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Mechanistic Studies on the Rearrangement of a Boron Cation: From a nido-Carborane to a Planar Boracycle Ya-Fan Lin,‡ Chao-Tang Shen,‡ Yu-Ting Hsiao, Yi-Hung Liu, Shie-Ming Peng, and Ching-Wen Chiu* Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan, 10617 S Supporting Information *

ABSTRACT: The reaction mechanism of the [Et3BH]−-induced conversion of a dicationic boron compound ([Cp*B-IMes]2+, [1]2+) to a planar neutral carbene-coordinated borabenzene (2) is investigated experimentally and theoretically. Owing to the steric congestion around the boron center, bulky [Et3BH]− attacks the less encumbered carbon atom of Cp*, leading to a metastable 5-borabicyclo[2.1.1]hex-2-ene borenium cation ([3-CH]+) at low temperature. Upon raising the temperature, the bicyclic-borenium ion undergoes sequential pericyclic reactions, including suprafacial [1,3]-sigmatropic shift, two-electron electrocyclic ring opening, and then [1,2]-hydrogen shift to a planar cyclic borenium ion, [4-CH]+, which can then be deprotonated to yield the neutral aromatic borabenzene. The attack of hydride at the boron center of [1]2+ was excluded through the preparation and isolation of 6-borabicyclo[3.1.0]hexenylium, [3-BH]+, which could not be transformed into [3-CH]+ or [4CH]+. The borenium ion rearrangement follows first-order kinetics with activation parameters of ΔH⧧ = 19.6 (±0.31) kcal/mol and ΔS⧧ = −2.03 (±1.15) cal/mol·K. The observed kinetic isotope effect of 0.87 at 265 K is consistent with the DFT-calculated reaction mechanism, which predicts an overall KIE value of 0.90. These results show that [1]2+ reacts with [Et3BH]− as a carbonbased electrophile and the formation of an electron-deficient borenium center is responsible for the skeletal rearrangement of the nido-cluster.

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INTRODUCTION Owing to the incomplete valence octet electron configuration of tricoordinate boron centers, neutral boranes can be utilized as optoelectronic materials,1 molecular sensors,2 and homogeneous catalysts for small-molecule activation.3 The avidity of tricoordinate boron centers for electrons also plays an important role in the rearrangement reactions of boranes.4 As demonstrated in the early studies on the sigmatropic rearrangement of borylated cyclopentadienes, the increase in electron population of the boron 2p orbital through π-donation or base coordination resulted in a significant increase in the activation barrier of the pericyclic reaction.5 The reaction that involves structural rearrangement of boranes could also be very complicated. For example, thermal skeletal rearrangement of a 7-borabicyclo[2.2.1]heptadiene led to an aromatic borepine, which underwent further rearrangement to yield 5-bora-3a,4dihydro-5H-benzindene (Scheme 1a).6 In addition to tricoordinate boranes, rearrangement reactions have also been identified in the photolysis of tetracoordinated borate and borane adducts.7 However, the structural rearrangements of borenium ions are relatively limited, despite being isoelectronic to carbenium ions, which are generally unstable and prone to structural rearrangements. Substituting one of the anionic substituents of a borane with a neutral ligand affords the so-called borenium ion. The added positive charge renders the borenium ion considerably more © XXXX American Chemical Society

electron deficient and therefore more reactive than neutral boranes.8 Indeed, the borenium ion has been recognized as the active species in electrophilic borylation of arene,9 hydrogenation of imine,10 and other Lewis acid activated reactions.11 Interestingly, investigations on the reactivity of [(acridine)BCl2]+ revealed that borenium ions could serve not only as boron-based Lewis acids but also as carbon-based Lewis acids.12 Although the borenium ion is isoelectronic to the carbenium ion, rearrangement reactions of boron cations were reported only a few times. In the base-coordination-induced 1,2migration of diborane (4), the reaction is proposed to proceed through a transition state that features a borate−borenium zwitterionic structure (Scheme 1b).13 Thermolysis of boracyclohexadiene boronium ion affords the NHC-stabilized borabenzene.14 Our group has also demonstrated that structural rearrangement of a hypercoordinated boron dication ([Cp*B-IMes]2+, [1]2+) to a planar neutral borabenzene (2) could be initiated through the addition of Li[Et3BH] (Scheme 1c).15 Similar boron atom insertion to the Cp* ring has been observed only within the coordination sphere of transition metals.16 While the related boron monocations ([Cp*BR]+) Special Issue: Organometallics in Asia Received: January 29, 2016

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DOI: 10.1021/acs.organomet.6b00077 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

temperature experiments and theoretical calculations were carried out, and the results are summarized herein.

