Synthesis of Transient and Stable <i>C</i>-Amino Phosphorus Ylides and Their Fragmentation into Transient and Stable Carbenes

Only basic phosphines, such as tris(dimethylamino)phosphine, allow for the synthesis of a stable
acyclic β-amino phosphonium salt 1c, which upon deprotonation with butyllithium affords the corresponding
stable C-amino phosphorus ylide 2c. In contrast, cyclic β-amino phosphonium salts 5a and 5b are stable
despite the presence of weakly basic triarylphosphine fragments. They are prepared by intramolecular
insertion of the carbene center of (amino)(phosphonio)carbenes into the CH bond of a phosphorus
substituent. Deprotonation of 5a leads to the corresponding cyclic C-amino phosphorus ylide 6a, which
has been fully characterized including an X-ray diffraction study. Deprotonation of 5b affords enamine 8,
probably via fragmentation of ylide 6b into transient carbene 7b and a subsequent 1,2-hydrogen shift.
Transient cyclic C-amino phosphorus ylides 6c and 6d have been prepared by intramolecular addition of
a carbanion generated by deprotonation of a phosphorus substituent. Three-membered heterocycle 6c
rearranges into alkene 9, whereas the four-membered ring system undergoes a ring opening affording the
stable carbene 7d. The latter results pave the route for the synthesis of various mixed carbene−phosphine
bidentate ligands.