2-Pyridyl substituents enhance the activity of palladium–phospha-adamantane catalysts for the methoxycarbonylation of phenylacetylene

Abstract

The synthesis of a series of CgPAr ligands is reported, where CgP is the 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl moiety and Ar = 2-pyridyl (L2), 3-pyridyl (L3), 2-pyrimidyl (L4), 4-R-2-pyridyl [R = Me (L5a), CF (L6a), SiMe (L7a)] or 6-R-2-pyridyl [R = Me (L5b), CF (L6b), SiMe (L7b). Testing of these ligands in the Pd-catalysed methoxycarbonylation of phenylacetylene reveals that the activity and branched selectivity of the catalysts derived from these ligands varies as a function of the N-heterocycle, with the catalyst derived from L5b being the most active of those tested. This, together with the poor performance of catalysts derived from L3 supports the hypothesis that the catalysis proceeds by a "proton shuttling" mechanism, an idea that previously had only been applied to arylphosphines. Reaction of [PtCl(cod)] with L where L = L2 or L4-7 yields a rac/meso mixture of the trans-[PtCl(L)] (1a-h) complexes, three of which are structurally characterised. P NMR spectroscopy shows that reaction of L3 with [PtCl(cod)] gives a mixture of mononuclear and binuclear metal complexes in solution. The complex trans-[PdCl(L2)] (4) reacts with AgBF to give the [PdCl(κ-L2)(κ-L2)]BF (5) with spectroscopic and structural characterisation confirming the presence of a P,N-chelate. H andP NMR evidence supports the assignment of a pyridyl-protonated species being formed upon treatment of 4 with TsOH·HO in CDCl; both the protonated species and chelate 5 are observed when the reaction is carried out in MeOH.

Tailored Palladium Catalysts for Selective Synthesis of Conjugated Enynes by Monocarbonylation of 1,3‐Diynes

“…On the basis of these results and our previous work on the mechanism of alkoxycarbonylation using ligands L1-L4 [12][13][14] , as well as the mechanistic studies by Cole-Hamilton, Drent, and Sparkes, [24] we propose the following catalytic cycle for ligand L12 [Figure 1 (c)]. Initially, the stable Pd II salt is reduced in situ to Pd 0 in the presence of an excess amount of phosphine ligands, [25] followed by a protonation process to afford the active complex A.…”

Tailored Palladium Catalysts for Selective Synthesis of Conjugated Enynes by Monocarbonylation of 1,3‐Diynes

“…On the basis of these results and our previous work on the mechanism of alkoxycarbonylation using ligands L1-L4 [12][13][14] , as well as the mechanistic studies by Cole-Hamilton, Drent, and Sparkes, [24] we propose the following catalytic cycle for ligand L12 [Figure 1(c)].I nitially,t he stable Pd II salt is reduced in situ to Pd 0 in the presence of an excess amount of phosphine ligands, [25] followed by ap rotonation process to afford the active complex A.I ti sl ikely that this palladium hydride species is in equilibrium with the N-protonated pyridinium complex. [13] Subsequently, p-coordination of one carbon À carbon triple bond to the metal center occurs, followed by the insertion of the alkyne into the palladium hydride bond, affording the alkenyl-Pd intermediate B.After CO insertion, the complex C is formed and N-assisted methanolysis of intermediate C via transition state D provides the desired monocarbonylation product 3ab and regenerates the active palladium hydride species,tofinish cycle I.…”

New insights into the alkoxycarbonylation of propargyl alcohol

“…Looking for an higher catalytic efficiency we replaced diphenyl-(pyridin-2-yl)phosphine with diphenyl-(6-methylpyridin-2-yl)phosphine (2MePyPPh 2 ) which usually allows to obtain higher activities and regioselectivies [28][29][30]40].…”