Ring Expansion of 7-Boranorbornadienes by ... - ACS Publications

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Ring Expansion of 7‑Boranorbornadienes by Coordination with an N‑Heterocyclic Carbene Holger Braunschweig,* Jonathan Maier, Krzysztof Radacki, and Johannes Wahler Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany S Supporting Information *

ABSTRACT: In the context of our longstanding interest in subvalent boron compounds, we targeted the liberation of a carbene-stabilized borylene from a suitable organoboron precursor. For this purpose, we tested 7-borabicyclo[2.2.1]hepta-2,5-dienes (or 7-boranorbornadienes in short) obtained from facile [4 + 2] cycloaddition of boroles and alkynes. By formation of a Lewis adduct with an N-heterocyclic carbene (NHC), we intended to block previously reported pericyclic reactions leading to undesired ring expansion and achieve a cheletropic elimination of the borylene species instead. Our results presented herein indicate that coordination of the NHC to 7boranorbornadienes indeed weakens the bridgehead boron−carbon bonds. However, ring expansion to form borepine-NHC Lewis adducts remains the favorable reaction pathway. This process is independent of excessive NHC in the reaction mixture, which precludes participation of a free borylene species. As an alternative, an intramolecular mechanism driven mainly by molecular strain and steric factors is most plausible. Our investigations are based on spectroscopic measurements and singlecrystal X-ray diffraction analyses.

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INTRODUCTION

Scheme 1. Proposed Sequence of Pericyclic Reactions between Diphenylacetylene and 1

Cyclopentadiene (I) is a textbook example for intermolecular dimerization of a conjugated olefinic system.1 The observed reaction is classified as a thermal pericyclic reaction, which can generally be understood by analyzing the frontier orbital symmetry and energy of a diene interacting with a dienophile.2,3 Regarding I, both diene and dienophile are provided by the same molecule. In a similar manner, boroles, such as 1-phenyl-2,3,4,5tetramethylborole, that are not kinetically stabilized by bulky aryl groups around the central C4B framework undergo instant dimerization.4−6 As a consequence of antiaromatic destabilization within the conjugated system, the tendency to undergo [4 + 2] cycloaddition is highly enhanced compared to I, as it results in disruption of conjugation within the five-membered borole ring and consequent stabilization of the system.7 It was also shown for sterically shielded boroles such as 1,2,3,4,5pentaphenylborole (1) that thermal pericyclic reactions can be used to synthesize novel boron-containing molecules (Scheme 1).8−10 First investigations by Eisch et al. were performed combining 1 and diphenylacetylene at 25 °C to initially generate the symmetrically substituted 7-borabicyclo[2.2.1]hepta-2,5-diene 2 in a [4 + 2] cycloaddition reaction.8 Upon warming of 2 to 110 °C, a series of pericyclic reactions were discerned.10 The 7-borabicyclo[4.1.0]heptadiene 3 was postulated as an intermediate formed by a [1,3]-suprafacial sigmatropic shift. The latter could not be observed, as a disrotatory 6π-electrocyclic ring-opening succeeds to form the aromatic 1,2,3,4,5,6,7-heptaphenylborepine 4. Prolonged heat© 2013 American Chemical Society

ing finally gave the isomer 5 as the final product resulting from an intramolecular ene-type reaction.10 Further studies on 1,2,3,4,5-pentakis(pentafluorophenyl)borole (6) toward differently substituted alkynes were reported recently by the group of Piers.7,11,12 The reaction of 6 with 3hexyne resulted in isolation and structural characterization of the unsymmetrical 7-boranorbornadiene 1,2-Et2-7, whereas the kinetic isomer 2,3-Et2-7 was not observed (Scheme 2). The enhanced thermodynamic stability of the 1,2-isomer over the 2,3-isomer has been previously proposed by Eisch et al.10 and Received: July 18, 2013 Published: October 21, 2013 6353

dx.doi.org/10.1021/om400708y | Organometallics 2013, 32, 6353−6359

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that reported in the literature (δ = −5 ppm).29 The previously reported 1H NMR spectrum, however, featuring multiplets between δ = 7.2 and 7.8 ppm,10 is significantly different from that of our compound (δ = 6.7−7.2 ppm) when measured in the same solvent (CDCl3). In order to clarify this mismatch, we completed the set of NMR data by recording a 13C{1H} NMR spectrum that is consistent with the structure of 2 in C2v symmetry. Furthermore, we were able to obtain single crystals of 2 suitable for X-ray diffraction that conclusively proved the constitution of 2 (Figure 1).30

