A modular route to boron doped PAHs by combining borylative cyclisation and electrophilic C–H borylation

Starting from simple alkynes, sequential borylative cyclisation/electrophilic C–H borylation represents a simple modular route to novel B-doped PAHs.


1
H, 101 MHz 13 C{ 1 H}, 128 MHz 11 B, 376 MHz 19 F{ 1 H}). The chemical shift values of the 1 H NMR and 13 C{ 1 H} NMR spectra are reported in ppm relative to residual protio solvent (e.g. CHCl 3 in CDCl 3 δH = 7.27 or δC = 77.2) as internal standards. The 19 F{ 1 H} NMR spectra were referenced to C 6 F 6 , 11 B NMR spectra were referenced to external BF 3 :Et 2 O. Unless otherwise stated all NMR spectra are recorded at 293 K.
Broad features in the 11 B NMR spectra are due to boron present in borosilicate glass.
Carbon atoms directly bonded to boron are not always observed in the 13 C{ 1 H} NMR spectra due to quadrupolar relaxation leading to signal broadening. Coupling constants J are given in Hertz (Hz) as positive values regardless of their real individual signs. The multiplicity of the signals are indicated as "s", "d", "t", "q", "quin", "sept" or "m" for singlet, doublet, triplet, quartet, quintet, septet, or multiplet, respectively. (br.) denotes a broad signal.
Matrix assisted laser desorption/ionization time of flight (MALDI-TOF), atmospheric pressure chemical ionization (APCI) and Electrospray ionization (ESI) measurements was performed by the Mass Spectrometry Service, School of Chemistry, University of Manchester. MALDI-TOF analyses were performed using a Shimadzu Axima Confidence spectrometer using a 4k PPG as a calibration reference. 1 µL of a solution of dopant NaI in THF (10 mg mL -1 ) was spotted onto a well of the MALDI plate and the solvent left to evaporate. Solutions were made up to 10 mg mL -1 in DCM. A solution of matrix dithranol was made up to 10 mg mL -1 in THF. 2 µL of sample solution and 20 µL of matrix solution were thoroughly mixed and 1 µL of this solution was spotted onto a well with no dopant and 1 µL spotted by S3 a layered method with the NaI. The solvent was allowed to evaporate before being placed in the spectrometer. Samples were run in positive polarity mode in either linear or reflection mode. High resolution mass spectra (HRMS) were recorded on a Waters QTOF mass spectrometer. Microanalysis was performed by Mr Stephen Boyer at the London Metropolitan University microanalytical service.
All UV-vis absorption spectra were recorded on a Varian Cary 5000 UV-vis-NIR spectrometer at room temperature in spectroscopic grade solvents. Emission spectra were recorded on a Varian Cary Eclipse Fluorimeter at room temperature in spectroscopic grade solvents, the solutions were excited at their relative absorbance maxima. Absolute quantum yield values were recorded on an Edinburgh Instruments FP920 Phosphorescence Lifetime Spectrometer equipped with a 5 watt microsecond pulsed xenon flashlamp (with single 300 mm focal length excitation and emission monochromators in Czerny Turner configuration) and a red sensitive photomultiplier in peltier (air cooled) housing and determined using a calibrated Edinburgh Instruments integrating sphere.
Cyclic voltammetry was performed using a CH-Instrument 1110C Electrochemical/Analyzer potentiostat under a nitrogen flow. Measurements were made using a 1 mM analyte solution (compound 9 ~0.5 mM) with 0.1 M tetra n butylammonium hexafluorophosphate (Fluka ≥99.0 %) as the supporting electrolyte in THF that had been degassed prior to use and obtained from a dry solvent system. A glassy carbon electrode served as the working electrode and a platinum wire as the counter electrode. An Ag/AgNO 3 non-aqueous reference electrode was used. All scans were calibrated against the ferrocene/ferrocenium (Fc/Fc + ) redox couple, which in this work is taken to be 4.8 eV below vacuum. S4 The half-wave potential of the ferrocene/ferrocenium (Fc/Fc + ) redox couple (E 1/2 , Fc,Fc+ ) was estimated from E 1/2, Fc,Fc+ = (E ap + E cp )/2, where E ap and E cp are the anodic and cathodic peak potentials, respectively.
