Broad Scope [4 + 2] Cycloaddition of o-Carboryne with Pentafulvenes

Sep 26, 2017 - o-Carboryne (1,2-dehydro-o-carborane) generated in situ from 1-Li-2-OTf-o-C2B10H10 undergoes an efficient [4 + 2] cycloaddition with pe...
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Broad Scope [4 + 2] Cycloaddition of o‑Carboryne with Pentafulvenes Using 1‑Li-2-OTf‑o‑C2B10H10 as Precursor Jie Zhang,† Zaozao Qiu,*,‡ and Zuowei Xie*,†,‡ †

Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China ‡ Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai, China S Supporting Information *

ABSTRACT: o-Carboryne (1,2-dehydro-o-carborane) generated in situ from 1-Li-2-OTf-o-C2B10H10 undergoes an efficient [4 + 2] cycloaddition with pentafulvenes at room temperature to give a series of carboranonorbornenes in good to high isolated yields. This reaction is compatible with many functional groups and has a very broad substrate scope from 6-mono- to 6,6′disubstituted pentafulvenes and from alkyl to aryl substituents. Further transformations of the resultant [4 + 2] cycloaddition products have been carried out, affording various multifunctionalized o-carboranes.

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Scheme 1. [4 + 2] Cycloaddition of o-Carboryne with Pentafulvenes

INTRODUCTION Diels−Alder reaction is a commonly used strategy for the preparation of various synthetically important carbocycles and heterocycles, which is also an important reaction type of benzyne. 1 Benzyne is well-known as a very reactive intermediate which can react with a large class of dienes owing to its high electrophilicity,2 in which pentafulvenes are a very important class of dienes with unique electronic and chemical properties.3 However, o-carboryne (1,2-dehydro-ocarborane) can be viewed as a three-dimensional analog to benzyne, with very comparable energies of formation.4 They exhibit similar reactivities toward unsaturated molecules, including the Diels−Alder reaction with dienes, though ocarboryne has shown some unique properties of its own because of steric reasons.5 As a very reactive species, o-carboryne can be produced in situ via the elimination of LiBr or LiI from 1-Br-2-Li-1,2C2B10H10 or 1-I-2-Li-1,2-C2B10H10 or from the reaction of 1Me3Si-2-[IPh(OAc)]-1,2-C2B10H10 with CsF, respectively.6 Our earlier work suggests that o-carboryne can be described as a resonance hybrid of both the bonding form and biradical form, which undergo different types of reactions such as [4 + 2]/[3 + 2]/[2 + 2] cycloaddition, ene reaction, hydrogen abstraction, and sp2/sp3 C−H bond insertion reaction with a broad array of substrates including alkynes, alkenes, (hetero)aromatics, amines, ethers, and ferrocenes to afford a large series of o-carborane derivatives.5−11 In this regard, o-carboryne is a very important synthon for the functionalization of ocarboranes that have received growing interests.12 We previously reported a [4 + 2] cycloaddition reaction of ocarboryne with 6,6′-diarylpentafulvenes, giving a class of carboranonorbornenes in good yields (Scheme 1).13 Though © XXXX American Chemical Society

excellent regioselectivity was observed, the substrate scope was limited to 6,6′-diarylpentafulvenes only, probably due to the relatively low efficiency in generating o-carboryne employing 1Li-2-I-o-C2B10H10 as precursor. Very recently, a highly efficient precursor for o-carboryne, 1-Li-2-OTf-o-C2B10H10, was reported by our laboratory.14 We spectulated that such a precursor may solve the problems related to reaction efficiency and substrate scope. Herein we report a broad scope Diels−Alder reaction of o-carboryne with a large variety of pentafulvenes using 1-Li-2OTf-o-C2B10H10 as precursor (Scheme 1) and subsequent transformations of the resultant carboranonorbornenes. Received: July 28, 2017

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DOI: 10.1021/acs.organomet.7b00574 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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RESULTS AND DISCUSSION We initiated our study by treatment of 1.2 equiv of 6,6′dimethylfulvene 2a with o-carboryne generated in situ from 1OTf-o-C2B10H11 (1) using 1.05 equiv of nBuLi as base in cyclohexane. The results were compiled in Table 1, which

desired products 3a−e were isolated in 70−80% yields. For 6,6′-methylphenylfulvene 2f, corresponding [4 + 2] adduct 3f was isolated in 60% yield. These results are sharply contrast to those using 1-I-2-Li-o-C2B10H10 as precursor, in which only Cbridged cyclopentadienyl carboranes 1-[C(Me)R-C5H5]-oC2B10H11 (R = Me and Ph) were generated in low yields without any [4 + 2] adduct formation.13 In addition, heterocyclic substituent in 2g was tolerant to give 3g in 42% yield. 6,6′-Diarylpentafulvenes underwent [4 + 2] cycloaddition smoothly to afford 3h−3m in good yields (61−75%), which are considerably higher than those obtained using 1-I-2-Li-oC2B10H10 as precursor.13 After examining the [4 + 2] cycloaddition reaction of ocarboryne with 6,6′-disubstituted pentafulvenes, we then evaluated the reaction with 6-substituted pentafulvenes 4. The results are summarized in Table 3. A variety of electron-

