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

Abstract

The search for efficient, active and universal catalyst systems for transition-metal catalyzed cross-coupling reactions, continues to be of interest to researchers world-wide. Herein, we report two new Pd(II) complexes, effortlessly prepared from N,Ndialkylamino-phosphines for Suzuki-Miyaura cross-coupling reactions. These sufficiently hindered (% V Bur = 30.0-31.5 %) and electron rich (v CO = 1947.41-1946.29 cm À 1 ) aminophosphines formed active catalysts for Suzuki-Miyaura coupling of aryl bromides and chlorides. Palladium-catalyzed cross-coupling reactions are indispensable tools in synthesis, providing easy access to CÀ C and CÀ X (X=N, O, S, F, Si, and B) bonds. [1,2] Among them is the Suzuki-Miyaura reaction, widely used in pharmaceuticals, [3] agrochemicals, [4] natural products, [5] materials science [6] and process chemistry. [7] As the application of this versatile reaction broadens, the need for robust, highly active catalyst systems increases. Pioneering contributions in this regard have been trialkyl-, biaryl-, ferrocenyl-and adamantyl-based phosphines developed by Fu, [8] Buchwald, [9] Hartwig [10] and Beller, [11] respectively. Notable contributions have subsequently been made by Kwong, [12] Verkade, [13] Reetz, [14] Stradiotto, [15] Hong, [16] and many more.Our focus is on aminophosphines, [17,18] pioneered as ligands in Suzuki-Miyaura coupling by Woolins [19] and now applicable in other catalyst systems such as intermolecular hydroamination of alkenes, [20] alkylation of amines by alcohols, [21] and amination of aryl-substituted allylic alcohols. [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. Their studies also revealed the alkyl-based [P(NC 4 H 8 ) 3 ] (1951 cm À 1 ) as a better donor compared to its aromatic counterpart [P(NC 4 H 4 ) 3 ] (2012 cm À 1 ) (Table 1). Most notable was the finding that Pbasicity increases with decrease of amino-substituents, Ph 2 P-(NR 2 ) > PhP(NR 2 ) 2 > P(NR 2 ) 3 . This trend was further confirmed by Burrows and co-workers, [33] in their parameterization studies of N-carbazolyl phosphines which showed a decrease in donor character with increase in number of N-carbazolyl substituents in the order [PPh 2 (NC 12 H 8 )] < [PPh(NC 12 H 8 ) 2 ] < [P(NC 12 H 8 ) 3 ] (Table 1). Furthermore, Singh and co-workers identified [PPh (NEt 2 ) 2 )] as a poor donor than [PPh 2 (NEt 2 )], with electron descriptors of v CO = 2007 cm À 1 and 1974 cm À 1 , and 1 J PSe = 761 Hz an...

The bis(Biphenyl)phosphorus Fragment in Trivalent and Tetravalent P-Environments

“…Therefore, it was concluded that the biphenyl ligands sterically shield the phosphorus atom which was confirmed by single-crystal X-ray analysis (see Section 2.2). The phenyl derivative of bis(biphenyl)diisopropylamino phosphine is widely used as ligand for transition-metal catalyzed cross-coupling reactions [31][32][33] or for coordination chemistry [34,35].…”