Reactions of Methyl Diazoacetate with ( E )‐ and ( Z )‐1,2‐Bis(trifluoromethyl)ethene‐1,2‐dicarbonitrile: Novel and Unanticipated Pathways

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‐1 H ‐pyrazoles 13 . The retention of dipolarophile configuration proceeds for ( E )‐ BTE with > 99.93% and for ( Z )‐ BTE with > 99.8% (CDCl 3 , 25°), 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‐1 H ‐pyrazoles. Competing with the cis / trans isomerization of 13 is the formation of a bis(dehydrofluoro) dimer (two diastereoisomers), 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 − 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 carbanionic substituent. Mechanistic comparison with the reaction of diazomethane and dimethyl 2,3‐dicyanofumarate, a related tetra‐acceptor‐ethylene, brings to light unanticipated divergencies.