Evidence for a Boroxinate Based Brønsted Acid Derivative of VAPOL as the Active Catalyst in the Catalytic Asymmetric Aziridination Reaction

Abstract

Studies are described that were designed to determine the structure of the active catalyst in the asymmetric catalytic aziridination of imines with ethyl diazoacetate (AZ reaction). Evidence suggests that the active catalyst contains a boroxine ring in which one of the three boron atoms is spiro-fused with the two phenol groups of the VAPOL ligand. (11)B and (1)H NMR evidence supports the boroxinate structure B in which the counterion to the boroxinate is the protonated form of the imine. The boroxinate structure is also supported by two solid state structures of a VAPOL boroxinate in which the gegen cation is tetramethyl ammonium and 4-dimethylaminopyridinium.

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

“…1a Treatment of the mixture of 10 and 11 with imine 1 at room temperature leads to the formation of the chiral boroxinate 9 (B3). 3b,7 Since this species has a boroxine ring as its core, we have decided to classify this family of catalysts as BOROX catalysts. The BOROX species 9 is an ion pair consisting of a protonated base (imine) as the cation and a chiral polyborate as the anion in the form of a boroxinate: a boroxine in which one of the borons is four-coordinate.…”

“…8 We have recently reported that the chiral boroxinate 9a (B3) can also be generated upon treatment of a mixture of the VAPOL ligand 3 and 3 equivalents of B(OPh) 3 with the imine 1a at room temperature in a few minutes, conditions under which VAPOL and B(OPh) 3 do not react. 7a The question then naturally arises: what other species can induce the formation of the chiral boroxinate complex 9 ? 9 …”

Three‐Membered Heterocycles. Structure and Reactivity

“…Curiously, impure commercial samples of BH 3 ÁTHF give the best results [104]. Recent crystallographic evidence in an analogous system suggests an unexpected boroxinate species as the active catalyst, the formation of which should be facilitated by adventitious water in the commercial boron reagent [105]. Table 2.8)…”

To Protonate or Alkylate? Stereoselective Brønsted Acid Catalysis of CC Bond Formation Using Diazoalkanes

“…After submission of this manuscript, Wulff and co-workers [55] presented evidence for a boroxinate-based Brønsted acid derivative of VAPOL ((2,2-diphenyl-[3,3-biphenanthrene]-4,4-diol) as the active catalyst in asymmetric aziridination of imines [Scheme 1, Eq. (4) and Ref.…”