Table 1. Optimization of Reaction Conditionsa

entry 1 2 3 4 5 6 7

base

T (°C)

time (h)

yield (3a, %)b

BuLi BuLi n BuLi n BuLi n BuLi NaH CH3MgBr

−40 0 rt rt rt rt rt

12 12 0.5 0.5 0.5 0.5 0.5

65 64 66 80 50 − 21

2a (equiv) 1.2 1.2 1.2 1.5 1.05 1.5 1.5

n n

Table 3. [4 + 2] Cycloaddition of o-Carborynes with 6Substituted Pentafulvenesa,b

a

Reactions were conducted using 0.5 mmol of 1 in 5 mL of cyclohexane in a Schlenk flask. bIsolated yields.

indicated that reaction temperatures did not have obvious effects on the isolated yields of expected [4 + 2] cycloaddition adduct 3a (Table 1, entries 1−3). The yield of 3a was increased to 80% if 1.5 equiv of 2a was employed (Table 1, entry 4). No [4 + 2] cycloadduct was observed when NaH was utilized as base (Table 1, entry 6), and the yield of 3a was decreased sharply to 21% with CH3MgBr as base (Table 1, entry 7). Under the optimal reaction conditions (Table 1, entry 4), a variety of 6,6′-disubstituted pentafulvenes 2 were examined, and the results were compiled in Table 2. When R1/R2 at 6,6′positions of pentafulvenes were alkyl/cyclic alkyl groups, Table 2. [4 + 2] Cycloaddition of o-Carboryne with 6,6′Disubstituted Pentafulvenesa,b

a

General conditions: 1 (0.50 mmol), 4 (0.75 mmol), nBuLi (0.52 mmol), cyclohexane (5 mL), rt, 0.5 h. bYields of the isolated products.

donating and -withdrawing groups at different positions of the aromatic ring in 4 were well-tolerated, furnishing carboranonorbornene derivatives 5a−g in up to 75% isolated yields. It was noted that oxygen-containing substituents generally offered low yields of the desired products, due probably to the side reactions. Gratifyingly, pentafulvenes bearing a furanyl or alkyl group also gave the desired products 5h−k in moderate to high isolated yields, further expanding the scope of this [4 + 2] cycloaddition reaction. In contrast, no such cycloaddition products were obtained if 1-I-2-Li-o-C2B10H10 was used as carboryne precursor.13 The resultant [4 + 2] cycloadducts bear olefin functionality that can be further transformed to other functional groups. For example, 3a underwent a [3 + 2] cycloaddition reaction with phenyl azide15 or phenyl nitrile oxide16 generated in situ to give the corresponding heterocyclic product 6 or 7 in high yields, respectively (Scheme 2). The thiolation17 and epoxidation18 of the norbornene unit in 3a went smoothly to generate the

a General conditions: 1 (0.50 mmol), 2 (0.75 mmol), nBuLi (0.52 mmol), cyclohexane (5 mL), rt, 0.5 h. bYields of the isolated products.

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DOI: 10.1021/acs.organomet.7b00574 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

