Activation of Epoxides by a Cooperative IronâThiolate Catalyst

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Activation of Epoxides by a Cooperative Iron-Thiolate Catalyst: Intermediacy of Ferrous Alkoxides in Catalytic Hydroboration Heng Song, Ke Ye, Peiyu Geng, Xiao Han, Rong-Zhen Liao, Chen-Ho Tung, and Wenguang Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02527 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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Activation of Epoxides by a Cooperative IronThiolate Catalyst: Intermediacy of Ferrous Alkoxides in Catalytic Hydroboration Heng Song,† Ke Ye,†† Peiyu Geng,† Xiao Han,† Rongzhen Liao,*†† Chen-Ho Tung,† and Wenguang Wang*† †

School of Chemistry and Chemical Engineering, Shandong University, 27 South Shanda Road, Jinan, 250100, China ††

School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, 430074, China

ABSTRACT. This paper describes a cooperative iron-thiolate catalyst

Cp*Fe(1,2-

Ph2PC6H4S)(NCMe) (Cp* = C5Me5, [1(NCMe)]) for regioselective hydroboration of aryl epoxide by pinacolborane (HBpin). The critical catalytic step involves the direct addition of epoxide to the catalyst rather than activation of the B-H bond of HBpin. Through iron-thiolate cooperation, [1(NCMe)] opens the aryl epoxide rings affording ferrous alkoxide compounds. Notably, the ferrous alkoxide intermediate (4) was structurally characterized after its isolation from the reaction of [1(NCMe)] with 2,3-diphenyloxirane. The more Lewis acidic hydroboranes such as H3B.THF and 9-BBN (BBN = borabicyclononane) can also be captured by [1(NCMe)]. The resulting iron-borane adducts [1H(BH2)] and [1H(BBN)] feature agnostic Fe---B-H interaction. DFT calculations indicate that the addition of HBpin across the iron-thiolate sites is endergonic by 12.9 kcal/mol, whereas it is exergonic by 20.2 kcal/mol with BH3 and 4.6 kcal/mol with 9-BBN. Combining the experimental data with theoretical studies, a mechanism of the substrates activation by [1(NCMe)], followed by HBpin addition is proposed for the catalysis. KEYWORDS：iron catalysis, metal-ligand cooperation, epoxide hydroboration, ferrous-borane adduct, ferrous alkoxide INTRODUCTION

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Cooperative metal-ligand reactivity in catalysis is an important concept in the development of homogeneous catalytic systems based on transition metal complexes.1 In contrast to the classical transition-metal catalysis in which all key transformations occur solely at the metal center, in cooperative catalysis both the metal and the ligand are functional sites, and are responsible for activation of the substrate or formation of new bonds.2 Early examples of cooperative metal-ligand catalysts include Shvo’s Ru-cyclopentadienone complex,3-5 and Noyori’s Ru-diamine systems6 for catalytic or transfer hydrogenation of carbonyl compounds and imines. More recently, metalligand cooperative strategy has expanded to cover a variety of catalysis and transformations.7-9

Chart 1. Selected Bifunctional Monomeric Iron(II) Catalysts with a Metal Hydride and Ligand Proton With regard to earth-abundant metal catalysis,10 iron-based cooperative catalysis is important in both enzymatic transformations and organic synthesis. For example, natural enzymes such as [FeFe] or [NiFe] hydrogenases reversibly catalyze proton production and H2 oxidation through the metal-ligand cooperation and with a fascinating activity.11 There has been noteworthy progress made in iron-based cooperative catalysts12 and some cases are shown in Chart 1. Well-known examples include the Fe-cyclopentadienone systems (I-III) based on the Knölke complex13 for catalytic hydrogenation and transfer hydrogenation reactions.14 Recently iron catalysts supported


