Novel chiral phosphine ligands and complexes from amino acid esters

Article abstract of DOI:10.1039/b009074n

N-Diphenylphosphinoamino acid methyl/ethyl esters RCH(NHPPh₂)CO₂R′ la-le were prepared in high yield from equimolar amounts of amino acid methyl/ethyl ester hydrochloride and chlorodiphenylphosphine. The reaction proceeds with configurational retention. Oxidation of la (R = R′ = CH₃) with H₂O₂ or S₈ leads to the N-diphenyl-phosphinoylalanine methyl ester 2a or N-diphenylthiophosphinoylalanine methyl ester 3a. la-le function as monodentate ligands to [PtC1₂(cod)], [RhCl(cod)]₂ and [AuCl(tht)] to give cis-[PtC1₂(1-P)₂], [RhCl(cod)(1-P)] and [AuCi(1-P)]. Two equivalents of chlorodiphenylphosphine react with alanine methyl ester hydrochloride in CH₂C1₂ to give N,N-bis(diphenylphosphino)alanine methyl ester, bdppal. This is an excellent bidentate ligand, the chelate complexes [MC1₂(bdppal-P,P′)] (M =Pd or Pt) being prepared and studied by X-ray analysis. Oxidation at one P atom in bdppal with either H₂O₂ or S₈ gives N-diphenylphosphino-N-diphenylphosphinoylalanine methyl ester, bdppalO, and N-diphenylphosphino-N-diphenylthiophosphinoylalanine methyl ester, bdppalS, respectively, bdppalO is very difficult to isolate but does react in situ with [PdC1₂(cod)] to give [PdC1₂(bdppalO-O,P)]. bdppalS reacts with [MCl₂(cod)] (M = Pd or Pt) giving [MC1₂(bdppalS-P,S)]. The dioxidised product N,N-bis(diphenylphosphinoyl)alanine methyl ester, bdppalO₂, does not react with [MC1₂(cod)] (M =Pd or Pt), whereas N,N-bis(diphenylthiophosphinoyl)alanine methyl ester, bdppalS₂, reacts with [PdC1₂(cod)] to give [PdC1₂(bdppal-S,S′)]. All the compounds are chiral and have been fully characterised by microanalysis, IR, ³¹P-{¹H}, ¹H NMR, and FAB⁺ mass spectroscopies, and in several cases structures are confirmed by X-ray analysis. The crystal structures described and those previously determined reveal the configurational retention of the reactions.