Efficient and chemoselective ethene hydromethoxycarbonylation catalysts based on Pd-complexes of heterodiphosphines o-C6H4(CH2PtBu2)(CH2PR2)

Abstract

The synthesis and properties of a series of unsymmetrical diphosphines o-C₆H₄(CH₂P^tBu₂)(CH₂PR₂) are reported where R = C₆H₅ (L_a); p-CH₃C₆H₄ (L_b); o-CH₃C₆H₄ (L_c); o-CH₃CH₂C₆H₄ (L_d); o-CH₃OC₆H₄ (L_e); 2,4,6-(CH₃)₃C₆H₄ (L_f); CH(CH₃)₂ (L_g); or PR₂ = PCg (L_h) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions is catalysed by the Pd complexes of each of the ligands L_a–ha–h to give methyl propanoate in >99% selectivity with catalytic activities comparable to those obtained with o-C₆H₄(CH₂P^tBu₂)₂ (L₁) or o-C₆H₄(CH₂PCg)₂ (L₅5). The catalysts derived from L_c, L_d and L_h are more active than the catalyst derived from L_a or L₁; these ligands have in common, a PR₂ donor that is more bulky than the PPh₂ present in L_a. Treatment of [PtCl(CH₃)(cod)] with L_a–ha–h gave [PtCl(CH₃)(L)] as mixtures of 2 isomers 1a–h and 2a–h. The major isomer in each case was 1a–h with the CH₃ ligand trans to the P^tBu₂ group; the diastereoselectivity of this reaction for products 1a–h ranges from 88% to over 99%. The crystal structures of 1b, 1c and 1e have been determined. Fluxionality associated with chelate ring inversion has been detected for 1c by variable temperature ³¹P NMR spectroscopy. The ³¹P NMR data are compared for the complexes [PtCl(CH₃)(L)], [Pt(CH₃)₂(L)] and [PtCl₂(L)] where L = L_h, L₁ or L₅5. The crystal structure of [Pt(CH₃)₂(L_h)] (4h) has been determined and shows that PCg is sterically less demanding than P^tBu₂ in this complex. Treatment of 4h with HCl gave a 1 : 1 mixture of 1h and 2h that equilibrated over 5 h to a 70 : 1 mixture. Treatment of an equilibrium mixture of 1h and 2h with isotopically labelled ¹³CO gas gave a single acyl complex [PtCl(¹³COCH₃)(L_h)] (5h) with retention of configuration at Pt, i.e. the ¹³COCH₃ is trans to the P^tBu₂ group. Mechanisms for the CO insertion are discussed which are consistent with the observed stereochemistry. The palladium complexes [PdCl(CH₃)(L_h)] (7/8) also reacted with labelled ¹³CO to give a single acyl species [PdCl(¹³COCH₃)(L_h)]. Treatment of [PdCl(¹³COCH₃)(L_h)] with MeOH rapidly gave CH₃¹³COOMe.