Article Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
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Fluorescence Switching of Intramolecular Lewis Acid−Base Pairs on a Flexible Backbone Yang Cao, Nicole E. Arsenault, Duane Hean, and Michael O. Wolf* Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada
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S Supporting Information *
ABSTRACT: A class of intramolecular Lewis acid−base pairs containing Lewis acidic dimesitylboranes paired with phosphine oxide, sulfoxide, and sulfone Lewis basic groups are explored. The absorption and emission properties of the compounds are investigated in different solvent environments, and switching of the photophysical behavior between the Lewis acid−base adducts and free acid− base pairs is examined. We find that phosphine oxide Lewis base groups are effective partners in flexible Lewis pairs; however, the analogous sulfoxide and sulfone groups are not. Additionally, the absorption and emission wavelengths can be systematically tuned by varying the conjugation length and electron-donating and -accepting substituents on the backbone.
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INTRODUCTION
We have reported a bithiophene-based system with tunable interactions between pendant dimesitylboryl (−BMes2) Lewis acid and phosphoryl (−P(O)R2, where R = phenyl or isopropyl) Lewis base moieties.27 This “flexible” fluorescent system exhibits a dynamic equilibrium of competitive binding of the Lewis base, to either the internal Lewis acid or external hydrogen bond (HB) donors, inducing a change in the emission profile. The equilibrium is found to be temperaturedependent and can be shifted by changing the solvent’s HB donating strength. These dual emissive compounds, called FlexFluors due to the flexible nature of the fluorophores to switch emission in varying media, have been successfully applied in optical fluorescence microscopy as dyes to image biological samples.28 It was shown that the dyes exhibited differing emission behavior in various lipid/water environments of adipose and brain tissue, where structural details based on emission wavelength could be observed. In this work, this Lewis acid−base pair system is further explored with incorporation of other Lewis bases, as well as variations on the bithiophene backbone. We further the scope of these flexible Lewis pairs as emissive fluorophores by investigating the fundamental absorption and emission characteristics of this system with changing substituents (Scheme 2). Only the Lewis base is varied because the dimesitylboryl group is important for the stability of the Lewis acidic boron center. Variations of the Lewis base moiety give rise to compounds in series 1 and 2. The color tunability was systematically explored by comparing compounds 3−7, and the effect of varying electron density in the backbone was investigated by comparing compounds 5 and 8−11.
Three-coordinate triarylboranes feature an empty p orbital that overlaps with adjacent aromatic π orbitals and gives rise to Lewis acidity and the propensity for intramolecular photoinduced charge transfer (CT). Triarylborane chromophores have been incorporated into small organic molecules, as well as the backbone or side chains of oligomeric and polymeric materials.1,2 It is well established that Lewis acidic threecoordinate boranes can interact with Lewis bases to form Lewis adducts; the resulting change in the Lewis acidity of the boron center can have a pronounced effect on CT within the molecule. These interactions have been used as the basis for chemical sensors,3−5 catalytic applications,6,7 OLEDs,8−10 and solar cell materials.11,12 Due to their promising luminescence properties, boronnitrogen-based intramolecular Lewis pairs involving N-heterocycles such as pyridine and thiazole-based units have been investigated for applications in optical materials.13−16 Recent interest in these B-N pairs has included systems that exhibit dual emission from a conformational change in the photoexcited state from four-coordinate to three-coordinate boron through bond-cleavage-induced intramolecular charge transfer,17−19 as well as systems that exhibit a dynamic equilibrium between free Lewis pair/adduct formation in the ground state, in order to exploit their different photophysical and electrochemical properties.20−24 Other B-X Lewis pairs that exhibit a dynamic equilibrium between free and bound forms in the ground state include BP6,25 and B-O donor−acceptor systems (Scheme 1),26,27 with limited examples of controllable tunability between the two forms. Recently, Wang et al. reported a series of boronaldehyde B-O Lewis pairs that can switch between the bound and unbound forms with varying solvent, temperature, or pressure and exhibit different optical properties.26 © XXXX American Chemical Society
Received: February 9, 2019
A
DOI: 10.1021/acs.joc.9b00398 J. Org. Chem. XXXX, XXX, XXX−XXX
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The Journal of Organic Chemistry Scheme 1. Examples of Dynamic Equilibria Involving Three- and Four-Coordinate Triarylboranes
Scheme 3. Synthetic details for precursors to compounds 4−11 are outlined in Schemes S1−S3 in the Supporting Information. The sulfur-containing O-donor Lewis bases are prepared from the corresponding sulfide (Scheme 4, see Scheme S4 in
Scheme 2. FlexFluor System and Structural Variations Explored in This Work
Scheme 4. Synthesis of Sulfur-Containing O-donor Lewis Bases
the Supporting Information for experimental details on 2-S). Oxidation of 2-S with 1 equiv of m-CPBA at low temperature results in selective oxidation into the sulfoxide with good yield (>80%). Increasing the number of equivalents of m-CPBA gave 2-SO2 in good yield (>80%), demonstrating the high stability of 2-S to these reaction conditions. Selected Crystal Structures. Single crystals suitable for Xray analysis were obtained by slow diffusion of MeOH into a THF solution of the corresponding compounds. Structures of 2-SO, 2-SO2, 3, 5, 6, and 10 have been resolved (Figure 1) and compared with previously obtained crystal structures of the closed form of 1-PPh and the open compound 1-PMes (see the Supporting Information Section S3 for complete details).27 Crystal structures of 2-SO and 2-SO2 showed the compounds in the open form, with interannular bithiophene torsion angles of 125.66 and −127.19°, respectively (Table S5). These are similar to the torsion angle in 1-PMes at 130.11°. The S−O bond length of 2-SO is slightly shorter (1.500 Å) than the analogous P−O bond length in the closed form of 1-PPh (1.5194 Å), is slightly longer than the P−O bond length in 1-PMes (1.4855 Å), and corresponds well with the length of a typical S−O sulfoxide bond.29 In 2-SO2, the S− O bond lengths (1.4303 and 1.4357 Å) are significantly shorter than the P−O bonds in 1-PPh, 1-PMes, and the S−O bond in 2-SO and agree well with the literature value for S−O sulfone bonds.29 FTIR data for 2-SO (Figure S9) shows a strong SO stretching resonance due to a free sulfoxide group at 1048 cm−1, and the data for 2-SO2 (Figure S10) shows both symmetric and asymmetric stretching peaks of free SO bonds at 1138 and 1311 cm−1, respectively. The findings
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RESULTS AND DISCUSSION Synthesis. The phosphine oxide Lewis bases with varying R groups on the phosphine were synthesized according to Scheme 3. Compounds 1-PPh, 1-PIPr, and 1-PMes were synthesized according to Wolf and co-workers.27 Compounds 3−11 were synthesized in the same general manner where the flexible Lewis pairs are added in a one-pot reaction following Scheme 3. Synthesis of Phosphine Oxide-Borane Lewis Pairs
B
DOI: 10.1021/acs.joc.9b00398 J. Org. Chem. XXXX, XXX, XXX−XXX
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Figure 1. Crystal structures of compounds 1-PPh, 1-PMes, 2-SO, 2-SO2, 3, 5, 6, and 10.
