Palladium-Catalyzed Selective Five-Fold Cascade Arylation of the 12

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Cite This: J. Am. Chem. Soc. 2018, 140, 13798−13807

Palladium-Catalyzed Selective Five-Fold Cascade Arylation of the 12-Vertex Monocarborane Anion by B−H Activation Furong Lin,†,§ Jing-Lu Yu,†,§ Yunjun Shen,†,§ Shuo-Qing Zhang,† Bernhard Spingler,‡ Jiyong Liu,† Xin Hong,*,† and Simon Duttwyler*,† †

Department of Chemistry, Zhejiang University, 38 Zheda Road, 310027 Hangzhou, People’s Republic of China Department of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland

‡

J. Am. Chem. Soc. 2018.140:13798-13807. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 11/15/18. For personal use only.

S Supporting Information *

ABSTRACT: A series of cage penta-arylated carboranes have been synthesized by palladium-catalyzed intermolecular coupling of the Ccarboxylic acid of the monocarba-closo-dodecaborate anion [CB11H12]− with iodoarenes by direct cage B−H bond functionalization. These transformations set a record in terms of one-pot directing group-mediated activation of inert bonds in a single molecule. The methodology is characterized by high yields, good functional group tolerance, and complete cage regioselectivity. The directing group COOH can be easily removed during or after the intermolecular coupling reaction. The mechanistic pathways were probed using density functional theory calculations. A Pd(II)−Pd(IV)− Pd(II) catalytic cycle is proposed, in which initial coupling is followed by preferred B−H activation of the adjacent boron vertex, and continuation of this selectivity results in a continuous walking process of the palladium center. The methodology opens a new avenue toward building blocks with 5-fold symmetry.

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INTRODUCTION 12-Vertex monocarboranes and dicarbaboranes based on the frameworks [CB11H12]− and C2B10H12 are icosahedral clusters in which one or two BH vertices are replaced by a CH unit.1,2 Their sphere-like delocalization of electron density can be compared to the π system of classical aromatic relatives such as benzene.2c,d In general, boron cluster compounds exhibit remarkable chemical and thermal stability as well as low toxicity. These properties have led to applications in diverse areas such as the development of extremely weakly coordinating anions3 as well as novel ligands,4 supramolecular structures,5 medicinal chemistry,6 fluorescence/phosphorescence,7 and materials science.8 The progress in all of these fields heavily relies on synthetic procedures that provide access to cage derivatives with a desired substitution pattern in an efficient manner. In order to further explore fundamental aspects of boron clusters and leverage their role as useful building blocks for chemists and materials scientists, new methodologies for their selective preparation are essential. For the introduction of new substituents onto carborane clusters, C−H vertices can in many cases be functionalized by deprotonation and subsequent reaction with electrophiles. The traditional approach to modify B−H bonds relies on electrophilic substitution or halogenation followed by crosscoupling reactions.9 In analogy to the concept of transitionmetal-catalyzed C−H activation,10 the directing-group-mediated functionalization of B−H vertices of boron cage compounds has emerged as a concept of enormous utility. It allows the construction of B−X bonds (X = C, O, N, halogen) under mild conditions and without prior halogenation. It has © 2018 American Chemical Society

become a powerful strategy to prepare otherwise inaccessible cluster derivatives with high selectivity and an overall improved step economy. The majority of the recent reports on catalytic B−H activation have focused on ortho-dicarbaboranes. The research groups of Xie, Yan, and Cao, among others, have pioneered methodologies for the selective derivatization of {C2B10}-based starting materials, leading to a variety of new products and materials.11,12 For example, palladium-catalyzed arylation procedures have been developed that provide access to different types of substitution patterns, depending on the particular directing group and reaction conditions (Scheme 1a).12f,l,p In contrast to the progress made in the chemistry of neutral {C2B10} cages, transition-metal-mediated B−H functionalization of anionic boron clusters is much less precedented.11c,13−15 In one study, we developed Rh(III)and Ir(III)-catalyzed derivatization reactions promoted by the pyrrolidine directing group.13c These transformations enabled the direct conversion of B−H vertices to B−C, B−N, and B− Cl units, occurring regioselectively at the B2−5 positions (Scheme 1b). The [CB11H12]− anion with its C5v symmetry possesses B2−6 and B7−11 positions of equal reactivity, which in principle allows for 5-fold substitution of the upper and/or lower boron belt. Until now, reactions in which the 5-fold rotational axis is retained have been restricted to electrophilic halogenations or alkylations under harsh conditions. The first position to react is B12, followed by B7−11 and eventually B2−6, based on the decreasing nucleophilicity in this Received: August 2, 2018 Published: September 19, 2018 13798

