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Highly Efficient Synthesis of Alkylboronate Esters via Cu(II)-Catalyzed Borylation of Unactivated Alkyl Bromides and Chlorides in Air Shubhankar Kumar Bose, Simon Brand, Helen Oluwatola Omoregie, Martin Haehnel, Jonathan Maier, Gerhard Bringmann, and Todd B Marder ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02918 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016
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Highly Efficient Synthesis of Alkylboronate Esters via Cu(II)Catalyzed Borylation of Unactivated Alkyl Bromides and Chlorides in Air Shubhankar Kumar Bose,† Simon Brand,† Helen Oluwatola Omoregie,‡ Martin Haehnel,† Jonathan Maier,§ Gerhard Bringmann,§ Todd B. Marder*† †
§
Institute of Inorganic Chemistry and Institute for Sustainable Chemistry & Catalysis with Boron, Institute of Organic Chemistry, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
‡
Department of Chemistry, Faculty of Science, University of Ibadan, Ibadan, Oyo 200284, Nigeria
KEYWORDS: C-X activation, boron, cross-coupling, N-heterocyclic carbenes, Suzuki–Miyaura ABSTRACT: A copper(II)-catalyzed borylation of alkyl halides with bis(pinacolato)diboron (B2pin2) has been developed, which can be carried out in air, providing a wide range of primary, secondary and some tertiary alkylboronates in high yields. A variety of functional groups are tolerated and the protocol is also applicable to unactivated alkyl chlorides (including 1,1- and 1,2-dichlorides). Preliminary mechanistic investigations show that this borylation reaction involves one electron processes.
Alkylboronic esters are very important reagents in synthetic organic/medicinal chemistry due to their unique reactivity, functional group tolerance, and accessibility,1-3 and recent efforts have been committed to their efficient preparation.4-7 Transition metal-catalyzed borylation of alkyl halides has emerged as a versatile and powerful approach for the synthesis of alkylboronate esters.8-14 In 2012, our group reported the metal-catalyzed borylation of alkyl halides with diboron reagents giving alkylboronate esters in good to excellent yields using CuI/PPh3, as well as the application of the products in Suzuki-Miyaura cross-couplings.8 Subsequently, Ito et al. reported a CuCl/Xantphos catalyst system for the borylation of alkyl halides,9a and the scope has been expanded by others using Ni,11 Pd12 and Fe13 catalysts. We also developed a Zn-catalyzed borylation of unactivated alkyl halides giving alkylboronates at room temperature.14a However, these methods have limitations, such as relatively high catalyst loadings and, more importantly, the reactions are sensitive to air. The use of alkyl iodides or bromides is often necessary, whereas unactivated alkyl chlorides are typically unsuitable precursors in the absence of added iodide.8,11a,13 Very recently, Cook et al. reported the MnCl2-catalyzed borylation of alkyl chlorides using 1.3 equiv of the Grignard reagent, EtMgBr.15 Cu(II) salts are attractive because of their ease of handling, low cost and resistance to oxidation in air. Cu(II)-mediated C-B bond formation reactions are mostly known for boron conjugate additions to α,β-unsaturated carbonyl and related compounds.16 Herein we describe a novel catalytic system based on a Cu(II)-NHC precursor for the borylation of alkyl halides, including unactivated chlorides with alkoxy diboron reagents under mild conditions and open to air,
which shows high functional group compatibility and broad substrate scope. Using compound 1a as the model substrate, a range of Cu(II) sources, bases, and ligands were evaluated (Table 1 and Tables S1-S8). The desired product 1b was obtained in 74% yield at room temperature using 1,10-phenanthroline (L1) as the ligand and inexpensive CuCl2 as the copper source in THF (entry 1). Different Cu(II) salts were tested (entries 2-6) which failed to improve the reaction. In the absence of a Cu(II) source, no 1b was obtained; thus, any uncatalyzed reaction is minimal (entry 7). Bases other than KOMe proved inferior (entries 8-10), except for KOtBu (entry 11). When no base was present, no alkylboronic ester was formed (entry 12). We investigated the ligand effect (entries 13-17) using 2,2′-bipyridine and a substituted 2,2′-bipyridine, which provided lower yields than L1 (entries 13 and 14). Changing the ligand to IMes (L4; IMes = 1,3-bis(2,4,6trimethylphenyl)imidazol-2-ylidene) gave the target compound 1b in 88% yield (entry 15).17 The expensive diphosphine ligand, Xantphos (L5), gave a comparable yield (entry 16). There was no reaction in the absence of a ligand (entry 17). Excellent yields were obtained when the catalyst loading was reduced to 1 mol % (98% yield after 30 min, entry 18) or even 0.1 mol % (90% yield after 2 h, entry 19). Reducing the amount of B2pin2 and KOMe to 1.2 equiv also provided 1b in 98% yield (entry 20). The possible involvement of Pd, Ni, Fe or Zn contamination was eliminated by the observation that Pd, Ni and Fe salts did not show any catalytic activity (entries 21-23), and ZnCl2 provided a low yield of 1b (entry 24). The reaction is moderately sensitive to moisture, as the addition of 4 equiv of H2O reduced the yield to 39% (entry 25). Im-
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portantly, the reaction can be performed in air, giving the desired alkylboronic ester 1b in 93% yield (entry 26). Table 1. Conditions for the Cu(II)-Catalyzed Borylation a of 3-Phenylpropyl Bromide, 1a.