Scheme 1. Selected Rearrangement Reactions of BoronContaining Molecules

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RESULTS AND DISCUSSION Detection of [3-CH]+ and [4-CH]+. Initial attempts in the isolation or detection of the reaction intermediate in the 1:1 and 1:2 reactions of [1][AlCl4]2 and Li[Et3BH] in d8-THF were unsuccessful due to the instability of [AlCl4]−. In THF, [1][AlCl4]2 decomposes to yield the less soluble Cp*BCl2− IMes adduct slowly at room temperature and rapidly at low temperature. This result not only shows that the chloride abstraction of Cp*BCl2−IMes with AlCl3 to give [1][AlCl4]2 is reversible but also confirms that the boron center of [1]2+ is indeed Lewis acidic. In THF, no species other than unreacted starting material ([1]2+), reaction product (2), and decomposition products (Cp*BCl2−IMes and imidazolium) were identified. However, when the reaction was carried out in CD2Cl2 at −78 °C, a clean transformation of [1]2+ to a new species ([3-CH]+) was identified (Figure 1a). The characterization of [3-CH]+ was carried out at 243 K using multinuclear NMR spectroscopy. Cation [3-CH]+ features a sharp 11B resonance at −32.5 ppm, suggesting the boron center remains highly shielded. The methyl groups of Cp* split into three singlets at 1.50, 0.45, and 0.92 ppm with integration of six, six, and three hydrogen atoms, respectively. In addition, a broad hydrogen signal with integration of one hydrogen atom was observed at 1.58 ppm. When the reaction mixture was warmed to 265 K, a new set of signals appeared and the formation of the new species was accompanied by the disappearance of [3-CH]+ (Figure 1). The newly formed species at 265 K is assigned as a boracyclohexa2,5-diene borenium ([4-CH]+). The 1H NMR signal of the para-methine hydrogen detected at 2.93 ppm as a quartet is coupled to the para-methyl group (1.06 ppm) with a 3JH−H coupling constant of 7.4 Hz (Figure S4). The ortho- and metamethyl groups of [4-CH]+ are respectively detected at 1.43 and 1.91 ppm. In addition to the detection of a 11B NMR resonance at 50.1 ppm, [4-CH]+ possesses an unusually downfield shifted 13 C NMR signal at 175.4 ppm, which is assigned to the electropositive meta-carbon of [4-CH]+. These spectroscopic data are comparable to that reported for the pyridine-stabilized cyclic borenium salts.18 Such base-stabilized cyclic borenium salts have been shown to play an important role in the ligand substitution reaction of borabenzene.14,19 Interestingly, while sequential protonation and chloride abstraction of borabenzene−pyridine adducts give rise to the formation of 2,4- and 2,5-boracyclohexadiene isomers, rearrangement of [3-CH]+ gives rise to the 2,5-isomer exclusively. In CD2Cl2, [4-CH]+ is stable at room temperature for at least several days, and no borabenzene (2) formation could be identified in the absence of bulky bases. Addition of PtBu3 to [4-CH]+ in CD2Cl2 leads to the formation of borabenzene 2 and [HPtBu3]+ (Figure S8). These results imply that the hydride-induced transformation of [1]2+ to 2 proceeds through two detectable intermediates, a bicyclic and a cyclic borenium cation. In addition, the reaction with PtBu3 also confirms the two-electron oxidation of hydride to proton through the sequential hydride addition and deprotonation process. Spontaneous deprotonation of [4-CH]+ can be triggered by changing the solvent from noncoordinating CD2Cl2 to Lewis basic THF, but the yield of 2 is significantly lower due to the undesired decomposition of [4-CH]+ in THF.

