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• W3089572678 abstract "Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2021Chemoselective Dual Functionalization of Phenols via Relay Catalysis of Borane with Different Forms Xiaowen Tang†, Xiang Luo†, Qiong Su, Gaofei Wei, Shan-Shui Meng and Albert S. C. Chan Xiaowen Tang† School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006 School of Pharmacy, Qingdao University, Qingdao 266021 †X. Tang and X. Luo contributed equally to this work.Google Scholar More articles by this author , Xiang Luo† School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006 †X. Tang and X. Luo contributed equally to this work.Google Scholar More articles by this author , Qiong Su School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author , Gaofei Wei *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected]ysu.edu.cn Institute of Medical Research, Northwestern Polytechnical University, Xi’an 710072 Google Scholar More articles by this author , Shan-Shui Meng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author and Albert S. C. Chan *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000404 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail A highly efficient and chemoselective dual functionalization of unprotected phenols with α- or β-hydroxyl acids is presented. A variety of valuable benzofuranones and dihydrocoumarins are delivered in moderate to high yields. Density functional theory (DFT) calculations and control experiments indicate that an untypical Friedel–Crafts alkylation and the subsequent lactonization are catalyzed by the Lewis acid form and the Brønsted acid form of borane, respectively. Gram-scale experiments and late-stage functionalization of complex molecules have been performed to highlight the utility of this reaction. Download figure Download PowerPoint Introduction Relay catalysis, which is defined as a cascade process in which two or more sequential bond-forming events are independently promoted by different catalysts in a cascade manner,1–7 since first introduced by Rovis’s group in 2007,8,9 has become a powerful tool in the construction of many appealing organic frameworks. However, to the best of our knowledge, the study of relay catalysis, which is realized by the same catalyst in different forms, has rarely been investigated. B(C6F5)3 has been shown to be a privileged Lewis acid with good thermal stability, low toxicity, and high steric hindrance, which is the essential attribute for so-called “frustrated Lewis pair” chemistry.10–12 Unlike other Lewis acids, the borane is not only water-tolerant but its hydrate is also a powerful Brønsted acid that has been employed in diverse organic reactions.13–18 Even so, the area of relay catalysis with B(C6F5)3 and its hydrate is still underdeveloped.19–26 Organic compounds that contain phenol motifs are not only fundamental chemical feedstocks, but also have many impressive biological activities.27–31 The most direct and effective way to obtain these valuable compounds is via chemoselective and site-selective functionalization of phenols.32–34 Zhang and Liu’s group35–38 reported the ortho C–H insertion of phenols with α-diazoesters using a borane catalyst, and in their proposition, the ortho-selectivity was derived from a hydrogen bond between the fluorine atom of the catalyst and the hydroxyl group of phenol (Scheme 1a). Li and co-workers also used the same catalyst realizing an elegant ortho-selective alkylation of phenols with 1,3-dienes39,40 (Scheme 1b). Apparently, the undesired ortho- and para-functionalized products in these precedents not only hamper the efficiency, but also pose a big challenge in the isolation. Thus, developing a highly ortho-selective functionalization of unprotected phenols is still of great importance. Scheme 1 | (a–c) Alkylation of unprotected phenols catalyzed by borane. Download figure Download PowerPoint Based on our previous work on the chemoselective alkylation of arylamines with benzylic alcohols catalyzed by B(C6F5)3,41,42 we wondered if the challenging ortho-selective alkylation of phenol could also be achieved. Before that, however, some problems needed to be addressed: (1) the dominant para-selectivity in the Friedel–Crafts alkylation