DOI: 10.1021/jacs.8b07872 J. Am. Chem. Soc. 2018, 140, 13798−13807

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Journal of the American Chemical Society

order.2a,16,17 To the best of our knowledge, there has only been ́ and coone report on penta-arylation, published by Š tibr workers.16a In this case, hexa-iodination of the parent carborane was followed by palladium-mediated cross coupling with tolyl magnesium bromide to afford the B7−11-arylated product (Scheme 1c). Arylation of the [CB11H12]− anion of any degree of substitution by B−H activation has not been accomplished. In the present study, we demonstrate that the carborane-C-carboxylic acid of this anion undergoes selective arylation of the upper boron belt using palladium catalysis and aryl iodides as the coupling partners (Scheme 1d). Under mild conditions, penta-substitution is achieved in high yields and with remarkable functional group tolerance. This is the first demonstration of regioselective catalytic 5-fold B−H functionalization of a boron cluster. More generally speaking, it is also the record for directing-group-mediated, metal-catalyzed activation of multiple equal inert bonds in a single molecule.18 Density functional theory (DFT) calculations indicate that the substitution occurs with a high probability of consecutive arylation at adjacent boron vertices. Furthermore, the removal of the carboxylate directing group was probed experimentally and computationally.

Scheme 1. Boron Vertex Functionalization of {C2B10}- and {CB11}-Based Cage Compoundsa

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RESULTS AND DISCUSSION Optimization of Penta-Arylation Reaction Conditions. At the outset of our study, we combined carborane carboxylic acid 1 with iodobenzene in the presence of Pd(OAc)2, AgOAc, and HOAc (Table 1). The reactions were monitored by ESI-mass spectrometry and, in some cases, NMR spectroscopy. At 60−80 °C, a solvent screen showed that dimethylformamide (DMF) gave the desired product 3a relatively cleanly, with a 68% isolated yield (entries 1−6; mass and 11B{1H} NMR spectra are depicted next to the scheme in Table 1). Slightly higher yields were obtained when the temperature and catalyst loading were decreased (entries 7−

a

DG = directing group.

Table 1. Optimization of Penta-arylation Conditions of 1 to 3aa

entry

Pd(OAc)2 (mol %)

additives (equiv)

T (°C)

solvent

resultb

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

10 10 10 10 10 10 10 5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

AgOAc (3), HOAc (6) AgOAc (3), HOAc (6) AgOAc (3), HOAc (6) AgOAc (3), HOAc (6) AgOAc (3), HOAc (6) AgOAc (3), HOAc (6) AgOAc (3), HOAc (6) AgOAc (3), HOAc (6) AgOAc (3), HOAc (6) AgOAc (6), HOAc (6) AgOAc (1), HOAc (6) NaOAc (3), HOAc (6) Cu(OAc)2 (6), HOAc (6) HOAc (6) AgOAc (6) AgOAc (6), HOAc (6)

80 80 80 80 60 60 25 25 25 25 25 25 25 25 25 25

MeCN DCE Tol (CH3)3CCN MeCN/DMF (1:1) DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF

MS: mixture MS: mixture N.R. MS: mixture MS: 3a + trace tetra 68% isolated 71% isolated 66% isolated 72% isolated 86% isolated MS: 1:3a = ca. 1:1 N.R. N.R. N.R. 22% isolated N.R.

