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Catalytic Regioselective Cage B(8)-H Arylation of o-Carboranes via “Cage-Walking” Strategy Hairong Lyu, Jie Zhang, Jingting Yang, Yangjian Quan, and Zuowei Xie J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00302 • Publication Date (Web): 24 Feb 2019 Downloaded from http://pubs.acs.org on February 24, 2019

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Journal of the American Chemical Society

Catalytic Regioselective Cage B(8)-H Arylation of o-Carboranes via “Cage-Walking” Strategy Hairong Lyu, Jie Zhang, Jingting Yang, Yangjian Quan* and Zuowei Xie* Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The Chinese University of Hong Kong, Shatin, N. T., Hong Kong, China

Supporting Information Placeholder ABSTRACT: A proof-of-concept example of catalytic

regioselective cage B(8)-H functionalization of o-carboranes has been disclosed for the first time. Under the help of an acylamino directing group at cage B(3), a series of B(8)arylated, B(4,7,8)-triarylated and B(4,7,8)-trifluorinated ocarborane derivatives were conveniently prepared. On the basis of isolation of a key intermediate, deuterium labelling experiments and DFT calculations, a reaction mechanism involving a high-valent palladium induced “cage-walking” from B(4) to B(8) vertex is proposed to account for the regioselective B(8)-H activation.

Carboranes, a class of boron-carbon molecular clusters, possess extraordinary properties including icosahedral geometry, three-dimensional aromaticity conjugated by σbonds, and inherent robustness.1 These endowments render carboranes remarkable and valuable molecular building blocks for applications ranging from functional materials to pharmaceuticals.2,3,4 In this connection, many efforts have been devoted to the development of efficient synthetic routes for the selective functionalization of carboranes, especially for those concerning selective cage BH derivatization. Vertex-specific functionalization among ten very similar B-H bonds is challenging yet critical to effectively construct carborane-featuring functional molecules. Our group and others

have recently developed transition metal catalyzed vertexspecific functionalization of o-carboranes at most BH vertices5,6,7 except cage B(8)-H (see Scheme 1 for numbering system),8,9,10 since the difference in electronic density between B(8) and B(9) is extremely small. We thought a suitable directing group at B(3) position may lead to electrophilic cage B(8)-H metalation preferentially according to our previous work,9a as B(8)-H bond is more electron-rich than B(4)-H, so as to achieve selective and direct cage B(8)-H functionalization. In fact, our mechanistic studies show that the initial B-H palladation occurs at the B(4)-H bond, followed by “cage-walking”11,12 of the Pd center to the B(8)position to realize direct B(8)-arylation. The higher thermodynamic stability of B(8)-substituted Pd(IV)intermediate is considered to provide the driving force for such migration. Detailed results are reported in this Communication (Scheme 1).

Table 1 Optimization of reaction conditions for cage B(8)-H arylationa O

O Ph-I (2a, 1.2 equiv)

HN H

H H 1a

Pd(OAc)2 (5 mol%) Cu(OTf)2 (0.5 equiv) Ag2CO3 (1.5 equiv) 1,2-DCE, 60 oC, 18 h 'standard' conditions

HN Ph

H H

3a, 92% (87% isolated)

Entry

Variations from the ‘standard’ conditions

3a (%)b

Scheme 1 Regioselective and direct cage B-H functionalization of o-carboranes

1

Without Pd(OAc)2

0

2

Without Cu(OTf)2

0

Previous work:

3

Cu(OTf)2 (0.2 equiv)

87

4

Cu(OTf)2 (1.0 equiv)

93

5

Cu(OAc)2 instead of Cu(OTf)2

0

6

Toluene instead of 1,2-DCE

50

7

r.t. instead of 60 oC

trace

8

80 oC instead of 60 oC

91

9

Without Ag2CO3

26

10

AgOAc (3.0 equiv) instead of Ag2CO3

86

Direct and selective cage B(3)-H, B(4)-H or B(9)-H functionalization R1 R2

FG cat.

R1 FG R2 FG = functional groups

numbering system 4 9

5

3

8

12

1

6

10

7

2

other

C B BH

11

This work: The first direct and selective B(8)-H functionalization Ar/F R1 O cat. [Pd] Ar/F N + R2 xs Ar-I or F H Ar/F R1 B(4,7,8)-H triarylation or trifluorination

R1 O cat. [Pd] N Ar-I R2 H R1

R1 O Ar N R2 H R1 B(8)-H arylation

aReactions

were conducted at 0.05 mmol scale in 1.5 mL of

ACS Paragon Plus Environment

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solvent in a closed flask; 1,2-DCE = 1,2-dichloroethane. bGC yield.

