Premium
[4 + + 2] Cycloadditions with an N ‐Methyl‐4 H ‐pyrazolium cation as electron‐deficient diazadiene system
Author(s) -
Frenzena Gerlinde,
Gerninghaus Christian,
MeyerDulheuer Christiane,
Paulus Erich F.,
Seitz Gunther
Publication year - 1995
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.1995199507174
Subject(s) - chemistry , tetrafluoroborate , cycloaddition , azine , pyridazine , cationic polymerization , medicinal chemistry , alkylation , adduct , yield (engineering) , stereochemistry , organic chemistry , ionic liquid , materials science , metallurgy , catalysis
3,6‐Diphenyl‐1,2,4,5‐tetrazine ( 6 ) reacts with dimethoxycarbene ( 7 ), generated from the 1,3,4‐oxadiazoline 8 , to yield the 4 H ‐pyrazole 10. This is alkylated to the N ‐methyl‐4 H ‐pyrazolium tetrafluoroborate 12 in high yield. The alkylated, cationic azine system of 12 serves as electron‐deficient diene in a [4 + + 2] cycloaddition with inverse electron demand and reacts diastereoselectively with several cyclic olefins as electron‐rich dienophiles. The initially formed cationic [4 + + 2] adduct, e.g. 15 , is unstable and rearranges probably via 16 to a racemic phthalazinecarboxylic ester of type 17. The crystal structures of the epoxyphthalazinecarboxylic ester 25 and of the trans ‐annulated cycloocta[ d ]pyridazine 27 were determined by X‐ray diffraction. In contrast to the cyclic olefins, cyclooctyne ( 28 ) reacts with the pyrazolium cation 12 to yield the cyclooctapyridiazinium tetrafluoroborate 30 via the cycloadduct 29 .