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The synthesis and characterization of a distally bridged, bidentate resorcinarene bis(carbene) palladium com- [a]

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...

Resorcin[4]arenes are not as extensively studied as their sister compounds, calix[4]arenes, owing to challenges in synthetic methodologies for their derivatization from the perceptive of molecular economy. In this tutorial review, we briefly illustrate the summary of their access methodologies together with their limitations, focusing at the derivatization of the ortho‐positions in the upper rim. We hope to spark a refreshed surge in the development of these derivatization strategies, allowing access to novel molecules and futuristic applications, putting resorcin[4]arenes at the center stage of research for which they were meant.