Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper(II)-Catalyzed Selective Hydroboration of Ketones and Aldehydes Haisu Zeng,†,‡ Jing Wu,†,‡ Sihan Li,†,‡ Christina Hui,† Anita Ta,† Shu-Yuan Cheng,*,† Shengping Zheng,*,‡ and Guoqi Zhang*,† †
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Department of Sciences, John Jay College and Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New York, New York 10019, United States ‡ Department of Chemistry, Hunter College, The City University of New York, New York, New York 10065, United States S Supporting Information *
ABSTRACT: A novel nonanuclear copper(II) complex obtained by a facile one-pot self-assembly was found to catalyze the hydroboration of ketones and aldehydes with the absence of an activator under mild, solvent-free conditions. The catalyst is air- and moisture-stable, displaying high efficiency (1980 h−1 turnover frequency, TOF) and chemoselectivity on aldehydes over ketones and ketones over imines. This represents a rare example of divalent copper catalyst for the hydroboration of carbonyls.
using a CuI−carbene catalyst ((IPr)CuOtBu) with high efficiency in C6D6.12 Although this catalyst was tolerant of some reducible functional groups, it was unselective for ketone vs aldehyde hydroboration (Scheme 1). In addition, CuI− carbene catalysts often suffer from poor air and moisture stability and high expenses in ligand synthesis. In this work, we wish to report a novel, air- and moisturestable nonanuclear copper(II) complex assembled readily from
I
n sustainable chemistry, an urgency for replacement of precious metals with Earth-abundant elements toward fundamental catalytic processes has prompted enormous research efforts leading to observation of effective nonprecious metal catalysts.1 Recent progress in industrially important catalytic processes including (transfer) hydrogenation, dehydrogenation, and hydrofunctionalization using well-defined iron and cobalt molecular catalysts has shown great promise for the future utilization of Earth-abundant metals in important organic transformations.2,3 Catalytic hydroboration represents a highly valuable approach toward synthesizing functionalized organoborates that play a key role in organic synthesis.4,5 A variety of base metal (Fe, Co, Mn, and Cu) catalysts have been recently developed to carry out the hydroboration of alkenes and alkynes with pinacolborane (HBpin).5,6 Hydroboration of carbonyl compounds with HBpin provides an alternative route to access alcohols. The introduction of a Bpin group can also serve as versatile directing and protecting group in synthetic chemistry.7 To date, the hydroboration of carbonyls has been achieved by various metal catalysts including transition metal (Ti, Mo, Ru, Co, Fe, Cu, Ni, and Zn), main group (Li, Mg, Ca, Al, Ga, Ge, Sn, and P), and lanthanide catalysts.8 In addition, several heterogeneous catalysts based on Zr, Co, and Fe coordination frameworks are also known.9 In some examples, good chemoselectivity between ketones and aldehydes was also reported.7,9 It was noted that although monovalent copper catalysts based on phosphine or carbene ligands have been extensively exploited for the regio- and/or enantioselective hydroboration of alkene and alkynes,10 copper-catalyzed hydroboration of carbonyls is surprisingly underexplored.11 The only copper(I)-catalyzed hydroboration of carbonyls was reported by the Mankad group © XXXX American Chemical Society
Scheme 1. Copper-Catalyzed Hydroboration of Ketones and Aldehydes
Received: November 8, 2018
A
DOI: 10.1021/acs.orglett.8b03583 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