of unprotected phenols, (2) the undesired multialkylation process, and (3) the homoetherification of alcohol substrates under catalytic conditions.42 Herein, we disclose a protocol for the highly chemoselective and ortho-selective functionalization of unprotected phenols with α- or β-hydroxyl acids via the relay catalysis of the borane and its hydrate (Scheme 1c). In this borane-catalytic system, the initial ortho-alkylation process not only delivers an unusual dearomatized complex III, but also produces the requisite Brønsted acid catalyst ( B3) in the subsequent lactonization. This transformation affords a series of benzofuranones43–49 and dihydrocoumarins,50,51 which are important motifs in natural products, biologically significant compounds, and materials for chemical biology, in moderate to high yields and high chemoselectivities. Experimental Methods To a 10 mL Schlenk tube was added phenol 1b (0.22 mmol, 1.1 equiv), 4a (0.20 mmol, 1.0 equiv) and B(C6F5)3 (11 mg, 0.1 eq). Then 1 mL dichloromethane (DCM) was added. The mixture was stirred at 50 °C after 48 h. Then the solvent was removed, and the residue was purified by silica gel column chromatography (PE:EA = 40∶1) to afford product 5a (27.3 mg, 90% yield). Analytical data for 5a:1H NMR (400 MHz, CDCl3, δ): 7.21 (dd, J = 8.3, 1.8 Hz, 1H), 7.17–7.12 (m, 2H), 7.08 (d, J = 8.3 Hz, 1H), 7.05 (s, 1H), 6.93–6.87 (m, 2H), 4.81 (s, 1H), 3.80 (s, 3H), 2.89 (dt, J = 13.8, 6.9 Hz, 1H), 1.22 (dd, J = 6.9, 4.3 Hz, 6H). 13C NMR (100 MHz, CDCl3, δ): 175.9, 159.5, 152.0, 145.5, 129.4, 127.3, 127.2, 127.2, 123.2, 114.6, 110.5, 55.3, 49.3, 33.9, 24.4, 24.1. High-resolution mass spectrometry (HRMS) (electrospray ionization [ESI]) (m/z): [M + Na]+ calcd for C18H18NaO3, 305.1154; found, 305.1153. All calculations were performed with the Gaussian 09 (Gaussian, Inc., Wallingford CT, USA) package. The coordinates of the species involved in the catalytic cycle were constructed by using GaussView 5.0 software (Gaussian, Inc.), and all three-dimensional (3D) structures of the optimized species were generated with CYLview (version 1.0.561, University of Sherbrooke, Canada) software. Geometry optimizations and vibrational frequencies of all the stationary points were performed at the M06-2X/6-311G(d,p) level. The temperature was set to 323.15 K to be consistent with the actual reaction conditions. Solvent effect was treated with the solute electron density model with dichloromethane as the solvent. With geometry optimization at each point, the relaxed potential energy surface (PES) scan was performed to estimate the existence of a transition state at each step. A lower level, M06-2X/6-31G(d) was adopted for the PES scan. Activation-free energy barriers here were defined as the free-energy difference between the transition state and the lowest energy stationary point preceding it on the reaction pathway. Results and Discussion Initially, the alkylation with 1-phenylethan-1-ol ( 2a) and phenol ( 1a) was tested with 10 mol % B(C6F5)3, and no desired alkylated products were produced. Under the same condition, a more nucleophilic phenol ( 1b) gave a mixture of monoalkylated and bisalkylated products (1.4∶1, 52% yield), and 31% of the phenol substrate was recovered. Based on the elegant precedents and our previous work, we reasoned that the ortho-selective alkylation of unprotected phenols might be achieved with an enhanced interaction between phenols and alcohol substrates. However, the phenol 1b was intact when alcohol substrates were installed with a CN-group or carbonyl group at α-position. A mixture of undefined products was obtained when methyl 2-hydroxyl-2-phenylacetate was used as the alkylating reagent (see Supporting Information Figure S1). Fortunately, the phenol underwent a moderate conversion with the commercially available mandelic acid. To our surprise, the cyclized product 5a was delivered as the sole product without any 5a′ (Table 1, Entry 1).35 In contrast to our previous work, polar solvents did not facilitate ortho-alkylation (Entries 2 and 3). A more suitable solvent, DCM, gave a better result (Entry 5, 86% yield). Unexpectedly, a lower temperature was more favored in this transformation (Entries 6 and 7). The yield had an obvious decrease with the addition of