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Addition and Cycloaddition Reactions of Arenediazonium Ions with 1,3‐Dienes: A Shift From a Concerted to a Stepwise Mechanism
Author(s) -
Hartnagel Manfred,
Grimm Klaus,
Mayr Herbert
Publication year - 1996
Publication title -
liebigs annalen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0947-3440
DOI - 10.1002/jlac.199719970112
Subject(s) - cycloaddition , chemistry , regioselectivity , electrophile , acetonitrile , medicinal chemistry , ion , photochemistry , organic chemistry , catalysis
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.