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by PNP pincer ligands (IV) have been developed for hydrogenation of various unsaturated compounds such as esters,15 amides,16 nitriles,17 N-heterocycles18 and alkynes.19 Especially, Iron complexes bearing tetradentate chiral PNNP ligands (V) have been explored as catalysts of asymmetric transfer hydrogenation and asymmetric hydrogenation of ketones and imines.12b, 12d, 20 In such iron-amido cooperative catalysis, the generation of iron hydrides (Fe-H) together with the secondary amine (N-H) is essential for H2 splitting. Through modification of the first cooperation sphere, the iron complex VI with a pendant amine in the diphosphine ligand has been reported to be an efficient catalyst in the electrochemical oxidation of H2.12e, 21 Capturing the B-H bond by metal-ligand cooperation in catalytic hydroboration has been reported for several systems based on noble metals such as Ru,22 Rh and Ir.23 In particular, ferraboranes such as HFe4(CO)12BH2 and (μ-H)Fe3(CO)9BH3R have been known since the 1980s.24 Iron catalysts include an iron-boryl compound CpFe(CO)2(Bcat) for the borylation of alkenes under photolysis,25 iron-borohydride (Fe-H---BH3)26 bearing pincer type ligands for catalytic semi-hydrogenation of alkynes,19a and relevant reactions include hydrogenation of esters to alcohols,27 and dehydrogenation of N-heterocycles.18 With respect to metal-ligand cooperation for catalytic hydroboration, an iron(II) complex bearing alkoxy-tethered NHC ligands has been reported.28 The alkoxy site in this complex could act in conjunction with the metal center to activate HBpin for the hydroboration of terminal alkenes.

Scheme 1. Cooperative Iron-Thiolate Catalyst


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So called piano-stool iron(II) complexes comprising phosphine-based chelating ligands are well-known in organometallic chemistry.29 Although there have been intensive studies on the reactivity of CpFe (Cp = C5H5) and Cp*Fe iron complexes,30,31 examples of iron catalysis in this series remain rare.32 In our previous studies, we reported the reduction of N-benzylpyridinium cation (BNA+) to BNAH by iron(II) hydrides of the Cp*(P-P)FeH type (P-P = chelating diphosphine).33 These iron hydrides, however, are stable toward organic carbonyl compounds. By switching the diphosphine to phosphine-thiolate chelating ligand as shown in Scheme 1, we have found and report here a new cooperative iron-thiolate catalyst Cp*Fe(1,2-Ph2PC6H4S)(NCMe) ([1(NCMe)]) for regioselective hydroboration of aryl epoxides by HBpin, a mild borane reagent.34 Through iron-thiolate cooperation, [1(NCMe)] directly activates epoxides rather than the B-H bond of HBpin. The resulting ferrous-alkoxide compounds are the key intermediates in this catalysis. RESULTS AND DISCUSSION Synthesis and Characterization of [1(NCMe)]. Compound [1(NCMe)] was prepared in a straightforward manner by the reaction of [Cp*Fe(NCMe)3]PF633a,35 and sodium phosphinothiolate36 in MeCN at room temperature, and isolated as an pale brown solid in 84% yield. It is reactive toward CH2Cl2, being converting to the ferric complex [1Cl]0 (Supporting Information , Figure S1). A solution of [1(NCMe)] in THF is paramagnetic, and always NMR silent, even at 213 K. A concentrated solution of [1(NCMe)] in MeCN was stored at -30 oC overnight, and X-ray quality crystals were obtained. Crystallographic structural analysis revealed the neutral framework of Cp*Fe(1,2-Ph2PC6H4S)(NCMe) shown in Figure 1. It should be noted that the paramagnetic piano-stool iron(II) complexes in an 18-electron configuration are known.37 By Evan’s method,38 the magnetic moment of [1(NCMe)] was determined to be 3.11 μB in solution. The measurement


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is consistent with 3.18 μB for [Cp*Fe(dppe)(acetone)]CF3SO3, an intermediate spin iron(II) compound.37a

