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Transition metal-induced B–H functionalization of o-carborane
Coordination Chemistry Reviews xxx (2017) xxx–xxx
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Review
Transition metal-induced B–H functionalization of o-carborane Xiaolei Zhang a,b, Hong Yan a,⇑ a State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Jiangsu Key Laboratory of Advanced Organic Materials, Nanjing University, Nanjing, Jiangsu 210023, China b School of Pharmaceutical Sciences, Jiangnan University, Wuxi, Jiangsu 214122, China
a r t i c l e
i n f o
Article history: Received 29 August 2017 Received in revised form 3 November 2017 Accepted 5 November 2017 Available online xxxx In memory of Professor Bernd Wrackmeyer. Keywords: B–H functionalization Carborane Half-sandwich complex Mechanism
a b s t r a c t Transition metal-mediated B–H functionalization of carboranes has drawn increasing interests during the last two decades. This review describes how to incorporate B–H functionalization into the derivation of half-sandwich transition metal complexes containing o-carborane dichalcogenolates. The detailed mechanistic elucidations for metal-induced versatile B–H functionalization are also reviewed. Examples of selective B–H activation, stepwise B–H functionalization, cobalt-mediated B–H functionalization as well as dimetal-mediated B–H activation of o-carborane are discussed. Ó 2017 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of half-sandwich 16e complexes bearing o-carborane dichalcogenolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactivity of 16e cobalt half-sandwich complex towards dimethyl but-2-ynedioate, methyl diazo acetate and tosyl azide . . . . . . . . . . . . . . . . . Functionalization of B–H bond of o-carborane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Selective B–H functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Mechanistic implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Stepwise B–H functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Cobalt-mediated B–H activation and functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Dimetal-mediated B–H activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Dicarba-closo-dodecaboranes (C2B10H12) are a class of icosahedral clusters composed of boron, carbon and hydrogen atoms (Scheme 1) [1–4]. Owing to their useful properties such as rigidity, robustness, hydrophobicity, enriched boron content and delocalized three-dimensional aromaticity, a lot of potentials or practical applications have been explored in energy, catalysis, medicine and materials [5–14]. The carboranyl units, as highly structural analogs ⇑ Corresponding author. E-mail address: [email protected] (H. Yan).
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of aromatic hydrocarbons, can serve as rigid scaffolds upon which to build molecules with well-defined, three-dimensional conformations [15–20]. A wide range of C–H and B–H functionalized carborane derivatives have been prepared under different reaction conditions. Owing to the significantly different electronic properties of the carbon and boron substituting effects (electronwithdrawing for carbon substitution and electron-donating for boron substitution) [21,22], it is especially interesting to study the properties of the boron-substituted carboranes in comparison to the carbon-substituted ones. The electronic deficient property of the carborane cage and the stronger electronegativity of the carbon atoms in the carborane
https://doi.org/10.1016/j.ccr.2017.11.006 0010-8545/Ó 2017 Elsevier B.V. All rights reserved.
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Scheme 1. Molecular geometry of o-, m-, p-dicarba-closo-dodecarboranes, IUPAC numbering of the cage atoms and their chemical transformations.
cluster cause acidic nature of the C–H bonds and hydridic nature of the B–H bonds around the icosahedral sphere. In sharp contrast to the functionalization of carboranes at the carbon site [23–29], the B–H functionalization is much more challenging owing to the less acidic nature and the chemo-similarity of the ten B–H bonds [30– 35]. Therefore, selective B–H bond activation and functionalization of carboranes has drawn increasing interests during the last two decades [36–44]. Notably, a wide range of o-carborane-based organometallic complexes containing half-sandwich metal moieties (Co, Rh, Ir, Ru and Os) have been prepared through transition metal-induced B–H bond activation and functionalization. Half-sandwich complexes of group VIII transition metals bearing 1,2-dicarba-closo-dodecarborane-dichalcogenolates (ocarborane dichalcogenolates) are a versatile class of organometallic compounds. Similar to the two-dimensional super-aromatic analog of the benzene counterpart [45], o-carborane dichalcogenolates can serve as excellent scaffolds for the construction of metal dichalcogenolate complexes with unique chemical and physical properties. They have the advantages of accessibility, robustness, chemo-stability, tunable hydrophilicity or hydrophobicity that allow their applications in catalysis, supramolecular chemistry, as well as biological field [46–50]. The chemistry of o-carborane dichalcogenolates in coordinative self-assembly [51], metal–metal bonding formation [52] and B–H activation [53], has been developed, which closely links the areas of polyhedral boranes and organometallic chemistry. Here, we presented in this review some recent advances on the metal-induced B–H bond activation and functionalization by the reactions of half-sandwich metal complexes and organic reagents. Various analytical tools such as X-ray crystallography, NMR spectroscopy and high-resolution mass spectroscopy were used to confirm B–H activation and study reaction routes. Based on our systematic research work in this area during the last two decades, this review will mainly focus on several key points in the very recent investigation of B–H function-
alization including selective and stepwise B–H functionalization, cobalt-mediated B–H functionalization and dimetal-mediated B–H activation.
2. Synthesis of half-sandwich 16e complexes bearing ocarborane dichalcogenolates A series of group VIII metal (Ru [54], Os [54], Co [55], Rh [56] and Ir [57]) complexes bearing o-carborane 1,2-dichalcogenolate ligands have been synthesized. Our recent research advances [58–61] are the generation of a new kind of half-sandwich transition metal complexes containing a 1,2-dicarba-closododecarborane-9,12-dithiolate (o-carborane 9,12-dithiolate) in sharp contrast to the previously reported ones with o-carborane 1,2-dichalcogenolates. o-Carborane 1,2-dichalcogenolates are readily prepared by insertion of elemental chalcogen (S or Se) into the Li–C bond of dilithium carborane after deprotonation by nbutyllithium (Scheme 2) [25]. 1,2-Dicarba-closo-dodecarborane9,12-dithiol is generated by the Friedel-Crafts reaction of o-carborane with excess sulfur powder under the catalysis of anhydrous aluminum chloride at 160 °C (Scheme 2) [62]. o-Carborane 1,2-dithiolate exhibits electron poorer property at the sulfur atoms than that in o-carborane 9,12-dithiolate, as the carborane cage of electron-withdrawing nature in the former, whereas the carborane cage is of the electron-donating nature in the latter [21,22]. Therefore, the different attachment of sulfur atom to the boron or carbon site of o-carborane offers the possibility to adjust electrondonating or withdrawing property without altering the steric property of the ligand. The synthesis of 16e half-sandwich Ru, Os, Co, Rh, Ir complexes (1–10) based on dilithium o-carborane 1,2-dichalcogenolate Li2E2C2B10H10 (E = S, Se) has been previously reviewed [63]. Herein, we compared these typical 16e half-sandwich complexes with the
Scheme 2. Synthesis of o-carborane 9,12-dithiolate and 1,2-dichalcogenolates.
