Stereospecific Syntheses and Structures of Planar Chiral Bidentate η⁵:κS-Indenyl-Sulfanyl and -Sulfinyl Complexes of Rhodium(III)

Abstract

Axially chiral rac-1-(2-methyl-1H-inden-3-yl)-2-(methylsulfanyl)naphthalene (rac-3) was synthesized from methyl 2-(methylsulfanyl)-1-naphthoate through reaction with the di-Grignard reagent derived from 1-bromo-2-(2-bromopropyl)benzene, followed by acid-catalyzed dehydration of the intermediate indanol. Oxidation of rac-3 with m-CPBA gave the diastereomeric sulfoxides (aR*,R S *)-5 and (aR*,S S *)-6, with the relative configuration of 5 established using single-crystal X-ray diffraction. The dichloro[η 5 :κS-indenylsulfanyl and -sulfinyl]rhodium complexes rac-4, (pR*,S S *)-7, and (pR*,R S *)-8 were synthesized through reaction of the ligands rac-3, (aR*,R S *)-5, and (aR*,S S *)-6, respectively, with rhodium trichloride in 9:1 methanol/water solution heated under reflux. The use of water as a cosolvent was found to be critical for obtaining good yields in the complexation reactions. Solid-state structures for the racemic rhodium complexes were determined through single-crystal X-ray diffraction. The enantiomers of the ligands 3, 5, and 6 were obtained in high enantiopurity through subjecting rac-3 to a series of Kagan asymmetric sulfoxidation, deoxygenation, and resulfoxidation reactions. The enantiomeric relationship of the rhodium complexes derived from the enantio-enriched ligands was confirmed by CD spectroscopy, and the high enantiopurity of the complexes established by 1 H NMR analysis using the chiral shift reagent Eu(hfc) 3 . The absolute configurations of the nonracemic ligands and rhodium complexes were established by a singlecrystal X-ray diffraction determination of the solid-state structure of (pS,S S )-8, with the Flack parameter refining to 0.00(2). ■ INTRODUCTIONPlanar chiral ferrocenyl ligands have been successfully utilized in a wide range of asymmetric transition-metal-catalyzed reactions. While the iron center does not participate directly in the catalytic cycle of these reactions, the chemical robustness of ferrocenes has the advantage of enabling numerous synthetic transformations that allow for the introduction of a variety of different coordinating groups, together with specific strategies for the stereoselective creation of the planar chiral element. 1 Planar chiral transition-metal complexes where the metal involved in the planar chiral element is also the site for catalysis have also been examined as asymmetric catalysts. The most successful examples to date are early transition-metal and lanthanide metallocenes, in particular ansa-metallocenes, where a tether between the cyclopentadienyl (Cp; we also use this abbreviation to cover related indenyl and fluorenyl systems) rings prevents the normally facile rotation around the Cp−metal axis, creating a well-defined steric and electronic environment around the metal. 2 As an extension of this development, planar chiral constrainedgeometry half-sandwich metal complexes would appear to be an attractive target for use in asymmetric catalysis. Takahashi and Onitsuka and their co-workers have, for example, demonstrated that planar chiral ruthen...

The emergence of sulfoxides as efficient ligands in transition metal catalysis

“…Nevertheless, at the time and until our own first report (see below), this was the best enantioselectivity known in transition metal catalysis employing a chiral bissulfoxide ligand. (72) were evaluated in palladium catalysed AAS reaction. 149 Moderate results were achieved (up to 69% ee) during the substitution of dimethyl malonate with rac-(E)-1,3diphenylallyl acetate (Table 2, Entry 15).…”

“…More recently, ferrocenyl ligands having chiral sulfinyl and phosphinyl groups (72) were evaluated in palladium catalysed AAS reaction (Fig. 47).…”

“…Fig. 47 Sulfoxide-phosphine hybrid ligands for AAS More recently, ferrocenyl ligands having chiral sulfinyl and phosphinyl groups(72) were evaluated in palladium catalysed AAS reaction 149. Moderate results were achieved (up to 69% ee) during the substitution of dimethyl malonate with rac-(E)-1,3diphenylallyl acetate (Table2, Entry 15).Ligand 73, which is structurally very closely related to ligands…”

Redox Chemistry of Bis(oxazolinyl)cyclopentadienyl and -fluorenyl Rhodium and Iridium Organometallic Compounds

“…The second approach, ring slip of the cyclopentadienyl group from pentahapto to tri- or monohapto, could also allow the supporting ligand to respond to electronic changes at the metal center. Indenyl (Ind) and fluorenyl (Flu) derivatives provide coordination modes and reactivity patterns that are difficult for the parent Cp ligand system to access. − A derivative combining bis­(oxazoline) and fluorenyl groups could potentially coordinate in a piano-stool configuration, bidentate C,N (Chart B, bottom), or tridentate C,N,N modes, and these coordination modes could support the multiple electronic configurations of redox-active metal centers. In addition, we thought flexibility within the anionic π donor group, via ring slip, could allow reversible coordination of pendent oxazoline groups in substitution reactions of donor ligands.…”

Steric Tuning of Sulfinamide/Sulfoxides as Chiral Ligands withC1, Pseudo-meso, and Pseudo-C2Symmetries: Application in Rhodium(I)-Mediated Arylation

“…25 However, it must be taken into 79 account the fact that the methylsulfinyl carbanion used as a 80 nucleophile also has a stereogenic sulfur that could also 81 influence (matched and mismatched pairs), or even control, 82 the stereochemistry of the new stereogenic center generated in 83 the process, as it has been previously demonstrated in the case 84 of N-p-tolylsulfinylimines as substrates. 30 The additions of the 85 corresponding racemic tert-butyl and phenyl methyl sulfoxides, 86 rac-2a or rac-2b, on different (R)-N-tert-butylsulfinilimines, 87 1a(R S )−1c(R S ), were carried out at −78 °C, adding LHMDS s1 88 (300 mol %) to a solution of both reagents in THF (Scheme s1 89 1). In all cases, the reactions were completed in <0.5 h, 90 yielding high to quantitative chemical yields, a 1:1 mixture of 91 two of the four possible diastereoisomers (Scheme 1).…”