English

• 1.   Introduction

  Carboranes are finding wide applications in medicine as boron neutron capture therapy (TNCT) agents, ^[1] coordination/organometallic chemistry as versatile ligands, ^[2] in materials as functional units, ^[3] and in supramolecular design as building blocks.^[4] Especially, cage carbon-arylated o-carboranes are used as a class of AIE (aggregation induced emission) active and stimuli-responsive luminescent materials, where the carborane moiety plays a unique role.^[5] In this regard, 1, 2-diaryl- o-carboranes were prepared by transition metal-catalyzed Kumada-type cross-coupling of 1, 2-di-magnesium chloride-o-carboranes with aryl iodides, ^[6] or via the condensation reaction of decaborane with diarylacetylenes.^[7] For cage boron arylation, direct palladium-catalyzed B-H substitution only afforded a mixture of B(8)- or B(9)-mono-arylation compounds.^[8] Pre-iodinaion is necessary for the most electron-rich B(9, 12)-position to achieve diarylation by a palladium-catalyzed coupling with aryl Grignard reagents.^[9] More recently, 1-CO₂H, 1-CHO and 1-CH₂NH₂ groups guided transition metal catalyzed cage B(4, 5)-diarylation^[10] of o-carboranes has been achieved by direct B—H bond activation.^[11] On the other hand, cage B(3, 6)-diarylaed-o-carboranes are generally achieved via multistep reaction of deboration-capping- deboration-capping, which is suffered from the narrow substrate scope and low yields (Scheme 1).^[12]

  Scheme 1

  

  Scheme 1.  Synthesis of diaryl-o-carboranes

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  Suzuki-Miyaura coupling is one of the most widely used selective C—C bond forming methods, in which organoboron reagents react with a series of electrophilic coupling partners, such as aryl or vinyl halide, triflate, diazonium/iodonium salt and carboxylic acid derivatives.^[13] Very recently, we have developed an iridium-catalyzed regioselective B(3, 6)-diborylation of o-carborane via direct B—H activation.^[14] These novel cage B-Bpin compounds can act as a new kind of boron nucleophiles. Herein we report our studies on the cross-coupling reactions of 3, 6-(Bpin)₂-1, 2- C₂B₁₀H₁₀ with aryl halides for an efficient synthesis of B(3, 6)-diarylated-o-carboranes.

  2.   Results and discussion

  The initial optimization of reaction conditions for the reaction of 3, 6-(Bpin)₂-1, 2-C₂B₁₀H₁₀ (1) with phenylbromide (2a) was summarized in Table 1. In the presence of 20 mol% Pd(PPh₃)₄ and 3 equiv. of Cs₂CO₃, a quantitative formation of coupling product 4a was observed by GC when cyclohexane was used as the solvent (Table 1, Entry 4). Further screening of other bases in cyclohexane solution did not afford better results (Table 1, Entries 5~8). Screening the Pd catalysts indicated that Pd₂(dba)₃/PPh₃ offered a lower conversion, while Pd₂(dba)₃ showed no reactivity (Table 1, Entries 9 and 10). Among the Pd(Ⅱ) catalysts, Pd(OAc)₂/PPh₃ worked as well as Pd(PPh₃)₄ to achieve the quantitative conversion (Table 1, Entries 11~15). Lowering the catalyst loading or reaction temperature greatly affected the yield of 4a (Table 1, Entries 16 and 17), although the catalyst loading is relative higher than classic Suzuki-Miyaura cross-coupling with a C—C bond forming process. P(o-tol)₃ offered poor result. It was noted that the combination of Pd(OAc)₂ or PdCl₂(cod) (cod＝1, 5-cy-clooctadiene) with PCy₃ resulted in the formation of 4a in the yields of 84% and 83% (Table 1, Entries 20 and 21).

  Table 1

  