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Scheme 3. Synthesis of 16e half-sandwich Ru, Os, Co, Rh, Ir complexes (1–14) containing an o-carborane 1,2-dichalcogenolate or an o-carborane 9,12-dithiolate.
newly developed ones containing an o-carborane 9,12-dithiolate (Scheme 3). The reaction of o-carborane-9,12-(SH)2 with 2 equiv. Et3N in CH2Cl2 at room temperature leads to o-carborane 9,12dithiolate. Then, the in situ treatment with metal moieties such as [Cp*MCl(l-Cl)]2 (M = Rh, Ir), and [Cp#Co(CO)I2] (Cp# = C5H5, C5Me5) gives rise to the corresponding boron-substituted 16e half-sandwich complexes [Cp*M(9,12-S2C2B10H10)] (M = Rh (11), Ir (12)) [58,59] and Cp#Co(9,12-S2C2B10H10) (Cp# = C5H5 (13) C5Me5, (14)) [60,61], respectively. The red complex CpCo(9,12S2C2B10H10) (13) is only characterized by NMR and MS-ESI data owing to its less stability in solution. The steric hindered pentamethylcyclopentadienyl (Cp*)-coordinated species Cp*M(9,12S2C2B10H10) (M = Co, Rh, Ir) (11, 12, 14) exhibit enhanced stability for further isolation and characterization. The molecular structure
of Cp*Ir(9,12-S2C2B10H10) (12) can be compared with its structural analog Cp*Ir(1,2-S2C2B10H10) (3) (Fig. 1). Both complexes show pseudo aromatic carbon fused or boron-fused MS2C2/MS2B2 metallacycles with coordinatively and electronically unsaturated property at the metal centers [57,59].
3. Reactivity of 16e cobalt half-sandwich complex towards dimethyl but-2-ynedioate, methyl diazo acetate and tosyl azide In spite of the coordinatively unsaturated metal center and the nucleophilic behavior of chalcogen atom, these neutral 16e halfsandwich complexes act as active organometallic species and react with small organic molecules at the metal–chalcogen (M–E) bonds
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Fig. 1. Molecular structures of Cp*Ir(1,2-S2C2B10H10) (3) (left) and Cp*Ir(9,12S2C2B10H10) (12) (right) (Hydrogen atoms are omitted for clarity).
(Scheme 4). For example, the mononuclear 16e dithio-o-carboranyl cobalt complex CpCo(1,2-S2C2B10H10) (7) readily reacts with one equiv. of dimethyl but-2-ynedioate to form the alkene inserted product CpCo(CO2MeC@CCO2Me)(1,2-S2C2B10H10) (15) [64]. This trend of forming Co–S bond inserted complexes also occurs in the reactions of diazo compounds or azides. The reaction of complex 7 and methyl diazo acetate or tosyl azide in dichloromethane gives rise to the corresponding product 16 [65] or 17 [66], respectively. These direct insertion reactions indicate the tendency of the metal center to form an electronically saturated 18e configuration. Furthermore, the formation of the C–S and N–S bonds at the ocarboranyl thiolate is accompanied by the generation of a M S bond. This provides the possibility for B–H activation since the weak M S coordination bond makes the central structure flexible and offers the metal to approach the B–H bond. To investigate the potential and scope of this kind of reactivity, systematic investigations of reaction conditions and mechanisms for B–H activation have been conducted based on half-sandwich transition metal o-carborane dichalcogenolate precursors.
Scheme 4. Reactivity of CpCo(1,2-S2C2B10H10) (7) towards dimethyl but-2-ynedioate, methyl diazo acetate and tosyl azide.
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Scheme 5. Reactivity of Cp*Rh(1,2-S2C2B10H10) (1) and Cp*Rh(1,2-Se2C2B10H10) (2) towards HC„CC(O)OMe to afford 18–21.
4. Functionalization of B–H bond of o-carborane 4.1. Selective B–H functionalization Selective B–H mono- or di-functionalization of o-carborane dichalcogenolates can be achieved by reactions of the 16e halfsandwich complexes containing o-carborane dichalcogenolates with alkynes [67–73]. As shown in Scheme 5, Cp*Rh(1,2S2C2B10H10) (1) reacts with methyl propiolate (HC„CC(O)OMe) in boiling CHCl3 to give 18 containing two vinyl groups substituted at the B(3)/B(6) sites of the o-carboranyl unit (Scheme 5) [53]. In
18, as expected for a 16e species, a planar rhodadithiolene fivemembered ring is present (Fig. 2). The reactivity of 2, as a structural analog of 1, with methyl propiolate has been proven slower in boiling CHCl3. The product has been isolated and characterized to be compound 21 with a methylene group substituted at the B (3) site (Scheme 5) [67]. In particular, two reactive intermediates 19 and 20 were isolated as structural isomers if the reaction was conducted at room temperature. Each contains a Rh–B bond, and can not only transform to the thermodynamically stable product 21 (Fig. 2), but also transmute to each other between the trans and cis configuration. Complex 20 takes a cisoid arrangement
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Fig. 2. Molecular structures of 18 (left) and 21 (right) (Hydrogen atoms are omitted for clarity).
Scheme 6. Proposed mechanism for Rh-induced B–H functionalization of o-carborane-1,2-dithiolate.
between the C(1)–B(6) and the g-SeC(CO2Me)@CH2 bonds, in contrast to a transoid arrangement of the corresponding units in 19. Although only complex 20 possessed the correct stereochemistry to afford 21 by an intramolecular rearrangement, neither 19 nor 20 were left after the individual was heated in boiling CHCl3. 4.2. Mechanistic implications The proposed mechanism for the reaction of 1 with HC„CC(O) OMe is illustrated in Scheme 6 [53,67]. Species A is generally considered as the first intermediate, generated by initial coordination of the alkyne to the 16e metal center. Then alkyne insertion into one of the M–S bonds is followed up to form intermediate B. If the coordinative S ? M bond is weak enough, for example in C, the Rh center enables to approach the carborane cage to activate the B–H bond (D), followed by the formation of M–B bond and the B–H hydrogen migration to the olefinic carbon atom (E). The cleavage of the M–B bond in E leads to a C–B bond to generate the 16e complex F. The repetition of the sequence from A to E leads
to complex 18 with boron-disubstitution in the B(3) and B(6) positions. 4.3. Stepwise B–H functionalization The 16e iridium complexes 3 and 4 are less reactive than their rhodium analogs. This gives a chance to study the stepwise substitution of the carborane cage at B(3/6) site (Scheme 7) [73]. Both 3 and 4 react with 1.0 equiv. of HC„CC(O)OMe at 110 °C to give both Z- and E-configurations of the 16e complexes (22 and 24) and (23 and 25). The carborane cage is selectively substituted in the B(3) position. The mono-substituted 16e complexes of 22–25 are able to take reaction with another equivalent of HC„CC(O)OMe at 110 °C in toluene to afford B(3)/B(6) disubstituted products (26– 31) with all possible Z and E configurations (Fig. 3). The 16e complex 7 with a cobalt core is more reactive towards alkynes than its analogous rhodium and iridium species. Complex 7 took reaction with MeO2C–C„C–CO2Me in CH2Cl2 at room temperature to afford the alkyne insertion complex 15 (Scheme 8) [74].