have been isolated for decades,17 conversion of the nidocarborane structure to planar borabenzene has been observed only for [1]2+. The unprecedented conversion of a boron dication ([1]2+) to a planar borabenzene (2) has motivated us to launch a mechanistic study. Since [1]2+ could serve as either a boron- or a carbon-based Lewis acid,12 two reaction mechanisms are proposed (Scheme 2). [3-BH]+ and [3-CH]+ represent the Scheme 2. Proposed Reaction Mechanisms for the HydrideInduced Transformation of [1]2+ to 2

hydride addition products derived from reactions at the boron and the carbon Lewis acidic sites, respectively. The bicyclic borenium cations then undergo skeletal rearrangement to yield the cationic six-membered-ring molecules, [4-BH]+ and [4CH]+. Subsequent deprotonation of [4-BH]+ and [4-CH]+ affords the aromatic borabenzene 2. In order to elucidate the hydride-induced rearrangement reaction, a series of lowB


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Figure 1. Conversion of [3-CH]+ to [4-CH]+ in CD2Cl2 at 265 K. 1H NMR spectrum of [3-CH]+ at 265 K: (a) initial, (b) after 60 min, and (c) after 300 min. (Signals of Et3B are denoted by *.)

Theoretical Computations on [3-BH]+ and [3-CH]+. Additional information on the species observed at low temperature was obtained from DFT calculations. [3-BH]+ and [3-CH]+ were optimized at the B3LYP/6-311g(d,p) level using the polarizable continuum model in CH2Cl2 medium (Figure 2). The optimized geometry of [3-BH]+ features a 6-

Structural features of [3-CH]+ are reminiscent of that determined for the isoelectronic bicyclo[2.1.1]hex-2-en-5ylium ions.20 The planar boron center is significantly tilted toward the CC double bond as a result of through-space homoconjugation,21 leading to a hypercoordinated boron center with a highly shielded boron nucleus resonance at −34.7 ppm. In addition to the 11B chemical shift, the calculated 1 H and 13C NMR resonances are comparable to that determined experimentally, suggesting that the species detected at 243 K is likely [3-CH]+ (Figure S12). Isolation of [3-BH]+. While the low-temperature NMR studies favor the “carbon-based Lewis acid” pathway for the conversion of [1]2+ to 2, the possibility of forming [3-BH]+ prior to the detection of [3-CH]+ still cannot be excluded. To verify whether [3-CH]+ results from rearrangement of [3BH]+, an alternative synthetic route to [3-BH]+ was developed. Borenium ion [3-BH]+ was prepared from chloride abstraction of Cp*BHCl−IMes with K[B(C6F5)4] in benzene (Scheme 3). The 11B resonance of [3-BH]+ detected at 21 ppm is comparable to that predicted by DFT calculation, and the presence of a B−H bond is unambiguously recognized by the detection of a 1JB−H coupling of 120 Hz. The observed coupling

Figure 2. Optimized geometries of [3-BH]+ and [3-CH]+.

borabicyclo[3.1.0]hexenylium structure with a tetracoordinated boron center. The boron center is bonded to Cp* in a η2 fashion, resulting in an isosceles CBC triangular structure. The calculated 11B chemical shift of [3-BH]+ is 19.9 ppm, which is much downfield shifted from that observed experimentally (− 32.5 ppm). On the basis of the 11B NMR data, the presence of [3-BH]+ at 243 K would be a less likely scenario. On the other hand, isomer [3-CH] + is a 5borabicyclo[2.1.1]hex-2-ene borenium cation and is 8.5 kcal/ mol less stable than [3-BH]+, showing that the central boron atom of [1]2+ is more acidic than the sp2 carbons of Cp*.

