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On the Reaction of 1‐Aza‐2‐azoniaallene Salts with Acetylenes
Author(s) -
Wang Quanrui,
AlTalib Mahmoud,
Jochims Johannes C.
Publication year - 1994
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19941270313
Subject(s) - chemistry , cycloaddition , geminal , steric effects , protonation , medicinal chemistry , 1,3 dipolar cycloaddition , alkyl , hydrogen atom , stereochemistry , organic chemistry , catalysis , ion
1‐Aza‐2‐azoniaallene cations 3 , prepared in situ from geminal chloro(alkylazo) compounds 2 , react with acetylenes 4 to give either 1 H ‐pyrazolium salts 6 or 4 H ‐pyrazolium salts 7 or mixtures of both. 4 H ‐Pyrazolium salts with a hydrogen atom attached to C(4) rearrange to the protonated 1 H ‐pyrazoles 8 , from which the free bases 9 are obtained upon treatment with aqueous NaOH. According to AM1 calculations the cycloaddition of acetylenes to 1‐aza‐2‐azoniaallene cations is a concerted process, which can be classified as a “1,3‐dipolar cycloaddition with reverse electron demand”. The cycloaddition forming the intermediates 5 is followed by an [1,2] alkyl shift to furnish the final products 6–8. The direction of the [1,2] shift has been found to be governed by subtle steric effects.