Sulfoxide complexes of rhodium and iridium and their potential use as asymmetric hydrogenation catalysts
Creators: name: Morris, Robert Harold
Publisher: name: University of British Columbia
Year: 2010

description: Efficient preparative routes to several new rhodium complexes and some iridium compounds containing sulfoxide ligands are described. Chiral sulfoxide complexes of rhodium were tested as possible catalysts for the homogeneous asymmetric hydrogenation of prochiral olefins. Also tested were chiral sulfoxide-iridium complexes as potential catalysts for H2 transfer from isopropanol to prochiral olefins and ketones. The sulfoxides used include: the monodentate ligands dimethyl (DMSO), tetramethylene (TMSO), di-n-propyl (NPSO), methyl phenyl (MPSO), and diphenyl sulfoxide (DPSO); the monodentate chiral ligands (+)-(R)-methyl-p-tolyl sulfoxide (MPTSO), (+)-(R)-t-butyl-p-tolyl sulfoxide (TBPTSO), (-)-(S)-o-tolyl-p-tolyl sulfoxide (OTPTSO), and (+)-(S)-2-methylbutyl-(S,R)-methyl sulfoxide (MBMSO); and the potentially chelating ligands meso-l,2-bis(methyl sulfinyl)ethane (MSE), (R,R)-1,2-bis(p-tolyl sulfinyl)ethane (PTSE), and (-)-(2R,3R)-2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(methyl sulfinyl)butane (DIOS). Displacement of the labile acetone ligand from [Rh(diene)(PPh₃) (acetone)]A (diene=l,5-cyclooctadiene (COD), norbornadiene (NBD); A=PF₆ ⁻, SbF₆ ⁻ ) allows facile coordination of dialkyl or diaryl sulfoxides, and [Rh(diene)(PPh₃)L]⁺ complexes (L=DMSO,TMSO,NPSO,MBMSO,MPSO,MPTSO, and TBPTSO) have been synthesized; compounds with L=AsPh₃, py and (CO)₂ also form. Diaryl sulfoxides and DIOS coordinate, but no solids were isolated. The upfield shifts of the sulfoxide resonances (¹H nmr), reflecting shielding by the adjacent phenyl groups of PPh₃, and the decrease in v(SO) on coordination, are indicative of O-bonding in all cases. NMR data on the olefinic diene protons suggest the occurrence of some disproportionation of the mixed ligand complexes to [Rh(diene) (PPh₃)₂]⁺ and [Rh(diene)(L)2]⁺ depending on L, and the presence of 3-coordinate, and 5-coordinate (for diene=NBD only) intermediates. The hydrogenation of itaconic acid using catalysts with L=R-MPTSO or DIOS resulted in no asymmetric induction in the a-methyl succinic acid product because of disproportionation and catalysis via the bis(triphenylphosphine) system. Efficient hydroformylation of 1-alkenes is effected using [Rh(diene) (PPh₃)(CO)₂j⁺ as catalyst precursors. Aqueous isopropanol solutions of RhCl₃-3H₂O on treatment with sulfoxides provide an efficient route to RhCl₃L₃ complexes (L=DMSO, R-MPTSO,MPSO,TMSO) that contain in solution, at least for the first three systems, two S-bonded sulfoxides trans to a chloride, and an 0-bonded ligand. The 0-bonded sulfoxide is displaced by amides, amine oxides, and phosphine oxides to give mer- RhCl₃ (DMSO)₂(OL) complexes. The DMSO cis to OL in RhCl₃DMSO)₂(OL) or RhCl₃ (DMSO)₃(OL) can be identified in the nmr by using the ring current shielding effects of OPPh₂Me. RhCl₃L₃ react with H₂ (1:1) in base promoted reactions to yield Rh(I) presumably via undetected Rh(III)-H species. RhCl₃.3H₂O reacts with DPSO in isopropanol to give Rh(I) as the chloride-bridged species [RhCl(DPSO)₂]₂. The reaction with NPSO gives a Rh(I) dimer (indirect evidence) and a Rh(III) product, isolated as [H(NPSO)₂][RhCl₄(NPSO)₂] containing a symmetrical hydrogen-bridged cation. A crystal structure of trans-[H(DMSO)₂] [RhCl₄(DMSO)₂] reveals the short oxygen-oxygen distance (~2.45Å) in the cation expected for strong H-bonds. Such cations display intense v[sub a] (OHO) bands at 1700- 1100 and 900-600 cm⁻¹. The air-sensitive complexes [RhCl(C₈H₁₄)(DPSO)]₂, [RhCl(DMS0)₂]₂, [RhCl(DIOS)₂]₂ and [RhCl(MPSO)(PPh₃)]₂, isolated from [RhCl(cyclooctene)₂]₂/ ligand solutions, contain very labile Rh-S bonds that do not appear to involve Rh(dπ)+S (d-π) backbonding. Attempts at generating hydride complexes by oxidative addition of H₂ or HCl to Rh(I) resulted normally in either metal formation or sulfoxide reduction; even in the presence of prochiral olefins these complications occurred rather than catalytic asymmetric hydrogenation. The compound [Rh(MSE) ₂]PF₆ was isolated from the reaction of H₂ with [Rh(NBD) ₂]PF₆, and 2 MSE in alcohol solutions. The compounds mer-IrCl₂(H)(DMS0)₃ with trans chlorides, and mer-IrCl(H)₂(DMSO)₃ with cis hydrides, were obtained from oxidative addition reactions involving HCl and H₂, respectively, with [IrCl(C₈H₁₄)₂]₂ in DMSO. The former catalyzes the efficient selective reduction of α,β-unsaturated aldehydes to the unsaturated alcohols. Attempts at asymmetric synthesis using as catalysts IrCl₃H₂0/chiral sulfoxide mixtures failed. A simple bent M&lt;-O=L vibrational model is used to estimate from v(M0) and v(S0) the force constants F[sub MO] and F[sub OL] using data for seventy 0-bonded DMSO, DMSO-D₆, and TMSO complexes of several metals. The correlation F[sub OL]=-(1.24±0.12)F[sub MO].+(8.78±0.12) mdyne/Å appears to hold for all metal complexes excepting those of group IVA and VA elements.