Scheme 2. Observed Reactivity of 6 toward 3-Hexyne (Pf = C6F5)

found conclusive experimental evidence by Piers’ work.11 In addition, the solid-state structure of 1,2-Et2-7 revealed a significant bending of the boryl moiety toward the more electron-rich double bond.11 This preorganization, caused by orbital interactions between the empty pz orbital of boron and the π-system of the double bond,10 emphasizes the tendency of 2 and 1,2-Et2-7 to undergo further suprafacial [1,3]-sigmatropic rearrangement.11 Thus, these findings support the mechanistic assumptions proposed by Eisch (Scheme 1) that a 7borabicyclo[4.1.0]heptadiene intermediate (3) is involved in the formation of the borepine 4.10 As the vacant pz orbital at boron seems to play a directing role in the observed pericyclic reactions, we expect a distinct effect on reactivity upon coordination of the Lewis acidic boron center in 7-borabicyclo[2.2.1]hepta-2,5-dienes by a suitable Lewis base. Vague studies performed by Eisch et al. using derivatives of 2 indicate that, after quarternization of the boron center with organolithium reagents and protic workup, hexaarylbenzene derivatives were obtained under loss of the boryl unit.9,10 An attractive alternative would be a reversed [4 + 1] cycloaddition yielding a benzene derivative under extrusion of a reactive borylene species.13 Generation of an aromatic system combined with the relief of ring strain and gain of entropy potentially provides the thermodynamic driving force for such a process. Examples of these cheletropic reactions have been documented with stable small molecules such as CO, N2, or SO214−18 and even rather reactive species such as silylenes19−21 or germylenes22−24 functioning as a leaving group. In addition, the N-heterocyclic carbene (NHC) N,N′dimethylimidazol-2-ylidene (IMe) was used in our group to generate transient [BH(IMe)] by reduction of [BHCl2(IMe)], which was trapped in a [2 + 1] cycloaddition reaction.25 Furthermore, isolation of the parent borylene B−H has been achieved recently by Bertrand et al. using cyclic (alkyl)(amino)carbenes to stabilize the subvalent species.26,27 Mass spectrometric investigations also suggest a possible release of a diborene via [4 + 2] cycloreversion (retro-Diels−Alder reaction) starting from a B−B-bridged anthracene derivative.28 These experimental findings provide a promising basis for our endeavor. In this contribution we present our efforts in extruding a base-stabilized borylene fragment from a strained bicyclic borane.

Figure 1. Molecular structure of 2 in the solid state with hydrogen atoms omitted for clarity. Thermal ellipsoids are set at 50% probability. Selected bond lengths [Å] and angles [deg]: B1−C1 1.633(2), B1−C4 1.638(2), B1−C7 1.573(2), B1−C2 1.823(2), B1−C3 1.829(2), C1− C2 1.522(2), C2−C3 1.392(2), C3−C4 1.521(2), C4−C5 1.517(2), C5−C6 1.337(2), C6−C1 1.516(2); C1−B1−C4 96.5(1), C1−B1− C7 131.1(2), C4−B1−C7 131.7(2), B1−C1−C2 70.5(1), B1−C1−C6 106.6(1), B1−C4−C3 70.6(1), B1−C4−C5 106.3(1), C1−C2−C3 109.8(1), C2−C3−C4 110.5(1), C3−C4−C5 111.0(1), C4−C5−C6 111.2(1), C5−C6−C1 111.5(1), α 83.1(1), β 153.3(2).

