Efficient Synthesis of Aryl Boronates via Cobalt-Catalyzed Borylation

Apr 5, 2018 - An efficient catalytic system based on a Co(II)-NHC precursor has been developed for the cross coupling of bis(pinacolato)diboron with a...
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Efficient Synthesis of Aryl Boronates via CobaltCatalyzed Borylation of Aryl Chlorides and Bromides Piyush Kumar Verma, Souvik Mandal, and K. Geetharani ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00536 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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ACS Catalysis

Efficient Synthesis of Aryl Boronates via Cobalt-Catalyzed Borylation of Aryl Chlorides and Bromides Piyush Kumar Verma, Souvik Mandal, K. Geetharani* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India ABSTRACT: An efficient catalytic system based on a Co(II)-NHC precursor has been developed for the cross coupling of bis(pinacolato)diboron with aryl halides including aryl chlorides, affording the aryl boronates in good to excellent yields. A wide range of functional groups are tolerated under mild reaction conditions. The reaction shows excellent chemoselectivity for bromide over chloride. Preliminary mechanistic investigations show that the catalytic cycle relies on a cobalt(I)‒ (III) redox couple.

KEYWORDS: Boronate esters, Cobalt, C-X activation, N-heterocyclic carbenes, Suzuki–Miyaura

INTRODUCTION Aryl boronic acids and boronate esters have been recognized as indispensable building blocks in organic synthesis, particularly in transition-metal-catalyzed crosscoupling reactions for the formation of various C‒C and C-heteroatom bonds, owing to their ease of handling, bench stability, high functional group compatibility and low toxicity.1 Moreover, they have found broad applications as synthetic intermediates in pharmaceuticals, agrochemicals, liquid crystals and organic light-emitting diodes.1c Despite their versatility, the conventional methods for their synthesis involve aryl lithium compounds or Grignard reagents which are not compatible with numerous functional groups and are limited by the availability of the organometallic reagents.2 To expand the scope of arylboronates, a variety of precious metal catalyst systems, such as Rh, Re, Ru and especially Ir were developed over the past decades via direct C–H borylation of arenes.3 Complementary, highly selective metal-catalyzed borylation reactions of (sp2)C–X (X = Cl, Br, I, OTf) bonds have also been developed.4,5 However, the economic constrains, low natural abundance and environmental concerns of precious metal catalysts have demanded the investigation of earth-abundant and inexpensive base metal alternatives, such as Fe,6 Co,7 Ni,8 Cu,9 Zn,10 and even metal-free11 conditions for the borylation of aryl halides. The use of aryl iodides or bromides is often necessary, whereas, there are only few reliable methods available in the literature for the borylation of relatively inexpensive and broadly available aryl chlorides. The aryl chlorides borylation is always challenging, and there were no methods available for this transformation until 2001. The first example was reported by Ishiyama et al. using Pd as a catalyst12 and subsequently scope was expanded by others using mainly Pd13 and few Ni catalyst systems.8g-l In 2014, Bedford et al. reported iron catalyzed borylation of alkyl, allyl and aryl halides, but only chloro-

benzene could be borylated in a poor yield of 5 %.6e Metal-free photoinduced C-X and dual C-H/C-X borylation of haloarenes were reported by Larionov et al.14 and pyridine-catalyzed radical borylation of aryl halides was reported by Jiao et al.,15 but only two aryl chlorides have been described in each cases. Very recently, Nakamura and co-workers reported an iron catalyzed borylation which can effectively transform the biaryl chlorides into their corresponding boronate esters in the presence of KOtBu.6f Therefore, there remains a strong need to develop cost effective and environmentally benign base-metal catalysts, in particularly cobalt as catalyst for the borylation of aryl chlorides. The first cobalt-catalyzed borylation of aryl halides (including chloro-, bromo-, and iodoarenes) and pseudohalides was reported in 2016 by Hu, Huang and co-workers, which typically involve the use of oxazolinylferrocenylphosphine ligands and MeLi, a reactive organometallic reagent.16 Herein, we report an active catalyst composed of cobalt(II) and NHC as ligand that efficiently converts aryl chlorides to boronate esters under mild reaction conditions. RESULT AND DISCUSSION NHC supported Co(II)-complex, Co(IMes)2Cl2 (I; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) was prepared following literature procedure in good yield.17a Reaction conditions were optimized using chloroanisole (2a) as a model substrate by using 5 mol % of I as catalyst, 1.3 equiv of B2pin2 and a variety of solvents, bases and ligands. Among several solvents screened, MTBE (tertbutylmethyl ether) was found to be superior to other solvents with 97% yield (Table 1, entries 1-5). Bases other than KOMe proved inferior (entries 6-10). Weak bases like Cs2CO3 and KOAc could not produce even trace amount of desired product (entries 9-10). There was no reaction in the absence of a base (entry 11). Further, influence of the ligand was studied using ICy (1,3-bis-cyclohexylimidazol-