1-BCl (30 mg, 0.1 mmol), [Ph 3 C]BF 4 (72 mg, 0.2 mmol) and 2,4,6-tri-tert-butylpyridine (TBP) (55 mg, 0.2 mmol) were dissolved in DCE (3 mL) and the reaction mixture was heated for 72 hours at 60 o C. Upon cooling to room temperature the solvent was removed under reduced pressure and the resulting residue was suspended in toluene (5 mL). (2,4,6-triisopropylphenyl)magnesium bromide (0.72 M in THF) (1.1 mL, 0.8 mmol) was slowly added and the reaction mixture was then stirred for 18 h. The remaining Grignard was quenched with 2-propanol (1 mL, 13.1 mmol) and after 10 min of stirring all volatile components were removed under reduced pressure. The residue was dissolved in pentane and filtered through a layer of silica and eluated with pentane followed by pentane:DCM (1:9) and the yellow fractions were collected. The solvent was removed under reduced pressure and the remaining yellow solid was purified via silica gel column chromatography (eluent = Petroleum ether). The desired product was isolated as a yellow solid. Yield 30 mg, 65 %.
Cumene (12 mg, 0.1 mmol), [CPh 3 ]BF 4 (132 mg, 0.4 mmol) and 2,4,6-tri-tert-butylpyridine (98 mg, 0.4 mmol) was dissolved in DCE (0.8 mL) and was heated at 75 o C for 72 hours. After cooling to room temperature the reaction mixture was filtered through a short plug of silica gel and eluted with DCM. The solvent was removed under reduced pressure and the resulting residue was purified by preparative TLC using DCM:Hexane (1:9) as eluent. The desired product was isolated as a white solid. Yield 14 mg, 39 %.

Synthesis of 6-BMes
To a solution of 6 (109 mg, 0.265 mmol) in dichloromethane (5 mL) was added boron trichloride solution (1.0 M in dichloromethane, 0.29 mL, 0.29 mmol) and the mixture was left to stir for 10 minutes. The reaction mixture was dried in vacuo then dissolved in dichloromethane (5 mL). 2,6-Dichloropyridine (88 mg, 0.595 mmol, 2.2 eq.) and aluminium trichloride (81 mg, 0.608 mmol, 2.3 eq.) were added to the solution and the reaction mixture was stirred at 60 °C for 50 minutes. The mixture dried in vacuo then extracted in toluene (4 x 8 mL) and filtered to give an orange solution. To this was added dimesitylzinc (89 mg, 0.29 mmol) and the reaction mixture was left to stir at ambient temperature for three hours. The mixture was filtered through a silica plug and washed with toluene. The product was purified by flash column chromatography using ethyl acetate: hexane (20:80) eluent to yield the product at Rf = 0.41. Crystals were obtained by slow evaporation of a saturated hexane solution at ambient temperature (76.9 mg, 0.143 mmol, 54%).

Synthesis of 9-BCl
BCl 3 (1M in DCM) (2.30 mL, 2.30 mmol) was added to a solution of 8 (203 mg, 0.53 mmol) and 2,4,6-tri-tert-butylpyridine (262 mg, 1.06 mmol) in DCM (4 mL). The colour of the solution changed instantly from pale yellow to bronze. After 5 min of stirring AlCl 3 (155 mg, 1.16 mmol) was added to the reaction mixture. After stirring for 15 minutes a colour change from bronze to purple was observed and the reaction mixture was evaporated to dryness under reduced pressure (to remove BCl 3 ) and the resulting residue was redissolved in DCM (3 mL). 2,6-dichloropyridine (157 mg, 1.06 mmol) and AlCl 3 (155 mg, 1.16 mmol) were then added to the reaction mixture. The reaction mixture was then stirred until all solids had dissolved (~2 minutes) then the stirring was stopped. The product started to precipitate almost instantly and the mixture was allowed to stand for 18 hours. Single crystals suitable for X-Ray diffraction were obtained directly from this precipitation. The reaction mixture was filtered using a filter cannula and the remaining solid was washed two times with 2 mL dichloromethane. The solid was dried under reduced pressure to obtain the desired product as a purple powder. Yield 215 mg, 86%. The sample proved to be poorly soluble in benzene, DCM, ortho-dichlorobenzene.
