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Article

One-Pot Synthesis of B-Aryl Carboranes with Sensitive Functional Groups Using Sequential Cobalt- and Palladium-Catalyzed Reactions

1
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilov Str., 119991 Moscow, Russia
2
N.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninskii Av., 119991 Moscow, Russia
3
Basic Department of Chemistry of Innovative Materials and Technologies, G.V. Plekhanov Russian University of Economics, 36 Stremyannyi Line, 117997 Moscow, Russia
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(11), 1348; https://doi.org/10.3390/catal10111348
Submission received: 30 October 2020 / Revised: 15 November 2020 / Accepted: 17 November 2020 / Published: 19 November 2020
(This article belongs to the Special Issue Transition Metal Complexes as Catalysts in Organic Chemistry)

Abstract

:
The simple and efficient method was developed for the one-pot synthesis of B-substituted aryl derivatives of ortho-carborane with functional groups sensitive to organolithium and organomagnesium reagents using 9-iodo-ortho-carborane and generated in situ organozinc compounds. The method proposed was used to prepare a series of 9-aryl-ortho-carboranes, including those containing nitrile and ester groups, 9-RC6H4-1,2-C2B10H11 (R = p-Me, p-NMe2, p-OCH2OMe, o-OMe, p-OMe, o-CN, p-CN, o-COOEt, m-COOEt, and p-COOEt). It was demonstrated that the same approach can be used for synthesis of diaryl derivatives of ortho-carborane 9,12-(RC6H4)2-1,2-C2B10H10 (R = H, p-Me). The solid-state structures of 9-RC6H4-1,2-C2B10H11 (R = p-NMe2, p-OCH2OMe, o-OMe, o-CN, p-CN, m-COOEt, and p-COOEt) and 9,12-(p-MeC6H4)2-1,2-C2B10H10 were determined by single crystal X-ray diffraction.

Graphical Abstract

1. Introduction

Aryl derivatives of icosahedral carboranes C2B10H12 (Figure 1) are of interest for a wide variety of applications, starting with medical chemistry [1,2,3,4] and ending with the development of new materials [5,6,7,8,9,10,11,12]. This necessitates the development of new convenient methods for their synthesis. Since the properties of the CH and BH groups in carboranes differ significantly, different methods are used to synthesize their C- and B-aryl derivatives.
There are several methods for synthesis of C-arylcarboranes, where an aryl group is directly bonded to the cage carbon [13]. A general method involves the use of decaborane nido-B10H14 and reacting it with Lewis bases, such as dialkyl sulfides, acetonitrile or alkylamines. This leads to the formation of compounds with a general formula of 6,9-arachno-B10H12L2. On reacting this species with alkynes with aromatic substituents, the corresponding C-aryl-ortho-carboranes are formed [14]. However, this reaction gives very low yields for some alkynes, especially those containing two aromatic units [15,16]. This reaction cannot be used when the aryl groups have acidic or readily reduced substituents. This method is also unsuitable for synthesis of aryl derivatives of meta- and para-carboranes. Therefore, for the synthesis of aryl derivatives of meta- and para-carboranes an Ullmann-type copper-coupling reaction is used. In this way, mono- and diaryl derivatives of meta- and para-carboranes can be obtained, as well as monoaryl derivatives of ortho-carborane [15,17,18,19,20]. An alternative method includes Ni-catalyzed cross-coupling reactions of aryl iodides with carboranyl Grignard reagents. In such way both monoaryl- and diaryl derivatives of ortho-carborane can be prepared [21,22]. Another approach is based on SNAr reactions of carborane-derived carbanions with fluorinated aromatics [23,24,25]. This approach is widely used for synthesis of asymmetrically substituted diaryl derivatives of carboranes [19,26].
Unlike C-aryl derivatives, the synthesis of B-aryl derivatives of carboranes is mainly based on Pd-catalyzed cross-coupling reactions of their iodo derivatives with aryl Grignard reagents (Kumada cross-coupling) [27,28,29,30,31,32,33,34,35,36]. However, available substituents on the aromatic ring in these reactions are strictly limited because of the high reactivity of Grignard reagents. Mild B-arylation of carboranes via Suzuki cross-coupling reactions of aryl boronic acids with 9-iodo-meta-, 2-iodo-para-, and 3-iodo-ortho-carboranes were reported [37,38,39]. It is important that these cross-coupling reactions can be used for direct introduction of functionalized aryl substituents that are not compatible with the previously mentioned Kumada reaction conditions. However, this approach was found to be ineffective for 9-iodo-ortho-carborane. Moreover, Suzuki cross-coupling reactions normally employ inorganic bases such as F- or OH- to facilitate the transmetallation step, but these bases are strong nucleophiles that can result in deboronation of the ortho-carborane cage during the cross-coupling. The possibility of B-arylation of ortho- and meta-carboranes using organozinc reagents (Negishi cross-coupling) has been demonstrated as well [40,41], however the arylzinc reagents were obtained by transmetallation of the corresponding Grignard reagents that decreases the synthetic utility of this method.
Direct arylation of ortho-carborane derivatives via Pd-catalyzed B-H activation has been recently reported [42]. This reaction is tolerant to various functional groups including acyl and ester; however, it is not selective leading to mixtures of 8- and 9-aryl derivatives and is not applicable to the parent ortho-carborane. A series of aryl derivatives of ortho-carborane were prepared by Pd-catalyzed B-H activation reactions with aryl iodides under functional group assistance or by intramolecular cyclization via Pd-catalyzed B-H activation involving pendant ortho-bromoaryl groups [43]; however, these reactions are currently of academic interest rather than real synthetic methods. Of greater interest is the direct arylation of 1,2-bis(dimethylphenylsilyl)-ortho-carborane 1,2-(PhMe2Si)2-1,2-C2B10H10 with arylmagnesium chlorides in diethyl ether, followed by the removal of the silyl protective groups with K2CO3 in acetone [44]; however, this approach gives only moderate yields of 9-aryl-ortho-carboranes and is not tolerant to many functional groups.
In this contribution we describe convenient and mild synthesis of B-aryl derivatives of ortho-carborane containing various functional groups including ester and nitrile ones.

