Catalytic Enantioselective Construction of Chiroptical Boron-Stereogenic Compounds

Abstract

The construction of main group heteroatom-stereogenic compounds is of great importance due to their intriguing chemical, physical, biological, and stereoelectronic properties. Despite that organoboron compounds are widely used in organic chemistry, the creation of a tetrahedral boron-stereogenic center in one enantiomeric form remains highly challenging. Given the labile nature of ligands attached to the tetracoordinate boron atom, only a handful of enantioenriched boron-stereogenic compounds have been reported via resolution or a chiral substrate-induced diastereoselective approach. To date catalytic asymmetric synthesis of boron-stereogenic compounds has remained unknown. Here, we demonstrate the first catalytic enantioselective construction of boron-stereogenic compounds via an asymmetric copper-catalyzed azide−alkyne cycloaddition (CuAAC) reaction. This enantioselective CuAAC reaction not only gives access to a wide range of novel highly functionalized boron-stereogenic heterocycles in high yields with good to excellent enantioselectivities but also produces optically active terminal alkyne and triazole moieties with various potential application prospects. Further transformation of the chiral tetracoordinate boron compounds delivers several complex heterocyclic entities bearing boron-stereogenic centers without the loss of enantiopurity. Moreover, the X-ray structure, the barrier to racemization, and the HOMO/LUMO gap of selected tetracoordinate boron compounds are investigated. Notably, these novel N,N π-conjugated boron-stereogenic compounds exhibit bright fluorescence. The optical properties, including circular dichroism, quantum yield, and circular polarized luminescence spectroscopies, are examined. These features expand the chemical space of the chiroptical boron-based dye platform, which could have great potential applications in chiral optoelectronic materials.

Synthesis and stereochemistry of chiral aza-boraspirobifluorenes with tetrahedral boron-stereogenic centers

“…58–60 The kinetic parameters (145 °C: Δ G ‡ = 33.7 kcal mol −1 , Δ H ‡ = 26.8 kcal mol −1 , and Δ S ‡ = −16.6 cal K −1 mol −1 ), which were determined by an Eyring-Polanyi plot at three different temperatures (135–145 °C), were higher than those of other reported BC 3 N systems. 39,40…”

“…[58][59][60] The kinetic parameters (145 °C: ΔG ‡ = 33.7 kcal mol −1 , ΔH ‡ = 26.8 kcal mol −1 , and ΔS ‡ = −16.6 cal K −1 mol −1 ), which were determined by an Eyring-Polanyi plot at three different temperatures (135-145 °C), were higher than those of other reported BC 3 N systems. 39,40 The kinetic parameters for racemization of 1a-i are summarized in Table 4. For 1a-c, which carry a substituent at the 2-position of the borafluorene ring (R 1 ), ΔG ‡ increases from 32.9 kcal mol −1 to 34.9 kcal mol −1 (145 °C) in the order 1b (R 1 = OMe) < 1a (R 1 = Me) < 1c (R 1 = CF 3 ).…”

“…Recently, He has reported the enantioselective synthesis of BC 3 N and BC 2 N 2 boranes via asymmetric copper-catalyzed azide–alkyne cycloaddition (CuAAc) reactions and studied their stereochemical stability. 39…”

“…Recently, He has reported the enantioselective synthesis of BC 3 N and BC 2 N 2 boranes via asymmetric copper-catalyzed azide-alkyne cycloaddition (CuAAc) reactions and studied their stereochemical stability. 39 Aza-boraspirobifluorenes, synthesized first by Ingleson et al in 2015, 40 are neutral tetrahedral boranes with a BC 3 N framework, in which the boron atom links an orthogonal π-conjugated pair of a borafluorene and an aza-borafluorene (Fig. 1c).…”

Enantioselective Rh-Catalyzed Azide-Internal-Alkyne Cycloaddition for the Construction of Axially Chiral 1,2,3-Triazoles

“…In comparison to the well-explored racemic CuAAC, , the catalytic asymmetric version of the [3 + 2] cycloaddition of alkynes and azides is still challenging because of the linear geometry of the alkynes and azides, the complex mechanism of cycloaddition, − and no new chiral center generation from this reaction . Remarkable progress on the enantioselective CuAAC (E-CuAAc) of terminal alkynes using prochiral substrates has been achieved by Fokin, Zhou, Topczewski, Fossey, and others, especially Zhou’s elegant work for the construction of quaternary carbon central chirality (Scheme A). However, the exploration in the asymmetric [3 + 2] cycloaddition of internal alkynes and azides remained extremely rare.…”