Understanding the mechanism of procedure-controlled enantioselectivity switch in cinchona alkaloid thiourea catalyzed [3 + 2] cycloaddition: H-bonds controlled enantioselectivity

Abstract

Density functional theory has been applied to the novel procedure-controlled enantioselectivity switch in the synthesis of chiral 2-oxazolidinone (3) through [3+2] cycloaddition between γ-hydroxy-α,β-unsaturated carbonyl compound (1) and isocyanate (2) catalyzed by a cinchona-alkaloid based aminothiourea catalyst. The study shows that three stages are involved in the whole process. In stage one, for Procedure A, 1 reacts with 2 to give carbamate intermediate. Then catalyst 4 is added. For Procedure B, catalyst 4 first activates 2 via quinuclidine N attacking isocyanate C. Then 1 is involved. In stage two, hydrogen is transferred from carbamate N to Ts O atom which is different from the quinuclidine N protonation mechanism. The last step is ring closure in which carbamate N attacks C atom of CC bond. From free energy profiles, the enantioselectivities of the reaction are determined by stage two. Further analysis shows that H-bond is the key in the procedure-controlled enantioselectivity switch. Different H-bonds (NAHA⋯OAC and NBHB⋯OAC in Procedure A vs NAHA⋯OAC and NBHB⋯OBS in Procedure B) lead to different relationships between thiourea and carbamate planes (vertical or parallel). In Procedure A, CC listing above carbamate plane is the favorable pathway to give R-3. However, in Procedure B, CC listing behind carbamate plane is more favorable to generate S-3.