  Table 1.  Optimization of reaction conditions with phenylbromide^a

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  Entry	Cat.	L	Base	Solvent	Yieldb/%
  					1	3a	4a
  1	Pd(PPh3)4	—	Cs2CO3	THF	10	—	90
  2	Pd(PPh3)4	—	Cs2CO3	Dioxane	9	16	75
  3	Pd(PPh3)4	—	Cs2CO3	DCE	93	7	—
  4	Pd(PPh3)4	—	Cs2CO3	Cyclohexane	—	—	100
  5	Pd(PPh3)4	—	K2CO3	Cyclohexane	92	8	—
  6	Pd(PPh3)4	—	AgOAc	Cyclohexane	—	—	—
  7	Pd(PPh3)4	—	K2HPO3	Cyclohexane	100	—	—
  8	Pd(PPh3)4	—	KOAc	Cyclohexane	100	—	—
  9	Pd2(dba)3	—	Cs2CO3	Cyclohexane	100	—	—
  10	Pd2(dba)3	PPh3	Cs2CO3	Cyclohexane	7	7	86
  11	Pd(acac)2	PPh3	Cs2CO3	Cyclohexane	100	—	—
  12	Pd(MeCN)4(BF4)2	PPh3	Cs2CO3	Cyclohexane	72	20	8
  13	[PdCl(C3H5)]2	PPh3	Cs2CO3	Cyclohexane	28	21	51
  14	PdCl2(cod)	PPh3	Cs2CO3	Cyclohexane	2	1	97
  15	Pd(OAc)2	PPh3	Cs2CO3	Cyclohexane	—	—	100
  16c	Pd(OAc)2	PPh3	Cs2CO3	Cyclohexane	21	17	62
  17d	Pd(OAc)2	PPh3	Cs2CO3	Cyclohexane	20	18	62
  18	Pd(OAc)2	P(C6H4OMe-o)3	Cs2CO3	Cyclohexane	39	22	39
  19	Pd(OAc)2	PCy3	Cs2CO3	Cyclohexane	8	8	84
  20	PdCl2(cod)	PCy3	Cs2CO3	Cyclohexane	7	10	83
  a Reaction condition: 1 (0.1 mmol), PhBr (0.25 mmol), 20 mol% cat., 40 mol% L, base (0.3 mmol), 110 ℃, 8 h; DCE＝dichloroethane; dba＝dibenzylideneacetone, cod＝1, 5-cyclooctadiene; Cy＝cyclohexyl. b GC yield. c10 mol% Pd(OAc)2, 20 mol% PPh3. d 80 ℃.

  Under the optimal reaction conditions (Table 1, Entry 15), coupling reaction of 1 with aryl bromides was examined (Table 2). The reactions of p- and m-tolyl bromide proceeded as well as phenyl bromide to give, unfortunately, an inseperatable mixture of 4 and 5, resulting from the aryl-phenyl exchange between palladium center and the large amount of phosphine ligand (Table 2, Entries 2 and 3).^[15] The exchange product 3-(p-MeOC₆H₄)-6-Ph-1, 2-C₂B₁₀H₁₀ was isolated in 26% yield using p-MeOC₆H₄Br as the reagent under aforementioned reaction conditions (Table 2, Entry 4). Its structure was confirmed by single-crystal X-ray analyses (Figure 1). On the other hand, no aryl exchange product was formed using electron-withdrawing group substituted p-ClC₆H₄Br and p-CF₃C₆H₄Br as coupling partners (Table 2, Entries 5 and 6), which agrees well with the trends observed by Novak on the substituent effects of the aryl-aryl interchange reaction.^[15h] We next studied the reaction using PCy₃ ligand to avoid this aryl exchange. However, another unexpected inseparable mixture of diarylated by-products 6 was obtained along with the desired compound 4 (Table 2, Entries 7~10). This structurally undetermined by-product may be ascribed to palladium- catalyzed direct B—H arylation of electron-rich boron vertexes. ^[8]

  Table 2

  

  Table 2.  Initial study of the reaction of 1 with arylbromide under different conditions^a

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  Entry	Ar	L	Yieldb/%
  1	C6H5	PPh3	84 (4)
  2	4-MeC6H4	PPh3	85 (4:5＝64:21)
  3	3-MeC6H4	PPh3	87 (4:5＝73:14)
  4c	4-MeOC6H4	PPh3	48 (4), 26 (5)
  5	4-ClC6H4	PPh3	69 (4)
  6	4-CF3C6H4	PPh3	60 (4)
  7	4-MeC6H4	PCy3	61 (4:6＝49:12)
  8	3-MeC6H4	PCy3	51 (4:6＝48:3)
  9c	4-MeOC6H4	PCy3	25 (4:6＝22:3)
  10	4-FC6H4	PCy3	65 (4:6＝62:3)
  a Reaction condition: 1 (0.5 mmol), ArBr (1.25 mmol), Cs2CO3 (1.5 mmol), 20 mol% Pd(OAc)2, 40 mol% L, 110 ℃, 24 h. b Isolated yield, ratio determined by GC. c130 ℃.

  Figure 1

  

  Figure 1.  Molecular structure of 3-(p-MeOC₆H₄)-6-Ph-1, 2- C₂B₁₀H₁₀ (5i)

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  We next used p-tolyl halide as a model electrophilic coupling partner to further optimize the reaction conditions (Table 3). Alkyl phosphine ligand PⁿBu₃ offered a very low conversion of 1 (Table 3, Entry 1). If PCy₃ was employed as the ligand, PdCl₂(cod) showed both high reactivity and selectivity for 4b (Table 3, Entries 2~4).

