Ylide‐Functionalized Phosphines: Strong Donor Ligands for Homogeneous Catalysis

Abstract

Phosphines are important ligands in homogenous catalysis and have been crucial for many advances, such as in cross‐coupling, hydrofunctionalization, or hydrogenation reactions. Herein we report the synthesis and application of a novel class of phosphines bearing ylide substituents. These phosphines are easily accessible via different synthetic routes from commercially available starting materials. Owing to the extra donation from the ylide group to the phosphorus center the ligands are unusually electron‐rich and can thus function as strong electron donors. The donor capacity surpasses that of commonly used phosphines and carbenes and can easily be tuned by changing the substitution pattern at the ylidic carbon atom. The huge potential of ylide‐functionalized phosphines in catalysis is demonstrated by their use in gold catalysis. Excellent performance at low catalyst loadings under mild reaction conditions is thus seen in different types of transformations.

Pyridinylidenaminophosphines: Facile Access to Highly Electron‐Rich Phosphines

“…[15][16][17][18][19][20] More recently, the family of highly electron-rich phosphines with p-donor substituents has been extended by the groups of Gessner and Sundermayer using phosphoniumylidyl (R 3 P= CRÀ) and phosphazenyl (R 3 P=NÀ) groups, respectively. [21,22] Although strongly donating phosphines have great potential as ligands in coordination chemistry and catalysis, [12,23] their broad application as ligands, but more importantly in stoichiometric reactions, is often hampered by their rather difficult synthesis. In this respect, readily available, cheap phosphines like PPh 3 or P(nBu) 3 are typically used in phosphine-mediated transformations such as Wittig, [24] Mitzunobu, [25] Appel, [26] or Staudinger [27] reactions.Given these considerations, we envisaged that pyridinylidenaminophosphines (PyAPs) might be a potentially very useful family of electron-rich phosphines owing to the following beneficial factors: 1) Aminopyridines are commercially available, cheap compounds which should enable a very short synthetic route to aminopyridin-substituted phosphines; 2) the pyridinylidenamino groups can be regarded as remote carbene analogues of imidazoline-2-ylidenamino groups and should therefore similarly enhance the electron density at the phosphorus atom; 3) The selection of the R group at the pyridine N atom and the position relative to the exocyclic N should provide an easy means for stereoelectronic finetuning of the resulting phosphines (Figure 1 a).With respect to the straightforward access, it is surprising that very little is known about the synthesis of PyAPs and their properties are unexplored: Nifantyev and co-workers prepared two PyAPs from the reaction of 1-ethylpyridin-2-imine with dialkylchlorophosphines when they studied the prototropic equilibrium of phosphorylated aminopyridines (Figure 1 c).…”

“…[15][16][17][18][19][20] More recently, the family of highly electron-rich phosphines with p-donor substituents has been extended by the groups of Gessner and Sundermayer using phosphoniumylidyl (R 3 P= CRÀ) and phosphazenyl (R 3 P=NÀ) groups, respectively. [21,22] Although strongly donating phosphines have great potential as ligands in coordination chemistry and catalysis, [12,23] their broad application as ligands, but more importantly in stoichiometric reactions, is often hampered by their rather difficult synthesis. In this respect, readily available, cheap phosphines like PPh 3 or P(nBu) 3 are typically used in phosphine-mediated transformations such as Wittig, [24] Mitzunobu, [25] Appel, [26] or Staudinger [27] reactions.…”

Group 9 and 10 Metal Complexes of an Ylide-Substituted Phosphine: Coordination versus Cyclometalation and Oxidative Addition

“…Preliminary attempts to determine the Tolman electronic parameter of 1 via synthesis of the corresponding nickel tricarbonyl complex gave no reliable data. 7 Thus, the isolation of the corresponding complex was attempted. Treatment of Y S PPh 2 with 1 equiv of [Ni(CO) 4 ] at first resulted in the expected formation of the complex [( κP - 1 )Ni(CO) 3 ] as evidenced by NMR spectroscopy (Scheme 1).…”

“…13 The CO stretching frequency of 2060.2 cm –1 determined by IR spectroscopy confirms the strong donor property of the YPhos ligand, as recently reported based on the [LRh(acac)Cl] complex. 7 The stronger donor property of 2 compared to 1 (TEP( 1 ) = 2066.5 cm –1 ) is probably also the reason for the selective formation of the tricarbonyl complex. Due to the resulting higher electron density at nickel the carbonyl ligands should be stronger bound in [( κP - 2 )Ni(CO) 3 ] than in [( κP - 1 )Ni(CO) 3 ] and thus more difficult to be replaced by arene coordination.…”

“…Recently, we reported on ylide-substituted phosphines (YPhos), formed by incorporation of an ylide group directly at the phosphorus center. 7 Due to the strong electron donation from the ylide these phosphines become more electron-rich than classical phosphines with donor strengths comparable to those of N -heterocyclic carbenes. 8 The YPhos ligands turned out to be ideal supporting ligands for transition metal catalysts and showed excellent performance in gold catalyzed hydroamination reactions as well as Buchwald Hartwig aminations under mild reaction conditions.…”