Abstract
Electron‐rich tertiary phosphines are valuable species in chemical synthesis. However, their broad application as ligands in catalysis and reagents in stoichiometric reactions is often limited by their costly synthesis. Herein, we report the synthesis and properties of a series of phosphines with 1‐alkylpyridin‐4‐ylidenamino and 1‐alkylpyridin‐2‐ylidenamino substituents that are accessible in a very short and scalable route starting from commercially available aminopyridines and chlorophosphines. The determination of the Tolman electronic parameter (TEP) value reveals that the electron donor ability can be tuned by the substituent pattern at the aminopyridine backbone and it can exceed that of common alkylphosphines and N‐heterocyclic carbenes. The potential of the new phosphines as strong nucleophiles in phosphine‐mediated transformations is demonstrated by the formation of Lewis base adducts with CO2 and CS2. In addition, the coordination chemistry of the new phosphines towards CuI, AuI, and PdII metal centers has been explored, and a convenient procedure to introduce the most basic phosphine into metal complexes starting from air‐stable phosphonium salt is described.
Highlights
Electron-rich tertiary phosphines are valuable species in chemical synthesis
We discovered that the electron-donating ability of phosphines can be significantly increased, beyond the range of N-heterocyclic carbenes,[13] by attaching up to three strong p-donating imidazoline-2-ylidenamino substituents to the phosphorus atom.[14]
The family of highly electron-rich phosphines with p-donor substituents has been extended by the groups of Gessner and Sundermayer using phosphoniumylidyl (R3P= CRÀ) and phosphazenyl (R3P=NÀ) groups, respectively.[21,22]
Summary
[a] Values in cmÀ1 recorded in CH2Cl2 solution. [b] Literature values.[14,19] [c] Recorded using solid samples. We recently showed that the p-donor ability of substituents R5 strongly responds to the interaction of the exocyclic nitrogen atom with protons.[16] To exclude that the TEP values might be influenced by such interactions with CH protons in dichloromethane, we recorded the IR spectra of the nickel complexes of phosphines 2 a, 4 a, and 4 d in the solid state. The decrease in TEP values observed for the solid compounds (4 a: DTEP = 1.3 cm, 4 d: DTEP = 1.7, 2 a: DTEP = 2.9 cmÀ1) suggests that the TEP values in CH2Cl2 are affected by the steric accessibility of the exocyclic nitrogen atom. CS2, a new 31P NMR resonance appeared at 28.7 ppm which is significantly highfield shifted compared with the free phosphine 2 d (d = 56.4 ppm). Given the combination of exceptional easy synthesis and highly basic character, PyAPs are appealing ligands in coordination chemistry and catalysis, and provide new opportunities for stoichiometric phosphine-mediated transformations
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