a

Reactions were conducted on a 0.032 mmol (10 mg) scale in 1 mL of solvent in a sealed vial under air. bResults primarily based on ESI-MS; isolated yields are values for purified 3a after silica gel column chromatography. 13799


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Table 3. Synthesis of Penta-Arylated Carboranes H-3a

9). Increasing the amount of AgOAc to 6 equiv improved the isolated yield to 86% (entry 10), while 1 equiv of AgOAc did not lead to full conversion (entry 11). Other acetate salts such as NaOAc and Cu(OAc)2 were ineffective as additives (entries 12 and 13). Control experiments revealed that in the absence of Pd(II), Ag(I), or HOAc, the reaction did not proceed in an efficient manner (entries 14−16). Based on these findings, the conditions from entry 10 were chosen to synthesize arylated products 3 and 4 on a preparative scale. Scope of Penta-Arylation with Respect to Iodoarenes. Subsequently, the substrate scope in terms of iodoarenes was investigated, and the results are compiled in Tables 2 and Table 2. Synthesis of Penta-Arylated Carboranes 3a

yield (%)b

entry

Ar

1 2 3 4 5 6 7 8

3-C6H4-Br 4-C6H4-CN 4-C6H4-CF3 3,5-C6H3-Cl2 3,4-C6H3-F2 4-C6H4-COOMe 4-C6H4-COOEt 4-C6H4-CHO

84 82 85 89 82 94 99 82

(H-3u) (H-3v) (H-3w) (H-3x) (H-3y) (H-3z) (H-3aa) (H-3ab)

a

Reactions were conducted on a 0.3 mmol scale in DMF (5 mL) in a 20 mL vial sealed with a screw cap under 60 °C. bYield of the isolated product after silica gel chromatography.

entry

R

Ar

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14c 15 16 17c 18 19 20

COOH COOH COOH COOH COOH COOH COOH COOH COOH COOH COOH COOH COOH COOMe COOH COOH COOMe COOH COOH COOH

Ph 4-C6H4-Me 4-C6H4-Et 4-C6H4-n-hex 4-C6H4-t-Bu 3-C6H4-Me 3,5-C6H3-Me2 2-C6H4-Me 4-C6H4-F 3-C6H4-F 4-C6H4-Cl 3-C6H4-Cl 4-C6H4-Br 4-C6H4-Br 4-C6H4-Ph 4-C6H4-CH2Ph 4-C6H4-OMe 4-C6H4-NHAc 4-C6H4-CH2OH 2-Nap

86 3a 91 3b 87 3c 86 3dd 85 3ed 88 3f 88 3g 3he 93 3i 92 3j 89 3k 86 3l 94 3m 86 3n 88 3o 78 3pd 76 3q 70 3r 80 3s 59 3td

directing group was transformed to the carboxylic acid ester in a one-pot procedure to furnish 3n in 86% yield. 4-Phenyl and 4-benzyl substitution provided access to 3o and 3p in 88% and 78% yield, respectively. Substrates bearing methoxy, NHAc, and CH2OH functionalities were tolerated as well to give the 5-fold arylated products in 70−80% yield (3q−s); isolation of analytically pure 3q was only achieved after conversion of the directing group to the methyl ester. A moderate yield of 59% was observed for coupling with 2-iodonaphthalene (3t). Monitoring the penta-arylation with 1,3-bromoiodobenzene as the substrate at 25 °C indicated formation of ca. 15% of a byproduct that seemed to arise from decarboxylation based on ESI-MS analysis of the reaction. This initial observation encouraged us to develop a one-pot procedure comprising 5fold substitution followed by decarboxylation. Simply raising the temperature to 60 °C under otherwise identical conditions cleanly afforded product H-3u (Table 3, entry 1). This compound with a cage C−H vertex was isolated in 84% yield. Generally, successful combined penta-arylation−decarboxylation occurred with electron-deficient iodoarenes. Cyano, trifluoromethyl, dichloro, difluoro, and ester substituents were fully tolerated with this catalytic system, affording the desired products in high yields (H-3v to H-3aa). Notably, the unprotected aldehyde functionality also remained intact during the reaction to give H-3ab in 82% yield. These results demonstrate that the C1−COOH moiety can be used as a traceless directing group, allowing for an even higher number of synthetic steps to be combined in the selective derivatization of carboranes. Compounds 3 and H-3 were fully characterized by multinuclear NMR spectroscopy (1H, 1H{11B}, 11B, 11B{1H}, 13 C{1H}, 11B−11B COSY for 3a) and mass spectrometry (for characterization details, see the Supporting Information). In the 11B NMR spectra, the resonances of B2−6 were usually observed around −4 ppm and those of B7−11 around −11.5 ppm (−14 and −13 ppm for 1, respectively). The B12 position experienced a slight deshielding from −6.5 ppm in 1 to ca. −5 ppm in the penta-arylated structures. A detailed analysis of cage C−H NMR signals of products H-3 is provided toward the end of this article. Bulk purity of the products, in particular the degree of substitution, could reliably be verified by fullrange (m/z = 100−1200) (−)-ESI-mass spectroscopy because

