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 F-19- and C-13-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 1211; 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 H2SO4-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-1H-pyrazole derivative 25. The reaction of 4 with dimethyl acetylenedicarboxylate leads to pyrazole 29 + isobutene.