Scheme 2 Control experiments a)

O NH

1. Pd(OAc)2 (1.0 equiv), Cu(OTf)2 (1.0 equiv), MeCN, r.t., 10 h

HN H

N

2. 1,10-phenanthroline (2.0 equiv)

O

B'

Ph-I (2a, 1.2 equiv) Cu(OTf)2 (0.5 equiv)

Pd N

1b b)

O

67% (LPdII-B(4), B') c) O

HN

Ag2CO3 (1.5 equiv) 1,2-DCE, 60 oC, 18 h

H

1b

Cu(OTf)2 (0.5 equiv) Ag2CO3 (1.5 equiv) 1,2-DCE, 60 oC, 18 h

no reaction


B'

O HN Ph

O

Ag2CO3 (1.5 equiv) 1,2-DCE, 60 oC, 18 h

HN D

R H H

H 1c-d4

O

Ph-I (2a, 1.2 equiv) Pd(OAc)2 (5 mol%) Cu(OTf)2 (0.5 equiv) Ag2CO3 (1.5 equiv) 1,2-DCE, 60 oC, 5 h R = p-Cl-C6H4

HN Ph D (93%)

O

HN Ph

HN Ph D

25% (51% b.r.s.m.) (3b)

f)

3b

85% (3b) O

D2O (2.0 equiv) Ph-I (2a, 1.2 equiv) Cu(OTf)2 (0.5 equiv)

e)

d)

cation in B'

B' (5 mol%) Ph-I (2a, 1.2 equiv)

HN

Ph

29% (59% b.r.s.m.) (3b)

B'

[OTf]

not observed

numbering system

R H H

3 8

7

4

1

9

2

11

12

5 10

6

other

C B BD BH

57% (3c-d4)

g)

cation in LPdII-B(8) (C', 0.7 kcal/mol)

cation in LPdII-B(4) (B', 0.0 kcal/mol)

We chose 3-acetylamino-o-carborane (1a) as the model compound to evaluate the feasibility of directed cage B(8)-H functionalization, due to its convenient preparation. Under the ‘standard’ conditions, reaction of 1a with iodobenzene (2a) afforded the target 3-acetylamino-8-phenyl-o-carborane (3a) in 92% yield with excellent regioselectivity (Table 1). Both catalyst Pd(OAc)2 and additive Cu(OTf)2 were proved very essential to produce 3a (entries 1 and 2, Table 1).13 Lowering the loading of Cu(OTf)2 to 0.2 equiv led to a decreased yield of 3a, while replacement of it by Cu(OAc)2 gave no desired arylation product (entries 3 and 5, Table 1). Screening of other solvents or temperatures did not offer better results (entries 6-8, Table 1). In the absence of Ag2CO3, the yield of 3a was dramatically dropped to 26%, suggesting that the silver salt might act as an efficient agent for iodide capture to regenerate active Pd-catalyst (entry 9, Table 1).

cation in LPdIV-B(8) (E', 0.0 kcal/mol)

cation in LPdIV-B(4) (D', 5.1 kcal/mol)

To understand the reaction pathway, several control experiments were carried out (Scheme 2). The stoichiometric reaction of 1,2-dimethyl-3-acetylamino-o-carborane 1b with Pd(OAc)2 and Cu(OTf)2, followed by the addition of 2.0 equiv of 1,10-phenanthroline, afforded a key Pd(II)-B(4) intermediate B’, rather than the hypothesized Pd(II)-B(8) one C’ (Scheme 2a). Single-crystal X-ray analyses unambiguously confirm the molecular structure of B’, in which the Pd atom is σ-bonded to the B(4) atom and coordinated to the oxygen atom from acetylamino group and two nitrogen atoms from phenanthroline in a square-planar geometry. It was noted that such kind of B-H palladation intermediate could not be isolated in the absence of 1,10phenanthroline ligand. Treatment of B’ with 1.2 equiv of iodobenzene in DCE at 60 oC for 18 h generated the B(8)phenylated carborane 3b in 29% yield as a sole product (Scheme 2b). The low conversion may be ascribed to the