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Scheme 7. Stepwise controlled reactions of Cp*Ir(1,2-E2C2B10H10) [E = S (3), Se (4)] with HC„CC(O)OMe leading to selective and stepwise functionalization of o-carborane dichalcogenolates.
Fig. 3. Molecular structures of 23 (left) and 29 (right) (Hydrogen atoms are omitted for clarity).
Upon heating to 70 °C in toluene, 15 rearranges to 32 and 33 in 1:1 ratio. Both complexes are geometrical isomers with boronsubstitution at the carborane cage. The formation of 32 follows well-documented process via transition metal-induced B–H activation. Owing to electron deficient nature at the cobalt core, a subsequent reaction of complex 32 with MeO2C–C„C–CO2Me at room temperature led to 34. Experimental results reveal that the presence of one boron substitution can accelerate the reactivity of the 16e metal center in 32. The configurations of the two olefinic substituents take trans (E) arrangement in 34, in contrast to the
cis (Z) configuration in 32 (Fig. 4). This strategy of stepwise and selective functionalization on carborane can be extended to other alkynes, providing a facile route to the B-functionalized ocarborane derivatives. The reaction of the 16e cobalt complex 7 towards alkynones (HC„C–C(O)Ph) in a 1:1 ratio at room temperature lead to B(3)–H activation and one of the products is monosubstituted complex 35. 35 is still a 16e half-sandwich complex, thus it can further react with HC„C–C(O)Ph or HC„C–CO2Me at room temperature, leading to 36 or 37, respectively. Again, both complexs 36 and 37 are still 16e half-sandwich complexes with
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Scheme 8. Reactivity of CpCo(1,2-S2C2B10H10) (7) towards MeO2C–C„C–CO2Me and HC„C–C(O)Ph.
B(3,6)-disubstitution at the o-carborane1,2-dithiolate ligand. Similar functionalization model can be extended to MeO2C–C„ C–CO2Me, which leads to stepwise formation of 38 and the final stable B(3,6)-disubstituted product 39 [75].
4.4. Cobalt-mediated B–H activation and functionalization As indicated in the aforementioned context, the 16e halfsandwich cobalt complex CpCo(1,2-S2C2B10H10) (7) exhibits the
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Fig. 4. Molecular structures of 32 (left) and 34 (right) (Hydrogen atoms are omitted for clarity).
Scheme 9. Three-component reactions of 7, HC„CC(O)OMe and organic molecules a–g; the ligand substitution leads to 40 or 41 from 43, 44 and 45 upon the addition of a or b.
Fig. 5. Molecular structures of 40 (left) and 45 (right) (Hydrogen atoms are omitted for clarity).
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Scheme 10. Proposed mechanism on the formation of 40.
Fig. 6. Molecular structures of 46a (R = Me, up-left), 47d (R = Fc, up-right), 48a (R = Me, down-left), 54bd (R = Ph, R0 = Fc, down-right) (Hydrogen atoms are omitted for clarity).
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Scheme 11. Generation of o-carboranyl-norbornyl derivatives through cobalt-induced B–H activation.
highest chemical reactivity towards alkynes compared with its ruthenium, osmium, rhodium and iridium analogs [76–83,66]. Considering low cost of cobalt starting compounds and the fact that boron-functionalization of o-carborane can occur at room temperature by using cobalt, efficient methods have been established through cobalt-mediated B–H activation to generate B-functionalized carboranes, such as B-cyclopentadienyl, Bnorbornadienyl and B-norbornyl species [84–86]. These novel compounds are unavailable through conventional routes. The reactions of 7 with alkynes and 3e-donor ligands led to facile B–C coupling between carboranyl and cyclopentadienyl at ambient temperature (Scheme 9) [84]. Particularly, the addition of a to the mixture of 7 and HC„CC(O)OMe in dichloromethane afforded 40, featuring a linkage between the carboranyl cage and the Cp unit via a covalent B–C bond (Fig. 5). A series of small organic molecules have been examined to understand their role in the formation of 40–45 (Scheme 9). Ligand b was able to lead to 41, which is the analog of 40. The weakly coordinative ligands c and d could also give rise to 42 with a C–B bond, but the ligand does not exist in the structure. Allenes (e and f) could also afford the similar products 43 and 44 with a 3e-donor allyl unit. Cyclopentadiene (g), containing a 3e-donor o-carborane dithio ligand in situ-generated from a [4+2] Diels–Alder cycloaddition, could also lead to a B–C coupled complex (45). The coordination ability of the carborane-based bulky dimercapto ligand in 45 is weaker and can be replaced by the stronger ones a or b to quantitatively furnish 40 or 41. Similarly, 43 and 44 can be readily
transformed to 40 or 41 upon treatment with the stronger ligand a or b, but 42 remains unreactive, demonstrating the different binding ability of these organic ligands. A plausible mechanism for the generation of 40 is shown in Scheme 10. The B–H activation at B(3) site of carborane and hydrogen transfer to the alkyne should take place as the earlier steps (I–III) after alkyne insertion into one Co–S bond. The chelating coordination of a would induce the binding mode conversion of Cp from g5 to g1 (IV), followed by a–H elimination (V), Co–C and Co–B cleavage and B–C formation as well as Cp switching back to g5 to lead to 40 [84]. The 16-electron complex 7 reacts with alkynones such as HC„CC(O)R (R = methyl (Me), phenyl (Ph), styryl (St), ferrocenyl (Fc)) at ambient temperature to give two novel types of 17electron cobalt complexes 46a–d and 47a–d (Scheme 11) [87,88]. Both contain B(3)/B(6)-norbornyl carborane structures. Furthermore, these 17-electron complexes can serve as precursors for the synthesis of various B-functionalized carboranes. Under the condition of air, moisture and silica, complexes 46a–d took place alkyl C–S cleavage to form 16-electron complexes 48a–d with a B-norbornadienyl unit and 49a–d containing an organic tricyclic moiety. However, 48a–d cannot be transformed to 49a–d in the presence of water. Furthermore, complexes 46a–d can also undergo carboranyl C–S cleavage to produce metal-free species 50a–d containing a B-norbornyl unit (Fig. 6). Compounds 46a–d and 47a–d can take place further alkyne insertions into the Co–S bond to furnish the cobalt-free B-norbornyl carborane derivatives (Z/E)-51 and (Z/E)-52, which contain a vinyl sulfido group.