self-assembly process, we found that when 4-tert-butyl-2,6diformylphenol was utilized in the one-pot reaction (ratio of aldehyde/amine/Cu = 1:2:6), green needle-like crystals of a novel nonanuclear CuII complex, namely (R)-Cu9 (2), were afforded in high yield (91%) after slow evaporation of solvents in air within 6 days (Scheme 2, see the SI). Compound 2 was characterized by IR spectroscopy, elemental analysis, X-ray crystallography, and X-ray photoelectron spectroscopy (XPS) (see the SI). Microanalytic data of vacuum-dried bulk sample of 2 are well consistent with a formula of Cu9(L)2(CH3COO−)8(CH3OH)2(μ3-OH)4 containing a Cu9 core (where L represents the fully deprotonated bis-Schiff base ligand formed in situ). The oxidation states of all copper centers are confirmed to be +2 by XPS analysis, in agreement with the charge balance in the formula. The IR spectrum showed a strong absorption at ∼1600 cm−1, assigned to the imine bonds in 2. The in vitro cytotoxicity measurements indicate that 2 is moderately toxic toward human breast cancer (MCF-7) and human leukemia (K562) cells, displaying significantly lower LD50 values than the tetranuclear 1 against both cells (see the SI). Single-crystal structural analysis of 2 confirmed the formation of a nonanuclear supramolecular complex composed of two Schiff base ligands and nine CuII centers (Figure 1a). Compound 2 crystallizes in the monoclinic space group P21, and two independent cluster molecules are included in the asymmetric unit, except for a number of crystalline solvent molecules. One of the Cu9 cluster structure with an N,Ocoordination environment is depicted in Figure 1. In each molecule, there contain nine CuII centers surrounded by two
Schiff base ligands and its extremely high activity in catalytic hydroboration of carbonyls without the use of extra activators.13 Notably, very high TOFs exceeding 1980 h−1 and excellent chemoselectivity for aldehydes over ketones were revealed, in contrast to the known CuI−carbene catalyst. This represents a rare example of a divalent copper complex that catalyzes hydroboration of carbonyl compounds.8 In search of readily available, air-stable CuII complexes for catalysis, we have previously reported the one-pot synthesis of a tetranuclear CuII cluster, namely (R)-Cu4 (1), resulting from the mixture of 3,5-di-tert-butyl-2-hydroxybenzaldehyde, (R)-2amino-2-phenylethanol, and Cu(OAc)2·2H2O in a 1:1:1 ratio in a CH2Cl2−MeOH solution and explored its catalytic activity in aerobic alcohol oxidation (Scheme 2).14 We were curious Scheme 2. One-Pot Synthesis of Complexes (R)-Cu4 (1) and (R)-Cu9 (2)
whether such a multinuclear CuII complex would be effective for reduction catalysis, such as hydroboration of unsaturated bonds. Hence, we examined the catalytic hydroboration of acetophenone with HBpin by using 1 as a catalyst in THF; to our delight, 30 turnover numbers (TONs) were found after 16 h (entry 1, Table 1). Encouraged by the moderate catalytic turnover, we sought to modify the structure of the CuII cluster complex and improve the catalytic efficiency. Interestingly, by varying the starting aldehyde in the CuII-mediated Schiff base Table 1. Reactivity Test of Copper(II)-Catalyzed Hydroboration of Acetophenone with HBpina
entry
catalyst
loading (mol %)
time (h)
solvent
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11c
1 2 Cu(OAc)2 none 2 2 2 2 2 2 2
1.0 1.0 5.0 − 1.0 1.0 1.0 1.0 0.05 0.001 0.05
16 16 16 16 16 16 16 1 1 24 16
THF THF THF THF Et2O toluene pentane neat neat neat neat
30 81 0 0 75 60 62 >99 99 35 20
Figure 1. (a) ORTEP representation of 2 plotted at 50% thermal ellipsoid probability level, (b) ball−stick model of the Cu9 core showing the coordination environment, and (c) side view of the cluster molecule. Crystalline solvent molecules and H-atoms are omitted for clarity.
a
Conditions: acetophenone (1.0 mmol), HBpin (1.2 mmol), catalyst (indicated loading amount), and solvent (1 mL), rt, 1−24 h, N2. b Determined by GC analysis with hexamethylbenzene as an internal standard. cReaction run in the air. B
DOI: 10.1021/acs.orglett.8b03583 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Table 2. Substrate Scope of Copper(II)-Catalyzed Ketone Hydroborationa
a
Conditions: ketone (1.0 mmol), HBpin (1.2 mmol), and 2 (0.05 mol %), neat, rt, 1 h, N2. bYields of hydroborated products (3) were determined by GC−MS analysis. Isolated yields of alcohols (4) are shown in parentheses. cReaction run at a 10 mmol scale. dReaction run for 16 h. e0.5 mol % of 2 was used.