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To verify our hypothesis of the aforementioned “cagewalking” process, B(8,9,10,12)-tetradeuterated carborane 1cd4 was synthesized. Treatment of 1c-d4 with iodobenzene under the reaction conditions shown in Scheme 2f gave the corresponding B(8)-phenylated product 3c-d4 in 57% yield with 93% B(4)-deuteration, in which the deuterium at B(4) was considered to originate from B(8)-D via the concerted Pd/D-migration process. The formation of 3c-d4 was confirmed by multinuclear NMR spectroscopy (see Figures S6-9 in the SI for detail), which provided a solid evidence for the involvement of “cage-walking” in the reaction. The driving force of such “cage-walking” phenomenon was further elucidated by DFT calculations (Scheme 2g). Although the difference in calculated free energies between B’ and C’ was small, the corresponding Pd(IV)-B(8) intermediate E’10b was significantly more stable than its Pd(IV)-B(4) isomer D’ by 5.1 kcal/mol, due probably to the stronger electron donating ability of the B(8) atom that benefited the stabilization of highly electron-deficient Pd(IV) center. In other words, the generation of electron-deficient high-valent palladium species would serve as the driving force to induce the “cage-walking” from electron-deficient cage BH vertex to the more electron-rich one.

Pd(OAc)2 + Cu(OTf)2 1

3 + AgI Pd(OAc)(OTf)

R2

pathway c

NH

O I

R2

AgOAc

Ar

TfO

ArI [OTf]

NH R1 R1

H

R2

LPdIV-B(8) (E)

A NH

O

R1 R1

[Pd]

Ag2CO3 [OTf]

LPdII-B(8) (C) R2

AgOAc + AgHCO3

R2 NH

NH

O

O

R1 R1

Pd

I

O Pd

AcO R1 R1

Pd

b

presence of 1,10-phenanthroline. However, in the absence of oxidant iodobenzene,10b no reaction was observed (Scheme 2d). Under standard conditions, complex B’ catalyzed efficiently cage B(8)-H arylation, leading to 3b in a yield of 85% with excellent regioselectivity (Scheme 2c). Furthermore, no deuterium was observed to be incorporated into the B(4) position of the B(8)-arylated product 3b when 2.0 equiv of D2O was used as an additive to the reaction of B’ with iodobenzene (Scheme 2e). These results indicated 1) B’ is an important intermediate participating in the catalytic cycle; 2) the initial cage B-H palladation occurs preferentially at the B(4)H vertex over B(8)H one under the reaction conditions; 3) a migration of Pd-moiety from B(4) to B(8) should be involved in the reaction pathway to account for the generation of B(8)-arylated product from the B(4)palladation intermediate;11,14,15 and 4) the migration of Pdmoiety might follow a concerted process preferentially over a B-H metalation-protonation pathway.14d,15

ay w th pa

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Journal of the American Chemical Society

R1 R1

[Pd]

[OTf]

[OTf]

Ar

pathway a

LPdIV-B(4) (D)

LPdII-B(4) (B)

ArI

On the basis of experimental results and DFT calculations, a plausible reaction mechanism is proposed in Scheme 3. The acylamino group guided Pd(II)-induced B(4)-H activation generates an intermediate [LPdII-B(4)] (B). Subsequent oxidative addition of aryl iodide onto Pd(II) affords the intermediate [LPdIV-B(4)] (D).7c,10b A “cage-walking” occurs from B(4) to B(8) to give another intermediate [LPdIV-B(8)] (E), which undergoes reductive elimination to yield the B(8)arylated product 3 (pathway a). The silver salt serves as the iodide capture reagent to regenerate the active Pd(II)catalyst and finish the catalytic cycle. We tend to prefer pathway a, however, other two pathways (pathways b and c) cannot be absolutely ruled out.

Table 2 Synthesis of B(8)-arylated o-carboranesa O

O Ar-I (2, 1.2 equiv)

R2

HN

Pd(OAc)2 (5 mol%)

R1 R1

H

Scheme 3 Proposed mechanism

1

87% (3a)

Ph

R2

HN Ph

H H 82% (3e) O HN

76% (3m)

Cl

HN

83% (3d)

3i O HN

H H 84% (3n)

Ph H H

Ph

2 O R = CH3, 83% (3f) t Bu, 73% (3g) HN OCH3, 78% (3h) H Cl, 80% (3i) H F, 68% (3j) Br, 62% (3k) 3 Ph, 77% (3l) O

H H

aReactions

HN H H

83% (3c)

86% (3b)

O

O

HN

HN Ph

H H

3

O

O

HN

R2 R1 R1

Ar


O

Ph

HN

H H 85% (3o)

were conducted at 0.10 mmol scale in 2 mL of 1,2dichloroethane in a closed flask under argon (isolated yield).