In agreement with Piers’ observations,11 the solid-state structure of 2 displays a pronounced tilt of the trigonal planar boron center toward one double bond. The underlying through-space π-interaction, also referred to as “homoconjugation” in the literature,31 is reflected by the low-frequency 11B chemical shift.10,29 In contrast, the 13C{1H} NMR spectrum suggests the bridge to be in a symmetric environment. This situation presumably arises from an exchange process of interaction between the boron center and the identical double bonds at C2 and C5, which is faster than the NMRspectroscopic time scale as a consequence of a rather low inversion barrier.31 This dynamic bridge-flipping process is present in solution at ambient temperature and inhibited in the solid state at −100 °C. The crystallographic data also show that, unlike 1,2-Et2-7,11 the boron atom is oriented symmetrically toward C2 and C3, as indicated by the nearly equal B1−C1 (1.633(2) Å) and B1−C4 (1.638(2) Å) separations (compare 1,2-Et2-7: B1−C1 1.651(4) Å and B1−C4 1.628(4) Å) as well as the B1−C2 (1.823(2) Å) and B1−C3 (1.829(2) Å) distances. The angles between the least-squares planes defined by B1, C1, and C4 versus those including C1, C2, C3, and C4 (α = 83.1(1)°, Figure 1) as well as C1, C4, C5, and C6 (β = 153.3(2)°) are close to those of 1,2-Et2-7 (83.0(2)° and 152.7(2)°). The consequential

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RESULTS AND DISCUSSION In order to avoid formation of constitutional isomers such as those derived from 7, we chose the established 7borabicyclo[2.2.1]hepta-2,5-diene derivative 2 for our studies. Synthesis of 2 was performed based on the published procedure (see Experimental Section).10 The 11B NMR spectrum of 2 displays a single broad resonance at δ = −4.2 ppm, which is comparable to that in 1,2-Et2-7 (δ = −10.3 ppm)11 and close to 6354


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is best exemplified by the resonance of the boron-bound bridgehead carbon atoms (C1 and C4, Figure 3). At ambient

elongation of the double bond between C2 and C3 (1.392(2) Å) referenced to that between C5 and C6 (1.337(2) Å), is similar to that of 1,2-Et2-7 (C2−C3: 1.388(4) Å, C5−C6: 1.329(4) Å).11 Quarternization of the boron center of 2 was achieved using IMe as a Lewis base with high σ-donor strength and the least steric bulk compared to other NHC ligands.32 The factor of minimized steric congestion of the NHC was assumed to be vital in order to achieve coordination, as the strained geometry combined with the peripheral phenyl rings already implies an elevated spatial demand around the boron center of 2. The synthesis of colorless solids of [2(IMe)] was accomplished by addition of a solution of IMe to a cold solution of 2 and subsequent precipitation of the product (Scheme 3).

Figure 2. Relevant excerpt of the 13C{1H} NMR spectra of [2(IMe)] at 23 and −80 °C (measured in CD2Cl2), depicting the resonances of the bridgehead carbon atoms.

Scheme 3. Formation of the Lewis Adduct [2(IMe)]