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Table 1. Co(II)-Catalyzed Borylation of chloroanisole, 2a.a

entry

deviation from standard conditions

1

H NMR b yield (%) c 1 none 97 (85) d 2 THF instead of MTBE 50 d 3 DME instead of MTBE 76 d 4 toluene instead of MTBE 44 d 5 benzene instead of MTBE 41 d t 6 KO Bu instead of KOMe 63 d t 7 NaO Bu instead of KOMe 68 d 8 LiOMe instead of KOMe 38 d 9 Cs2CO3 instead of KOMe 0 d 10 KOAc instead of KOMe 0 d 11 in absence of KOMe 0 12 II instead of I 81 13 III instead of I 95 14 CoCl2 instead of I trace 15 in absence of I trace 16 2.5 mol % of I used 92 a Standard conditions: 2a (0.1 mmol), I (5 mol %), KOMe (1.3 equiv), B2pin2 (1.3 equiv), MTBE (0.5 mL) at 50 °C for 8 h (see b 1 Supporting Information (SI) for details). Determined by H c NMR using nitromethane as an internal standard. Isolated d yield. Reaction was performed at 70 °C for 20 h.

Table 2. Substrate Scope of Co(II)-Catalyzed Borylation of Aryl Chlorides.a

2b was obtained; thus, any uncatalyzed reaction is minimal (entry 15). Gratifyingly, with a reduced catalyst loading of 2.5 mol % (entry 16), we obtained a comparable yield of 92 %. Having identified the optimum conditions with chloroanisole as a standard substrate, we examined the scope of the present borylation reaction with different aryl chlorides, as depicted in Table 2. Electron-neutral (1a, 15a), electron-rich (2a-5a) and electron-deficient (6a-9a) aryl chlorides are converted to corresponding arylboronic esters in good to excellent yields. Notably, 4-fluorophenyl boronate (6b) was the sole borylated product from the reaction of 1-chloro-4-fluorobenzene, leaving the fluoro group intact. Furthermore, 1,4-dichlorobenzene with 2.5 equiv of B2pin2/KOMe gave 1,4-diborylated product (9b) in excellent yield. This protocol was also applicable to substrates having sterically hindered environment around chloro substituent. Reaction of 2-chlorotoluene (10a) under standard conditions afforded the corresponding borylated product in 92% yield. Though, sterically congested 2-chloro-m-xylene 11a, substituted at both ortho positions, afforded borylation product in only 32% yield. The system shows excellent chemoselectivity in case of chlorobenzyl chloride (12a), where the borylation was selective for (sp2)C-Cl bond, leaving the (sp3)C-Cl bond intact. Substrate having heterocyclic-arene in the system (13a) was also tolerable, affording the desired arylboronate in 88% yield. In addition, 3-chloropyridine was transformed to the desired boronate, but in lower yield (14b). Table 3. Substrate Scope of C0(II)-Catalyzed Borylation of Aryl Halides.a

a

Conditions: aryl chloride (1.0 mmol, 1 equiv), I (5 mol %), B2pin2 (1.3 equiv), KOMe (1.3 equiv), MTBE (5 mL), at 50 °C for 8 h unless otherwise stated. Isolated yield after chroma1 tographic workup. Yield determined by H NMR using nib tromethane as internal standard is given in parentheses. c The reaction was performed at 70 °C. The reaction was performed using 10 mol % of I, 2.5 equiv of B2pin2/KOMe for 20 h.

2-ylidene) supported Co-complex (Co(ICy)2Cl2, II)17b gave slightly lower yield than I (entry 12). The Co(IMes)2Br2 complex (III)17c displayed comparable reactivity to I (entry 13). Ligand is essential for this catalytic process; no borylated product was observed when the ligand was omitted (entry 14). In the absence of a Co(II) source, no

a

Conditions: aryl halide (1.0 mmol, 1 equiv), I (2.5 mol %), B2pin2 (1.3 equiv), KOMe (1.3 equiv), MTBE (5 mL), at 50 °C for 8 h unless otherwise stated. Isolated yield after chroma1 tographic workup. Yield determined by H NMR using nib tromethane as internal standard are given in parentheses. c The reaction was performed using 5 mol % of I at 70 °C. The d reaction was performed using 1.0 equiv of B2pin2 and KOMe. e 4-chlorophenyl boronate (22b) was obtained. The reaction was performed using 10 mol % of I, 2.5 equiv of B2pin2/KOMe f at 70 °C for 20 h. 1,4-bis(Bpin)benzene was obtained.