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Scheme 12. Energy profile for the generation of 46. The related free energy and the electronic energy (in the bracket) for the reaction intermediate or transition state are given in kcal/mol.
Moreover, thermal promotion or Lewis acid mediation of (Z/E)-51 can lead to retro-Diels–Alder products with differing B-norbornyl groups and excellent selectivity. Addition of AlCl3 promotes the conversion of 50 to 53 as predominant retro-Diels–Alder products. The isomerization from E to Z-configuration on the vinyl group and reorganization of the norbornyl unit of (Z/E)-51 lead to (Z)-53 after heating. As mentioned above, the 17e cobalt(II) complexes (46a–d) are the key intermediates to transform to the products in Scheme 11. A formation mechanism of 46a–d via a tandem sequence of processes such as metal-induced B–H activation, the coordination change of Cp ligand, boron–Cp linkage, and Diels–Alder reaction are proposed based on DFT calculations. The calculated energy profile is shown in Scheme 12 [86]. The reaction of 7 with 1.0 equiv. ethyl diazoacetate (EDA) at room temperature gives rise to 55 (Fig. 7), in which one EDA molecule inserted into one Co–S bond to form a three-membered metallacyclic ring (Scheme 13) [65]. Compound 55 can take further reaction with excess amount of EDA to generate a series of boron-functionalized organometallic species 56–60 (Scheme 13). Both of 56 and 57 contain a stable Co–B bond. Particularly, the dual activation of B(3)–H and B(4)–H bonds of the carborane cage has been observed in 56. One EDA insertion into the Co–B bond leads
Fig. 7. Molecular structures of 55 (Hydrogen atoms are omitted for clarity).
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to the formation of a C–B bond in 58. Another EDA insertion into the Co–S bond in 58 gives rise to 59. Upon heating, deboronation is observed for 59, leading to 60 containing a nido-C2B9 unit. Therefore, the cobalt-induced alkylation of o-carborane 1,2-dithiolate can be realized by using organic diazo compounds [65,87]. The three-component reactions of CpCo(S2C2B10H10)(7), ethyl diazoacetate (EDA) and alkynes at ambient temperature led to 61–64 (Scheme 14) [87], in which one alkyne is stereoselectively inserted into the Co–B bond. At ambient temperature, 61–64 undergo rearrangement to 65–68 by migratory insertion of the inserted EDA moiety. If weak base is present, 61–64 can lose an apex BH to give rise to complexes 69–71. Particularly, the reactions of complex 7, diazo ester and 1,6-diyne [PhN(CH2C„CH)2] was investigated. A pyrrole ring is formed in 74 by in situ ring-closure
13
of the 1,6-diyne, along with the formation of the B–H activated species 72 and 73 [88].
4.5. Dimetal-mediated B–H activation In contrast to the widely developed o-carborane-1,2-dithiolate for the generation of dinuclear metal clusters, o-carborane-9,12dithiolate is also suitable for the generation of dinuclear rhodium complexes 75–77 (Scheme 15). The 16e half-sandwich rhodium complex [Cp*Rh{9,12-S2C2(B10H10)}] (Cp* = g5-C5Me5) (11) takes reaction with Rh(PPh3)3Cl in the presence of NH4PF6 to deliver a cationic dinuclear complex 75. Deboronation of 75 under basic conditions affords complex 76 containing a nido-carborane unit.
Scheme 13. Reaction of CpCo(1,2-S2C2B10H10) (7) with ethyl diazoacetate (EDA).
Scheme 14. Three-component reactions of CpCo(1,2-S2C2B10H10) (7), EDA and alkynes.
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Scheme 15. Preparation of dirhodium complexes with o-carborane-9,12-dithiolate.
Fig. 8. Molecular structures of 79 and 81 (Hydrogen atoms are omitted for clarity).
One bulky PPh3 in 75 can be replaced by CO, leading to the carbonyl product 77 containing an agnostic Rh–H–B interaction [58].
Both dinuclear rhodium complexes 76 and 77 can lead to B–H activation under mild conditions. Treatment of complex 76 or 77 with 1.0 equiv. of FcPF6 in CH2Cl2 at ambient temperature gave rise
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X. Zhang, H. Yan / Coordination Chemistry Reviews xxx (2017) xxx–xxx
15
Scheme 16. Dinuclear rhodium mediated B–H bond activation, B–Cl bond and B–O bond formation.
to complex 78 or 79 containing a Rh–B bond and a l2-bridged Rh– Cl–Rh bond (Scheme 16). Furthermore, the reductive elimination of 78 or 79 affords products 80 or 81 in high yields (>80%) to generate a B–Cl bond and a normal Rh–Rh bond (Fig. 8). Consistent with the structural analysis, a DFT calculation also suggests much weaker Rh–Rh interaction in 79 than those in 77 or 81, which demonstrates the dimetal mediated B–H activation by fitting the Rh–Rh distance which is restored after B–Cl reductive elimination. The reaction of 77 towards CH3OH or water at 60 °C followed by bubbling CO led to B–H activation and B–O coupling as shown in 82 or 83 with the delivery of dihydrogen (Scheme 16). A plausible mechanism was proposed in Scheme 17 by taking 76 as an example. Metal-induced B–H bond activation by an agnostic interaction should be in the earlier step. Then a M–B bond is formed and the B–H hydrogen becomes a bridging hydride between the two metal centers (II) (a Rh–H–Rh bond is more stable than a terminal Rh–H bond). The OH moiety of the coordinating substrate H2O takes the position of the bridging hydrogen in II, accompanied by release of
H2 to generate species III. After reductive elimination, the product containing a B–O bond (83) is generated (Scheme 17).
5. Conclusion and outlook In the very recent years, considerable contributions have been made on transition metal-induced B–H activation and functionalization of o-carborane. This approach provides an unconventional way for the generation of B-substituted carborane derivatives. The detailed mechanistic investigations offer an insight into design of new types of B–H functionalization. Given the mild reaction conditions and interesting structures of products, it is anticipated that transition-metal induced B–H activation is a promising approach for generation of new types of boron-substituted carborane derivatives. The extensive studies on the synthetic methodologies of carborane system reinforce the relationship between inorganic boron clusters and organic synthetic methods, which may ignite
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Scheme 17. Proposed reaction mechanism for B–O bond formation via dimetal-induced B–H activation.
versatile potential applications of carboranes in material science, medical applications and supramolecular chemistry.