the CT is from a boron localized LUMO and a bithiophene π and mesityl π localized HOMO and HOMO-1 transition.27 In THF, the Stokes shift for compounds 1-PPh, 1-PIPr, and 1PCy is significantly smaller, with the emission peak between 440 and 460 nm. The emission of the closed structure was previously assigned27 to a combination of a bithiophene-based π−π* transition and a mesityl-based π-bithiophene-based π* CT transition. Only 1-PMes remains open, regardless of the solvent, due to steric hindrance, with CT emission maxima at 550 nm in both solvents. By comparison, in MeOH, 1-PIPr and 1-PCy with dialkyl phosphine oxide moieties show significant emission from the closed structure near 425 nm due to the incomplete opening of the Lewis adducts. Compound 1-PPh shows only a very small amount of emission from the closed form in MeOH (emits at 450 nm in THF). The different amounts of open and closed forms in 1-PIPr, 1-PCy, and 1-PPh can be attributed to the more polarized PO bonds in the dialkyl phosphine oxide group compared to the diaryl compound 1-PPh, resulting in stronger B−O bonds that are more difficult for MeOH to disrupt. Even though hydrogen bonding between solvent and PO groups may be stronger in these cases, the overall effect still favors the closed structure. Compared to 1-PIPr, 1-PCy shows slightly less emission from the closed form in MeOH. This is suggestive of the closed form of 1-PCy being less favorable due to greater steric bulk of the Lewis base. UV−vis spectra of 2-SO and 2-SO2 (Figure 3a) are almost identical in both solvents, showing only slightly weaker low energy absorbance near 370 nm in MeOH. Even though 2SO2 emits at a slightly higher wavelength than 2-SO (Figure 3b), these compounds show similar excitation and emission features in different solvents, with large Stokes shifts typical of the open structures. From the crystal structure, FTIR, and photophysical data, it can be concluded that SO bonds in the sulfoxide or sulfone do not form closed Lewis adducts with the dimesitylboryl Lewis acid center. This may be attributed to weaker polarization of the SO bond compared to the PO bond, making the partially negatively charged oxygen atom a weaker Lewis base that cannot form a Lewis adduct with the −BMes2 group.31 Effect of Conjugation Length. UV−vis spectra of compounds 3−7 in MeOH are shown in Figure 4a. All compounds show weaker CT absorptions at higher wavelengths relative to the lower wavelength absorptions. The
indicate no significant interaction of the sulfoxide or sulfone groups with the Lewis acidic dimesitylboryl group in the solid state. Crystal structures of the closed forms of 3, 5, 6, and 10 were obtained, and the B−O bond lengths were compared with the closed form of 1-PPh. The B−O bond lengths in compounds 3 (1.626 Å), 5 (1.643 Å), and 6 (1.6435, 1.6399 Å) are similar to that in 1-PPh (1.630 Å, see Table S3), whereas compound 10 has a slightly shorter B−O bond length (1.605 Å), suggesting that the B−O interaction is stronger in 10 compared to in compounds 1-PPh, 3, 5, and 6. Compound 10 also has a smaller S−C−C−S interannular bithiophene torsion angle (13.78°) compared to the other closed compounds that have been characterized (20.95−30.46°) (Table S5). The significantly decreased torsion angle of 10 may be attributed to the methoxy substituent increasing the effective conjugation along the backbone through the electron-donating p orbital on the oxygen, stabilizing the planar structure.30 The decreased torsion angle may promote the formation of the closed structure, thus resulting in a shorter B−O bond length for 10 compared to in other FlexFluor structures. Photophysical Properties. Excitation and emission spectra of Lewis pairs with different phosphine oxide Lewis bases are shown in Figure 2. All compounds in MeOH show a prominent CT emission peak at 550 nm, characterized by a large Stokes shift, which is a feature resulting from the compounds being predominantly in the open form. The origin of the CT emission peak has been previously probed for 1-PPh by DFT and TD-DFT studies, where it was determined that
Figure 2. Normalized excitation and emission spectra of 1-PPh, 1PMes, 1-PIPr, and 1-PCy in (a) MeOH, λem = 520 nm and λex = 320 nm and (b) THF, λem = corresponding emission maxima and λex = 350 nm. C
DOI: 10.1021/acs.joc.9b00398 J. Org. Chem. XXXX, XXX, XXX−XXX
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Figure 3. (a) UV−vis spectra of 1.0 × 10−6 M solutions of 2-SO and 2-SO2 in MeOH and THF. (b) Normalized excitation and emission spectra of 2-SO and 2-SO2 in MeOH and THF.
absorptions of the more planar conjugated backbone in the closed Lewis adduct in THF. The same trend as in MeOH is observed in THF, where molecules with longer conjugation lengths show increasingly red-shifted absorptions. Electronic excitation and emission spectra of compounds 3− 7 in MeOH and THF are shown in Figure 5a,b, respectively. They all show large Stokes shifts in MeOH, which is in line with energy loss in CT processes of the open structures. However, in THF where the closed structure is dominant, a much smaller Stokes shift is observed due to less energy loss between absorption and emission in the π−π* transitions. The emission colors of compounds 3−7 range from blue to orange in the open structures and violet to green in the closed structures. Effect of Electron-Withdrawing/Electron-Donating Groups. The UV−vis spectra of compounds 5 and 8−11 are shown in Figure 6. In MeOH, all the compounds show a
Figure 4. UV−vis spectra of 1.0 × 10−5 M solutions of compounds 3−7 (a) in MeOH with open structures and (b) in THF with closed structures.
shoulder, which is typical for oligothiophene derivatives with pendant three-coordinate −BMes2 groups,32 red-shifts from 3 (∼350 nm) to 4 (∼370 nm), to 5 (∼390 nm), to 7 (∼420 nm). This is consistent with increasing backbone conjugation since compound 4 with the mesityl group out of the bithiophene plane is expected to show less conjugation than 5, and the absorbance of 7 is red-shifted relative to 5 as the thiophene rings offer more effective conjugation than phenyl rings in this context.33 The CT absorption of 6 is only slightly lower in energy than that of 5, indicating that the alkynyl groups do not contribute significantly to the increase in effective conjugation length. However, the molar absorption coefficient of 6 is much higher than that for 5 as a result of the presence of two more bridging alkynyl groups. UV−vis spectra of compounds 3−7 in THF are shown in Figure 4b. The lowest energy bands of the molecules in THF are more intense and red-shifted than those of corresponding open structures in MeOH. This feature is attributed to predominantly π−π*
Figure 6. UV−vis spectra of 1.0 × 10−5 M solutions of compounds 5 and 8−11 (a) in MeOH with open structures and (b) in THF with closed structures.
