Diastereoselective Synthesis of Chiral Octahedral Iridium(III) Phosphano–Oxazoline Complexes

Abstract

Reaction of the dimer [Ir(μ-Cl)(η 2 -coe) 2 ] 2 (coe = cyclooctene) with chiral phosphano−oxazoline ligands (PN*) renders neutral mononuclear iridium(I) complexes of the formula [IrCl(η 2 -coe)(PN*)] (1, 2), which in turn are oxidized to the corresponding iridium(III) hydride species [IrCl 2 H(η 2coe)(PN*)] (3, 4) by treatment with aqueous HCl. The latter react with diphosphanes (PP) in the presence of NaSbF 6 to afford cationic complexes of stoichiometry [IrClH(PN*)(PP)][SbF 6 ] (5−16). The fluorophenyldiphosphane-containing compounds [IrClH(PN*)(dfppe)][SbF 6 ] (15, 16) evolve to the corresponding Ir(I) species [Ir(PN*)(dfppe)][SbF 6 ] (17, 18) by HCl loss. The new compounds have been fully characterized by analytical and spectroscopic means, including the molecular structure determination of [IrCl 2 H(η 2 -coe)(PNInd)] (3), [IrClH(PNInd)(dppen)][SbF 6 ] (6a,b), [IrClH(PNInd)(dppp)][SbF 6 ] (7a), [IrClH(PNiPr)-(dppp)][SbF 6 ] (13a), [IrClH(PNInd)(dfppe)][SbF 6 ] (15), [Ir(PNInd)(dfppe)][SbF 6 ] (17), and [Ir(PNiPr)(dfppe)][SbF 6 ] (18) by X-ray diffractometric methods.

Chiral Octahedral Phosphano–Oxazoline Iridium(III) Complexes as Catalysts in Asymmetric Cycloaddition Reactions

“…The puckering amplitude of the six-membered PN* metallacycle is very similar to that of complexes 1 , 2 and 10a ; it exhibits the same 1 S 6 screw-boat conformation already noted . In contrast, significant differences have been observed in the deviation from planarity and in the conformation of the PP metallacycles of the complexes reported in this paper (and in ref ), reflecting their flexibility . In 11 the five-membered Ir–P(1)–C(13)–C(14)–P(2) metallacycle adopts a twisted 4 T 3 conformation.…”

“…This structural feature, together with the close values of dihedral angles between NIrP and N–C(45)–C(44)–C(39)–P planes (130.8(2)° ( 1 ), 136.7(2)° ( 2 ), and 135.1(2)° ( 10a )) seems to be characteristic of these phosphano–oxazoline fragments. We have already pointed out the marked rigidity of these ligands and their coordination toward the metal (see ref ). In particular, this conformation forces the pro - R (C(33)–C(38)) and pro - S (C(27)–C(32)) phenyl groups of the PPh 2 moiety of the PN* ligand to occupy pseudo-axial and pseudo-equatorial positions, respectively.…”

“…By analogy with the related chloride isomers (see ref ), we assign an A configuration to the minor isomer of complex 3 ( 3b ) and a C configuration to the b isomer of the PN i Pr-containing complex 8 . It is interesting to note that only for dppp derivatives are both epimers at metal obtained.…”

“…On the other hand, assignment of the stereochemistry to the a , a′ pair of isomers of the ( R )-prophos-containing compounds 5 and 9 was accomplished by comparison of the 13 C NMR resonance of the asymmetric carbon of the ( R )-prophos ligand with that of the corresponding chlorides (see ref ). The phosphorus atom nearest to the asymmetric carbon of the diphosphane is trans to the phosphorus atom of the PN* ligand in isomers a′ and cis in the corresponding a isomers (Figure ).…”

“…Phosphano–oxazolines PNInd and PN i Pr and the nitrone N -benzylidenephenylamine N -oxide were prepared using literature procedures. For the preparation of the starting chloride complexes [IrClH(PN*)(PP)][SbF 6 ] see ref . General labeling for NMR assignments is given in Scheme , and labeling of the ( R )-prophos ligand for NMR assignments is given in Scheme .…”