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.