Reductive Activation of C70 Equatorial Carbons and Structurally

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Reductive Activation of C70 Equatorial Carbons and Structurally Characterized C70 δ‑Adduct with Closed [5,6]-Ring Fusion Wei-Wei Yang,† Zong-Jun Li,† Shu-Hui Li,†,§ Sheng-Li Wu,† Zujin Shi,*,‡ and Xiang Gao*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun, Jilin 130022, China ‡ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: The C70 δ-adducts with closed [5,6]-ring fusion are an important type of compound in classifying bond delocalization in the equatorial belt of C70. However, the formation of such compounds is severely restricted due to the low reactivity of the carbon atoms in the flat equatorial region. Such a restriction is lifted when reduced anionic C70 species are used, where the inert equatorial carbon atoms are activated.

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availability of the compounds. Instead, single crystal structure of the δ-homofullerene (fulleroid) with [5,6]-open configuration was obtained,20 complicating the assignment of the δ-bond nature and arousing interest in an explicit structural characterization of the C70 δ-isomer with a closed [5,6]-bond. Alteration of the regioselectivity of the C70 reaction is therefore desirable to have a sufficient amount of the δ-isomer for a more comprehensive study. In fact, such regioselectivity modification is of importance because the structure of the fullerene derivative is closely related to the properties of the compound. However, previous work with microwave10 or light13b radiation is not effective for enhancing the yield of the δ-isomer even though it can modify the yield of the α- and β-isomers. Previous work on C 70 functionalization with anionic6d,22,23,25−27 or cationic28 C70 species has shown the preferential formation of equatorial para adducts (addition at d,d-carbons across the equator), suggesting that the inert d-type of carbon atoms are likely activated in charged C70 species. However, such reactions are limited to the para additions only, and no work on cycloaddition reactions has been reported. It is therefore of interest to employ the charged C70 species for cycloaddition reactions and to examine whether the regioselectivity of C70 cycloaddition is affected. The cycloaddition reaction of charged C70 species was examined by the reaction of C702− with α,α′-dibromo-o-xylene under argon at room temperature. A control experiment of neutral C70 with α,α′-dibromo-o-xylene was performed at 160 °C in refluxing o-DCB solution, following reported procedures.29 The crude product obtained from each reaction was first eluted over a semipreparative (10 ID × 250 mm) Buckyprep column with toluene, which afforded the fraction consisting of regioisomeric C70 o-quinodimethanemonoadducts (C70QM) along with fractions of C70 and regioisomeric C70 o-

he chemistry of fullerenes is governed by the release of the strain energy caused by the deviation from the planarity of sp2-hybridized carbon atoms.1,2 The principle is well observed for the reactions of C70, where the more strained carbons in the polar region (types a−c in Figure 1) are more reactive with

Figure 1. Schematic illustration of C70. Carbon atoms of the five different types in C70 are assigned conventionally as a−e. Double bonds of four different types are assigned conventionally as α, β, γ, and δ.

preferential formation of derivatives at the most and second most curved bonds (α- and β-bonds, Figure 1) for orthoaddition reactions.2−19 In contrast, the less strained carbon atoms in the flat equatorial region of C70 are inert with very limited reports so far on the addition at the δ-bond.6b,17,20 Notably, the δ-adduct is the fifth most stable isomer predicted theoretically using C70H2 as a model, next only to the α- and βadducts and two para isomers with additions at d,d-carbons across the equator and a,c-carbons,21 which have all been obtained in significant amounts,2−17,19a,22,23 suggesting that the unavailability of the δ-isomer is likely due to the kinetic rather than thermodynamic factor. The δ-isomer with closed [5,6]-ring fusion is an important type of C70 adduct, as it provides key evidence for the unique bond delocalization and aromaticity in the equatorial belt of C70.6b,24 However, no X-ray single crystal structure of this type of compound has been obtained so far due to the limited © 2017 American Chemical Society

Received: July 13, 2017 Published: July 27, 2017 9253

DOI: 10.1021/acs.joc.7b01756 J. Org. Chem. 2017, 82, 9253−9257

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The Journal of Organic Chemistry

