BOROX Catalysis: Self-assembled amino-BOROX and imino-BOROX Chiral Brønsted Acids in a Five Component Catalyst Assembly/Catalytic Asymmetric Aziridination

Abstract

A five-component catalyst assembly/aziridination reaction is described starting from an aldehyde, an amine, ethyl diazoacetate, B(OPh)3 and a molecule of a vaulted biaryl ligand (VAPOL or VANOL). A remarkable level of chemo-selectivity was observed since while 10 different products could have resulted from various reactions between the five components, an aziridine was formed in 85% yield and 98% ee and only two other products could be detected in 3% yield. Studies reveal that the first in a sequence of three reactions is an exceedingly rapid amine induced assembly of an amino-boroxinate (AMINO-BOROX) species from VAPOL and B(OPh)3 which is followed by imine formation from the amine and aldehyde and the concomitant formation of an IMINO-BOROX species, and finally the reaction of the imine with ethyl diazoacetate mediated by the IMINO-BOROX catalyst to give aziridine-2-carboxylic esters with very high diastereo- and enantioselectivity.

β-Amino Esters from the Reductive Ring Opening of Aziridine-2-carboxylates

“…5 To set the stage for the regiocomplementary reductive ring opening of aziridine 54 , the MEDAM group was cleaved with trifluoracetic acid in anisole and the resulting N–H aziridine was not purified but rather directly protected by TsCl or SESCl to give N-protected aziridines 57 and 58 in 70 and 78% yield, respectively, for the two steps. The samarium diiodide-mediated ring opening of both tosyl aziridine 57 and SES-aziridine 58 gave complex mixtures because of the cleavage of zero, one, or two of the acetoxy groups on the benzene ring.…”

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

“…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 …”

“…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 …”

“…The BOROX catalyst in most of our aziridination studies was generated from B(OPh) 3 as the boron source, 1,3,4 which was also the method in the preliminary screens in Table 1. It was found in the present study that replacing B(OPh) 3 with the combination of BH 3 • SMe 2 and phenol provided identical results (compare result with phenol in Scheme 2 with that in entry 2 of Table 1).…”