a

Reactions were conducted on a 0.3 mmol scale in DMF (5 mL) in a 20 mL vial sealed with a screw cap at 25 °C. bYield of the isolated product after silica gel chromatography. cProduct was separated after methylation by using K2CO3 and MeI. dReaction conducted at 60 °C. e Monosubstituted product was observed by ESI-MS, and the product was isolated in 12% yield and ca. 85% purity.

3. Generally speaking, electron-withdrawing and electrondonating iodo coupling partners allowed the functionalization of B−H bonds at the B2−6 of the carborane cage with complete regioselectivity, affording the corresponding pentaarylated species in good to excellent yields. The products derived from unsubstituted iodobenzene (3a, 86%) and analogues alkylated at the aryl-3−5 positions (3b−g) could all be obtained with comparable high yields of 85−91%.19 In contrast, 2-methyliodobenzene did not afford the desired product; primarily, monosubstitution was observed by ESI-MS, and monoarylated 3h was isolated in low yield and ca. 85% purity (see the Supporting Information for details). Furthermore, the transformation was well compatible with fluoro, chloro, and bromo substitution, providing the desired products 3i−m in yields of 89−94%. In addition, in one case the 13800


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Figure 1. X-ray crystal structures of (a) 3b, (b) 3q, and (c) H-3x; cations and H atoms (except for cage C−H and COOH) omitted for clarity; 30% displacement ellipsoids. Selected distances (Å) for (a) C(cage)−C2 1.527(3), C(cage)−B5 1.771(3), B5−C24 1.599(3).; (b) C(cage)−C2 1.507(4), C(cage)−B4 1.779(4), B4−C25 1.586(4); (c) C(cage)−B2 1.723(8), B2−C8 1.583(8).

Figure 2. (a) DFT-computed free energy changes of the Pd-catalyzed arylation of carborane carboxylic acid. (b) DFT-computed free energy changes of concerted metalation−deprotonation of carborane carboxylate anion. Gibbs free energies are in kcal/mol. (c) Optimized structures of key catalytic species.

respect to the plane defined by B2−6. Generally speaking, the penta-arylation did not impose a significant strain on the carborane cage, as evidenced by C1−B and B−B bond lengths similar to those of the parent cluster. Additional ORTEP representations are given in the Supporting Information (Figures S4−S10). Mechanistic Studies. On the basis of the above experimental results and previous mechanistic studies, we next explored the reaction mechanism and the origins of regioselectivity and decarboxylation with DFT calculations.20 The DFT-computed free energy changes of the most favorable

the products carry a negative charge and do not fragment easily. Crystals suitable for X-ray diffraction were obtained for 3a, 3b, 3f, 3q, H-3u, and H-3x. Representative molecular structures of the anions of 3b, 3q, and H-3x are shown in Figure 1. In the cases with an acid or ester moiety at cage C1, the aryl rings are arranged in a propeller-like geometry (3a, 3f, and 3q) or do not exhibit a particular preference in terms of the rotation of the aryl planes (3b). On the other hand, in the solid-state structures of decarboxylated H-3u and H-3x, the aryl rings were found in a perpendicular orientation with 13801