Figure 5. Normalized excitation (solid, λem = corresponding emission maxima) and emission spectra (dashed, λex = corresponding excitation maxima) of 1.0 × 10−5 M solutions of the compounds 3−7 (a) in MeOH with open structures and (b) in THF with closed structures. D
DOI: 10.1021/acs.joc.9b00398 J. Org. Chem. XXXX, XXX, XXX−XXX
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The Journal of Organic Chemistry weak, low energy CT absorption shoulder typical of open structures. This feature has a relatively consistent intensity for all compounds and red-shifts from compounds 5 and 8−11. The shift to lower energy is consistent with increasing electron density in the π-conjugated backbone resulting in a lower energy charge transfer to the electron-deficient boron center. The absorptions of the closed forms of compounds 5 and 8− 11 in THF (Figure 6b) show a similar trend where more electron-rich substitution on the backbone gives rise to a lower energy absorption, except in 8 with −CF3 groups. In both solvents, the absorption energy differences between these compounds are much smaller than those between compounds 3 and 7. The only exception is compound 11 with a strongly electron-donating −NMe2 group, which behaves very differently from 5 and 8−10. Compound 11 also shows a strong effect of added conjugation as seen in compounds 3−7 in that the −NMe2 group is coplanar with the backbone with a notable contribution to its conjugation length, consistent with DFT-calculated frontier molecular orbitals (entry 11, Tables 2 and 3). Electronic excitation and emission spectra of compounds 5 and 8−11 in MeOH and THF are shown in Figure 7. Again,
Table 1. Photophysical Data for 3−11 in MeOH and THF open structure (in MeOH)
closed structure (in THF)
entry
λabs (nm)
λem (nm)
Φem
λabs (nm)
λem (nm)
Φem
3 4 5 6 7 8 9 10 11
350a 370a 392 400 418 389 396 402 435
512 536 558 560 605 548 572 586 645
0.52 0.84 0.75 0.61 0.17 0.41 0.54 0.55 0.18
345 366 402 417 431 405 409 415 445
429 472 488 498 536 468 498 513 552
0.23 0.55 0.58 0.063 0.060 0.089 0.16 0.52 0.49
a
Maxima of shoulder-like features are estimated.
the unusual photophysical data of compound 11. These include red-shifted excitation and emission features of compound 11 in THF and MeOH, as well as slightly redshifted low-energy absorptions. DFT calculations were carried out at the PBE0/6-31G* level of theory to simulate the ground state structure of Lewis pairs 5 and 8−11. For all simulations, the solvation effect of either MeOH or THF was implemented using the polarizable continuum model solvation method. DFT-calculated frontier orbitals of the optimized ground state structures of 5 and 8−11 are summarized in Tables 2 (open structure in MeOH, see the Table 2. HOMO-1s, HOMOs, and LUMOs of the Open Structures of Compounds 5 and 8−11
Figure 7. Normalized excitation (solid) and emission (dashed) spectra of 1.0 × 10−5 M solutions of compounds 5 and 8−11 (a) in MeOH with open structures and (b) in THF with closed structures.
compound 11 shows a significant red-shift from 5 and 8−10 due to the reasons discussed above. The emission colors of these compounds range from green to red in the open structures and cyan to yellow in the closed structures. Emission of 11 in MeOH shows a shoulder feature at around 540 nm possibly due to a small amount of the closed structure, which is much more emissive than the open structure (11 shows a much higher emission quantum yield in THF than in MeOH, see Table 1). It is also possible that the electron-rich compound is oxidized by oxygen under excitation and forms highly emissive impurities. The photophysical properties of flexible Lewis pairs 3−11 are summarized in Table 1. Interestingly, for the open and closed structures of the same compound, there is almost no overlap between the absorption of one structure and the emission of the other structure. This effectively prevents energy transfer between adjacent closed and open structures in the case of an equilibrium so that the existence of one structure does not decrease/quench the emission of the other. This can be important for imaging applications where both forms are present and may allow quantification of the open/closed ratio via measurement of emission intensities. DFT Calculations. DFT calculations were performed to obtain insight into the different emission colors of the flexible Lewis pairs with different substituents and to help understand the photophysical trends of compounds 5 and 8−11, including
Supporting Information Table S6 for enlarged frontier orbital images) and 3 (closed structure in THF). The HOMO and LUMO energy levels of compounds 5 and 8−11 are summarized in Table S6 for qualitative comparisons. For the open structures modeled with MeOH solvation, the LUMOs of 5 and 8−11 are heavily boron-centered, while the LUMO of 8 also contains a strong contribution from the π* orbital of the relatively electron-poor backbone (Table 2, LUMO entry 8) due to the presence of the electronwithdrawing −CF3 groups. This leads to relatively consistent LUMO energy levels of 5 and 9−11, while the LUMO of 8 is E
DOI: 10.1021/acs.joc.9b00398 J. Org. Chem. XXXX, XXX, XXX−XXX
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electron-donating groups, for both the open and closed forms. These findings have introduced new insights into the relationship between the photophysical properties and backbone design of these emissive flexible Lewis pairs. Furthermore, the emission spectra can be tuned through almost the entire visible region, demonstrating the versatility of these flexible Lewis pair systems as emissive fluorophores. The ability to tune the absorption and emission properties toward lower energy wavelengths makes these compounds promising for application in the field of biological imaging of deep tissue samples, where longer wavelengths are required for effective light penetration.
Table 3. HOMOs and LUMOs of the Closed Structures of Compounds 5 and 8−11a
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EXPERIMENTAL SECTION
General. 1H NMR, 13C{1H} NMR, and 31P{1H} NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer. Residual protio-solvent peaks were used in the calibration of the 1H NMR spectra. Peaks of the deuterated solvents were used to calibrate the 13C{1H} NMR spectra. All 31P{1H} NMR spectra were referenced to 85% H3PO4 as an external standard. Absorption spectra were obtained on a Varian Cary 5000 UV−vis−NIR spectrophotometer, and emission measurements were performed on a PTI QuantaMaster 50 fluorimeter. Infrared (IR) spectroscopy was performed on an attenuated total reflection (ATR) crystal using a PerkinElmer Frontier FTIR spectrometer. DFT Calculations. DFT calculations were carried out using the Gaussian 09 Rev.D01 suite of programs.34 The PBE0 hybrid functional35 with the 6-31G* basis set was employed to simulate the ground state structure of Lewis pairs. Optimized structures were confirmed to be the minimum on the potential energy surface by vibrational frequency calculations. For all simulations, the solvation effect of either methanol (ε = 32.613, for the open structures) or THF (ε = 7.4257, for the closed structures) was implemented using the polarizable continuum model in Gaussian 09. Synthesis. All syntheses involving air-sensitive compounds were carried out using standard Schlenk-type procedures under an atmosphere of N2. Anhydrous tetrahydrofuran (THF) and diethyl ether (Et2O) were obtained using a solvent purification system. Chlorodiphenylphosphine (ClPPh2, 98%), chlorodicyclohexylphosphine (Cy2PCl, 97%), dimethyl disulfide (CH3S-SCH3, 99%), 5bromo-2-hexylthiophene (97%), 3-chloroperbenzoic acid (m-CPBA, ≤77%), hydrogen peroxide solution (H2O2, 30 wt % in H2O), NMe3 solution (∼45 wt % in H2O), phenylboronic acid (95%), 4trifluoromethyl-phenylboronic acid (95%), 4-tert-butylphenylboronic acid (95%), 4-methoxyphenylboronic acid (95%), and 4(dimethylamino)phenylboronic acid (95%) were all purchased from Sigma-Aldrich. Dimesitylfluoroborane (Mes2BF, 98%) was purchased from TCI America. All purchased chemicals were used without further purification. The phosphine oxide-borane Lewis pairs were synthesized using a general one-pot method described in Scheme 3 from corresponding substituted dibromo bithiophene precursors (Schemes S1−S3), which were made from either 3,3′,5,5′-tetrabromo-2,2′-bithiophene (T2Br4) 3 6 or 3,3′-dibromo-5,5′-diiodo-2,2′-bithiophene (T2Br2I2)37 via Suzuki, Sonogashira, or Negishi cross-coupling reactions. Compounds 3,3′-dibromo-5,5′-dibutyl-2,2′-bithiophene (1Br),24 3,3′-dibromo-2,2′-bithiophene (3-Br),38 and 3,3′-dibromo5,5′-bis(4-methoxyphenyl)-2,2′-bithiophene (10-Br)39 were synthesized according to literature procedures. The sulfur-borane Lewis pair 2-S was synthesized according to Scheme S4. 3,3′-Dibromo-5,5′-dimesityl-2,2′-bithiophene (4-Br). With stirring, 50 mL of deaerated H2O−THF (v/v = 1:1) mixed solvent was added under N2 into a three-necked round-bottom flask equipped with a condenser containing T2Br4 (964 mg, 2.00 mmol), K2CO3 (2.21 g, 16.0 mmol), and 2,4,6-trimethyl-phenylboronic acid (688 mg, 4.20 mmol). Pd(PPh3)4 (230 mg, 0.20 mmol) was then added under high N2 flow. The mixture was slowly warmed to 70 °C and stirred at the same temperature for 24 h. THF was removed under reduced
a
In the ball-and-stick models, the colors for atoms are gray (C), red (O), yellow (S), orange (P), pink (B), and blue (N). Hydrogens are omitted.
