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. 2014 Jun 20;16(12):3340-3.
doi: 10.1021/ol501362w. Epub 2014 Jun 11.

Rhodium-catalyzed B-H activation of 1,2-azaborines: synthesis and characterization of BN isosteres of stilbenes

Alec N Brown  1 , Lev N Zakharov, Tanya Mikulas, David A Dixon, Shih-Yuan Liu
Affiliations

Rhodium-catalyzed B-H activation of 1,2-azaborines: synthesis and characterization of BN isosteres of stilbenes

Alec N Brown et al. Org Lett. .

Abstract

The first example of catalytic B-H activation of azaborines leading to a new family of stilbene derivatives through dehydrogenative borylation is reported. Ten 1,2-azaborine-based BN isosteres of stilbenes have been synthesized using this method, including a BN isostere of a biologically active stilbene. It is demonstrated that BN/CC isosterism in the context of stilbenes can lead to significant changes in the observed photophysical properties such as higher quantum yield and a larger Stokes shift. Direct comparative analysis of BN stilbene 3g and its carbonaceous counterpart 6g is consistent with a stronger charge-transfer character of the excited state exhibited by 3g in which the 1,2-azaborine heterocycle serves as a better electron donor than the corresponding arene.

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Figures

Scheme 1
Scheme 1. BN Stilbene and Its Retrosynthetic Analysis
Scheme 2
Scheme 2. Synthesis of BN Stilbenes through Rh-Catalyzed Dehydrogenative Borylation
The yields of isolated products are given as the average of two runs.
Scheme 3
Scheme 3. Synthesis of a BN Isostere of 4-Methoxy-trans-stilbene
Figure 1
Figure 1
ORTEP illustration, with thermal ellipsoids drawn at the 35% probability level, of BN stilbene 3a. All hydrogens except for the alkenyl hydrogens at C12 and C13 have been omitted for clarity.
Figure 2
Figure 2
Normalized absorption and emission spectra (in MeCN) of select BN stilbenes in direct comparison with their corresponding carbonaceous counterparts. Quantum yields were determined in EtOH at room temperature.
Figure 3
Figure 3
Normalized emission spectra of (a) BN stilbene 3g and (b) carbonaceous analogue 6g in various solvents (DMSO, orange; MeCN, red; black, CH2Cl2; pink, EtOH; blue, THF; green, Et2O; cyclohexane, pale blue). All measurements were taken at 1 × 10–5 M.
Figure 4
Figure 4
HOMO and LUMO (B3LYP Kohn–Sham orbitals) of 3g and 6g.
See this image and copyright information in PMC

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