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nistic information, whereas formation of 15b from 1bromo-3-methyl-2-butene (15a) and 16b from 1-bromo-2octene (16a) suggests an SN2' type process. In contrast, this is not the case for 14b, presumably due to the stability of the conjugated styryl moiety. Table 2. Substrate Scope of Cu(II)-Catalyzed Borylation a of Alkyl Halides.
entry catalyst (mol %) 1 CuCl2 (10) 2 CuBr2 (10) 3 Cu(OAc)2·H2O (10) 4 Cu(acac)2 (10) 5 CuSO4·5H2O (10) 6 Cu(OTf)2 (10) 7 8 CuCl2 (10) 9 CuCl2 (10) 10 CuCl2 (10) 11 CuCl2 (10) 12 CuCl2 (10) 13 CuCl2 (10) 14 CuCl2 (10) 15 CuCl2 (10) 16 CuCl2 (10) 17 CuCl2 (10) 18c CuCl2 (1) 19c CuCl2 (0.1) 20c,d CuCl2 (1) 21 Pd(OAc)2 (1) 22 NiCl2 (1) 23 FeCl3 (1) 24 ZnCl2 (1) 25f CuCl2 (1) 26g CuCl2 (1)
ligand (mol %) L1 (10) L1 (10) L1 (10) L1 (10) L1 (10) L1 (10) L1 (10) L1 (10) L1 (10) L1 (10) L1 (10) L1 (10) L2 (10) L3 (10) L4 (10) L5 (10) L4 (1) L4 (0.1) L4 (1) L4 (1) L4 (1) L4 (1) L4 (1) L4 (1) L4 (1)
base
t (h)
KOMe KOMe KOMe KOMe KOMe KOMe KOMe LiOMe NaOMe NaOtBu KOtBu KOMe KOMe KOMe KOMe KOMe KOMe KOMe KOMe KOMe KOMe KOMe KOMe KOMe KOMe
12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 0.5 2 0.5 12 12 12 12 1 1
yield (%)b 74 72 52 70 70 74 trace 17 60 56 72 0 23 8 88 86 trace 98 90 98 (88)e 0 0 0 5 39 93
a
Conditions: 1a (0.5 mmol, 1 equiv), catalyst (10 mol %), ligand (10 mol %), base (1.5 equiv), B2pin2 (1.5 equiv), THF (2 b mL), at RT. Yields determined by GCMS vs. a calibrated c internal standard; average of 2 runs. 1 mmol scale. Complete d conversion (GCMS). Using 1.2 equiv of B2pin2 and 1.2 equiv e f of KOMe. Isolated yield. 36 μL (2 mmol) of water added. g Reaction performed in air.