slightly lower in energy (Table S7, left). The HOMO energies of compounds 5 and 8−11 consistently increase with more electron density on the backbone in the order 8 < 5 < 9 < 10 < 11. Therefore, the HOMO−LUMO energy gap (EgOPEN) in MeOH decreases as the result of the presence of more electron-donating groups. Frontier orbitals of the closed structures of compounds 5 and 8−11 are modeled with THF solvation and show very consistent π−π* character (Table 3). The HOMO−LUMO energy gaps also do not show significant differences from one another except for compound 11 (Table S7, right), indicating that the electron-donating and -withdrawing groups of the closed structure have similar effects on the π orbitals (HOMO) and π* orbitals (LUMO). Compound 11 shows different behavior since there is some degree of charge transfer from HOMO to LUMO, and the sp2 nitrogen atoms also contribute significantly to extended conjugation of the backbone.
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CONCLUSIONS In this work, the bithiophene-based Lewis pair system was systematically studied by varying the Lewis base from different alkyl and aryl phosphine oxide to sulfoxide and sulfone groups. Only the phosphine oxides were shown to effectively form the B−O bond with the −BMes2 moiety, and the equilibrium between the open and closed structures is shifted toward the closed structures when the R2P(O)− group is more polarized and less sterically hindered. We also demonstrate that the emission colors of the open and closed forms of the Lewis pairs can be systematically tuned by altering the substituents on the thiophene backbone. The absorption and emission spectra were red-shifted with increasing backbone conjugation, or with F
DOI: 10.1021/acs.joc.9b00398 J. Org. Chem. XXXX, XXX, XXX−XXX
Article
The Journal of Organic Chemistry pressure, and the remaining mixture was extracted with diethyl ether (4 × 20 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was filtered, and the solvent was removed from the filtrate under reduced pressure. Purification by flash column chromatography (hexanes) gave a white solid. Yield 680 mg, 61%. 1H NMR (CDCl3, 400 MHz): δ 2.24 (s, 12H), 2.35 (s, 6H), 6.82 (s, 2H), 6.98 (s, 4H). 13C{1H} NMR (CDCl3, 100 MHz): δ 20.9, 21.3, 111.5, 128.4, 129.2, 129.5, 130.0, 138.3, 138.7, 143.5. HRMS (EI-MS) m/z: [M]+ calcd for C26H24Br2S2 557.9686; found: 557.9684. 3,3′-Dibromo-5,5′-diphenyl-2,2′-bithiophene (5-Br). A similar method as for 4-Br was followed except phenylboronic acid (513 mg, 4.20 mmol) was used. Purification by flash column chromatography (hexanes/chloroform = 5:1) gave pure 5-Br as yellow needles. Yield 626 mg, 66%. 1H NMR (CDCl3, 400 MHz): δ 7.30 (s, 2H), 7.36−7.62 (m, 10H). 13C{1H} NMR (CDCl3, 100 MHz): δ 112.7, 125.7, 126.6, 128.0, 128.6, 129.1, 132.8, 145.5. HRMS (EI-MS) m/z: [M]+ calcd for C20H1279Br2S2 473.8747; found: 473.8745. 3,3′-Dibromo-5,5′-bis(2-phenylethynyl)-2,2′-bithiophene (6-Br). With stirring, 60 mL of deaerated diisopropylamine−THF (v/v = 1:3) mixed solvent was added under N2 into a three-necked roundbottom flask containing T2Br2I2 (1.15 g, 2.00 mmol), CuI (34.4 mg, 0.18 mmol), and Pd(PPh3)2Cl2 (112 mg, 0.16 mmol). Phenylacetylene (430 mg, 4.20 mmol) was added via syringe, and the mixture was stirred at room temperature for 24 h. THF was removed under reduced pressure, and the remaining mixture was extracted with diethyl ether (4 × 20 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was then filtered, and the solvent was removed from the filtrate under reduced pressure. Purification by column chromatography (hexanes/CH2Cl2 = 10:1) gave a yellow powder. Yield 960 mg, 92%. 1H NMR (CDCl3, 400 MHz): δ 7.23 (s, 2H), 7.36−7.38 (m, 6H), 7.51−7.54 (m, 4H). 13 C{1H} NMR (CDCl3, 100 MHz): δ 81.0, 96.2, 111.9, 122.3, 125.6, 128.6, 129.2, 129.6, 131.7, 134.9. HRMS (EI-MS) m/z: [M]+ calcd for C24H1279Br2S2 521.8747; found: 521.8751. 3,3′-Dibromo-5,5′-bis(5-hexylthienyl)-2,2′-bithiophene (7-Br). To an oven-dried 100 mL round-bottom flask under N2 were added 5-bromo-2-hexylthiophene (1.04 g, 4.20 mmol) and 40 mL of dry THF. The solution was cooled to −78 °C, and n-BuLi (1.6 M in hexanes, 2.90 mL, 4.60 mmol) was added slowly. The mixture was stirred at −78 °C for 1 h after which ZnBr2 (1.14 g, 4.60 mmol) was added under high N2 flow. The mixture was warmed up to room temperature and stirred for another 1 h. Pd(PPh3)4 (230 mg, 0.20 mmol) and T2Br4 (964 mg, 2.00 mmol) were added under high N2 flow. The mixture was slowly warmed up to 50 °C and stirred for 24 h. The solvent was removed under reduced pressure, and the remaining mixture was added with 50 mL of H2O and 5 mL of 1 M HCl solution and extracted with diethyl ether (4 × 20 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was then filtered, and the solvent was removed from the filtrate under reduced pressure. Purification by column chromatography (hexanes) gave a yellow waxy solid. Yield 1.03 mg, 79%. 