED DIELS-ALDER REACTION

The tris(4-bromophenyl)aminium salt-catalysed Diels–Alder additions of stilbenes to 2, 3-dimethylbuta-1,3diene are shown to proceed via ionization of the stilbenes to the corresponding cation radicals by demonstrating that the reactions involve the generation of a unit positive charge in an essentially symmetrical distribution on the stilbene substrates; a hypothetical electrophilic mechanism is unequivocally ruled out by the present results.

The development of strong covalent interactions in inner sphere electron transfer reactions between cation radicals and neutral molecules

The cation radical Diels–Alder reactions of a series of meta and para substituted aryl cis-prop-1-enyl ethers with 2,3-dimethylbuta-1,3-diene catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate have been studied mechanistically. The reactions are found to occur primarily via an indirect route involving cation radical cyclobutanation followed by cation radical vinylcyclobutane rearrangement. Kinetic studies reveal that the ionization of these ethers to the corresponding cation radicals occurs by an inner sphere mechanism.

Mechanistic aspects of the substrate ionization step in aminium salt catalyzed cyclopropanation

The cyclopropanations of a series of m- and p-substituted trans-β-methylstyrenes (3) by ethyl diazoacetate (4), catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (1·+) and also by tris(2,4-dibromphenyl) aminium hexachloroantimonate (2·+) have been studied by competition kinetics. For the reactions catalyzed by the milder aminium salt (1·+), the Hammett-Brown ρ values and the fact that the absolute rates are independent of the concentration of 4 establish that ionization to 3·+ is not reversible, but rate-determining. The dependence of the magnitude of ρ upon the absolute concentration of 3 indicates the operation of competing chain and catalytic mechanisms, i.e. the ionization of 3 by both product cation radicals and by the catalyst. The extremely low ρ value observed in the reactions catalyzed by 2·+ indicates the exclusive operation of a relatively unselective chain mechanism. These mechanistic assignments are further supported by the observation of the formation of the same products under electrochemical conditions, in the absence of a chemical catalyst, in closely comparable diastereoisomer ratios and with ρ values which correspond nicely with the ρ values observed for equipotential aminium salt catalysts.

Electrochemical mechanistic criteria for the diagnosis and elucidation of cation radical mechanisms: the quantitative simulation of aminium salt catalysis by anodes of the same potential

Intermolecular selectivities in the cyclopropanation of a series of trans-stilbenes by ethyl diazoacetate are essentially identical when an aminium salt catalyst or an anode of approximately the same potential is used.