Preparation and DFT studies of chiral Cu (I)-complexes of biphenyl bisoxazolines and their application in enantioselective Kharasch-Sosnovsky reaction

Abstract

Effect of a range of t-butyl perbenzoates bearing electron-withdrawing and electron-donating substitutions on the phenyl ring and HZSM-5 as a porous additive at 0 °C in enantioselective allylic C-H bond oxidation of cyclic and acyclic olefins in the presence of Cu (I)-(S,aS,S) complexes of biphenyl bisoxazoline ligands, produced easily through the chelation-induced process, were investigated. The enantioenriched allylic esters were obtained in reasonable times with excellent enantioselectivities and yields using electron-withdrawing substituted peresters in the presence of Cu (I)-(S,aS,S)-1a complex, containing phenyl groups at the stereogenic centers of the oxazoline moieties. To reach a better insight on geometry, chemical activity, enantioselectivity, and thermodynamic stability of the Cu (I)-BOX complexes, DFT calculations with B3LYP-D3/6-31G (d, p) level of theory were applied to them. Moreover, NBO analysis was used to illustrate interactions between orbitals.

Scheme 1

Resolution of (S,aS,S)-bipheny bisoxazoline complex through chelation-induced process.

Scheme 2

Preparation of bipheny bisoxazoline ligands 1.

Figure 1

Comparison of the chiral ligands 1a and 1b in ee, yield and reaction time.

Scheme 3

A rational mechanism of enantioselective allylic oxidation of cyclohexene using tert-butyl p-nitrobenzoperoxoate, catalyzed by Cu (I)-(S,aS,S)-1a complex.

Figure 2

Gibbs free energy and equilibrium constant values for Cu (I)-1a complexes.

Figure 3

Optimized structures of the Cu (I)-(S,aS,S) and (S,aR,S)-1a complexes in CHCl₃.

Figure 4

Bond lengths and partial charges on selected atoms in the Cu (I)-(S,aS,S)-1a and -1b complexes.

Figure 5

HOMO–LUMO energy gaps of the Cu (I)-(S,aS,S)-1a and Cu (I)-(S,aS,S)-1b complexes.

Figure 6

Steric repulsion caused by phenyl and isopropyl substitutions.

Figure 7

Steric hindrance and Gibbs free energies of the Si-face and Re-face of the key reaction intermediate containing cyclohexene reactant. To better visibility, some atoms of biphenyl backbone are not shown.

Figure 8

Geometries and Gibbs free energies of the Si-face and Re-face of the key reaction intermediate containing 1-hexene reactant. To better visibility, some atoms of biphenyl backbone are not shown.