Mechanism and Solvent Catalysis of the Degenerate 1,12-Metalations of [1.1]Ferrocenophanyllithium and [1.1]Ferrocenophanylsodium Studied by NMR Spectroscopy

Abstract

Insight into the detailed mechanism of carbon lithiation by an organolithium reagent and of carbon sodiation by an organosodium reagent has been obtained using [1.1]ferrocenophanyllithium (1) and [1.1]ferrocenophanylsodium (3), respectively. In tetrahydrofuran (THF) 1 and 3 undergo rapid 1,12-proton transfer reactions which are coupled with 1,12-lithium ion and 1,12-sodium ion transfers, respectively. It is concluded that the degenerate rearrangement of 1 does not make use of a pseudorotation mechanism, but occurs by direct conversion of a syn-conformer to another syn-conformer. Activation parameters (ΔH ⧧ = 19 kJ mol-1 and ΔS ⧧ = −93 J K-1 mol-1) for the degenerate reaction of 1 in THF have been measured by dynamic NMR spectroscopy. The primary isotope effect (k H/k D) of the 1,12-hydron transfer is 7.4 ± 1.5 at 320 K. The degenerate rearrangement of 1 shows strong solvent dependence, e.g. the reaction is 4 × 103 times faster in THF than in dimethyltetrahydrofuran (DMTHF). Thus, the rearrangement may be catalyzed by THF in DMTHF. The catalysis is first order in THF at low concentrations of THF. The results show that in the rate-limiting activated complex the lithium ion is coordinating one solvent molecule more than in the initial state. It is paired with a carbanion in which the proton is symmetrically located between the bridge carbons. Compound 3 shows a behavior similar to that of 1, but it is more fluxtional. It also shows a solvent catalysis that is weaker than for 1. It is concluded that 3 also exchanges using an activated complex that contains one solvent molecule more than the initial complex.

Li vs Na: Divergent Reaction Patterns between Organolithium and Organosodium Complexes and Ligand-Catalyzed Ketone/Aldehyde Methylenation

“…Compared with other popular CH 2 transfer reagents, such as phosphorus ylides (Wittig), sulfones (Julia and Kocieński), titanium (Tebbe), zirconium, rare-earth, molybdenum (Kauffmann), or chromium (Takai) reagents, the organosodium reagent [NaCH 2 SiMe 3 ] ∞ features significant sustainability advantages and, potentially, a much lower cost (for comparison, Tebbe reagent solution (0.5 M in toluene) is priced at £229 for 25 mL at Merck). The concept of ligand-catalysis (and the relevant solvent-catalysis) is based on reversible coordination of a ligand (e.g., Me 6 Tren in this case) to the metal center, which has been seldom reported in Group-1 metal chemistry as early as in the 1960s − until today, − mostly for deprotonation reactions.…”

An NMR and Computational Study of a Lithium Amide and Ethereal Ligand Exchange

“…This is compensated for by the entropy term, which is calculated to be % 25 cal K À1 mol À1 , in agreement with other experimental data. [18] Geometry optimizations of DEE and THF: There are several local energy minima or conformations for DEE (Scheme 4 and B3LYP/6-31(d)//B3LYP/6-31(d) level were calculated to be the planar trans-trans and the trans-gauche minima. The transgauche minimum is 1.34 kcal mol À1 higher in DFT energy than the planar trans-trans minimum.…”

Ketone Enolization with Sodium Hexamethyldisilazide: Solvent- and Substrate-Dependent E–Z Selectivity and Affiliated Mechanisms

“…Over a century in which organolithium chemistry flourished, organosodium chemistry languished despite the prevalence and low cost of sodium. , It is difficult to find examples in which sodium-based reagents are used for sophisticated or nuanced applications in stereo- or regiocontrolled organic synthesis . While it might be tempting for the mechanistically minded to attribute this to a dearth of underlying solution structural and mechanistic principles, − there is scant evidence that a lack of physical principles impedes the empiricism-based development of new methods. A perceived inconvenience of the obvious foundational reagents n -butylsodium ( n -BuNa) and sodium diispropylamide (NaDA), when compared with commercially available n -butyllithium ( n -BuLi) and lithium diispropylamide (LDA) could have stifled development, but even that impediment was probably exaggerated and is now largely resolved. , …”