The Steric Course of SE2 Reactions of Unstabilized α-Aminoorganolithiums:  Distinguishing between SET and Polar Mechanisms

The competing mechanisms that determine the steric course of electrophilic aliphatic substitution reactions (SE2) of configurationally stable, unstabilized α-aminoorganolithiums are compared. The steric course of the reaction of lithiated pyrrolidines and piperidines with electrophiles is variable, such as when comparisons are made between two electrophiles and a single organolithium, or between two organolithiums and a single electrophile. The possible pathways considered are single electron transfer (SET) and competing polar substitutions (SE2ret vs SE2inv). Catalysis of organolithium racemization by the electrophile was eliminated as a possible source of racemic products. When the products are completely racemic, our evidence suggests that SET is the most likely mechanism; when polar pathways are operative, stereoselectivities vary from 75 to 100%, and may be invertive or retentive at the carbanionic carbon, depending on the electrophile.