compounds was confirmed via NMR (1H, 13C, and 11B) spectra as well as HRMS. General Procedures for Synthesis of 3 or 5. To a cyclohexane solution (5 mL) of 1-OTf-o-C2B10H11 (146 mg, 0.5 mmol) and pentafulvene (0.75 mmol) was added a hexane solution of nBuLi (328 μL, 1.6 M in hexane, 0.525 mmol) with stirring at room temperature. After stirring for 0.5 h, the resultant reaction mixture was quenched with water and extracted with diethyl ether (5 mL × 3). The ether solutions were combined and concentrated to dryness under vacuum. The residue was then subjected to flash column chromatography on silica gel using hexane as eluent to afford 3 or 5. 3a. Colorless crystals. 80% yield. 1H NMR (400 MHz, CDCl3): δ 6.55 (t, J = 2.0 Hz, 2H) (CHCH), 3.92 (t, J = 2.0 Hz, 2H) (C− CH), 1.50 (s, 6H) (CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 156.0, 141.1, 106.2 (alkenyl C), 79.2 (Ccage), 51.0 (C−CH), 18.8 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −1.5 (2B), −3.0 (1B), −4.7 (1B), −11.8 (4B), −12.9 (2B). Anal. Calcd for C10H2010B211B8: C, 48.36; H, 8.12. Found: C, 48.63; H, 8.42. 3b. Colorless crystals. 77% yield. 1H NMR (400 MHz, CDCl3): δ 6.56 (m, 2H) (CHCH), 3.92 (m, 2H) (C−CH), 1.87 (m, 2H) (CH2CH3), 1.50 (s, 3H) (C = C−CH3), 0.93 (t, J = 7.6 Hz, 3H) (CH2CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 155.7, 141.2, 141.1, 110.6 (alkenyl C), 79.3, 79.1 (Ccage), 50.9, 50.6 (C−CH), 26.2 (CH2CH3), 16.2 (C = C-CH3), 11.2 (CH2CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −2.1 (2B), −3.5 (1B), −5.2 (1B), −12.4 (4B), −13.5 (2B). HRMS: m/z calcd for C11H2210B211B8 [M − H]−: 261.2657. Found 261.2653. 3c. Colorless crystals. 74% yield. 1H NMR (400 MHz, CDCl3): δ 6.56 (m, 2H) (CHCH), 3.78 (m, 2H) (C−CH), 2.17 (m, 4H) (CH2), 1.62 (m, 4H) (CH2), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 152.2, 140.9, 116.9 (alkenyl C), 79.4 (Ccage), 51.9 (C−CH), 29.5, 26.4 (CH2). 11B{1H} NMR (128 MHz, CDCl3): δ −1.8 (2B), −3.1 (1B), −4.9 (1B), −12.0 (4B), −13.3 (2B). HRMS: m/z calcd for C12H2210B211B8 [M − H]−: 273.2658. Found 273.2654. 3d. Colorless crystals. 76% yield. 1H NMR (400 MHz, CDCl3): δ 6.55 (s, 2H) (CHCH), 3.95 (s, 2H) (C−CH), 1.99 (m, 2H), 1.90 (d, J = 7.8 Hz, 2H), 1.50 (m, 6H) (CH2), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 153.0, 141.3, 112.8 (alkenyl C), 79.2 (Ccage), 50.4 (C−CH), 29.7, 26.2, 26.1(CH2). 11B{1H} NMR (128 MHz, CDCl3): δ −1.4 (2B), −3.0 (1B), −4.5 (1B), −11.8 (4B), −12.9 (2B). HRMS: m/z calcd for C13H2410B211B8 [M]+: 288.2882. Found 288.2881. 3e. Colorless crystals. 70% yield. 1H NMR (400 MHz, CDCl3): δ 6.56 (t, J = 2.4 Hz 2H) (CHCH), 3.93 (t, J = 2.4 Hz, 2H) (C− CH), 2.05 (t, J = 2.0 Hz, 4H), 1.58 (m, 2H), 1.43 (m, 6H) (CH2), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 156.4, 141.2, 113.9 (alkenyl C), 79.3 (Ccage), 50.8 ( C−CH), 30.8, 29.2, 26.6 (CH2). 11B{1H} NMR (128 MHz, CDCl3): δ −0.4 (2B), −1.8 (1B), −3.5 (1B), −10.6 (4B), −11.7 (2B). HRMS: m/z calcd for C14H2610B211B8 [M]+: 302.3039. Found 302.3038. 3f. Colorless crystals. 60% yield. 1H NMR (400 MHz, CDCl3): δ 7.37 (t, J = 7.6 Hz, 2H), 7.27 (m, 1H), 7.14 (d, J = 7.6 Hz, 2H) (phenyl CH), 6.66 (m, 1H), 6.59 (m, 1H) (CHCH), 4.09 (m, 1H), 3.92 (m, 1H) (C−CH), 1.89 (s, 3H) (CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 157.1, 141.7, 140.8, 140.3, 128.5, 127.1, 127.0, 110.1 (phenyl and alkenyl C), 78.8, 78.6 (Ccage), 51.8, 51.4 (C−CH), 18.6 (CH3). 11 1 B{ H} NMR (128 MHz, CDCl3): δ −1.4 (2B), −3.1 (1B), −4.8 (1B), −11.5 (4B), −12.8 (2B). Calcd for C15H2210B211B8: C, 58.03; H, 7.14. Found: C, 57.84; H, 7.14. 3g. Colorless crystals. 42% yield. 1H NMR (400 MHz, CDCl3): δ 7.33 (d, J = 5.2 Hz, 2H), 7.03 (m, 2H), 6.89 (d, J = 3.2 Hz, 2H) (thiophenyl CH), 6.69 (t, J = 2.0 Hz, 2H) (CHCH), 4.29 (t, J = 2.0 Hz, 2H) (C−CH), ten BH signals were very broad and unresolved. 13 C{1H} NMR (101 MHz, CDCl3): δ 158.6, 141.5, 141.0, 127.6, 127.2, 126.3, 105.2 (thiophenyl and alkenyl C), 78.0 (Ccage), 52.9 ( C−CH). 11B{1H} NMR (128 MHz, CDCl3): δ −1.1 (2B), −3.8 (1B),

Scheme 2. Chemical Transformation of 3a

desired products 8 and 9 in 82 and 72% yields. They are all exoproducts, and no endoisomers are observed due to the steric hindrance of the cage. However, treatment of 3a with 4-phenyl4H-1,2,4-triazole-3,5-dione afforded unprecedented multiheterocyclic product 10 in 77% isolated yield, which is presumably formed via a usual [4 + 2] cycloaddition reaction.19 These transformations demonstrate that carboranonorbornenes can serve as useful starting materials for the preparation of highly functionalized carboranes for possible applications in medicine. All new compounds of 3 and 5−10 were characterized by 1H, 13C, and 11B NMR spectroscopy and HRMS. In addition, the molecular structures of 3a, 3b, 3f, 5c, 5g, 6, 7, 9, and 10 were confirmed by single-crystal X-ray analyses.

