Synthesis of Transient and Stable C-Amino Phosphorus Ylides and Their Fragmentation into Transient and Stable Carbenes

Abstract

Only basic phosphines, such as tris(dimethylamino)phosphine, allow for the synthesis of a stable acyclic beta-amino phosphonium salt 1c, which upon deprotonation with butyllithium affords the corresponding stable C-amino phosphorus ylide 2c. In contrast, cyclic beta-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.

Imidazoliophosphines are True N‐Heterocyclic Carbene (NHC)–Phosphenium Adducts

“…Displacement of a NHC unit from the free phosphenium end of BIMIONAP (1) by a chloride ion is expected to afford chlorodiphenylphosphine (Ph 2 PCl), which can be easily identified by 31 P NMR spectroscopy. The relevance of such a transformation in the field of coordination chemistry is a priori supported by evidence for the dative character of P-halogen bonds.…”

“…At 50 8C, however, a slow degradation of 2 was observed. Monitoring the reaction by 31 P NMR spectroscopy indicated the complete disappearance of the signals of 2 (d p = + + 18.8, + 24.0 ppm) after 3 h and the appearance of two new signals at d p = + + 23.2 and + 80.0 ppm. The latter chemical shift is characteristic of ClPPh 2 , whereas the former could be consistently attributed to the NHC-phosphine palladium complex 3 (Scheme 3).…”

“…The dicationic complex 4 (4 a + 4 b) was then reacted with TEACl in MeCN, and by contrast to complex 2, the reaction proceeded at room temperature, due to the enhanced electrophilicity of 4. Monitoring the reaction by 31 P NMR spectroscopy indicated the disappearance of 4 and the appearance of two new signals at d P = + + 17.76 (d) and + 15.13 ppm (d) with a large P-P coupling constant (J PP = 457.9 Hz), along with the signals of the above described complex 3. After column chromatography, the complexes 3 and 5 were isolated in 29 and 60 % yield, respectively.…”

“…To trap the expected but elusive free NHC product, the reaction was carried out in the presence of elemental sulfur (S 8 ), a classical trapping agent of carbenes (Scheme 6). Monitoring the reaction by 31 P NMR spectroscopy indicated the immediate disappearance of the starting BIMIONAP (1) and the presence of two new signals at dp = + + 42.7 and + 43.5 ppm. After purification, these two signals were assigned to thioA C H T U N G T R E N N U N G (butyl)diphenylphosphine and thiophosphinethiourea 7 which was isolated in 71 % yield, respectively.…”

“…Alternatively, and especially for the nBuLi nucleophile from which the a-elimination process is less likely, the initial attack could occur at the phosphorus center to give a neutral phosphonium C-diamino-ylide intermediate (Scheme 7b). The C-diA C H T U N G T R E N N U N G amino-ylide being highly destabilized, [31] it could then undergo dissociation to the neutral phosphine and the neutral diA C H T U N G T R E N N U N G aminocarbene, which would be stabilized by protonation, sulfurization, or hydrolysis (Scheme 7b).…”