First optically active PC-palladacycle bearing a phosphorus atom in an axially chiral environment

Abstract

Direct aromatic C-H bond activation in the (S a ) BINOL derived phosphite (S a ) HL afforded the dimeric cyclopalladated complex (S a ,S a ) {Pd(η 2 L)(μ Cl)} 2 (2) which is the first optically active PC palladacycle bearing a phosphorus atom in an axially chiral environment. ortho Palladated structure of dimer 2 was confirmed by spectral ( 1 H and 31 P NMR) examina tion of its mononuclear derivatives and by X ray diffraction analysis of the phosphine adduct (η 2 L)PdCl(PPh 3 ) (4). The enantiomeric purity of the starting ligand remained unchanged in the PC palladacycle under the thermal conditions used for the cyclopalladation (∼110°C); this fact was confirmed by the 31 P NMR spectroscopy after chiral derivatization in situ of dimer (S a ,S a ) 2 with the (R C ) valinate chiral auxiliary (Val). trans(N,C) Configuration of the valinate complex (η 2 L)Pd(Val) (5) was established by 1 H NMR and supported by DFT calculations. The chirality transfer in the new PC palladacycle was discussed on the basis of X ray diffrac tion data for the phosphine adduct rac 4 and DFT calculations performed for both phosphine and valinate mononuclear derivatives.Transition metal coordination complexes with mono 1 and bidentate 2 P donor ligands, atropoisomeric or bear ing axially chiral moieties, are widely known as highly effective enantioselective catalysts. In particular, phos phites, 3 phosphoramidites 3e,4 and other P donor ligands 5 derived from commercially available enantiomerically pure diol BINOL 6 or its analogs provide up to 99.9% ee selectivity in the asymmetric catalysis of various transfor mations including palladium catalyzed reactions. 3h,4d,5c,7 Monodentate ligands of the above type often prove more effective than bidentate ones, affording optical yields exceeding 99% ee. 3a,b,d,e,k-m,4a,b,5b,7e A number of processes catalyzed by coordination compounds of iridium and palladium were found 8a,b or supposed 8с to be mediated by the appropriate phosphametallacycles. In particular, for the allylic amination involving phosphoramidite HL´, the cyclometalated derivative of the latter, I formed in situ under catalysis conditions was shown 8a,b to be a true catalyst (Scheme 1). These results may contribute to the explanation of the high enantioselectivity exhibited by monodentate P donor ligands of this type.Despite the high catalytic activity of achiral phosphite palladacycles 9 and the excellent results achieved in asym metric catalysis with the BINOL derived transition metal complexes with monodentate phosphites, 3 cyclopalladated derivaives of such chiral ligands remained unknown until this time. Only recently there have emerged the reports 10 on synthesis and application in catalysis of several РСP Scheme 1 B is base, cod is cycloocta 1,5 diene