Pd(II)-catalyzed B(9)-alkynylation of o/m-carboranes

Abstract

Alkyne motifs are important building blocks in organic syntheses. Therefore, introducing alkynyl groups into molecules is highly desirable. Herein, we developed a Pd(II)-catalyzed selective B(9)-alkynylation of o-carboranes and m-carboranes with ⁱPr₃SiC≡CBr as the alkynylated reagent. Ratios of 6 : 1 for o-carboranes and >20 : 1 for m-carboranes were detected in this reaction. Desilylation of the resultant product affords carboranyl acetylene, which can undergo further transformation to provide a diverse range of functional carboranes.

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Carboranes, a class of carbon–boron molecular clusters,¹ have potential applications in the fields of functional materials,² coordination chemistry,³ organic catalysis,⁴ and biomedicine.⁵ Therefore, the development of methods for the selective functionalization of carboranes has received much attention.⁶ Compared to the acidic C–H bonds, the selective functionalization of B–H bonds is more challenging due to the weak polarity and similar reactivity of the B–H bonds. To overcome these problems, methods for transition-metal-catalyzed selective B–H bond activation of o-carboranes were developed. Generally speaking, the electron-rich transition metal catalysts can activate the electron-deficient B(3,6)–H bonds.^6h–j However, the electrophilic transition metal catalysts prefer to react with the electron-rich B(8,9,10,12)–H bonds.^6k Employing a directing group, the transition-metal catalyzed B(4,5,7,11)–H bond functionalization has been developed.^6l–n

With electron-deficient Pd(II) as the catalyst, our group achieved the selective B(9)–H amination⁷ and B(9)–H/B(9)–H oxidative dehydrogenation coupling⁸ of carboranes through an electrophilic palladation process. Additionally, we also achieved the electrophilic halogenation,⁹ hydroxylation¹⁰ and electrophilic amination¹¹ of carboranes. In all these reactions, 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) was used as solvent. HFIP can increase the electrophilicity of electrophiles and the Pd(II) catalyst due to its excellent hydrogen bond donating ability and weak nucleophilicity.¹²

Alkyne motifs are not only important building blocks in natural products, pharmaceuticals and materials but also essential functional groups in cross-coupling, metathesis and addition reactions.¹³ Therefore, the development of efficient methods to construct molecules containing an alkynyl group has received much attention.¹⁴ Meanwhile, alkynyl carboranes have proved to be useful basic units in supramolecular design, metal organic frameworks and optoelectronic functional materials.¹⁵ The most-used alkyne moieties in these studies are generally connected to cage carbon atoms, due to the lack of direct and efficient methodologies for the synthesis of B-alkynylated carboranes. With B-iodo-carboranes as the substrates, the alkynyl group was firstly introduced to the boron vertex of carboranes by transition-metal-catalyzed cross-coupling with alkynyl Grignard reagents (Scheme 1a).¹⁶ In this strategy, the installation of an iodo atom on the carborane is necessary. Under the assistance of a directing group, Xie's group developed an efficient method for the selective B(4) alkynylation of o-carboranes by palladium- and copper-catalyzed B–H bond activation (Scheme 1b).¹⁷ Employing a nitrogen-centered radical-mediated hydrogen atom transfer (HAT) strategy, the direct B(9) alkynylation of m-carborane has been achieved by Yan's group (Scheme 1c).¹⁸ On the basis of our previous work on the selective B(9) functionalization of o-carboranes, the Pd(II)-catalyzed selective B(9)-alkynylation of o-carboranes and m-carboranes was developed (Scheme 1d).

Scheme 1 Synthetic methods of B-alkynylated carboranes.

We started our investigation by using 1,2-Me₂-o-carborane 1a and (bromoethynyl)triisopropylsilane 2a as the model compounds to optimize the reaction conditions. As the solvent HFIP shows an excellent result in the selective B(9)–H functionalization of o-carboranes and m-carboranes, it was selected as the solvent, and different silver salts were examined with Pd(OAc)₂ as the catalyst (Table 1, entries 1–6). When AgOAc, AgPF₆ and AgTFA were used, and the desired product 3a was obtained in moderate to good yields with a 6 : 1 selectivity for B(9) : B(8). When AgPF₆ was used, the starting material o-carborane was consumed, and a total 70% yield of B(9) and B(8) products was obtained. A byproduct (M = 368 by GC-MS) was detected under this reaction condition, but we could not isolate and confirm it. Then, different Pd catalysts were examined, which indicated that Pd(0) could also promote this reaction to give the desired product in moderate yield (Table 1, entries 7–9). Next, we screened different solvents, and the results showed that CF₃COOH (TFA) was slightly better than HFIP, with the desired B(9) product isolated in 60% yield (Table 1, entries 10–14). It is noted that the starting material 1,2-Me₂-o-carborane was consumed, and a total isolated yield of 78% for the two isomers B(9) and B(8) was obtained. Finally, other alkynyl sources were explored, but no products were detected (Table 1, entries 15–19).

