Premium
[2 + +4] Cycloadditions of Iminium Ions − Concerted or Stepwise Mechanism of Aza Diels−Alder Reactions?
Author(s) -
Mayr Herbert,
Ofial Armin R.,
Sauer Jürgen,
Schmied Bernhard
Publication year - 2000
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200006)2000:11<2013::aid-ejoc2013>3.0.co;2-a
Subject(s) - chemistry , iminium , cycloaddition , cyclopentadiene , stepwise reaction , pericyclic reaction , concerted reaction , enthalpy , medicinal chemistry , reaction mechanism , yield (engineering) , computational chemistry , diels–alder reaction , ion , photochemistry , reaction rate constant , organic chemistry , kinetics , catalysis , order of reaction , thermodynamics , physics , quantum mechanics
The N , N ‐dimethylmethyleneammonium ion 1 reacts regio‐ and 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 concluded that these cycloadditions proceed in a stepwise manner or through pericyclic transition states that are not significantly stabilized by the concerted formation of two new σ‐bonds. In contrast, the reaction of the iminium ion 1 with cyclopentadiene ( 2g ) yields 2,2‐dimethyl‐2‐azoniabicyclo[2.2.1]hept‐5‐ene ( 3g ) 2×10 4 times more rapidly than predicted for the stepwise cycloaddition process, indicating a free enthalpy of concert of 27 kJ mol −1 .