QUANTUM-CHEMICAL MODELING OF CHIRAL CATALYSIS .18. CONFORMATIONAL STUDIES ON CHIRAL N-SULFONYLATED 1,3,2-OXAZABOROLIDINES AND RELATED ALDEHYDE COMPLEXES POTENTIALLY INVOLVED IN THE CATALYTIC ASYMMETRIC DIELS-ALDER REACTIONS

Properties of Lewis acidic oxazaborolidines 1 were investigated by mea ns of ab initio MO methods (RHF, N-sulfonylated 1,3,2-oxazaborolidine 1' as a model). Energies of the coordination of aldehydes to 1 were es timated by using formaldehyde and acrolein adducts of 1' as models. En ergies (MP2/6-31G//6-31G) of the coordination of formaldehyde to 1' we re -1 kJ mol(-1) (2'b; axial S=O) and +26 kJ mol(-1) (2'a; equatorial S=O). The corresponding energies of the formation of acrolein adducts (3'a and 3'b, both s-trans, equatorial S=O) were higher, +43 kJ mol(-1 ) (vinyl equatorial) and +54 kJ mol(-1) (vinyl axial). An s-cis confor mer of 3'a was found to be unstable.