Cyclic Seven‐Membered Ketene Imines from Hindered ‘Thiocarbonyl Ylides’ and 2,3‐Bis(trifluoromethyl)fumaronitrile: Properties and Surprising Reactions

‘Thiocarbonyl ylide’ 2B is accessible from 1,1,3,3‐tetramethylindanthione and CH 2 N 2 , and subsequent N 2 extrusion at 40–50°. In situ cycloaddition of the sterically hindered 2B with 2,3‐bis(trifluoromethyl)fumaronitrile (( E )‐ 3 ) affords the isolable spirocyclic ketene imine 4B in high yield. X‐Ray analysis discloses angle deformation, with ring strain being responsible for the high reactivity of 4B . In CDCl 3 at 40°, 4B isomerizes to the trans‐ spirothiolane 6B and fragments to 1,2‐bis(trifluoromethyl)cyclopropane‐1,2‐dicarbonitrile ( 12 ) (+thioketone 11B ) in parallel reactions with a ratio of ca. 4:1. Van der Waals strain limits the thermal stability of 6B , which, at 60° in CDCl 3 , is slowly converted to 11B and 12 . Kinetics studies are in accordance with the zwitterionic intermediate 5B in gauche and anti conformations. In CD 3 CN, 4B enters into a third parallel reaction, which leads to a formal adduct with CD 3 CN. X‐Ray analysis reveals the amidine 13 ; this deep‐seated, unexpected structural change is rationalized with the assumption of a nine‐membered cyclic intermediate. In CD 3 CN at 80°, 13 and 12 (+ 11B ) are stable and occur in a ratio of ca. 1 : 1. Hindered ‘thiocarbonyl ylides’ 2C and 2D produce, in reactions with ( E )‐ 3 or ( Z )‐ 3 , the cyclic ketene imines 4C and 4D , respectively, which likewise isomerize to trans ‐thiolanes 6C and 6D . In contrast to 6B and 6C , the less‐hindered thiolane 6D is thermostable and does not fragment; it undergoes trans ⇌ cis equilibration in PhCN at 139°. Structural assignments of thiolanes are based on 19 F‐NMR spectroscopy.