Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Catalytic Oxidative Dehydrogenative Coupling of Cage B−H/B−H Bonds for Synthesis of Bis(o‑carborane)s Ji Wu,† Ke Cao,*,† Cai-Yan Zhang,† Tao-Tao Xu,† Li-Fang Ding,† Bo Li,‡ and Junxiao Yang† †
State Key Laboratory of Environment-Friendly Energy Materials and School of Material Science and Engineering, Southwest University of Science and Technology, 59 Qinglong Road, Mianyang, Sichuan 621010, P. R. China ‡ Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang, Sichuan 621900, P. R. China
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S Supporting Information *
ABSTRACT: An efficient and succinct protocol for synthesis of bis(o-carborane) connected by a B−B bond via palladium catalyzed oxidative dehydrogenative coupling of cage B−H/B−H bonds was developed for the first time. A series of bis(ocarborane)s connected by B(4)−B(4)′ and B(4)−B(5)′ bonds was synthesized with moderate to good yields. This work opens the door for miscellaneous applications of bis(o-carborane) in related disciplines and has important value in design and synthesis of different kinds of biscarboranes.
C
a general and environmentally friendly protocol for synthesis of bis(o-carborane) connected by a B−B bond (BBC), which keeps the property evaluation and potential applications of BBC in its infancy. Transition metal catalyzed C−H activation/cross dehydrogenative coupling for direct functionalization of carbon-based molecules has emerged as a powerful strategy over the past decades, which represents the most succinct and atomeconomical synthetic route for construction of carbon−carbon and carbon−heteroatom bonds. Due to the three-dimensional aromaticity of o-carborane analogues to benzene, utilization of the transition metal catalyzed cage B−H activation for regioselective boron functionalization of o-carbrane has resulted in many advancements in recent years.14−17 Synthetically, transition metal catalyzed cage B−H activation and following cross dehydrogenative coupling of B−H/B−H bonds is the most efficient strategy for construction of BBC. However, the 10 B−H bonds of o-carborane are not fully equal, and the electrophilic reactivity is reduced in the following order: B(9, 12) > B(8, 10) > B(4, 5, 7, 11) > B(3, 6).18 Therefore, the method to control the selectivity of the formation of B−B bonds via specific B−H/B−H coupling is the key problem to be considered. Recently, Xie’s group and ours accomplished the regioselective construction of B−C and B−O bonds via cross dehydrogenative coupling of the cage B− H bond with C−H and O−H bonds,14j,16e respectively. These works indicated that the direct selective functionalization of
arboranes are a class of carbon−boron molecular clusters with three-dimensional (3D) aromaticity analogues to benzene, which have extensive applications in pharmaceuticals for boron neutron capture therapy (BNCT),1 building blocks in functional materials,2 and ligands in organometallic as well as coordination chemistry.3 Correspondingly, biscarboranes can be viewed as 3D analogues to biphenyl and have also found important applications as unique bidentate ligands,4 structural units for the construction of rigid rod molecules,5 metallacarboranes,6 and supraicosahedral bis(heteroborane)s.7 Additionally, they might also have potential application in asymmetric catalysis by design and synthesis of ligand analogues to BINAP. For bis(o-carborane), 1,1′-bis(o-carborane) connected by a cage C−C bond is the most well-known and first synthesized from the reaction of B10H12(CH3CN)2 with diacetylene by Hawthorne half a century ago.8 After then, the CuCl mediated coupling of 1,2-Li2C2B10H10 is the dominating route for the synthesis of 1,1′-bis(o-carborane).9 For biscarborane connected by a B−B bond, there were some