Stereochemistry of Solvation of Benzylic Lithium Compounds:  Structure and Dynamic Behavior

Abstract

Several sec-benzylic lithium compounds, both externally coordinated, [α-(trimethylsilyl)benzyl]lithium·PMDTA (12) and p-tert-butyl-α-(dimethylethylsilyl)benzyllithium·TMEDA (13), and internally coordinated, [α-[[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl]dimethylsilyl]-p-tert-butylbenzyl]lithium (14) and [α-[[[(S)-2-(methoxymethyl)-1-pyrrolidinyl]methyl]dimethylsilyl]benzyl]lithium (15), have been prepared. Ring 13C NMR shifts indicate that 12−15 have partially delocalized structures. Externally solvated allylic lithium compounds are found to be delocalized, and only some internally coordinated species are partially delocalized. Compound 15 exists as >95% of one stereoisomer of the two invertomers at Cα. This is in accord with a published ee of >98% in products of the reactions of 15 with aldehydes. All four compounds show evidence of one-bond 13C−6Li spin coupling, ca. 3 Hz, which indicates a small detectable C−Li covalence. Averaging of the 13C−6Li coupling of 12 with increasing temperature provides the dynamics of intermolecular C−Li bond exchange, with Δ = 9 ± 0.5 kcal mol-1. Carbon-13 NMR line shape changes due to geminal methyls, and ligand carbons gave similar rates of inversion at Cα in 13 (externally solvated) and 14 (internally solvated), Δ ≈ 4.9 ± 0.5 kcal mol-1. By contrast, barriers to rotation around the ring-Cα bonds vary widely, depending on the mode of lithium coordination, Δ ≈ 8 ± 0.5 to 19 ± 1.0 kcal mol-1. Some mechanisms for these processes are proposed.

Dinuclear Dicyclopentadienyl Titanium Complexes with Bridging Cyclopentadienylsiloxo Ligands

“…However, the description of the organolithium reagent required some test calculations. Solvent effects are very important in organolithium chemistry, and they dramatically influence the aggregation state and reactivity of alkyllithium compounds. − Test calculations proved than only one molecule of toluene is coordinated to the lithium atom of LiMe. For this reason LiMe(tol) ( H ) was chosen as reagent (see Supporting Information for further discussion on the choice of the model system).…”

Ligand effects on coordination properties of organolithium compounds: insights from computational experiments on a “weakened” Li+

“…long time concern and it is supposed to depend on the solvent and other additives present in the reaction medium. [23] Figure 5: Allyl anions, naked (left) and in interaction with a lithium cation in a bridged (center) or terminal (right) mode.…”

Contact vs solvent‐separated ion pairs: a 7Li and 13C solid‐state NMR study of organolithium compounds

“…of [12]crown-4 were added to the THF solution of the organolithium precursor; 3c crystallized at 27°C (orange powder). Compound 3b was prepared according to the procedure published by Fraenkel et al 21 and crystallized at 27°C as colorless needles. All solids obtained were washed three times with hexane, dried in vacuo (ca 10 2 mbar) and transferred into 4 mm zirconium dioxide rotors equipped with Kel-F caps; ca 200 mg of compound were used for each sample.…”