Acknowledgments We gratefully acknowledge the financial support from the Natural Science Foundation of China (NSFC, 21531004 and 21472086) and the Major State Basic Research Development Program of China (2013CB922100).
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Review
Transition metal-induced B–H functionalization of o-carborane Xiaolei Zhang a,b, Hong Yan a,⇑ a State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Jiangsu Key Laboratory of Advanced Organic Materials, Nanjing University, Nanjing, Jiangsu 210023, China b School of Pharmaceutical Sciences, Jiangnan University, Wuxi, Jiangsu 214122, China
a r t i c l e
i n f o
Article history: Received 29 August 2017 Received in revised form 3 November 2017 Accepted 5 November 2017 Available online xxxx In memory of Professor Bernd Wrackmeyer. Keywords: B–H functionalization Carborane Half-sandwich complex Mechanism
a b s t r a c t Transition metal-mediated B–H functionalization of carboranes has drawn increasing interests during the last two decades. This review describes how to incorporate B–H functionalization into the derivation of half-sandwich transition metal complexes containing o-carborane dichalcogenolates. The detailed mechanistic elucidations for metal-induced versatile B–H functionalization are also reviewed. Examples of selective B–H activation, stepwise B–H functionalization, cobalt-mediated B–H functionalization as well as dimetal-mediated B–H activation of o-carborane are discussed. Ó 2017 Elsevier B.V. All rights reserved.
Contents 1. 2. 3. 4.
5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis of half-sandwich 16e complexes bearing o-carborane dichalcogenolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactivity of 16e cobalt half-sandwich complex towards dimethyl but-2-ynedioate, methyl diazo acetate and tosyl azide . . . . . . . . . . . . . . . . . Functionalization of B–H bond of o-carborane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Selective B–H functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Mechanistic implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Stepwise B–H functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Cobalt-mediated B–H activation and functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Dimetal-mediated B–H activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Dicarba-closo-dodecaboranes (C2B10H12) are a class of icosahedral clusters composed of boron, carbon and hydrogen atoms (Scheme 1) [1–4]. Owing to their useful properties such as rigidity, robustness, hydrophobicity, enriched boron content and delocalized three-dimensional aromaticity, a lot of potentials or practical applications have been explored in energy, catalysis, medicine and materials [5–14]. The carboranyl units, as highly structural analogs ⇑ Corresponding author. E-mail address: [email protected] (H. Yan).
00 00 00 00 00 00 00 00 00 00 00 00
of aromatic hydrocarbons, can serve as rigid scaffolds upon which to build molecules with well-defined, three-dimensional conformations [15–20]. A wide range of C–H and B–H functionalized carborane derivatives have been prepared under different reaction conditions. Owing to the significantly different electronic properties of the carbon and boron substituting effects (electronwithdrawing for carbon substitution and electron-donating for boron substitution) [21,22], it is especially interesting to study the properties of the boron-substituted carboranes in comparison to the carbon-substituted ones. The electronic deficient property of the carborane cage and the stronger electronegativity of the carbon atoms in the carborane
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Scheme 1. Molecular geometry of o-, m-, p-dicarba-closo-dodecarboranes, IUPAC numbering of the cage atoms and their chemical transformations.
cluster cause acidic nature of the C–H bonds and hydridic nature of the B–H bonds around the icosahedral sphere. In sharp contrast to the functionalization of carboranes at the carbon site [23–29], the B–H functionalization is much more challenging owing to the less acidic nature and the chemo-similarity of the ten B–H bonds [30– 35]. Therefore, selective B–H bond activation and functionalization of carboranes has drawn increasing interests during the last two decades [36–44]. Notably, a wide range of o-carborane-based organometallic complexes containing half-sandwich metal moieties (Co, Rh, Ir, Ru and Os) have been prepared through transition metal-induced B–H bond activation and functionalization. Half-sandwich complexes of group VIII transition metals bearing 1,2-dicarba-closo-dodecarborane-dichalcogenolates (ocarborane dichalcogenolates) are a versatile class of organometallic compounds. Similar to the two-dimensional super-aromatic analog of the benzene counterpart [45], o-carborane dichalcogenolates can serve as excellent scaffolds for the construction of metal dichalcogenolate complexes with unique chemical and physical properties. They have the advantages of accessibility, robustness, chemo-stability, tunable hydrophilicity or hydrophobicity that allow their applications in catalysis, supramolecular chemistry, as well as biological field [46–50]. The chemistry of o-carborane dichalcogenolates in coordinative self-assembly [51], metal–metal bonding formation [52] and B–H activation [53], has been developed, which closely links the areas of polyhedral boranes and organometallic chemistry. Here, we presented in this review some recent advances on the metal-induced B–H bond activation and functionalization by the reactions of half-sandwich metal complexes and organic reagents. Various analytical tools such as X-ray crystallography, NMR spectroscopy and high-resolution mass spectroscopy were used to confirm B–H activation and study reaction routes. Based on our systematic research work in this area during the last two decades, this review will mainly focus on several key points in the very recent investigation of B–H function-
alization including selective and stepwise B–H functionalization, cobalt-mediated B–H functionalization and dimetal-mediated B–H activation.
2. Synthesis of half-sandwich 16e complexes bearing ocarborane dichalcogenolates A series of group VIII metal (Ru [54], Os [54], Co [55], Rh [56] and Ir [57]) complexes bearing o-carborane 1,2-dichalcogenolate ligands have been synthesized. Our recent research advances [58–61] are the generation of a new kind of half-sandwich transition metal complexes containing a 1,2-dicarba-closododecarborane-9,12-dithiolate (o-carborane 9,12-dithiolate) in sharp contrast to the previously reported ones with o-carborane 1,2-dichalcogenolates. o-Carborane 1,2-dichalcogenolates are readily prepared by insertion of elemental chalcogen (S or Se) into the Li–C bond of dilithium carborane after deprotonation by nbutyllithium (Scheme 2) [25]. 1,2-Dicarba-closo-dodecarborane9,12-dithiol is generated by the Friedel-Crafts reaction of o-carborane with excess sulfur powder under the catalysis of anhydrous aluminum chloride at 160 °C (Scheme 2) [62]. o-Carborane 1,2-dithiolate exhibits electron poorer property at the sulfur atoms than that in o-carborane 9,12-dithiolate, as the carborane cage of electron-withdrawing nature in the former, whereas the carborane cage is of the electron-donating nature in the latter [21,22]. Therefore, the different attachment of sulfur atom to the boron or carbon site of o-carborane offers the possibility to adjust electrondonating or withdrawing property without altering the steric property of the ligand. The synthesis of 16e half-sandwich Ru, Os, Co, Rh, Ir complexes (1–10) based on dilithium o-carborane 1,2-dichalcogenolate Li2E2C2B10H10 (E = S, Se) has been previously reviewed [63]. Herein, we compared these typical 16e half-sandwich complexes with the
Scheme 2. Synthesis of o-carborane 9,12-dithiolate and 1,2-dichalcogenolates.
