Abstract
As a non-covalent interaction of a chiral scaffold in catalysis, pnicogen bonding of epi-cinchonidine (epi-CD), a cinchona alkaloid, was simulated to consider whether the interaction can have the potential controlling enantiotopic face like hydrogen bonding. Among five reactive functional groups in epi-CD, two stable complexes of the hydroxyl group (X-epi-CD1) at C17 and of the quinoline ring (X-epi-CD2) at N16 with pnictide family analytes [X = substituted phosphine (PX), i.e., F, Br, Cl, CF3, CN, HO, NO2, and CH3, and pnictide family analytes, i.e., PBr3, BiI3, SbI3, and AsI3] were predicted with intermolecular interaction energies, charge transfer (QMulliken and QNBO), and band gap energies of HOMO–LUMO (Eg) at the B3LYP/6-31G(d,p) level of density functional theory. It was found that the dominant site of pnicogen bonding in epi-CD is the quinoline ring (N16 atom) rather than the hydroxyl group (O36 atom). In addition, the UV-Vis spectra of the complex were calculated by time-dependent density functional theory (TD-DFT) at the B3LYP/6-31+G(d,p) level and compared with experimental measurements. Through these calculations, two intermolecular interactions (H-bond vs. pnicogen bond) of epi-CD were compared.
Highlights
Cinchona alkaloids have played an important role as privileged chiral sources in the history of chemistry due to their diverse chiral skeletons and tunable reactions
An oxyanion hole between the hydroxyl group at C17 and the nitrogen atom of quinuclidine (N43) was proposed (Bürgi and Baiker, 1998), and a density functional theory (DFT) study of quinine-catalyzed aza-Henry reaction explained the mechanism through the hydrogen bonding interaction (Xue et al, 2016)
We studied the possibility of pnicogen bonding between a Lewis base of epi-cinchonidine, a type of cinchona alkaloid, and covalently bonded P, As, Sb, and Bi of the pnictide family as a Lewis acid
Summary
Cinchona alkaloids have played an important role as privileged chiral sources in the history of chemistry due to their diverse chiral skeletons and tunable reactions (like chiral ligands in Sharpless asymmetric dihydroxylation; Maeda et al, 2011; Tian et al, 2004; Szollosi et al, 2011; Sim et al, 2015; Marcelli and Hiemstra, 2010). The delicate differences in the reactivity of their five functional groups have not been sufficiently investigated when compared with intensive use of cinchona alkaloids Despite their potential controlling asymmetric reactions through the non-covalent interaction (NCI) (Manna and Mugesh, 2012; Pal et al, 2016; Wheeler et al, 2016; Benz et al, 2017; Breugst et al, 2017; Li and Hong, 2018), the scope and limitation of the NCIs in these five functional groups has not been properly studied. An oxyanion hole between the hydroxyl group at C17 and the nitrogen atom of quinuclidine (N43) was proposed (Bürgi and Baiker, 1998), and a density functional theory (DFT) study of quinine-catalyzed aza-Henry reaction explained the mechanism through the hydrogen bonding interaction (Xue et al, 2016)
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