Configurational Equilibria in Amido and Lithioamido Complexes of Formulas (η⁵-C₅H₅)Re(NO)(PAr₃)(N̈HCHRR‘) and (η⁵-C₅H₅)Re(NO)(PAr₃)(N̈LiR‘‘):  Epimerization Occurs at Rhenium via Phosphine Dissociation

Abstract

The diastereomerically and enantiomerically pure amido complex (SR)-(η5-C5H5)Re(NO)(PPh3)(N̈HCH(CH3)Ph) ((SR)-5) converts to (RR)-5 (inversion at rhenium, retention at carbon) in THF-d 8 at 49.4 °C with k 1 = 2.34 × 10-4 s-1 and k - 1 = 0.90 × 10-4 s-1. Similarly, (SS)-5 converts to (RS)-5 with k 1 = 0.90 × 10-4 s-1 and k - 1 = 2.30 × 10-4 s-1. Both epimerizations give equilibrium ratios (RR/SR or SS/RS) of 70:30. Reactions with HOTf yield [(η5-C5H5)Re(NO)(PPh3)(NH2CH(CH3)Ph)]+TfO-, and subsequent additions of Et4N+CN- afford (η5-C5H5)Re(NO)(PPh3)(CN) and NH2CH(CH3)Ph (all steps with retention at rhenium and carbon). Enantiomeric purities and absolute configurations are assayed by chiral NMR shift reagents and (−)-menthyl chloroformate derivatives, respectively, establishing configurations of epimerized 5. Reaction of (SR)-5 and P(p-tol)3 in THF-d 8 at 49.4 °C gives (η5-C5H5)Re(NO)(P(p-tol)3)(N̈HCH(CH3)Ph) (50:50 (t 0) → 66:34 (t ∞) RR/SR) with k = 4.6 × 10-4 s-1, twice that for the conversion of (SR)-5 to (RR)-5. Rate data for the latter at 32.3−59.1 °C give ΔH ⧧ = 26 kcal/mol and ΔS ⧧ = 6 eu. These results are best modeled by mechanisms involving initial and rate determining PPh3 dissociation, with anchimeric assistance by the amido lone pair, to give an intermediate that is trigonal planar at rhenium and combines with PAr3 without significant diastereoselectivity. Reactions of n-BuLi with 5 and related complexes give N̈LiR species from which PPh3 is lost at lower temperatures, and are presumed to be much less configurationally stable.

Syntheses of Enantiopure Bifunctional 2-Guanidinobenzimidazole Cyclopentadienyl Ruthenium Complexes: Highly Enantioselective Organometallic Hydrogen Bond Donor Catalysts for Carbon–Carbon Bond Forming Reactions

“…Calcd for C 40 H 29 Cl 12 N 6 O 7 PRu·CHCl 3 : C, 35.62; H, 2. 19;Cl,38.49;N,6.08. Found: C,35.04;H,2.05;Cl,38.24;N,5.84.…”

Asymmetric Induction and Configurational Stability at the Metal Centre in Half‐Sandwich (η6‐p‐Cymene)ruthenium(II) and (η5‐C5Me5)rhodium(III) Complexes Containing Chiral N‐N* Ligands with Different Rigidity and Flexibility

“…[15] Generally, the results are best explained by mechanisms involving initial dissociation of a coordinated ligand, with possible anchimeric assistance, [15,16] as a rate-determining step, to give chiral pyramidal intermediates which can interconvert through a planar species. The conformational stability of such species has been established by theoretical studies, [17] and experimental results [16,18] have shown that the stereochemical outcome can also be determined after the dissociative rate-determining step and that it can be influenced by the steric features of the incoming ligand. In fact, experimental data show that, in some cases, increasing the bulk of the incoming ligand also increases the diastereomeric excess; this supports an associative Ia-type interchange mechanism.…”

“…In fact, experimental data show that, in some cases, increasing the bulk of the incoming ligand also increases the diastereomeric excess; this supports an associative Ia-type interchange mechanism. [18] We have found previously [4] that the kinetics, under pseudo-first-order conditions, of the chelation process in the neutral species [Ru(η 6 -arene)(η 1 -P-N*)Cl 2 ] [P-N* = (β-aminoalkyl)phosphanes; arene = benzene, hexamethylbenzene, p-cymene], in CDCl 3 solution containing variable amounts of methanol, or of alcohols with different hydrogen-bonding properties, follows a first-order course, and the k obs values are linearly correlated to the nucleophile (methanol) concentration, with no significant intercept. These kinetic results led us to propose a mechanism in which the rate-determining step of the process is solvolysis of the starting complex in methanol.…”

Synthesis and Characterization of Chiral Bis(aminato)iridate(I) and Aminato-Bridged Iridium(I) Complexes

“…Solvent molecules such as THF and Et 2 O cannot be removed completely under vacuum, but no NMR shifts of the solvent molecules due to coordination are observed. However, even the Li -specific crown ether [12]crown-4 did not separate the ion pairs. Low-temperature 2D-NMR experiments aimed at elucidating the structures of 4 and 5, with emphasis on the location of the Li cation were not conclusive.…”

“…Thus, our goal was the isolation and characterization of reactive aminatoiridium complexes that could be intermediates or precursors of (1) our Ir-catalyzed enantioselective hydroamination [3a], and (2) that could function as N-analogs of cuprates. To the best of our knowledge, only one chiral, enantiomerically pure aminato complex has been reported so far [12]. We describe herein the synthesis and structural characterization of novel chiral aminatoiridates and aminato-bridged iridium(I) complexes and the, however unsuccessful, attempts to use them in hydroamination and 1,4-N-additions.…”