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CONCLUSION 1-OTf-2-Li-o-C2B10H10 is an excellent precursor for the generation of o-carboryne which undergoes Diels−Alder reaction with 6-substituted and 6,6′-disubstituted pentafulvenes to give a large variety of carboranonorbornenes. This reaction is compatible with a very broad substrate scope and tolerant of many functional groups. The resultant carboranonorbornenes can be further functionalized via [3 + 2]/[4 + 2] cycloaddition, thiolation, and epoxidation to afford multifunctionalized carboranes.

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EXPERIMENTAL SECTION

General Procedures. All reactions were performed under dry argon with the rigid exclusion of moisture and oxygen using standard Schlenk techniques unless otherwise specified. 1H and 13C{1H} NMR spectra were recorded on a Bruker DPX 400 spectrometer at 400 and 100 MHz, respectively. 11B{1H} NMR spectra were recorded on a Bruker DPX 300 spectrometer at 96 MHz or a Varian Inova 400 spectrometer at 128 MHz. All signals were reported in ppm with reference to the residual solvent resonances of the deuterated solvents for proton and carbon chemical shifts: to external BF3·OEt2 (0.00 ppm) for boron chemical shifts and to external CFCl3 (0.00 ppm) for fluorine chemical shifts. Elemental analyses were carried out by the Analytical Laboratory of Shanghai Institute of Organic Chemistry, the Chinese Academy of Sciences, China. Mass spectra were obtained on a Thermo Finnigan MAT 95 XL spectrometer or Waters Micromass GCT Premier or Q Exactive Focus Orbitrap. All organic solvents were freshly distilled from Na−K alloy or CaH2 immediately prior to use. 1OTf-o-carborane,14 fulvenes,20 and benzohydroximinoyl chloride21,22 were synthesized according to the reported procedures. Other chemicals were purchased from either Aldrich or Acros Chemical Co. and used as received unless otherwise specified. The purity of new C

DOI: 10.1021/acs.organomet.7b00574 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