  Table 3

  

  Table 3.  Optimization of reaction conditions with p-tolyl halides^a

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  Entry	Cat.	L	Yieldb/%
  			1	3b	4b	6b
  1	Pd(OAc)2	PnBu3	75	20	5	—
  2	Pd(OAc)2	PCy3	8	9	65	16
  3	Pd2(dba)3	PCy3	16	19	62	3
  4	PdCl2(cod)	PCy3	7	12	78	3
  5	PdCl2(PCy3)2	—	51	31	18	0
  a Reaction condition: 1 (0.1 mmol), p-tolyl halide (0.25 mmol), 20 mol% cat., 40 mol% L, Cs2CO3 (0.3 mmol), 110 ℃, 24 h. b GC yield.

  Under the further optimal reaction conditions (Table 3, Entry 4), the substrate scope was examined and the results were compiled in Table 4. Both electron-withdrawing and electron-donating groups on phenyl ring afforded the corresponding products in moderate isolated yields. 2-Na- phthyl bromide also worked well, giving 4x in 57% yield. Steric factors played an important role in these reactions. Only trace product was observed in the reaction with sterically hindered o-methyl/o-fluorophenyl or 1-naphthyl bromide (2d, 2p and 2y), while o-methoxyl and o-triflu- oromethyl groups on phenyl ring (2k and 2u) completely suppressed the reaction. For heteroatom containing substituents, the p- and m-OMe containing products 4i and 4j were isolated in relatively lower yields. And only a trace amount of product was observed from GC-MS for p-bromo-N, N-dimethylaniline 2l, probably owing to the interactions of the heteroatom with the metal center.

  Table 4

  

  Table 4.  Synthesis of 3, 6-diarylated-o-carboranes^{a, b}

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  Compounds 4 and 5i were fully characterized by ¹H NMR, ¹³C NMR and ¹¹B NMR spectra, HRMS as well as elemental analyses. For 4, the NMR spectra exhibited symmetric structures. Their ¹¹B{¹H} NMR spectra showed the same pattern of 2:2:2:4. The substituted cage B was found at about δ －4 as a singlet in the ¹H coupled ¹¹B NMR spectra.

  A proposed mechanism for the above 3, 6-diarylation of o-carborane is shown in Scheme 2. The catalysis is likely to be initiated by a Pd⁰ species that is generated from the reduction of Pd^Ⅱ with phosphine ligand.^[16] Oxidative addition of Ar—Br on Pd(0), followed by the subsequent reaction with base, produces Pd^Ⅱ intermediate B. Transmetalation of B with B-carboranyl boronate 1 forms the carboranylpalladium intermediate C. Reductive elimination yields the arylation product 3 and releases the Pd⁰ species to complete the catalytic cycle. Compound 3 undergoes another catalytic cycle to afford cage B(3, 6)-diarylated product 4. On the other hand, in the presence of PPh₃, aryl-phenyl exchange between the palladium center in A and phosphine ligands produces intermediate D, which undergoes a similar reaction cycle to give the unsymmetrical 3, 6-diarylated- o-carborane 5.

  Scheme 2

  

  Scheme 2.  Proposed mechanism for the formation of 4 and 5

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  3.   Conclusions

  In summary, an efficient Pd-catalyzed coupling of borylated o-carborane with different aryl bromide has been achieved, giving a series of B(3, 6)-diaryl-o-carboranes.

  This represents the first general method for the synthesis of B(3, 6)-diaryl-o-carboranes. This work may also shed some light on developing new methodologies for the B(3, 6)-func- tionalization using cage B-Bpin compounds as a new kind of boron nucleophile.

  4.   Experimental section

  4.1   General procedures

  All reactions were carried out in oven-dried glassware under an atmosphere of dry N₂ with the rigid exclusion of air and moisture using standard Schlenk techniques or in a glovebox. Organic solvents were freshly distilled from sodium benzophenone ketyl immediately prior to use. Compound 1a was prepared according to literature procedures.^[14] All other chemicals were purchased from either Aldrich or J & K Chemical Co. and used as received unless otherwise specified. The melting points were measured on an Electrothermal Melt Temp instrument of Shanghai INESA Physico-Optical Instrument Co., Ltd and were uncorrected. ¹H, ¹³C{¹H}, ¹¹B{¹H}, ¹¹B and ¹⁹F NMR spectra were recorded on a Bruker 400 spectrometer at 400, 101, 128 and 376 MHz, respectively. All signals were reported with references to the residual solvent resonances of the deuterated solvents for proton and carbon chemical shifts, to external BF₃•OEt₂ (0.00) for boron chemical shifts and to external CFCl₃ (0.00) for fluorine chemical shifts. Mass spectra were obtained on a Waters Micromass GCT Premier CAB088 spectrometer. Elemental analyses were performed with an elementary VARIO EL Ⅲ elemental analyzer, Shanghai Institute of Organic Chemistry, CAS.