1H NMR (CDCl3, 400 MHz): δ 0.91 (t, J = 6.0 Hz, 6H), 1.32−1.41 (m, 12H), 1.65−1.73 (m, 4H), 2.81 (t, J = 7.5 Hz, 4H), 6.70 (d, J = 3.4 Hz, 2H), 7.02 (d, J = 3.4 Hz, 2H), 7.05 (s, 2H). 13C{1H} NMR (CDCl3, 100 MHz): δ 14.2, 22.7, 28.9, 30.3, 31.6, 31.7, 112.3, 124.5, 125.2, 126.1, 126.6, 133.0, 139.4, 147.1. HRMS (EI-MS) m/z: [M]+ calcd for C28H3279Br2S4 653.9754; found: 653.9750. 3,3′-Dibromo-5,5′-bis(4-trifluoromethylphenyl)-2,2′-bithiophene (8-Br). A similar method as for 4-Br was followed except 4trifluoromethyl-phenylboronic acid (797 mg, 4.20 mmol) was used. Purification by flash column chromatography (hexanes) gave an offwhite solid. Yield 707 mg, 58%. 1H NMR (CDCl3, 400 MHz): δ 7.38 (s, 2H), 7.66−7.71 (m, 8H). 13C{1H} NMR (CDCl3, 100 MHz): δ 113.3, 124.1 (q, 1JFC = 270.3 Hz), 126.0, 126.3 (q, 3JFC = 3.8 Hz), 128.1, 129.2, 130.5 (q, 2JFC = 32.5 Hz), 136.1, 143.9. 19F{1H} NMR (CDCl3, 376 MHz): δ −62.4. HRMS (EI-MS) m/z: [M]+ calcd for C22H1079Br2F6S2 609.8495; found: 609.8491. 3,3′-Dibromo-5,5′-bis(4-tert-butylphenyl)-2,2′-bithiophene (9Br). A similar method as for 4-Br was followed except 4-tert-
butylphenylboronic acid (748 mg, 4.20 mmol) was used. Purification by flash column chromatography (hexanes) gave a yellow solid. Yield 762 mg, 65%. 1H NMR (CDCl3, 400 MHz): δ 1.36 (s, 18H), 7.25 (s, 2H), 7.45 (d, J = 8.4 Hz, 4H), 7.53 (d, J = 8.4 Hz, 4H). 13C{1H} NMR (CDCl3, 100 MHz): δ 31.4, 34.9, 112.6, 125.6, 126.2, 126.3, 127.8, 130.2, 145.7, 152.0. HRMS (EI-MS) m/z: [M]+ calcd for C20H2879Br2S2 473.8747; found: 585.9998. 3,3′-Dibromo-5,5′-bis(4-methoxyphenyl)-2,2′-bithiophene (10Br). A similar method as for 4-Br was followed except 4methoxyphenylboronic acid (638 mg, 4.20 mmol) was used. Purification by column chromatography (hexanes/THF = 5:1) gave a yellow solid. Yield 588 mg, 55%. 1H NMR (CDCl3, 400 MHz): δ 3.85 (s, 6H), 6.94 (d, J = 8.8 Hz, 4H), 7.17 (s, 2H), 7.52 (d, J = 8.8 Hz, 4H). HRMS (EI-MS) m/z: [M]+ calcd for C22H16O279Br2S2 533.8959; found: 533.8954. This compound is known.39 3,3′-Dibromo-5,5′-bis(4-dimethylaminophenyl)-2,2′-bithiophene (11-Br). A similar method as for 4-Br was followed except 4(dimethylamino)phenylboronic acid (693 mg, 4.20 mmol) was used and T2Br2I2 (1.15 g, 2.00 mmol) was added instead of T2Br4. Purification by column chromatography (hexanes/THF = 5:1) gave an orange powder. Yield 559 mg, 49%. 1H NMR (CD2Cl2, 400 MHz): δ 3.00 (s, 12H), 6.73 (d, J = 8.9 Hz, 4H), 7.13 (s, 2H), 7.47 (d, J = 8.9 Hz, 4H). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 40.5, 112.6, 112.7, 121.0, 124.2, 126.1, 126.9, 147.0, 151.2. HRMS (EI-MS) m/z: [M]+ calcd for C24H2279Br2N2S2 559.9591; found: 559.9594. 5,5′-Dibutyl-3-dicyclohexylphosphinyl-3′-dimesitylboryl-2,2′-bithiophene (1-PCy). A similar method as for 1-PPh27 was followed except T2Br2I2 (870 mg, 2.00 mmol) was used as the starting material, THF was used as the solvent, and chlorodicyclohexylphosphine was used instead of chlorodiphenylphosphine. Purification by flash column chromatography (hexanes/THF = 4:1) gave a yellow powder. Yield 1.20 g, 81%. 1H NMR (CDCl3, 400 MHz): δ 0.88− 0.95 (m, 6H), 1.21−1.76 (m, 30H), 2.05 (s, 12H), 2.20 (s, 6H), 2.66−2.72 (m, 4H), 6.30 (d, 3JPH = 2.4 Hz, 1H), 6.44 (s, 1H), 6.62 (br, 4H). 13C NMR (CD2Cl2, 100 MHz): δ 13.9, 13.9, 21.1, 21.9, 22.2, 24.1, 25.6 (d, 3JPC = 3.1 Hz), 25.6, 25.9, 26.5 (d, 2JPC = 12.8 Hz), 26.7 (d, 2JPC = 13.5 Hz), 29.4, 29.6, 33.5, 34.0, 37.3 (d, 1JPC = 68.8 Hz), 121.4 (d, 1JPC = 111.3 Hz), 125.7 (d, 2JPC = 13.5 Hz), 128.5, 132.9, 136.2, 136.2, 141.1, 145.3 (d, 2JPC = 17.6 Hz), 145.5, 146.5, 148.7, 154.8. 31P NMR (CDCl3, 162 MHz): δ 49.1. HRMS (ESITOF) m/z: [M + H]+ calcd for C46H65OPS211B 739.4308; found: 739.4323. 3′-Dimesitylboryl-3-diphenylphosphinyl-2,2′-bithiophene (3). 