−4.9 (1B), −11.5 (4B), −12.6 (2B). HRMS: m/z calcd for C14H1910B211B8Cl [M]+: 331.2154. Found 331.2158. 5e. Colorless crystals. 27% yield. 1H NMR (400 MHz, CDCl3): δ 6.85 (d, J = 8.1 Hz, 1H), 6.70 (m, 3H), 6.62 (m, 1H), 5.50 (s, 1H) (phenyl CH and = CH), 4.30 (m, 1H), 3.90 (s, 1H) (C−CH), 3.89 (s, 3H), 3.88 (s, 3H) (OCH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 159.6, 149.1, 148.4, 141.4, 141.2, 128.2, 120.1, 111.5, 110.9, 105.1 (phenyl and alkenyl C), 78.6, 77.7 (Ccage), 55.9, 55.8 (C−CH), 54.6, 50.9 (OCH3). 11B{1H} NMR (128 MHz, CDCl3): δ −1.3 (2B), −4.5 (2B), −11.4 (4B), −12.6 (2B). Calcd for C16H2410B211B8O2: C, 53.91; H, 6.79. Found: C, 53.73; H, 6.79. 5f. Colorless crystals. 75% yield. 1H NMR (400 MHz, CDCl3): δ 7.27 (m, 2H), 7.13 (s, 1H), 7.05 (d, J = 7.4 Hz, 1H) (phenyl CH), 6.71 (m, 1H), 6.66 (m, 1H) (CH), 4.29 (d, J = 3.2 Hz, 1H), 3.90 (d, J = 3.2 Hz, 1H) (C−CH), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 161.0, 141.4, 140.9, 137.1, 134.6, 130.0, 127.7, 127.3, 125.7, 104.1 (phenyl and alkenyl C), 77.9 (Ccage), 54.5, 50.6 (C−CH), another Ccage was not observed. 11 1 B{ H} NMR (128 MHz, CDCl3): δ −1.4 (2B), −4.8 (2B), −11.5 (6B). HRMS: m/z calcd for C14H1910B211B8Cl [M]+: 330.2076. Found 330.2077. 5g. Colorless crystals. 72% yield. 1H NMR (400 MHz, CDCl3): δ 7.39 (d, J = 7.7 Hz, 1H), 7.25 (m, 2H), 7.17 (d, J = 7.4 Hz, 1H) (phenyl CH), 6.72 (m, 1H), 6.64 (m, 1H), 5.74 (s, 1H) (CH), 4.15 (d, J = 2.2 Hz, 1H), 3.96 (d, J = 2.4 Hz, 1H) (C−CH), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 161.2, 141.3, 141.0, 133.7, 133.5, 129.8, 128.6, 128.5, 126.8, 102.2 (phenyl and alkenyl C), 78.1, 77.4 (Ccage), 54.3, 50.6 (C− CH). 11B{1H} NMR (128 MHz, CDCl3): δ −1.1 (2B), −3.4 (1B), −4.7 (1B), −11.1 (2B), −11.7 (2B), −12.5 (2B). HRMS: m/z calcd for C14H1910B211B8Cl [M]+: 330.2076. Found 330.2079. 5h. Colorless crystals. 41% yield. 1H NMR (400 MHz, CDCl3): δ 7.38 (d, J = 1.2 Hz, 1H), 6.66 (m, 2H) (furanyl CH), 6.37 (dd, J = 3.2, 1.8 Hz, 1H), 6.17 (d, J = 3.2 Hz, 1H), 5.34 (s, 1H) (CH), 4.71 (s, 1H), 3.83 (s, 1H) (C−CH), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 142.3, 141.4, 141.2, 111.3, 109.4, 95.5 (furanyl and alkenyl C), 77.2 (Ccage), 54.5, 51.7 ( C−CH), another Ccage was not observed. 11B{1H} NMR (128 MHz, CDCl3): δ 0.2 (2B), −1.9 (1B), −3.3 (1B), −10.5 (4B), −11.3 (2B). HRMS: m/z calcd for C12H1810B211B8O [M]+: 286.2361. Found 286.2362. 5i. Colorless crystals. 68% yield. 1H NMR (400 MHz, CDCl3): δ 6.60 (m, 1H), 6.55 (m, 1H), 4.47 (t, J = 7.5 Hz, 1H) (CH), 3.95 (m, 1H), 3.70 (m, 1H) (C−CH), 1.85 (m, 2H), 1.35 (m, 2H) (CH2), 0.87 (t, J = 7.3 Hz, 3H) (CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 160.3, 141.7, 140.8, 104.8 (alkenyl C), 78.7, 78.3 (Ccage), 53.9, 50.0 (C−CH), 29.5, 22.1 (CH2), 13.9 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −1.5 (2B), −3.2 (1B), −4.8 (1B), −11.8 (4B), −12.8 (2B). HRMS: m/z calcd for C11H2210B211B8 [M]+: 262.2725. Found 262.2727. 5j. Colorless crystals. 72% yield. 1H NMR (400 MHz, CDCl3): δ 6.57 (m, 2H), 4.27 (d, J = 9.3 Hz, 1H) (CH), 3.95 (s, 1H), 3.67 (s, 1H), 2.24 (m, 1H) (C−CH), 1.00 (d, J = 6.7 Hz, 3H), 0.87 (d, J = 6.7 Hz, 3H) (CH3), ten BH signals were very broad and unresolved. 13 C{1H} NMR (101 MHz, CDCl3): δ 158.1, 141.7, 140.7, 111.3 (alkenyl C), 78.7, 78.3 (Ccage), 53.8, 50.1, 27.6 (C−CH), 22.4, 22.0 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −1.5 (2B), −3.3 (1B), −4.9 (1B), −11.8 (4B), −12.8 (2B). HRMS: m/z calcd for C11H2210B211B8 [M]+: 262.2725. Found 262.2722. 5k. Colorless crystals. 76% yield. 1H NMR (400 MHz, CDCl3): δ 6.60 (m, 1H), 6.56 (m, 1H), 4.46 (t, J = 7.2 Hz, 1H) (CH), 3.94 (m, 1H), 3.70 (m, 1H) (C−CH), 1.90 (m, 4H), 1.35 (m, 6H) (CH2), 0.88 (t, J = 3.6 Hz, 3H) (CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 160.2, 141.7, 140.8, 105.0 (alkenyl C), 78.7, 78.3 (Ccage), 53.9, 50.0 (C−CH), 31.5, 28.9, 28.8, 27.4, 22.6 (CH2), 14.0 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −0.2 (2B), −2.0 (1B), −3.5 (1B), −10.5 (4B), −11.6 (2B). HRMS: m/z calcd for C14H2810B211B8 [M]+: 304.3196. Found 304.3199.