The 11B NMR spectrum of [2(IMe)] shows a sharp resonance at δ = 11.5 ppm, which is remarkably deshielded compared to the uncomplexed 2. Although the π-interaction of the boron atom with the double bonds is interrupted upon quarternization of the boron center with IMe, the observed 11B chemical shift is rather unexpected. The resonance was estimated to be found further shifted to lower frequencies similar to strained Lewis adducts of IMe with borirenes, which are typically found around δ = −20 ppm.33 The 1H NMR spectrum indicates a free rotation of IMe about the B−C bond as deduced from the presence of only one singlet associated with the NHC methyl protons at δ = 3.02 ppm. As expected, the 13C{1H} NMR spectrum is in accord with a change of symmetry from C2v (2) to Cs in solution upon coordination of IMe to 2. The bridgehead carbon resonances (C1 and C4) of [2(IMe)] were detected at δ = 72.68 ppm, remaining virtually unaffected by rehybridization of the adjacent boron atom (compare 2: δ = 72.51 ppm). The solid-state structure was determined by single-crystal X-ray diffraction. Obviously, tilting of the bridging boryl unit toward a CC double bond is prevented by coordination with IMe. Hence, the angles α = 126.4(2)° and β = 120.0(2)°, respectively, are comparatively balanced. Accordingly, the CC double bonds show relatively similar lengths (C2−C3, 1.333(3) Å; C5−C6, 1.344(3) Å). The boron atom was found in a considerably distorted tetrahedral geometry with a rather acute C1−B1−C4 angle of 87.6(2)°. In addition, the bond lengths between B1 and the bridgehead carbon atoms vary significantly. While the B1−C1 distance (1.682(3) Å) is moderately elongated compared to 2, in accord with a changing hybridization from sp2 to sp3 at boron,34 the B1−C4 separation (1.743(3) Å) is remarkably long.25 In order to exclude the presence of solid-state packing effects, additional VT NMR experiments were performed. Except the increasing line broadening at low temperatures, the 11 B NMR chemical shift remains virtually unaffected when the sample is cooled to −80 °C (δ = 11.0 ppm). However, the 13 C{1H} NMR spectrum of [2(IMe)] measured at −80 °C comfirms an unsymmetric structure in solution at low temperature similar to the X-ray diffraction data. This behavior

Figure 3. Molecular structure of [2(IMe)] in the solid state with hydrogen atoms and solvent molecules (2 × C6H6) omitted for clarity. Thermal ellipsoids are set at 50% probability. Selected bond lengths [Å] and angles [deg]: B1−C1 1.682(3), B1−C4 1.743(3), B1−C7 1.643(3), B1−C8 1.654(3), C1−C2 1.547(3), C2−C3 1.333(3), C3− C4 1.553(3), C4−C5 1.547(3), C5−C6 1.344(3), C6−C1 1.548(3); C1−B1−C4 87.6(2), C1−B1−C7 111.5(2), C1−B1−C8 123.6(2), C4−B1−C7 121.0(2), C4−B1−C8 106.7(2), C7−B1−C8 106.6(2), B1−C1−C2 101.5(2), B1−C1−C6 96.2(2), B1−C4−C3 98.3(2), B1−C4−C5 96.5(2), C1−C2−C3 110.1(2), C2−C3−C4 108.8(2), C3−C4−C5 105.3(2), C4−C5−C6 108.8(2), C5−C6−C1 109.9(2), α 126.4(2), β 120.0(2).

temperature, only one broadened resonance is detected (δ = 72.7 ppm). At −80 °C this resonance is split up into two distinct resonances at δ = 70.2 ppm and δ = 72.6 ppm, indicating a decrease in symmetry (Figure 2). Hence, we again observe a dynamic effect where two enantiomeric forms of [2(IMe)] in C1 symmetry interconvert via the averaged structure (Cs symmetry) detected at 23 °C. We believe that this situation arises from the combination of steric effects and molecular strain within the framework. Consequently, the B1−C4 bond is assumed to be relatively weak, suggesting possible further reaction paths initiated by cleavage of this bond. In addition, the high-frequency 11B NMR chemical shift might result from a reduced B1−C4 bonding 6355


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interaction accompanied with a polarization of the bond and concomitant deshielding of the boron nucleus. Indeed, heating solutions of [2(IMe)] in benzene to 80 °C for about 9 h gave a new product in high yield, as deduced from 11 B NMR spectroscopy. Hence, the resonance of [2(IMe)] completely vanished, while a single new resonance was found at δ = −13.4 ppm in the expected range for a Lewis adduct of IMe with a borane. The solid-state structure was determined by single-crystal X-ray diffraction, which revealed the presence of the Lewis adduct [4(IMe)] formed by the borepine 4 and IMe (Scheme 4, 85% isolated yield). Scheme 4. Thermal Ring Expansion of [2(IMe)] to Yield the Borepine Derivative [4(IMe)]