The substrate scope of such B(8)-arylation was then investigated. Methyl groups at two cage carbons did not influence the yield and regioselectivity of this coupling reaction (3b, Table 2). Different acylamino directing groups were compatible to give the corresponding B(8)-phenylated products 3c, 3d and 3e in > 80% yields. Aryl iodides with electron-donating or electron-withdrawing substituents at the phenyl ring reacted smoothly, leading to the desired B(8)-arylated o-carboranes in 62-87% yields, and no obvious electronic effects were observed (3f to 3n, Table 2). This cross-coupling was tolerant of various functional groups including -OMe, -F, -Cl and -Br. 2-Iodo-naphthalene served as a competent coupling partner, giving the product 3o in a yield of 85%. As cage B(4)-H was initially activated under the standard reaction conditions (Scheme 2a), we envisioned multifunctionalization would be achieved by increasing the loadings of coupling reagents. After many attempts (see Table S2 in the SI for detail), the optimal reaction conditions were established to produce the target B(4,7,8)-triphenylated o-carborane 4a in 80% yield (Table 3). All three cage B-H bonds adjacent to the acetylamino directing group were facilely arylated in this one-pot process. Substrates bearing methyl or chloro groups were also compatible, giving the corresponding products 4b and 4c in 74% and 53% isolated yields, respectively. Compound 1d offered a yield of 77% for triphenylated product 4d. In addition, we examined the B(4,7,8)-trifluorination under the reaction conditions similar to that of our previously reported B(8,9,10,12)tetrafluorination.6a Accordingly, a yield of 76% for B(4,7,8)trifluorinated o-carborane 6a was obtained (Table 3).

Table 3 Synthesis carboranesa H R1 O N R2 H R1

H H 1

of

B(4,7,8)-trisubstituted

Ar-I (2, 6.0 equiv) Pd(OAc)2 (15 mol%) Cu(OTf)2 (1.0 equiv) Ag2CO3 (4.0 equiv) 1,2-DCE, 80 oC, 36 h

Ar R1 O N R2 H R1

Ar Ar 4

Ar Ar Ar

H H H

Ph

H O N H H

Ar = C6H5, 80% (4a) 4-Me-C6H4, 74% (4b) 4-Cl-C6H4, 53% (4c)

o-

H O N Ph H Ph H 77% (4d)

Ph

4c

F (5, 5.0 equiv) H O H O N F BF4N N F H H Pd(MeCN)4(OTf)2 (15 mol%) H F H o 1,2-DCE, 70 C, 36 h 1a 76% (6a)

aReactions

were conducted at 0.10 mmol scale in 2 mL of 1,2dichloroethane in a closed flask under argon (isolated yield). In summary, selective cage B(8)-H functionalization of ocarboranes has been realized via Pd-catalyzed direct B-H activation for the first time. Under the help of an acylamino directing group at cage B(3), a series of B(8)-arylated, B(4,7,8)-triarylated and B(4,7,8)-trifluorinated o-carborane derivatives were conveniently prepared, which cannot be synthesized by any other means. On the basis of experimental results and DFT calculations, a high-valent palladium induced “cage-walking” from B(4) to B(8) vertex is

proposed to account for the regioselective B(8)-H activation. This work not only provides new insights to the “cagewalking” process, but also opens up new avenues for the derivatization of boron clusters via “cage-walking” strategy.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, characterization of the products (PDF) Crystal structures (CIF)

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by grants from the Research Grants Council of The Hong Kong Special Administration Region (Project Nos. 14305017, 14305018 and 14305918) and Incentive Fund from Faculty of Science, CUHK.

