Article pubs.acs.org/Organometallics
Cobalt-Catalyzed Borylation of Aryl Halides and Pseudohalides Wubing Yao,†,‡ Huaquan Fang,‡ Sihan Peng,‡ Huanan Wen,‡ Lei Zhang,‡ Aiguo Hu,*,† and Zheng Huang*,‡ †
Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Road, Shanghai 200032, People’s Republic of China S Supporting Information *
ABSTRACT: We report the first Co-catalyzed borylation of aryl halides and pseudohalides with bis(pinacolato)diboron (B2pin2). The synthesis of two new Co(II) complexes of oxazolinylferrocenylphosphine ligands is described. Upon activation with LiMe, the Co complex catalyzes the borylation reactions of aryl bromides, iodides, sulfonates, arenediazonium salts, and even aryl chlorides under mild conditions, providing the borylated products in excellent to moderate yields and with high functional group tolerance.
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INTRODUCTION Due to their stability, low toxicity, and ease of handling, arylboronic acids and arylboronate esters have become indispensable synthetic intermediates for C−C bond-forming reactions.1 The traditional procedures for the preparation of these boron compounds involve the reaction of aryllithium or Grignard reagents with suitable organoboron nucleophiles.2 However, these methods suffer from poor functional group tolerance and multistep synthesis. Thus, recently there has been increasing interest in the development of transition-metalcatalyzed methods for the synthesis of arylboronic acid derivatives. Two important classes of metal-catalyzed transformations are catalytic borylations of aryl halides3 and C−H bond activation/borylations.4,5 The majority of the catalyst systems for borylations of aryl halides use the noble metal Pd6 or Ni.7 During the past decade, environmentally more friendly and low-cost base-metal catalysts, including Cu,8 Zn,9 and Fe10 complexes, have been developed.11 These systems in general show lower catalytic efficiency than the Pd catalysts. Moreover, the borylations of inexpensive, widely available aryl chlorides using base metals other than Ni7g−k are rare. Thus, the development of costeffective and environmentally benign base-metal catalysts for efficient borylation of aryl halides, in particular for transformations of aryl chlorides, is of great interest. Cobalt is an attractive alternative to the widely used Pd metal because it is abundant and inexpensive. To the best of our knowledge, the Co-catalyzed borylations of aryl halides have remained unknown, although stoichiometric borylation of aryl bromides using Co complexes has previously been reported.12 Driven by our interest in employing base-metal catalysts for the preparation of synthetically useful organoboronic acid derivatives,13 we report here the preparation of Co complexes of © XXXX American Chemical Society
oxazolinylferrocenylphosphine ligands and their applications in the first Co-catalyzed borylations of aryl (pseudo)halides with bis(pinacolato)diboron (B2pin2). Importantly, the Co catalyst is effective for borylations of the challenging aryl chlorides under mild conditions.
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RESULTS AND DISCUSSION The synthesis of the ferrocenyloxazoline-derived N,P ligands and the corresponding Co complexes is outlined in Scheme 1. Deprotonation of ferrocenyloxazoline, followed by the addition of diphenylchlorophosphine or diisopropylchlorophosphine, formed ligands 1a,b containing Ph and iPr phosphino substituents, respectively.14 Treatment of the ligands with CoCl2 in THF afforded complexes 2a (87%) and 2b (90%) in Scheme 1. Synthesis of Oxazolinylferrocenylphosphine Ligands and the Corresponding Co Complexes
Special Issue: Organometallics in Asia Received: February 26, 2016
A
DOI: 10.1021/acs.organomet.6b00161 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
and 63% yields, respectively (Table 1, entries 1−3). Varying amounts of benzene (6−16%) were also detected as the protodehalogenation product in these reactions (see Table S7 in the Supporting Information for the product distributions). Encouraged by these results, we studied Co complexes of various bidentate ligands, such as bis(phosphine), bipyridine, pyridinyl oxazoline, phosphino pyridine, and phosphino oxazoline, for the borylation of 6a. While most complexes of bidentate ligands afforded the desired product 7a in low to moderate yields (see Table S7), the reaction using the new Co complex 2a ligated by the oxazolinylferrocenylphosphine ligand gave 7a in 80% yield (entry 4), along with 7% of benzene. The effects of solvent, base, and activator on the conversion of 6a to 7a were next examined by using 2a as the catalyst precursor. The solvents have marked impact on the borylation reaction. MTBE proved to be the best medium among the solvents studied; dichloromethane (DCM) and toluene are not suitable for the borylation (Table 1, entries 4−9). The catalytic efficiency was improved by substitution of the base from NaOMe to KOMe (89%, entry 10), while the reactions using NaOH and K2CO3 gave