Pyridinylidenaminophosphines: Facile Access to Highly Electron‐Rich Phosphines

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.

Effective Control of the Electron‐donating Ability of Phosphines by using Phosphazenyl and Phosphoniumylidyl Substituents

“…Note that the determination of the donor strength of 3 using a solid sample of [Ni(CO) 3 ( 3 )] gives an even lower TEP value of 2037.1 cm –1 . It has been observed that the interaction of the basic nitrogen atoms adjacent to the phosphorus atom with acidic CH protons such as those in dichloromethane can increase the TEP value , …”

“…It has been observed that the interaction of the basic nitrogen atoms adjacent to the phosphorus atom with acidic CH protons such as those in dichloromethane can increase the TEP value. [15,32] In order to compare the stability of the phosphines towards oxidation with molecular oxygen, phosphines 1a and 3 were exposed to an oxygen atmosphere at room temperature. 31 P NMR analysis of the reaction mixture showed partial decomposition (30 %) of 1a and complete decomposition of 3 after nine hours (see the Supporting Information).…”

“…We recently expanded this concept to pyridinylidenamino phosphines (PyAPs) which are accessible in a much shorter synthetic route than IAPs starting from commercially available ARTICLE aminopyridines and chlorophosphines (Scheme 1a). [15] The potential of IAPs as strongly donating ligands in catalysis has been demonstrated in Au-catalyzed hydroamination reactions and Pd-catalyzed cross-coupling reactions. [11,16] Moreover, owing to their highly basic character, IAPs became valuable tools in stoichiometric and catalytic phosphine-mediated transformations.…”

Easily Prepared Mono(N,N‐dialkylamino)phosphine Palladium(II) Complexes: Structural and Catalytic Evaluation

“…[22] This class of ligands allows for easy tuning of donor and steric capacities by varying substituents on the N-(abundance of amines, including easily accessible imidazol-2-ylidene-and pyridinylide amines), the Pcenter or both. [23][24][25][26][27][28][29] Arylamino groups attenuate the basicity of the P-donor atom through resonance and σ-electron withdrawal, while aliphatic amino groups increase the basicity due to the absence of the π-system. [30] This was idependently demonstrated by Ziółkowski [31] and Woollins, [32] studying the electronic properties of N-pyrrolyl-and N-pyrrolidinyl-phosphines employing v CO of trans-[RhCl(CO)L 2 ], respectively.…”

Iminophosphorano‐Substituted Bispyridinylidenes: Redox Potentials and Substituent Constants from Tolman Electronic Parameters

“…In terms of weaker reductants, this equationy ields E 1/2 values for 8R of À0.76 Vf or R = F( TEP = 2110.8 cm À1 ), or À1.01 Vf or more synthetically accessible R = O-p-C 6 H 4 CN (TEP = 2092.8 cm À1 ). On the other hand, the recent reports [32][33][34][35][36][37][38] of av ariety of phosphines that are more donating than the classical benchmark PtBu 3 (TEP = 2056.1 cm À1 )p rovide new possibilities for highly reducing derivatives of 8R with pre-dicted redoxp otentials as low as À2.12 Vv s. SCE. (see Table 2).…”