fully deprotonated bis-Schiff base ligands, eight CH3COO−, and two CH3OH groups and four bridging −OH groups (through μ3-O coordination) to form a {Cu9N4O25} core, where all CuII centers are bridged by O atoms (Figure 1b). Interestingly, a side view of one independent molecule reveals a saddle-like shape and accessible CuII sites are available, indicating its potential in activating substrates in catalysis (vide infra). Next, complex 2 was examined for catalytic hydroboration of acetophenone with HBpin under the same conditions used for 1, without the use of an activator. The results for condition screening are summarized in Table 1. It was delightful to find that 2 (1 mol %) led to much more effective hydroboration of acetophenone than 1 without the presence of any additives, and 81% GC yield of boronate ester 3a was detected (entry 2, Table 1). In contrast, the control experiments show that Cu(OAc)2 alone (5 mol %) does not catalyze this reaction, and no product was observed in the absence of any catalysts (entries 3 and 4, Table 1). These results indicate the unique role of the nonanuclear CuII cluster in enabling catalytic reactivity. We further tested other solvents and solvent-free conditions for the 2-catalyzed hydroboration. The results reveal that the reaction proceeded most efficiently under neat conditions (entry 8, Table 1). To this end, quantitative yield was observed for 3a within 1 h under solvent-free conditions. It was further found that even when the the catalyst loading was lowered to 0.05 mol % the reaction could be completed in 1 h,
achieving a TOF of 1980 h−1 (entry 9, Table 1). The reaction was also tested with an extremely low loading of catalyst (0.001 mol %) under neat conditions, and 35% yield of 3a was detected by using GC after 24 h, corresponding to a TON of 35000 (entry 10, Table 1), which is comparable to the most effective metal catalysts including precious metals for HBpinbased reduction.2−8 Finally, it was noted that when the reaction was performed open to the air, a very low yield of 3a was observed (entry 11, Table 1). The applicability of 2-catalyzed hydroboration was then studied by using a variety of functionalized carbonyl compounds. First, a series of aryl and aliphatic ketones were employed under the optimized conditions (0.05 mol % of 2, neat). The results are shown in Table 2. The hydroborated products (3) were analyzed by GC−MS, and the subsequent purification by column chromatography on silica led to hydrolysis of boronate esters, affording the corresponding alcohols (4).5 It is worth mentioning that the current copper(II) precatalyst promoted the hydroboration of acetophenone smoothly on a gram scale (entry 1, Table 2). Differently substituted acetophenones with groups including methyl, halo, and methoxy (electron-donating) all proceeded well, affording the hydroborated products in high yields (entries 2−6, Table 2). However, acetophenones bearing 4trifluoromethyl and 4-nitro (electron-withdrawing) groups were less active substrates, with moderate yields of the desired products being isolated even after elongated reaction time C
DOI: 10.1021/acs.orglett.8b03583 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters (entries 7 and 8, Table 2). Isobutyrophenone, cyclopropyl phenyl ketone, and benzophenone were suitable substrates for the hydroboration, and the corresponding alcohols were isolated in appreciable yields (entries 9−11, Table 2). The reaction was also tolerant of cyclic or acyclic aliphatic ketones, affording boronate esters 3l−n with moderate to good yields when 0.5 mol % of catalyst was used (entries 12−14, Table 2). Interestingly, the heteroatomic aryl ketone 2-acetylpyrazine was fully hydroborated with high isolated yield of alcohol 4o, whereas 2- or 3-acetylpyridine and α,β-unsaturated ketone were challenging substrates under standard conditions (entries 15−18, Table 2). We also examined a variety of aldehyde substrates under the optimized conditions (Table 3). It was found that