molecular sieves, which might mean that water promotes this transformation (Entry 8). Recently, similar transformations have been reported with metallic Lewis acids.52–54 However, high temperatures (140–160 °C) and vacuum conditions are absolutely essential for the reaction. As the optimal conditions (Entry 7) in our system were apparently different from those reports, we considered that a new pathway and mechanism must be active in our reaction. Table 1 | Optimization of the Cyclization of Unprotected Phenolsa Entry Solvent T (°C) Yield% ( 5a)b 1 DCE 70 67 2 CH3NO2 70 43 3 HFIP 70 32 4 Toluene 70 56 5 DCM 70 86 6 DCM 90 78 7 DCM 50 90 8c DCM 50 71 Note: DCE, 1,2-dichloroethane; HFIP, 1,1,1,3,3,3-hexafluoro-2-propanol; DCM, dichloromethane. a 1b (0.22 mmol), 4a (0.2 mmol), B(C6F5)3, 1 mL solvent, 48 h. bIsolated yield. c100 mg 4 Å molecular sieve. Density functional theory (DFT) calculations were employed to explore the mechanism. The PES scanning was performed at the M06-2X55–58/6-31G(d) level, and a larger basis set, 6-311G(d,p), was adopted to evaluate the free-energy profile and configurational information of the catalytic process. The solvent effects were considered with dichloromethane as the solvent, and the temperature was set to 323.15 K, which was consistent with the actual reaction conditions. In view of the dual functionalization, distinguishing the sequence of alkylation and esterification was unavoidable when determining the whole catalytic mechanism.13 Regarding esterification of 1b and 4a, various mechanisms that refer to different coordination modes, electron-transfer modes, and catalytic modes were estimated, and results indicated that esterification of 1b and 4a cannot occur under the catalyzation of neither B(C6F5)3 nor its hydrate in all proposed mechanisms (see Supporting Information Figure S3–S5).a Therefore, we prefer to think that the alkylation was more likely to be the first step in this dual functioanlization. Because multiple O-atoms exist in our reaction, the borane catalyst ( B1) has three different coordination modes with 4a ( I, I1, and I2, see Figure 1). The interaction energies for the three modes were calculated via DFT and PES scanning for delivering carbocation 4a + from the three modes shown in Figure 1. Apparently, only coordinating with the alcoholic hydroxyl group (coordination mode I) can activate the substrate to deliver a carbocation. Meanwhile, we also estimated the catalytic ability of the hydrate form of the borane catalyst [B(C6F5)3–OH2, B3] toward activating the substrate, and the result showed that it was incapable of activating the substrate (see Supporting Information Figure S6). As revealed by Li,39 the B(C6F5)3-catalyzed alkylation of phenols can be triggered by coordination with the phenolic hydroxy group, which delivers a proton that can activate 1,3-dienes to generate a carbocation. We also calculated the potential energy profile of the mechanism that 4a is activated through receiving a proton from B1-coordinated 1b complex (coordination mode I3 in Supporting Information Figure S6). However, such a mechanism is not possible in the current reaction system, as indicated by the DFT calculations. Therefore, this coordination mode I was adopted as the most suitable coordination mode in our current reaction, the free-energy barrier for the activation of 4a was estimated to be 13.9 kcal/mol, and the carbocation intermediate 4a + was considered to be a metastable state with a reaction energy of 11.6 kcal/mol. Figure 1 | Gibbs free-energy profile for the activation of 4a. The feasible pathway is labeled in black, and the comparative pathways are labeled in gray. Chemical structures of I, I1, and I2 are provided with different O-atoms labeled by colors. The structure of TSI is displayed with a ball-and-stick model. Units of energy and distance are used in kcal/mol and Å, respectively. Download figure Download PowerPoint The subsequent step was ortho-alkylation of 1b by electrophilic attack of 4a + to form cation intermediate II. The free-energy profile of this step and optimized reactant structures for ortho-/para-alkylation ( pre-o- II and pre-p- II) are displayed in Figure 2. Obviously, some electronic interactions existed between the carboxyl and phenolic