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Scheme 3. Synthesis of 16e half-sandwich Ru, Os, Co, Rh, Ir complexes (1–14) containing an o-carborane 1,2-dichalcogenolate or an o-carborane 9,12-dithiolate.
newly developed ones containing an o-carborane 9,12-dithiolate (Scheme 3). The reaction of o-carborane-9,12-(SH)2 with 2 equiv. Et3N in CH2Cl2 at room temperature leads to o-carborane 9,12dithiolate. Then, the in situ treatment with metal moieties such as [Cp*MCl(l-Cl)]2 (M = Rh, Ir), and [Cp#Co(CO)I2] (Cp# = C5H5, C5Me5) gives rise to the corresponding boron-substituted 16e half-sandwich complexes [Cp*M(9,12-S2C2B10H10)] (M = Rh (11), Ir (12)) [58,59] and Cp#Co(9,12-S2C2B10H10) (Cp# = C5H5 (13) C5Me5, (14)) [60,61], respectively. The red complex CpCo(9,12S2C2B10H10) (13) is only characterized by NMR and MS-ESI data owing to its less stability in solution. The steric hindered pentamethylcyclopentadienyl (Cp*)-coordinated species Cp*M(9,12S2C2B10H10) (M = Co, Rh, Ir) (11, 12, 14) exhibit enhanced stability for further isolation and characterization. The molecular structure
of Cp*Ir(9,12-S2C2B10H10) (12) can be compared with its structural analog Cp*Ir(1,2-S2C2B10H10) (3) (Fig. 1). Both complexes show pseudo aromatic carbon fused or boron-fused MS2C2/MS2B2 metallacycles with coordinatively and electronically unsaturated property at the metal centers [57,59].
3. Reactivity of 16e cobalt half-sandwich complex towards dimethyl but-2-ynedioate, methyl diazo acetate and tosyl azide In spite of the coordinatively unsaturated metal center and the nucleophilic behavior of chalcogen atom, these neutral 16e halfsandwich complexes act as active organometallic species and react with small organic molecules at the metal–chalcogen (M–E) bonds
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Fig. 1. Molecular structures of Cp*Ir(1,2-S2C2B10H10) (3) (left) and Cp*Ir(9,12S2C2B10H10) (12) (right) (Hydrogen atoms are omitted for clarity).
(Scheme 4). For example, the mononuclear 16e dithio-o-carboranyl cobalt complex CpCo(1,2-S2C2B10H10) (7) readily reacts with one equiv. of dimethyl but-2-ynedioate to form the alkene inserted product CpCo(CO2MeC@CCO2Me)(1,2-S2C2B10H10) (15) [64]. This trend of forming Co–S bond inserted complexes also occurs in the reactions of diazo compounds or azides. The reaction of complex 7 and methyl diazo acetate or tosyl azide in dichloromethane gives rise to the corresponding product 16 [65] or 17 [66], respectively. These direct insertion reactions indicate the tendency of the metal center to form an electronically saturated 18e configuration. Furthermore, the formation of the C–S and N–S bonds at the ocarboranyl thiolate is accompanied by the generation of a M S bond. This provides the possibility for B–H activation since the weak M S coordination bond makes the central structure flexible and offers the metal to approach the B–H bond. To investigate the potential and scope of this kind of reactivity, systematic investigations of reaction conditions and mechanisms for B–H activation have been conducted based on half-sandwich transition metal o-carborane dichalcogenolate precursors.
Scheme 4. Reactivity of CpCo(1,2-S2C2B10H10) (7) towards dimethyl but-2-ynedioate, methyl diazo acetate and tosyl azide.
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Scheme 5. Reactivity of Cp*Rh(1,2-S2C2B10H10) (1) and Cp*Rh(1,2-Se2C2B10H10) (2) towards HC„CC(O)OMe to afford 18–21.
4. Functionalization of B–H bond of o-carborane 4.1. Selective B–H functionalization Selective B–H mono- or di-functionalization of o-carborane dichalcogenolates can be achieved by reactions of the 16e halfsandwich complexes containing o-carborane dichalcogenolates with alkynes [67–73]. As shown in Scheme 5, Cp*Rh(1,2S2C2B10H10) (1) reacts with methyl propiolate (HC„CC(O)OMe) in boiling CHCl3 to give 18 containing two vinyl groups substituted at the B(3)/B(6) sites of the o-carboranyl unit (Scheme 5) [53]. In
18, as expected for a 16e species, a planar rhodadithiolene fivemembered ring is present (Fig. 2). The reactivity of 2, as a structural analog of 1, with methyl propiolate has been proven slower in boiling CHCl3. The product has been isolated and characterized to be compound 21 with a methylene group substituted at the B (3) site (Scheme 5) [67]. In particular, two reactive intermediates 19 and 20 were isolated as structural isomers if the reaction was conducted at room temperature. Each contains a Rh–B bond, and can not only transform to the thermodynamically stable product 21 (Fig. 2), but also transmute to each other between the trans and cis configuration. Complex 20 takes a cisoid arrangement
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Fig. 2. Molecular structures of 18 (left) and 21 (right) (Hydrogen atoms are omitted for clarity).
Scheme 6. Proposed mechanism for Rh-induced B–H functionalization of o-carborane-1,2-dithiolate.
between the C(1)–B(6) and the g-SeC(CO2Me)@CH2 bonds, in contrast to a transoid arrangement of the corresponding units in 19. Although only complex 20 possessed the correct stereochemistry to afford 21 by an intramolecular rearrangement, neither 19 nor 20 were left after the individual was heated in boiling CHCl3. 4.2. Mechanistic implications The proposed mechanism for the reaction of 1 with HC„CC(O) OMe is illustrated in Scheme 6 [53,67]. Species A is generally considered as the first intermediate, generated by initial coordination of the alkyne to the 16e metal center. Then alkyne insertion into one of the M–S bonds is followed up to form intermediate B. If the coordinative S ? M bond is weak enough, for example in C, the Rh center enables to approach the carborane cage to activate the B–H bond (D), followed by the formation of M–B bond and the B–H hydrogen migration to the olefinic carbon atom (E). The cleavage of the M–B bond in E leads to a C–B bond to generate the 16e complex F. The repetition of the sequence from A to E leads
to complex 18 with boron-disubstitution in the B(3) and B(6) positions. 4.3. Stepwise B–H functionalization The 16e iridium complexes 3 and 4 are less reactive than their rhodium analogs. This gives a chance to study the stepwise substitution of the carborane cage at B(3/6) site (Scheme 7) [73]. Both 3 and 4 react with 1.0 equiv. of HC„CC(O)OMe at 110 °C to give both Z- and E-configurations of the 16e complexes (22 and 24) and (23 and 25). The carborane cage is selectively substituted in the B(3) position. The mono-substituted 16e complexes of 22–25 are able to take reaction with another equivalent of HC„CC(O)OMe at 110 °C in toluene to afford B(3)/B(6) disubstituted products (26– 31) with all possible Z and E configurations (Fig. 3). The 16e complex 7 with a cobalt core is more reactive towards alkynes than its analogous rhodium and iridium species. Complex 7 took reaction with MeO2C–C„C–CO2Me in CH2Cl2 at room temperature to afford the alkyne insertion complex 15 (Scheme 8) [74].