Figure 1. Structure of [1(NCMe)] with 50% probability thermal ellipsoids. For clarity, hydrogen atoms are omitted, and the two phenyl groups bonded at the phosphorus atom are drawn as lines. Selected bond distances (Å) and angles (): Fe-N(1), 1.903(2); Fe-S(1), 2.2797(5); Fe-P(1), 2.2070(5); N(1)-C(1): 1.140(2), P(1)-Fe-S(1), 86.84(2). Catalytic Hydroboration of Epoxides. The reduction of epoxides to alcohols is an important transformation in organic synthesis, because it facilitates selective hydration of alkenes via C=C bond epoxidation.39 The hydrogenolysis of epoxides is usually carried out with stoichiometric metal hydrides40 or with heterogeneous catalytic systems based on Raney Ni or Pd/C.41 There have been few examples for the selective hydrogenolysis of epoxides with homogenous catalysts.42 Especially, the homogenous catalysis based on an iron catalyst has never been reported. We began by examining [1(NCMe)] in the catalytic hydroboration of the simplest aryl epoxide, 2phenyloxirane, with HBpin. At a loading of 1.0 mol% in benzene, [1(NCMe)] catalyzed 2phenyloxirane hydroboration with up to quantitative conversion to the linear borate ester 3a within 15 min at room temperature. A control experiment showed under the same reaction conditions, that no reaction was observed in a catalyst-free system.


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The scope of epoxides substrates was further investigated. Although inactive toward aliphatic epoxides, compound [1(NCMe)] is an extremely effective catalyst for the hydroboration of aryl epoxides, and delivers ring opening with complete regioselectivity. These results are summarized in Table 1. The catalytic efficiency is not influenced by electron-donating or withdrawing groups at the para- or meta-position of the aromatic ring. In 15 min, all the aryl epoxides are hydroborated quantitatively to the corresponding linear borate esters 3b-3e (>99%). The hydroboration of substrates doubly substituted at C1 also leads to excellent results (3f, 3g). In addition to terminal epoxides, internal epoxides bearing various substituents such as methyl (3h), phenyl (3i) or an ester group (3j) at C2, are tolerated. The structure of 3i, shown in Table 1 was confirmed crystallographically. A cyclic internal epoxide gives preferentially, the -boron ester product (3k) in 10 h. The hydroboration of the disubstituted aryl epoxides requires a prolonged reaction time owing to steric factors. The epoxide unit is chemoselectively hydroborated in the presence of amide C=O groups to give 3l. Table 1. Scope of Hydroboration of Aryl Epoxides Catalyzed by [1(NCMe)]a


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a

Reaction conditions: epoxide substrates (0.5 mmol), HBpin (0.55 mmol), 1.0% mmol catalyst loading relative to the substrates in 0.6 mL C6D6 at room temperature. Yields were determined by 1H NMR spectroscopy using 1,3,5trimethoxybenzene as an internal standard.

Mechanistic Insights. We initially thought our catalytic reactions involve B-H bond activation by [1(NCMe)] through iron-thiolate cooperation. [1(NCMe)] captures the B-H bond of some hydroboranes. The formation of stable iron-borane adducts is however very dependent on the Lewis acidity of the hydroboranes. Hydroboranes such as H3BTHF and the dimer [(9-BBN)2] react with [1(NCMe)] providing the stable iron-borane adducts [1H(BH2)] and [1H(BBN)], respectively (Scheme 2). The 1H NMR spectrum of [1H(BH2)] exhibits distinct signals at  -16.9 (br) for coordinated B-H-Fe and  -0.6 (br) for the uncoordinated H2B-S hydrogen atoms. For [1H(BBN)], the proton resonance of Fe-H-B is observed at  -18.5.


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Scheme 2. Reactions of [1(NCMe)] with H3BTHF and [(9-BBN)2] Dimer In molecular structures of [1H(BH2)] and [1H(BBN)], the BH3 and 9-BBN moieties are stabilized by their B-H bonds coordinating with the Fe-S unity (Figure 2). Both structures feature a four-membered ring, consisting of Fe, H, B and S atoms. The binding of BH3 and 9-BBN at the iron-thiolate sites is strong, and transfer of the boron fragments to aryl oxides was not observed.

