Terarylenes as Photoactivatable Hydride Donors - The Journal of

Article Cite This: J. Org. Chem. 2018, 83, 13700−13706

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Terarylenes as Photoactivatable Hydride Donors Colin J. Martin,*,†,‡ Miho Minamide,† Jan Patrick Dela Cruz Calupitan,†,‡,§ Ryosuke Asato,† Jumpei Kuno,† Takuya Nakashima,† Gwénaël Rapenne,†,‡,§ and Tsuyoshi Kawai*,†,‡ †

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Graduate School of Science and Technology, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan ‡ International Collaborative Laboratory for Supraphotoactive Systems, NAIST-CEMES, CNRS, UPR 8011, 29 rue J. Marvig, Toulouse F-31055, France § CEMES, Université de Toulouse, CNRS, Toulouse F-31055, France S Supporting Information *

ABSTRACT: Terarylene frameworks containing benzothiazole as a photoprecursor of hydride donors are presented. We here report on two new scaffolds along with their photoreactivity in solution. Through use of selected external oxidants, the photogeneration of hydride donors is monitored using UV−visible, NMR, and TEM methods. As a proof-ofconcept, photogeneration of hydride in the presence of Ag+ gave rise to the formation of Ag nanoparticles.

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INTRODUCTION The use of photoactive molecules as precursors toward the in situ generation of reactive species has become a common technique for materials applications in recent years.1 Such methodologies have been employed in wide ranging areas including polymer synthesis,2 biological probes,3 and photodynamic therapy.4 The majority of systems reported thus far have been optimized toward the release of electrophilic radicals and acids; however, much fewer examples have been reported in which the targets are nucleophilic in nature. The few nucleophilic photogenerators that have been reported exploit photoinduced homolytic bond cleavage pathways, leading to intermediates that undergo reactions with surrounding media to produce the desired photoproducts.5 Both ionic and nonionic examples including carbamates,6,7 O-acyloximes,8,9 and ammonium salts10,11 have been exploited in the production of photobases as well as decarboxylation systems.12 In the past few years, a series of photoacid generators (PAGs) exploiting cyclization processes of 6π-electron frameworks and subsequent elimination of acids have been designed and investigated by the present group (Scheme 1a).13−16 The open form precursor (o) consists of structures containing hydrogen-conjugate base (H-cb) pairs adjacent to the sites of photocyclization. Upon illumination, pericyclization takes place leading to the formation of the cyclohexa-1,3-diene closed form (c) intermediate which, driven by formation of a chemically stable aromatic polycycle (a), undergoes spontaneous elimination of an acid molecule (H-cb) in dark conditions. Recently, two groups have proposed that certain substituted benzothiazoles can undergo photocyclization to cationic benzothiazolylphenylethenes with the formation of a new C−N bond, which would formally release a hydride ion (Scheme 1b).17−19 © 2018 American Chemical Society

Scheme 1. (a) Acid Generator and (b) Hydride Donor (Base) Frameworks

In this system, the open form (o) showed favorable geometric and electronic properties to allow photochemical ring closure to a short-lived closed intermediate (c). In the presence of oxygen it underwent conversion to the aromatically stabilized oxidized form (x). Their results motivated us to explore new photoactivatable formal hydride (H−) donors, which promote hydride transfer upon association with an appropriate acceptor under illumination. This represents the first example of a self-contained hydride photogenerator operating via an analogous mechanism to existing acid generation frameworks with an initial pericyclic ring closing rather than intermediate radical states. Phototriggered hydride Received: July 23, 2018 Published: October 8, 2018 13700

DOI: 10.1021/acs.joc.8b01877 J. Org. Chem. 2018, 83, 13700−13706

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The Journal of Organic Chemistry donors are typically exemplified in NAD/NADH,20,21 which has been studied for the photochemical reduction of CO2.22 Organic substances based on arylamine with a preconjugated sp3/C−H group, such as acridine, phenylbenzolimidazole and leucomalachite green, have also been used as efficient reductants for fullerenes and carbon nanotubes, which further motivated us to explore the phototriggered formation of hydride donors.23−25 In an attempt to test these frameworks, the open forms of two new model compounds 1o and 2o have been synthesized using the route shown in Scheme 2 and their