controversies on the reproducibility in its early stage.10 Until 1983, the 9,9′-bis(1,2Me2-o-carborane) was precisely synthesized in the presence of a catalytic amount of Pd(OAc)2 and stoichiometric Tl(OCOCF3)2 by Usyatinskii and coworkers11 (Scheme 1b). Then, the 9,9′-bis(m-carborane) was further synthesized from 9,9′-bis(m-carboranyl) mercury12 (Scheme 1c). However, the 4,4′-bis(o-carborane) is only a proposed byproduct in the reaction of dinido-carborane with diborane.13 We can see that the high toxicity thallium and mercury salts are indispensable for synthesis of biscarboranes connected by B−B bonds in the traditional methods. To the best of our knowledge, there is not © XXXX American Chemical Society
Received: June 20, 2019
A
DOI: 10.1021/acs.orglett.9b02129 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. Strategies for Synthesis of Biscarboranes
Table 1. Optimized Conditions for the Selective Oxidative Dehydrogenative Coupling of B(4)−H Bond of 9Benzamide-o-carboranea
the B−H bond via dehydrogenative coupling is a potential strategy for construction of BBC. However, as the B−H bonds of o-carborane are much less polar than the cage C−H bonds, the direct dehydrogenative coupling of cage B−H/B−H bonds for construction of BBC with good selectivity is still an intricate project. Recently, we found that when the benzamide was introduced on B(9), the charge distribution of 9-benzamide-o-carborane has a great change, and B(4, 5, 8, 10, 12) were much more negative than those in o-carborane. More importantly, the B(4)−H and B(5)−H bonds were more polarized when comparing the relative charge change of other B−H bonds. Additionally, the B(4)−H bond is slightly more polarized than the B(5)−H bond. On the basis of this characteristic, we developed a selective arylation of 9-amide-o-carborane on B(4) with arylboronic acid; meanwhile, the electrophilic reaction on B(8, 10, 12) was inhibited completely.16f Inspired by this result, we envisioned a general method for the synthesis of BBC via intermolecular oxidative dehydrogenative coupling of B(4)−H bonds with good selectivity. To initiate our research, 9-benzamide-o-carborane was selected as the model substrate to screen conditions, and the results are summarized in Table 1. After many efforts, we found such an oxidative dehydrogenative coupling of B(4)−H bond could be accomplished in the presence of Pd(OAc)2 and AgF in toluene at 60 °C for 24 h, which gave the desired bis(ocarborane) connected by a B(4)−B(4)′ bond (2a) in 31% yield. Meanwhile, the isomer 3a connected by a B(4)−B(5)′ bond was also formed in 8% yield (entry 1), which might ascribed to the slight difference in reactivity of B(4)−H and B(5)−H bonds. Importantly, 2a and 3a could be easily separated by column chromatography. Further studies indicated that Pd(OAc)2 was more favorable for this transformation in toluene (entries 2−6). Subsequently, the silver salts were also examined, and the results demonstrated that the coupling proceeded smoothly in the presence of 0.5
entry
catalyst (10%)
oxidant
solvent
2a/3a (%)b
1 2 3 4 5 6 7 8 9 10
Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 PdCl2 Pd(MeCN)4(BF4)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2
toluene DCE MeCN THF toluene toluene toluene toluene toluene toluene
31/8 23/8
11
Pd(OAc)2
toluene
43/20
12
Pd(OAc)2/NiCl2c
toluene
57/23
13
NiCl2c
2 equiv AgF 2 equiv AgF 2 equiv AgF 2 equiv AgF 2 equiv AgF 2 equiv AgF 2 equiv AgBF4 2 equiv AgOTf 2 equiv AgOAc 0.5 equiv AgF 2 equiv AgOAc 0.5 equiv AgF 3 equiv AgOAc 0.5 equiv AgF 3 equiv AgOAc 0.5 equiv AgF 2 equiv AgOAc
23/4 6/0 4/4 4/8 8/0 12/17 38/21
toluene
a
All reactions were carried out using 0.2 mmol 1a, 10 mol % catalyst, and 2 mL of solvent at 60 °C for 24 h under an argon atmosphere. b Isolated yields. c10 mol % NiCl2.
