Isotope Effects and Mechanism of the Asymmetric BOROX Brønsted Acid Catalyzed Aziridination Reaction

Abstract

The mechanism of the chiral VANOL-BOROX Brønsted acid catalyzed aziridination reaction of imines and ethyldiazoacetate has been studied using a combination of experimental kinetic isotope effects and theoretical calculations. A stepwise mechanism where reversible formation of a diazonium ion intermediate precedes rate-limiting ring-closure to form the cis-aziridine is implicated. A revised model for the origin of enantio- and diastereoselectivity is proposed based on relative energies of the ring closing transition structures.

Self-assembly of a library of polyborate chiral anions for asymmetric catalytic quinoline reduction

“…1 The catalyst is a polyborate chiral anion that contains a boroxine ring in which one of the borons is four coordinate producing a boroxinate core (Figure 1). 2 These boroxinate or BOROX catalysts, are self-assembled by a substrate of the catalyst, which is an imine or amine in the case of the aziridination reaction (Scheme 1).…”

“…2 These boroxinate or BOROX catalysts, are self-assembled by a substrate of the catalyst, which is an imine or amine in the case of the aziridination reaction (Scheme 1). 3 The base induced assembly of the BOROX catalyst can be achieved from: 1) the ligand, water, and triphenylborate, 3-5 2) the ligand and triphenoxyboroxine 1b and 3) the ligand, borane dimethysulfide complex, phenol and water. 4,6 …”

“…1c,d In addition to rendering proton catalysis enantioselective, it was proposed that the VAPOL/VANOL-BOROX counterion also plays the role of orchestrating the orientation of the substrates at the key transition state via multiple H-bonding interactions with oxygens O1, O2 and O3 in the boroxinate core. 1c,d In addition to aziridinations, the BOROX catalyst is also involved in an asymmetric catalytic aza-Cope rearrangement in the aminoallylation of aldehydes, 7 in a catalytic asymmetric 3-componnent Ugi reaction 8 and presumably in a heteroatom Diels-Alder reaction as well. 9 Since the BOROX catalyst is self-assembled in-situ when the substrate is added, it thus becomes possible to generate a vast library of the BOROX catalysts on the spot by the incorporation of different alkoxy groups in the boroxinate core.…”

“…1,3 The 11 B and 1 H NMR spectra of the VAPOL-BOROX complex with 1a were distinctive (Figure 2). The two 3- coordinate borons appear as a broad peak at 15.97 ppm, and the 4-coordinate boron as a sharp peak at 5.76 ppm, with an integration of 2:1 respectively (not shown).…”

“…These observations are in accord with those made previously for the BOROX catalysts in our aziridination studies. 1,3 …”