Quantitative Line‐Shape Analysis of Temperature‐ and Concentration‐Dependent ¹³C‐NMR Spectra of ⁶Li‐ and ¹³C‐Labelled Organolithium Compounds. Kinetic and Thermodynamic Data for Exchange Processes in Dibromomethyllithium, (Phenylthio)methyllithium, and Butyllithium

Abstract

(1 9.W 1.85)Using the 'permutation of indices' method proposed by Kupian and Fruenkel, we could formulate the density-matrix equations required to lit the temperature-dependent I3C-NMR spectra observed with the title compounds. For 6Li'3CHBr2 (1) and 6Li'3CH2SC,H, (2) an exchange mechanism is proposed by which monomers interchange C-and Li-atoms via a non-observed dimeric intermediate; the activation parameters of these intermolecular dynamic processes have been found to be A H # = 10.2 kcal/mol, AS' = 13.7 cal/mol.K for 1 and A H # = 11.1 kcal/mol, A S # = 20.6 cal/mol.K for 2 ((D,)THF as solvent). In the case of ('Li)butyllithium (3), the observed low-temperature spectra indicate that dimeric (3b) and tetrameric (3a) species are in dynamic equilibrium interchanging the C3H,"CH2 groups (and THF molecules) bonded to the 6Li-atonis. The relative concentrations of the dimer and of the tetramer have been determined by peak integration or by line-shape fitting; the 'pseudo'-equilibrium constant, defined by KLq = [3bI2/[3a], was found to be 2.6 -mol/l (at -88") and corresponds to dG, (-88") = 2 AGi(3b) -dGi(3a) = 1.34 kcdl/mol. The activation parameters of the dynamic process responsible for the exchange were estimated as A H # = 3.78 kcal/mol and A S z = -31.3 cal/mol.K. Tentative interpretation of thc thermodynamic and kinetic parameters is given.

Wherefore Art Thou Copper? Structures and Reaction Mechanisms of Organocuprate Clusters in Organic Chemistry

“…The spectroscopic observations are consistent with the experimental and theoretical thermodynamic considerations. Dissociation of the (Me 2 CuLi) 2 dimer to the monomer is enthalpically highly disfavored by gas-phase calculations, [138] and must also be entropically disfavored due to solvent participation, [139,140] as is known experimentally for alkyllithium oligomers (see Equation (11); the dissociation process is accompanied by a loss of 18.8 cal mol À1 K À1 [140] ).…”

“…[21] 2) Due to the fractional RÀLi bonds, [140] clusters and polymers can reversibly form open clusters, which trap the unsaturated substrate through multiple-point binding (see Schemes 10 and 11). Lithium cations assist electron flow from the organocuprate to the electrophile, and to achieve such cooperative action, a cluster of appropriate size may be necessary.…”

Mechanisms of Copper‐mediated Addition and Substitution Reactions

“…In ethereal solution, R 2 CuLi exists as higher aggregates, the LiaR bonds of which are fractional [26,[177][178][179]. It is invariably possible to identify a neutral fragment, RaCuaRaLi, in crystals of cyclic oligomers and higher polymers (Scheme 10.16).…”

Formation mechanism of acetaldehyde lithium enolate by reaction of n‐butyllithium with tetrahydrofuran: infrared and Raman spectroscopy and density functional theory calculations

“…McGarrity and Ogle 4 and Heinzer et al 31 measured the n-BuLi tetramer-dimer equilibrium in THF and their calculated thermodynamic parameters are summarized in Table 6. Large discrepancies exist between the two data sets, mainly between the values of the calculated equilibrium constant (K).…”