Host-guest interactions and their role in enantioselective hydrogenation of alpha-keto esters - An analysis of model systems

The interaction between cinchonidine and methyl pyruvate has been proposed as the key step leading to enantiodifferentiation in the enantioselective h ydrogenation of alpha -ketoesters, In the present work, we employ ab initio MP2/6-31G(d) and MP2/6-31G(d,p) methods to carry out an analysis of the mo st relevant kind of interactions operating in representative model systems. These interactions are discussed in terms of orbital superposition and dip olar interaction. When approaching H2CO to NH3 at distances lower than 3.4 Angstrom, orbital superposition is the predominant interaction, while at di stances above 3.4 Angstrom, both orbital superposition and dipolar interact ions may contribute to stabilization, with a small prevalence of dipolar in teractions. The stabilization energy at large distances (above 4.5 Angstrom ) is very small (about 0.5 kcal mol(-1)), probably not enough to be respons ible for the enantiodifferentiation process. Semiempirical calculations on the parent systems were also unable to reveal any special interaction which could be attributed to the enantiodifferentiation process. (C) 2001 Elsevi er Science B.V. All rights reserved.