Dual H-bond activation of NHC–Au(i)–Cl complexes with amide functionalized side-arms assisted by H-bond donor substrates or acid additives

Abstract

Novel approach with amide-tethered H-bond donor NHC ligands enabled Au(I)-catalysis via H-bonding. The plain NHC-Au(I)-Cl complex catalysed conversions of terminal N-propynamides to oxazolines, and enyne cycloisomerization with an acid additive,...

Silver‐Free Au(I) Catalysis Enabled by Bifunctional Urea‐ and Squaramide‐Phosphine Ligands via H‐Bonding

“…It was found that chloride abstraction occurs with a remarkable low barrier of 6.7 kcal/mol, via an associative ligand exchange mechanism, wherein the displaced chloride ion is stabilized by a triple H‐bond established with the two NH groups of the squaramide and the NH functionality of amide 12 a . Helaja and coworkers computed a very similar mechanism for the Au−Cl bond activation of a NHC−Au(I) chloride complex possessing a pendant tosylamide, finding a 13.2 kcal/mol barrier with their system [13] . Therefore, computations suggest that the bidentate HBD on complex 1 facilitates the chloride abstraction step.…”

“…The reaction was found to be faster for more electron‐rich substrates, while it required more than 24 h to reach completion with electron‐poor amides 12 c and 12 d . Whereas electronic variations on the standard substrate 12 were tolerated, complexes 1 – 5 , 8 b and 9 b did not to catalyze the silver‐free cyclizations of N ‐(but‐3‐yn‐1‐yl)benzamide, bearing a homopropargylic substituent, and of the less reactive [13,25] N ‐(3‐phenylprop‐2‐yn‐1‐yl)benzamide possessing an internal alkyne [29] …”

“…This trend between reactivity and H‐bonding ability can be inferred also from the following two observations: (i) complexes 7 b – 9 b possessing electron‐poor urea rings systematically outperform their non‐fluorinated counterparts 7 a – 9 a (Table 1, entries 7–12), and (ii) within each a/b series, the activity of catalysts 8 and 9 , bearing an additional trifluoromethyl substituent on the internal aryl ring, surpasses that of 7 . The greater acidity of these groups might also speed up the protodeauration step, thought to be turnover‐limiting in this transformation at least in some cases [13,27] . It is worth specifying that all complexes except 7 a are soluble in CH 2 Cl 2 under the reaction conditions, so the difference in activity cannot be attributed to solubility profiles.…”

“…To obviate the practical issues associated with the use of silver salts, the development of silver‐free methods for the activation of Au(I) chloride precatalysts has lately become the focus of intense research efforts [4] . Thus, several groups have reported the use of other external activators (e. g. NaBAr F 4 , salts of other metals [5] and halogen‐bond donors [6] ) and self‐activating [LAuCl] complexes possessing tailored ligands, such as phosphaalkenes, [7] phosphinines, [8] carbenes or phosphines with redox‐switchable metallocenyl groups, [9] pendant Z‐type ligands [10] and tethered monodentate H‐bond donor (HBD) groups (Figure 1, top) [11,12,13] . In this context, the group of Gabbaï disclosed that Au(I) chloride complex A bearing a trifluoroacetamide group on a modified PPh 3 scaffold could catalyze the silver‐free cyclization of N ‐propargyl benzamide [11] .…”

“…Marinetti, Guinchard and coworkers described another isolated example of a phosphine Au(I) chloride complex with a pendant phosphoric acid moiety ( B ), capable of catalyzing the silver‐free tandem cycloisomerization‐indole addition reaction of 2‐alkynyl enones [12] . Regarding NHC‐based Au(I) complexes, the group of Helaja very recently presented complexes C equipped with an amide side arm, which displayed excellent performances in the absence of additives, but once again only in the cyclization of terminal N ‐propargyl benzamides [13] . Based on calculations, the authors proposed that the NH groups of both the benzamide substrate and the ligand concurred to stabilize the displaced chloride ion.…”