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Journal of the American Chemical Society pathway are shown in Figure 2a, and the optimized structures of key catalytic species are shown in Figure 2c. From the [Pd(OAc)2]3 precatalyst, the endergonic exchange with carborane carboxylic acid 6 leads to the intermediate 7. From 7, the classic concerted metalation−deprotonation (CMD) step via TS8 generates the cyclometalated intermediate 9, which requires a barrier of 22.9 kcal/mol compared to the separate [Pd(OAc)2]3 precatalyst and carborane carboxylic acid 6. Subsequent oxidative addition of iodobenzene occurs via TS11 and produces the Pd(IV) intermediate 12. This intermediate undergoes C−C reductive elimination through TS13, leading to the B2-monoarylated species 14. From 14, silver acetate irreversibly abstracts the iodide anion, leading to the intermediate 15. Complex 15 can undergo the same sequence of CMD, oxidative addition, reductive elimination, and iodide abstraction for the second arylation at B3, until the penta-arylation cascade is completed (intermediate 16). The oxidative additions with the assistance of carboxylic acid or solvent are less favorable (Supporting Information, Figure S11). Based on the free energy changes of the first arylation, the barriers of oxidative addition and reductive elimination are significantly lower than that of the CMD step; thus the CMD step limits the overall efficiency of the first arylation. The same sequence of the transformations applies to the subsequent four arylations, and the free energy changes of all four arylations are included in the Supporting Information (Figures S12−S15). Figure 2b shows the free energy barriers of the second and third arylations. From 15, the second arylation can in principle occur at the B3 position (adjacent to the first arylation event) via TS17 or at the B4 position (vicinal to the first arylation event) via TS18. Transition state TS17 is 1.9 kcal/mol more favorable than TS18, suggesting that the regioselectivity of the secondary arylation favors the B3 position. Similarly, the third arylation of intermediate S14 also favors the adjacent position by 2.8 kcal/mol (TS19 vs TS20). These computational results indicate that the pentaarylation proceeds through a very specific manner, either clockwise or anticlockwise with a high preference for consecutive adjacent (vs geminal) bond activation. This regioselectivity is caused by the electron-withdrawing effect of the aryl substituent. Initial arylation at B2 increases the acidity of the adjacent hydrogen atom at B3, making the CMD step at this position more favorable than at the vicinal B4 position. Subsequently, the electron-withdrawing effect by aryl substitution further decreases the CMD barriers for the following coupling events. Therefore, the first arylation has the highest CMD barrier among the five sequential arylations (Figure S16), and the catalytic sequence does not stop until the penta-arylation is completed. To confirm the computed reaction mechanism, we performed experimental studies to capture the hypothesized cyclometalated species. A stoichiometric reaction of 1 with Pd(OAc)2 in acetonitrile at room temperature was carried out, giving the corresponding palladacycle 1-Pd containing two MeCN ligands (Figure 3a). The structure of this Pd(II) intermediate was confirmed by X-ray diffraction and spectroscopic methods. Single-crystal analysis clearly suggested B2−H bond activation, resulting in a direct B−Pd bond with a B2−Pd distance of 2.016(3) Å (Figure 3b). The computed structure of 1-Pd was in very good agreement with the experimentally determined structure (Figure 3c). Furthermore, 1-Pd, when subjected to the catalytic conditions with 1,4-

Figure 3. (a) Formation and reactivity of palladacycle 1-Pd; the NMR yield was inferred by 19F NMR spectroscopy upon addition of 1fluoro-4-methylbenzene as an internal standard. (b) X-ray crystal structure of 1-Pd; cation and H atoms omitted for clarity. Selected bond distances [Å]: C1−C2 1.511(3), C1−O2 1.237(3), C1−O1 1.279(3), Pd−O1 2.0467(17), Pd−B1 2.016(3), N1−C3 1.132(3), C3−C4 1.454(3). (c) Overlay of computed and experimental structures (RMSD = 0.201 Å).