−4.9 (1B), −11.1 (4B), −12.4 (2B). Calcd for C16H2010B211B8S2: C, 49.97; H, 5.24. Found: C, 49.90; H, 5.20. 3h. Colorless crystals. 69% yield. 1H NMR (400 MHz, CDCl3): δ 7.32 (m, 6H), 7.07 (d, J = 6.7 Hz, 4H) (phenyl CH), 6.70 (br s, 2H) (alkenyl CH), 4.06 (br s, 2H) (C−CH), ten BH signals were very broad and unresolved. These data are the same as those reported.13 3i. Colorless crystals. 61% yield. 1H NMR (400 MHz, CDCl3) δ 6.99 (d, J = 8.6 Hz, 4H), 6.86 (d, J = 8.6 Hz, 4H) (phenyl CH), 6.69 (t, J = 2.0 Hz, 2H) (CHCH), 4.05 (t, J = 2.0 Hz, 2H) (C−CH), 3.82 (s, 6H) (OCH3), ten BH signals were very broad and unresolved. 13 C{1H} NMR (101 MHz, CDCl3): δ 158.9, 156.8, 141.7, 131.9, 130.1, 113.8 (phenyl and alkenyl C), 78.8 (Ccage), 55.3 (C−CH), 52.4 (OCH3). 11B{1H} NMR (128 MHz, CDCl3): δ −1.5 (2B), −4.5 (2B), −11.5 (6B). Anal. Calcd for C22H2810B211B8O2: C, 61.09; H, 6.52. Found: C, 61.11; H, 6.52. 3j. Colorless crystals. 66% yield. 1H NMR (400 MHz, CDCl3): δ 7.14 (d, J = 8.0 Hz, 4H), 6.96 (d, J = 8.0 Hz, 4H) (phenyl CH), 6.69 (t, J = 2.0 Hz, 2H) (CHCH), 4.06 (t, J = 2.0 Hz, 2H) (C−CH), 2.36 (s, 6H) (CH3), ten BH signals were very broad and unresolved. These data are the same as those reported.13 3k. Colorless crystals. 71% yield. 1H NMR (400 MHz, CDCl3): δ 7.03 (m, 8H) (phenyl CH), 6.71 (t, J = 1.5 Hz, 2H) (CHCH), 4.01 (t, J = 1.5 Hz, 2H) (C−CH), ten BH signals were very broad and unresolved. These data are the same as those reported.13 3l. Colorless crystals. 75% yield. 1H NMR (400 MHz, CDCl3): δ 7.32 (d, J = 8.8 Hz, 4H), 6.98 (d, J = 8.8 Hz, 4H) (phenyl CH), 6.71 (t, J = 2.1 Hz, 2H) (CHCH), 4.01 (t, J = 2.1 Hz, 2H) (C−CH), ten BH signals were very broad and unresolved. These data are the same as those reported.13 3m. Colorless crystals. 72% yield. 1H NMR (400 MHz, CDCl3): δ 7.48 (d, J = 8.4 Hz, 4H), 6.92 (t, J = 8.4 Hz, 4H) (phenyl CH), 6.71 (t, J = 2.1 Hz, 2H) (CHCH), 4.01 (t, J = 2.1 Hz, 2H) (CH), ten BH signals were very broad and unresolved. These data are the same as those reported.13 5a. Colorless crystals. 69% yield. 1H NMR (400 MHz, CDCl3): δ 7.36 (t, J = 7.4 Hz, 2H), 7.28 (m, 1H), 7.18 (d, J = 7.5 Hz, 2H) (phenyl CH), 6.70 (s, 1H), 6.64 (s, 1H), 5.56 (s, 1H) (CH), 4.33 (s, 1H), 3.89 (s, 1H) (C−CH), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 160.2, 141.4, 141.0, 135.2, 128.7, 127.6, 127.2, 105.3 (phenyl and alkenyl C), 78.4, 77.8 (Ccage), 54.6, 50.7 (C−CH). 11B{1H} NMR (128 MHz, CDCl3): δ −1.5 (2B), −3.6 (1B), −5.1 (1B), −11.6 (4B), −12.9 (2B). HRMS: m/z calcd for C14H2010B211B8 [M]+: 296.2570. Found 296.2570. 5b. Colorless crystals. 40% yield. 1H NMR (400 MHz, CDCl3): δ 7.09 (d, J = 8.2 Hz, 2H), 6.88 (d, J = 8.4 Hz, 2H) (phenyl CH), 6.68 (m, 1H), 6.63 (m, 1H), 5.49 (s, 1H) (CH), 4.30 (s, 1H), 3.86 (s, 1H) (C−CH), 3.81 (s, 3H) (OCH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 159.3, 158.8, 141.5, 141.0, 128.8, 127.7, 114.2, 104.9 (phenyl and alkenyl C), 78.6, 77.9 (Ccage), 55.3 (OCH3), 54.7, 50.7 (C−CH). 11B{1H} NMR (128 MHz, CDCl3): δ −1.3 (2B), −3.2 (1B), −4.9 (1B), −11.4 (4B), −12.7 (2B). HRMS: m/z calcd for C15H2210B211B8O [M]+: 326.2676. Found 326.2671. 5c. Colorless crystals. 65% yield. 1H NMR (400 MHz, CDCl3): δ 7.17 (d, J = 7.9 Hz, 2H), 7.07 (d, J = 8.0 Hz, 2H) (phenyl CH), 6.69 (m, 1H), 6.63 (m, 1H), 5.53 (s, 1H) (CH), 4.33 (d, J = 2.5 Hz, 1H), 3.88 (d, J = 2.5 Hz, 1H) (C−CH), 2.37 (s, 3H) (CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 159.9, 141.4, 141.0, 137.1, 132.3, 129.4, 127.5, 105.3 (phenyl and alkenyl C), 78.5, 77.8 (Ccage), 54.7, 50.7 (C−CH), 21.1 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −1.5 (2B), −3.5 (1B), −4.9 (1B), −11.6 (4B), −12.8 (2B). HRMS: m/z calcd for C15H2210B211B8 [M]+: 310.2727. Found 310.2727. 5d. Colorless crystals. 72% yield. 1H NMR (400 MHz, CDCl3): δ 7.32 (d, J = 7.6 Hz, 2H), 7.08 (d, J = 7.6 Hz, 2H) (phenyl CH), 6.70 (s, 1H), 6.64 (s, 1H), 5.50 (s, 1H) (CH), 4.25 (s, 1H), 3.87 (s, 1H) (C−CH), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 160.8, 141.5, 140.9, 133.8, 133.2, 129.0, 128.9, 104.4 (phenyl and alkenyl C), 78.0, 77.5 (Ccage), 54.7, 50.7 ( C−CH). 11B{1H} NMR (128 MHz, CDCl3): δ −1.4 (2B), −3.6 (1B), D