Figure 4. Molecular structure of [4(IMe)] in the solid state with hydrogen atoms and solvent molecule (C6H6) omitted for clarity. Thermal ellipsoids are set at 50% probability. Selected bond lengths [Å] and angles [deg]: B1−C1 1.640(3), B1−C6 1.650(3), B1−C7 1.645(3), B1−C8 1.630(3), C1−C2 1.363(3), C2−C3 1.487(3), C3− C4 1.368(3), C4−C5 1.488(3), C5−C6 1.362(3); C1−B1−C6 97.5(2), C1−B1−C7 114.8(2), C1−B1−C8 113.5(2), C6−B1−C7 115.9(2), C6−B1−C8 110.8(2), C7−B1−C8 104.7(2), B1−C1−C2 122.4(2), B1−C6−C5 122.0(2), C1−C2−C3 123.4(2), C2−C3−C4 122.3(2), C3−C4−C5 123.8(2), C4−C5−C6 123.6(2).

The molecule displays a boat-like structure similar to that of the Lewis adduct [8(Py)] formed by 1,2,3,4,5,6,7-heptakis(pentafluorophenyl)borepine (8) and pyridine (Py).11 The boron-bound phenyl group resides in equatorial position, and the Lewis base was found in an axial arrangement. The angle between C1, B1, and C6 [97.5(2)°] is more obtuse than the respective C1−B1−C4 angle of [2(IMe)], indicating a considerable relief of strain around the tetrahedral boron center. The bond distances in the conjugated backbone (C1 to C6) reflect those of a rather localized π-system with alternating CC double and C−C single bonds similar to the situation in [8(Py)]. The B1−C1 [1.640(3) Å] and B1−C6 [1.650(3) Å] bonds, however, are slightly longer in [4(IMe)] compared to [8(Py)] (B1−C1: 1.604(7) Å; B1−C6: 1.640(7) Å).11 According to 1H and 13C{1H} NMR spectroscopy, both the IMe and the phenyl group freely rotate around the B1−C8 and B1−C7 axes. Some phenyl groups of the organic backbone are rotationally hindered at ambient temperature, as indicated by broadened resonances in both spectra. Furthermore, the protons in ortho-position of the boron-bound phenyl group are highly shielded (δ = 5.4 ppm), presumably as a consequence of spacial proximity to the electron-rich NHC. Additionally, the presence of constitutional isomerism, i.e., IMe in equatorial instead of axial position, was not observed. In the context of our goal to liberate a borylene derivative from [2(IMe)] instead of synthesizing [4(IMe)], we attempted some further reactivity studies. In an effort to trap a potential subvalent boron species appearing in the course of transformation to [4(IMe)], 2 equivalents of IMe was added to solutions of [2(IMe)] before the reaction was performed under otherwise identical conditions. Unfortunately, the outcome of the reaction is independent of additional IMe in the reaction mixture, so that [4(IMe)] was identified as the only product by means of 11B NMR spectroscopy. This finding indicates that intramolecular rearrangement processes are favored over a cheletropic reaction. It is also conceivable that at elevated temperatures a dissociation of IMe occurs accompanied with pericyclic reactions outlined in Scheme 1 and subsequent quarternization of the borepine 4. For such a reaction path an increase of the activation energy is expected due to additional