Table 1 Screening of reaction conditions^a

Entry	Si, X	Catalyst	Ag salt	Solvent	Yield (B9 : B8)^b
a Conditions: 1a (0.2 mmol, 1 equiv.), 2 (0.4 mmol, 2 equiv.), Pd(OAc)2 (10 mol%), Ag salt (0.6 mmol, 3 equiv.) in 1 mL solvent at room temperature, under air atmosphere. b Detected by GC. c Isolated yield of B(9) product.
1	TIPS, Br (2a)	Pd(OAc)2	AgOAc	HFIP	49% (6 : 1)
2	TIPS, Br (2a)	Pd(OAc)2	AgF	HFIP	Trace
3	TIPS, Br (2a)	Pd(OAc)2	AgPF6	HFIP	70% (6 : 1)
4	TIPS, Br (2a)	Pd(OAc)2	AgTFA	HFIP	50% (6 : 1)
5	TIPS, Br (2a)	Pd(OAc)2	AgOTf	HFIP	Trace
6	TIPS, Br (2a)	Pd(OAc)2	Ag3PO4	HFIP	Trace
7	TIPS, Br (2a)	Pd(TFA)2	AgPF6	HFIP	43% (6 : 1)
8	TIPS, Br (2a)	PdCl2	AgPF6	HFIP	Trace
9	TIPS, Br (2a)	Pd2(dba)3	AgPF6	HFIP	49% (6 : 1)
10	TIPS, Br (2a)	Pd(OAc)2	AgPF6	THF	Trace
11	TIPS, Br (2a)	Pd(OAc)2	AgPF6	CH3CN	n.r.
12	TIPS, Br (2a)	Pd(OAc)2	AgPF6	DCE	Trace
13	TIPS, Br (2a)	Pd(OAc)2	AgPF6	CF3CH2OH	Trace
14	TIPS, Br ( 2a )	Pd(OAc) 2	AgPF 6	TFA	72% (6 : 1), 60%
15	TMS, Br (2b)	Pd(OAc)2	AgPF6	TFA	0%
16	TES, Br (2c)	Pd(OAc)2	AgPF6	TFA	0%
17	TBS, Br (2d)	Pd(OAc)2	AgPF6	TFA	0%
18	TIPS, I (2e)	Pd(OAc)2	AgPF6	TFA	0%
19	TIPS, H (2f)	Pd(OAc)2	AgPF6	TFA	0%

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With the optimal conditions in hand (Table 1, entry 14), we next explored the substrate scope of carboranes (Table 2). Firstly, the scope of o-carboranes was examined. Generally speaking, the C-alkyl substituted o-carboranes provided the desired products in moderate to good yields (3a, 3c–h). The free o-carborane gave the corresponding product 3b in 59% yield at 40 °C. The yields decreased slightly when C-aryl substituted o-carboranes were used because some substrates remained (3i–j). It is noted that the ratio of B(9) : B(8) was about 6 : 1 for the o-carborane substrates, but the B(9) isomers could be easily isolated in pure form by column chromatography, and the results in Table 2 are the isolated yields of the B(9) products. Encouraged by these results, we studied the reaction of m-carborane. To our delight, the desired product 3k was obtained in 48% yield with excellent B(9) selectivity (>20 : 1) when m-carborane was used. It is noted that some of the starting m-carborane remained, which is the main reason for the low yield. Next, the scope of m-carboranes was further explored, which suggested that di-C-alkyl substituted m-carboranes gave the desired products in moderate to good yields, and the mono-C-hydroxymethyl and formyl groups gave relatively lower yields (3l–s). The substrates with a substituent on the boron vertex were also examined, which indicated that 9-Me-m-carborane provided the corresponding product 3t in 27% yield, and no or trace products were detected when a halogen atom or hydroxyl group was introduced to the boron vertex of m-carborane.

Table 2 Reaction scope^a

a Conditions: 1 (0.2 mmol, 1 equiv.), 2a (0.4 mmol, 2 equiv.), Pd(OAc)₂ (10 mol%), and AgPF₆ (0.6 mmol, 3 equiv.) in 2 mL CF₃COOH, room temperature, air atmosphere, 24 hours. Isolated yields. b Reaction was conducted at 40 °C.

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To gain some insight into the reaction mechanism, two control experiments were conducted (Scheme 2). In the absence of Ag salt, the desired product 3a was obtained in 20% yield under Pd(OAc)₂ catalysis (Scheme 2a). On the other hand, under the same reaction conditions, replacement of Pd(OAc)₂ with Pd₂dba₃ resulted in no reaction being detected (Scheme 2b). These results suggested that this cross-coupling reaction was initiated by Pd(II), not Pd(0).

