Letter pubs.acs.org/OrgLett
Isonicotinate Ester Catalyzed Decarboxylative Borylation of (Hetero)Aryl and Alkenyl Carboxylic Acids through N‑Hydroxyphthalimide Esters Wan-Min Cheng, Rui Shang,*,† Bin Zhao, Wei-Long Xing, and Yao Fu* Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy, iChEM, University of Science and Technology of China, Hefei 230026, China S Supporting Information *
ABSTRACT: Decarboxylative borylation of aryl and alkenyl carboxylic acids with bis(pinacolato)diboron was achieved through N-hydroxyphthalimide esters using tert-butyl isonicotinate as a catalyst under base-free conditions. A variety of aryl carboxylic acids possessing different functional groups and electronic properties can be smoothly converted to aryl boronate esters, including those that are difficult to decarboxylate under transition-metal catalysis, offering a new method enabling use of carboxylic acid as building blocks in organic synthesis. Mechanistic analysis suggests the reaction proceeds through coupling of a transient aryl radical generated by radical decarboxylation with a pyridine-stabilized persistent boryl radical. Activation of redox active esters may proceed via an intramolecular single-electron-transfer (SET) process through a pyridine−diboron−phthalimide adduct and accounts for the base-free reaction conditions.
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ecarboxylative cross-coupling reactions1 have been extensively studied in the past decades. Aryl and alkenyl carboxylic acids and their salts have been applied as carbanion equivalents in various transformations.2 However, compared with transformations of boronic acids and boronates,3 the scope of decarboxylative transformation is still limited regarding both the scope of aryl carboxylic acids and the types of transformations. The limitation is mainly due to the high activation energy for redox neutral decarboxylation4 in transition-metal catalysis (Cu, Ag, Pd). Recently, new redox activation modes for decarboxylation have enabled application of various aliphatic carboxylic acids in cross-coupling reactions5 either through photoredox catalysis6 or by using redox activation groups (e.g., NHPI ester).7 Though the concept was mainly applied in decarboxylative coupling of aliphatic carboxylic acids, we wondered whether this new redox activation mode can achieve a transformation which is a challenge in a previous decarboxylative coupling of aryl and alkenyl carboxylates to expand the amenable substrate scope and transform versatility. With this in mind, a radical type decarboxylative borylation of C(sp2)−COOH to deliver C(sp2)−Bpin as a platform8 for diverse transformation may solve the problem faced in transition-metal-catalyzed decarboxylative couplings (Scheme 1a). Enlightened by the recent work by Jiao et al. depicting a pyridine-catalyzed borylation of aryl halides,9 we envisioned that aryl carboxylic acid N-hydroxyphthamide esters may also be suitable to deliver aryl radicals through radical decarboxylation to react with a pyridine-stabilized boryl radical, to enable decarboxylative borylation of aryl carboxylic acids (Scheme 1b). Decarboxylative borylation of NHPI esters was recently reported by Baran,10a Glorius,10b Li,10c and Aggarwal10d groups using visible-light irradiation or nickel catalysis. We reported herein that isonicotinate tert-butyl ester acts as an © XXXX American Chemical Society
Scheme 1. Decarboxylative Borylation as a Platform To Utilize Aryl/Alkenyl Carboxylic Acids
effective catalyst to decarboxylatively borylate aryl carboxylic acid N-hydroxyphthamide esters in refluxing trifluorotoluene (TFT).10b The reaction system is extremely simple without any additive or base. Various aryl carboxylic acids and alkenyl carboxylic acids were successfully transformed to the corresponding pinacolboronates, including electron-rich and unbiased aryl carboxylic acids, which are challenging substrates in transitionmetal-catalyzed decarboxylative transformations. The reaction offers a fully metal-free method to prepare valuable aryl and alkenyl boronate reagents from readily available, environmentally benign carboxylic acids and demonstrates the potential of utilizing one electron redox process11 to solve problems faced in decarboxylative couplings of C(sp2)−COOH. We demonstrate the optimized reaction conditions in Table 1. Heating a mixture of 4-phenylbenzoic acid N-hydroxyReceived: June 26, 2017
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DOI: 10.1021/acs.orglett.7b01950 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
heteroarenes, such as pyrazine and isoquinoline, failed to act as catalysts. 3-Hydroxy-1,2,3-benzotriazin-4(3H)-one ester can also be used but is less effective. N-Hydroxysuccinimide ester is totally ineffective. It is interesting to note that the Lewis acidity and steric properties of the boronate ester significantly affect the reaction outcomes. As shown at the bottom of Table 1. With the optimized reaction conditions in hand, we investigated the reaction scope with respect to different carboxylic acids. The scope of this reaction is broad as shown in Scheme 2. A
Table 1. Investigation of Reactions Parameters
Scheme 2. Reaction Scopea
a Reaction conditions: NHPI ester (0.2 mmol), B2Pin2 (0.4 mmol), isonicotinate tert-butyl ester (20 mol %), PhCF3 (1.0 mL), 100 °C, 10 h.