2. Results and Discussion

The possibility of synthesis of B-aryl derivatives of ortho-carborane using organozinc reagents has been demonstrated earlier [40,41]. However, the preparation of these organozinc compounds was achieved by a transmetallation reaction of preformed organolithium or magnesium counterparts with zinc halide. Later, various methods were elaborated for preparation of zinc organometallics bearing highly sensitive functional groups [45,46]. However, most of these methods are based on the use of bimetallic zinc-lithium or zinc-magnesium reagents and, despite their indisputable synthetic utility, the use of such activated zinc reagents requires specific conditions together with careful handling. Therefore, such methods for the synthesis of organozinc compounds that do not require the use of organolithium and organomagnesium reagents are of particular interest. We chose the method based on Co-catalyzed preparation of arylzinc organics starting from readily available aryl bromides and zinc dust [47,48,49]. Thus, the obtained organozinc species can be easily coupled with various aryl iodides in the presence of a catalytic amount of (Ph3P)2PdCl2 [49]. In addition to the formation of the C–C bond, this approach can be used to form the C–B bond in the reaction of aryl bromides with haloboronic esters, leading to the formation of the corresponding arylboronates [50]. The reaction was found to be tolerate to many functional groups including nitriles and esters. Earlier this approach was used for synthesis of 1,4-bis(ortho-carboran-8′-yl)benzene staring from 8-iodo-ortho-carborane [51].
Arylzinc bromides containing various substituents including sensitive functional groups (-CN, -COOEt) were prepared by the reaction of the corresponding aryl bromides with allyl zinc chloride/bromide generated from allyl chloride and zinc metal in the presence of 25 mol % of CoBr2 and catalytic amount of trifluoroacetic acid in acetonitrile at ambient temperature (Scheme 1).
The reactions of the prepared arylzinc bromides with 9-iodo-ortho-carborane in the presence of 2 mol.% of [(Ph3P)2PdCl2] in acetonitrile at room temperature lead to its nearly quantitative conversion to the corresponding 9-aryl-ortho-carboranes with isolated yields varying from 60 to 91% (Scheme 2 and Scheme 3).
The synthesized 9-aryl-ortho-carboranes were characterized by the methods of 1H, 13C and 11B NMR and IR spectroscopy. The 1H and 13C NMR spectra of all compounds contain signals of the corresponding aryl substituents as well as the signals of two non-equivalent carborane CH groups in the range of 3.3–3.7 ppm and 48–53 ppm, respectively. The 11B NMR spectra of the 9-aryl-ortho-carboranes contain singlet of the C-substituted boron atom at ~ 6–8 ppm and five doublets with a total integral ratio of 1:1:2:2:2:2 (except for the spectra of 9-cyanophenyl and 9-ethoxycarbonylphenyl derivatives with a ratio of 1:1:2:4:2 ratio). The solid-state structures of all new 9-aryl-ortho-carboranes (except for 6, which is liquid) were determined by single crystal X-ray diffraction (Figure 2 and Figure 3).
The same approach can be used for synthesis of disubstituted aryl derivatives: the reactions of phenyl- and p-tolylzinc bromides with 9,12-diiodo-ortho-carborane in the presence of 4 mol. % of [(Ph3P)2PdCl2] in acetonitrile at room temperature lead to the corresponding 9,12-diaryl-ortho-carboranes (Scheme 4).
The solid-state structures of 9,12-di(p-tolyl)-ortho-carborane 14 was determined by single crystal X-ray diffraction (Figure 4).