3Br (745 mg, 2.30 mmol) was dissolved in 40 mL of dry diethyl ether and cooled to −78 °C. n-BuLi (1.6 M in hexanes, 1.58 mL, 2.53 mmol) was added, and the mixture was stirred for 1 h at the same temperature. Ph2PCl (560 mg, 2.53 mmol) was then added, and the mixture was slowly warmed up to room temperature and stirred for another 1 h. The mixture was cooled to −78 °C again, and n-BuLi (1.6 M in hexanes, 1.73 mL, 2.76 mmol) was added followed by 1 h of stirring. Solid Mes2BF (800 mg, 3.04 mmol) was directly added under a high N2 flow, and the reaction was stirred for 1 h before it was warmed up to room temperature and stirred for an additional 18 h. The reaction was then quenched by adding 5 mL of H2O, and an excess of H2O2 (1 mL, 30 wt % in H2O) was added. The mixture was stirred for 4 more hours at 0 °C and then extracted with diethyl ether (4 × 10 mL). The organic phases were combined and washed with water and brine before drying over anhydrous MgSO4. The mixture was filtered, and the solvent was removed from the filtrate under vacuum. Purification by flash column chromatography (hexanes/THF = 5:1) gave a white solid. Yield 1.06 g, 76%. 1H NMR (CDCl3, 400 MHz): δ 2.04 (s, 12H), 2.23 (s, 6H), 6.49 (dd, 3JHH = 5.3 Hz, 3JPH = 3.7 Hz, 1H), 6.61 (br, 4H), 6.82 (d, 3JHH = 5.0 Hz, 1H), 7.05 (dd, 3 JHH = 5.4 Hz, 4JPH = 2.4 Hz, 1H), 7.18 (d, 3JHH = 5.0 Hz, 1H), 7.39− 7.55 (m, 10H). 13C{1H} NMR (CDCl3, 100 MHz): δ 21.1, 24.0, 123.9 (d, 1JPC = 105.5 Hz), 125.1 (d, 2JPC = 16.4 Hz), 126.4, 128.4 (d, 3 JPC = 13.6 Hz), 128.5, 131.9 (d, 3JPC = 17.8 Hz), 132.0 (d, 4JPC = 2.8 Hz), 132.0 (d, 1JPC = 109.3 Hz), 132.2 (d, 2JPC = 10.5 Hz), 136.0, 136.5, 138.5 (d, 3JPC = 2.2 Hz), 141.3, 144.7, 150.5 (d, 3JPC = 10.4 Hz), 156.1. 31P{1H} NMR (CDCl3, 162 MHz): δ 29.1. HRMS (ESIG
DOI: 10.1021/acs.joc.9b00398 J. Org. Chem. XXXX, XXX, XXX−XXX
Article
The Journal of Organic Chemistry TOF) m/z: [M + H]+ calcd for C38H37OPS210B 614.2153; found: 614.2143. 5,5′-Dimesityl-3′-dimesitylboryl-3-diphenylphosphinyl-2,2′-bithiophene (4). A similar method as for 3 was followed except 4-Br (1.12 g, 2.00 mmol) was used as the starting material and THF as the solvent. Purification by flash column chromatography (hexanes/THF = 5:1) gave a light-yellow solid. Yield 1.14 g, 67%. 1H NMR (CD2Cl2, 400 MHz): δ 2.05 (s, 6H), 2.09 (s, 12H), 2.17 (s, 6H), 2.24 (s, 6H), 2.29 (s, 6H), 6.27 (d, 3JPH = 4.0 Hz, 1H), 6.44 (s, 1H), 6.64 (br, 4H), 6.90 (s, 2H), 6.91 (s, 2H), 7.43−7.45 (m, 4H), 7.52−7.56 (m, 6H). 13 C{1H} NMR (CD2Cl2, 100 MHz): δ 20.9, 21.0, 21.2, 21.2, 24.2, 124.3 (d, 1JPC = 103.3 Hz), 128.3, 128.5, 128.7 (d, 3JPC = 12.6 Hz), 129.0, 129.4, 131.1, 132.0 (d, 2JPC = 17.1 Hz), 132.4 (d, 4JPC = 2.5 Hz), 132.5 (d, 2JPC = 10.6 Hz), 132.5 (d, 1JPC = 109.2 Hz), 136.4, 136.8, 138.1, 138.2, 138.4, 138.9, 138.9, 141.7 (d, 3JPC = 16.1 Hz), 141.8, 142.8, 145.4, 150.1 (d, 2JPC = 9.7 Hz), 157.4. 31P{1H} NMR (CD2Cl2, 162 MHz): δ 28.6. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C56H57OPS210B 850.3718; found: 850.3723. 3′-Dimesitylboryl-3-diphenylphosphinyl-5,5′-diphenyl-2,2′-bithiophene (5). A similar method as for 3 was followed except 5-Br (943 mg, 2.00 mmol) was used as the starting material and THF as the solvent. Purification by flash column chromatography (hexanes/ THF = 4:1) gave a yellow solid. Yield 859 mg, 56%. 1H NMR (CDCl3, 400 MHz): δ 2.10 (s, 12H), 2.27 (s, 6H), 6.63 (s, 4H), 6.74 (d, 3JPH = 4.4 Hz, 1H), 7.10 (s, 1H), 7.25−7.58 (m, 20H). 13C{1H} NMR (CDCl3, 100 MHz): δ 21.1, 24.4, 123.7 (d, 1JPC = 104.6 Hz), 125.9, 126.2, 127.0 (d, 2JPC = 17.8 Hz), 127.5, 128.3, 128.5 (d, 3JPC = 12.8 Hz), 128.7, 128.8, 129.1, 131.1 (d, 1JPC = 110.6 Hz), 132.3 (d, 4 JPC = 2.5 Hz), 132.5 (d, 2JPC = 10.6 Hz), 132.7, 132.9, 134.2, 136.1, 137.0, 141.6, 143.1 (d, 3JPC = 16.0 Hz), 144.9, 145.4, 150.4 (d, 2JPC = 10.0 Hz), 159.7. 31P{1H} NMR (CDCl3, 162 MHz): δ 30.2. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C50H44OPS211BNa 789.2562; found: 789.2573. 3′-Dimesitylboryl-5,5′-bis(2-phenylethynyl)-3-diphenylphosphinyl-2,2′-bithiophene (6). A similar method as for 3 was followed except 6-Br (1.12 g, 2.0 mmol) was used as the starting material and THF as the solvent. Purification by flash column chromatography (hexanes/THF = 4:1) gave a yellow powder. Yield 944 mg, 58%. 1H NMR (CDCl3, 400 MHz): δ 1.98 (s, 12H), 2.25 (s, 6H), 6.58 (br, 4H), 6.74 (d, 3JPH = 8.7 Hz, 1H), 7.02 (s, 1H), 7.32−7.59 (m, 20H). 13 C{1H} NMR (CDCl3, 100 MHz): δ 20.9, 24.4, 80.9, 83.2, 94.8, 95.6, 121.7 (d, 1JPC = 105.8 Hz), 122.1, 122.6 (d, 3JPC = 18.9 Hz), 122.9, 124.0, 128.3, 128.3, 128.5 (d, 3JPC = 13.0 Hz), 128.5, 128.9, 129.0, 129.1 (d, 1JPC = 112.7 Hz), 131.3, 131.4, 132.5 (d, 2JPC = 10.1 Hz), 136.4 (d, 4JPC = 2.8 Hz), 135.3, 135.5 (d, 2JPC = 18.8 Hz), 136.4 (d, 3JPC = 2.2 Hz), 141.6, 142.5, 145.8, 152.1 (d, 2JPC = 9.2 Hz), 161.3. 