DOI: 10.1021/acs.organomet.7b00574 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

NMR (101 MHz, CDCl3): δ 147.1, 146.8 (CO), 131.1, 129.1, 128.1, 125.7 (phenyl C), 80.7, 74.3 (Ccage), 66.1 (CH), 61.6 (quaternary C−N), 56.5, 48.4, 44.7 (CH), 24.4 (CH3), 20.5 (quaternary C), 20.4 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −1.9 (2B), −4.1 (2B), −8.6 (2B), −11.8 (2B), −13.9 (2B). HRMS: m/z calcd for C18H2510B211B8N3O2 [M]+: 423.2953. Found 423.2955. X-ray Structure Determination. All data were collected at 293 K on a Bruker SMART 1000 CCD diffractometer using Mo Kα radiation. An empirical absorption correction was applied using the SADABS program.23 All structures were solved by direct methods and subsequent Fourier difference techniques and refined anisotropically for all non-hydrogen atoms by full-matrix least-squares calculations on F2 using the SHELXTL program package.24 All hydrogen atoms were geometrically fixed using the riding model. Data collection and structural refinement were included in the Supporting Information. CCDC 1565384−1565392 for 3a, 3b, 3f, 5c, 5g, 6, 7, 9, and 10, respectively, contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif.

Synthesis of 6. To a DMF solution (2 mL) of 3a (24.8 mg, 0.1 mmol) and benzohydroximinoyl chloride (15.6 mg, 0.1 mmol) was slowly added NEt3 (10.1 mg, 0.1 mmol). A white precipitate was immediately formed. The reaction was finished in 5 min as monitored by TLC. The reaction was then diluted with EtOAc. The organic phase was washed with water, dried over Na2SO4, and concentrated under vacuum. The residue was purified using flash chromatography on silica gel using hexane as eluent to afford 6 as colorless crystals (27.5 mg, 75% yield). 1H NMR (400 MHz, CDCl3): δ 7.59 (dd, J = 6.6, 2.9 Hz, 2H), 7.43 (m, 3H) (phenyl CH), 5.03 (d, J = 7.5 Hz, 1H) (O−CH), 4.21 (d, J = 7.5 Hz, 1H), 3.71 (s, 1H), 3.61 (s, 1H) (CH), 1.68 (s, 3H), 1.27 (s, 3H) (CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 155.4, 140.0, 130.7, 129.3, 128.1, 126.4, 126.1 (phenyl and alkenyl C), 87.9 (O-CH), 78.8, 76.2 (Ccage), 60.5, 50.1, 48.4 (CH), 20.5, 20.4 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −1.7 (2B), −6.5 (2B), −10.5 (3B), −13.3 (1B), −14.9 (2B). HRMS: m/z calcd for C17H2510B211B8NO [M]+: 367.2942. Found 367.2941. Synthesis of 7. To a solution of 3a (24.8 mg, 0.1 mmol) in hexane (2 mL) was added PhN3 (23.8 mg, 0.2 mmol). After stirring at room temperature overnight, a white precipitate was formed. The solid was filtered off and washed with hexane to afford 7 as a white solid (31.3 mg, 85% yield). 1H NMR (400 MHz, CDCl3): δ 7.38 (m, 2H), 7.19 (m, 2H), 7.08 (m, 1H) (phenyl CH), 5.05 (d, J = 8.4 Hz, 1H), 4.29 (d, J = 8.4 Hz, 1H) (N−CH), 3.89 (s, 1H), 3.68 (s, 1H) (CH), 1.57 (s, 3H), 1.27 (s, 3H) (CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 139.0, 138.9, 129.8, 126.5, 123.2 (phenyl and alkenyl C), 88.2, 61.7 (N−CH), 49.4, 47.4 (CH), 20.7, 20.1 (CH3), Ccage were not observed. 11B{1H} NMR (128 MHz, CDCl3): δ −2.0 (2B), −7.4 (2B), −10.4 (3B), −13.2 (1B), −14.9 (2B). HRMS: m/z calcd for C16H2510B211B8N3 [M + H]+: 368.3132. Found 368.3116. Synthesis of 8. To a solution of 3a (24.8 mg, 0.1 mmol) in THF (2 mL) was added PhSH (22.0 mg, 0.2 mmol) and AIBN (16.4 mg, 0.1 mmol) at room temperature. After heating at 100 °C for 12 h and removal of the solvent under vacuum, the residue was purified by flash column chromatography on silica gel using hexane as eluent to afford 8 as colorless crystals (29.4 mg, 82% yield). 