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Scheme 7. Stepwise controlled reactions of Cp*Ir(1,2-E2C2B10H10) [E = S (3), Se (4)] with HC„CC(O)OMe leading to selective and stepwise functionalization of o-carborane dichalcogenolates.
Fig. 3. Molecular structures of 23 (left) and 29 (right) (Hydrogen atoms are omitted for clarity).
Upon heating to 70 °C in toluene, 15 rearranges to 32 and 33 in 1:1 ratio. Both complexes are geometrical isomers with boronsubstitution at the carborane cage. The formation of 32 follows well-documented process via transition metal-induced B–H activation. Owing to electron deficient nature at the cobalt core, a subsequent reaction of complex 32 with MeO2C–C„C–CO2Me at room temperature led to 34. Experimental results reveal that the presence of one boron substitution can accelerate the reactivity of the 16e metal center in 32. The configurations of the two olefinic substituents take trans (E) arrangement in 34, in contrast to the
cis (Z) configuration in 32 (Fig. 4). This strategy of stepwise and selective functionalization on carborane can be extended to other alkynes, providing a facile route to the B-functionalized ocarborane derivatives. The reaction of the 16e cobalt complex 7 towards alkynones (HC„C–C(O)Ph) in a 1:1 ratio at room temperature lead to B(3)–H activation and one of the products is monosubstituted complex 35. 35 is still a 16e half-sandwich complex, thus it can further react with HC„C–C(O)Ph or HC„C–CO2Me at room temperature, leading to 36 or 37, respectively. Again, both complexs 36 and 37 are still 16e half-sandwich complexes with
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Scheme 8. Reactivity of CpCo(1,2-S2C2B10H10) (7) towards MeO2C–C„C–CO2Me and HC„C–C(O)Ph.
B(3,6)-disubstitution at the o-carborane1,2-dithiolate ligand. Similar functionalization model can be extended to MeO2C–C„ C–CO2Me, which leads to stepwise formation of 38 and the final stable B(3,6)-disubstituted product 39 [75].
4.4. Cobalt-mediated B–H activation and functionalization As indicated in the aforementioned context, the 16e halfsandwich cobalt complex CpCo(1,2-S2C2B10H10) (7) exhibits the
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Fig. 4. Molecular structures of 32 (left) and 34 (right) (Hydrogen atoms are omitted for clarity).
Scheme 9. Three-component reactions of 7, HC„CC(O)OMe and organic molecules a–g; the ligand substitution leads to 40 or 41 from 43, 44 and 45 upon the addition of a or b.
Fig. 5. Molecular structures of 40 (left) and 45 (right) (Hydrogen atoms are omitted for clarity).
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Scheme 10. Proposed mechanism on the formation of 40.
Fig. 6. Molecular structures of 46a (R = Me, up-left), 47d (R = Fc, up-right), 48a (R = Me, down-left), 54bd (R = Ph, R0 = Fc, down-right) (Hydrogen atoms are omitted for clarity).
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Scheme 11. Generation of o-carboranyl-norbornyl derivatives through cobalt-induced B–H activation.
highest chemical reactivity towards alkynes compared with its ruthenium, osmium, rhodium and iridium analogs [76–83,66]. Considering low cost of cobalt starting compounds and the fact that boron-functionalization of o-carborane can occur at room temperature by using cobalt, efficient methods have been established through cobalt-mediated B–H activation to generate B-functionalized carboranes, such as B-cyclopentadienyl, Bnorbornadienyl and B-norbornyl species [84–86]. These novel compounds are unavailable through conventional routes. The reactions of 7 with alkynes and 3e-donor ligands led to facile B–C coupling between carboranyl and cyclopentadienyl at ambient temperature (Scheme 9) [84]. Particularly, the addition of a to the mixture of 7 and HC„CC(O)OMe in dichloromethane afforded 40, featuring a linkage between the carboranyl cage and the Cp unit via a covalent B–C bond (Fig. 5). A series of small organic molecules have been examined to understand their role in the formation of 40–45 (Scheme 9). Ligand b was able to lead to 41, which is the analog of 40. The weakly coordinative ligands c and d could also give rise to 42 with a C–B bond, but the ligand does not exist in the structure. Allenes (e and f) could also afford the similar products 43 and 44 with a 3e-donor allyl unit. Cyclopentadiene (g), containing a 3e-donor o-carborane dithio ligand in situ-generated from a [4+2] Diels–Alder cycloaddition, could also lead to a B–C coupled complex (45). The coordination ability of the carborane-based bulky dimercapto ligand in 45 is weaker and can be replaced by the stronger ones a or b to quantitatively furnish 40 or 41. Similarly, 43 and 44 can be readily
transformed to 40 or 41 upon treatment with the stronger ligand a or b, but 42 remains unreactive, demonstrating the different binding ability of these organic ligands. A plausible mechanism for the generation of 40 is shown in Scheme 10. The B–H activation at B(3) site of carborane and hydrogen transfer to the alkyne should take place as the earlier steps (I–III) after alkyne insertion into one Co–S bond. The chelating coordination of a would induce the binding mode conversion of Cp from g5 to g1 (IV), followed by a–H elimination (V), Co–C and Co–B cleavage and B–C formation as well as Cp switching back to g5 to lead to 40 [84]. The 16-electron complex 7 reacts with alkynones such as HC„CC(O)R (R = methyl (Me), phenyl (Ph), styryl (St), ferrocenyl (Fc)) at ambient temperature to give two novel types of 17electron cobalt complexes 46a–d and 47a–d (Scheme 11) [87,88]. Both contain B(3)/B(6)-norbornyl carborane structures. Furthermore, these 17-electron complexes can serve as precursors for the synthesis of various B-functionalized carboranes. Under the condition of air, moisture and silica, complexes 46a–d took place alkyl C–S cleavage to form 16-electron complexes 48a–d with a B-norbornadienyl unit and 49a–d containing an organic tricyclic moiety. However, 48a–d cannot be transformed to 49a–d in the presence of water. Furthermore, complexes 46a–d can also undergo carboranyl C–S cleavage to produce metal-free species 50a–d containing a B-norbornyl unit (Fig. 6). Compounds 46a–d and 47a–d can take place further alkyne insertions into the Co–S bond to furnish the cobalt-free B-norbornyl carborane derivatives (Z/E)-51 and (Z/E)-52, which contain a vinyl sulfido group.
