P-Stereogenic monophosphines with the 2-p-terphenylyl and 1-pyrenyl substituents. Application to Pd and Ru asymmetric catalysis

Abstract

tThe synthesis of five optically pure P-stereogenic monophosphines of the type PPhArR (Ar = 2-pterphenylyl (a), 1-pyrenyl (b); R = OMe, Me, i-Pr) is described. The ligands were fully characterisedand the absolute configurations of PPh(1-pyrenyl)R (3b and 5b; R = OMe and Me respectively) wereconfirmed by X-ray diffraction. The complexation of the monophosphines to Pd and Ru organometal-lic units yielded the neutral complexes [PdCl( 3-2-Me-allyl)P] (10-12) and [RuCl2( 6-pcymene)P](16-18). Complete characterisation, including the crystal structure determination of [RuCl2( 6-p-cymene)(PMePh(2-p-terphenyl))] (17a) is provided. Neutral palladium complexes appeared as mixturesof two diastereomers in solution according to NMR. The synthesis and characterisation of four cationic[Pd( 3-2-Me-allyl)(P)2]PF6(13 and 14) is also described. The application of neutral Pd complexes tocatalytic styrene hydrovinylation afforded moderate conversions, high chemoselectivities (>92%) to 3-phenyl-1-butene and up to 43% ee with precursor 12a. Cationic Pd complexes were tested as catalyticprecursors in allylic substitution of rac-3-acetoxy-1,3-diphenyl-1propene (rac-I), with the anion ofdimethylmalonate and benzylamine as nucleophiles, obtaining full conversions and up to 80% ee in alkyl-ation and 60% ee in amination with precursor 13a. Finally, ruthenium complexes were used as catalyticprecursors in transfer hydrogenation of acetophenone, with complete conversions after several hoursbut low enantioselectivities

Neutral and Cationic Palladium Complexes of P‐Stereogenic Phosphanes Bearing a Heterocyclic Substituent

“…The complexes were characterised by IR, chemical microanalysis (or MS) and multinuclear NMR in solution. As expected,, the complexes were found to exist as mixtures of two diastereomeric species in solution, due to the presence of the chiral ligand and the allyl moiety. Hence, two singlets in the 31 P{ 1 H} NMR spectra, often partially overlapped, could be observed.…”

“…They were characterised by the usual techniques. The NMR spectra showed that a single species was present in solution, as previously found for analogous compounds , . The presence of the allyl group and the chirality of the phosphane makes it the case that the atoms in the molecule are all different.…”

“…The next type of Pd complexes prepared were cationic bis‐phosphanes of the type [Pd(η 3 ‐2‐methylallyl)( P ) 2 ]PF 6 . As described in previous reports,, they were obtained by splitting dimer D with slightly more than four equivalents of phosphane in the presence of an excess of ammonium hexafluorophosphate (Scheme ).…”

“…It has been found that the only ligands capable of acting as bidentate are those containing a 1‐thianthrenyl group. The obtained Pd complexes were used in catalytic asymmetric hydrovinylation of styrene and allylic substitution on rac ‐ I with the aim of comparing the performance of the new ligands with that of previously reported systems based on P ‐stereogenic PArPhR (Ar = polycyclic aromatic group) 15c], It was found that, in general, the new heterocyclic ligands give more active systems for the hydrovinylation reaction but that they are less selective towards 3‐phenylbut‐1‐ene and none of them improves on the best enantioselectivity achieved with the previous systems.…”

“…As previously described for other monophosphanes, [5] treatment of the well-known Pd-dimer D with slightly more than two equivalents of phosphane in dichloromethane yielded the expected neutral complexes Pd1-13, of the type [PdCl(η 3 -2methylallyl)(P)] (Scheme 2). Scheme 2.…”