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Journal of the American Chemical Society Metal Oxides, Nat. Mater. 2018, 17, 341. (h) Mukherjee, S.; Thilagar, P. Boron Clusters in Luminescent Materials. Chem. Commun. 2016, 52, 1070. (i) Li, X.; Yan, H.; Zhao, Q. Carboranes as a Tool to Tune Phosphorescence. Chem. Eur. J. 2016, 22, 1888. (j) Núñez, R.; Tarrés, M.; Ferrer-Ugalde, A.; de Biani, F. F.; Teixidor, F. Electrochemistry and Photoluminescence of Icosahedral Carboranes, Boranes, Metallacarboranes, and Their Derivatives. Chem. Rev. 2016, 116, 14307. (k) Guo, J.; Liu, D.; Zhang, J.; Zhang, J.; Miao, Q.; Xie, Z. oCarborane Functionalized Pentacenes: Synthesis, Molecular Packing and Ambipolar Organic Thin-Film Transistors. Chem. Commun. 2015, 51, 12004. (l) Grzelczak, M. P.; Danks, S. P.; Klipp, R. C.; Belic, D.; Zaulet, A.; Kunstmann-Olsen, C.; Bradley, D. F.; Tsukuda, T.; Viñas, C.; Teixidor, F.; Abramson, J. J.; Brust, M. Ion Transport across Biological Membranes by Carborane-Capped Gold Nanoparticles. ACS Nano 2017, 11, 12492. (m) Cioran, A. M.; Musteti, A. D.; Teixidor, F.; Krpetić, Z.; Prior, I. A.; He, Q.; Kiely, C. J.; Brust, M.; Viñas, C. Mercaptocarborane-Capped Gold Nanoparticles: Electron Pools and Ion Traps with Switchable Hydrophilicity. J. Am. Chem. Soc. 2012, 134, 212. (3) Selected references for applications in medicine: (a) Hawthorne, M. F. The Role of Chemistry in the Development of Boron Neutron Capture Therapy of Cancer. Angew. Chem., Int. Ed. 1993, 32, 950. (b) Hawthorne, M. F.; Maderna, A. Applications of Radiolabeled Boron Clusters to the Diagnosis and Treatment of Cancer. Chem. Rev. 1999, 99, 3421. (c) Armstrong, A. F.; Valliant, J. F. The Bioinorganic and Medicinal Chemistry of Carboranes: from New Drug Discovery to Molecular Imaging and Therapy. Dalton Trans. 2007, 4240. (d) Issa, F.; Kassiou, M.; Rendina, L. M. Boron in Drug Discovery: Carboranes as Unique Pharmacophores in Biologically Active Compounds. Chem. Rev. 2011, 111, 5701. (e) Scholz, M.; HeyHawkins, E. Carbaboranes as Pharmacophores: Properties, Synthesis, and Application Strategies. Chem. Rev. 2011, 111, 7035. (4) Selected references for applications in coordination chemistry: (a) Hosmane, N. S.; Maguire, J. A. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, 2007; Vol. 3, Chapter 5. (b) Xie, Z. Cyclopentadienyl−Carboranyl Hybrid Compounds: A New Class of Versatile Ligands for Organometallic Chemistry. Acc. Chem. Res. 2003, 36, 1. (c) Yao, Z.-J.; Jin, G.-X. Transition Metal Complexes Based on Carboranyl Ligands Containing N, P, and S Donors: Synthesis, Reactivity and Applications. Coord. Chem. Rev. 2013, 257, 2522. (d) Qiu, Z.; Ren, S.; Xie, Z. Transition Metal−Carboryne Complexes: Synthesis, Bonding, and Reactivity. Acc. Chem. Res. 2011, 44, 299. (e) Xie, Z.; Jin, G. X. Carborane Themed Issue Dalton Trans. 2014, 43, 4924. (f) Estrada, J.; Lavallo, V. Fusing Dicarbollide Ions with NHeterocyclic Carbenes. Angew. Chem., Int. Ed. 2017, 56, 9906. (g) ElHellani, A.; Lavallo, V. Fusing N-Heterocyclic Carbenes with Carborane Anions. Angew. Chem., Int. Ed. 2014, 53, 4489. (5) Selected examples for transition metal catalyzed B(3,6)-H functionalization: (a) Mirabelli, M. G. L.; Sneddon, L. G. TransitionMetal-Promoted Reactions of Boron Hydrides. 9. Cp*Ir-Catalyzed Reactions of Polyhedral Boranes and Acetylenes. J. Am. Chem. Soc. 1988, 110, 449. (b) Cheng, R.; Qiu, Z.; Xie, Z. Iridium-Catalysed Regioselective Borylation of Carboranes via Direct B–H Activation. Nat. Commun. 2017, 8, 14827. (c) Li, C.-X.; Zhang, H.-Y.; Wong, T.-Y.; Cao, H.-J.; Yan, H.; Lu, C.-S. Pyridyl-Directed Cp*Rh(III)-Catalyzed B(3)–H Acyloxylation of o-Carborane. Org. Lett. 2017, 19, 5178. (d) Quan, Y.; Xie, Z. Palladium-Catalyzed Regioselective Intramolecular Coupling of o-Carborane with Aromatics via Direct Cage B–H Activation. J. Am. Chem. Soc. 2015, 137, 3502. (6) Selected examples for transition metal catalyzed B(8,9,10,12)-H functionalization: (a) Qiu, Z.; Quan, Y.; Xie, Z. Palladium-Catalyzed Selective Fluorination of o-Carboranes. J. Am. Chem. Soc. 2013, 135, 12192. (b) Cao, K.; Huang, Y.; Yang, J.; Wu, J. Palladium Catalyzed Selective Mono-Arylation of o-Carboranes via B–H Activation Chem. Commun. 2015, 51, 7257. (c) Xu, T.-T.; Cao, K.; Wu, J.; Zhang, C.-Y.; Yang, J. Palladium-Catalyzed Selective Mono-/Tetraacetoxylation of o-Carboranes with Acetic Acid via Cross Dehydrogenative Coupling of Cage B–H/O–H Bonds. Inorg. Chem. 2018, 57, 2925. (7) Selected examples for transition metal catalyzed B(4,5,7,11)-H functionalization: (a) Quan, Y.; Xie, Z. Iridium Catalyzed