Scheme 3. Synthesis of [3-BH]+

C


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Organometallics constant is comparable to that determined for a hydrido boron dication ([py3BH]2+, 115 ± 10 Hz)22 and the naphthalenetrapped parent borylenes (∼118 Hz).23 Unlike the 1H NMR spectrum of Cp*BHCl−IMes, where the five methyl groups of Cp* were split into five distinct signals, the Cp* of [3-BH]+ was detected as a sharp singlet at 1.45 ppm, suggesting the existence of rapid sigmatropic rearrangement of the [BHIMes]+ fragment on the Cp ring. The η2-coordination of Cp* to the boron center of [3-BH]+ was established by a single-crystal X-ray diffraction analysis (Figure 3). In the solid state, the boron center of [3-BH]+

Scheme 4. Proposed Reaction Mechanism for the Transformation of [1]2+ to 2

addition of bases to deprotonate the borenium to yield the carbene-stabilized borabenzene. The proposed mechanism predicts a one-to-one conversion of [3-CH]+ to [4-CH]+, and the reaction rate should be firstorder to the concentration of [3-CH]+. Indeed, monitoring the conversion process at 265 K reveals that the summation of concentrations of [3-CH]+ and [4-CH]+ remains unchanged throughout the reaction course (Figure S9). In addition, the natural logarithm of the concentration of [3-CH]+ is linear with respect to the reaction time with a negative slope of 1.31 × 10−4 s−1 at 265 K. To determine the activation parameters of the rearrangement process, a set of kinetic experiments were performed. As derived from the Eyring plot of the reaction (Figure 4), the

Figure 3. Molecular structure of [3-BH][B(C6F5)4]. Counteranion and hydrogen atoms except for the boron hydride are omitted for clarity. Thermal ellipsoid probability is set at 50%. Selected bond distances (Å) and angles (deg): B(1)−C(1) 1.622(8), B(1)−C(22) 1.758(8), B(1)−C(26) 1.740(9), C(22)−C(23) 1.416(8), C(23)− C(24) 1.410(8), C(24)−C(25) 1.387(8), C(25)−C(26) 1.439(8), C(22)−C(26) 1.511(8); C(1)−B(1)−C(22) 119.7(5), C(1)−B(1)− C(26) 130.6(5), C(22)−B(1)−C(26) 51.2(3), C(22)−C(26)−B(1) 65.0(4), C(26)−C(22)−B(1) 63.8(4).

adopts a distorted tetrahedral geometry with a sum of the C− B−C angles of 301.5°. The B(1)−C(22) and B(1)−C(26) bond distances of 1.758(8) and 1.740(9) Å are comparable to that determined in [1]2+ (av 1.759 Å), but are markedly longer than that determined for the naphthalene-trapped NHCstabilized parent borylene (av 1.616 Å),23 THF-trapped nonclassical borane (av 1.607 Å),24 and 7-boratabicyclo[4.1.0]hepta-2,4-diene (av 1.60 Å).7f The elongation of B−C bonds of the CBC triangular structure is attributed to the lack of electron density on the boron center of [3-BH]+ to form strong σ-bonds with the π-system. The most important property of [3-BH]+ is that it is stable in CD2Cl2 at room temperature, indicating that the formation of [3-CH]+ at low temperature does not result from rearrangement of [3-BH]+. These experimental results further support that [1]2+ reacts with Li[Et3BH] as a carbon-based Lewis acid instead of a boron-based Lewis acid. Proposed Mechanism. With the aforementioned results, we propose that the Li[Et3BH]-induced transformation of boron dication ([1]2+) to borabenzene (2) is a three-stage process (Scheme 4). Owing to the excessive steric congestion around the boron center, the bulky [Et3BH]− attacks the less protected Cp* ring, leading to the 5-borabicyclo[2.1.1]hex-2ene borenium ([3-CH]+). At temperatures higher than 260 K, [3-CH]+ undergoes a sequential [1,3]-sigmatropic migration, disrotatory electrocyclic ring opening, and [1,2]-hydrogen migration to yield the planar cyclic borenium [4-CH]+. The final stage of the borabenzene formation process requires the

Figure 4. Eyring plot of the [3-CH]+ to [4-CH]+ reaction carried out at 260, 265, 273, and 278 K.

conversion process features an activation enthalpy and entropy of 19.6 (±0.31) kcal/mol and −2.03 (±1.15) cal/mol·K, respectively. With these two activation parameters, the ΔG⧧ is calculated to be 20.2 (±0.43) kcal/mol at 298 K. The relatively small ΔS⧧ value is also consistent with the proposed unimolecular reaction process. Finally, a kinetic isotope effect of the borenium-involved rearrangement process was investigated by monitoring the reaction progress at 265 K using Li[Et3BH] and Li[Et3BD] (Figure 5). The measured kinetic isotope effect (KIE) value of D