1H NMR (400 MHz, CDCl3): δ 7.32 (m, 4H), 7.26 (m, 1H) (phenyl CH), 3.70 (dd, J = 7.5, 4.2 Hz, 1H), 3.45 (d, J = 4.2 Hz, 1H), 3.31 (s, 1H) (CH), 2.40 (dd, J = 13.8, 7.5 Hz, 1H) (CH2), 1.69 (s, 3H), 1.68 (s, 3H) (CH3), 1.57 (dt, J = 13.8, 4.2 Hz, 1H) (CH2), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 144.3, 135.8, 129.3, 129.2, 126.8, 120.9 (phenyl and alkenyl C), 80.4, 79.9 (Ccage), 49.9, 49.4, 45.3 (CH), 39.4 (CH2), 20.7, 20.1 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −2.5 (1B), −2.9 (1B), −7.1 (2B), −10.0 (1B), −10.7 (2B), −14.0 (1B), −15.0 (2B). HRMS: m/z calcd for C16H2610B211B8S [M]+: 358.2761. Found 358.2764. Synthesis of 9. To a solution of 3a (24.8 mg, 0.1 mmol) in CHCl3 (2 mL) was added diethyl diazenedicarboxylate (174 mg, 10 mmol). After heating at 100 °C for 4 days and removal of the solvent under vacuum, the residue was purified by flash column chromatography on silica gel using hexane as eluent to afford 9 as colorless crystals (19.0 mg, 72% yield). 1H NMR (400 MHz, CDCl3): δ 3.61 (s, 2H), 3.44 (s, 2H) (CH), 1.61 (s, 6H) (CH3), ten BH signals were very broad and unresolved. 13C{1H} NMR (101 MHz, CDCl3): δ 135.4, 126.2 (alkenyl C), 79.1 (Ccage), 55.4, 47.0 (CH), 19.7 (CH3). 11B{1H} NMR (128 MHz, CDCl3): δ −2.1 (2B), −5.3 (1B), −8.2 (1B), −11.7 (3B), −13.0 (1B), −14.6 (2B). HRMS: m/z calcd for C10H2010B211B8O [M]+: 264.2517. Found 264.2517. Synthesis of 10. A Schlenk tube was charged with 3a (24.8 mg, 0.1 mmol), phenyl-4H-1,2,4-triazole-3,5-dione (35 mg, 0.2 mmol) and CH2Cl2 (2 mL), and then closed. The solution was stirred at 50 °C for 24 h in the dark. After removal of the solvent under vacuum, the residue was purified by flash column chromatography on silica gel using a mixture of hexane/CH2Cl2 (2/1 in V/V) as eluent to afford 10 as colorless crystals (32.6 mg, 77% yield). 1H NMR (400 MHz, CDCl3): δ 7.46 (m, 4H), 7.37 (m, 1H) (phenyl CH), 5.30 (s, 1H), 5.18 (s, 1H), 3.37 (m, 1H), 3.14 (m, 1H) (CH), 1.96 (s, 3H), 1.41 (s, 3H) (CH3), ten BH signals were very broad and unresolved. 13C{1H}

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00574. NMR spectra of all new compounds (PDF) Accession Codes

CCDC 1565384−1565392 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

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

Zaozao Qiu: 0000-0001-9094-7027 Zuowei Xie: 0000-0001-6206-004X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from RGC of the Hong Kong Special Administration Region (project no. 14306114 to Z.X.), the National Natural Sciences Foundation of China (project no. 21372245 to Z.Q.), NSFC-RGC Joint Research Scheme (project no. N_CUHK442/14 to Z.X.), and CASCroucher Funding Scheme. We thank Ms. Hoi-Shan Chan for single-crystal X-ray analyses.

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REFERENCES

(1) (a) Norton, J. A. Chem. Rev. 1942, 31, 319−523. (b) Martin, J. G.; Hill, R. K. Chem. Rev. 1961, 61, 537−562. (c) Nicolaou, K. C.; Snyder, S. A.; Montagnon, T.; Vassilikogiannakis, G. Angew. Chem., Int. Ed. 2002, 41, 1668−1698. (d) Gampe, C. M.; Carreira, E. M. Angew. Chem., Int. Ed. 2012, 51, 3766−3778. (e) Sumida, Y.; Kato, T.; Hosoya, T. Org. Lett. 2013, 15, 2806−2809. (2) (a) Dockendorff, C.; Sahli, S.; Olsen, M.; Milhau, L.; Lautens, M. J. Am. Chem. Soc. 2005, 127, 15028−15029. (b) Buszek, K. R.; Luo, D.; Kondrashov, M.; Brown, N.; VanderVelde, D. Org. Lett. 2007, 9, E

DOI: 10.1021/acs.organomet.7b00574 Organometallics XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.organomet.7b00574 Organometallics XXXX, XXX, XXX−XXX