  4.2   Preparation of 3, 6-diaryl-o-carborane (4)

  An oven-dried Schlenk flask equipped with a stir bar was charged with 1a (198 mg, 0.5 mmol), aryl bromide (1.5 mmol), PdCl₂(cod) (29 mg, 0.1 mmol), PCy₃ (56 mg, 0.2 mmol) and Cs₂CO₃ (490 mg, 1.5 mmol), followed by dry cyclohexane (5 mL). The flask was closed under an atmosphere of nitrogen and stirred at 110 ℃ (bath temperature) for 24 h. After cooled to room temperature, water (10 mL) and 30% H₂O₂ (5 mL) aqueous solution were successively added and the mixture was stirred at room temperature overnight to oxidize the phosphine ligand. After extraction with diethyl ether (10 mL×3), the ether solutions were combined and concentrated to dryness in vacuo. The residue was subjected to flash column chromatography on silica gel (230~400 mesh) using n-hexane and ethyl acetate (V/V＝100/1) as eluent to give 4.

  3, 6-(C₆H₅)₂-1, 2-C₂B₁₀H₁₀ (4a): White solid, yield 58%. m.p. 175~176 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.62 (m, 4H, ArH), 7.40 (m, 6H, ArH), 3.85 (s, 2H, cage CH). These data are identical with those reported in the literature.^[14]

  3, 6-(p-CH₃C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4b): White solid, yield 67%. m.p. 192~194 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.51 (d, J＝8.0 Hz, 4H, ArH), 7.19 (d, J＝8.0 Hz, 4H, ArH), 3.81 (s, 2H, cage CH), 2.37 (s, 6H, CH₃); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 140.0, 133.3, 129.2 (aromatic C), 59.3 (cage CH), 21.5 (CH₃), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.3 (d, J_BH＝160 Hz, 2B, B_cageH), －3.6 (s, 2B, ArB_cage), －11.4 (d, J_BH＝164 Hz, 2B), －12.9 (d, J_BH＝174 Hz, 4B, B_cageH). Anal. calcd for C₁₆H₂₄B₁₀: C 59.23, H 7.46; found C 59.14, H 7.38.

  3, 6-(m-(CH₃)₂C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4c): White solid, yield 56%. m.p. 207~209 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.42 (s, 2H, ArH), 7.41 (d, J＝6.8 Hz, 2H, ArH), 7.25 (m, 4H, ArH), 3.83 (s, 2H, cage CH), 2.37 (s, 6H, CH₃); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 138.0, 134.0, 130.7, 130.3, 128.4 (aromatic C), 59.2 (cage CH), 21.6 (CH₃), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.3 (d, J_BH＝149 Hz, 2B, B_cageH), －3.7 (s, 2B, ArB_cage), －11.3 (d, J_BH＝182 Hz, 2B), －12.8 (d, J_BH＝177 Hz, 4B, B_cageH). Anal. calcd for C₁₆H₂₄B₁₀: C 59.23, H 7.46; found C 58.94, H 7.46.

  3, 6-(3', 5'-(CH₃)₂C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4e): White solid, yield 65%. m.p. 228~229 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.23 (s, 4H, ArH), 7.06 (s, 2H, ArH), 3.82 (s, 2H, cage CH), 2.34 (s, 12H, CH₃); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 137.9, 131.6, 131.1 (aromatic C), 59.3 (cage CH), 21.4 (CH₃), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.4 (d, J_BH＝154 Hz, 2B, B_cageH), －3.6 (s, 2B, ArB_cage), －11.4 (d, J_BH＝178 Hz, 2B), －12.9 (d, J_BH＝178 Hz, 4B, B_cageH). Anal. calcd for C₁₈H₂₈B₁₀: C 61.33, H 8.01; found C 61.14, H 7.76.

  3, 6-(p-C₂H₅C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4f): White solid, yield 58%. m.p. 95~96 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.57 (d, J＝7.2 Hz, 4H, ArH), 7.25 (d, J＝7.2 Hz, 4H, ArH), 3.84 (s, 2H, cage CH), 2.70 (q, J＝7.2 Hz, 4H, CH₂), 1.29 (t, J＝7.2 Hz, 6H, CH₃); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 146.3, 133.4, 128.0 (aromatic C), 59.3 (cage CH), 28.9 (CH₂), 15.6 (CH₃), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.9 (d, J_BH＝152 Hz, 2B, B_cageH), －4.2 (s, 2B, ArB_cage), －11.9 (d, J_BH＝175 Hz, 2B), －13.4 (d, J_BH＝177 Hz, 4B, B_cageH). HRMS (ESI) calcd for C₁₈H₂₈¹⁰B₂¹¹B₈: 352.3194, found 352.3189.

