Article Cite This: J. Org. Chem. 2017, 82, 13440−13448
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Benzo[b]thiophene Fusion Enhances Local Borepin Aromaticity in Polycyclic Heteroaromatic Compounds Reid E. Messersmith,† Sangeeta Yadav,‡ Maxime A. Siegler,† Henrik Ottosson,‡ and John D. Tovar*,†,§ †
Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States Department of Chemistry - Ångström Laboratory, Uppsala University, 751 20 Uppsala, Sweden § Department of Materials Science and Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States ‡
S Supporting Information *
ABSTRACT: This report documents the synthesis, characterization, and computational evaluation of two isomeric borepin-containing polycyclic aromatics. The syntheses of these two isomers involved symmetrical disubstituted alkynes that were reduced to Z-olefins followed by borepin formation either through an isolable stannocycle intermediate or directly from the alkene via the trapping of a transient dilithio intermediate. Comparisons of their magnetic, crystallographic, and computational characterization to literature compounds gave valuable insights about the aromaticity of these symmetrically fused [b,f ]borepins. The fusion of benzo[b]thiophene units to the central borepin cores forced a high degree of local aromaticity within the borepin moieties relative to other known borepin-based polycyclic aromatics. Each isomer had unique electronic responses in the presence of fluoride anions. The experimental data demonstrate that the local borepin rings in these two compounds have a relatively high amount of aromatic character. Results from quantum chemical calculations provide a more comprehensive understanding of local and global aromatic characters of various rings in fused ring systems built upon boron heterocycles.
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induced current density (ACID).15,16 Additionally, 1 and 2 display markedly different and unprecedented UV−vis spectra in response to fluoride additions. The expectation is that the aromaticity would be focused strongly in the borepin core according to the Clar−Robinson valence bond representation such that all π-electrons would be included in aromatic circuits, as has been used in other contemporary pi-electron material designs.17,18 This is in contrast to previously studied borepin polycyclic aromatics (e.g., 3 and 4) where the strongest localized aromaticity is at the fused peripheries.25 Three Clar sextets can be drawn within 1 and 2 if one of the sextets is formally in the borepin ring, while maximizing the number of sextets in 3 and 4 places them on the thiophene rings (Chart 1). Benzo[b]thiophene fusion in particular has been recently utilized, in the opposite manner, to enhance the antiaromaticity of indenofluorene scaffolds.19 These molecules continue the discussion regarding modern manifestations of aromaticity and contribute structural and magnetic evidence for enhanced local aromaticity in the borepin ring.
INTRODUCTION Seven-membered ring systems consisting of six π-electrons, six carbons, and a boron atom are known as borepins, and they have unusual aromatic behavior.1,2 When boron is trisubstituted, it has a vacant p-orbital that allows for π-electron conjugation and cyclic aromatic delocalization around the borepin ring.3−5 This vacant orbital is also frequently responsible for ambient degradation, so bulky protecting groups are often required to maintain chemical integrity.2,6 Tricoordinate boron has been previously incorporated into a wide variety of molecules for various and even selective anion sensing.6−8 The empty p-orbital on boron has also been shown to be particularly sensitive to the fluoride anion.9−14 The fluoride binding constant (Ka) is normally large in triarylboranes, but is impacted by steric bulk and electronic changes.12 Herein we describe the synthesis and characterization of two new borepin containing polycyclic aromatic compounds with isomerically different ring fusions to promote borepin-centered aromaticity. These C2v symmetric dibenzo[b]thiophene fused borepins (1 and 2) with B-mesityl capping groups were analyzed by 1H, 11B NMR spectroscopy, and single crystal Xray crystallography for evaluation of magnetic and structural aromaticity, respectively. These results are compared with computational aromaticity probes including nucleus-independent chemical shifts (NICS) and plots of the anisotropy of the © 2017 American Chemical Society
Received: October 3, 2017 Published: November 14, 2017 13440
DOI: 10.1021/acs.joc.7b02512 J. Org. Chem. 2017, 82, 13440−13448
Article
The Journal of Organic Chemistry
iodine monochloride (ICl) to install iodines at the silylated carbons, but ICl reacted with 10 to generated an inseparable mixture of undesired compounds due to the presence of the alkyne. Alkyne 10 yielded only starting material under the titanium reduction conditions, but after removal of the trimethylsilyl groups, the reduction of 11 to the Z-alkene 12 went in 86% yield. Photoisomerization was not observed in either alkene (8 and 12) under ambient light. Halogens were thought to be necessary because previous borepin syntheses utilized lithium-halogen exchange reactions. The borepincontaining polycyclic aromatic 2 could be obtained by direct deprotonation of 12 and quenching with MesB(OMe)2 in a toluene/THF mixture. The lithiation-borepin formation was again the most challenging aspect of the synthesis of 1 and 2.20 The stannocycle en route to 1 could only be formed after addition of TMEDA, which is unusual for t-butyllithium exchanges with bromines. The formation of MesLi from MesBr was completed by heating in toluene to avoid ethereal solvents that would have interfered with the B−Cl intermediate in the subsequent addition. The formation of 2 by direct deprotonation of 12 is the first example of borepin formation without the use of lithium-halogen exchange or tin−boron exchange. The dilithiated species generated from 12 was sensitive to solvent and temperature, and optimization was required to discover the specific successful conditions utilized (Figure S1). Electronic and Electrochemical Properties. Figure 1a displays the UV−vis spectra for 1 (solid line) and 2 (dashed
Chart 1. Chemical Structures Showing Clar−Robinson Sextets, Reflecting the Enhanced Aromaticity Expected within the Borepin Rings of 1 and 2
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RESULTS AND DISCUSSION Synthesis. The synthesis of 1 and 2 follows the general strategies outlined in our previous work (Schemes 1 and 2).20 Scheme 1. Synthesis of 1a
a
Conditions: (a) PdCl2(PPh3)2, CuI, iPr2NH, THF, TBAF, rt; (b) (i) Ti(OiPr)4, iPrMgCl, toluene, −40 °C, (ii) H2O; (c) (i) tBuLi, TMEDA, Et2O, −78 °C, (ii) Me2SnCl2; (d) (i) BCl3, toluene, (ii) MesLi.