hydroxyl groups in pre-o- II. The DFT calculations also showed that binding free energy of pre-o- II was estimated to be 3.1 kcal/mol lower than that in pre-p- II, namely, the binding pattern that prepared for ortho-alkylation is dominant when the carbocation intermediate 4a + combined with 1b. Compared with para-alkylation, ortho-alkylation overcame a lower barrier of only 6.1 kcal/mol and released more heat of 10.7 kcal/mol to form a stable intermediate II. In consequence, the high ortho-chemoselectivity in the current reaction system was a combined action of thermodynamic and kinetic effects. The distance involved in the alkylation was decreased from 2.98 Å in pre-o-II to 1.57 Å in II via 2.13 Å in TSII, indicating that the alkylation process was complete. To further confirm our conclusion, more comparative studies were conducted, as displayed in Supporting Information Figure S7. When phenol was used as the model substrate, the privileged ortho-selectivity was still observed. Unsurprisingly, the ortho-selectivity disappeared when the carboxyl group was replaced by –CH3. We also estimated whether the catalysts were involved in the alkylation process, and different binding patterns of B(C6F5)3 with substrates were considered. Results (see Supporting Information Figure S8) showed that the barrier of the alkylation process increased in all the proposed models. Therefore, the alkylation process that involves only 4a + and 1b is more reasonable. Figure 2 | Gibbs free-energy profile for the alkylation of 1b. The feasible pathway is labeled in black, and the comparative pathways are labeled in gray. Optimized reactant structure of pre-o-II, pre-p-II, TSII, and II are shown in a ball-and-stick model. Units of energy and distance are used in kcal/mol and Å, respectively. Download figure Download PowerPoint Our previous study revealed that aromatization occurred smoothly via proton transfer from the aromatic ring to B2.41,59 Surprisingly, in this system, intermediate II did not undergo the above process giving the aromatized product 5a′ (Figure 3). Two binding patterns with different stability ( III, −13.1 kcal/mol and III1, −3.9 kcal/mol in Figure 3) were found when B2 combined with II. Apparently, the complex III that the proton at the phenolic hydroxyl position extracted by B2 in a barrierless and highly exothermic manner was much more dominant than III1, inspired by the previous study. Therefore, complex III was indicated as an authentic intermediate in the aromatization process. Moreover, III1 can be aromatized with a negligible barrier of only 0.3 kcal/mol forming intermediate 5a′, which cannot be esterified under our current reaction conditions catalyzed by neither B1 nor B3 in the PES (see Supporting Information Figures S9 and S10). To further demonstrate that the ultimate product is not delivered from 5a′, the ortho-alkylated phenol was tested under optimal conditions, and no cyclized product was generated. Figure 3 | Gibbs free-energy profile for the aromatization of II. The feasible pathway is labeled in black, and the comparative pathways are labeled in gray. Structures of III, TSIII, IV, and III1 are displayed in a ball-and-stick model. Units of distance and energy are used in Å and kcal/mol, respectively. Download figure Download PowerPoint The actual aromatization occured in steps III– IV, during which the C=O acted as a proton accepter to promote aromatization and produce intermediate IV. Remarkably, the catalyst served as a proton shuttle60–63 to stabilize the reaction system. It captured the proton of phenolic hydroxyl to present as B3 [d(O–H1) = 1.03 Å and d(O1–H1) = 1.49 Å in III] when the aromatic ring was destroyed. In reverse, it released the proton to convert into B2 [d(O–H1) = 1.57 Å and d(O1–H1) = 1.01 Å In IV] when the aromatic ring was recovered. To further illustrate the effect of the catalyst, we constructed a new reaction model by truncating the B3 segment in complex III. The results revealed that aromatization could hardly proceed without the catalyst (forming IV2 with a barrier of 27.0 kcal/mol). Besides, the aromatization of intermediate II was also blocked by the formation of III with a high spontaneity, although only a moderate energy barrier of 13.3 kcal/mol had to be overcome. Besides, we