Elemental Analysis = Calculated: C = 76.18%, H = 4.26%; Found: C = 75.71%, H = 4.28% S15 Synthesis of 9-BMes BCl 3 (1M in DCM) (1.21 mL, 1.21 mmol) was added to a solution of 8 (100 mg, 0.26 mmol) and 2,4,6-tri-tert-butylpyridine (129 mg, 0.52 mmol) in DCM (5 mL). The solution changed colour instantly from pale yellow to bronze. After 5 min of stirring AlCl 3 (69 mg, 52 mmol) was added and the reaction mixture stirred for 10 min. All volatiles were then removed under reduced pressure (to remove excess BCl 3 ) and the resulting residue was redissolved in DCM (10 mL). 2,6-Dichloropyridine (76 mg, 0.52 mmol) and AlCl 3 (69 mg, 0.52 mmol) were then added to the reaction mixture. The reaction mixture was stirred for 30 min and the reaction mixture was evaporated to dryness under reduced pressure and the resulting residue containing 9-BCl was suspended in toluene. Mesitylmagnesium bromide (1 M in THF) (3.12 mL, 3.12 mmol) was then slowly added and the reaction mixture was stirred for 30 minutes. The excess MgBrMes was quenched with a mixture of IPA: toluene (1:9) (10 mL) and after 10 minutes of stirring all volatiles were removed under reduced pressure. The residue was redissolved in DCM, filtered through a plug of silica gel and eluted until the filtrate was colourless. The crude product was further purified by silica gel column chromatography (eluent = hexane:DCM 4:1). The desired product was isolated as a red solid. Slow evaporation of the chromatography fractions gave single crystals suitable for X-Ray diffraction. During column chromatography the compound undergoes protodeboronation resulting in an extremely low isolated yield. Furthermore, on standing in wet CDCl 3 9-BMes decomposes completely over 24 h.  147.5, 141.7, 140.3, 138.9, 138.2, 136.7, 133.7, 131.8, 131.7, 129.8, 128.3, 127.6, 126.9, 125.6, 29.0, 27.5, 22.8, 21 9-BCl (100 mg, 0.21 mmol) was suspended in DCM (2 mL) and (2,4,6-triisopropylphenyl)magnesium bromide (0.72 M in THF) (0.62 mL, 0.72 mmol) was added over a period of 5 min. The purple suspension was stirred for 16 hours and the solution changed colour to red. Additional (2,4,6-tri-isopropylphenyl)magnesium bromide (0.72 M in THF) (0.62 mL, 0.72 mmol) was slowly added and the suspension turned a dark red colour and the reaction mixture was stirred at room temperature for a further 6 hours. The excess MgBrTrip was then quenched with a mixture of IPA: toluene (1:9) (10 mL). The suspension was filtered through a plug of base treated (5% NEt 3 in hexane) silica gel and eluted with DCM until the filtrate was colourless. All volatiles were removed under reduced pressure giving a red powder. The crude product was purified using silica gel column chromatography (eluent = hexane:DCM 4:1). The desired product was obtained as a red powder. Yield 120 mg, 70 %.  3, 150.6, 149.0, 142.5, 141.2, 139.1, 134.2, 132.1, 131.9, 130.4, 128.8, 128.1, 126.1, 120.5, 36.0, 34.9, 29.4, 28.9, 24.6, 24.6, 24.5; 11  9-BCl (55 mg, 0.12 mmol), 2,4,6-tri-tert-butylpyridine (TBP) (117 mg, 0.47 mmol) and [Ph 3 C]BF 4 (154 mg, 0.47 mmol) were dissolved in DCE (10 mL) and the reaction mixture was stirred for 120 hours at 75 o C. Upon cooling the reaction mixture was evaporated to dryness under reduced pressure and the resulting residue was suspended in toluene (10 mL). (2,4,6-Tri-isopropylphenyl)magnesium bromide (0.72 M in THF) (1.22 mL, 0.88 mmol) then was added over a period of 5 minutes. The reaction mixture was then stirred overnight and the excess MgBrTrip was quenched with a mixture of IPA:toluene (1:9) (10 mL) and after 1 hour of stirring all volatiles were removed under reduced pressure. The residue was dissolved in DCM:hexane (2:8) and filtered through a layer of silica gel and eluted with hexane followed by DCM:hexane (2:8) retaining only the coloured fraction. The resulting solution was evaporated to dryness under reduced pressure and the resulting residue was purified by preparative TLC using pentane followed by pentane:acetone (98:2) as eluent collecting the orange/red band. The desired product was extracted from the silica using DCM and the resulting solution was evaporated to dryness under reduced pressure. The resulting red/orange residue was recrystallised from acetone. The resulting solid was then collected by filtration and washed with acetone followed by pentane to give the desired product as a red/orange powder. Yield 42 mg, 45%. Single crystals suitable for X-Ray diffraction were obtained by slow evaporation from DCM. 1-BCl (84 mg, 0.28 mmol), [Ph 3 C]BF 4 (184 mg, 0.56 mmol) and 2,4,6-tri-tert-butylpyridine (TBP) (140 mg, 0.56 mmol) were dissolved in DCE (10 mL) and the