New Phosphoranylideneketenes and Phosphoranylidenethioketenes‐Synthesis, Structure, Spectroscopy, and Lewis Base Properties

Posphoranylideneketene Ph 3 PCCO and its sulfur analogue, Ph 3 PCCS, are cumulated ylides that exhibit diverse reactivity and distinct structural features. Herein, we present the first systematic study of substituent effects on the structures and bonding of a series of aryl‐ and alkyl‐substituted phosphoranylideneketenes, R 3 PCCO [R = Et, cyclohexyl, NMe 2 , 4‐Me 2 N‐C 6 H 4 , 4‐MeO‐C 6 H 4 , 4‐CF 3 ‐C 6 H 4 , 3,5‐(CF 3 ) 2 ‐C 6 H 3 ], prepared from the corresponding phosphonium salts, [R 3 PCH 2 COOEt]X (X = Br, I) and two equivalents of Na[N(SiMe 3 ) 2 ]. Multinuclear NMR and IR spectroscopy, supported by single‐crystal X‐ray diffraction, revealed that the bond strength within the central P═C═C═O fragment increases with the electron‐deficiency of the aryl substituents at phosphorus. The corresponding phosphoranylidenethioketenes, R 3 PCCS (R = Et, cyclohexyl, NMe 2 , 4‐Me 2 N‐C 6 H 4 , 4‐MeO‐C 6 H 4 , 4‐CF 3 ‐C 6 H 4 ), were obtained by reactions of R 3 PCCO with CS 2 . The rate of conversion of R 3 PCCO into R 3 PCCS decreases with increasing electron deficiency at phosphorus. Both R 3 PCCO and R 3 PCCS act as relatively weak ambidentate Lewis donors, yet they form stable acid‐base adducts with strong Lewis acids such as B(C 6 F 5 ) 3 and Al(C 6 F 5 ) 3 .