also estimated the feasibility of another mechanism that B(C6F5)3 alone coordinated to the O-atom of phenols, and a water molecule served as a proton shuttle to complete the aromatization process. Our DFT calculation revealed that such a mechanism was also excluded due to the unfavorable binding pattern of the reactant compared with the highly exothermic pattern III (see Supporting Information Figure S11). With a protonated carboxyl group, intermediate IV can undergo intramolecular ring closure to generate gem-diol intermediate V1 with a negligible barrier of 0.6 kcal/mol. Apparently, the borane catalyst again served as a proton shuttle, converting from B2 [d(O–H1) = 1.57 Å and d(O1–H1) = 1.01 Å in IV] to B3 [d(O–H1) = 1.00 Å and d(O1–H1) = 1.66 Å in V1] in the ring-closure process. DFT calculations showed that the free-energy barrier for the intramolecular dehydration of gem-diol generating the ultimate lactonized product was 39.4 kcal/mol ( V4 in Figure 4), indicating that gem-diol V4 cannot undergo dehydration in the gas phase. When a water molecule was introduced to ease the strain ( V3), the dehydration barrier was significantly decreased to 22.5 kcal/mol. To our delight, the barrier was further decreased to only 9.8 kcal/mol when the water molecule was replaced by the Brønsted acid B3 ( V2). In addition, to give complex V2, which preferentially undergoes dehydration, V1 must undergo a conformational change, in which the hydrogen bond between H1 and O1 is transformed to H1 and O2. The stability was reduced as the binding pattern converted from V1 to V2 (−10.3 kcal/mol vs −8.5 kcal/mol). As displayed in Figure 4, the weakened π–π stacking of aromatic rings in catalyst and phenol segments (3.62 Å vs 4.17 Å) and a dramatic electrostatic repulsion (2.90 Å) between fluorine in catalyst and oxygen in methoxyl were responsible for it. Figure 4 | Gibbs free-energy profile for the esterification process. The feasible pathway is labeled in black, and the comparative pathways are labeled in gray. Key structures from IV to the final product 5a are displayed in a ball-and-stick model. Units of distance and energy are used in Å and kcal/mol, respectively. Download figure Download PowerPoint Taken together, the boron-catalyzed dual functionalization of phenol ( 1b) with mandelic acid ( 4a) is proposed to proceed through four stages on the “alkylation-to-lactonization” pathway, as displayed in Figure 5a: (1) activation of 4a delivers carbocation 4a + from B(C6F5)3-coordinated complex I, (2) alkylation of 1b by electrophilic attack of 4a + forms cation intermediate II, (3) aromatization of II via an unnormal intermediate III generates complex IV with the assistance of B(C6F5)3 and its hydrate, and (4) esterification of IV produces final benzofuranone product 5a. Apparently, the overall reaction cycle is mediated by the relay catalysis of B(C6F5)3 ( B1) and its hydrate B(C6F5)3–OH2 ( B3). The free-energy profile along the catalytic cycle is displayed in Figure 5b, and all structures involved in the catalytic cycle are shown in the Supporting Information. The whole reaction cycle is highly exothermic by −15.8 kcal/mol, with an overall barrier of only 19.3 kcal/mol, indicating that our proposed catalytic cycle is absolutely energetically feasible. Figure 5 | Catalytic cycle (a) and Gibbs free-energy profile (b) for the boron-catalyzed dual functionalization process. Some migratory atoms are traced by colors. The reaction stages catalyzed by B(C6F5)3 and its hydrate are marked by red and blue, respectively. The reaction stage catalyzed by B(C6F5)3 is labeled in red, and the one catalyzed by its hydrate is labeled in blue. Structures of I and III are displayed in a ball-and-stick model. Units of energy and distance are used in kcal/mol and Å, respectively. Download figure Download PowerPoint Because a plausible mechanism had been proposed, we began to explore the substrates scope in this dual functionalization. Phenols with electron-neutral substituents (Table 2, 5a–5d, 83 –91% yield) produced the desired benzofuranones in high yields; when the phenol rings were more electron rich, the reactions gave higher yields ( 5e–5g, 89 –92% yield). The unprotected alcoholic hydroxyl group was also tolerated well in the