reaction mixture was heated for 18 hours at 75 o C to form 2-BCl. Upon cooling to room temperature the solvent was removed under reduced pressure and the resulting residue was dissolved in toluene (10 mL). A solution of (2,6-di(prop-1-en-2-yl)phenyl)lithium was prepared by adding n BuLi (1.6 M in hexanes) (0.77 mL, 1.23 mmol) dropwise to stirred solution of 2-bromo-1,3-di(prop-1-en-2-yl)benzene (264 mg, 1.12 mmol) in toluene (3 mL) at 0 o C, this solution was then stirred for 6 hours at room temperature. The solution of (2,6-di(prop-1-en-2-yl)phenyl)lithium in toluene was added dropwise to the stirred solution of 2-BCl in toluene at room temperature and was stirred overnight. A solution of IPA:toluene (1:9) (10 mL) was added dropwise to the reaction mixture which was then evaporated to dryness under reduced pressure to give a yellow residue. Attempts to isolated 4 using silica gel chromatography were unsuccessful so the impure reaction residue was dissolved in DCE (3.5 mL) and Sc(OTf) 3 (117 mg, 0.24 mmol) (which had been heated under vacuum at 180 o C for 10 min) was added. The reaction mixture was then heated at 75 o C for 16 hours. Upon cooling to room temperature was passed through a short plug of silica gel and eluted with DCM. The solvent was removed under reduced pressure and the resulting residue was purified by base treated (5% NEt 3 in hexane) silica gel chromatography [eluent: DCM:Petroleum ether (1:9)]. The desired product was then isolated as a yellow powder. Yield 31 mg, 26%. In our hands, single crystals suitable for X-Ray diffraction could not be obtained. Synthesis of 11 9-BCl (75 mg, 0.16 mmol), [Ph 3 C]BF 4 (212 mg, 0.64 mmol) and 2,4,6-tri-tert-butylpyridine (TBP) (160 mg, 0.64 mmol) were dissolved in DCE (12 mL) and the reaction mixture was heated for 5 days at 75 o C to form 10-BCl. Upon cooling to room temperature the solvent was removed under reduced pressure and the resulting residue was suspended in toluene (15 mL). A solution of (2,6-di(prop-1-en-2-yl)phenyl)lithium was prepared by adding n BuLi (1.6 M in hexanes) (0.85 mL, 1.36 mmol) dropwise to stirred solution of 2-bromo-1,3di(prop-1-en-2-yl)benzene (294 mg, 1.28 mmol) in toluene (3 mL) at 0 o C, this solution was then stirred for 6 hours at room temperature. The solution of (2,6-di(prop-1-en-2yl)phenyl)lithium in toluene was added dropwise to the stirred solution of 10-BCl in toluene at room temperature and was stirred overnight. A solution of IPA:toluene (1:9) (10 mL) was added dropwise to the reaction mixture which was then evaporated to dryness under reduced pressure to give a yellow residue. Attempts to isolated 10DVP using silica gel chromatography were unsuccessful in addition to 10DVP decomposing on silica gel. The impure reaction residue was then dissolved in DCE (10 mL) and Sc(OTf) 3 (610 mg, 1.24 mmol) (which had been heated under vacuum at 180 o C for 10 min) was added. The reaction mixture was then heated at 75 o C for 16 hours. Upon cooling to room temperature was passed through a short plug of silica gel and eluted with DCM. The solvent was removed under reduced pressure and the resulting residue was purified by silica gel chromatography S20 [eluent: DCM:Petroleum ether (1:9)]. The desired product was then isolated as a red powder. Yield 12 mg, 11%. Slow evaporation of the chromatography fractions gave single crystals suitable for X-Ray diffraction. Compound 11 proved to be poorly soluble in a range on common organic solvents.  1 H NMR spectra of 5 in CD 2 Cl 2 (9.0-7.5 ppm).

Crystallographic Details
Data Collection. Synchrotron X-ray data were collected at beamline I19 (λ = 0.6889 Å) Diamond Light Source S10 for 1-BCl, 10-BTrip) and 11 at temperature of 100 K. Data were measured using GDA suite of programs. X-ray data for compound 1-BMes and 9-BCl was collected at a temperature of 150 K using Mo-Kα radiation on an Agilent Supernova, equipped with an Oxford Cryosystems Cobra nitrogen flow gas system. Data were measured using CrysAlisPro suite of programs. X-ray data for compound 2-BMes, 7-BMes and 9-BMes was collected at a temperature of 150 K using a using Mo-Kα radiation on an Agilent Supernova, equipped with an Atlas detector and Oxford Cryosystems Cobra nitrogen flow gas system. X-ray data for 6-BMes was collected at 150 K on a Rigaku FR-X diffractometer using Mo-Kα radiation. Data were measured using CrysAlisPro suite of programs.
The unexpected packing structure for 9-BCl aligns adjacent B atoms at close distances, but this is not positioning highly positive areas close in space, instead the electrostatic potential near boron is close to neutral.

S69
Optimised Structure Coordinates