Unactivated primary and secondary alkyl halides were converted to the corresponding alkylboronates in good to excellent yields (Table 2). A variety of functional groups, including ether (3b, 4b), ketal (5b), ester (6b, 7b), alcohol (8b) and cyano (9b) are tolerated, with yields of alkylboronates ranging from ca. 78% to 92%. Reactions with 1,1-dibromoethane (10a) gave gem-diborylethane (10b) in good yield. 1-Bromo-6-chlorohexane (11a) underwent predominantly monoborylation at the primary bromide when only 1 equiv of the borylating reagent was used. This procedure is also applicable to activated alkyl halides, as exemplified by the borylation of allyl (12a, 14a-17a) and benzyl bromide (13a), which gave the desired products in good yields. Products 12b and 17b do not provide mecha-
a
Conditions: alkyl halide (1.0 mmol, 1 equiv), CuCl2 (1 mol %), L4 (1 mol %), B2pin2 (1.2 equiv), KOMe (1.2 equiv), THF (2 mL), at RT for 0.5-1 h unless otherwise stated (see Supporting Information (SI) for details). Yields determined by GCMS vs. a calibrated internal standard. Isolated yields are given in parentheses. Some volatile products were isolated in lower b 1 yields. Yield determined by H NMR using 1,1,2,2c tetrachloroethane as internal standard. Using CuCl2 (2 mol %), L6 (2 mol %; L6: 1,3-bis-(2,6-diisopropylphenyl)imidazold 2-ylidene), 2.2 equiv of B2pin2 and 2.2 equiv of KOMe. Using 1.0 equiv of B2pin2 and 1.0 equiv of KOMe. 1,6e bis(Bpin)hexane (4%) detected by GCMS. Exo-2bromonorbornane was used resulting in an exo:endo product ratio of ca. 86:14. A similar result was obtained using endo-2f bromonorbornane (see SI). Using CuCl2 (10 mol %), L6 (10 mol %), 1.5 equiv of B2pin2 and 1.5 equiv of KOMe at 60 °C for g 2 h (Tables S10-S13). Using CuCl2 (20 mol %), L6 (20 mol %), 2.5 equiv of B2pin2 and 2.5 equiv of KOMe at 60 °C for 2 h. h Reaction was performed at 60 °C for 12 h.
Unactivated secondary alkyl halides were borylated in good yields (Table 2). Cyclic, bicyclic and acyclic secondary bromides can be smoothly borylated (18b-25b). The effects of the halide or pseudo halide were investigated with cyclohexyl substrates (22a); cyclohexyl iodide is readily converted into 22b (yield = 79%), while the tosylate is not effectively borylated. Reaction of cyclohexyl chloride gave the borylation product 22b in good yield (84%, vide infra). Alkyl halides with protected amines (26b, 27b) are readily borylated.
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Tertiary alkyl halides, e.g. 1-bromo- and 1,3dibromoadamantane gave the corresponding mono- and bisboryl products in high yields (28b, 29b), but 2-bromo-2methylbutane (30a) and (2-bromo-2-methylpropyl)benzene (31a) gave very low yields even after prolonged heating. The method enables convenient gram scale synthesis (5 mmol) with the same efficiency, as demonstrated for 1a (1b: 1.08 g, 88%). Table 3. Substrate Scope of Cu(II)-Catalyzed Borylation a of Alkyl Chlorides. R1 R2
Cl
B B O
R3 (1 equiv)
entry
R1 CuCl2 (1 mol %) L6 (1 mol %) O 2 B R KOMe (1.2 equiv) R3 O THF (2 mL)
O
O +
O
ArN
(1.2 equiv) product
Alkyl-Cl Cl
1
2 Cl
yield (%) entry Bpin
78
Alkyl-Cl
product
O
O
4
Br Bpin
11a
Cl
7
1b
1aa
3aa 4
11c
Bpin 74b,c (70)
8
Cl 35a
Cl
C
H H 32a
Bpin
Cl
C D
D
Bpin
33a
C
H H 32b
81b,c (67)
Cl
81
3b Bpin
(80)
Scheme 1. Borylation of 37a and 38a
35b Bpin (83)
9 21aa
C D
D
74b,c (58)
21b
77c Bpin
10aa
10b
Cl
22b Bpin
11 36a
36b
77b (30)
Scheme 2. Borylation of 39a
Bpin
Cl
(54)c
6 Bpin 34a
Bpin 84
22aa
Bpin Cl
Cl
Cl
10
33b
Cl 5
Bpin
Bpin
Cl 4
yield (%)
Bpin
Cl 3
NAr
L6 Ar = 2,6-di-i-propylphenyl