3. Materials and Methods

3.1. General Methods

9-Iodo-ortho-carborane (1) [52], 9,12-diiodo-ortho-carborane (12) [28] and bis(triphenylphosphine)palladium dichloride [53] were prepared according to the literature procedures. Anhydrous cobalt dibromide was prepared from cobalt bromide hexahydrate by heating at 160 °C under vacuum for 3 h and stored under argon atmosphere. Acetonitrile was dried using standard procedures [54]. All other chemical reagents were purchased from Sigma Aldrich, Acros Organics and ABCR and used without purification. All reactions were carried out at argon atmosphere. The reaction progress was monitored by thin layer chromatography (Merck F254 silica gel on aluminum plates) and visualized using 0.5 % PdCl2 in 1% HCl in aq. MeOH (1:10). Acros Organics silica gel (0.060–0.200 mm) was used for column chromatography. The NMR spectra at 400 MHz (1H), 128 MHz (11B) and 100 MHz (13C) were recorded with Varian Inova 400 spectrometer. The residual signal of the NMR solvent relative to tetramethylsilane was taken as the internal reference for 1H and 13C NMR spectra. 11B NMR spectra were referenced using BF3•Et2O as external standard. Infrared spectra were recorded on an IR Prestige-21 (SHIMADZU) instrument.
Single crystal X-ray diffraction experiments for compounds 35, 7, 8, 10, 11, and 14 were carried out using SMART APEX2 CCD diffractometer (λ(Mo-Kα) = 0.71073 Å, graphite monochromator, ω-scans) at 120 K. Collected data were processed by the SAINT and SADABS programs incorporated into the APEX2 program package [55]. The structures were solved by the direct methods and refined by the full-matrix least-squares procedure against F2 in anisotropic approximation. The refinement was carried out with the SHELXTL program [56]. The CCDC numbers (2041390, 2041385, 2041386, 2041387, 2041388, 2041389, 2044200, and 2044199 for 3, 4, 5, 7, 8, 10, 11, and 14, respectively) 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.