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containing 1,5-enynes to benzene derivatives.25 Later, a disrotatory ring opening of [A]+ yields a boraalkene containing cyclic carbenium ion [B]+, the meta-isomer of [4-CH]+. Finally, [B]+ undergoes a [1,2]-hydrogen shift to form [4-CH]+. The transition state TS2 is surprisingly low in energy considering the fact that a C−C bond is broken during the process. Being the highest transition state of the process, the energy difference between [3-CH]+ and TS2 also represents the activation energy of the rearrangement reaction. The calculated Gibbs free energy difference of 11.7 kcal/mol is slightly smaller than the experimental value obtained from the Eyring plot. This result could originate from the negligence of the ion-pairing effect in the DFT calculations. The KIE value (0.90) calculated between [3-CH]+ and [TS2]+ with the Bigeleisen−Mayer equation is consistent with that determined experimentally, implying that the identified rearrangement pathway is satisfactory.26 A wider Csp2−Me(H)C−Csp2 angle and shorter Me(H)C−Csp2 bonds of [TS2]+ result in a more congested environment around the C−H/D carbon, which might be responsible for the inverse KIE.

Figure 5. Plot of ln[3-CH/D]+ vs reaction time at 265 K.

0.87 shows a small effect on deuteration, implying that the breaking of the C−H/D bond is not involved in the ratedetermining step. In other words, the activation energy of the rearrangement process is less likely to be determined by the [1,2]-hydrogen migration from [B]+ to [4-CH]+. Since the C− H/D carbon remains sp3 during the transformation from [3CH]+ to [B]+, the observed inverse KIE might result from the presence of a congested transition state in the process. Calculations on the Rearrangement Process. To gain more insights into the ring-expansion process, we have carried out a series of DFT calculations. In all cases, molecules were optimized with the 6-311G(d,p) basis set at the B3LYP level of theory with the PCM solvation model (solvent = CH2Cl2). The results are summarized in Figure 6. The calculated rearrangement of [3-CH]+ to [4-CH]+ suggested a three-step pathway, which begins with an endothermic suprafacial [1,3]-sigmatropic migration of the B−C1 bond to form the 6-borabicyclic[3.1.0]hex-2-ene borenium cation [A]+. Compound [A]+ is isoelectronic to bicyclo[3.1.0]hexenium cation, a proposed reaction intermediate of the gold(I)-catalyzed cycloisomerization of cyclopropene

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CONCLUSIONS The reaction mechanism of the Li[Et3BH]-induced transformation of [1]2+ to 2 has been investigated. Although the boron center was predicted to be a stronger Lewis acidic site, steric crowdedness around the boron atom forces the relative bulky [Et3BH]− to attack the more exposed carbon atom of Cp*. The resulting borabicyclo[2.1.1]hex-2-ene borenium ([3CH]+) was identified and characterized using solution NMR spectroscopic methods at 243 K. Subsequent skeletal rearrangement of [3-CH]+ to a planar boracyclohexa-2,5-diene borenium ([4-CH]+) was observed upon raising the temperature. The [3CH]+ to [4-CH]+ transformation process follows first-order kinetics with a ΔG⧧ of 20.2 kcal/mol at 298 K and a KIE value of 0.87. A sequential pericyclic reaction was identified for the rearrangement of [3-CH]+ using the DFT method. [3-CH]+ undergoes a [1,3]-sigmatropic migration of the B−C1 bond, then an electrocyclic CBC ring opening, and then a 1,2-

Figure 6. Calculated Gibbs free energies (kcal/mol) for the rearrangement reaction of [3-CH]+ to [4-CH]+ in DCM at the B3LYP/6-311G(d,p) level of theory. E