Figure 2. HPLC traces of the C70QM mixture obtained from the reaction of α,α′-dibromo-o-xylene with (a) C702− and (b) neutral C70. Flow rate = 2.0 mL/min; wavelength = 380 nm.

bond length of the [5,6]-bond attaching to the o-xylene group is 1.620(5) Å in the δ-isomer, which is comparable to those of the single bonds obtained by direct addition to the α-bond of C70 (1.60115 and 1.66 Å6b) and is significantly shorter than the nonbond distance of 2.136 Å in a [70]fulleroid derivative,20 demonstrating unambiguously that direct addition of the oxylene group to the δ-bond of C70 results in a closed [5,6]bond, confirming the benzoid nature in the equatorial region of C70. The single crystal structure of the δ-isomer reveals a significant structural disturbance on the equatorial hexagons caused by the closed [5,6]-addition. The bond length ranges from 1.397 to 1.516 Å for the rest of the bonds in the equatorial hexagon bearing the o-xylene addend, and it varies from 1.328 to 1.572 Å and 1.353 to 1.478 Å for the bonds in the other two types of equatorial hexagons. In contrast, a much better bond delocalization is shown for the equatorial hexagons in the fulleroid with an open [5,6]-addition at the δ-bond, where the bond length ranges from 1.407 to 1.459 Å for the equatorial hexagons according to one of the reported single crystal structures.20 The result indicates that the well-conjugated electronic structure in the equatorial region of pristine C70 is likely disrupted in the δ-isomer. Figure 3b shows the single crystal structure of the β-isomer. The bond length of the added bond is 1.581(7) Å, which is well within the range of the C−C single bond, confirming the double bond nature of the β-bond in C70. Different from the [5,6]-closed δ-addition, the addition at the β-bond has little effect on the geometry of the equatorial hexagons, where the bond length is well averaged, ranging from 1.401 to 1.431 Å in the three different types of equatorial hexagons, indicating that the well-delocalized electronic structure in the equatorial region of C70 is preserved in the β-isomer. The C70QM isomers are also characterized by spectral methods. The HRMS spectra of the α-, β-, and δ-isomers (Figures S6, S10, and S14) show molecular ions at 944.0609, 944.0602, and 944.0608, respectively, in agreement with the theoretical value of 944.0621 for C78H8+. The 1H NMR spectrum of the α-isomer shows four broad methylene proton resonances and well-resolved signals due to the phenyl protons (Figure S4), consistent with the bonding of the rigid o-xylene addend at the C5 axis of C70 (Scheme 1). The 1H NMR spectra of the β- and δ-isomers (Figures S8 and S12a) exhibit broad peaks corresponding to the methylene and phenyl protons, where partial resonances arising from the methylene protons are incidentally overlapped. An increased shielding effect is

quinodimethane bisadducts (C70(QM)2) (Figure S1). The collected C70QM mixture was further purified by eluting over a semipreparative Buckyprep-D column with toluene, yielding the α-, β-, and δ-isomers. Figure 2 shows the HPLC traces of the C70QM fractions obtained from the reactions of C702− and C70 over a semipreparative Buckyprep-D column. Impressively, the formation of the δ-isomer is significantly enhanced from the reaction of C702−, where the α-, β-, and δ-isomers are obtained with yields of 26.3, 20.8, and 21.9%, respectively. In contrast, only a tiny amount of the δ-isomer is formed from the reaction of neutral C70, as shown in Figure 2b, where the α- and βisomers are the major products. Notably, the elution order of the three isomers is in agreement with the calculated dipole moments of 3.45, 3.77, and 4.06 D for the δ-, β-, and α-isomers, respectively (Gaussian09, B3LYP/6-31G), suggesting an increasing polarity for the isomers as the addend moves from the equatorial region to the polar region of C70. Single crystals suitable for X-ray diffraction were obtained by slowly diffusing methanol into the CS2 solution of the δ- and βC70QM mixture, which was obtained by removing the α-isomer via eluting the regioisomeric C70QM mixture over a BuckprepM column with toluene. The structure of 0.6(δ-C70QM)·0.4(βC70QM)·CS2 was obtained for the crystal mixture, which revealed the structure of δ-C70QM. Single crystals of β-C70QM were obtained by slowly diffusing methanol into the toluene/ CS2 solution of β-C70QM, and the structure of β-C70QM·CS2 was obtained. Figure 3 displays the single crystal structures of both the δ- and β-isomers. As shown in Figure 3a, the critical