3.2. General Synthetic Procedure and Characterization of Monosubstituted B-Aryl Derivatives of Ortho-Carborane

Allyl chloride (82 μL, 77 mg, 1.00 mmol) and trifluoroacetic acid (25 μL, catalytic amount) were added to a blue mixture of zinc powder (490 mg, 7.50 mmol) and anhydrous cobalt dibromide (55 mg, 0.25 mmol) in 2.5 mL of fresh distilled acetonitrile. The resulting dark orange mixture was stirred at room temperature for 15 min. Then corresponding aryl bromide (2.50 mmol) was added, and reaction was stirred at room temperature for another 1 h. Then 9-iodo-ortho-carborane (270 mg, 1.00 mmol) with bis(triphenylphosphine)palladium dichloride (14 mg, 0.02 mmol, catalytic amount) were added. The reaction was stirred at room temperature overnight. After removal of volatiles under reduced pressure, the residue was washed with water (25 mL), dichloromethane (3 × 25 mL) and acetone (until no trace of carborane appeared on TLC). The organic phases were combined, dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography on silica to give the corresponding B-aryl derivative of ortho-carborane.
9-(4-Methylphenyl)-ortho-carborane (2): 4-Methylphenyl bromide (315 μL, 435 mg, 2.50 mmol) was used; diethyl ether was used as eluent for column chromatography; a pale-yellow crystalline solid was obtained (207 mg, yield 88%). 1H NMR (400 MHz, CDCl3): δ 7.34 (2H, d, J = 7.7 Hz, CHAr), 7.12 (2H, d, J = 7.7 Hz, CHAr), 3.41 (1H, br s, CHCarb), 3.31 (1H, br s, CHCarb), 2.37 (3H, s, CH3) ppm; 13C NMR (100 MHz, CDCl3): δ 148.8 (CAr-B), 137.0 (CAr-CH3), 132.5 (CArH), 128.4 (CArH), 53.3 (CCarbH), 48.9 (CCarbH), 21.3 (CH3) ppm; 11B NMR (128 MHz, CDCl3): δ 7.6 (1B, s, B-C), −2.2 (1B, d, J = 148 Hz), −8.7 (2B, d, J = 150 Hz), −13.8 (2B, d, J = 194 Hz), −14.2 (2B, d, J = 156 Hz), −15.3 (2B, d, J = 174 Hz) ppm. The spectral data correspond to those described in the literature [36].
9-(4-N,N-Dimethylaminophenyl)-ortho-carborane (3): 4-N,N-Dimethylaminophenyl bromide (500 mg, 2.50 mmol) was used; a mixture of chloroform and hexane (3:1, v/v) was used as eluent for column chromatography; a pale-pink crystalline solid was obtained (170 mg, yield 65%). 1H NMR (400 MHz, CDCl3): δ 7.26 (2H, d, J = 8.4 Hz, CHAr), 6.66 (2H, d, J = 8.4 Hz, CHAr), 3.50 (1H, br s, CHCarb), 3.39 (1H, br s, CHCarb), 2.92 (6H, s, N(CH3)2) ppm; 13C NMR (100 MHz, CDCl3): δ 150.1 (CAr-N), 133.3 (CArH), 126.1 (CAr-B), 112.2 (CArH), 53.0 (CCarbH), 48.0 (CCarbH), 40.7 (N(CH3)2) ppm; 11B NMR (128 MHz, CDCl3): δ 8.1 (1B, s, B-C), −2.3 (1B, d, J = 147 Hz), −8.7 (2B, d, J = 149 Hz), −13.9 (2B, d, J = 148 Hz), −14.4 (2B, d, J = 150 Hz), −15.4 (2B, d, J = 190 Hz) ppm; IR (film): υmax 3047 (CCarb-H, CAr-H), 3028 (CCarb-H, CAr-H), 2816-2941 (CAlkyl-H), 2625 (B-H), 2590 (B-H), 2563 (B-H), 1601 (CAr-CAr), 1516 (CAr-CAr) cm–1. Crystallographic data: C10H21B10N are monoclinic, space group P21/c: a = 7.5405(2) Å, b = 13.0917(3) Å, c = 16.2594(4) Å, β = 102.3370(10)°, V = 1568.03(7) Å3, Z = 4, M = 263.38, dcryst = 1.116 g cm−3. wR2 = 0.1238 calculated on F2hkl for all 3094 independent reflections with 2θ < 52.2°, (GOF = 1.056, R = 0.0442 calculated on Fhkl for 2667 reflections with I > 2σ(I)).
9-(4-Methoxymethoxyphenyl)-ortho-carborane (4): 4-Methoxymethoxy bromide (381 μL, 543 mg, 2.50 mmol) was used; a mixture of diethyl ether and hexane (1:2, v/v) was used as eluent for column chromatography; a pale-yellow solid was obtained (189 mg, yield 67%). 1H NMR (400 MHz, CDCl3): δ 7.30 (2H, d, J = 8.4 Hz, CHAr), 6.92 (2H, d, J = 8.4 Hz, CHAr), 5.16 (2H, s, OCH2O), 3.55 (1H, br s, CHCarb), 3.47 (3H, s, OCH3), 3.44 (1H, br s, CHCarb) ppm; 13C NMR (100 MHz, CDCl3): δ 156.9 (CAr-O), 133.6 (CArH), 115.4 (CArH), 94.4 (OCH2O), 56.1 (OCH3), 53.2 (CCarbH), 48.7 (CCarbH) ppm; 11B NMR (128 MHz, CDCl3): δ 7.7 (1B, s, B-C), −2.2 (1B, d, J = 149 Hz), −8.7 (2B, d, J = 149 Hz), −13.8 (2B, d, J = 156 Hz), −14.3 (2B, d, J = 139 Hz), −15.4 (2B, d, J = 170 Hz) ppm; IR (film): υmax 3066 (CCarb-H, CAr-H), 3027 (CCarb-H, CAr-H), 2785-3002 (CAlkyl-H), 2633 (B-H), 2603 (B-H), 2589 (B-H), 2570 (B-H), 1600 (CAr-CAr), 1507 (CAr-CAr) cm−1. Crystallographic data: C10H20B10O2 are monoclinic, space group P21/c: a = 7.9988(2) Å, b = 12.7477(3) Å, c = 15.4187(4) Å, β = 91.4090(10)°, V = 1571.71(7) Å3, Z = 4, M = 280.36, dcryst = 1.185 g cm−3. wR2 = 0.1203 calculated on F2hkl for all 3830 independent reflections with 2θ < 56.2°, (GOF = 1.050, R = 0.0520 calculated on Fhkl for 2839 reflections with I > 2σ(I)).