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NMR (128.4 MHz, CD2Cl2, 299.9 K): δ = 21 ppm. 13C NMR (100.6 MHz, CD2Cl2, 299.9 K): δ = 152.7 (IMes-C2), 149.2 (bs, m-C6F5), 141.8 (p-C, Mes), 139.3 (bs, p-C6F5), 137.4 (bs, o-C6F5), 134.6 (o-C, Mes), 133.9 (C5, Cp*), 131.7 (ipso-C, Mes), 130.1 (m-C, Mes), 124.5 (NCH), 20.9 (p-Me, Mes), 17.6 (o-Me, Mes), 11.6 ppm (Me, Cp*). 11 B NMR (128.4 MHz, CD2Cl2, 298 K): δ = 21 ppm (d, 1JB−H = 120 Hz). Anal. Calcd (%) for C55H40B2F20N2: C 58.43, H 3.57, N 2.48. Found: C 58.39, H 3.79, N 2.39. Synthesis of [4-CH][AlCl4]. Li[Et3BH] (1.0 M in THF, 70 μL, 0.070 mmol) was added into a DCM suspension (5 mL) of [1][AlCl4]2 (50 mg, 0.067 mmol) at −78 °C. When the reaction mixture was slowly warmed to room temperature, the color of the solution gradually changed from colorless to pale yellow. Afterward, all volatiles were removed under vacuum to yield an opaque oil mixture. The crude sample showed a relatively clean NMR spectrum (Figure S4). However, further purifications of the mixture including extraction with DCM, exchange of counteranion with K[B(C6F5)4] or Ag[Al(OC(CF3)3)4], and recrystallization all resulted in an increase of impurities. Thus, only the solution NMR spectroscopic data of the unpurified [4-CH][AlCl4]2 is reported. 1H NMR (400.2 MHz, CD2Cl2, 298 K): δ = 7.60 (s, 2H, NCH), 7.07 (s, 2H, Mes), 7.00 (s, 2H, Mes), 2.93 (q, 3JH−H = 7.4 Hz, 1H, C(CH3)H), 2.39 (s, 3H, pMe, Mes), 2.35 (s, 3H, p-Me, Mes), 2.21 (s, 6H, o-Me, Mes), 2.15 (s, 6H, o-Me, Mes), 1.91 (s, 6H, BCMeCCH3CHMe), 1.43 (s, 6H, BCCH3CMeCHMe), 1.06 (d, 3JH−H = 7.4 Hz, 3H, C(CH3)H). 13C NMR (100.6 MHz, CD2Cl2, 298 K): δ = 175.4 (BCMeCMeCHMe), 156.7 (bs, IMes-C2), 141.7 (p-C, Mes), 141.6 (p-C, Mes), 134.4 (o-C, Mes), 134.2 (o-C, Mes), 132.3 (ipso-C, Mes), 131.6 (ipso-C, Mes), 130.9 (m-C, Mes), 130.6 (m-C, Mes), 126.5 (NCH), 126.0 (NCH), 53.1 (BCMeCMeCHMe), 21.4 (BCMeCMeCHMe), 21.2 (p-Me, Mes), 20.8 (BCMeCMeCHMe), 19.2 (o-Me, Mes), 19.0 (o-Me, Mes), 18.0 (BCMeCMeCHMe). 11B NMR (160.5 MHz, CD2Cl2, 298 K): δ = 50.1 ppm. Deprotonation of [4-CH][AlCl4]. A 0.5 mL amount of CD2Cl2 was added to a crude [4-CH][AlCl4] sample, which was prepared from 15 mg of [1][AlCl4]2, to yield a pale yellow solution. After addition of excess PtBu3, the solution immediately turned to bright yellow, and the formation of borabenzene (2) was confirmed by 1H NMR spectrum (Figure S8). Kinetic Study. In all cases, [1][AlCl4]2 (20 mg, 0.025 mmol) and 0.5 mL of CD2Cl2 were cooled to −78 °C in a Young’s NMR tube. Then, 27 μL of Li[Et3BH] (1.0 M, 0.027 mmol) was added under a nitrogen atmosphere. The solution was allowed to thermoequilibrate inside an NMR spectrometer, and the 1H NMR spectrum was recorded every 3, 5, or 15 min. Structural Determinations. Crystallographic data of [3-BH][B(C6F5)4] were collected with an Oxford Gemini Duo system diffractometer using graphite-monochromated Cu Kα radiation (λ = 1.541 78 Å) at 150 K. The structure was solved by the direct method and refined by least-squares cycles. The non-hydrogen atoms were refined anisotropically. All calculations were performed using the SHELXTL-97 package. Crystallographic data of [3-BH][B(C6F5)4] have been deposited at the Cambridge Crystallographic Data Center with deposition number CCDC 1414842. Theoretical Calculations. Structural optimizations of the intermediates and transition states were performed with the Gaussian 09 program package and B3LYP density functional. The basis set employed was 6-311G(d,p) for all atoms. In all cases, only the singlet electronic ground state was considered. The effect of CH2Cl2 solvation was taken into account using the polarized continuum model (PCM) with the universal force field (UFF) radii built from the UFF force field. Vibrational analysis was performed to verify the stationary-point structures as local minima or transition states. Intrinsic reaction coordinate (IRC) or quasi-IRC calculation was carried out to confirm that all transition states were appropriately connected to the corresponding minima. In the quasi-IRC calculation, the geometry of the transition state was slightly perturbed in the direction of the reactants or products and released for equilibrium optimization. Zeropoint energy correction was applied for the determination of the relative energies and the activation energies. The position-specific

