Reactions of Di( tert ‐butyl)diazomethane with Acceptor‐Substituted Ethylenes

Di( tert‐ butyl)diazomethane ( 4 ) is a nucleophilic 1,3‐dipole with strong steric hindrance at one terminus. In its reaction with 2,3‐bis(trifluoromethyl)fumaronitrile (( E )‐ BTE ), a highly electrophilic tetra‐acceptor‐substituted ethene, an imino‐substituted cyclopentene 9 is formed as a 1 : 2 product. The open‐chain zwitterion 10 , assumed as intermediate, adds the second molecule of ( E )‐ BTE . The 19 F‐ and 13 C‐NMR spectra allow the structural assignment of two diastereoisomers, 9A and 9B . The zwitterion 10 can also be intercepted by dimethyl 2,3‐dicyanofumarate ( 11 ) and furnishes diastereoisomeric cyclopentenes 12A and 12B ; an X‐ray‐analysis of 12B confirms the ‘mixed’ 1 : 1 : 1 product. Competing is an ( E )‐ BTE ‐catalyzed decomposition of 4 to give 2,3,4,4‐tetramethylpent‐1‐ene ( 7 )+N 2 ; the reaction of ( E )‐ BTE with a trace of water appears to be responsible for the chain initiation. The H 2 SO 4 ‐catalyzed decomposition of diazoalkane 4 , indeed, produced the alkene 7 in high yield. The attack on the hindered diazoalkane 4 by 11 is slower than that by ( E )‐ BTE ; the zwitterionic intermediate 21 undergoes cyclization and furnishes the tetrasubstituted furan 22 . In fumaronitrile, electrophilicity and steric demand are diminished, and a 1,3‐cycloaddition produces the 4,5‐dihydro‐1 H ‐pyrazole derivative 25 . The reaction of 4 with dimethyl acetylenedicarboxylate leads to pyrazole 29 +isobutene.