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.

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Several 1-sila allylic lithium compounds have been prepared with potential ligands for Li substituted at the 2-position. They are [2-[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl]-1-(trimethylsilyl)allyl]lithium (22), [2-[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl]-1-(dimethylethylsilyl)allyl]lithium (23), [2-[[bis(2-methoxyethyl)amino]methyl]-1-(dimethylethylsilyl)allyl]lithium (24), [2-[[bis(2-methoxyethyl)amino]methyl]-1-(tert-butyldimethylsilyl)allyl]lithium (25), and [2-[2-[bis(2-methoxyethyl)amino]-1,1-dimethylethyl](ethyldimethylsilyl)]allyllithium (26). Using diethyl ether or THF solutions all these compounds exhibited one bond 13C, 7Li spin coupling of ∼8 Hz, a 1:1:1:1 13C NMR pattern indicating monomeric structures; all show ligand resonances to be magnetically nonequivalent and reveal C1, C3 13C NMR shifts of about 40 and 75 δ which lie between those for model delocalized 1 and localized species 2. These compounds are concluded to be examples of the heretofore missing folded, internally tridentately coordinated partially delocalized structures with small detectable C, Li covalence. The exception is 25 which, in diethyl ether, consists of a rapidly interconverting equilibrium mixture of localized and delocalized more solvated forms, the former prevailing at 300 K and progressively converting mainly to the latter by 180 K. NMR line shape analysis of the diastereotopic gem methylsilyl 13C resonances as well as that due to the ligand carbons shows that all these line shape changes are due to the dynamics of inversion at lithium-bound carbon and that other ligand reorientation processes are slower than carbanide inversion; for inversion, ΔH ⧧ is found to be 6−9 kcal·mol-1, respectively. Averaging with increasing temperature of the 13C, 7Li spin coupling in 24 provides the dynamics of bimolecular carbon lithium bond exchange with ΔH ⧧ of 12 kcal·mol-1. Mechanisms are proposed on the basis of the data. We ascribe restricted stereochemistry of ligand lithium coordination to be responsible for these unusual internally coordinated structures.

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