Figure 3. Single crystal structures of (a) δ-C70QM and (b) β-C70QM with 25% thermal ellipsoids. The hydrogen atoms and solvent molecules are omitted for clarity. 9254

DOI: 10.1021/acs.joc.7b01756 J. Org. Chem. 2017, 82, 9253−9257

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The Journal of Organic Chemistry Scheme 1. Mechanism for the Reaction of C702− with α,α′Dibromo-o-xylene

densities are located at the types of d-, a-, and c-carbons. Computational calculations (B3LYP/6-31G) on oBrCH2C6H4CH2C70− predicted energy levels of 0.0, 6.6, 4.1, 3.2, and 19.2 kcal/mol for the a−e-bound intermediates. The result indicates that the typically inert d-carbons are activated in C70•− due to the location of large spin density and stability of the d-bound intermediate. Consequently, the d-bound and the a-, c-bound o-BrCH2C6H4CH2C70− are formed preferentially, which would result in the δ- and α-, β-isomers by subsequent intramolecular SN2 reaction via the ortho-addition manner. In contrast, the b- and e-bound intermediates are unlikely to form due to either the unfavorable spin density distribution at bcarbons or the instability of the e-bound intermediate. It is noteworthy that the d-bound o-BrCH2C6H4CH2C70− could in principle result in both δ- and γ-isomers. However, calculations at the B3LYP/6-31G level predicted energy levels of 0.0, 2.9, 9.0, and 19.5 kcal/mol for the α-, β-, δ-, and γ-isomers, respectively, implying that the γ-isomer is unlikely to be stable, rationalizing the unavailability of this isomer, which is consistent with previous work.12 Notably, the spin density distribution in C70•− indicated a symmetry reduction for the anionic species compared to that of the neutral fullerenes even though the 13C NMR spectra of anionic C60 and C70 species indicate that these species retain the symmetry of neutral fullerenes.34 The result is, however, consistent with previous computational work on the charge distribution in C602− and optimization of C60•−,32,35 where a symmetry reduction is also reported, and can be rationalized by the Jahn−Teller distortion associated with anionic fullerenes.36 As for the symmetry preservation exhibited by 13C NMR spectroscopy, it is likely because the spectroscopy represents a time-averaged structure rather than a global minimum one. Interestingly, the δ-isomer exhibits a stronger electron deficiency with respect to the α-, β-isomers and even pristine C70. An E1/2red1 (half-wave potential of the first reduction) of −0.97 V vs Fc/Fc+ is measured for the δ-isomer in o-DCB, which is anodically shifted compared to the E1/2red1 of −1.22, −1.19, and −1.08 V vs Fc/Fc+ for the α-, β-isomers and C70, respectively (Figures S15 and S16). The result is different from that of the typical fullerene derivatives, where the derivatives are usually less electron-deficient due to the cleavage of the electron-withdrawing fullerene π-conjugation.37 Notably, C70 equatorial para adducts exhibit similar anodic shift,27,38 suggesting that the unusual strong electron deficiency is likely related to the involvement of the d-carbons in the addition. As shown by the single crystal structure of the δ-isomer, the bond delocalization at the equatorial region is considerably disrupted by the closed [5,6]-addition to the δ-bond, whereas it is well retained when the addition occurs at the relatively far away β(Figure 4) and α-bond (refer to cif in ref 15), indicating that the unusual electron deficiency of the δ-isomer is likely related to the disruption of the conjugated electronic structure in the equatorial region, which, however, should be electron-donating rather than electron-withdrawing as observed in C60 and the polar region of C70. In summary, reductive activation of the less strained and inert d-carbons in C70 is demonstrated, where the location of large electron spin density at d-carbons in C70•− is critical. The reaction affords the otherwise hardly available δ-isomer, and the X-ray single crystal structure is obtained for the first time for this type of adduct, which shows explicitly a closed [5,6]addition and clarifies the bond delocalization in the equatorial belt of C70. The δ-isomer exhibits a strong electron deficiency

