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-(dimethyl ethylsilyl)benz (13), and internally coordinated, [alpha-[[[cis-2,5-bis(met hoxymethyl)-1-pyrrolidinyl]methyl] dimethylsilyl]-p-tert-butylbenzyl]lithiu m (14) and [alpha-[[[(S)-2-(methoxymethyl)-1-pyrrolidinyl]methyl]dimethylsi lyl]benzyl]lithium (15), have been prepared. Ring C-13 NMR shifts indicate that 12-15 have partially delocalized structures. Externally solvated allyl ic lithium compounds are found to be delocalized, and only some internally coordinated species are partially delocalized. Compound 15 exists as >95% o f one stereoisomer of the two invertomers at C-alpha. This is in accord wit h 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 o f 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 (extern ally solvated) and 14 (internally solvated), Delta H(inv)double dagger appr oximate to 4.9 +/- 0.5 kcal mol(-1). By contrast, barriers to rotation arou nd the ring-C, bonds vary widely, depending on the mode of Lithium coordina tion, Delta H(rot)double dagger approximate to 8 +/- 0.5 to 19 +/- 1.0 kcal mol(-1). Some mechanisms for these processes are proposed.