Figure 2. Structures (50% probability thermal ellipsoids) of [1H(BH2)] (left) and [1H(BBN)] (right). Owing to the decrease in Lewis acidity, the binding of HBpin through iron-thiolate cooperation is unfavorable. In the NMR spectra, there are no 1H or 31P signals observed for the reaction of [1(NCMe)] with HBpin (1:1 ratio in toluene-d8 solution). To compare the different reactivity of [1(NCMe)] toward the three hydroboranes, DFT calculations43 of free energy changes (G) were performed for each elementary reaction (Scheme 3) The calculated results show that the 18electron compound [1(NCMe)] is a quintet, and the dissociation of MeCN from the Fe center to


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form a 16-electron species (1) is slightly exergonic (-3.7 kcal/mol). This prediction is consistent with our experimental observation that the MeCN ligand in [1(NCMe)] is labile and replaceable for example by CO, affording [1(CO)] (Supporting Information, Figure S2).

Scheme 3. Calculated Free Energies Changes (G) for Reactions of [1(NCMe)] with Various Substrates Based on the dissociation of the MeCN ligand, the addition of hydroboranes to 1 was examined. For the iron-borane adducts [1H(BH2)] and [1H(BBN)], the optimized structures are very close to those determined by X-ray crystallographic analysis. The root-mean-square deviations (RMSD) for the key bond distances are all within 0.05 Å (Supporting Information, Table S1). According to these calculations, the reactions of 1 with BH3 and 9-BBN are exergonic by 20.2 kcal/mol and 4.6 kcal/mol, respectively. Both [1H(BH2)] and [1H(BBN)] were calculated to be singlets at the B3LYP*-D3 level, but the reaction of HBpin and 1 becomes endergonic by 12.9 kcal/mol. These results agree well with our experimental observations.


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To gain deeper insight into the initial step of the catalysis, the free energy change for the elementary reaction of 1 with 2,3-diphenyloxirane was calculated. Surprisingly, the reaction was found to be exergonic by 6.2 kcal/mol. This indicates that the addition of an epoxide to 1 is thermodynamically favorable. In agreement with the DFT predictions, the catalysis is initiated by the reaction of [1(NCMe)] with epoxides rather than with HBpin. When [1(NCMe)] is treated with trans-2,3-diphenyloxirane in benzene, the color of the solution immediately turns deep brown. The production of ferrous alkoxide complexes was deduced from ESI-MS spectral analysis (Figure 3).

Figure 3. ESI-MS spectroscopic analysis for the reaction solutions of [1(NCMe)] with 2,3diphenyloxirane.

Figure 4. Structure (50% probability thermal ellipsoids) of 4. Selected bond distances (Å) and angles (): Fe-S(1), 2.1997(9); Fe-P(1), 2.1914(8); Fe-O(1), 1.960(2). P(1)-Fe-S(1), 88.77(3); O(1)-Fe-S(1), 85.85(6); O(1)-Fe-P(1), 89.45(6).


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The ferrous alkoxide (4) was isolated from the reaction of [1(NCMe)] with 2,3-diphenyloxirane and submitted to further examination. X-ray crystallographic analysis confirmed the identity of a ferrous alkoxide compound. In 4, Fe-S cooperatively opens the ring of the epoxide forming a fivemembered genuine metallic-heterocycle with anti-configuration of phenyl groups and hydrogen atoms at C-1 and C-2 atoms (Figure 4). The Fe-O bond distance is 1.960(2) Å, consistent with the range from 1.94 to 1.97 Å reported for the Fe(Ph2PC6H4CHO)2(CO)L series.44

Scheme 4. Addition of 2,3-Diphenyloxirane to [1(NCMe)] Produces an Intermediate of Ferrous Alkoxide 4 for Hydroboration In C6D6 solution, the

31

P NMR spectrum of 4 exhibits a singlet at δ 92.8. In the 1H NMR

spectrum, the proton signals of OCH and SCH are displayed as two doublets at δ 3.93 (JHH = 10.6 Hz, 1H) and 2.38 (JHH = 10.6 Hz, 1H), respectively. By contrast, the proton resonances of the epoxide ring are shown as a singlet at δ 3.63 for 2,3-diphenyloxirane. As expected, the further reaction of complex 4 with HBpin provides the borate ester 3i, which was identified by GC-MS analysis. The recovery of 4 was achieved by the subsequent addition of 2,3-diphenyloxirane to the reaction mixture, judging from 31P NMR studies (Scheme 4). In our catalytic reactions, aryl epoxides are opened at the benzylic carbon giving -boron esters as the sole products. The retention of stereochemistry was established by transforming of (1R,2R)(+)-1-phenylpropylene oxide (2m) to (R)-1-phenyl-2-propanol (3m, eq 1) through the catalytic