shown for both the phenyl and methylene protons as the oxylene moves from the polar to the equatorial region (Figures S4, S8, and S12), which is likely associated with the strong equatorial ring currents in C7030 and consistent with previous work on C70 derivatives.6d,27,31 The 13C NMR spectra of the regioisomers (Figures S5, S9, and S13) are consistent with the structural assignment with the exhibition of two resonances for the sp3 C70 carbons of the α-isomer and one resonance for the sp3 C70 carbons of the β- and δ-isomers. The UV−vis spectra of the α-, β-, and δ-isomers (Figures S3, S7, and S11) show absorptions at 334, 395, 459, 535, and 662 nm; 322, 362, 396, 434, 584, and 697 nm; and 325, 374, 396, 511, and 646 nm, respectively, in agreement with the reported spectral feature for the regioisomers.6b The reactions of C60 and C70 dianions with alkyl halides (RX) are initiated by single electron transfer (SET) from C2n2− (n = 30 or 35) to RX, generating fullerene monoanion and alkyl radicals (C2n•− and R•), which would couple with each other to produce the RC2n− intermediate, followed by SN2 reaction with another RX to afford the final product.25,32 Consequently, the electron spin density distribution in C70•− would be critical in determining the regioselectivity of the reaction, similar to the case of reductive benzylation of dimetallohexaaryl[70]fullerenes, where the exhibited regioselectivity is in good agreement with the spin density distribution in the monoanion radicals of dimetallohexaaryl[70]fullerenes.26 Scheme 1 illustrates the possible mechanism for the reaction of C702− with α,α′-dibromo-o-xylene. Figure 4 shows the plot of the spin density distribution in C70•− generated with the Multiwfn program.33 The Mulliken spin density distribution in C70•− was calculated with Gaussian09 at the B3LYP/6-311G(d) level, where the highest spin densities for the a−e-carbons are 0.078, 0.007, 0.071, 0.104, and −0.045, respectively, suggesting that large spin

Figure 4. Illustration of spin density distribution in C70•− at an isosurface = 0.001. 9255

DOI: 10.1021/acs.joc.7b01756 J. Org. Chem. 2017, 82, 9253−9257

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The Journal of Organic Chemistry

