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

Abstract

Abstract‘Thiocarbonyl ylide’ 2B is accessible from 1,1,3,3‐tetramethylindanthione and CH2N2, and subsequent N2 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 CDCl3 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 CDCl3, is slowly converted to 11B and 12. Kinetics studies are in accordance with the zwitterionic intermediate 5B in gauche and anti conformations. In CD3CN, 4B enters into a third parallel reaction, which leads to a formal adduct with CD3CN. 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 CD3CN 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 19F‐NMR spectroscopy.