Ligand Effect and Control of E‐ and Z‐Selectivity in the Silver‐Catalyzed Synthesis of 4‐Bromooxazolines

Abstract

The stereospecific intramolecular hydroamination of alkynyl trichloroacetimidates catalyzed by bis(pyridyl)silver(I) complexes to form 1,3‐oxazolines and oxazines has been investigated in greater detail. The rate of the reaction is profoundly influenced by the electronic character of the pyridyl ligand; leading to optimized conditions whereby reactions can be completed at ambient temperature with catalyst loadings as low as 1 mol%, while maintaining stereoselectivity towards the (Z)‐isomer. Further investigations into establishing the role of the ligand, and the synthesis of (E)‐isomer, are also presented.magnified image

Theoretical NMR spectroscopy of N-heterocyclic carbenes and their metal complexes

“…In this case, the bond energy decomposition analysis also showed a good correlation between the magnetic shielding on the P atom and the orbital energy contribution, E π , see Figure 10. recently, see Chart 7 [80,81]. In contrast to Cu and Au, only a few Ag-NHC complexes have been experimentally investigated, due to the easier 'decomposition' reaction of ligand exchange yielding a bis-ligated complex [82].…”

Synthesis, Structure and Catalytic Activity of NHC–AgI Carboxylate Complexes

“…More recently, this was extended to the intramolecular addition of NÀHa nd OÀHb onds to alkynes to form substituted oxazolines at ambientc onditions (Scheme 1). [16,25,26] Catalytic turnoversw ere found to be very responsive to the electronic characteristics of the pyridine ligands. Notably,N ÀHa dditions were promoted by electron-deficient pyridyl ligands (Scheme 1a), while the opposite trend was observed for OÀH additions (Scheme 1b).…”

“…In the case of the NÀHa ddition (Y = NH, Z = O), the reaction promoted by 4acetyl pyridine afforded the fastest turnover. [16] This suggests that step (iii)i sl ikely to be turnover limiting, which can be facilitatedb yamore Brønsted acidic pyridinium salt for the protodemetallation step. Conversely,f or the OÀHa ddition (Y = O, Z = NH), the turnover rate is likely to be governed by step (i), as am ore electron-rich ligand (4-methoxyl pyridine) promotes greater p back donation of electrons from Ag to the CC moiety,l owering the energy of its LUMO orbital towards the nucleophilic attack.…”

“…In synthesis, Ag–NHC complexes have been used as latent catalysts to initiate the polymerization of lactides and transesterification reactions . In fact, there are no more than a handful of examples in which the metal center of Ag–NHC complexes has been implicated as the active site, and these include the carbene insertion of ethyl diazoacetate into C−H and ketones, aerobic oxidation of alcohols, O‐glycosidation of carbohydrates, the three‐component reaction of aldehyde, amines, and alkynes, and the hydroboraton of alkynes . In all these cases, either [Ag(NHC)X] or [Ag(NHC) 2 ]X (X=halide) can be employed as catalyst precursors.…”

“…Our group has an ongoing interest in the catalytic activity of cheaper coinage metal (Cu and Ag) catalysts for the heterofunctionalisation of unsaturated carbon–carbon bonds. More recently, this was extended to the intramolecular addition of N−H and O−H bonds to alkynes to form substituted oxazolines at ambient conditions (Scheme ) . Catalytic turnovers were found to be very responsive to the electronic characteristics of the pyridine ligands.…”