hydrogen migration to generate [4-CH]+, which can then be transformed into borabenzene 2 via deprotonation. In conclusion, the reaction mechanism of the hydride-induced transformation of [1]2+ to 2 via two novel borenium cation intermediates has been elucidated experimentally and computationally.

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EXPERIMENTAL SECTION

General Information. All reactions were carried out using standard Schlenk techniques or inside a glovebox under a N2 atmosphere. DCM was dried with a solvent purification system. CD2Cl2 was dried using P2O5 and distilled under nitrogen. Cp*BCl2− IMes and [1][AlCl4]2 were prepared according to the published procedure.15 Li[Et3BH] was prepared from BEt3 (1.0 M in THF) and LiH and consumed within one month to avoid the undesired disproportionation of [Et3BH]− to the less reactive Li2[Et2BH2][Et4B], which releases only 0.5 equiv of hydride at low temperature. Solution NMR spectra were collected with Bruker Avance III 400 MHz (1H: 400.2 MHz, 11B: 128.4 MHz, 13C: 100.6 MHz) and Bruker AVIII 500 MHz (1H: 500.1 MHz, 11B: 160.5 MHz, 13C: 125.8 MHz) FT-NMR spectrometers. Chemical shifts are reported in ppm with residual solvent signals (1H and 13C NMR) or external BF3−OEt2 (11B NMR) as reference. Elemental analyses were performed on a Heraeus VarioIII-NCH elemental analyzer. Generation and Characterization of [3-CH]+. A suspension of [1][AlCl4]2 (20 mg, 0.025 mmol) in 0.5 mL of CD2Cl2 in a Young’s NMR tube was cooled to −78 °C before the addition of 27 μL of Li[Et3BH] (1.0 M, 0.027 mmol). The solution was allowed to thermoequilibrate inside an NMR spectrometer. Since [3-CH]+ could not be isolated, spectroscopic characterization was performed on the reaction mixture at 243 K. 1H NMR (500.1 MHz, CD2Cl2, 243 K): δ = 7.23 (s, 2H, NCH) 7.11 (s, 4H, Mes), 2.38 (s, 6H, p-Me, Mes), 2.08 (s, 6H, o-Me, Mes), 1.58 (br, 1H, C(CH3)H), 1.50 (s, 6H, Me, C2 and C3), 0.92 (br, 3H, C(CH3)H), 0.45 ppm (s, 6H, Me, C1 and C4). 13C NMR (125.8 MHz, CD2Cl2, 243 K): δ = 142.0 (p-C, Mes), 135.4 (oC, Mes), 132.8 (ipso-C, Mes), 130.3 (m-C, Mes), 126.5 (NCH), 114.2 (C2, C3), 57.0 (C1, C4), 56.1 (C5), 21.3 (p-Me, Mes), 18.2 (o-Me, Mes), 14.9 (C(CH3)H), 9.3 ppm (Me, C1, C2, C3, and C4). 11B NMR (160.5 MHz, CD2Cl2, 243 K): δ = −32.5 ppm. Synthesis of Cp*BHCl-IMes. A solution of Cp*BCl2−IMes (40 mg, 0.076 mmol) in DCM (0.4 mL) was allowed to react with Li[Et3BH] (1.0 M, 80 μL, 0.08 mmol) at room temperature for 3 h. The solution was filtered and dried to yield Cp*BHCl−IMes as a white powder (30 mg). Yield: 80%. 1H NMR (400.2 MHz, C6D6, 298 K): δ = 6.74 (s, 2H, Mes), 6.73 (s, 2H, Mes), 5.75 (s, 2H, NCH), 2.13 (s, 6H, p-Me, Mes), 2.09 (s, 6H, o-Me, Mes), 1.97 (s, 6H, o-Me, Mes), 1.88 ppm (s, 3H, Me, Cp*), 1.79 ppm (s, 3H, Me, Cp*), 1.65 ppm (s, 3H, Me, Cp*), 1.51 ppm (s, 3H, Me, Cp*), 1.43 ppm (s, 3H, Me, Cp*). 13C NMR (100.6 MHz, C6D6, 298 K): δ = 149.3 (C5, Cp*), 144.7 (C5, Cp*), 139.2 (p-C, Mes), 135.5 (C5, Cp*), 135.3 (C5, Cp*), 129.2 (m-C, Mes), 128.9 (o-C, Mes), 122.6 (NCH), 55.5 (C5, Cp*), 25.5 (p-Me, Mes), 21.0 (Me, Cp*), 18.6 ppm (Me, Cp*), 18.0 (Me, Cp*), 12.1 (o-Me, Mes), 12.0 (Me, Cp*), 11.6 ppm (Me, Cp*). 11B NMR (128.4 MHz, C6D6, 298 K): δ = −6.1 ppm (bs). Anal. Calcd (%) for C31H40BClN2: C 76.47, H 8.28, N 5.75. Found: C 76.33, H 7.83, N 5.88. Synthesis of [3-BH][B(C6F5)4]. K[B(C6F5)4] (55 mg, 0.076 mmol) was added to a solution of Cp*BHCl-IMes (37 mg, 0.076 mmol) in benzene (0.4 mL) in a Young’s NMR tube at room temperature. The reaction progress was monitored using 1H NMR. After the reaction was complete, solvent was decanted, and the resulting pale yellow solids were washed with benzene and dissolved in dichloromethane. The solution was further filtered and dried under vacuum to afford [3-BH][B(C6F5)4] (67 mg). Yield: 78%. Single crystals of [3-BH][B(C6F5)4] were obtained from diffusion of pentane into a chloroform solution of [3-BH][B(C6F5)4] at ambient temperature in a glovebox. 1H NMR (400.2 MHz, CD2Cl2, 299.9 K): δ = 7.24 (s, 2H, NCH), 7.12 (s, 4H, Mes), 2.41 (s, 6H, p-Me, Mes), 2.09 (s, 12H, o-Me, Mes), 1.45 ppm (s, 15H, Me, Cp*). 11B F