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hydroboration followed by protonolysis of the resulting boron esters. A scale-up hydroboration of 2m (3.0 mmol, 400 mg) with HBpin (3.3 mmol) was conducted under the catalytic conditions. After reaction at room temperature for 5h, the solvent was removed under vacuum. The crude product was treated with aqueous HCl, and the mixture was refluxed in MeOH for 1 h.45 The above two-step transformation provided the optical purity of 3m (ee ~ 99%) in 93% yield (Figure S42).46

To understand the regioselectivity of the ring opening, the mechanism of catalytic hydroboration of 2-phenyloxirane, the simplest aryl epoxide, was investigated by DFT calculations. In the uncatalyzed reaction, the direct addition of HBpin to 2-phenyloxirane proceeds via a concerted transition state (Figure S46) that involves C-O bond cleavage of the epoxide, nucleophilic attack of the alkoxide on the boron moiety, and hydride transfer from boron to carbon C1 of the epoxide. The nature of the transition state has been confirmed by intrinsic reaction coordinate (IRC) calculations47 (Figure S49). The barrier was calculated to be 35.7 kcal/mol relative to the isolated reactants. Such a high barrier suggests the direct addition of HBpin to 2-phenyloxirane is kinetically unfavorable and requires a catalyst to ensure the reaction.


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Figure 5. Gibbs free energy diagram (in kcal/mol) for the catalytic addition of HBpin to 2phenyloxirane. Cis and Trans define the configuration of the substrate phenyl ring relative to thiophenolate ring of the catalyst. Schematic representations of intermediates are shown for the cis configuration. Based on the experimental results, we considered the pathway of addition of 2-phenyloxirane to 1, followed by HBpin addition. The catalyst is not chiral but the 2-phenyloxirane used in our experiment is racemic and, for simplicity (R)-2-phenyloxirane was used in all our calculations. The overall Gibbs free energy diagram is shown in Figure 5, and the structures of the transition states are shown in Figures 6 and Figures 7. First, the substrate oxygen can coordinate to the ferrous ion from two different sides of the catalyst, generating a chiral metal center with configurations corresponding to either Fe(R) (Int1C) or Fe(S) (Int1T) (for definition of the configuration, see supporting information Scheme S1). The labeling of C (Cis) and T (Trans) denotes the configuration of the substrate phenyl ring relative to the catalyst thiophenolate ring. In Int1, heterolytic ring opening of the substrate takes place, and this is coupled with nucleophilic attack of the thiolate on the epoxide carbon. The cleavages of both C1-O and C2-O bonds are considered. TS1C-C1 has the lowest barrier, which is only 9.9 kcal/mol relative to the catalyst and the epoxide


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substrate. Compared to the quintet state, the triplet and singlet barriers are 8.6 and 25.0 kcal/mol higher, respectively. The barriers reported are always relative to the spin state with the lowest energy. Consequently, the reaction proceeds in the quintet state. In TS1C-C1, the scissile C1-O bond is 2.42 Å, the nascent C1-S bond is 2.77 Å and the barrier for TS1T-C1 (quintet) is 4.7 kcal/mol higher. For the cleavage of C2-O, the barriers for TS1C-C2 and TS1T-C2 were calculated to be 26.3 and 22.8 kcal/mol, respectively. The origin of the regioselectivity can be understood from the heterolytic nature of the C-O bond cleavage at the transition state. A carbon cation is generated at either C1 or C2 of the transition state, and the substrate phenyl ring can stabilize the transition state by delocalizing the positive charge in TS1T-C1 and TS1T-C1. The addition of the epoxide to the catalyst is exergonic by 7.3 kcal/mol. In Int2, an alkoxide is coordinated to the ferrous ion.