fluoroiodobenzene as the coupling partner, afforded 3i in 60% NMR yield. This finding indicated that formation of a fivemembered palladacycle is a key step in the early stages of the catalytic cycle. The somewhat lower yield may be attributed to the presence of MeCN ligands as opposed to conditions where DMF is used as the single solvent. We were not able to isolate the palladacycle intermediate from pure DMF. Additional experiments were carried out in order to probe the preference for consecutive adjacent B−H bond activation. Specifically, 3h was subjected to catalytic conditions with iodobenzene as the coupling partner, and the transformation was monitored by ESI-mass spectrometry. Furthermore, 3h was treated with a stoichiometric amount of Pd(II) in a similar manner to the preparation of 1-Pd. Unfortunately, the results from these experiments did not allow for an unambiguous confirmation of the calculated reaction sequence; details are given in the Supporting Information on pp S29−S31. Next, we studied the mechanistic details of directing group removal. To our surprise, the palladium- and silver-mediated decarboxylation processes all require insurmountable barriers (Figures S17 and S18).21 Instead, we discovered that deprotonated penta-arylated products can undergo facile decarboxylation without any additional reagent (Table 4). With R = Ph (3a), the decarboxylation barrier is only 13.6 kcal/mol, and the reaction is exergonic with ΔGsolv = −0.5 kcal/mol (entry 1). With the stronger electron-withdrawing 4CN-C6H4 ring, both the kinetics and thermodynamics of the decarboxylation are energetically more favorable (entry 2). Without the aryl substituents, the generated carborane dianion is much less stable, and the reaction is endergonic by 19.7 kcal/mol (entry 3). Therefore, the charge-stabilizing aryl substituents play a key role in facilitating the decarboxylation process, rather than added metal ions. The above calculations also highlight the distinctive role of initial deprotonation. Only through the deprotonated carboxylate anion can the penta-arylated carborane undergo facile decarboxylation. Therefore, the acidity of the arylated carborane carboxylic acid is critical for directing group 13802


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Table 5. Optimization of Decarboxylation Conditionsa

Table 4. DFT-Computed Kinetics and Thermodynamics of the Decarboxylation of Selected Carborane Carboxylate Anions

entry

R

ΔG⧧sol

ΔGsol

1 2 3

Ph 4-C6H4-CN H

13.6 13.3

−0.5 −2.0 19.7

a

a

Transition state cannot be located.

removal, and this explains the effects of specific aryl substitution on the eventual outcome of the cascade, i.e., formation of product 3 vs H-3 (Tables 2 and 3). For 3a, the penta-arylated carborane carboxylic acid has a calculated pKa of 9.8 (Scheme 2, calculated for DMF solvent).22 The acidity of

entry

additivesb

resultc

1 2 3 4 5 6 7 8 9 10d 11d

Pd/Ag/H+ (0.25/1.1/2.0 equiv) Pd/H+ (0.25/2.0 equiv) Ag/H+ (1.1/2.0 equiv) Pd/Ag (0.25/1.1 equiv) Pd (0.25 equiv) Ag (1.1 equiv) Ag (0.05 equiv) Pd (0.025 equiv)

H-3a+3a+bpt H-3a+3a+bpt H-3a+3a+bpt H-3a+trace 3a+bpt H-3a+3a+trace bpt H-3a+trace 3a+bpt H-3a+trace bpt H-3a+trace 3a+trace bpt H-3a+trace 3a+trace bpt H-3a, 91% isolated yield H-3a

TEMPO (2.0 equiv)

a

Reactions were conducted on a 0.01 mmol (7 mg) scale in 0.5 mL of DMF in a sealed vial under air. bPd = Pd(OAc)2, Ag = AgOAc, H+ = HOAc, TEMPO = 2,2,6,6-tetramethylpiperidyl-1-oxy radical. cResults were inferred by ESI-MS, bpt = unknown byproduct(s). dUnder a N2 atmosphere.