Regioselective Cage Boron Alkenylation of o-Carboranes via Direct Cage B–H Activation. J. Am. Chem. Soc. 2014, 136, 15513. (b) Quan, Y.; Tang, C.; Xie, Z. Palladium Catalyzed Regioselective B–C(sp) Coupling via Direct Cage B–H Activation: Synthesis of B(4)Alkynylated o-Carboranes. Chem. Sci. 2016, 7, 5838. (c) Quan, Y.; Xie, Z. Palladium-Catalyzed Regioselective Diarylation of o-Carboranes by Direct Cage B−H Activation. Angew. Chem., Int. Ed. 2016, 55, 1295. (d) Lyu, H.; Quan, Y.; Xie, Z. Rhodium-Catalyzed Regioselective Hydroxylation of Cage B−H Bonds of o-Carboranes with O2 or Air. Angew. Chem., Int. Ed. 2016, 55, 11840. (e) Lyu, H.; Quan, Y.; Xie, Z. Transition Metal Catalyzed Direct Amination of the Cage B(4)–H Bond in o-Carboranes: Synthesis of Tertiary, Secondary, and Primary o-Carboranyl Amines. J. Am. Chem. Soc. 2016, 138, 12727. (f) Cheng, R.; Li, B.; Wu, J.; Zhang, J.; Qiu, Z.; Tang, W.; You, S.-L.; Tang, Y.; Xie, Z. Enantioselective Synthesis of Chiral-at-Cage o-Carboranes via PdCatalyzed Asymmetric B−H Substitution. J. Am. Chem. Soc. 2018, 140, 4508. (g) Quan, Y.; Lyu, H.; Xie, Z. Dehydrogenative Cross-Coupling of o-Carborane with Thiophenes via Ir-Catalyzed Regioselective Cage B–H and C(sp2)–H Activation. Chem. Commun. 2017, 53, 4818. (h) Lyu, H.; Quan, Y.; Xie, Z. Transition Metal Catalyzed, Regioselective B(4)-Halogenation and B(4,5)-Diiodination of Cage B−H Bonds in oCarboranes. Chem. Eur. J. 2017, 23, 14866. (i) Lyu, H.; Quan, Y.; Xie, Z. Palladium-Catalyzed Direct Dialkenylation of Cage B-H Bonds in o-Carboranes through Cross-Coupling Reactions. Angew. Chem. Int. Ed. 2015, 54, 10623. (j) Xu, T.-T.; Cao, K.; Zhang, C.-Y.; Wu, J.; Jiang, L.; Yang, J. Palladium Catalyzed Selective Arylation of o-carboranes via B(4)–H Activation: Amide Induced Regioselectivity Reversal. Chem. Commun. 2018, 54, 13603. (k) Zhang, X.; Zheng, H.; Li, J.; Xu, F.; Zhao, J.; Yan, H. Selective Catalytic B–H Arylation of o-Carboranyl Aldehydes by a Transient Directing Strategy. J. Am. Chem. Soc. 2017, 139, 14511. (l) Zhang, X.; Yan, H. Pd(II)-Catalyzed Synthesis of Bifunctionalized Carboranes via Cage B–H Activation of 1-CH2NH2-oCarboranes. Chem. Sci. 2018, 9, 3964. (m) Lyu, H.; Quan, Y.; Xie, Z. Rhodium Catalyzed Cascade Cyclization Featuring B–H and