phthalimide ester (0.2 mmol), bis(pinacolato)diboron (0.4 mmol), and isonicotinate tert-butyl ester in refluxing TFT in a sealed Schlenk tube under argon afforded the desired borylation product in 93% yield (GC). Ethyl acetate can also be used as the solvent to afford the borylation product in comparable yield (entry 1). Interestingly, a base is generally necessary to activate diboron reagents in various borylation reactions, while addition of a base exhibited a deteriorative effect to this reaction. Although product can be detected under irradiation conditions at room temperature, the yield is significantly lower (25%).10b Some representative examples of the catalyst screen are listed in Table 1. The reaction is highly sensitive to the electronic and steric properties of N-heteroarenes. When isonicotinate methyl ester and isonicotinate ethyl ester were used, the product was obtained in reduced yield. This observation may be due to the instability of methyl ester and ethyl ester toward SN2 substitution under the reaction conditions to consume the isonicotinate ester catalyst. Putting a substituent on the 2-position of isonicotinate tert-butyl ester entirely removes the catalytic activity. Picolinate ester was ineffective. Using 1.0 equiv of B2Pin2 caused a reduced yield. 4-Cyanopyridine, which was demonstrated to catalyze homolysis of bis(pinacolato)diboron,12 works as a less efficient catalyst. 4-Phenyl-pyridine and 4,4′-bipyridine are less effective catalysts. Testing electron-rich and simple pyridines as catalysts entirely failed, showing the requirement of the electron-deficient property of pyridine to be catalytically effective. Other N-
a
Reaction conditions: NHPI ester (0.2 mmol), B2Pin2 (0.4 mmol), isonicotinate tert-butyl ester (15 mol %), PhCF3 (1.0 mL), 110 °C, 15 h. bYield determined by 1H NMR. cReaction time was 24 h.
variety of aryl carboxylic acids can be used regardless of their electronic nature. It is important to note that both electron-rich and -deficient aryl carboxylic acids are amenable substrates and ortho-, meta-, para-substituted benzoic acids are all reactive. In transition-metal-catalyzed decarboxylative cross-coupling reactions, the reactive substrates required a specific electronic property13 (e.g., highly electronic-deficient14a or -rich14b with ortho-substituent14c), and a general scope including meta- or parasubstituted benzoic acids is still challenging. The reaction possesses excellent functional group compatibility, as demonstrated in Scheme 2: ether (6, 10, 20, 26), trifluoromethyl (7), B
DOI: 10.1021/acs.orglett.7b01950 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters cyano (8), aryl chloride (9, 19), aryl bromide (11), ketone (12), trifluoromethyl sulfide (13), sulfonamide (14), ester (15), and sulfone (16) are all well tolerated. Naphthalene carboxylic acids are also reactive (21, 22). For entries of moderate yields (4, 6, 12, 20), starting materials were recovered rather than forming side products (e.g., decarboxylative protonation, homocoupling). Heteroarene carboxylic acids such as pyridine-3-carboxylic acid (23), indole-6-carboxylic acid (25), and quinoline-6-carboxylic acid (24) are all amenable substrates. Testing pyridine-2carboxylic acid was unsuccessful, possibly due to the intrinsic instability of pyridine-2-boronate.15 Jiao et al. reported aryl halides, such as aryl bromides, can be borylated using a pyridine catalyst. However, in our reaction the NHPI ester was decarboxylatively borylated in preference to aryl bromide (11). For 3-bromobenzoate, the product of borylation on the aryl bromide was not detected. The different chemoselectivity may be attributed to the base-free conditions that distinguish it from the reported methods.16 Besides monosubstituted benzoic acids, disubstituted benzoic acids also work well (18, 19, 20). Alkenyl carboxylic acids also react under the optimized reactions to deliver valuable alkenyl boronates (27, 28, 29, 30).17 The simple reaction conditions and low cost of the isonicotinate catalyst make this reaction highly valuable to produce a boronate reagent on large scale. The reaction can be easily scaled up to gram scale without reducing the yield (Scheme 3).