Scheme 2. Calculated Acid−Base Equilibria and pKa Values of Substituted Carborane Carboxylic Acids in DMF

reaction was run under a N2 atmosphere (entry 10). In this case, H-3a was isolated in 91% yield. Furthermore, a radical trap experiment was conducted with 2 equiv of TEMPO (2,2,6,6-tetramethylpiperdyl-1-oxy radical); ESI-mass spectrometry indicated clean decarboxylation, suggesting that radical intermediates do not play a significant role in this process (entry 11). Applying the conditions from Table 5, entry 10, we investigated the decarboxylation of selected isolated acids 3.23 Aryl rings bearing groups with different electronic properties were all converted cleanly to the corresponding decarboxylated penta-aryl-carboranes H-3 in consistently excellent yields (Table 6). Starting from 3a, the product H-

this carboxylic acid is limited, and thus the corresponding penta-arylation product 3a is inert under the acidic catalytic conditions. With the more electron-withdrawing 4-C6H4-CN substituent, the pKa of the carborane carboxylate acid is 7.3, and acid ionization is significantly more favorable. Therefore, the arylation cascade involving 4-C6H4-CN directly leads to the decarboxylation product H-3v, as is the case for other electron-deficient aryl systems shown in Table 3. Decarboxylation of Isolated Penta-Arylated Carborane Carboxylic Acids. To verify the hypothesized decarboxylation process from computations, we next performed experimental studies to explore the possibility of decarboxylation of isolated products 3. We chose 3a as a model compound and subjected it to different conditions for directing group removal. Under typical penta-arylation conditions (DMF solvent, 2.5 mol % Pd(II), 6 equiv AgOAc, 6 equiv HOAc), no decarboxylation product was detected at 25 or 60 °C. However, increasing the temperature to 100 °C and screening reaction parameters allowed us to identify suitable decarboxylation conditions (Table 5). Under acidic conditions and in the presence of Pd(II) and/or Ag(I), partial formation of H-3a was observed, together with an unknown byproduct (entries 1−3). In the absence of Brønsted acid and with varying amounts of additives, larger amounts of desired H-3a were obtained, but the transformations still did not occur cleanly (entries 4−8). On the other hand, simple heating of 3a in DMF without any additives caused equally efficient decarboxylation (entry 9). These experimental results were in complete agreement with the conclusions from DFT calculations. Finally directing group removal without formation of unknown byproduts was found to be successful when the

Table 6. Decarboxylation of Penta-Arylated Carboranes 3a

entry

Ar

1 2 3 4 5

Ph 4-C6H4−F 4-C6H4−Br 4-C6H4-Et 4-C6H4-NHAc

yield (%)b 91 99 99 99 93

(H-3a) (H-3i)c (H-3n) (H-3c)c (H-3r)c

a

Reactions were conducted on a 0.05 mmol scale in DMF (2.5 mL) under N2 in a 20 mL round-bottom flask at 100 °C. bYield of the purified product. cWith the addition of KOAc (2 equiv).

3a was obtained in identical yield to the reaction on a smaller scale (entry 1). Substituents such as 4-F and 4-Br quantitatively gave the decarboxylated products H-3i and H3n (entries 2 and 3). Complete decarboxylation with 4-Et and 4-NHAc to afford H-3c and H-3r required the addition of 2 equiv of potassium acetate, and the corresponding yields were 13803


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workers,25 such that 4a and 4b are anticipated to present molecules en route to even more highly functionalized boron clusters. Furthermore, deacylation of H-3n furnished deprotected 5 with free amino groups in 99% yield (Scheme 3b). This product is a rare example of a C5-symmetrical ligand for the construction of frameworks involving transition metals or serving as a building block in covalent supramolecular chemistry. Finally, we tested the scalability of our catalytic protocol. When the penta-arylation of 1 with iodobenzene was run on a 2 mmol scale, the desired product 3a was isolated in 92% yield (1.28 g), matching that of the reactions on a smaller scale (Scheme 3c).

