Dedicated to Mordecai Rubin, Technion, Haifa, on the occasion of his 80th birthday
The cycloadditions of methyl diazoacetate to 2,3-bis(trifluoromethyl)fumaronitrile ((E)-BTE) and 2,3-bis(trifluoromethyl)maleonitrile ((Z)-BTE) furnish the 4,5-dihydro-1H-pyrazoles 13. The retention of dipolarophile configuration proceeds for (E)-BTE with > 99.93% and for (Z)-BTE with > 99.8% (CDCl 3 , 258), suggesting concertedness. Base catalysis (1,4-diazabicyclo[2.2.2]octane (DABCO), proton sponge) converts the cycloadducts, trans-13 and cis-13, to a 94 : 6 equilibrium mixture (CDCl 3 , r.t.); the first step is N-deprotonation, since reaction with methyl fluorosulfonate affords the 4,5-dihydro-1-methyl-1H-pyrazoles. Competing with the cis/trans isomerization of 13 is the formation of a bis(dehydrofluoro) dimer (two diastereoisomers), 2 C 9 H 4 F 6 N 4 O 2 (13) ! C 18 H 6 F 10 N 8 O 4 (22) + 2 HF, the structure of which was elucidated by IR, 19 F-NMR, and 13 C-NMR spectroscopy. The reaction slows when DABCO is bound by HF, but F À as base keeps the conversion to 22 going and binds HF. The diazo group in 22 suggests a common intermediate for cis/trans isomerization of 13 and conversion to 22: reversible ring opening of N-deprotonated 13 provides 18, a derivative of methyl diazoacetate with a carban-A C H T U N G T R E N N U N G ionic substituent. Mechanistic comparison with the reaction of diazomethane and dimethyl 2,3-dicyanofumarate, a related tetra-acceptor-ethylene, brings to light unanticipated divergencies.