C–H Activation: One-Step Construction of Carborane-Fused NPolyheterocycles. Chem. Sci. 2018, 9, 6390. (n) Chen, Y.; Au, Y. K.; Quan, Y.; Xie, Z. Copper Catalyzed/Mediated Direct B–H Alkenylation/Alkynylation in Carboranes. Sci. China Chem. 2018, https://doi.org/10.1007/s11426-018-9388-3. (8) Two examples were reported for transition metal induced cage B(8)-H activation, however, only deboronated products were obtained in both cases. It was believed that B-H activation occured after deboronation. (a) Zhang, X.; Zhou, Z.; Yan, H. Metal–Metal Redox Synergy in Selective B–H Activation of ortho-Carborane-9,12Dithiolate. Chem. Commun. 2014, 50, 13077. (b) Zhang, C.-Y.; Cao, K.; Xu, T.-T.; Wu, J.; Jiang, L.; Yang, J. A Facile Approach for the Synthesis of nido-Carborane Fused Oxazoles via One Pot Deboronation/Cyclization of 9-Amide-o-Carboranes. Chem. Commun. 2018, 54, DOI: 10.1039/c8cc07728b. (9) Selected reviews for transition metal catalyzed/mediated B-H activation: (a) Quan, Y.; Qiu, Z.; Xie, Z. Transition-Metal-Catalyzed Selective Cage B−H Functionalization of o-Carboranes. Chem. Eur. J. 2018, 24, 2795. (b) Zhang, X.; Yan, H. Transition Metal-Induced B–H Functionalization of o-Carborane. Coord. Chem. Rev. 2019, 378, 466. (c) Yu, W.-B.; Cui, P.-F.; Gao, W.-X.; Jin, G.-X. BH Activation of Carboranes Induced by Late Transition Metals. Coord. Chem. Rev. 2017, 350, 300. (d) Eleazer, B. J.; Peryshkov, D. V. Coordination Chemistry of Carborane Clusters: Metal-Boron Bonds in Carborane, Carboranyl, and Carboryne Complexes. Comments Inorg. Chem. 2018, 38, 79. (10) Selected examples for transition metal catalyzed B-H functionalization of other boron clusters: (a) Zhang, Y.; Sun, Y.; Lin, F.; Liu, J.; Duttwyler, S. Rhodium(III)-Catalyzed Alkenylation– Annulation of closo-Dodecaborate Anions through Double B−H Activation at Room Temperature. Angew. Chem., Int. Ed. 2016, 55, 15609. (b) Lin, F.; Yu, J.-L.; Shen, Y.; Zhang, S.-Q.; Spingler, B.; Liu, J.; Hong, X.; Duttwyler, S. Palladium-Catalyzed Selective Five-Fold Cascade Arylation of the 12-Vertex Monocarborane Anion by B–H Activation. J. Am. Chem. Soc. 2018, 140, 13798. (c) Molinos, E.; Kociok-Köhn, G.; Weller, A. S. Polyethyl Substituted Weakly Coordinating Carborane Anions: A Sequential Dehydrogenative