9-(2-Methoxyphenyl)-ortho-carborane (5): 2-Methoxyphenyl bromide (310 μL, 468 mg, 2.50 mmol) was used; a mixture of diethyl ether and hexane (1:2, v/v) was used as eluent for column chromatography; a pale-yellow crystalline solid was obtained (177 mg, yield 71%). 1H NMR (400 MHz, CDCl3): δ 7.37 (1H, d, J = 7.1 Hz, CHAr), 7.21 (1H, dd, J1 = 8.3 Hz, J2 = 7.2 Hz, CHAr), 6.87 (1H, dd, J1 = 7.2 Hz, J2 = 7.1 Hz, CHAr), 6.78 (1H, d, J = 8.3 Hz, CHAr), 3.76 (3H, s, OCH3), 3.55 (1H, br s, CHCarb), 3.51 (1H, br s, CHCarb) ppm; 13C NMR (100 MHz, CDCl3): δ 161.6 (CAr-O), 136.1 (CArH), 129.1 (CArH), 120.3 (CArH), 110.5 (CArH), 55.2 (OCH3), 53.1 (CCarbH), 50.1 (CCarbH) ppm; 11B NMR (128 MHz, CDCl3): δ 6.1 (1B, s, B-C), −1.9 (1B, d, J = 146 Hz), −8.6 (2B, d, J = 151 Hz), –13.5 (2B, d, J = 182 Hz), −14.6 (2B, d, J = 148 Hz), –15.6 (2B, d, J = 188 Hz) ppm; IR (film): υmax 3065 (CCarb-H, CAr-H), 2832-3003 (CAlkyl-H), 2600 (B-H), 2571 (B-H), 1589 (CAr-CAr), 1570 (CAr-CAr), 1483 (CAr-CAr), 1460 (CAr-CAr) cm–1. Crystallographic data: C9H18B10O are monoclinic, space group P21/n: a = 7.6038(2) Å, b = 12.5827(3) Å, c = 14.9903(4) Å, β = 91.5740(10)°, V = 1433.68(6) Å3, Z = 4, M = 250.33, dcryst = 1.160 g cm−3. wR2 = 0.1077 calculated on F2hkl for all 3502 independent reflections with 2θ < 56.3°, (GOF = 1.081, R = 0.0408 calculated on Fhkl for 3132 reflections with I > 2σ(I)).
9-(4-Methoxyphenyl)-ortho-carborane (6): 4-Methoxyphenyl bromide (312 μL, 468 mg, 2.50 mmol) was used; a mixture of diethyl ether and hexane (1:2, v/v) was used as eluent for column chromatography; a pale-yellow crystalline solid was obtained (228 mg, yield 91%). 1H NMR (400 MHz, CDCl3): δ 7.30 (2H, d, J = 8.6 Hz, CHAr), 6.79 (2H, d, J = 8.6 Hz, CHAr), 3.77 (3H, s, OCH3), 3.61 (1H, br s, CHCarb), 3.51 (1H, br s, CHCarb) ppm; 11B NMR (128 MHz, CDCl3): δ 7.7 (s, 1B, B-C), −2.2 (1B, d, J = 151 Hz), −8.7 (2B, d, J = 145 Hz), −13.9 (2B, d, J = 160 Hz), −14.3 (2B, d, J = 138 Hz), −15.4 (2B, d, J = 182 Hz) ppm. The spectral data correspond to those described in the literature [34].
9-(2-Cyanophenyl)-ortho-carborane (7): 2-Cyanophenyl bromide (455 mg, 2.50 mmol) was used; mixture of chloroform and hexane (1:1, v/v) was used as eluent for column chromatography; a pale-yellow crystalline solid was obtained (184 mg, yield 75%). 1H NMR (400 MHz, CDCl3): δ 7.58 (2H, m, CHAr), 7.42 (1H, dd, J1 = 7.7 Hz, J2 = 7.5 Hz, CHAr), 7.29 (1H, dd, J1 = 7.7 Hz, J2 = 9.6 Hz, CHAr), 3.72 (1H, br s, CHCarb), 3.65 (1H, br s, CHCarb) ppm; 13C NMR (100 MHz, CDCl3): δ 135.3 (CArH), 134.3 (CArH), 131.5 (CArH), 127.7 (CArH), 118.0 (CAr-CN), 114.6 (CN), 53.6 (CCarbH), 51.4 (CCarbH) ppm; 11B NMR (128 MHz, CDCl3): δ 5.5 (1B, s, B-C), −1.9 (1B, d, J = 151 Hz), −8.5 (2B, d, J = 155 Hz), −13.7 (2B, d, J = 148 Hz), −14.0 (2B, d, J = 140 Hz), −15.2 (2B, d, J = 184 Hz) ppm; IR (film): υmax 3053 (CCarb-H, CAr-H), 3034 (CCarb-H, CAr-H), 2634 (B-H), 2613 (B-H), 2598 (B-H), 2565 (B-H), 2224 (C≡N), 1591 (CAr-CAr), 1558 (CAr-CAr), 1476 (CAr-CAr), 1456 (CAr-CAr) cm–1. Crystallographic data: C9H15B10N are monoclinic, space group C2c: a = 20.6075(5) Å, b = 12.2065(3) Å, c = 13.9109(6) Å, β = 129.0580(10)°, V = 2717.17(15) Å3, Z = 8, M = 245.32, dcryst = 1.199 g cm−3. wR2 = 0.1201 calculated on F2hkl for all 3308 independent reflections with 2θ < 56.1°, (GOF = 1.066, R = 0.0445 calculated on Fhkl for 2835 reflections with I > 2σ(I)).
