Simple, efficient and reusable Pd–NHC catalysts for hydroamination

Abstract

A series of chelating NHC-palladium complexes with different alkane-bridges of the type Pd[NHC-(CH 2 ) n -NHC]X 2 (X = Br or Cl, n = 2-4) were synthesized, where NHC is a triazolyl-N-heterocyclic carbene donor ligand. The bromide complexes with n = 2 and 3 were characterized by X-ray crystallography. The effects of the length of the bridge and halide ligand on the catalytic reactivity in the hydroamination reaction were investigated. The best catalytic performance was observed with the propylene-bridged Pd-NHC bromide complex 2. The hydroamination of phenylacetylene or 4-methylphenylacetylene with various substituted anilines catalyzed by 2 yielded only Markovnikov addition products in good yield. Interestingly, this homogenous catalyst can be reused three times without significant loss in activity.

PdI2 as a Simple and Efficient Catalyst for the Hydroamination of Arylacetylenes with Anilines

“…Once the best reaction conditions (Table 1, entry 6) were determined, we then investigated the scope for variously substituted anilines (1) and arylacetylenes (2) (Scheme 1). In general, the yields attained using PdI2 as catalyst were comparable with the ones achieved with the previously employed more complex catalytic systems [30]. For example, different alkyl chains at the para position of the aniline provided good yields, irrespective of the length of the chain (yields of 3b, 67%; 3c, 79%; 3e, 67%) or the steric hindrance (3d, 80%).…”

“…Despite the versatility of palladium-catalyzed hydroaminations, elaborate and expensive catalytic systems are often required. In particular, in the reaction between an alkyne and an aromatic amine, palladium has been employed in the presence of NHC [30] or NHCP [31] ligands. In both these cases, the catalytic precursor needs to be activated with a silver salt that acts as halogen scavenger.…”

Pd(II)–Ni(II) Pyrazolate Framework as Active and Recyclable Catalyst for the Hydroamination of Terminal Alkynes

“…The sum of TONs for each individual recycle of the Pd@NiBDP catalyst results in a cumulative TON of 150, a comparable value to those reported for other Pd systems in literature, such as Pd-NHC. [14] Figure 6. Reusability of Pd@NiBDP and Pd@HY in 4 reaction cycles (a) and hot-filtration test of Pd@NiBDP (b) in the intramolecular hydroamination of 2-ethenylaniline.…”

The nature of Pd–carbene and Pd–halogen bonds in (bisNHC)PdX2 type catalysts: insights from density functional theory

“…[23][24][25][26][27] In general, a metal-NHC bond is formed through s-donation from carbene carbon to the unlled metal d-orbital and pbackbonding from occupied metal d orbitals to the vacant p orbital of carbene carbon. 28,29 A large number of Pd-NHC complexes have been reported [30][31][32] and particularly, these complexes are useful in Suzuki-Miyaura cross-coupling reaction. [33][34][35][36][37] Recently, Strassner et al published several papers relating to the synthesis of methylene bridged palladium biscarbene complexes.…”