Nucleophile Singulett‐Carbene in der [4 + 1]1‐Cycloaddition mit 1,2,4,5‐Tetrazinen: Eine neue Isopyrazol‐Synthese

Nucleophilic Singlet Carbenes in the [4 + 1] Cycloaddition with 1,2,4,5‐Tetrazines: a New Synthesis of Isopyrazoles A series of 3,6‐disubstituted 1,2,4,5‐tetrazines 6 , including C 6 H 5 , SCH 3 , SO 2 CH 3 , N(CH 3 ) 2 , CF 3 , CO 2 CH 3 groups, has been submitted to [4 + 1] cycloaddition with the nucleophilic singlet carbenes 4, 15 , and 20 , which are generated from the precursors 1, 11 and 19 , resp. In all cases isopyrazoles (4 H 1‐pyrazoles) 9, 10, 18 , and 21 are isolated in good yields. They are formed in a two‐step reaction sequence with the [4 + 1] cycloadducts of type 8 as intermediates which eliminate nitrogen by subsequent [4 + 2] cycloreversion. The acceptor‐substituted isopyrazole 9e is characterized as an electron‐deficient diene by some Diels‐Alder reactions with inverse electron demand leading to the expected azo‐bridged cycloadducts 23, 25, 27, 29 , and 31 without acid catalysis or application of higher pressure. With cyclooctyne ( 32 ), the [4 + 2] cycloaddition is followed by a [4 + 2] cycloreversion with formation of the cyclopentacyclooctene 34 .