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Scheme 12. Energy profile for the generation of 46. The related free energy and the electronic energy (in the bracket) for the reaction intermediate or transition state are given in kcal/mol.
Moreover, thermal promotion or Lewis acid mediation of (Z/E)-51 can lead to retro-Diels–Alder products with differing B-norbornyl groups and excellent selectivity. Addition of AlCl3 promotes the conversion of 50 to 53 as predominant retro-Diels–Alder products. The isomerization from E to Z-configuration on the vinyl group and reorganization of the norbornyl unit of (Z/E)-51 lead to (Z)-53 after heating. As mentioned above, the 17e cobalt(II) complexes (46a–d) are the key intermediates to transform to the products in Scheme 11. A formation mechanism of 46a–d via a tandem sequence of processes such as metal-induced B–H activation, the coordination change of Cp ligand, boron–Cp linkage, and Diels–Alder reaction are proposed based on DFT calculations. The calculated energy profile is shown in Scheme 12 [86]. The reaction of 7 with 1.0 equiv. ethyl diazoacetate (EDA) at room temperature gives rise to 55 (Fig. 7), in which one EDA molecule inserted into one Co–S bond to form a three-membered metallacyclic ring (Scheme 13) [65]. Compound 55 can take further reaction with excess amount of EDA to generate a series of boron-functionalized organometallic species 56–60 (Scheme 13). Both of 56 and 57 contain a stable Co–B bond. Particularly, the dual activation of B(3)–H and B(4)–H bonds of the carborane cage has been observed in 56. One EDA insertion into the Co–B bond leads
Fig. 7. Molecular structures of 55 (Hydrogen atoms are omitted for clarity).
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to the formation of a C–B bond in 58. Another EDA insertion into the Co–S bond in 58 gives rise to 59. Upon heating, deboronation is observed for 59, leading to 60 containing a nido-C2B9 unit. Therefore, the cobalt-induced alkylation of o-carborane 1,2-dithiolate can be realized by using organic diazo compounds [65,87]. The three-component reactions of CpCo(S2C2B10H10)(7), ethyl diazoacetate (EDA) and alkynes at ambient temperature led to 61–64 (Scheme 14) [87], in which one alkyne is stereoselectively inserted into the Co–B bond. At ambient temperature, 61–64 undergo rearrangement to 65–68 by migratory insertion of the inserted EDA moiety. If weak base is present, 61–64 can lose an apex BH to give rise to complexes 69–71. Particularly, the reactions of complex 7, diazo ester and 1,6-diyne [PhN(CH2C„CH)2] was investigated. A pyrrole ring is formed in 74 by in situ ring-closure
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of the 1,6-diyne, along with the formation of the B–H activated species 72 and 73 [88].
4.5. Dimetal-mediated B–H activation In contrast to the widely developed o-carborane-1,2-dithiolate for the generation of dinuclear metal clusters, o-carborane-9,12dithiolate is also suitable for the generation of dinuclear rhodium complexes 75–77 (Scheme 15). The 16e half-sandwich rhodium complex [Cp*Rh{9,12-S2C2(B10H10)}] (Cp* = g5-C5Me5) (11) takes reaction with Rh(PPh3)3Cl in the presence of NH4PF6 to deliver a cationic dinuclear complex 75. Deboronation of 75 under basic conditions affords complex 76 containing a nido-carborane unit.
Scheme 13. Reaction of CpCo(1,2-S2C2B10H10) (7) with ethyl diazoacetate (EDA).
Scheme 14. Three-component reactions of CpCo(1,2-S2C2B10H10) (7), EDA and alkynes.
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Scheme 15. Preparation of dirhodium complexes with o-carborane-9,12-dithiolate.
Fig. 8. Molecular structures of 79 and 81 (Hydrogen atoms are omitted for clarity).
One bulky PPh3 in 75 can be replaced by CO, leading to the carbonyl product 77 containing an agnostic Rh–H–B interaction [58].
Both dinuclear rhodium complexes 76 and 77 can lead to B–H activation under mild conditions. Treatment of complex 76 or 77 with 1.0 equiv. of FcPF6 in CH2Cl2 at ambient temperature gave rise
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Scheme 16. Dinuclear rhodium mediated B–H bond activation, B–Cl bond and B–O bond formation.
to complex 78 or 79 containing a Rh–B bond and a l2-bridged Rh– Cl–Rh bond (Scheme 16). Furthermore, the reductive elimination of 78 or 79 affords products 80 or 81 in high yields (>80%) to generate a B–Cl bond and a normal Rh–Rh bond (Fig. 8). Consistent with the structural analysis, a DFT calculation also suggests much weaker Rh–Rh interaction in 79 than those in 77 or 81, which demonstrates the dimetal mediated B–H activation by fitting the Rh–Rh distance which is restored after B–Cl reductive elimination. The reaction of 77 towards CH3OH or water at 60 °C followed by bubbling CO led to B–H activation and B–O coupling as shown in 82 or 83 with the delivery of dihydrogen (Scheme 16). A plausible mechanism was proposed in Scheme 17 by taking 76 as an example. Metal-induced B–H bond activation by an agnostic interaction should be in the earlier step. Then a M–B bond is formed and the B–H hydrogen becomes a bridging hydride between the two metal centers (II) (a Rh–H–Rh bond is more stable than a terminal Rh–H bond). The OH moiety of the coordinating substrate H2O takes the position of the bridging hydrogen in II, accompanied by release of
H2 to generate species III. After reductive elimination, the product containing a B–O bond (83) is generated (Scheme 17).
5. Conclusion and outlook In the very recent years, considerable contributions have been made on transition metal-induced B–H activation and functionalization of o-carborane. This approach provides an unconventional way for the generation of B-substituted carborane derivatives. The detailed mechanistic investigations offer an insight into design of new types of B–H functionalization. Given the mild reaction conditions and interesting structures of products, it is anticipated that transition-metal induced B–H activation is a promising approach for generation of new types of boron-substituted carborane derivatives. The extensive studies on the synthetic methodologies of carborane system reinforce the relationship between inorganic boron clusters and organic synthetic methods, which may ignite
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Scheme 17. Proposed reaction mechanism for B–O bond formation via dimetal-induced B–H activation.
versatile potential applications of carboranes in material science, medical applications and supramolecular chemistry.
Acknowledgments We gratefully acknowledge the financial support from the Natural Science Foundation of China (NSFC, 21531004 and 21472086) and the Major State Basic Research Development Program of China (2013CB922100).
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