1c, which immediately undergoes oxidation to form 1x, with either remaining starting material or other compounds present in solution and the resultant ejection of hydride. To examine the hydride release, a small excess (1.3 equiv) of the hydride acceptor methylene blue was added to 1o in a 9:1 THF/ chloroform solution (chosen due to solubilities). The typical absorption for methylene blue at 652 nm was observed in addition to the bands corresponding to 1o (Figure 2). Upon illumination at 365 nm, this band decays concurrently with the appearance of the bands corresponding to 1x. These changes are similar to those observed when the chemical proton source sodium borohydride is titrated against methylene blue (Figure S1), whereby methylene blue acts as the oxidant assisting in hydride loss. Next 1:1 mixtures of methylene blue with both 1o (Figure 3a) and the reference hydride donor leucomalachite green (Figure 3b) were illuminated above 600 nm to compare their hydride donation in this region. For the reference solution, a rapid decrease in the methylene blue absorption at 652 nm is seen along with the formation of a new band at 624 nm, indicative of the formation of malachite green through loss of hydride from leucomalachite green (Figure 3a). In the case of methylene blue with 1o, illumination above 600 nm again results in a decrease in the intensity of the band at 652 nm; however, here no new peaks are observed (Figure 3b). In this case, hydrogen extraction occurs; however, the product formed is not the same at that produced through illumination of 1o in the UV region. This indicates that photohydride donation from 1o is only activated upon illumination in the UV; above 600 nm illumination does not affect the transfer mechanism, which remains a purely thermal process. This compares well to leucomalachite green, which acts as a hydride donor even when no light is applied. To isolate the oxidized form 1x, we next examined the hydride donation in the presence of 5 equiv of NH4PF6 in THF (Figure 1b). Here any hydride generated should immediately undergo exchange with hexafluorophosphate, leading to the formation of the ion pair [1x][PF6] along with NH3 and H2. The new bands at 355, 374, and 393 nm are again seen upon illumination, and the isosbestic points were maintained throughout the conversion at 287, 303, and 344 nm. Similar spectra are also observed for the conversion of 2o to [2x][PF6] (Figure S2). The bathochromic shift of the absorption onset from 400 nm for 1o to 450 nm for 1x

Scheme 2. Synthesis of 1o and 2o

hydride donating ability compared to that of the reference hydride donor leucomalachite green. Both compounds have been fully characterized using 1D and 2D NMR spectroscopy (See Supporting Information) and show parent high-resolution mass spectral peaks corresponding to the desired mass value.

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RESULTS AND DISCUSSION To examine its photoinduced change to 1x, a solution of 1o was monitored using UV−visible spectroscopy during incremental periods of illumination (Figure 1a); due to the varying solubilities of 1o and 1x, this analysis was done in THF to prevent aggregation. Two main features can be observed: first the appearance of new bands at 355, 374, and 393 nm and second the presence of an isosbestic point at 345 nm. After 4 min, a decay in intensity of the spectrum can be seen, which has been attributed to the reaction of the photohydride with the remaining 1o. It is proposed that upon illumination, the open form converts to the short-lived closed form intermediate

Figure 1. UV−vis spectrum of 1o (4.66 × 10−5 M) upon illumination (a) in THF and (b) in the presence of excess NH4PF6 in THF. 13701

DOI: 10.1021/acs.joc.8b01877 J. Org. Chem. 2018, 83, 13700−13706

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The Journal of Organic Chemistry

Figure 2. UV−vis spectrum of 1o (2.35 × 10−5 M) upon illumination in 9:1 chloroform/THF in the presence of 1.3 equiv of methylene blue.

Figure 3. UV−vis spectrum of methylene blue in 9:1 chloroform/THF under illumination above 600 nm (a) 1:1 with leucomalachite green and (b) 1:1 with 1o.