Scheme 2. Synthesis of 2a
a
Conditions: (a) Pd(PPh3)4, toluene, bis(tributylstannyl)acetylene, reflux; (b) K2CO3, MeOH, THF; (c) (i) Ti(OiPr)4, iPrMgCl, toluene, −40 °C, (ii) H2O; (d) (i) tBuLi, toluene, THF, (ii) MesB(OMe)2, rt.
Chemoselective Sonogashira coupling of 2,3-dibromobenzo[b]thiophene with trimethylsilylacetylene afforded 5, which was then coupled chemoselectively with 6 after in situ desilylation to afford the symmetrical alkyne 7. Titanium-mediated alkyne reduction to the Z-alkene gave alkene 8 in good yield.21 The borepin could not be directly prepared by lithiation and quenching with MesB(OMe)2, but a stannocycle intermediate could be formed after lithiation of alkene 8. Subsequent addition of BCl3 followed by MesLi in toluene yielded 1 in 56% yield from 8.22 In a similar manner, 9 underwent Stille coupling with bis(tributylstannyl)acetylene to afford the symmetrical alkyne 10.23 We could not prepare 7 directly from 6 by this method, possibly due to competitive reactivity of the bromines at the 3-position. We envisioned that 10 could be treated with
Figure 1. UV−vis spectra (a) and normalized photoluminescence spectra (b) of 1 (solid line) and 2 (dashed line) recorded in CHCl3 at room temperature and excited at 350 and 360 nm, respectively. Cyclic voltammograms (c) for 1 (1.9 mM, solid line) and 2 (1.6 mM, dashed line) in dry and deoxygenated 0.1 M TBAPF6 in THF, obtained with a potential sweep rate of 100 mV/s and referenced to the Fc/Fc+ couple. 13441
DOI: 10.1021/acs.joc.7b02512 J. Org. Chem. 2017, 82, 13440−13448
Article
The Journal of Organic Chemistry line). The peak of maximum absorbance (1: 303 nm; 2: 312 nm) and the onset of absorption (1: 417 nm; 2: 437 nm) are both blue-shifted in 1 relative to 2. The bathochromic shift of spectral features in 2 suggests a longer conjugation throughout the 5-membered polycyclic core.24 The lower energy series of vibronic transitions (358−395 nm for 1, and 385−425 nm for 2) has a significantly higher molar absorptivity in 1. The absorption profiles were probed with time-dependent density functional theory (TD-DFT), which agree with the experimental data and that the calculated gap between the frontier molecular orbitals is smaller in 2 relative to 1 (see Supporting Information). In 1, the highest occupied molecular orbital (HOMO) has a calculated energy of −6.05 eV and a lowest unoccupied molecular orbital (LUMO) with a calculated energy of −2.39 eV. In 2, the HOMO and LUMO were calculated to be −5.84 eV and −2.23 eV, respectively. There is significant molecular orbital density on the boron atom in the LUMO of 2 (Figure S20), while there is a lack of orbital density on the boron atom in 1 (Figure S19). Figure 1b displays the photoluminescence spectra of 1 (solid line) and 2 (dashed line). The emission for 1 is broad with an emission maximum of 417 nm, while the emission of 2 has a more structured profile with defined maxima at 425 and 447 nm. The quantum yields were relatively similar for 1 (10%) and 2 (8%) relative to quinine sulfate (55%). These electronic properties are comparable to dithieno-fused borepins 3 and 4. The compounds with the trans vicinal orientation of sulfur and boron (1 and 3) have an increased absorption band with low energy vibronic features compared to the compounds that have sulfur and boron in a “terminal methylene”-like geminal position (2 and 4). Compounds 1 and 3 also have broader emission profiles than 2 and 4.25 The cyclic voltammograms (Figure 1c) for 1 (solid line) and 2 (dashed line) show one reversible redox event with formal reduction potentials (E1/2) of −2.14 V for 1 and −2.30 V for 2 referenced to the Fc/Fc+ redox couple.26 This is consistent with previously reported 3 being reduced at a less negative potential than 4.25 Neither 1 nor 2 showed any reversible anodic activity within the electrochemical window of the electrolyte solution (
