Analytical aspects of organolithium compounds

Lithium Enolates of Simple Ketones: Structure Determination Using the Method of Continuous Variation

“…A comprehensive survey of scaled procedures used by Pfizer Process over two decades shows that 68% of all C−C bond formations are carbanion-based and 44% of these involve enolates. , Even a casual survey of synthesis papers emanating from academic labs reinforces the notion that lithium enolates are indispensable . It may seem puzzling, therefore, that structure−reactivity relationships in enolatesthe influence of solvation and aggregation on reactivityare poorly understood when compared with other commonly used classes of organolithiums such as alkyllithiums and lithium amides. − The primary contributions have come from Jackman and co-workers, 4a,d, Streitwieser and co-workers, and several research groups focusing on methacrylate ester polymerizations . The limited progress toward understanding lithium enolates is glaringly simple: Despite extensive crystallographic determinations of lithium enolates, there are few methods for determining enolate structures in solution, and none are general (vide infra).…”

Synthesis, Structures, and Kinetics of Mixed-Donor Amido−Amino−Siloxo Ligands from Symmetrical Diamidosilyl Ether Ligands via a Retro-Brook Rearrangement

“…For all compounds, peaks in the 1 H NMR spectra show only slight shifting and broadening (i.e., no new peaks or coupling) as a function of temperature. However, the 7 Li NMR spectra in toluene- d 8 indicated that some fast intramolecular fluxionality and/or intermolecular aggregation equilibria (for 1c and 3c ) or a static system ( 4c ) is present, but the nature of the fluxionality or aggregation level could not be determined. ,, In THF- d 8 , the 7 Li NMR spectra all show fluxional behavior: 3c shows two peaks at 183 K (δ −0.68, −2.06) and 4c shows two peaks at 298 K (δ −1.83, −0.81), likely due to Li−amide and Li−siloxide groups. ,, Upon raising the temperature, coalescence to a single peak (δ −1.35 at 243 K for 3c and −0.86 at 323 K for 4c ) is observed (representative spectra for 3c are shown in Figure ). For 1c , the situation is more complex than for the bulkier 3c or 4c (see Experimental Section for 7 Li NMR data), and since both intramolecular fluxionality and some cluster dissociation/association equilibria are plausibly occurring simultaneously in 1c in THF- d 8 we have not made any further attempts to interpret its 7 Li NMR spectra.…”

“…5,73,74 In THF-d 8 , the 7 Li NMR spectra all show fluxional behavior: 3c shows two peaks at 183 K (δ -0.68, -2.06) and 4c shows two peaks at 298 K (δ -1.83, -0.81), likely due to Li-amide and Li-siloxide groups. 46,75,76 Upon raising the temperature, coalescence to a single peak (δ -1.35 at 243 K for 3c and -0.86 at 323 K for 4c) is observed (representative spectra for 3c are shown in Figure 8). For 1c, the situation is more complex than for the bulkier 3c or 4c (see Experimental Section for 7 Li NMR data), and since both intramolecular fluxionality and some cluster dissociation/association equilibria are plausibly occurring simultaneously in 1c in THF-d 8 we have not made any further attempts to interpret its 7 Li NMR spectra.…”

Diastereoselective Ortho Lithiation of Phosphinimidic Amides: A Multinuclear Magnetic Resonance and Computational Study

“…For compounds where a lithium cation is bound to a carbon, the number of C–Li contacts can often be determined from the detection of the 13 C, 7 Li coupling constant. Except for ortho-lithiated phosphinic amides, ortho-lithium compounds tend to aggregate into dimers via C–Li–C bridges, leading to the formation of C 2 Li 2 cores in the absence of strongly coordinating reagents. , The multiplicity observed for C13 indicates that dianion 10 exists as a monomer in THF solution. Most probably, the lithium atom is additionally coordinated to the nitrogen of the phosphazenyl moiety, thus forming a five-membered metallacycle, and would complete the preferred tetracoordination by solvation with two THF molecules.…”