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 (L_5a), CF₃ (L_6a), SiMe₃ (L_7a)] or 6-R-2-pyridyl [R = Me (L_5b), CF₃ (L_6b), SiMe₃ (L_7b). 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 L_5b 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 [PtCl₂(cod)] with L where L = L₂ or L_4–7 yields a rac/meso mixture of the trans-[PtCl₂(L)₂] (1a–h) complexes, three of which are structurally characterised. ³¹P NMR spectroscopy shows that reaction of L₃ with [PtCl₂(cod)] gives a mixture of mononuclear and binuclear metal complexes in solution. The complex trans-[PdCl₂(L₂)₂] (4) reacts with AgBF₄ to give the [PdCl(κ¹-L₂)(κ²-L₂)]BF₄ (5) with spectroscopic and structural characterisation confirming the presence of a P,N-chelate. ¹H and ³¹P NMR evidence supports the assignment of a pyridyl-protonated species being formed upon treatment of 4 with TsOH·H₂O in CD₂Cl₂; both the protonated species and chelate 5 are observed when the reaction is carried out in MeOH.