Figure 6. Structures of transition states for the ring-opening of the epoxide at both C-1 and C-2 with different configurations. For clarity, extraneous hydrogen atoms are omitted. Distances are given in Å and the imaginary frequencies are also indicated. Gibbs free energies of different spin states are given in kcal/mol relative to the quintet of TS1C-C1. From Int2C, the HBpin substrate can undergo electrophilic addition to the alkoxide, and the formation of Int3C is close to isoergonic. Subsequently, hydride transfer from the boron to the


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ferrous ion takes place via TS2C in the singlet state, and is associated with a barrier of only 6.7 kcal/mol relative to Int3C. The triplet and quintet barriers are 5.1 and 3.1 kcal/mol higher, respectively. At TS2C, the Fe-H and B-H distances are 1.56 Å and 1.81 Å, respectively. Homolytic cleavage of S-C1 bond through TS3C has a barrier of 16.3 kcal/mol relative to Int4C in the brokensymmetry open-shell singlet state. In TS3C, the critical S-C1 distance is 2.33 Å, and the spin densities on Fe and C1 are 0.49 and -0.38, respectively. Finally, transfer of hydrogen atoms from Fe to C1 produces the hydroborated product with a very minor barrier. Comparing the barriers between Cis and Trans configurations, the Cis configuration is preferred. The total barrier was calculated to be 16.3 kcal/mol, and the cleavage of S-C1 bond turns out to be the rate-limiting step of the entire catalysis. This mechanism differs from the Rh+-catalyzed hydroboration of cyclic γ,δunsaturated amide48 and Ni0-catalyzed hydroboration of aldehydes and imines,49 in which an oxidative addition of HBpin to the metal takes place first. For Ru-catalyzed hydroboration of alkynes, DFT calculations by Wu and co-workers50 reveal that a rate-limiting oxidative hydrogen migration from HBpin to alkyne proceeds prior to the reductive boryl migration reaction.

Figure 7. Structures of transition states for the hydride transfer, homolytic S-C bond cleavage, and hydrogen atom transfer. For clarity, extraneous hydrogen atoms are omitted. Distances are given in Å, spin densities on Fe, H, and C-1 are shown in red italics for the broken-symmetry singlet of TS3C and the triplet of TS4C, and the imaginary frequencies are also indicated. Relative Gibbs free energies of different spin states are given in kcal/mol.


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By theoretical studies, the possibility of the addition of HBpin to 1, followed by 2-phenyloxirane addition was also ruled out. The calculated energy diagram and structures of transition states are shown in Figure S47-S48 (see Supporting Information). Addition of HBpin to the catalyst already has a barrier of 17.5 kcal/mol, which is higher than the total barrier for the pathway initiated by 2phenyloxirane addition (16.3 kcal/mol). In the subsequent steps, the epoxide oxygen approaches the boron atom, and this is coupled with a heterolytic C-O bond cleavage and hydride transfer from Fe to C1. The closed-shell singlet state preferred has a barrier of 41.4 kcal/mol, which is even higher than that in the uncatalyzed reaction.

Scheme 5. Proposed Catalytic Cycle for Aryl Epoxides Hydroboration by the Iron-Thiolate Catalyst CONCLUSION


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Transition metal-catalyzed hydroboration reactions usually involve an initial step of B-H bond activation by the catalyst.23, 47, 51 In the case at hand, bifunctional catalyst [1(NCMe)] reacts directly with the epoxide substrate. Through iron-thiolate cooperation, [1(NCMe)] is capable of opening the aryl epoxide rings, affording ferrous alkoxide compounds. Theoretical studies support the experimental results showing that the activation of HBpin is thermodynamically unfavorable. On the basis of experimental data and DFT calculations, the catalytic mechanism is proposed in Scheme 5. It was predicted that HBpin undergoes electrophilic addition to the alkoxide (Int2C), and the formation of Int3C is essentially isoergonic. Subsequently, hydride transfer from the boron to the ferrous ion affords the iron(II) hydride intermediate Int4C. The release of hydroborated product could be through S-C1 bond cleavage steps, and transfer of hydrogen atoms from Fe to C1. Ferrous-alkoxide compounds are uncommon.52 Direct addition of unsaturated organic carbonyl substrates or epoxides to an iron(II) precursor has not been reported previously. Notably, the ferrous alkoxide (4) was isolated from the reaction of [1(NCMe)] with 2,3-diphenyloxirane, and characterized by X-ray crystallographic analysis. Furthermore, cooperative iron-thiolate reactivity allows [1(NCMe)] to capture BH3 and 9-BBN providing new iron-borane adducts. Such an iron complex with an agostic Fe∙∙∙H-B interaction53 is particularly of interest because of its relevance to the B-H bond activation.54 Although [1H(BH2)] and [1H(BBN)] are stable toward carbonyl substrates and epoxides, they provide valuable knowledge with which to optimize the catalyst by tuning the Lewis acid-base coordination sphere interactions. ASSOCIATED CONTENT