Borylation–Hydrogenation Route. Chem. Commun. 2005, 3609. (d) Rojo, I.; Teixidor, F.; Kivekäs, R.; Sillanpää, R.; Viñas, C. Formation of Bridging Alkene and Conjugated Dialkenes Exclusively Generated from Alkynes on the [3,3’-Co(1,2-C2B9H11)2]- Platform. The Unique Hydroboration Role of [3,3’-Co(1,2-C2B9H11)2]-. J. Am. Chem. Soc. 2003, 125, 14720. (11) (a) Dziedzic, R. M.; Martin, J. L.; Axtell, J. C.; Saleh, L. M. A; Ong, T.-C.; Yang, Y.-F.; Messina, M.-S.; Rheingold, A. L.; Houk, K. N; Spokoyny, A. M. Cage-Walking: Vertex Differentiation by PalladiumCatalyzed Isomerization of B(9)-Bromo-meta-Carborane. J. Am. Chem. Soc. 2017, 139, 7729. (b) Eleazer, B. J.; Smith, M. D.; Popov, A. A.; Peryshkov, D. V. Rapid Reversible Borane to Boryl Hydride Exchange by Metal Shuttling on the Carborane Cluster Surface. Chem. Sci. 2017, 8, 5399. (c) Liu, D.; Dang, L.; Sun, Y.; Chan, H.-S.; Lin, Z.; Xie, Z. Hydrogen-Mediated Metal−Carbon to Metal−Boron Bond Conversion in Metal−Carboranyl Complexes. J. Am. Chem. Soc. 2008, 130, 16103. (12) Selected references for transition metal catalyzed “chainwalking”: (a) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J. Chain Walking: A New Strategy to Control Polymer Topology. Science 1999, 283, 2059. (b) Juliá-Hernández, F.; Moragas, T.; Cornella, J.; Martin, R. Remote Carboxylation of Halogenated Aliphatic Hydrocarbons with Carbon Dioxide. Nature 2017, 545, 84. (c) Vasseur, A.; Bruffaerts, J.; Marek, I. Remote Functionalization through Alkene Isomerization. Nat. Chem. 2016, 8, 209. (d) Werner, E. W.; Mei, T.-S.; Burckle, A. J.; Sigman, M. S. Enantioselective Heck Arylations of Acyclic Alkenyl Alcohols Using a Redox-Relay Strategy. Science 2012, 338, 1455. (13) Wang, D.; Stahl, S. S. Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)2 and Discovery of Fe(OTf)3 as a Highly Effective Cocatalyst. J. Am. Chem. Soc. 2017, 139, 5704. (14) Selected examples for 1,2-palladium migration: (a) Lane, B. S.; Brown, M. A.; Sames, D. Direct Palladium-Catalyzed C-2 and C-3 Arylation of Indoles: A Mechanistic Rationale for Regioselectivity. J. Am. Chem. Soc. 2005, 127, 8050. (b) Grimster, N. P.; Gauntlett, C.; Godfrey, C. R. A.; Gaunt, M. J. Palladium-Catalyzed Intermolecular Alkenylation of Indoles by Solvent-Controlled Regioselective C-H Functionalization. Angew. Chem., Int. Ed. 2005, 44, 3125. (c) Mochida, K.; Shimizu, M.; Hiyama, T. Palladium-Catalyzed Intramolecular Coupling of 2-[(2-Pyrrolyl)silyl]aryl Triflates through 1,2-Silicon Migration. J. Am. Chem. Soc. 2009, 131, 8350. (d) Milner, P. J.; Kinzel, T.; Zhang, Y.; Buchwald, S. L. Studying Regioisomer Formation in the Pd-Catalyzed Fluorination of Aryl Triflates by Deuterium Labeling. J. Am. Chem. Soc. 2014, 136, 15757. (e) Seel, S.; Thaler, T.; Takatsu, K.; Zhang, C.; Zipse, H.; Straub, B. F.; Mayer, P.; Knochel, P. Highly Diastereoselective Arylations of Substituted Piperidines. J. Am. Chem. Soc. 2011, 133, 4774. (f) Hansen, A. L.; Ebran, J.-P.; Ahlquist, M.; Norrby, P.-O.; Skrydstrup, T. Heck Coupling with Nonactivated Alkenyl Tosylates and Phosphates: Examples of Effective 1,2-Migrations of the Alkenyl Palladium(II) Intermediates. Angew. Chem., Int. Ed. 2006, 45, 3349. (g) Kirchberg, S.; Fröhlich, R.; Studer, A. Stereoselective Palladium-Catalyzed Carboaminoxylations of Indoles with Arylboronic Acids and TEMPO. Angew. Chem., Int. Ed. 2009, 48, 4235. (15) Selected reviews for palladium migration: (a) Ma, S.; Gu, Z. 1,4-Migration of Rhodium and Palladium in Catalytic Organometallic Reactions. Angew. Chem., Int. Ed. 2005, 44, 7512. (b) Shi, F.; Larock, R. C. Remote C–H Activation via Through-Space Palladium and Rhodium Migrations. Top. Curr. Chem. 2010, 292, 123.


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Journal of the American Chemical Society DG

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H Arylation of o-Carboranes via - ACS Publications - American

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