31P{1H} NMR (CDCl3, 162 MHz): δ 33.7. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C54H44OPS210BNa 836.2598; found: 836.2593. 3′-Dimesitylboryl-5,5′-bis(5-hexylthienyl)-3-diphenylphosphinyl2,2′-bithiophene (7). A similar method as for 3 was followed except 7-Br (1.12 g, 2.0 mmol) was used as the starting material and THF as the solvent. Purification by flash column chromatography (hexanes/ THF = 5:1) gave a red powder. Yield 572 mg, 31%. 1H NMR (CD2Cl2, 400 MHz): δ 0.89−0.93 (m, 6H), 1.33−1.39 (m, 12H), 1.62−1.69 (m, 4H), 1.96 (s, 12H), 2.20 (s, 6H), 2.75−2.80 (m, 4H), 6.51 (d, 3JPH = 4.2 Hz, 1H), 6.53 (br, 4H), 6.65 (d, 3JHH = 3.6 Hz, 1H), 6.69 (d, 3JHH = 3.6 Hz, 1H), 6.71 (s, 1H), 6.90 (d, 3JHH = 3.6 Hz, 1H), 6.92 (d, 3JHH = 3.6 Hz, 1H), 7.40−7.61 (m, 10H). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 14.3, 14.3, 21.0, 23.0, 23.0, 24.6, 29.1, 29.1, 30.5, 30.5, 32.0, 32.0, 32.0, 122.0 (d, 1JPC = 106.3 Hz), 124.1, 124.9, 125.3, 125.5, 126.4 (d, 2JPC = 18.2 Hz), 129.0 (d, 3JPC = 13.0 Hz), 129.3, 129.8 (d, 1JPC = 110.7 Hz), 132.9, 132.9 (d, 4JPC = 2.9 Hz), 133.0 (d, 2JPC = 10.7 Hz), 133.4, 134.4 (d, 3JPC = 2.0 Hz), 134.8, 135.6, 137.0 (d, 3JPC = 17.0 Hz), 138.3, 141.9, 146.3, 146.5, 147.3, 150.3 (d, 2JPC = 9.8 Hz), 161.9. 31P{1H} NMR (CD2Cl2, 162 MHz): δ 33.6. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C58H65OPS411B 947.3749; found: 947.3755. 3′-Dimesitylboryl-3-diphenylphosphinyl-5,5′-bis(4-trifluoromethylphenyl)-2,2′-bithiophene (8). A similar method as for 3 was
followed except 8-Br (1.22 g, 2.00 mmol) was used as the starting material and THF as the solvent. Purification by flash column chromatography (hexanes/THF = 5:1) gave a light-yellow solid. Yield 1.04 g, 58%. 1H NMR (CDCl3, 400 MHz): δ 2.02 (s, 12H), 2.19 (s, 6H), 6.57 (br, 4H), 6.74 (d, 3JPH = 4.4 Hz, 1H), 7.10 (s, 1H), 7.44− 7.60 (m, 18H). 13C{1H} NMR (CDCl3, 100 MHz): δ 21.1, 24.4, 124.0 (q, 1JFC = 270.3 Hz), 124.3 (q, 1JFC = 270.0 Hz), 125.0 (d, 1JPC = 104.7 Hz), 125.0 (q, 3JFC = 3.9 Hz), 125.7 (q, 3JFC = 3.7 Hz), 126.0, 126.3, 128.4 (d, 2JPC = 17.6 Hz), 128.7 (d, 3JPC = 12.7 Hz), 128.9, 129.2 (q, 2JFC = 32.5 Hz), 130.1 (q, 2JFC = 32.6 Hz), 130.8 (d, 1JPC = 110.6 Hz), 132.4 (d, 2JPC = 10.8 Hz), 132.6 (d, 4JPC = 2.7 Hz), 134.1, 136.2, 136.5, 137.4, 138.0 (d, 3JPC = 2.3 Hz), 141.6, 141.6 (d, 3JPC = 15.7 Hz), 143.3, 145.0, 150.8 (d, 2JPC = 9.5 Hz), 160.1. 31P{1H} NMR (CDCl3, 162 MHz): δ 30.5. 19F{1H} NMR (CDCl3, 376 MHz): δ −62.2, −62.3. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C52H42OF6PS210BNa 924.2346; found: 924.2341. 3′-Dimesitylboryl-3-diphenylphosphinyl-5,5′-bis(4-tert-butylphenyl)-2,2′-bithiophene (9). A similar method as for 3 was followed except 9-Br (950 mg, 2.00 mmol) was used as the starting material and THF as the solvent. Purification by flash column chromatography (hexanes/THF = 5:1) gave a light-yellow solid. Yield 894 mg, 51%. 1 H NMR (CDCl3, 400 MHz): δ 1.32 (s, 9H), 1.34 (s, 9H), 2.03 (s, 12H), 2.22 (s, 6H), 6.57 (br, 4H), 6.65 (d, 3JPH = 4.4 Hz, 1H), 7.00 (s, 1H), 7.26−7.45 (m, 12H), 7.50−7.56 (m, 6H). 13C{1H} NMR (CDCl3, 100 MHz): δ 21.1, 24.4, 31.3, 31.4, 34.7, 34.8, 123.1 (d, 1JPC = 105.4 Hz), 125.6, 125.7, 125.9, 126.0, 126.7 (d, 2JPC = 17.8 Hz), 128.5 (d, 3JPC = 12.7 Hz), 128.8, 130.2, 131.0 (d, 1JPC = 110.5 Hz), 131.5, 132.3 (d, 4JPC = 2.7 Hz), 132.5 (d, 2JPC = 10.6 Hz), 132.6, 135.9, 136.4, 141.6, 143.0 (d, 3JPC = 15.8 Hz), 144.7, 145.6, 150.4 (d, 2 JPC = 9.6 Hz), 150.6, 151.6, 159.9. 31P{1H} NMR (CDCl3, 162 MHz): δ 31.2. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C58H60OPS210BNa 900.3850; found: 900.3850. 3′-Dimesitylboryl-5,5′-bis(4-methoxyphenyl)-3-diphenylphosphinyl-2,2′-bithiophene (10). A similar method as for 3 was followed except 10-Br (803 mg, 1.50 mmol) was used as the starting material and THF as the solvent. Purification by flash column chromatography (hexanes/THF = 2:1) gave an orange-colored solid. Yield 533 mg, 43%. 1H NMR (CDCl3, 400 MHz): δ 2.06 (s, 12H), 2.22 (s, 6H), 3.80 (s, 3H), 3.82 (s, 3H), 6.57 (d, 3JPH = 4.4 Hz, 1H), 6.59 (br, 4H), 6.84 (d, 3JHH = 8.8 Hz, 2H), 6.89 (d, 3JHH = 8.8 Hz, 2H), 6.95 (s, 1H), 7.33 (d, 3JHH = 8.8 Hz, 2H), 7.43 (d, 3JHH = 8.8 Hz, 2H), 7.41− 7.45 (m, 4H), 7.52−7.57 (m, 6H). 