Article

Organometallics

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proton kinetic isotope effect was calculated from the Gibbs energies at 265.0 K obtained by the isotopic replacement using the Bigeleisen− Mayer formalism.26 The gauge-including atomic orbital (GIAO) method was applied to compute the nuclear shieldings in the optimized structure of [3-CH]+ and [3-BH]+. All relative chemical shifts are given with respect to the absolute shielding values of a chosen reference compound obtained at the same computational level.

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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.organomet.6b00077. NMR spectra of [3-CH][AlCl4] and [4-CH][AlCl4], kinetic study data, crystallographic data of [3-BH][B(C6F5)4], and DFT calculations (PDF) Crystallographic data (CIF) Optimized Cartesian coordintes (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

Y.-F. Lin and C.-T. Shen contributed equally to this work and share the first authorship. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology of Taiwan (MOST 103-2113-M-002-005 and MOST 104-2113-M-002-018-MY3) and National Taiwan University (NTU-CESRP-104R7619). Y.-F.L. extends gratitude to MOST for a postdoctoral fellowship (MOST 104-2811-M002-130). We would like to thank Prof. Yeun-Min Tsai and Dr. Tzu-Pin Lin for the insightful discussions on the reaction mechanism and Ms. Shou-Ling Huang for the VT-NMR experiments.

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REFERENCES

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