Mechanism of Pd(NHC)-Catalyzed Transfer Hydrogenation of Alkynes

Abstract

The transfer semihydrogenation of alkynes to (Z)-alkenes shows excellent chemo-and stereoselectivity when using a zero-valent palladium(NHC)(maleic anhydride)-complex as pre-catalyst and triethylammonium formate as hydrogen donor. Studies on the kinetics under reaction conditions showed a broken positive order in substrate and first order in catalyst and hydrogen donor. Deuteriumlabeling studies on the hydrogen donor showed that both hydrogens of formic acid display a primary kinetic isotope effect, indicating that proton and hydride transfers are separate rate-determining steps. By monitoring the reaction with NMR we observed the presence of a coordinated formate anion and found that part of the maleic anhydride remains coordinated during the reaction. From these observations we propose a mechanism in which hydrogen transfer from formate anion to zero-valent palladium(NHC)(MA)(alkyne)-complex is followed by migratory insertion of hydride, after which the product alkene is liberated by proton transfer from the triethylammonium cation. The explanation for the 2 high selectivity observed lies in the competition between strongly coordinating solvent and alkyne for a Pd(alkene)-intermediate.

Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

“…Unfortunately, this complex showed poor activity of only 18% conversion for this system (Table 2, entry 1). Considering the proposed mechanism for the mono-NHC analogue in which both formic acid and the alkyne coordinate to the Pd 0 center, 15 this may be explained by the presence of only one available vacant site in this system, whereas two are needed if the same mechanism would hold here.…”

“…15,41,47 We applied complex 3a in this reaction, using 1-phenyl-1-propyne as the substrate and formic acid as a hydrogen donor at 70°C in acetonitrile. Unfortunately, this complex showed poor activity of only 18% conversion for this system (Table 2, entry 1).…”

Synthesis and characterization of palladacyclopentadiene complexes with N-heterocyclic carbene ligands

“…Complex a was observed by Elsevier and co-workers as a by-product in the Pd(NHC)-catalysed semihydrogenation of alkynes [10] whereas complex b was detected by our group when studying the reaction of [(MeeNHCeCH 2 Py)Pd(h 2 -MA)] (MA ¼ maleic anhydride) with an excess of dimethylbutynedioate [11].…”

Catalytic Hydrogenation of Hydrocarbons for Gasoline Production

“…Although alkene hydrogenation is inhibited by the presence of alkynes, in the absence of the alkyne, research indicates that the alkene will react more rapidly. 43 Research has also shown that in a competitive environment, the alkyne can influence the reactivity of other alkynes and alkenes. 44,45 Hydrogenation of unsaturated bonds is considered a very important catalytic technique in refinery operations.…”