  3, 6-(p-(CH₃)₂CHC₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4g): White solid, yield 66%. m.p. 135~137 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.56 (d, J＝7.6 Hz, 4H, ArH), 7.25 (d, J＝7.6 Hz, 4H, ArH), 3.83 (s, 2H, cage CH), 2.93 (m, 2H, CH(CH₃)₂), 1.27 (d, J＝6.8 Hz, 12H, CH(CH₃)₂); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 150.9, 133.4, 126.6 (aromatic C), 59.3 (cage CH), 34.2 (CH(CH₃)₂), 24.0 (CH(CH₃)₂), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.7 (d, J_BH＝148 Hz, 2B, B_cageH), －3.9 (s, 2B, ArB_cage), －11.8 (d, J_BH＝178 Hz, 2B), －13.3 (d, J_BH＝177 Hz, 4B, B_cageH). Anal. calcd for C₂₀H₃₂B₁₀: C 63.12, H 8.48; found C 63.07, H 8.52.

  3, 6-(p-(CH₃)₃C-C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4h): White solid, yield 68%. m.p. 245~247 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.55 (d, J＝8.0 Hz, 4H, ArH), 7.39 (d, J＝8.0 Hz, 4H, ArH), 3.83 (s, 2H, cage CH), 1.32 (s, 18H, CH₃); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 153.1, 133.2, 125.4 (aromatic C), 59.3 (cage CH), 34.8 (C(CH₃)₃), 31.3 (C(CH₃)₃), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.2 (d, J_BH＝152 Hz, 2B, B_cageH), －3.8 (s, 2B, ArB_cage), －11.2 (d, J_BH＝188 Hz, 2B), －12.8 (d, J_BH＝183 Hz, 4B, B_cageH). Anal. calcd for C₂₂H₃₆B₁₀: C 64.67, H 8.88; found C 64.68, H 8.79.

  3, 6-(p-OCH₃C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4i): White solid, yield 29%. m.p. 170~172 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.54 (d, J＝8.0 Hz, 4H, ArH), 6.90 (d, J＝8.0 Hz, 4H, ArH), 3.82 (s, 6H, OCH₃), 3.79 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 161.1, 134.7, 114.0 (aromatic C), 59.4 (cage CH), 55.3 (OCH₃), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.4 (d, J_BH＝142 Hz, 2B, B_cageH), －3.0 (s, 2B, ArB_cage), －11.5 (d, J_BH＝169 Hz, 2B), －13.0 (d, J_BH＝172 Hz, 4B, B_cageH). Anal. calcd for C₁₆H₂₄B₁₀O₂: C 53.91, H 6.79; found C 54.02, H 6.86.

  3, 6-(m-OCH₃C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4j): White solid, yield 43%. m.p. 168~170 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.25 (m, 2H, ArH), 7.13 (m, 4H, ArH), 6.92 (m, 2H, ArH), 3.80 (s, 8H, OCH₃ & cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 159.4, 129.7, 125.3, 119.3, 114.9 (aromatic C), 59.2 (cage CH), 55.4 (OCH₃), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.4 (d, J_BH＝150 Hz, 2B, B_cageH), －4.1 (s, 2B, ArB_cage), －11.4 (d, J_BH＝165 Hz, 2B), －13.0 (d, J_BH＝170 Hz, 4B, B_cageH). Anal. calcd for C₁₆H₂₄B₁₀O₂: C 53.91, H 6.79; found C 53.49, H 6.67.

  3, 6-(p-ClC₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4m): White solid, yield 61%. m.p. 187~188 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.54 (d, J＝8.4 Hz, , 4H, ArH), 7.35 (d, J＝8.4 Hz, 4H, ArH), 3.80 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 136.6, 134.6, 128.7 (aromatic C), 59.0 (cage CH), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －1.9 (d, J_BH＝148 Hz, 2B, B_cageH), －4.0 (s, 2B, ArB_cage), －11.0 (d, J_BH＝204 Hz, 2B), －12.8 (d, J_BH＝182 Hz, 4B, B_cageH). Anal. calcd for C₁₄H₁₈B₁₀Cl₂: C 46.03, H 4.97; found C 46.21, H 5.03.

  3, 6-(p-FC₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4n): White solid, yield 69%. m.p. 164~166 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.60 (dd, J＝8.0, 6.0 Hz, 4H, ArH), 7.07 (t, J＝8.0 Hz, 4H, ArH), 3.81 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 164.3 (d, ¹J_CF＝249 Hz), 135.2 (d, ³J_CF＝8 Hz), 115.6 (d, ²J_CF＝20 Hz, aromatic C), 59.3 (cage CH), the B_cage-C were not observed; ¹⁹F{¹H} NMR (376 MHz, CDCl₃) δ: －110.7; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.1 (d, J_BH＝148 Hz, 2B, B_cageH), －3.8 (s, 2B, ArB_cage), －11.1 (d, J_BH＝189 Hz, 2B), －12.8 (d, J_BH＝178 Hz, 4B, B_cageH). Anal. calcd for C₁₄H₁₈B₁₀F₂: C 50.59, H 5.46; found C 50.77, H 5.68.

