Abstract
The synthesis of a small family of six electronically and sterically modified 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene gold(I) chloride complexes is described. Additionally, the corresponding trans-[PdBr2(iPr2-bimy)(1,3,4-trisubstituted 1,2,3-triazol-5-ylidene)] complexes are also generated and used to examine the donor strength of the 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene ligands. All compounds have been characterized by 1H and 13C NMR and IR spectroscopy, high-resolution electrospray mass spectrometry (HR-ESI-MS), and elemental analysis. The molecular structures of four of the gold(I) and four of the palladium(II) complexes were determined using X-ray crystallography. Finally, it is demonstrated that these 1,2,3-triazol-5-ylidene gold(I) chloride complexes (Au(trz)Cl) are able to catalyze the cycloisomerization of 1,6-enynes, in high yield and regioselectivity, as well as the intermolecular direct etherification of allylic alcohols. Exploiting the Au(trz)Cl precatalysts allowed the etherification of allylic alcohols to be carried out under milder conditions, with better yield and regioselectivity than selected commercially available gold(I) catalysts.
Highlights
In the past two decades N-heterocyclic carbenes (NHCs) have become the ligands of choice for new catalyst development.[1]
Trisubstituted 1,2,3-triazol-5-ylidenes are some of the most recent additions to the NHC family.[7]. These NHCs have been termed abnormal NHCs/mesoionic carbenes (MICs) because no sensible uncharged resonance structures can be generated for these systems.[8]
Because of our interest in “click” coordination chemistry,[17] we recently reported the synthesis of the Au(trz)Cl complex 7a (Scheme 1) and showed that it was catalytically active.[18]
Summary
In the past two decades N-heterocyclic carbenes (NHCs) have become the ligands of choice for new catalyst development.[1]. 2-ylidenes (A) and the related 1,2,4-triazol-5-ylidenes (B) can be readily sterically modified, the electron-donating ability of these carbenes can only be tuned over a narrow range To overcome this limitation, a vast array of NHCs have been generated in the past 15 years, including ring-expanded carbenes (C and D),[3] cyclic alkylaminocarbenes (CAAC, E),[4] pyrid-2-ylidenes (F),[5] pyrid-4-ylidenes (G),[5] pyrazol-4-ylidenes (H),[6] and imidazol-4-ylidenes (I) (Figure 1a). The effect of this systematic modification of the Au(trz)Cl complexes is explored in two different gold(I)-catalyzed reactions: (1) the cyclo-
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