9-(4-Cyanophenyl)-ortho-carborane (8): 4-Cyanophenyl bromide (455 mg, 2.50 mmol) was used; a mixture of chloroform and hexane (3:1, v/v) was used as eluent for column chromatography; a pale-yellow crystalline solid was obtained (221 mg, yield 90%). 1H NMR (400 MHz, CDCl3): δ 7.47 (4H, m, CHAr), 3.69 (1H, br s, CHCarb), 3.60 (1H, br s, CHCarb) ppm; 13C NMR (100 MHz, CDCl3): 133.0 (CArH), 131.0 (CArH), 119.6 (CAr-CN), 110.9 (CN), 53.6 (CCarbH), 50.1 (CCarbH) ppm; 11B NMR (128 MHz, CDCl3): δ 6.4 (1B, s, B-C), −2.1 (1B, d, J = 154 Hz), −8.7 (2B, d, J = 148 Hz), −14.0 (4B, d, J = 157 Hz), −15.2 (2B, d, J = 176 Hz) ppm; IR (film): υmax 3075 (CCarb-H, CAr-H), 3049 (CCarb-H, CAr-H), 2598 (B-H), 2577 (B-H), 2226 (C≡N), 1603 (CAr-CAr), 1495 (CAr-CAr) cm−1. Crystallographic data: C9H15B10N are orthorhombic, space group Pca21: a = 10.2431(6) Å, b = 13.4908(5) Å, c = 10.1576(4) Å, V = 1403.65(11) Å3, Z = 4, M = 245.32, dcryst = 1.161 g cm−3. wR2 = 0.1283 calculated on F2hkl for all 3253 independent reflections with 2θ < 56.0°, (GOF = 1.038, R = 0.0520 calculated on Fhkl for 2243 reflections with I > 2σ(I)).
9-(2-Ethoxycarbonylphenyl)-ortho-carborane (9): 2-Ethoxycarbonyl bromide (398 μL, 573 mg, 2.50 mmol)was used; a mixture of diethyl ether and hexane (1:2, v/v) was used as eluent for column chromatography; a colorless liquid was obtained (176 mg, yield 60%). 1H NMR (400 MHz, CDCl3): δ 7.50 (1H, d, J = 7.5 Hz, CHAr), 7.28 (1H, m, CHAr), 7.23 (2H, m, CHAr), 4.29 (2H, q, J = 7.2 Hz, OCH2CH3), 3.54 (1H, br s, CHCarb), 3.49 (1H, br s, CHCarb), 1.36 (3H, t, J = 7.2 Hz, OCH2CH3) ppm; 13C NMR (100 MHz, CDCl3): δ 161.5 (CO), 137.9 (CAr-CO), 136.1 (CArH), 128.8 (CArH), 127.2 (CArH), 126.8 (CArH), 61.1 (OCH2CH3), 53.4 (CCarbH), 50.8 (CCarbH), 14.2 (OCH2CH3) ppm; 11B NMR (128 MHz, CDCl3): δ 6.1 (1B, s, B-C), −2.0 (1B, d, J = 148 Hz), −8.7 (2B, d, J = 150 Hz), −13.8 (2B, d), −14.3 (2B, d), −15.3 (2B, d, J = 178 Hz) ppm; IR (film): υmax 3065 (CCarb-H, CAr-H), 2855-2982 (CAlkyl-H), 2596 (B-H), 1713 (C=O), 1558 (CAr-CAr), 1474 (CAr-CAr) cm−1.
9-(3-Ethoxycarbonylphenyl)-ortho-carborane (10): 3-Ethoxycarbonyl bromide (401 μL, 573 mg, 2.50 mmol) was used; diethyl ether was used as eluent for column chromatography; a pale-yellow crystalline solid was obtained (226 mg, yield 70%). 1H NMR (400 MHz, CDCl3): δ 8.04 (1H, s, CHAr), 7.89 (1H, d, J = 7.8 Hz, CHAr), 7.56 (1H, d, J = 7.4 Hz, CHAr), 7.29 (1H, dd, J1 = 7.8 Hz, J2 = 7.4 Hz, CHAr), 4.36 (2H, q, J = 7.1 Hz, OCH2CH3), 3.65 (1H, br s, CHCarb), 3.55 (1H, br s, CHCarb), 1.39 (3H, t, J = 7.1 Hz, OCH2CH3) ppm; 13C NMR (100 MHz, CDCl3): δ 167.3 (CO), 137.1 (CArH), 133.4 (CArH), 129.5 (CAr-CO), 128.5 (CArH), 127.5 (CArH), 60.9 (OCH2CH3), 53.5 (CCarbH), 49.4 (CCarbH), 14.5 (OCH2CH3) ppm; 11B NMR (128 MHz, CDCl3): δ 7.1 (1B, s, B-C), –2.1 (1B, d, J = 143 Hz), −8.6 (2B, d, J = 151 Hz), –13.8 (2B, d, J = 148 Hz), −14.1 (2B, d, J = 154 Hz), −15.3 (2B, d, J = 186 Hz) ppm; IR (film): υmax 3070 (CCarb-H, CAr-H), 2854-2982 (CAlkyl-H), 2598 (B-H), 2574 (B-H), 1707 (C=O), 1597 (CAr-CAr), 1577 (CAr-CAr), 1477 (CAr-CAr), 1465 (CAr-CAr) cm−1. Crystallographic data: C11H20B10O2 are orthorhombic, space group Pna21: a = 18.4106(4) Å, b = 12.2532(3) Å, c = 7.2177(2) Å, V = 1628.23(7) Å3, Z = 4, M = 292.37, dcryst = 1.193 g cm−3. wR2 = 0.0951 calculated on F2hkl for all 3929 independent reflections with 2θ < 56.1°, (GOF = 1.080, R = 0.0452 calculated on Fhkl for 3543 reflections with I > 2σ(I)).
9-(4-Ethoxycarbonylphenyl)-ortho-carborane (11): 4-Ethoxycarbonyl bromide (408 μL, 573 mg, 2.50 mmol) was used; diethyl ether was used as eluent for column chromatography; a pale-yellow crystalline solid was obtained (252 mg, yield 86%). 1H NMR (400 MHz, CDCl3): δ 7.88 (2H, d, J = 8.2 Hz, CHAr), 7.44 (2H, d, J = 8.2 Hz, CHAr), 4.35 (2H, q, J = 7.1 Hz, OCH2CH3), 3.66 (1H, br s, CHCarb), 3.56 (1H, br s, CHCarb), 1.37 (3H, t, J = 7.1 Hz, OCH2CH3) ppm; 13C NMR (100 MHz, CDCl3): δ 167.1 (CO), 144.9 (CAr-B), 132.5 (CArH), 129.4 (CAr-CO), 128.5 (CArH), 60.8 (OCH2CH3), 53.5 (CCarbH), 49.7 (CCarbH), 14.5 (OCH2CH3) ppm; 11B NMR (128 MHz, CDCl3): δ 7.0 (1B, s, B-C), −2.1 (1B, d, J = 152 Hz), −8.7 (2B, d, J = 149 Hz), −14.0 (4B, d, J = 150 Hz), −15.2 (2B, d, J = 176 Hz) ppm; IR (film): υmax 3057 (CCarb-H, CAr-H), 2851-2994 (CAlkyl-H), 2602 (B-H), 2579 (B-H), 1694 (C=O), 1605 (CAr-CAr), 1558 (CAr-CAr), 1474 (CAr-CAr), 1445 (CAr-CAr) cm–1. Crystallographic data: C11H20B10O2 are monoclinic, space group P21/n: a = 10.701(3) Å, b = 7.0121(18) Å, c = 21.715(5) Å, β = 91.609(7)°, V = 1628.7(7) Å3, Z = 4, M = 292.37, dcryst = 1.192 g cm−3. wR2 = 0.2004 calculated on F2hkl for all 3089 independent reflections with 2θ < 52.0°, (GOF = 0.998, R = 0.0705 calculated on Fhkl for 1636 reflections with I > 2σ(I)).