7.42 (br, 1H), 7.36 (br, 1H), 4.22 (br, 1H), 3.90 (br, 3H). 13C NMR (150 MHz, in CS2 with CDCl3 as the external lock solvent) δ 155.5, 155.2, 154.5, 154.3, 152.0, 151.8, 150.6, 150.4, 149.5, 149.3, 149.2, 148.4, 148.0, 147.4, 147.2, 147.0, 146.8, 146.4, 146.0, 145.4, 145.2, 144.9, 144.7, 144.3, 144.1, 143.5, 142.7, 142.4, 141.9, 141.5, 141.0, 140.7, 137.6, 137.1, 132.7, 132.5, 131.6, 128.5, 128.2, 128.0, 127.7, 126.5, 126.1, 56.2, 44.3, 44.1. UV−vis (n-hexane) λmax/nm: 322, 362, 396, 434, 584, 697. X-ray Single Crystal Diffraction of β-C70QM. Black lamellar crystals of β-C70QM suitable for X-ray analysis were obtained by slowly diffusing methanol into a toluene/CS2 solution of β-C70QM at room temperature. Single-crystal X-ray diffraction data were collected with an instrument equipped with a CCD area detector using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) in the scan range 1.67° < θ < 28.29°. The structure was solved with direct methods using SHELXS-97 and refined with full-matrix least-squares techniques using the SHELXL-97 program within WINGX. Nonhydrogen atoms were refined anisotropically. Crystal data of β-C70QM·CS2: C79H8S2, Mw = 1020.97, black, tetragonal, space group = I4/m, a = 24.3708(14) Å, b = 24.3708(14) Å, c = 13.8797(15) Å, α = 90°, β = 90°, γ = 90°, V = 8243.7(11) Å3, z = 8, Dcalcd = 1.645 Mg m−3, μ = 0.192 mm−1, T = 190(2) K, crystal size 0.21 × 0.19 × 0.17 mm; reflections collected 25101, independent reflections 5201; 2695 with I > 2σ(I); R1 = 0.0942 [I > 2σ(I)], wR2 = 0.1519 [I > 2σ(I)]; R1 = 0.1754 (all data), wR2 = 0.1812 (all data), GOF (on F2) = 1.034. Spectral Characterization of δ-C70QM. MALDI TOF MS: m/z calcd for M+ (C78H8+) 944.0626, found 944.0608. 1H NMR (600 MHz, in CS2 with D2O as the external lock solvent) δ 7.47 (br, 4H), 3.85 (br, 2H), 3.60 (br, 1H), 3.20 (br, 1H). 13C NMR (150 MHz, in CS2 with CDCl3 as the external lock solvent) δ 151.5, 149.6, 149.5, 148.8, 148.3, 148.1, 147.8, 147.1, 146.7, 146.5, 146.4, 146.2, 146.2, 146.1, 145.4, 144.5, 143.8, 141.3, 133.2, 133.1, 132.3, 128.2, 57.1, 45.9. UV−vis (n-hexane) λmax/nm: 325, 374, 396, 511, 646. X-ray Single Crystal Diffraction of δ-C70QM. The C70QM regioisomeric mixture resulted from the Buckyprep column purification was eluted over a semipreparative Buckyprep-M column with toluene, which afforded a mixture of β-C70QM and δ-C70QM by removing the α-C70QM. Black lamellar crystals suitable for X-ray analysis were obtained by slowly diffusing methanol into a CS2 solution of this mixture containing β-C70QM and δ-C70QM at room temperature. Single-crystal X-ray diffraction data were collected with an instrument equipped with a CCD area detector using graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) in the scan range 1.68° < θ < 26.05°. The structure was solved with the direct methods using SHELXS-97 and refined with full-matrix least-squares techniques using the SHELXL-97 program within WINGX. Nonhydrogen atoms were refined anisotropically. Crystal data of 0.6(δ-C70QM)·0.4(βC70QM)·CS2: C79H8S2, Mw = 1020.97, dark brown, tetragonal, space group = I4/m, a = 24.3119(7) Å, b = 24.3119(7) Å, c = 13.9782(8) Å, α = 90°, β = 90°, γ = 90°, V = 8262.1(6) Å3, z = 8, Dcalcd = 1.642 Mg m−3, μ = 0.191 mm−1, T = 187(2) K, crystal size 0.21 × 0.19 × 0.09 mm; reflections collected 22735, independent reflections 4260; 1544 with I > 2σ(I); R1 = 0.0685 [I > 2σ(I)], wR2 = 0.1555 [I > 2σ(I)]; R1 = 0.1036 (all data), wR2 = 0.1707 (all data), GOF (on F2) = 1.100.

property, and it is likely related to the disruption of the conjugated electronic structure in the equatorial region. The work may provide insight into the intrinsic electronic structure of C70 and shed light on the regioselectivity modification of fullerene reactions.