contributions from the dissociation of IMe. However, the lower temperatures (80 °C) and shorter reaction times (8.75 h) used in the reaction of the Lewis adduct [2(IMe)] to [4(IMe)] compared to the reaction of 2 to 4 (110 °C, 24 h)8 are contradictory to this mechanism.35 In order to gain further insights into these reaction pathways, we targeted the synthesis of a 7-borabicyclo[2.2.1]hepta-2,5-diene with increased steric congestion at the boron center. This geometry should lead to more facile transformations after generation of the Lewis adduct with IMe. Therefore, the reaction of 1-mesityl-2,3,4,5tetraphenylborole (9) with diphenylacetylene in benzene solution was tested under various conditions. No significant reaction progress was observed at ambient temperature within the time frame of 24 h or by heating to 60 °C for 5 h, as indicated by 1H NMR spectroscopy as well as the persistent dark green color of the reaction mixture. The color of the mixture remained even after heating to 160 °C in a microwave for 2.5 h, and we were not able to isolate any pure product. Hence, we used the smaller alkyne 2-butyne, leading to formation of both isomers 2,3-Me2-10 and 1,2-Me2-10, which were obtained in the ratio of 1:2.4 after a smooth reaction between borole 9 and the alkyne at ambient temperature and subsequent recrystallization (Scheme 5). When kept in benzene solution, the ratio equilibrated to about 1:9 within 2 d, as determined by 1H NMR spectroscopy. For the subsequent Lewis adduct formation with IMe we used the initially obtained kinetic mixture of 2,3- and 1,2-Me2-10. Upon addition of 1 equivalent of IMe to a solution of 10 at room temperature, a mixture of several products was obtained. The 11B NMR spectrum of the crude product indicated the presence of at least five different species resonating between δ = 12.2 and −27.6 ppm (see SI for details) with different relative intensities. Unable to separate the unidentified products, we heated the reaction mixture to 105 °C for 6 h. Under thermal treatment a transformation to one major thermodynamic product (11B NMR: δ = −11.6 ppm) was observed that was already present 6356


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repeated recrystallization. The 1H and 13C{1H} NMR spectra allowed the assignment of [11(IMe)eq] to the Cs symmetric major isomer, as the resonances associated with IMe show an asymmetric environment of the methyl and methylene groups. In contrast to the NHC, the mesityl moiety shows only one ortho-methyl resonance. These results are in agreement with an equatorial position of IMe in accord with the solid-state structure of [11(IMe)eq]. In the minor species the situation seems to be reversed, as deduced from 1H NMR spectroscopy. Hence, we assume to observe a mixture of [11(IMe)eq] and [11(IMe)ax] (Scheme 5) with a ratio of approximately 11:1 as determined by integration of the 1H NMR spectrum. Presumably, the two isomers are in equilibrium via a rocking motion of the boat conformation facilitated by similar spatial requirements of the mesityl group and IMe as well as reduced steric bulk at C3 and C4 compared to [4(IMe)]. Similar to the synthesis of [4(IMe)], the reaction of 10 with an excess of IMe (3 equivalents) is invariant compared to the stoichiometric reaction. Although not all species were identified, the observed mixture of products present after addition of IMe to 10 at ambient temperature and subsequent thermal isomerization to the thermodynamic product [11(IMe)] indicate that Lewis-base-induced ring expansion of 7-borabicyclo[2.2.1]hepta-2,5-dienes is the favored reaction path over the desired cheletropic reaction. Nevertheless, this is in agreement with the above-mentioned assumption that further weakening of the B1−C4 bond in [2(IMe)] can be accomplished by increased steric bulk at boron. Hence, the Lewis adducts of 10 are not persistent and undergo the observed intramolecular transformations.

Scheme 5. Reaction of 9 with 2-Butyne and Subsequent Formation of Lewis Adducts with IMe

as a component of the mixture. This product was obtained in 67% yield after crystallization, and the composition was elucidated by single-crystal X-ray diffraction. The solid-state structure confirms the presence of a Lewis adduct [11(IMe)] formed by IMe and borepine system 11, which accommodates two methyl groups at C3 and C4 of the boracycle. In addition, the mesityl group occupies the axial position, whereas IMe was found in equatorial arrangement (Figure 5). The B1−C1