13C{1H} NMR (CDCl3, 100 MHz): δ 21.1, 24.4, 55.4, 55.5, 114.1, 114.5, 122.9 (d, 1JPC = 105.7 Hz), 125.7, 125.8 (d, 2JPC = 18.0 Hz), 127.10, 127.2, 127.4, 128.5 (d, 3 JPC = 12.6 Hz), 128.7, 131.0 (d, 1JPC = 110.5 Hz), 131.8, 132.2 (d, 4 JPC = 2.7 Hz), 132.5 (d, 2JPC = 10.7 Hz), 135.8, 136.0 (d, 3JPC = 2.4 Hz), 141.6, 142.8 (d, 3JPC = 16.1 Hz), 144.5, 145.6, 149.8 (d, 2JPC = 9.8 Hz), 159.2, 159.7, 159.7. 31P{1H} NMR (CDCl3, 162 MHz): δ 31.0. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C52H49O3PS210B 827.2954; found: 827.2955. 3′-Dimesitylboryl-5,5′-bis(4-dimethylaminophenyl)-3-diphenylphosphinyl-2,2′-bithiophene (11). A similar method as for 3 was followed except 11-Br (840 mg, 1.50 mmol) was used as the starting material and THF as the solvent. Purification by flash column chromatography (hexanes/THF = 2:1) gave an orange-red solid. Yield 410 mg, 33%. 1H NMR (CDCl3, 400 MHz): δ 2.27 (s, 12H), 2.45 (s, 6H), 3.19 (s, 6H), 3.22 (s, 6H), 6.75 (d, 3JPH = 4.3 Hz, 1H), 6.79 (br, 4H), 6.89 (d, 3JHH = 8.7 Hz, 2H), 6.91 (d, 3JHH = 8.7 Hz, 2H), 7.10 (s, 1H), 7.52 (d, 3JHH = 9.0 Hz, 2H), 7.61 (d, 3JHH = 8.8 Hz, 2H), 7.64−7.66 (m, 4H), 7.73−7.78 (m, 6H). 13C{1H} NMR (CDCl3, 100 MHz): δ 21.1, 24.4, 40.5, 40.6, 112.4, 112.5, 121.2, 121.8 (d, 1JPC = 107.3 Hz), 122.9, 124.6 (d, 2JPC = 18.2 Hz), 126.8, 127.1, 128.4 (d, 3JPC = 12.6 Hz), 128.7, 130.5, 131.0 (d, 1JPC = 111.2 Hz), 132.1 (d, 4JPC = 2.5 Hz), 132.6 (d, 2JPC = 10.7 Hz), 135.0 (d, 3 JPC = 2.3 Hz), 135.5, 141.7, 143.4 (d, 3JPC = 16.2 Hz), 145.2, 146.0, 149.4 (d, 2JPC = 9.9 Hz) 150.0, 150.3, 159.8. 31P{1H} NMR (CDCl3, 162 MHz): δ 31.7. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C54H55OPS210BNa 852.3623; found: 852.3646. 3-Bromo-5,5′-dibutyl-3-thiomethyl-2,2′-bithiophene (2-Br). Under N2, 1-Br (2.10 g, 4.80 mmol) was dissolved in 100 mL of H
DOI: 10.1021/acs.joc.9b00398 J. Org. Chem. XXXX, XXX, XXX−XXX
Article
The Journal of Organic Chemistry dry THF and cooled to −78 °C. With stirring, n-BuLi (1.6 M in hexanes, 3.30 mL, 5.28 mmol) was added to the solution dropwise, and the mixture was stirred at the same temperature for 1 h. Dimethyl disulfide (543 mg, 5.76 mmol) was added via a syringe, and the mixture was slowly warmed up to room temperature and stirred for 12 h. The reaction was quenched with water, and the solvent was removed under reduced pressure. The remaining mixture was added with 20 mL of H2O and extracted with hexanes (4 × 15 mL). The organic phases were combined and dried over anhydrous MgSO4. The mixture was then filtered, and the solvent was removed from the filtrate under reduced pressure. Purification by column chromatography in hexanes gave a colorless oil. Yield 1.74 g, 90%. 1H NMR (CDCl3, 400 MHz): δ 0.94−0.99 (m, 6H), 1.40−1.47 (m, 4H), 1.64−1.73 (m, 4H), 2.40 (s, 3H), 2.77−2.82 (m, 4H), 6.74 (s, 1H), 6.77 (s, 1H). 13C{1H} NMR (CDCl3, 100 MHz): δ 13.8, 13.9, 18.6, 22.2, 22.3, 29.9, 30.0, 33.2, 33.4, 110.4, 126.1, 127.0, 127.5, 127.6, 133.0, 146.6, 147.0. HRMS (ESI-TOF) m/z: [M + H]+ calcd for C17H2479BrS3 403.0224; found: 403.0228. 5,5′-Dibutyl-3′-dimesitylboryl-3-thiomethyl-2,2′-bithiophene (2S). 2-Br (1.30 g, 2.21 mmol) was dissolved in 30 mL of dry diethyl ether and cooled to −78 °C. n-BuLi (1.6 M in hexanes, 1.52 mL, 2.43 mmol) was added, and the mixture was stirred for 1 h at the same temperature. Solid Mes2BF (710 mg, 2.65 mmol) was then added under a high N2 flow, and the reaction was stirred for 1 h before it was warmed up to room temperature and stirred for an additional 18 h. The reaction was then quenched with H2O. The organic phases were combined and washed with water and brine before drying over anhydrous MgSO4. The mixture was filtered, and the solvent was removed from the filtrate under vacuum. Purification by flash column chromatography in hexanes gave a yellow solid. Yield 920 mg, 73%. 1 H NMR (CD2Cl2, 400 MHz): δ 0.94−1.00 (m, 6H), 1.29−1.30 (m, 2H), 1.33−1.49 (m, 2H), 1.51−1.54 (m, 2H), 1.64−1.71 (m, 2H), 2.11 (s, 12H), 2.24 (s, 6H), 2.61 (t, 3JHH = 7.6 Hz, 2H), 2.81 (t, 3JHH = 7.4 Hz, 2H), 6.47 (s, 1H), 6.62 (s, 1H), 6.70 (s, 4H). 13C{1H} NMR (CD2Cl2, 100 MHz): δ 14.1, 14.2, 19.1, 21.4, 22.5, 22.7, 23.6, 30.0, 30.2, 34.1, 34.5, 126.4, 128.4, 130.8, 131.5, 131.7, 138.7, 141.1, 142.9, 143.0, 146.4, 146.8, 150.5. HRMS (ESI-TOF) m/z: [M + Na]+ calcd for C35H45S311BNa 595.2674; found: 595.2678. 5,5′-Dibutyl-3′-dimesitylboryl-2,2′-bithiophene-3-methylsulfoxide (2-SO). 2-S (570 mg, 1.00 mmol) was dissolved in 10 mL of CH2Cl2, and solid m-CPBA (
