Abstract
Two bulky triazolium salts, namely 1-{4(24),6(10),12(16),18(22)-tetramethylenedioxy- 2,8,14,20-tetrapentylresorcin[4]arene-5-yl}-4-phenyl-3-methyl-1H-1,2,3-triazolium tetrafluoro borate (1) and 1,4-bis{4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentyl resorcin[4]arene-5-yl}-3-methyl-1H-1,2,3-triazolium iodide (2), have been synthesized and assessed in the palladium-catalyzed Suzuki–Miyaura cross-coupling of aryl chlorides, with aryl boronic acids. As a general trend, the reaction rates obtained with 1 were significantly higher (up to 5 times) than those observed for 2, this mainly reflected a sterically more accessible metal center in the catalytic intermediates formed with 1. The presence of flexible pentyl chains in these intermediates, which might sterically interact with the metal center, when the latter adopts an exo-orientation with respect to the cavity, were likely responsible for the observed good performance.
Highlights
In the last two decades N-heterocyclic carbenes (NHCs) have emerged as powerful ligands for the palladium-catalyzed Suzuki–Miyaura cross-coupling reactions [1,2]
The resorcin[4]arene-derived triazolium salts 1 and 2 were synthesized, stepwise, according to the sequences shown in Scheme 1
We have described the first triazolium salts substituted by resorcinarene units (1 and 2)
Summary
In the last two decades N-heterocyclic carbenes (NHCs) have emerged as powerful ligands for the palladium-catalyzed Suzuki–Miyaura cross-coupling reactions [1,2] Their performance mainly relies on their strong σ-donor properties, generally considered to be superior to that of phosphines, and relies on the ease with which they can be made sterically bulky [3], this, generally being achieved by tethering appropriate substituents on their nitrogen atoms. Often referred to as abnormal (aNHCs) or mesoionic carbenes (MICs), such ligands typically display a stronger electron donating capacity, when compared to that of the classical NHCs. Often referred to as abnormal (aNHCs) or mesoionic carbenes (MICs), such ligands typically display a stronger electron donating capacity, when compared to that of the classical NHCs In this context, following the pioneering work of Albrecht [4] (who synthesized the first aNHC-transition metal complexes) and Bertrand [5] (who isolated the first free, non-conventional carbenes), 1,2,3-triazol-5-ylidenes (tzNHCs) have been studied extensively. Their precursors, namely 1,2,3-triazoles, are accessible through copper-catalyzed Huisgen [3 + 2]
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