carboxylic acids is very much limited; through the platform of boronate, various types of decarboxylative couplings can be achieved. One intriguing mechanistic aspect of the reaction system is very simple without any additive or base. From literature studies we know that cleavage of the B−B bond in diboron species generally requires coordination of two Lewis bases with diboron.19 By 11B NMR analysis of the completed reaction mixture, we detected a peak at 22 ppm, which suggests the formation of a phthalimidecoordinated Bpin species.20 It is reasonable that the NHPI redox ester may form a complex with pyridine-coordinated diboron (B) to form an adduct (C) (Figure 2). Single electron transfer may
Scheme 3. Gram-Scale Transformation of Aryl Carboxylic Acid to Ar-Bpin
proceed intramolecularly in C to activate the NHPI ester through B−B cleavage to deliver a carboxylate radical (D) and a pyridinestabilized persistent boryl radical (E).9 The persistent boryl radical (E) rapidly reacts with a transient aryl radical21 generated after radical decarboxylation to form the product and regenerate the isonicotinate ester catalyst (A). Since a three-component, acid−base adduct C is supposedly formed, it is easy to rationalize the sensitivity of this reaction toward the steric hindrance and Lewis acidity of both N-heteroarenes and diboron reagents observed in the optimization study. The deleterious effect of additional base can be ascribed to the coordination with diboron impeding the formation of adduct C (Table 1, entries 2−4). Since isonicotinate ester is used as a catalyst (15 mol %) in the presence of a large excess of B2Pin2, formation of a diboron complex coordinated with two pyridines to induce homolysis of the B−B bond is less probable. The efficacy of isonicotinate ester as a catalyst can be rationalized from two aspects. First, it coordinates to bis(pinacolato)diboron and induces a negative charge on the boron atom for electron transfer to activate the NHPI ester. The effect of the para-ester substituent can be explained by its subtle turning of the rate of single electron transfer in complex C, making the rate match that of the decarboxylation step. Second, isonicotinate ester is crucial for generation of the persistent boryl radical.22 The ester substituent may also affect the stability of the persistent radical (E) and enhance its reactivity with the transient aryl radical, suppressing undesired side reactions. In summary, we discovered that isonicotinate tert-butyl ester efficiently catalyzes decarboxylative borylation of a variety of aryl and alkenyl carboxylic acid redox active esters to form synthetically useful aryl and alkenyl pinacol boronates. A broad scope of aryl carboxylic acids possessing various functional groups, including challenging substrates in metal-catalyzed decarboxylation, are amenable substrates. The reaction proceeds under base- and additive-free conditions and features operational simplicity and easy scale-up ability. The reaction proceeds through a radical coupling mechanism via formation of a
Figure 2. Plausible mechanism.
The decarboxylative borylation method offers a route to achieve a series of challenging decarboxylative transformations such as trifluoromethylation,18a sulfenylation,18b amination,18c and hydroxylation18d (Figure 1). These transformations are highly valuable in current organic synthesis. Though several transition-metal-catalyzed methods are applicable, the scope of
Figure 1. Diverse transformation enabled by decarboxylative boryation. Reaction conditions: (a) CuTc (10 mol %), 1,10-phenanthroline (10 mol %), Togni’s reagent (100 mol %), LiOH·H2O (200 mol %), 45 °C, CH2Cl2, 6 h; (b) CuCl (10 mol %), 2,2′-bipyridine (10 mol %), disulfide (66 mol %), 80 °C, DMSO/H2O, under air, 12 h; (c) Cu2O (10 mol %), imidazole (150 mol %), 40 °C, MeOH, under air, 12 h; (d) [Ru(bpy)3Cl2]·6H2O (2 mol %), i-Pr2NEt (200 mol %), 36 W CFL, DMF, under air, 12 h. C
DOI: 10.1021/acs.orglett.7b01950 Org. Lett. XXXX, XXX, XXX−XXX
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persistent pyridine-stabilized boryl radical. Complexation of Nhydroxyphthalimide ester with diboron facilitates intramolecular single electron transfer to activate the redox active ester leading to radical decarboxylation. Considering the diverse transformations of organoboronates, the reaction offers a new strategy to achieve diverse decarboxylative functionalization.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01950. Experimental details and characterization data for all products (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Rui Shang: 0000-0002-2513-2064 Yao Fu: 0000-0003-2282-4839 Present Address †
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by NSFC (21325208, 21572212), Ministry of Science and Technology of China (2017YFA0303500), CAS (XDB20000000), the Key Technologies R&D Programme of Anhui Province (1604a0702027), FRFCU, and PCSIRT.
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