Palladium(0) Complexes with Unsymmetric Bidentate Nitrogen Ligands for the Stereoselective Hydrogenation of 1-Phenyl-1-propyne to (Z)-1-Phenyl-1-propene

Abstract

A series of zerovalent palladium complexes Pd(NN)(alkene) of bi-or tridentate nitrogen ligands of the general formula 6-R′′-C 5 H 3 N-(C(R′)dNR)-2 (R′′ ) H, Me, CHdNR′; R′ ) H, Me; R ) alkyl, aryl, or amino group) and dimethylfumarate (dmfu) have been prepared and were subsequently employed as precatalysts in the homogeneous stereoselective semihydrogenation of 1-phenyl-1-propyne. An X-ray structure of Pd(C 5 H 4 N-(C(Me)dNi-Pr)-2)(dmfu) was obtained. Whereas only relative small changes in substituents apply, the various complexes show very different stabilities under hydrogenation conditions. The complex Pd-(C 5 H 4 N-(C(H)dN(CH 2 ) 2 OH)-2)(dmfu) exhibits a good selectivity for the (Z)-alkene but decomposes just before full conversion of the alkyne, whereas the complex Pd(C 5 H 4 N-(C(H)d Ni-Pr)-2)(dmfu) exhibits a good selectivity and stability under hydrogenation conditions and is a suitable catalyst for the stereoselective hydrogenation of 1-phenyl-1-propyne to (Z)-1phenyl-1-propene.

Homogeneous Hydrogenation of Alkynes and Dienes

“…Only the latter are responsible for the observed minor amounts of (E)-alkene, which was shown to be a secondary reaction product formed by a subsequent palladium-catalyzed, hydrogen-assisted isomerization reaction. Since no n-octane was detected in the reaction mixture, only a tiny [34]. Most of the obtained inherently high cis-selectivities in the semihydrogenation of 1-phenyl-1-propyne are comparable to each other and to the results of the [Pd(Ar-bian)] system.…”

“…However, the surface of these catalysts is a rather complex assembly of various domains, each of which contributes to the product distribution in its own way, often resulting in unpredictable overall results. In order to circumvent the presence of different palladium species, much attention has been devoted to the application of homogeneous mononuclear palladium complexes; either in the form of a Pd(II) complex [32,33] or a Pd(0) compound [34][35][36]. In most of the reported cases, these mononuclear palladium complexes display a higher chemo-and (sometimes) higher stereoselectivity than the heterogeneous counterparts.…”

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…X-ray diffractometric studies 10.1. Complexes bearing mono-bi-and ter-dentate nitrogen ligands A significant number of palladium(0) olefin compounds bearing the title ligands was synthesized by several authors [30,31,35,37,75,[78][79][80][81][82][83][84][85][86]. All the structures display the typical trigonal arrangement around the palladium centre with the Pd-N bond lengths being scarcely influenced by the olefin nature, spanning within 2.10 and 2.18 Å.…”

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

“…Despite the usefulness of this reaction, it has been studied far less extensively than the similar hydrogenation of carbon-carbon double bonds. 1,2 Several homogeneous Pd-complexes that catalyze this reaction have been reported by us [3][4][5] and others 6 , and examples of other metal-complexes that are able to catalyze this reaction are known based on Ru-, 7,8 Rh-, 9 or Fe-complexes. 10 However, this process is often plagued by overreduction after full conversion, leading to saturated compounds.…”