Figure 4. (a) UV−vis spectrum of 1o (2.0 × 10−5 M) upon illumination in the presence of excess NH4PF6 in THF under 1% illumination (inset fitted linear plot at 393 nm). (b) Reaction rate vs light intensity.

suggests an extended π-conjugation framework upon cyclization. This lowers the HOMO−LUMO gap consistent with DFT predictions for the absorption spectra of both 1x and 2x (Figure S3).26 Interestingly, we did not observe the broad UV−vis peaks above 450 nm (Figure S3), which is typically expected for the c forms. The short lifetime leads us to postulate that c forms are rapidly converting either back to the open form or to the oxidized form x. DFT calculations show that 1o, 1c, 2o, and 2c all optimize to local minima (see Supporting Information) with the difference in energies between the open and closed forms of each system less than

15 kJ/mol (Table S1). In order to further understand the photoprocesses involved, this system was next examined using different intensities of UV light. A significant change in reaction rate for photosaturation was observed upon variation in the light intensity: at 9.9 mW (1% of source intensity), illumination saturation took 11 min (Figure 4 a), and at 19.8 mW (2%), saturation occurred within 3.5 min (Figure S4a). At 4% and 6% source intensity, it occurred after 2 and 1.5 min, respectively (Figure S4b,c). Plotting the absorption at λmax (393 nm) against time allowed for extrapolation of an apparent rate constant at each intensity (Figure 4a, inset). This rate was 13702

DOI: 10.1021/acs.joc.8b01877 J. Org. Chem. 2018, 83, 13700−13706

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Figure 5. 1H NMR spectrum of 1o (in CDCl3) and [1x][PF6] (in DMSO-d6).

found to increase linearly with light intensity. This first-order relationship infers that the photoconversion from 1o to 1c is the one photon process and that the spontaneous conversion of 1c to 1x seems independent of light intensity. As such, the quantum yield for this process could be estimated using a conventional single photon first-order QYM procedure.13−15 The production of hydride from 1o and 2o was found to have a quantum yield of about 4%. In order to isolate the [1x][PF6] salt for NMR analysis, 1o was dissolved in THF along with 5 equiv of NH4PF6 and irradiated at 365 nm for 2.5 h with the solution turning orange. The solvent was then removed, and the solid obtained was washed with methanol to remove any unreacted 1o along with remaining NH4PF6, leaving only [1x][PF6]. Due to the solubilities of 1o and [1x][PF6], 1H NMR could only be recorded in CDCl3 and DMSO-d6,6 respectively. By comparison, the loss of hydride can be clearly observed (Figure 5) with a decrease from 14 to 13 protons with the disappearance of the peak at 7.36 ppm (signal l in Figure 5). A systematic downfield shift of all protons is also observed, resulting from both the more extended π-conjugation and the cationic charge, with the loss of rotational symmetry from the o to x form, leading to the separation of some peaks (signals c−e and h−k in Figure 3). A similar analysis of 2o led to the formation of [2x][PF6] (Figure S5) with a decrease in the total proton integration from 15 to 14. Electrospray ionization (ESI) mass spectral analysis of 1o and 2o before and after irradiation also show the formation of the cationic oxidized structures with a change from 449.02 m.u. for [1o + Na]+ to 425.02 m.u. for [1x]+ and 454.12 m.u. for [2o + H]+ to 452.12 m.u. for [2x]+. In order to follow the hydride formation, it was decided to observe the effect of illumination on a sample under 1 H NMR conditions in the presence of a small amount of scavenger. Aqueous ammonia and water have been employed for this, whereby the photoreaction of 1o or 2o results in a shift in the scavenger signals as they react with the hydride generated. For 1o, shifts of 0.35 and 0.05 within the first 8 min are observed in the scavenger for aqueous ammonia and water, respectively, with each peak also broadening upon illumination (Figures 6 and S6). Similar photoresponses were also observed for 2o (Figures S7 and S8). To estimate the ground-state conformation, DFT-based quantum chemical calculations were employed to model the photoactive precursors 1o and 2o. Geometrical optimization of 1o gave a structure containing interatomic S−N and N−H distances of 2.9 and 2.2 Å, respectively. These are both less than the sum of van der Waals radii of these respective atoms

Figure 6. 1H NMR spectrum of 1o (in CDCl3) under illumination in the presence of an aqueous ammonia (28%) scavenger.