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ACS Catalysis The reaction conditions optimized for aryl chlorides were found to be compatible with both aryl bromides and iodides, and they even need less catalyst loading of 2.5 mol % (Table 3). Bromobenzene (16a) gave the desired boronate ester in 95 % yield. Aryl bromides with electron donating (17a-19a) and withdrawing (20a-22a) substituents were borylated in moderate to high yield. Reaction of 1-bromo-4-chlorobenzene (22a) with 1.3 equiv of B2pin2/KOMe gave mixture of mono and diborylated products, in ca. 80:20 ratio. To achieve chemoselective CBr bond activation over C-Cl bond, reaction was performed using 1.0 equiv of B2pin2/KOMe, yielded solely monoborylated product (4-chlorophenyl boronate), verified by 1H NMR spectroscopy and GC-MS analysis. Using 2.5 equiv of B2pin2/KOMe at elevated temperature (70 ºC) and longer reaction time (20 h) gave exclusively 1,4bis(Bpin)benzene in good yield. To study the steric effect of substituents, we investigated the borylation of 2bromo-1,3-dimethylbenzene (23a), which gave moderate yield of arylboronate. 4-Bromo-3-methylaniline (24a) gave the desired product in 49% yield. Biaryl system (25a) worked well to afford excellent yield of arylboronic ester. A variety of heteroaryl bromides, such as thiophene, pyridine, quinoline and indole systems (26a-29a) produced the corresponding boronate esters in moderate to high yield. Furthermore, aryl iodides (30a and 31a) produced good yields of the corresponding arylboronic esters. The method enables convenient gram scale synthesis (6 mmol) with the same efficiency, as demonstrated for 2a (2b: 1.07 g, 75%), 7a (7b: 1.15 g, 70 %), 15a (15b: 1.3 g, 84 %) and 19a (19b: 1.16 g, 78 %). To gain insight into the mechanism, first we conducted the reactions in the presence of radical scavenger. When the reaction of 2a with B2pin2 was performed in the presence of radical scavenger 9,10-dihydroanthracene (1.3 equiv), the borylation product was obtained without any significant decrease in the yield (85%), indicating that the borylation does not appear to be a radical process (see SI for details). In addition, no detected amount of expected borylated as well as radical cyclization product was observed by NMR and GC-MS analysis, when a radical-clock experiment was performed using 1-(allyloxy)-2iodoobenzene as a substrate under the standard reaction conditions. Given the propensity of cobalt complexes to undergo redox reaction via Co(I)-Co(III) catalytic cycle, as observed in Co-catalysed (sp2)C–H functionalization,18 we speculated that this borylation reaction might have involvement of Co(I) species as the active catalyst. Thus, adopting a modified procedure,19 we synthesized Co(IMes)2Cl (IV) by the reaction of Co(IMes)2Cl2 (I) using Mg dust as a reducing agent. To our delight, using IV as a catalyst also produced 92% of the borylated product from 2a under standard conditions. The involvement of Co(I) complex in this borylation process was further confirmed by a series of NMR monitoring experiments. Treatment of equimolar amount of I with KOMe and B2pin2 in C6D6 shows the formation of IV, determined by 1H NMR spectroscopy (δ = 107.27 ppm).19 To provide further evidence

that IV is the active catalyst in catalytic cycle, EPR experiments were performed (see SI). The EPR active IV could be produced by the reaction of I with the preformed diboron ate complex, but not with diboron itself (see SI for details). A similar EPR signal was observed for the pure IV as well as for the borylation reaction mixture. These results further supports that Co(I) species is involved as an active catalyst in this borylation process. Based on the experimental results and related reports on Co-catalyzed borylation reactions,18 a possible mechanism for this C-X activation is depicted in Figure 1. The active catalyst IV generated from the reduction of I by K+[B2pin2OMe]¯ (generated via reaction between B2pin2 and KOMe),11f which undergo ligand exchange reaction with KOMe to produce a cobalt-OMe complex A. Reaction of A with B2pin2 may produce Co-boryl intermediate B together with formation of MeO-Bpin. Oxidative addition of aryl halide to B followed by reductive elimination gives the product Ar-Bpin and regenerates the Co(I) active species IV.

Figure 1. A plausible mechanism of Co-catalyzed borylation of aryl halides with B2pin2. CONCLUSIONS In summary, we have demonstrated an efficient Cocatalyzed borylation of aryl halides, including aryl chlorides with B2pin2. The reaction proceeds under mild conditions, displays broad scope and functional group tolerance, and furnishes arylboranates in good to excellent yields. From a mechanistic standpoint, based on preliminary experimental results it show that the catalytic cycle operates via a Co(I)−Co(III) redox couple. Current investigations are aimed to explore the reactivity of NHC supported cobalt systems toward more diverse set of substrates.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest. Supporting Information

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Experimental and spectroscopic data, copies of H, C and B NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT K.G. thanks AllyChem Co. Ltd., China for a gift of B2pin2, and the SERB (EMR/2016/000367) and Indian Institute of Science (IISc) start-up research grant, for funding. P.K.V. thanks IISc Bangalore for a research fellowship. S.M. thanks Kishore Vaigyanik Protsahan Yojana (KVPY) for fellowship. We also thank Prof. S. Ramakrishnan, IISc Bangalore for the GC-MS analysis and Prof. S. Maheswaran, IIT Bombay for his help in obtaining the EPR Spectra.

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