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

Several sec-benzylic lithium compounds, both externally coordinated, [alpha-(trimethylsilyl)benzyl]lithium PMDTA (12) and p-terr-butyl-alpha-(dimethylethylsilyl)benz (13), and internally coordinated, [alpha-[[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl] dimethylsilyl]-p-tert-butylbenzyl]lithium (14) and [alpha-[[[(S)-2-(methoxymethyl)-1-pyrrolidinyl]methyl]dimethylsilyl]benzyl]lithium (15), have been prepared. Ring C-13 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 C-alpha. 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 C-13-Li-6 spin coupling, ca. 3 Hz, which indicates a small detectable C-Li covalence. Averaging of the C-13-Li-6 coupling of 12 with increasing temperature provides the dynamics of intermolecular C-Li bond exchange, with Delta H(ex)double dagger = 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 C-alpha in 13 (externally solvated) and 14 (internally solvated), Delta H(inv)double dagger approximate to 4.9 +/- 0.5 kcal mol(-1). By contrast, barriers to rotation around the ring-C, bonds vary widely, depending on the mode of Lithium coordination, Delta H(rot)double dagger approximate to 8 +/- 0.5 to 19 +/- 1.0 kcal mol(-1). Some mechanisms for these processes are proposed.