3.3. General Synthetic Procedure and Characterization of Disubstituted B-Aryl Derivatives of Ortho-Carborane

Allyl chloride (82 μL, 77 mg, 1.00 mmol) and trifluoroacetic acid (25 μL, catalytic amount) were added to a blue mixture of zinc powder (490 mg, 7.50 mmol) and anhydrous cobalt dibromide (55 mg, 0.25 mmol) in 2.5 mL of freshly distilled acetonitrile. The resulting dark orange mixture was stirred at room temperature for 15 min. Then corresponding aryl bromide (2.50 mmol) was added, and reaction was stirred at room temperature for another 1 h. Then 9,12-diiodo-ortho-carborane (198 mg, 0.50 mmol) with bis(triphenylphosphine)palladium dichloride (14 mg, 0.02 mmol, catalytic amount) were added. The reaction mixture was stirred at room temperature overnight and filtered, the solid was washed with hot acetonitrile (until no trace of carborane appeared on TLC). The organic phases were combined and concentrated under reduced pressure. The crude product was washed by 5% HCl and water to remove inorganic solids and by Et2O and acetone to remove starting materials to give the corresponding diaryl derivatives of ortho-carborane.
9,12-Diphenyl-ortho-carborane (13): Phenyl bromide (262 μL, 393 mg, 2.50 mmol) was used. White powder was obtained (76 mg, yield 51%). 1H NMR (400 MHz, CD3COCD3): δ 7.19 (4H, m, CHAr), 7.07 (6H, m, CHAr), 4.69 (2H, br s, CHCarb) ppm; 11B NMR (128 MHz, CDCl3): δ 7.3 (2B, s, B-C), −9.7 (2B, d, J = 144 Hz), −13.9 (4B, d, J = 161 Hz), −16.1 (,2B d, J = 174 Hz) ppm. The spectral data correspond to those described in the literature [32,35].
9,12-Di(4-Methylphenyl)-ortho-carborane (14): 4-Methylphenyl bromide (315 μL, 435 mg, 2.50 mmol) was used; a white powder was obtained (98 mg, yield 60%). 1H NMR (400 MHz, CD3COCD3): δ 7.09 (4H, d, J = 6.7 Hz, CHAr), 6.89 (4H, d, J = 6.7 Hz, CHAr), 4.63 (2H, br s, CHCarb), 2.18 (6H, s, CH3) ppm; 13C NMR (100 MHz, CD3COCD3): δ 148.8 (CAr-B), 137.0 (CAr-CH3), 132.5 (CArH), 128.4 (CArH), 53.3 (CCarbH), 48.9 (CCarbH), 21.3 (CH3) ppm; 11B NMR (128 MHz, CD3COCD3): δ 7.4 (2B, s, B-C), −9.4 (2B, d, J = 152 Hz), −14.0 (4B, d, J = 156 Hz), −16.3 (2B, d, J = 193 Hz) ppm. The spectral data correspond to those described in the literature [32,35]. Crystallographic data: C16H24B10 are monoclinic, space group C2/c: a = 21.421(6) Å, b = 7.7182(18) Å, c = 14.405(4) Å, β = 127.139(14)°, V = 1898.6(9) Å3, Z = 4, M = 324.45, dcryst = 1.135 g cm−3. wR2 = 0.1983 calculated on F2hkl for all 1886 independent reflections with 2θ < 52.4°, (GOF = 1.034, R = 0.0801 calculated on Fhkl for 1034 reflections with I > 2σ(I)).

4. Conclusions

In conclusion, the simple and efficient method was developed for the one-pot synthesis of B-substituted aryl derivatives of ortho-carborane with functional groups sensitive to organolithium and organomagnesium reagents using 9-iodo-ortho-carborane and generated in situ organozinc compounds. The method proposed provides near quantitative conversion of the starting 9-iodo-ortho-carborane to the corresponding aryl derivativs. A series of 9-aryl-ortho-carboranes, including those containing nitrile and ester groups, 9-RC6H4-1,2-C2B10H11 (R = p-Me, p-NMe2, p-OCH2OMe, o-OMe, p-OMe, o-CN, p-CN, o-COOEt, m-COOEt, p-COOEt) was synthesized. The same approach was used for synthesis of the diaryl derivatives of ortho-carborane 9,12-(RC6H4)2-1,2-C2B10H10 (R = H, p-Me). The study of the applicability of this approach for the synthesis of aryl derivatives using other iodo derivatives of carboranes and metallacarboranes is in progress.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/11/1348/s1: 1H, 13C and 11B NMR spectra of compounds 211, 13, and 14.