very low yields of 7a (entries 11 and 12). Using NaBHEt3 as the catalyst activator resulted in the formation of 60% of 7a and 18% of benzene (entry 13). Interestingly, the reaction occurred in the absence of the activator, albeit with a lower yield of 7a (entry 14, 54%) in comparison to the reaction with the activator. Under the optimized conditions, the Co complex 2b bearing the iPr substituent on the P atom is even more effective than the Phsubstituted analogue 2a; the reaction using 2b gave 95% of 7a and 4% of benzene (90% isolated yield, entry 15). Finally, a control experiment with KOMe and LiMe, but with no Co precatalyst, was conducted. In this case no borylation product was observed (entry 16). Using the catalyst generated from 2b and LiMe, and with KOMe as the base, we explored the substrate scope with respect to aryl halides for the borylation reactions (Table 2). Aryl bromides containing both electron-donating and -withdrawing groups in the para positions (6c−j) underwent borylations to form the arylboronate products in good to excellent yields. The ortho-substituted substrate 6b gave the desired product 7b (54%) in moderate yield. Importantly, a wide range of functional groups including ether (6d), amine (6e,f), cyanide (6h), ketone (6i), and amide (6j) can be tolerated. Furthermore, heteroaromatic bromides, such as bromo-substituted 1,3-benzodioxole (6k), benzofuran (6l), thiophene (6m), and quinoline (6n), were efficiently borylated. The Co-catalyzed borylations of aryl iodides (6a-I, 6o-I, and 6b-I) produced the corresponding arylboronate esters in useful yields. More importantly, borylations of aryl sulfonates including aryl mesylates (6a-OMs, 6c-OMs, and 6p-OMs) and aryl tosylate (6a-OTf) proceeded smoothly under the standard conditions. It is worth noting that the sterically demanding aryl mesylate (6p-OMs) bearing a bulky trimethylsilyl group in the ortho position gave the borylated product 7p in 87% isolated yield. In addition, aryldiazonium salts are suitable substrates, as demonstrated by the direct coupling between 6d-N2BF4 and B2pin2 to form 7d in 72% yield after 24 h at 50 °C (Table 2). Thus, the Co-catalyzed borylation reaction provides a mild method for transforming various aryl pseudohalides into arylboronate esters. Despite the cost advantages of using aryl chlorides as the coupling reagents, there have been only a few reports of borylations of aryl chlorides using base-metal (Ni) catalysts.
high yields. The solid-state structures of 2a,b adopt distortedtetrahedral coordination geometries at the Co center (Figure 1).
Figure 1. Crystal structures of complexes 2a,b.
We commenced the studies of the Co-catalyzed borylation of aryl halide by assessing the catalytic activities of previously reported PNN15 and PNP16 pincer Co complexes (3−5, see Table 1), including 3 developed by our group for efficient alkene hydroborations.13c Using NaOMe (1.5 equiv) as the base and LiMe (10 mol %) as the catalyst activator,13b the coupling between bromobenzene (6a) and B2pin2 (1.5 equiv) in the presence of the pincer Co precatalyst (5 mol %) occurred at 50 °C in MTBE. After 24 h, the reactions with 3−5 gave the desired borylation product, phenylboronate ester 7a, in 34, 51, Table 1. Borylation of Bromobenzene (6a) with B2pin2: Catalyst Screening and Condition Optimizationa
entry
[Co]
solvent
base
activator
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
3 4 5 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2b
MTBE MTBE MTBE MTBE THF Et2O DME DCM PhMe MTBE MTBE MTBE MTBE MTBE MTBE MTBE
NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe NaOMe KOMe NaOH K2CO3 KOMe KOMe KOMe KOMe
LiMe LiMe LiMe LiMe LiMe LiMe LiMe LiMe LiMe LiMe LiMe LiMe NaBHEt3
34 51 63 80 60 70 38 0 14 89 16 4 60 54 95 (90) 0
LiMe LiMe
a
Reaction conditions: 6a (0.26 mmol), B2pin2 (0.39 mmol), base (0.39 mmol), [Co] (0.013 mmol), activator (0.026 mmol), and solvent (1.5 mL) under Ar at 50 °C for 24 h.
b Yields were determined by GC. The value in parentheses is the isolated yield.
B
DOI: 10.1021/acs.organomet.6b00161 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
electron-rich and -neutral substrates for borylations.6d The reactions of aryl chlorides with ester and CF3 groups in the para positions gave the desired products (7t−v) in moderate yields. The reaction is also sensitive to the steric effects of the substrates: an aryl chloride possessing an o-methyl group gave the product 7b in only 43% yield. Mechanistic studies of this Co-catalyzed borylation of aryl halides are underway in our laboratory. To probe the possibility of a radical-mediated reaction pathway, we conducted the reactions of 6a with B2pin2 in the presence of commonly used radical scavengers (Table 4). The addition of 9,10-dihydroan-
Table 2. Co-Catalyzed Borylation of Aryl Bromides and Iodides and Aryl Pseudohalidesa
Table 4. Effects of Radical Scavengers on the Borylation of 6aa
entry
additive
amt of additive (equiv)
yield (%)
1 2
9,10-dihydroanthracene
3
BHT
4
TEMPO
0.5 2.0 0.5 2.0 0.5 2.0
95 79 60 74 81 53