(3.3 and 2.7 Å for S−N and N−H), indicating strong intramolecular S−N and N−H interactions (Figure S9a,b). 2o is also observed to have a planar optimized structure containing multiple S−N and N−H interactions. The S−N distances between the thiazole groups are 2.9 Å, while N−H between the releasable H and the N on the adjacent benzothiazole is 2.1 Å, indicating a strong interaction that locks the molecule in this flat conformation (Figure S10a,b). This flat structure supports the insolubility of 2o even in THF and also implies an extended π-electronic structure compared to 1o. This is consistent with the lower HOMO−LUMO gap seen for 2o with the absorbance onset coming closer to the visible region compared to 1o. It must be noted, however, that this flat structure is not predicted to be very reactive as the N− H interactions can prevent the molecule from reaching a geometry at which photocyclization is favored. For diarylethenes and related analogues, the equilibrium among rotational conformers has been shown to strongly influence the cyclization quantum yield.27 We thus probed the strength of intramolecular interactions by scanning the relevant dihedral angle in a series of DFT calculations (Figures S9c and S10c, dark rectangles).28 For 1o, a conformer in which the side thiazole and benzothiazole rings are skewed around the central benzothiophene ring (Figure S9d) was found to be the most stable form. This lies 2.87 kJ mol−1 lower in energy than the conformation favorable for photocyclization (Figure S9a). With the sites of cyclization lying 5.3 Å apart, well above the 4.0 Å limit for diarylethenes,27 further explaining the low quantum yields observed. Similar scans on 2o again showed 13703

DOI: 10.1021/acs.joc.8b01877 J. Org. Chem. 2018, 83, 13700−13706

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that a nonreactive structure is the most stable (Figure S10d). Here the reactive conformation lies 15 kJ mol−1 higher in energy, implying that only a small population lie at this reactive conformation, again implying low quantum yields. We here only examined the quantum yield from 1o to 1x, which should be given as a product of the photochemical cyclization quantum yield of 1o to the c-form and the reaction yield of dissociation to form 1x. If the cycloreversion reaction from the c-form to 1o competes with the dissociation, the overall quantum yield should be suppressed. As it has been shown that illumination in the presence of suitable scavengers leads to photogeneration of reactive species, it was proposed, for an initial test of utility, to exploit such systems in the preparation of silver nanoparticles (AgNP). A solution containing 2o and 5 equiv of AgPF6 in THF was illuminated for 30 min. UV−vis spectroscopy shows the formation of peaks in the 400−450 nm region (Figure 7a)