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Experimental details, NMR (1H, 31P) and IR spectra, and crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org Accession Codes CCDC 1558573 and 1555601-1555605 and 1555607 contain the supplementary crystallographic data

for

this

paper.

These

www.ccdc.cam.ac.uk/data_request/cif,

data

can

be

obtained

free

of

charge

via

or by emailing [email protected],

or

contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] [email protected] Notes The authors declare no competing financial interest. Supporting Information Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org and includes detailed experimental procedures, characterization of products, computational details, and further details on reaction pathways (PDF) ACKNOWLEDGMENT We gratefully acknowledge the financial support from the “1000 Youth Talents Plan”, the Natural Science Foundation of China (21402107, 91427303, and 21503083), the Natural Science


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ACS Catalysis

(46) Vitale, P.; Perna, F. M.; Perrone, M. G.; Scilimati, A. Tetrahedron: Asymmetry 2011, 22, 1985–1993. (47) Koren-Selfridge, L.; Londino, H. N.; Vellucci, J. K.; Simmons, B. J.; Casey, C. P.; Clark, T. B. Organometallics 2009, 28, 2085–2090. (48) Yang, Z.-D.; Pal, R.; Hoang, G. L.; Zeng, X. C.; Takacs, J. M. ACS Catal. 2014, 4, 763−773. (49) King, A. E.; Stieber, S. C. E.; Henson, N. J.; Kozimor, S. A.; Scott, B. L.; Smythe, N. C.; Sutton, A. D.; Gordon, J. C. Eur. J. Inorg. Chem. 2016, 1635−1640. (50) Song, L.-J.; Wang, T.; Zhang, X.; Chung, L. W.; Wu, Y.-D. ACS Catal. 2017, 7, 1361−1368. (51) Chong, C. C.; Kinjo, R. ACS Catal. 2015, 5, 3238−3259. (52) (a) Chambers, M. B.; Groysman, S.; Villagrán, D.; Nocera, D. G. Inorg. Chem. 2013, 52, 3159–3169. (b) Bellow, J. A.; Martin, P. D.; Lord, R. L. Groysman, S. Inorg. Chem. 2013, 52, 12335−12337 (c) Bryndza, H. E.; Tam, W. Chem. Rev. 1988, 88, 1163−1188. (53) Lalaoui, N.; Woods, T.; Rauchfuss, T. B.; Zampella, G. Organometallics 2017, 36, 2054– 2057. (54) (a) Contemporary Metal Boron Chemistry I: Borylenes, Boryls, Borane -complexes, and Borohydrides, Marder, T. B.; Lin, Z., Eds.; springer-Verlag: Berlin, 2008. (b) Alcaraz, G.; SaboEtienne, S. Angew. Chem. Int. Ed. 2010, 49, 7170–7179. (c) Maity, A.; Teets, T. S. Chem. Rev. 2016, 116, 8873–8911.

Table of Contents Graphic and Synopsis


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This paper describes a cooperative iron-thiolate catalyst [1(NCMe)]) for regioselective hydroboration of aryl epoxide by pinacolborane (HBpin) reagent. Instead of activating the B-H bond of the reagent, the critical step of catalysis involves the direct addition of the substrates to the catalyst affording ferrous alkoxides. The ferrous alkoxide intermediate 4 was isolated from the reaction of [1(NCMe)] with 2,3-diphenyloxirane, and structurally characterized.


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Activation of Epoxides by a Cooperative IronâThiolate Catalyst

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