  3, 6-(m-FC₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4o): White solid, yield 41%. m.p. 120~121 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.34 (m, 6H, ArH), 7.12 (m, 2H, ArH), 3.83 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 162.7 (d, ¹J_CF＝247 Hz) (CF), 130.3 (d, ³J_CF＝7 Hz), 128.8 (d, ⁴J_CF＝3 Hz), 120.1 (d, ²J_CF＝20 Hz), 117.0 (d, ²J_CF＝21 Hz, aromatic C), 59.1 (cage CH), the B_cage-C were not observed; ¹⁹F{¹H} NMR (376 MHz, CDCl₃) δ: －112.7; ¹¹B NMR (128 MHz, CDCl₃) δ: －1.8 (d, J_BH＝150 Hz, 2B, B_cageH), －4.2 (s, 2B, ArB_cage), －10.8 (d, J_BH＝152 Hz, 2B), －12.6 (d, J_BH＝163 Hz, 4B, B_cageH). Anal. calcd for C₁₄H₁₈B₁₀F₂: C 50.59, H 5.46; found C 50.57, H 5.49.

  3, 6-(3', 5'-C₆H₃F₂)₂-1, 2-C₂B₁₀H₁₀ (4q): White solid, yield 74%. m.p. 133~135 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.11 (m, 4H, ArH), 6.87 (tt, J＝8.8, 2.4 Hz, 2H, ArH) 3.81 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 163.1 (dd, J_CF＝253, 12 Hz), 116.0 (dd, J_CF＝18, 6 Hz), 105.7 (t, J_CF＝25 Hz, aromatic C), 58.9 (cage CH), the B_cage-C were not observed; ¹⁹F{¹H} NMR (376 MHz, CDCl₃) δ: －108.9; ¹¹B NMR (128 MHz, CDCl₃) δ: －1.5 (d, J_BH＝150 Hz, 2B, B_cageH), －4.7 (s, 2B, ArB_cage), －10.6 (d, J_BH＝150 Hz, 2B), －12.6 (d, J_BH＝165 Hz, 4B) (B_cageH). Anal. calcd for C₁₄H₁₆B₁₀F₄: C 45.65, H 4.38; found C 45.73, H 4.44.

  3, 6-(p-CF₃C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4s): White solid, yield 42%. m.p. 141~142 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.75 (d, J＝8.0 Hz, 4H, ArH), 7.63 (d, J＝8.0 Hz, 4H, ArH), 3.88 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 133.7, 132.1 (q, ²J_CF＝32 Hz), 125.2 (q, ³J_CF＝4 Hz, aromatic C), 124.0 (q, ¹J_CF＝274 Hz, CF₃), 58.9 (cage CH), the B_cage-C were not observed; ¹⁹F{¹H} NMR (376 MHz, CDCl₃) δ: －63.1; ¹¹B NMR (128 MHz, CDCl₃) δ: －1.6 (d, J_BH＝151 Hz, 2B, B_cageH), －4.3 (s, 2B, ArB_cage), －10.6 (d, J_BH＝145 Hz, 2B), －12.6 (d, J_BH＝169 Hz, 4B, B_cageH). Anal. calcd for C₁₆H₁₈B₁₀F₆: C 44.44, H 4.20; found C 44.30, H 4.31.

  3, 6-(m-CF₃C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4t): White solid, yield: 61%. m.p. 125~126 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.84 (sm, 4H, ArH), 7.70 (d, J＝8.0 Hz, 2H, ArH), 7.52 (t, J＝8.0 Hz, 2H, ArH), 3.89 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 136.8, 130.9 (q, ²J_CF＝32 Hz), 129.7 (q, ³J_CF＝4 Hz), 129.0, 126.9 (q, ³J_CF＝4 Hz, aromatic C), 124.1 (q, ¹J_CF＝274 Hz) (CF₃), 59.0 (cage CH), the B_cage-C were not observed; ¹⁹F{¹H} NMR (376 MHz, CDCl₃) δ: －62.6; ¹¹B NMR (128 MHz, CDCl₃) δ: －1.5 (d, J_BH＝150 Hz, 2B, B_cageH), －4.1 (s, 2B, ArB_cage), －10.5 (d, J_BH＝151 Hz, 2B), －12.6 (d, J_BH＝164 Hz, 4B, B_cageH). Anal. calcd for C₁₆H₁₈B₁₀F₆: C 44.44, H 4.20; found C 44.41, H 4.24.