Article

EXPERIMENTAL SECTION

For the synthesis of compounds 1o and 2o, 6 (86.6 mg, 0.30 mmol), tripotassium phosphate (64 mg, 0.30 mmol) ,and 5 (75 mg, 0.20 mmol) or 8 (78 mg, 0.20 mmol) were placed under a vacuum and argon flushed 3 times. Then tetrakis(triphenylphosphine)palladium (6.97 mg, 0.006 mmol) was added, and the system cycled vacuum/ argon two more times. 1,2-Dimethoxyethane (10 mL) and distilled water (5 mL) were added, and the system was heated for 24 h at 70 °C. The reaction was stopped, and the product was extracted with ethyl acetate. It was washed with a saturated aqueous solution of sodium bicarbonate and then a saturated aqueous solution of sodium chloride, dried over anhydrous magnesium sulfate, and filtered, and the solvent was evaporated. Purification by silica gel column chromatography (1o Rf = 0.4 in 2:1 dichloromethane/hexane; 2o Rf = 0.8 in 4:1 hexane/ethyl acetate) gave white solids, which were further purified by washing with acetonitrile. Compound 1o: 90 mg, 0.211 mmol, 37% yield; 1H NMR (600 MHz, CDCl3, 25 °C, TMS) δ 8.23 (dt, 1H, J = 8.1, 0.9 Hz), 7.98 (ddd, 1H, J = 8.0, 1.3, 0.7 Hz), 7.95−7.88 (m, 4H), 7.67−7.56 (m, 1H), 7.53−7.47 (m, 1H), 7.46− 7.38 (m, 5H), 7.35 (s, 1H); 13C NMR (150 MHz, CDCl3, 25 °C, TMS) δ 167.6, 162.2, 153.6, 148.4, 134.0, 139.4, 138.9, 136.5, 133.2, 130.5, 129.1, 126.8, 126.4, 125.8, 125.6, 125.5, 125.3, 123.8, 123.6, 122.3, 121.8, 117.7; HRMS (ESI-TOF) m/z [M + Na]+ calcd for C24H14N2S3Na 449.0211, found 449.0209. Compound 2o: 30 mg, 0.0659 mmol, 70% yield; 1H NMR (600 MHz, CDCl3, 25 °C, TMS) δ 9.88 (s, 1H), 8.15 (d, 1H, J = 8.1 Hz), 8.10 (d, 2H, J = 8.0 Hz), 8.05 (d, 2H, J = 8.0 Hz), 7.97 (d, 1H, J = 8.1 Hz), 7.55−7.43 (m, 9H); 13C NMR (150 MHz, CDCl3, 25 °C, TMS) δ 207.2, 167.0, 165.6, 164.6, 154.1, 146.6, 142.7, 136.0, 134.4, 133.2, 133.0, 130.7, 130.5, 129.2, 126.9, 126.7, 126.2, 125.6, 123.4, 121.8, 120.4; HRMS (ESI-TOF) m/ z [M]+ calcd for C25H15N3S3 453.0428, found 453.0409. Leucomalachite green, methylene blue, and 3 were commercially available, and 6 was prepared according to a previously reported method.15 For the synthesis of compounds 4 and 7, to 2-bromobenzothiazole (750 mg, 3.5 mmol) and tripotassium phosphate (1.12g, 5.25 mmol) was added 3 (935 mg, 5.25 mmol) or 6 (1.51 g, 5.25 mmol), and the flask cycled with vacuum/nitrogen three times. Then tetrakis(triphenylphosphine)palladium (121 mg, 0.105 mmol), dimethoxyethane (70 mL), and deionized water (35 mL) were added, and the system was heated to 70 °C for 24 h. The solution was cooled to room temperature, and water was added. Then the mixture was extracted with ethyl acetate and dried over magnesium sulfate, and the solvent evaporated. The crude residue was dissolved in chloroform, passed through a short silica column, and then recrystallized from ethanol to give 4 or 7 as white solids. Compound 4: 524 mg, 1.96 mmol, 57% yield; 1H NMR consistent with published reports.30 Compound 7: 754 mg, 2.56 mmol, 73% yield; 1H NMR (400 MHz, CDCl3, 25 °C, TMS) δ 8.18 (s, 1H), 8.09−8.09 (m, 3H), 7.95 (ddd, 1H, J = 10.8, 2.0, 0.8 Hz), 7.56−7.38 (m, 5H). For the synthesis of compounds 5 and 8, 4 (364 mg, 1.36 mmol) or 7 (400 mg, 1.36 mmol) were dissolved in dry tetrahydrofuran, and the mixture was cooled to −78 °C. To this was added n-butyllithium (1.9 M, 0.86 mL, 1.63 mmol) dropwise, and the solution stirred at −78 °C for 1 h. Neat bromine (0.084 mL, 1.63 mmol) was added, and the solution stirred a further 1 h at −78 °C and then 4 days at room temperature. The solution was quenched via addition of a saturated aqueous solution of sodium thiosulfate, extracted into ethyl acetate, and dried over magnesium sulfate, and the solvent was removed under a vacuum. The crude residue was dissolved in chloroform, passed through a short silica column, and then recrystallized from ethanol to give 5 or 8 as orange solids. Compound 5: 173 mg, 0.50 mmol, 37% yield; 1H NMR (400 MHz, CDCl3, 25 °C, TMS) δ 8.66 (m, 1H), 8.21 (m, 1H), 8.00 (m, 1H), 7.77 (m, 1H), 7.60−7.36 (m, 4H). Compound 8: 356 mg, 0.95 mmol, 70% yield; 1H NMR (400 MHz, CDCl3, 25 °C, TMS) δ 8.27 (d, 1H, J = 11.3 Hz), 8.05−7.97 (m, 3H), 7.61−7.45 (m, 5H).