Author Contributions

Experiment design, synthesis, and NMR spectroscopy studies, S.A.A.; synthesis, A.V.S.; single crystal X-ray diffraction experiments, K.Y.S.; supervision and manuscript concept, I.B.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Russian Science Foundation (Grant No. 19-73-00353).

Acknowledgments

The NMR spectral data were obtained by using equipment from the Center for Molecular Structure Studies at A.N. Nesmeyanov Institute of Organoelement Compounds, operating with support from the Ministry of Science and Higher Education of the Russian Federation.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. Atom numeration in ortho-carborane 1,2-C2B10H12.
Figure 1. Atom numeration in ortho-carborane 1,2-C2B10H12.
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Scheme 1. Synthesis of arylzinc bromides.
Scheme 1. Synthesis of arylzinc bromides.
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Scheme 2. Synthesis of 9-aryl-ortho-carboranes.
Scheme 2. Synthesis of 9-aryl-ortho-carboranes.
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Scheme 3. Proposed mechanism of Pd-catalyzed arylation of 9-iodo-ortho-carboranes.
Scheme 3. Proposed mechanism of Pd-catalyzed arylation of 9-iodo-ortho-carboranes.
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Figure 2. General views of 9-(p-Me2NC6H4)-1,2-C2B10H11 (3, top left), 9-(p-MeOCH2OC6H4)-1,2-C2B10H11 (4, top right), and 9-(o-MeOC6H4)-1,2-C2B10H11 (5, bottom) showing atomic numbering. Thermal ellipsoids are drawn at 50% probability level.
Figure 2. General views of 9-(p-Me2NC6H4)-1,2-C2B10H11 (3, top left), 9-(p-MeOCH2OC6H4)-1,2-C2B10H11 (4, top right), and 9-(o-MeOC6H4)-1,2-C2B10H11 (5, bottom) showing atomic numbering. Thermal ellipsoids are drawn at 50% probability level.
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Figure 3. General views of 9-(o-NCC6H4)-1,2-C2B10H11 (7, top left), 9-(p-NCC6H4)-1,2-C2B10H11 (8, top right), 9-(m-EtOOCC6H4)-1,2-C2B10H11 (10, bottom left), and 9-(p-EtOOCC6H4)-1,2-C2B10H11 (11, bottom right) showing atomic numbering. Thermal ellipsoids are drawn at 50% probability level.
Figure 3. General views of 9-(o-NCC6H4)-1,2-C2B10H11 (7, top left), 9-(p-NCC6H4)-1,2-C2B10H11 (8, top right), 9-(m-EtOOCC6H4)-1,2-C2B10H11 (10, bottom left), and 9-(p-EtOOCC6H4)-1,2-C2B10H11 (11, bottom right) showing atomic numbering. Thermal ellipsoids are drawn at 50% probability level.
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Scheme 4. Synthesis of 9,12-diaryl-ortho-carboranes.
Scheme 4. Synthesis of 9,12-diaryl-ortho-carboranes.
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Figure 4. General view of 9,12-(p-MeC6H4)2-1,2-C2B10H11 (14) showing atomic numbering. Thermal ellipsoids are drawn at 50% probability level. The molecule occupies special position. Numbering of symmetrically dependent part are marked with letter “A”.
Figure 4. General view of 9,12-(p-MeC6H4)2-1,2-C2B10H11 (14) showing atomic numbering. Thermal ellipsoids are drawn at 50% probability level. The molecule occupies special position. Numbering of symmetrically dependent part are marked with letter “A”.
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Anufriev, S.A.; Shmal’ko, A.V.; Suponitsky, K.Y.; Sivaev, I.B. One-Pot Synthesis of B-Aryl Carboranes with Sensitive Functional Groups Using Sequential Cobalt- and Palladium-Catalyzed Reactions. Catalysts 2020, 10, 1348. https://doi.org/10.3390/catal10111348

AMA Style

Anufriev SA, Shmal’ko AV, Suponitsky KY, Sivaev IB. One-Pot Synthesis of B-Aryl Carboranes with Sensitive Functional Groups Using Sequential Cobalt- and Palladium-Catalyzed Reactions. Catalysts. 2020; 10(11):1348. https://doi.org/10.3390/catal10111348

Chicago/Turabian Style

Anufriev, Sergey A., Akim V. Shmal’ko, Kyrill Yu. Suponitsky, and Igor B. Sivaev. 2020. "One-Pot Synthesis of B-Aryl Carboranes with Sensitive Functional Groups Using Sequential Cobalt- and Palladium-Catalyzed Reactions" Catalysts 10, no. 11: 1348. https://doi.org/10.3390/catal10111348

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