  3, 6-(p-C₆H₅C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4v): White solid, yield 65%. m.p. 216~217 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.72 (m, 4H, ArH), 7.61 (m, 8H, ArH), 7.47 (m, 4H, ArH), 7.39 (m, 2H, ArH), 3.93 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 142.9, 140.6, 133.8, 129.0, 127.9, 127.3, 127.1 (aromatic C), 59.3 (cage CH), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.1 (d, J_BH＝157 Hz, 2B, B_cageH), －3.6 (s, 2B, ArB_cage), －11.1 (d, J_BH＝214 Hz, 2B), －12.7 (d, J_BH＝184 Hz, 4B, B_cageH). Anal. calcd for C₂₆H₂₈B₁₀: C 69.61, H 6.29; found C 69.27, H 6.24.

  3, 6-(m-C₆H₅-C₆H₄)₂-1, 2-C₂B₁₀H₁₀ (4w): White solid, yield 47%. m.p. 235~237 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.86 (s, 2H, ArH), 7.64 (m, 8H, ArH), 7.48 (m, 6H, ArH), 7.40 (m, 2H, ArH), 3.96 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 141.4, 140.8, 132.2, 132.1, 129.0, 128.9, 128.8, 127.7, 127.4 (aromatic C), 59.2 (cage CH), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －0.8 (d, J_BH＝157 Hz, 2B, B_cageH), －2.6 (s, 2B, ArB_cage), －9.8 (d, J_BH＝214 Hz, 2B), －11.5 (d, J_BH＝184 Hz, 4B, B_cageH). HRMS (DART-nagative mode) calcd for C₂₆H₂₇¹⁰B₂¹¹B₈: 447.3121, found 447.3134.

  3, 6-(2'-C₁₀H₇)₂-1, 2-C₂B₁₀H₁₀ (4x): White solid, yield 57%. m.p. 154~156 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 8.15 (s, 2H, ArH), 7.86 (m, 6H, ArH), 7.71 (d, J＝8.4 Hz, 2H), 7.54 (m, 4H, ArH), 4.03 (s, 2H, cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 133.9, 132.9, 129.5, 128.2, 127.9, 127.8, 127.1, 126.7 (aromatic C), 59.4 (cage CH), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.2 (d, J_BH＝195 Hz, 2B, B_cageH), －3.6 (s, 2B, ArB_cage), －11.2 (d, J_BH＝177 Hz, 2B), －12.7 (d, J_BH＝180 Hz, 4B, B_cageH). Anal. calcd for C₂₂H₂₄B₁₀: C 66.64, H 6.10; found C 66.30, H 6.04.

  4.3   Preparation of byproduct 3-(p-CH₃OC₆H₄)-6- (p-CH₃C₆H₄)-1, 2-C₂B₁₀H₁₀ (5i)

  An oven-dried Schlenk flask equipped with a stir bar was charged with 1 (198 mg, 0.5 mmol), 4-bromoanisole (281 mg, 1.5 mmol), Pd(OAc)₂ (22 mg, 0.1 mmol), PPh₃ (52 mg, 0.2 mmol) and Cs₂CO₃ (490 mg, 1.5 mmol), followed by dry cyclohexane (5 mL). The flask was closed under an atmosphere of nitrogen and stirred at 130 ℃ (bath temperature) for 12 h. After cooled to room temperature, water (10 mL) and 30% H₂O₂ (5 mL) aqueous solution were successively added and the mixture was stirred at room temperature overnight. After extraction with diethyl ether (10 mL× 3), the ether solutions were combined and concentrated to dryness in vacuo. The residue was subjected to flash column chromatography on silica gel (230~400 mesh) using n-hexane and ethyl acetate (V/V＝100/1) as eluent to give 4i (86 mg, 48%) and 5i (43 mg, 26%). 5i: White solid, m.p. 136~137 ℃; ¹H NMR (400 MHz, CDCl₃) δ: 7.62 (d, J＝7.2 Hz, 2H), 7.55 (d, J＝8.4 Hz, 2H), 7.40 (m, 3H), 6.91 (d, J＝8.4 Hz, 2H) (aromatic CH), 3.83 (m, 5H) (OCH₃ & cage CH); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ: 161.2, 134.7, 133.3, 129.9, 128.4, 114.1 (aromatic C), 59.3 (cage CH), 55.4 (s, OCH₃), the B_cage-C were not observed; ¹¹B NMR (128 MHz, CDCl₃) δ: －2.3 (d, J_BH＝152 Hz, 2B, B_cageH), －3.5 (s, 2B, ArB_cage), －11.3 (d, J_BH＝188 Hz, 2B), －12.9 (d, J_BH＝174 Hz, 4B, B_cageH). Anal. calcd for C₁₅H₂₂B₁₀O: C 55.19, H 6.79; found C 54.91, H 6.90.

  4.4   X-ray structure determination

  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.^[17] The structure was 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.^[18] All hydrogen atoms were geometrically fixed using the riding model.

  Supporting Information NMR spectra of B(3, 6)-dia-rylated-o-carboranes 4 and 5i. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn/. CCDC 1827267 for 5i contain the supplementary crystallographic data. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html(or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or deposit@ccdc.cam.ac.uk).

