The stereochemistry of aziridine borane lithiation: diastereoselectivity and enantioselectivity

Lithiation of 1-methylaziridine borane, 1-(tert-butyldimethylsiloxyethyl)aziridine borane, or 1-(tert-butyldimethylsiloxyethyl)-2-methylaziridine borane occurs syn to the boron substituent, while lithiation of 1-(tert-butyldimethylsiloxyethyl)-2-trimethylstannylaziridine borane occurs anti to boron as well as silicon due to the steric effect of trimethylsilyl group (s-butyllithium was used in all cases). Kinetically controlled lithiation in the first three cases results from a combination of steric and electrostatic effects. Enantioselective lithiation occurs in the presence of (−)-sparteine, with product enantioselectivities near 70% ee.