Abstract
The present work describes the synthesis of mono- and bis-pyrazole compounds bearing flexible p-tolyl ether and rigid 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene backbones as pyrazolyl analogues of DPEphos and Xantphos ligands, respectively. The synthesis of new pyrazolyl analogues was accomplished following an Ullmann coupling protocol, and the resulting products were isolated in overall good yields. In addition, a hybrid imidazolyl–pyrazolyl analogue bearing a xanthene backbone was synthesized using the same protocol, whereas a hybrid selanyl–pyrazolyl analogue with a xanthene backbone was synthesized in a good yield employing a second C–H activation step. The symmetrical bis-pyrazolyl and the hybrid imidazolyl–pyrazolyl analogues were found to be the most active among the new ligands evaluated in the Pd-catalysed Suzuki-Miyaura cross-coupling of aryl halides with aryl boronic acids. A simple catalytic system based on Pd(OAc)2/2a was developed, which efficiently catalyses the Suzuki–Miyaura reaction of aryl halides and aryl boronic acids and provides moderate to excellent yields of the corresponding cross-coupling products.
Highlights
Transition metal complexes of phosphorous (P), nitrogen (N), sulphur (S), and oxygen (O)-based ligands and their hybrids have provided chemists with a wide opportunity and a quasi-exhaustive tool for creating C–C bonds of significant interest [1,2]
A two-step coupling reaction sequence was designed in order to two-stepbromination/Ullmann bromination/Ullmann coupling reaction sequence was designed insynthesize order to the new pyrazolyl analogue of p-tolyl ether was chosen as the starting synthesize the new pyrazolyl analogue of DPEphos (Scheme 1). p-tolyl ether was chosen as the material to avoid the o/p selectivity issues in the bromination step through blockage of starting material to avoid the o/p selectivity issues in the bromination step through blockage of thethe pp-position
This work highlights the synthesis of new mono- and bis-pyrazole compounds bearing a flexible p-tolyl ether or a rigid xanthene backbone as pyrazolyl analogues of DPEphos and Xantphos ligands, respectively, as well as their potential as ligands leading to active catalyst systems for the Pd-catalysed Suzuki–Miyaura reaction
Summary
Transition metal complexes of phosphorous (P), nitrogen (N), sulphur (S), and oxygen (O)-based ligands and their hybrids have provided chemists with a wide opportunity and a quasi-exhaustive tool for creating C–C bonds of significant interest [1,2]. The fine-tuning of these properties in order to synthesize ligands of particular interest has been an interesting strategy. In this perspective, a plethora of ligands with flexible backbones, such as DPEphos, rigid backbones containing bulky substituents, such as t Bu-Xantphos, and other xanthene scaffolds have been widely described in the literature [5,6,7]. Since these xanthene-derived ligands possess wide bite angles, they can coordinate with a variety of metals, and the corresponding metal
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