also gave the desired alkylboronates in good yields (entries 9-12). In analogy to Cu(I)-catalyzed borylations,7-9,20 an NHCCu(II) complex might activate the diboron reagent forming a Cu boryl complex, promoting boryl addition to the electrophilic alkyl halides (see SI for details). To explore the possibility of a radical-mediated mechanism using L4, we performed the borylation on 6-bromohex-1-ene (37a) and 4-bromobut-1-ene (38a), and both reactions afforded the cyclized alkylboronates 37c and 38c as the major product; these results are similar to those reported by Ito et al.9 (Scheme 1). Borylation of cyclopropylmethyl bromide (39a, Scheme 2), gave predominantly 3butenylboronate 38b (73%; ca. 1% of cyclobutyl-Bpin was also observed (GCMS)). The formation of the ring-closure products 37c and 38c, as well as ring-opened product 38b (from 39a), suggests a radical mechanism, but addition of a Cu boryl bond to a terminal alkene, followed by intramolecular cyclization cannot be excluded.9b
34b
12
Cl 28aa
Bpin
66d
28b
a
Conditions: same as Table 2 except using L6 (1 mol %), at 60 °C for 0.5-1 h (see SI for details). Yields determined by GCMS vs. a calibrated internal standard. Isolated yields are given in b 1 parentheses. Yield determined by H NMR using 1,1,2,2c tetrachloroethane as internal standard. Using CuCl2 (2 mol %), L6 (2 mol %), 2.2 equiv of B2pin2 and 2.2 equiv of KOMe. d Using CuCl2 (10 mol %), L6 (10 mol %), 1.5 equiv of B2pin2 and 1.5 equiv of KOMe.
Gratifyingly, our Cu(II)-catalyzed system is not restricted to alkyl iodides and bromides, but can also be extended to more readily available unactivated alkyl chlorides. The best catalyst system we identified for the alkyl chloride borylation comprises CuCl2, L6, and 1.2 equiv of B2pin2 and KOMe in THF at 60 °C (Tables 3 and S14). Reaction of 1-bromo-6-chlorohexane (11a) with 2.2 equiv of B2pin2/KOMe gave the 1,6-diborylated product 11c in good yield. Gem-diborylalkanes are valuable synthetic building blocks in organic synthesis, particularly C−C bond formation via cross-coupling reactions.18,19 Dichloromethane (32a) gave diborylmethane in 81% yield using 2.2 equiv of B2pin2/KOMe. 1,1-Dichloroethane yielded 1,1-diborylethane (10b) in good yield. Using 1 equiv of the borylating reagent also gave bis(boronate) products (32b and 10b), but in 39% and 37% yields, respectively. Benzyl chloride (35a) gave the desired product in 80% yield. Unactivated secondary and tertiary alkyl chlorides
When the reaction of 1a was performed in the presence of the radical scavenger 9,10-dihydroanthracene (5 equiv), the desired product 1b was obtained in only trace amounts; a similar result was obtained using 1 equiv of TEMPO. Analysis of the crude reaction mixture with added TEMPO revealed the formation of a small amount of 3phenyl-1-(2',2',6',6'-tetramethyl-1'-piperidinyloxy)-propane (1c),21 but 1c was not formed with TEMPO alone. Furthermore, reaction of the enantiomerically pure (R)-2bromo-5-phenylpentane 40a gave the racemic alkylboronate 40b in 88% yield (see SI).9a Thus, the borylation reaction seems to involve one-electron processes, although more than one active catalyst and mechanism maybe involved. We have demonstrated a novel and efficient Cu(II)catalyzed borylation of alkyl halides, including the effective couplings of unactivated alkyl chlorides with B2pin2 under mild conditions, and even open to air. These reactions can be used to prepare primary, secondary and some tertiary alkyl boronic esters with diverse structures and functional groups in good yields. Preliminary mechanistic investigations suggest that this borylation reaction involves one-electron processes.
ASSOCIATED CONTENT Supporting Information
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Experimental and crystallographic details, spectroscopic 1 13 1 11 1 data, copies of H, C{ H}, B{ H} spectra and GCMS data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT T.B.M. thanks AllylChem Co. Ltd. for a gift of B2pin2, and the DFG for funding. S.K.B. thanks the Alexander von Humboldt Foundation for a postdoctoral fellowship. H.O.O. thanks the DFG and TWAS for funding.
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