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  Scheme 1  Synthesis of diaryl-o-carboranes

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  Figure 1  Molecular structure of 3-(p-MeOC₆H₄)-6-Ph-1, 2- C₂B₁₀H₁₀ (5i)

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  Scheme 2  Proposed mechanism for the formation of 4 and 5

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  Table 1.  Optimization of reaction conditions with phenylbromide^a

  Entry	Cat.	L	Base	Solvent	Yieldb/%
  					1	3a	4a
  1	Pd(PPh3)4	—	Cs2CO3	THF	10	—	90
  2	Pd(PPh3)4	—	Cs2CO3	Dioxane	9	16	75
  3	Pd(PPh3)4	—	Cs2CO3	DCE	93	7	—
  4	Pd(PPh3)4	—	Cs2CO3	Cyclohexane	—	—	100
  5	Pd(PPh3)4	—	K2CO3	Cyclohexane	92	8	—
  6	Pd(PPh3)4	—	AgOAc	Cyclohexane	—	—	—
  7	Pd(PPh3)4	—	K2HPO3	Cyclohexane	100	—	—
  8	Pd(PPh3)4	—	KOAc	Cyclohexane	100	—	—
  9	Pd2(dba)3	—	Cs2CO3	Cyclohexane	100	—	—
  10	Pd2(dba)3	PPh3	Cs2CO3	Cyclohexane	7	7	86
  11	Pd(acac)2	PPh3	Cs2CO3	Cyclohexane	100	—	—
  12	Pd(MeCN)4(BF4)2	PPh3	Cs2CO3	Cyclohexane	72	20	8
  13	[PdCl(C3H5)]2	PPh3	Cs2CO3	Cyclohexane	28	21	51
  14	PdCl2(cod)	PPh3	Cs2CO3	Cyclohexane	2	1	97
  15	Pd(OAc)2	PPh3	Cs2CO3	Cyclohexane	—	—	100
  16c	Pd(OAc)2	PPh3	Cs2CO3	Cyclohexane	21	17	62
  17d	Pd(OAc)2	PPh3	Cs2CO3	Cyclohexane	20	18	62
  18	Pd(OAc)2	P(C6H4OMe-o)3	Cs2CO3	Cyclohexane	39	22	39
  19	Pd(OAc)2	PCy3	Cs2CO3	Cyclohexane	8	8	84
  20	PdCl2(cod)	PCy3	Cs2CO3	Cyclohexane	7	10	83
  a Reaction condition: 1 (0.1 mmol), PhBr (0.25 mmol), 20 mol% cat., 40 mol% L, base (0.3 mmol), 110 ℃, 8 h; DCE＝dichloroethane; dba＝dibenzylideneacetone, cod＝1, 5-cyclooctadiene; Cy＝cyclohexyl. b GC yield. c10 mol% Pd(OAc)2, 20 mol% PPh3. d 80 ℃.

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  Table 2.  Initial study of the reaction of 1 with arylbromide under different conditions^a

  Entry	Ar	L	Yieldb/%
  1	C6H5	PPh3	84 (4)
  2	4-MeC6H4	PPh3	85 (4:5＝64:21)
  3	3-MeC6H4	PPh3	87 (4:5＝73:14)
  4c	4-MeOC6H4	PPh3	48 (4), 26 (5)
  5	4-ClC6H4	PPh3	69 (4)
  6	4-CF3C6H4	PPh3	60 (4)
  7	4-MeC6H4	PCy3	61 (4:6＝49:12)
  8	3-MeC6H4	PCy3	51 (4:6＝48:3)
  9c	4-MeOC6H4	PCy3	25 (4:6＝22:3)
  10	4-FC6H4	PCy3	65 (4:6＝62:3)
  a Reaction condition: 1 (0.5 mmol), ArBr (1.25 mmol), Cs2CO3 (1.5 mmol), 20 mol% Pd(OAc)2, 40 mol% L, 110 ℃, 24 h. b Isolated yield, ratio determined by GC. c130 ℃.

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  Table 3.  Optimization of reaction conditions with p-tolyl halides^a

  Entry	Cat.	L	Yieldb/%
  			1	3b	4b	6b
  1	Pd(OAc)2	PnBu3	75	20	5	—
  2	Pd(OAc)2	PCy3	8	9	65	16
  3	Pd2(dba)3	PCy3	16	19	62	3
  4	PdCl2(cod)	PCy3	7	12	78	3
  5	PdCl2(PCy3)2	—	51	31	18	0
  a Reaction condition: 1 (0.1 mmol), p-tolyl halide (0.25 mmol), 20 mol% cat., 40 mol% L, Cs2CO3 (0.3 mmol), 110 ℃, 24 h. b GC yield.

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  Table 4.  Synthesis of 3, 6-diarylated-o-carboranes^{a, b}

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