benzaldehyde and functionalized benzaldehyde were suitable for copper(II)-catalyzed hydroboration with HBpin. Halogenated benzaldehydes (4-chloro- and pentafluoro-) are suitable substrates, and the corresponding primary alcohols were obtained in 81 and 90% yields, respectively (entries 2 and 3, Table 3). While benzaldehyde bearing an electron-donating group is a less active substrate affording only a moderate yield of alcohol, benzaldehydes containing strongly electron-withdrawing groups (−CF3 and −NO2) were converted to the corresponding alcohols in high yields (entries 4−6, Table 3). Ester-functionalized aldehyde was selectively hydroborated in a reasonable yield (entry 7, Table 3). In addition, furfural showed complete conversion, although 2-pyridinecarboxaldehyde was almost inactive (entries 8 and 9, Table 3). transCinnamaldehyde containing both CC and CO bonds was selectively hydroborated on the CO (entry 10, Table 3), giving the desired allylic alcohol in 80% yield. Finally, two aliphatic aldehydes were tested, and high conversions to boronate esters were found based on GC analysis (entries 11 and 12, Table 3). Chemoselectivity is a critical and challenging issue in developing practical methodology for carbonyl hydroboration.7 Previously, the Mankad’s CuI−carbene catalyst was found to show poor selectivity toward ketone vs aldehyde hydroboration.12 Our CuII catalyst was therefore examined for the chemoselective hydroboration (Scheme 3). Thus, an equi-
Table 3. Substrate Scope of Copper(II)-Catalyzed Aldehyde Hydroborationa
Scheme 3. Intermolecular and Intramolecular Chemoselective Hydroboration of Different Functionalities
molar mixture of acetophenone and benzaldehyde was first tested under standard conditions, and the quantitative hydroboration of benzaldehyde was observed, with acetophenone being fully recovered. Likewise, the same selectivity was observed in the competing hydroboration between 1-octanal and 2-heptanone. In addition, good selectivity on the ketone hydroboration over imine was also confirmed. Toward this end, the intramolecular competing hydroboration of ketone vs aldehyde was conducted using 4-acetylbenzaldehyde and selective hydroboration occurred only on the aldehyde part, offering alcohol 7 with 83% isolated yield. These results indicate excellent chemoselectivity of 2 toward aldehyde vs ketone and ketone vs imine hydroboration. To investigate whether 2-catalyzed hydroboration proceeded through a homogeneous or heterogeneous mechanism, we conducted a mercury poisoning experiment (see the SI).15 It was found that adding elemental Hg to the reaction of
a
Conditions: aldehyde (1.0 mmol), HBpin (1.2 mmol), and 2 (0.05 mol %), neat, rt, 1 h, N2. bYields of hydroborated products (5) were determined by GC−MS analysis. Isolated yields of primary alcohols (6) are shown in parentheses. cReaction run for 16 h. D
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acetophenone with HBpin under standard conditions resulted in complete suppression of the reaction, indicating that the reaction was likely to take place through a heterogeneous mechanism. We also noticed that the reaction between 2 and HBpin (10 equiv) in THF gave immediately a black precipitate which was insoluble in common organic solvents. We assume that an active copper cluster hydride species which is responsible for the effective hydroboration might have formed,12 although the nature of this species remains to be determined. We are currently in the progress of a detailed mechanistic investigation as well as the potential of this and relevant chiral copper catalysts in asymmetric hydroboration. In conclusion, we report a divalent copper-catalyzed chemoselective hydroboration of ketones and aldehydes using a novel nonanuclear copper(II) cluster complex. The method features low catalyst loading, mild, solvent-free and activator-free conditions, a broad scope of substrates, and good functional group compatibility, which imply promising practical applicability of this catalyst. The observation of the easy-to-make, bench-stable CuII−Schiff base complex as a highly efficient reduction catalyst would pave the way to future catalyst design based on Earth-abundant and inexpensive copper.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03583. Additional experimental details and copies of NMR spectra (PDF) Accession Codes
CCDC 1870930 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Guoqi Zhang: 0000-0001-6071-8469 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to the donors of the American Chemical Society Petroleum Research Fund for partial support of this work (54247-UNI3). We also acknowledge the support from the PSC−CUNY awards (69069-0047, 60328-0048), the Seed grant from the Office for Advancement of Research, and the PRISM program at John Jay College, CUNY. Partial support from National Science Foundation (CHE-1464543) is gratefully acknowledged. E
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DOI: 10.1021/acs.orglett.8b03583 Org. Lett. XXXX, XXX, XXX−XXX
