Chiral Diamines 4: A Computational Study of the Enantioselective Deprotonation of Boc-pyrrolidine with an Alkyllithium in the Presence of a Chiral Diamine
2001-07-27T00:00:00Z (GMT) by
The enantioselective deprotonation of <i>N</i>-Boc-pyrrolidine (<b>1</b>) with i-PrLi−(−)-sparteine has been studied at theoretical levels up through B3P86/6-31G*. Four low-energy intermediate complexes involving i-PrLi−(−)-sparteine and <b>1</b> were located via geometry optimizations; two of these complexes would lead to abstraction of the <i>pro</i>-S hydrogen from <b>1</b>, and the other two complexes would lead to loss of the <i>pro</i>-R hydrogen. The lowest-energy intermediate complex was found to lead to loss of the <i>pro</i>-S hydrogen as observed experimentally. Transition states for the deprotonations were located using the synchronous transit-guided quasi-Newton method. The calculated activation enthalpy for transfer of the <i>pro</i>-S hydrogen within the lowest-energy intermediate complex, 10.8 kcal/mol, is reasonable for a reaction that occurs at a relatively low temperature, and the calculated kinetic hydrogen isotope effect is in agreement with experimental data. The lower enantioselectivity observed experimentally for deprotonation of <b>1</b> using t-BuLi−(−)-sparteine is attributed to a transition-state effect due to increased steric interaction engendered by the bulky t-BuLi. Replacement of the <i>tert</i>-butoxycarbonyl group in <b>1</b> by a methoxycarbonyl is predicted to result in a slower deprotonation with somewhat decreased enantioselectivity. Asymmetric deprotonation of <b>1</b> using i-PrLi in combination with the <i>C</i><sub>2</sub>-symmetric diamine, (<i>S,S</i>)-1,2-bis(<i>N</i>,<i>N</i>-dimethylamino)cyclohexane, was calculated to be much less selective than is the deprotonation mediated by (−)-sparteine as observed experimentally. The relative energies of the intermediate complexes were fairly well-reproduced by ONIUM calculations in which the sparteine ligand less its nitrogen atoms was treated by molecular mechanics and the remainder of the complex was treated by quantum mechanics.