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EXPERIMENTAL SECTION

General Methods. All reactions were carried out under an argon atmosphere. All reagents were obtained commercially and used without further purification unless otherwise noted. Tetra-nbutylammonium perchlorate (TBAP) was recrystallized from absolute ethanol and dried in a vacuum at 303 K prior to use. 1H NMR spectra were recorded with a 600 MHz spectrometer, and 13C NMR spectra were recorded with a 150 MHz spectrometer. HRMS measurement was performed using a MALDI TOF mass spectrometer. Controlled-potential bulk electrolysis was carried out with a potentiostat/galvanostat using an “H”-type cell that consisted of two platinum gauze electrodes (working and counter electrodes) separated by a sintered glass frit. A three-electrode cell was used for CV measurements and a glassy carbon, a platinum, and a saturated calomel electrode (SCE) were used as working, counter, and reference electrodes, respectively. A fritted-glass bridge of low porosity, which contained the solvent/supporting electrolyte mixture, was used to separate the SCE from the bulk of the solution. Synthesis of C70QM from C702−. Typically, 500 mg (0.595 mmol) of C70 was electrolyzed at −1.00 V vs SCE in 300 mL of PhCN solution containing 0.1 M TBAP under an argon atmosphere. The potentiostat was switched off after the electrolytic formation of C702− was completed, and a 10-fold excess of α,α′-dibromo-o-xylene (C8H8Br2) was added to the solution under argon. The reaction was allowed to proceed for approximately 1 h with stirring. The reaction mixture was dried with a rotary evaporator under reduced pressure, and the residue was washed with methanol to remove TBAP and excessive α,α′-dibromo-o-xylene. The obtained crude mixture was put into toluene and sonicated for 10 min, and the soluble part was purified by HPLC with a two-stage chromatographic procedure. At the first stage, the crude mixture was eluted with toluene over a semipreparative (10 ID × 250 mm) Buckyprep column at a flow rate of 4.0 mL/min, where the fraction consisting of C70QM regioisomers was collected. At the second stage, the fraction consisting of C70QM regioisomers collected during the first stage was eluted over a semipreparative Buckyprep-D column with toluene at a flow rate of 2.0 mL/min, where a separation of the α-, β-, and δ-isomers was achieved. The isolated yields for the α-, β-, and δ-isomers were 26.3% (148 mg), 20.8% (117 mg), and 21.9% (123 mg), respectively, and 63 mg of C70 was recovered. Synthesis of C70QM from C70. Typically, 420 mg of C70 (0.5 mmol) was added to 580 mg of KI (3.6 mmol), 950 mg of 18-crown-6 (3.6 mmol), and 238 mg of α,α′-dibromo-o-xylene (0.9 mmol) into 120 mL of o-DCB. The solution was refluxed at 160 °C in the dark under argon for 6 h. The reaction mixture was dried with a rotary evaporator under reduced pressure, and the residue was washed with methanol to remove the nonfullerene species. The purification of the reaction mixture was carried out with procedures similar to that used for purifying the reaction mixture of C702−. The reaction afforded 176 mg (37.3%) of α-C70QM, 54 mg (11.4%) of β-C70QM, and 2 mg (0.4%) of δ-C70QM, and 79 mg of C70 was recovered. Spectral Characterization of α-C70QM. MALDI TOF HRMS: m/z calcd for M+ (C78H8+) 944.0626, found 944.0609. 1H NMR (600 MHz, in CS2 with CDCl3 as the external lock solvent) δ 7.82 (d, 1H), 7.72 (t, 1H), 7.66 (t, 1H), 7.59 (d, 1H), 4.51 (br, 1H), 4.35 (br, 1H), 4.16 (br, 1H), 3.90 (br, 1H). 13C NMR (150 MHz, in CS2 with CDCl3 as the external lock solvent) δ 151.8, 151.7, 151.0, 150.2, 149.9, 149.7, 149.3, 149.2, 148.2, 147.8, 147.4, 147.3, 146.7, 146.1, 143.5, 143.4, 143.2, 140.5, 137.0, 137.0, 134.1, 131.6, 128.5, 128.1, 128.0, 59.5, 57.8, 45.4, 41.8. UV−vis (n-hexane) λmax/nm: 334, 395, 459, 535, 662. Spectral Characterization of β-C70QM. MALDI TOF HRMS: m/z calcd for M+ (C78H8+) 944.0626, found 944.0602. 1H NMR (600 MHz, in CS2 with CDCl3 as the external lock solvent) δ 7.73 (br, 2H),

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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.7b01756. Experimental data, HPLC traces, spectra of the α-, β-, and δ-isomers, and computational details (PDF) Crystallographic data for the δ-isomer (CIF) Crystallographic data for the β-isomer (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. 9256

DOI: 10.1021/acs.joc.7b01756 J. Org. Chem. 2017, 82, 9253−9257

Note

The Journal of Organic Chemistry *E-mail: [email protected].

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ORCID

Zong-Jun Li: 0000-0002-7944-8371 Sheng-Li Wu: 0000-0002-3133-3751 Xiang Gao: 0000-0002-0624-3279 Present Address §

S.-H.L.: College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21472183, 21471010, 21302179, and 21602192), MOST (2013CB933402 and 2017YFA0204901), and the Jilin Provincial Science & Technology Department (20170101172JC and 20160520128JH).

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DOI: 10.1021/acs.joc.7b01756 J. Org. Chem. 2017, 82, 9253−9257