Figure 7. (a) UV−vis spectrum of 2o in THF (black), in the presence of excess NH4PF6 in THF (red) and in the presence of excess AgPF6 in THF (blue). (b) TEM image of Ag nanoparticles.

corresponding to the plasmon peak of silver nanoparticles generated through reduction of Ag+ to Ag.29 When 2o was replaced with benzothiaole as a control, these peaks are not observed, indicating the reduction occurs only in the presence of a photogenerator. The broadness of these bands indicates that the nanoparticles have a large polydispersity, which was confirmed by transmission electron microscope (TEM) measurements. Nanoparticles ranging from 30 to 100 nm were found as well as aggregates (Figures 7b and S11); optimization of the nanoparticle size remains as a subject of future study. In summary, two new scaffolds 1o and 2o have been prepared and their potential applications as photoactivatable hydride donors have been tested. Initial experiments show they both undergo photocyclization and subsequent oxidation to the planar cationic 1x and 2x in the presence of a hydride acceptor, and their hydride donation compares well to the reference leucomalachite green. The isolation of the oxidized forms of both compounds offers a new methodology for the generation and study of such hydride donors. Their formation has been followed through exploitation of their reactivity with external oxidants present in solution. Through the use of scavengers such as NH4PF6, water, and aqueous ammonia, the formation of the oxidized form has been monitored through both UV−visible and NMR spectroscopy. Such systems have been used in the preparation of Ag nanoparticles from AgPF6 and show the formation of particles of sizes between 30 and 100 nm. Although initial quantum yields are low at about 4%, these frameworks offer a potential pathway toward the development of novel photohydride donors. 13704

DOI: 10.1021/acs.joc.8b01877 J. Org. Chem. 2018, 83, 13700−13706

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The Journal of Organic Chemistry Silver Nanoparticle Preparation. To a 30 μM solution of 2o in THF was added 5 equiv of AgPF6. The solution was then bubbled with N2 for 15 min and illuminated under UV light for 30 min. Ag+ Ion Control Experiment. To a 30 μM solution of benzothiazole in THF was added 5 equiv of AgPF6. The solution was then bubbled with N2 for 15 min and illuminated under UV light for 30 min. Light Sources. UV illumination was provided using a Panasonic UJ30/35 LED Spot Type UV Curing System operating at 365 nm placed 10 mm from the sample cell. UV light of 990 mW intensity (100%) was modulated to 1−8% using an internal ANUJ3000 intensity controller. Red illumination (above 600 nm) was provided using an “Ignus” wide band hand-held torch fitted with a R-60 filter removing light below 600 nm, placed 10 mm from the sample cell. All illuminations were performed in the dark at room temperature. Computational Methods. All calculations were performed with the Gaussian09 package.26 We worked at the ωB97xD 6-31G(d,p) level of theory in THF, since it is known to give accurate descriptions of DAE derivatives while taking into account dispersion forces.16 Moreover, to model the bulk THF solvent effect on DFT, TD-DFT, and transient state calculations, we used the cost-effective PCM model. The open form was drawn with the reactive carbons relatively close to each other, which were drawn as input structures. Frequency calculations showed no negative frequencies, confirming that resulting structures were in the absolute minima of the potential map. Dihedral angle scans were performed by rotating an angle of 20° (dark rectangle, Figures S9a and S10a) 18 times.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b01877. Additional UV−vis, NMR, DFT, and TEM data along with NMR spectra of 1o and 2o (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Colin J. Martin: 0000-0001-9419-9898 Jan Patrick Dela Cruz Calupitan: 0000-0003-3044-2603 Takuya Nakashima: 0000-0002-5311-4146 Gwénaël Rapenne: 0000-0002-4993-4213 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MEXT Program for Promoting the Enhancement of Research Universities in NAIST, the CNRS, and the University Paul Sabatier (Toulouse). This research was also partly supported by the JSPS KAKENHI Grant (JP26107006) in Scientific Research on Innovative Areas “Photosynergetics” and the Programme Investissements d’Avenir ANR-11-IDEX-0002-02, reference ANR-10-LABX-0037-NEX.

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REFERENCES

(1) Martin, C. J.; Rapenne, G.; Nakashima, T.; Kawai, T. Recent progress in development of photoacid generators. J. Photochem. Photobiol., C 2018, 34, 41−51. (2) Yang, P.; Yang, W. Surface Chemoselective Phototransformation of C−H Bonds on Organic Polymeric Materials and Related HighTech Applications. Chem. Rev. 2013, 113, 5547−5594. 13705

DOI: 10.1021/acs.joc.8b01877 J. Org. Chem. 2018, 83, 13700−13706

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DOI: 10.1021/acs.joc.8b01877 J. Org. Chem. 2018, 83, 13700−13706