reaction ( 5h, 77% yield). Phenol gave a moderate yield (45% yield), but an ortho-substituted phenol (o-cresol) could only deliver trace cyclization product after 48 h. Meta-substituted phenols produced the desired cyclic products smoothly ( 5j–5l, 76 –87% yield). ortho-alkylated naphthol is an important scaffold in many natural products37; fortunately, both α- and β-naphthol gave the desired products without any byproducts ( 5m and 5n). Polysubstituted phenols were tested, and to our delight, the corresponding benzofuranones were obtained in satisfactory results ( 5o–5r, 85 –92% yield) even when the substrate ( 1q) had a bulky substituent at the ortho-position. The cyclizations with different mandelic acids were also investigated. Different electron-neutral and electron-donating substituents were tolerated in this transformation. An unprotected phenolic hydroxyl group at the para-position was also well tolerated ( 6b), and the remaining hydroxyl group could be further derivatized. Polysubstituted mandelic acids ( 4d–4f) smoothly delivered benzofuranone products (86 –91%). The electron-deficient phenol or mandelic acids (Br, Cl, F, etc.) did not deliver any desired products under the typical condition. Fortunately, a higher temperature gave the corresponding products in moderate yields ( 5s 67% yield, 6f 51% yield). This method was also applied in the late-stage functionalization of drug-like molecules [ 4t, estrone and 4u, (+)-γ-tocopherol], and the corresponding cyclization products were obtained in moderate yields. β-Hydroxyl acids were subjected to our reaction conditions, producing the desired dihydrocoumarins also in moderate to high yields and excellent chemoselectivities (Table 2, 8a–8e, 67 –88% yield). Table 2 | Substrate Scopea Note: DCM, dichloromethane; DCE, 1,2-dichloroethane. a 1b (0.22 mmol), 4a (0.2 mmol), B(C6F5)3 10 mol %, 1 mL DCM, 50 °C, 48 h. b0.2 mL DCM, B(C6F5)3 20 mol %, RT, 48 h. c0.2 mL DCE, 120 °C, 15 mL sealed tube, 48 h. To further demonstrate the utility of this cyclization, the optimal reaction was performed on a gram scale with a lower catalyst loading (5 mol %), and a slightly decreased yield was achieved after 48 h (Scheme 2a). Irganox HP-136,64,65 one of the most important carbon-centered radical antioxidants, is widely used to inhibit the degradation of oils and stabilize polymers at high temperatures. A one-pot synthesis of HP-136 was attempted, and to our delight, the target Irganox was generated in 95% yield (Scheme 2b). Via our approach, the molecule 9,66–68 which is isolated as a minor constituent from the leaf extract of elaeagnoidea, could be synthesized in a moderate yield (Scheme 2c). Scheme 2 | (a) Gram-scale reaction with 5 mol % catalyst. (b) One-pot synthesis of HP-136. (c) Synthesis of 9. Download figure Download PowerPoint Conclusion We have achieved the dual functionalization of unprotected phenols with α- and β-hydroxyl acids to produce an array of valuable benzofuran-2-(3H)-ones and dihydrocoumarins. Based on experimental results and DFT calculations, a plausible mechanism containing untypical Friedel–Crafts alkylation and subsequent lactonization is suggested. Gram-scale experiments and the late-stage functionalization of complex molecules highlight the high utility of this transformation. Moreover, this protocol successfully provides an example of relay catalysis, realized by the same catalyst with different forms. Footnote a There was also no esterification product observed when 1b and 2-(4-methoxyphenyl)acetic acid were tested under optimal conditions. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (no. 21903089), the Research Foundation for Natural Science of Guangdong Province (no. 2018A0303130178), the Advanced Talents of Sun Yat-sen University (no. 36000-18821101), and the Guangdong Provincial Key Laboratory of Chiral Molecule and Drug Discovery (no. 2019B030301005) for financial support of this program. References 1. 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• W3089572678 date "2021-08-01" @default.
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• W3089572678 title "Chemoselective Dual Functionalization of Phenols via Relay Catalysis of Borane with Different Forms" @default.
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