[2++4] Cycloadditions of Iminium Ions − Concerted or Stepwise Mechanism of Aza Diels−Alder Reactions?

Abstract

Keywords: Carbocations / Cycloadditions / Kinetics / Linear free-energy relationships / Reaction mechanismsThe N,N-dimethylmethyleneammonium ion 1 reacts regioand stereoselectively with the 1,3-dienes 2a−2f to yield the 1,2,5,6-tetrahydropyridinium ions 3. The kinetics of these hetero Diels−Alder reactions, which have been followed by dilatometry and 1 H-NMR spectroscopy, obey second-order rate laws. Since the observed rate constants are only 30−300 times larger than those calculated for the stepwise process by the linear free-enthalpy relationship lg k = s (E + N), it is

Mechanistic studies on the formal aza-Diels–Alder reactions of N-aryl imines: evidence for the non-concertedness under Lewis-acid catalysed conditions

“…They concluded that a reaction between cyclopentadiene and protonated N-methylpyridine-2carboxaldehyde imine proceeded by a stepwise mechanism according to theoretical calculations. Such conclusions are also supported by Sauer et al, 17 who proposed that such aza-Diels-Alder reactions proceeded through transition structures involving allyl cations. A more recent Density Functional Theory (DFT) calculation on an iminium ion reacting with cyclopentadiene 18 also concludes that the reaction "takes place along a highly asynchronous concerted process characterised by the nucleophilic attack of the cyclopentadiene on the ylidene ammonium cation instead of a pericyclic process."…”

Mechanistic studies on the formal aza-Diels–Alder reactions of N-aryl imines: evidence for the non-concertedness under Lewis-acid catalysed conditions

“…They concluded that a reaction between cyclopentadiene and protonated N-methylpyridine-2carboxaldehyde imine proceeded by a stepwise mechanism according to theoretical calculations. Such conclusions are also supported by Sauer et al, 17 who proposed that such aza-Diels-Alder reactions proceeded through transition structures involving allyl cations. A more recent Density Functional Theory (DFT) calculation on an iminium ion reacting with cyclopentadiene 18 also concludes that the reaction "takes place along a highly asynchronous concerted process characterised by the nucleophilic attack of the cyclopentadiene on the ylidene ammonium cation instead of a pericyclic process."…”

Electrophilicity index revisited

“…However, it is highly unlikely that diverse molecules with varied range of structural, electronic and bonding properties will accommodate a universal model/scale for electrophilicity and nucleophilicity. Several works reported by Mayr and co-workers [24][25][26][27][28][29][30][31][32][33][34] have illustrated this fact. Their contribution [32][33][34][35][36][37][38][39] has been to describe the absolute rate constants of various electrophile-nucleophile reactions in terms of the electrophilicity (E) and nucleophilicity (N) parameters along with a nucleophile-specific slope parameter s (Equation 13).…”

Understanding the Mechanism of Diels–Alder Reactions with Anionic Dienophiles: A Systematic Comparison of [ECX] − (E = P, As; X = O, S, Se) Anions

“…A sy of the TSs also offers information on the localization of the interactions. The high A sy is typically associated with the polar or ionic nature and is the result of a nonsymmetrical substitution pattern of the diene and/or dienophile. ,− …”

Mechanistic studies on the formal aza-Diels–Alder reactions of N-aryl imines: evidence for the non-concertedness under Lewis-acid catalysed conditions

“…They concluded that a reaction between cyclopentadiene and protonated N-methylpyridine-2carboxaldehyde imine proceeded by a stepwise mechanism according to theoretical calculations. Such conclusions are also supported by Sauer et al, 17 who proposed that such aza-Diels-Alder reactions proceeded through transition structures involving allyl cations. A more recent Density Functional Theory (DFT) calculation on an iminium ion reacting with cyclopentadiene 18 also concludes that the reaction "takes place along a highly asynchronous concerted process characterised by the nucleophilic attack of the cyclopentadiene on the ylidene ammonium cation instead of a pericyclic process."…”

Electrophilicity index revisited

“…However, it is highly unlikely that diverse molecules with varied range of structural, electronic and bonding properties will accommodate a universal model/scale for electrophilicity and nucleophilicity. Several works reported by Mayr and co-workers [24][25][26][27][28][29][30][31][32][33][34] have illustrated this fact. Their contribution [32][33][34][35][36][37][38][39] has been to describe the absolute rate constants of various electrophile-nucleophile reactions in terms of the electrophilicity (E) and nucleophilicity (N) parameters along with a nucleophile-specific slope parameter s (Equation 13).…”

Understanding the Mechanism of Diels–Alder Reactions with Anionic Dienophiles: A Systematic Comparison of [ECX] − (E = P, As; X = O, S, Se) Anions

“…A sy of the TSs also offers information on the localization of the interactions. The high A sy is typically associated with the polar or ionic nature and is the result of a nonsymmetrical substitution pattern of the diene and/or dienophile. ,− …”

Mechanistic studies on the formal aza-Diels–Alder reactions of N-aryl imines: evidence for the non-concertedness under Lewis-acid catalysed conditions

“…They concluded that a reaction between cyclopentadiene and protonated N-methylpyridine-2carboxaldehyde imine proceeded by a stepwise mechanism according to theoretical calculations. Such conclusions are also supported by Sauer et al, 17 who proposed that such aza-Diels-Alder reactions proceeded through transition structures involving allyl cations. A more recent Density Functional Theory (DFT) calculation on an iminium ion reacting with cyclopentadiene 18 also concludes that the reaction "takes place along a highly asynchronous concerted process characterised by the nucleophilic attack of the cyclopentadiene on the ylidene ammonium cation instead of a pericyclic process."…”

Electrophilicity index revisited

“…However, it is highly unlikely that diverse molecules with varied range of structural, electronic and bonding properties will accommodate a universal model/scale for electrophilicity and nucleophilicity. Several works reported by Mayr and co-workers [24][25][26][27][28][29][30][31][32][33][34] have illustrated this fact. Their contribution [32][33][34][35][36][37][38][39] has been to describe the absolute rate constants of various electrophile-nucleophile reactions in terms of the electrophilicity (E) and nucleophilicity (N) parameters along with a nucleophile-specific slope parameter s (Equation 13).…”

Understanding the Mechanism of Diels–Alder Reactions with Anionic Dienophiles: A Systematic Comparison of [ECX] − (E = P, As; X = O, S, Se) Anions

“…A sy of the TSs also offers information on the localization of the interactions. The high A sy is typically associated with the polar or ionic nature and is the result of a nonsymmetrical substitution pattern of the diene and/or dienophile. ,− …”