99% and 93% (entries 4 and 5). The effect of base addition is attributed to facilitated formation of the carboxylate anion of 3. Detailed analytical data for decarboxylated products H-3 enabled the study of substituent effects through the cage on the carborane CH position. To that end, a plot 1H NMR chemical shifts δ(C1−H) vs Hammett σ constants24 of the aryl rings was created (Figure 4). A fairly good linear correlation

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CONCLUSION In conclusion, we have synthesized a series of cage pentaarylated carboranes by palladium-catalyzed intermolecular coupling of monocarborane carboxylic acid with iodoarenes by direct cage B−H bond functionalization in high yields and with complete regioselectivity. For the first time, directinggroup-mediated, metal-catalyzed activation of five equivalent inert bonds within a single molecule has been accomplished. The directing group can easily be removed during or after the intermolecular coupling reaction. The mechanistic manifold was investigated in detail using DFT calculations, which strongly suggest consecutive B−C bond formation at adjacent B2−6 vertices. Relevant transition states and intermediates were calculated for each part of the cascade. The decarboxylation was additionally related to the electronic properties of the substituents, indicating more facile directing group removal in the case of electron-deficient aryl systems. The above-described functionalized penta-aryl carborane derivatives represent attractive building blocks that have unique 5-fold symmetry for the synthesis of supramolecular structures by subcomponent assembly, setting the stage for the construction of novel types of molecular architectures.

Figure 4. 1H NMR shifts of carborane cage CH positions of decarboxylated products H-3 vs Hammett σ constant of the aryl substituents (NMR conditions: DMSO-d6, 23 °C).

was obtained with a correlation factor of R2 = 0.79; that is, going from electron-donating to electron-withdrawing systems was associated with increasing δ(C1−H). This finding is in agreement with the computational results above and the transformations leading from products 3 to H-3, suggesting that electronic effects of ring substituents are transmitted through the cage onto the remote apical position. Additional Transformations. Subsequent reactions were performed to probe the opportunity for late-stage modification of penta-arylated products. Bromination of 3a and also H-3v with N-bromosuccinimide in acetonitrile proceeded smoothly to afford the corresponding B12-brominated compounds 4a and 4b in 98% and 91% yield, respectively (Scheme 3a). Successful palladium-catalyzed cross coupling of B-brominated carboranes has been demonstrated by Spokoyny and co-

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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/jacs.8b07872. X-ray data for 3a (CIF) X-ray data for 3b (CIF) X-ray data for 3f (CIF) X-ray data for 3q (CIF) X-ray data for H-3u (CIF) X-ray data for H-3x (CIF) X-ray data for 1-Pd (CIF) Experimental details, computational methods, compound characterizations (PDF)

Scheme 3. (a and b) Subsequent Transformations of 3a, H3v, and H-3n and (c) Gram-Scale Synthesis of 3a

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Shuo-Qing Zhang: 0000-0002-7617-3042 Bernhard Spingler: 0000-0003-3402-2016 Xin Hong: 0000-0003-4717-2814 Simon Duttwyler: 0000-0001-9851-4920 Author Contributions §

F. Lin, J.-L. Yu, and Y. Shen contributed equally.

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Journal of the American Chemical Society Notes

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The authors declare no competing financial interest. The X-ray crystallographic data files for the compounds in this paper deposited at the Cambridge Crystallographic Database have CCDC numbers 1854793−1854799.

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ACKNOWLEDGMENTS This research was supported by the Natural Science Foundation of China (grant 21472166, S.D.; 21702182, X.H.), the National Basic Research Program of China (973 Project 2015CB856500, S.D.), and the Chinese “1000 Young Talents Plan” (S.D. and X.H.). Calculations were performed on the high-performance computing system at the Department of Chemistry, Zhejiang University.

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Palladium-Catalyzed Selective Five-Fold Cascade Arylation of the 12

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