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Addition and Cycloaddition Reactions of Arenediazonium Ions with 1,3-Dienes: A Shift From a Concerted to a Stepwise Mechanism

✍ Scribed by Hartnagel, Manfred ;Grimm, Klaus ;Mayr, Herbert

Publisher
John Wiley and Sons
Year
1997
Tongue
English
Weight
1002 KB
Volume
1997
Category
Article
ISSN
0947-3440

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✦ Synopsis

Abstract

Arenediazonium ions 1 undergo [2^+^ + 4] cycloadditions with (E)‐1,3‐pentadiene (2a), 2,3‐dimethylbutadiene (2b), and (E)‐2‐methyl‐1,3‐pentadiene (2c) to give dihydropyridazines or pyridazinium salts. While highly electrophilic diazonium ions and the unsymmetrical dienes 2a and 2c predominantly yield those regioisomers that are expected for a stepwise cycloaddition process, the opposite regioselectivity is found in the cycloadducts of less electrophilic diazonium ions. Kinetic investigations and product studies indicate that all cycloaddition reactions proceed concertedly. The dienes 2d and 2e, in contrast, undergo ordinary azo coupling reactions in acetonitrile/methanol with formation of the hydrazones 17 and the azo compounds 18, respectively. It is demonstrated that the linear free enthalpy relationship lg__k__ = s (N + E) can also be used to roughly estimate the rates of ionic cycloaddition reactions.

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