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CONCLUSION In summary, we have investigated the behavior of 7borabicyclo[2.2.1]hepta-2,5-dienes 2 and 10 toward quaternization with the N-heterocyclic carbene IMe. The solid-state structure of [2(IMe)] displays significant structural changes compared to the uncoordinated 2. The bridgehead boron atom was found in highly strained geometry with an exceptionally long B−C bond (1.743(3) Å). Hence, [2(IMe)] undergoes a facile ring-opening reaction to form the borepine Lewis adduct [4(IMe)], when moderately heated. This ring-opening reaction, induced by coordination with the Lewis base, can be accelerated by increasing the steric congestion at boron. Synthesis of 10, accommodating a mesityl group at the boron center, proved to be useful for this purpose. In this case, a mixture of products was obtained upon reaction with IMe including the ring-expanded borepine Lewis adduct [11(IMe)]. Thermal treatment of this mixture gave [11(IMe)] as the main product, indicating a chemical reversibility of the involved side reactions. Other thermal reaction pathways such as cheletropic elimination of an NHC-stabilized borylene13 were not detected.

Figure 5. Molecular structure of [11(IMe)eq] in the solid state with hydrogen atoms omitted for clarity. Thermal ellipsoids are set at 50% probability. Selected bond lengths [Å] and angles [deg]: B1−C1 1.679(2), B1−C6 1.670(2), B1−C7 1.675(2), B1−C8 1.668(2), C1− C2 1.358(2), C2−C3 1.472(2), C3−C4 1.353(2), C4−C5 1.478(2), C5−C6 1.364(2); C1−B1−C6 99.7(2), C1−B1−C7 110.0(2), C1− B1−C8 111.8(2), C6−B1−C7 113.8(2), C6−B1−C8 111.3(2), C7− B1−C8 109.9(2), B1−C1−C2 118.1(2), B1−C6−C5 119.8(2), C1− C2−C3 123.4(2), C2−C3−C4 123.3(2), C3−C4−C5 124.2(2), C4− C5−C6 122.9(2).

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EXPERIMENTAL SECTION

General Considerations. All syntheses were carried out under an argon atmosphere with standard Schlenk and glovebox techniques. 1,36 9,37 and IMe38 were prepared according to the published procedures. Diphenylacetylene and 2-butyne were purchased from commercial sources and used without further purification. Hexane, pentane, toluene (Na/K alloy), Et2O (Na), and benzene (K) were dried by distillation over suitable alkali metals under argon and stored over molecular sieves. C6D6 and CD2Cl2 were degassed with three freeze− pump−thaw cycles and stored over molecular sieves. CDCl3 was dried by distillation over P2O4. Elemental analyses were obtained from an Elementar Vario MICRO cube instrument. NMR spectra were

(1.679(2) Å) and B1−C6 (1.670(2) Å) distances are further elongated compared to [4(IMe)], whereas the bond lengths within the hexatriene moiety basically reflect those of [8(Py)].11 The 11B NMR spectrum of isolated [11(IMe)] shows a major resonance at δ = −11.6 ppm accompanied by a minor resonance at δ = −13.3 ppm, which could not be eliminated by 6357