equiv of AgF and 3 equiv of AgOAc and gave the combined yield of 2a and 3a in 63% yield (entry 11). Furthermore, we found that when 10 mol % of NiCl2 was added, the combined yield was further improved to 80% (entry 12). A control experiment demonstrated that NiCl2 could not catalyze this dehydrogenative coupling in the absence of Pd(OAc)2 (entry 13). This result implies that the nickel might not be the catalytic active center. Under the optimized conditions (Table 1, entry 12), the scope of 9-amide-o-carboranes was then examined. As can be seen from Scheme 2, for 9-(4-R-benzamide)-o-carboranes (1a−1f), the benzamides substituted with electron-donating or electron-withdrawing groups were all compatible with this transformation and gave the corresponding bis(o-carborane)s connected by B(4)−B(4)′ and B(4)−B(5)′ bonds in moderate to good yields (2a−2f). Additionally, the exact structures of 2d and 3d were unambiguously confirmed by X-ray crystallographic analysis (Figure 1).19 For 9-(3-R-benzamide)-ocarboranes (1g−1i), the electron-donating group was more favorable for the coupling, and the expected products were generated in moderate to good yields (2g−2i). The electronic and steric effect of substituents has a distinct influence on the coupling of 9-(2-R-benzamide)-o-carboranes (1j−1n), 1o and 1p, and the desired products were afforded in moderate yields (2j−2p). Interestingly, 9-alkylamide-o-carboranes were also compatible with this coupling (1q−1r), and only the bis(ocarborane)s connected by a B(4)−B(4)′ bond were formed but with a lower conversion ratio (2q−2r). These above synthesized BBCs are potential synthons for synthesis of diazonium salts of BBC via removal of the acyl group and a diazotization reaction, which would be a universal intermediate B
DOI: 10.1021/acs.orglett.9b02129 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Synthesis of Bis(o-carborane)s via Selective Oxidative Dehydrogenative Coupling of 9-Amide-ocarboranesa,b
Scheme 3. Proposed Mechanism for Synthesis of Bis(ocarborane)s via Oxidative Dehydrogenative Coupling of Cage B−H/B−H Bonds
B(5)−H bond by PdIV. Additionally, the NiCl2 might be a Lewis acid, which activates the carbonyl of 9-amide-ocarborane and facilitates the formation of PdIV intermediate. In conclusion, we developed a palladium catalyzed selective oxidative dehydrogenative coupling of cage B−H/B−H bonds for the synthesis of bis(o-carborane) connected by a B−B bond for the first time. A series of bis(o-carborane)s connected by B(4)−B(4)′ and B(4)−B(5)′ bonds was synthesized with moderate to good yields. A plausible mechanism involving one B−H activation at PdII and another B−H activation at PdIV was proposed for the formation of B(4)−B(4)′ and B(4)− B(5)′ bonded bis(o-carborane)s. This work represents the first efficient and general strategy for synthesis of bis(o-carborane) connected by a B−B bond via oxidative dehydrogenative coupling of cage B−H/B−H bonds, which would be useful for design and synthesis of many kinds of biscarboranes and opens the door for evaluating the properties and miscellaneous applications of BBCs in related disciplines.
a
All reactions were carried out on a 0.2 mmol scale in 2 mL of toluene at 60 °C for 24 h under an argon atmosphere. bIsolated yields; the ratios of isomers 2 and 3 were calculated based on the isolated yields and are given in parentheses. cIsolated yields were calculated based on the consumed 9-amide-o-carboranes, and the conversion ratios of 9amide-o-carboranes are given in parentheses.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b02129. Detailed experimental procedures, complete characterization data, and copies of 1H, 13C, 11B{H}, and 11B NMR spectra (PDF)
Figure 1. Crystal structure of 2d (left) and 3d (right).
for the synthesis of many kinds of BBC derivatives20 and would have important value in promoting the applications of BBCs in related disciplines. To elucidate the possible mechanism, two control experiments for 1a by addition of radical scavenger (1,4benzoquinone and TEMPO) were carried out under the standard conditions, and the desired products were obtained in 69 and 61% yields, respectively. This result indicates the selective B−H/B−H coupling would not be a radical mechanism. Based on the experimental results and our previous work on selective arylation of B(4)−H of 9-amideo-carboranes, we considered the cyclopalladium intermediate A should form first by amide guided selective B(4)−H activation.14k,16f The bis(o-carborane) connected by a B(4)− B(5)′ bond is generated along with the B(4)−B(4)′ one; therefore, the highly electrophilic PdIV intermediate B should form in the presence of silver salts and had less selectivity following activation of B(4)′−H and B(5)′−H bonds of another 9-amide-o-carborane via intermediates C and D, respectively21 (Scheme 3). In view of the ratio of regioisomers, we consider these results to be reasonable as the slightly more polarized B(4)−H bond should be activated prior to the
Accession Codes
CCDC 1904327 and 1904329 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, 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 Author
*E-mail:
[email protected]. ORCID
Ke Cao: 0000-0003-0348-2386 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21602182), the Longshan academic talent research supporting program of SWUST C
DOI: 10.1021/acs.orglett.9b02129 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
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(Grants 17LZX324, 18LZX305, and 18LZXT02), and the Project of State Key Laboratory of Environment-Friendly Energy Materials, SWUST (Grants 17fksy0102 and 18fksy0206).
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DOI: 10.1021/acs.orglett.9b02129 Org. Lett. XXXX, XXX, XXX−XXX