Scheme 2 Control experiments.

To illustrate the role of Ag salt, the ¹¹B{¹H} NMR spectra were measured (Fig. 1), which revealed that a B–O coupling product was formed in the presence of Pd(OAc)₂, and Pd(II) was reduced to Pd(0) (Fig. 1b). The B–O coupling product was determined to be B(9)-OOCCF₃-o-carborane by GC-MS. The addition of AgPF₆ increased the stability of o-carborane and inhibited the reduction of Pd(II) (Fig. 1c). Surprisingly, the ¹¹B{¹H} NMR signals changed significantly in comparison with that with only 1a in CF₃COOH (Fig. 1a and c). To find out the real reason for this phenomenon, we monitored the ¹¹B{¹H} NMR of 1a and AgPF₆ in CF₃COOH, and a similar spectrum was obtained (Fig. 1d). We think this is because AgPF₆, acting as a Lewis acid, can bind with the negative BH vertexes of o-carborane, especially the B(8,9,10,12)H vertexes. These results suggest that AgPF₆ acts as both a Lewis acid and an oxidant in this reaction.

Fig. 1 ¹¹B{¹H} NMR.

Based on the above results and our previous work,^7,8 a plausible reaction mechanism is proposed as shown in Scheme 3. Firstly, a highly active electron-deficient Pd(II) species I is formed by the reaction of Pd(OAc)₂, AgPF₆ and TFA. Then, electrophilic palladation between o-carborane and I produces the B–Pd intermediate II. Oxidative addition of (bromoethynyl)triisopropylsilane 2a affords Pd(IV) intermediate III. Reductive elimination of III provides the final product 3a and Pd(TFA)Br, which further transforms to Pd(II) species I in TFA solvent and regenerates the catalyst.

Scheme 3 Proposed mechanism.

To examine the practicality and synthetic utility of this method, further transformation of the product 3l was carried out (Scheme 4). Firstly, the silyl group TIPS was removed by treatment with tetra-n-butylammonium fluoride (TBAF) in THF,^17b and the corresponding terminal alkyne 4 was obtained in 60% yield. Like other terminal alkynes, compound 4 can undergo various transformations to give different kinds of carborane-incorporated functional molecules. Reaction of 4 with decarborane generated the bis-carborane 5 in 77% yield.¹⁹ The stereoselective 1,2-dibromination of 4 provided the dibromoalkene 6 in 60% yield, employing N-bromosuccinimide (NBS) as the brominating reagent in the presence of PPh₃.²⁰ Sonogashira coupling with iodo-benzene gave 7 in 90% isolated yield.^17b Oxidation with CrO₃ under acidic conditions formed 9-COOH-1,7-Me₂-m-carborane 8 in 65% yield.²¹ Deprotonation with ⁿBuLi, followed nucleophilic addition to ketones, generated propargyl alcohols 9 and 10 in 76% and 80% yields, respectively. Finally, the bromoacetenyl carborane 11 was obtained in 83% yield by the reaction of 4 with NBS in the presence of AgNO₃.²² The success of these transformations suggested that alkynyl carboranes have a potential application in the synthesis of functional carboranes.

Scheme 4 Synthetic application. (a) 3l, TBAF, at 0 °C, in THF. (b) 4, B₁₀H₁₄, PhNMe₂, reflux, in toluene. (c) 4, NBS, PPh₃, in DCM. (d) 4, PhI, Pd(PPh₃)₂Cl₂, CuI, Et₃N, in toluene. (e) 4, H₂SO₄, CrO₃, in HOAc. (f) 4, acetophenone, ⁿBuLi, in THF. (g) 4, acetone, ⁿBuLi, in THF. (h) 4, AgNO₃, NBS, in acetone.

Conclusions

In conclusion, we have developed a simple and efficient method for the selective B(9) alkynylation of o-carboranes and m-carboranes in the absence of any directing groups by Pd(II)-catalyzed B–H activation in CF₃COOH. This reaction features good B(9) selectivity and mild reaction conditions. The importance and synthetic application of the alkynyl group was also studied, which provided a series of carborane derivatives.

Author contributions

Y. N. M. and X. C. conceived and designed the study; H. T. Z. and Y. G. performed the experiments. Y. N. M. and X. C. prepared the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the Natural Science Foundation of Henan Province (232300421088), Science and Technology Research and Development Plan Joint Fund (Cultivation of Superior Disciplines) Project (232301420046), and the National Natural Science Foundation of China (No. 22271256 and 22171246).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo01428f

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