Convenient Transfer Semihydrogenation Methodology for Alkynes Using a Pd^II-NHC Precatalyst

Abstract

A convenient and easy-to-use protocol for the Z-selective transfer semihydrogenation of alkynes was developed, using ammonium formate as the hydrogen source and the easily prepared and commercially available, highly stable complex PdCl­(η3-C3H5)­(IMes) (1) as the (pre)­catalyst. Combined with triphenyl posphine as an additional ligand, this system provides a robust catalytic synthetic method that shows little to no over-reduction or isomerization after full substrate conversion. The system allows the direct use of solvents and reagents, as received from the supplier without drying or purification, thus providing a practical method for semihydrogenation of a broad range of alkynes. The mechanism behind these high and enhanced selectivities was determined through a set of kinetic experiments.

Pd‐Catalyzed Z‐Selective Semihydrogenation of Alkynes: Determining the Type of Active Species

“…[102] In this case, the obtained percentage of active Pd fits well with am olecular active compound. The data obtained in the previous mechanistic studies are also circumstantiale videnceo famolecular catalyst.…”

“…[4,108] Efficient well-defined molecular catalysts ystems are expected to give values greatert han 40 %o fa ctive metal.However,i f also inactive NPs are formed, ap art of the added palladium is deactivated, thus leadingt oal ower percentage of active Pd atoms. Furthermore, we previously proposed that the phosphine coordinationt ot he [Pd 0 (NHC)] species leads to an inactive state, [102] which was also proposed for aP d(NHC)PCy 3 system by Cazin et al [41,109] The presence of an inactive species could also explainw hy al ower fraction of Pd that is active in the reactioni so btained for am olecular catalyst. The degree of Pd that is in ap oison-independent inactive state depends on the bindings trength of the phosphine ligand compared to that of the alkynes ubstrate.…”

“…We reporteda nother system for the transfer semihydrogenation of alkynes. [102] Thiss ystem applies ap recatalyst that is transformed in situ into a[ Pd 0 (IMes)] species by triethylammonium formate, whichi ss tabilized by additional selectivity-enhancing PPh 3 ligands (Scheme 7).…”

“…As mentioned before,s uch kinetic behavior does not unequivocally prove that the reaction is catalyzed by amolecular catalyst, but at the time we nevertheless interpreted the data as such. [102] From mechanistic studies we concluded that the high selectivities that are induced by the additive are the resulto ft he relative coordination strengths of the substrate, the phosphine and the products. The coordination of the phosphinel igand to the catalystp revents the isomerization and over-reduction of the Z-alkene product.…”

Bonding and Catalytic Application of Ruthenium N-Heterocyclic Carbene Complexes Featuring Triazole, Triazolylidene, and Imidazolylidene Ligands

“…16 A palladium(II) mono-NHC complex featuring a similar ligand motif, was described recently, however, no chelation was reported. 17 Interestingly, it was not possible to isolate the related neutral mono-NHC complex with two non-coordinated triazole moieties in our case.…”