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recorded on a Bruker Avance 400 or a Bruker Avance 500 NMR spectrometer. Chemical shifts (δ) are given in ppm and are referenced against external Me4Si (1H, 13C) and [BF3(Et2O)] (11B). Broadened resonances in 13C{1H} NMR spectra were unambiguously identified by means of two-dimensional 1H−13C-correlation NMR experiments (HMBC, HSQC). Synthesis of 2. A solution of diphenylacetylene (150 mg, 842 μmol) in toluene (5 mL) was added to a cooled (0 °C) suspension of 1 (367 mg, 826 μmol) in toluene (4 mL). The mixture was stirred for 2.5 h at rt, resulting in a color change from dark blue to pale yellow. The solvent was reduced to about 2 mL, and pentane (5 mL) was added to precipitate a colorless solid. After filtration the solid was washed with pentane (3 × 1 mL) and dried under vacuum to yield 2 (441 mg, 708 μmol, 86%) as a colorless solid. Single crystals suitable for X-ray diffraction were obtained by diffusion of Et2O into a saturated solution of 2 in toluene. 1H NMR (400 MHz, CDCl3, 296 K): δ 6.72−6.76 (m, 4H, C6H5), 6.85−6.90 (m, 4H, C6H5), 6.92−7.02 (m, 10H, C6H5), 7.04−7.10 (m, 13H, C6H5), 7.15−7.20 (m, 4H, C6H5). 11B NMR (128 MHz, CDCl3, 296 K): δ −4.2. 13C{1H} NMR (101 MHz, CDCl3, 296 K): δ 125.31, 126.69, 127.14, 127.35, 127.74, 130.13, 132.20, 135.85 (CH), 72.51 (br), 136.65, 137.27 (br), 137.79 (C). Anal. Calcd (%) for C48H35B: C 92.60, H 5.67. Found: C 92.50, H 5.80. Synthesis of [2(IMe)]. A solution of IMe (42.0 mg, 437 μmol) in toluene (2 mL) was added to a cooled (0 °C) suspension of 2 (250 mg, 402 μmol) in toluene (5 mL) to give a yellow solution. After 10 min the cooling bath was removed, and stirring was continued for 1 h at rt, resulting in formation of a colorless solid. Pentane (3 mL) was added and the mixture was stored at −30 °C for 2 d. The precipitate was filtered, washed with pentane (3 × 1 mL), and dried under vacuum to yield [2(IMe)] (235 mg, 327 μmol, 81%) as a colorless solid. Single crystals suitable for X-ray diffraction were obtained by diffusion of pentane into a saturated solution of [2(IMe)] in benzene. 1 H NMR (500 MHz, CD2Cl2, 296 K): δ 3.02 (s, 6H, CH3), 6.51−6.53 (m, 4H, C6H5), 6.68−6.85 (m, 16H, C6H5 and NCHCHN), 6.94− 6.98 (m, 12H, C6H5), 7.03−7.07 (m, 1H, C6H5), 7.13−7.16 (m, 2H, C6H5) 7.74 (br s, 2H, C6H5). 11B NMR (160 MHz, CD2Cl2, 296 K): δ 11.5. 13C{1H} NMR (126 MHz, CD2Cl2, 296 K): δ 39.06 (CH3), 122.82, 123.71, 124.69, 125.13, 125.46, 126.02, 126.73, 127.27, 127.31, 130.39, 131.40, 132.88, 136.61 (CH), 72.68 (br), 140.73, 141.48, 142.99, 152.65, 155.80, 165.37 (br) (C). Anal. Calcd (%) for C53H43BN2: C 88.57, H 6.03, N 3.90. Found: C 88.73, H 6.21, N 3.88. Synthesis of [4(IMe)]. A solution of [2(IMe)] (130 mg, 181 μmol) in benzene (8 mL) was heated to 80 °C for 8 h 45 min under formation of a pale yellow solution. The solvent was reduced to about 2 mL, resulting in precipitation of colorless crystals within the time frame of 12 h. The solvent was decanted, and the crystals were washed with pentane (3 × 1 mL) and dried under vacuum to yield [4(IMe)] (111 mg, 154 μmol, 85%) as a colorless, crystalline solid. Single crystals suitable for X-ray diffraction were obtained by diffusion of hexane into a solution of [4(IMe)] in benzene. 1H NMR (500 MHz, CD2Cl2, 296 K): δ 4.15 (s, 6H, NCH3), 5.41−5.43 (m, 2H, C6H5), 6.33−6.35 (m, 4H, C6H5), 6.47−6.50 (m, 4H, C6H5), 6.56−6.66 (m, 4H, C6H5), 6.68−6.75 (m, 15H, C6H5), 6.88−6.92 (m, 2H, C6H5), 7.12 (s, 2H, NCHCHN), 7.23 (br s, 2H, C6H5), 7.47 (br s, 2H, C6H5). 11 B NMR (160 MHz, CD2Cl2, 296 K): δ −13.4. 13C{1H} NMR (126 MHz, CD2Cl2, 296 K): δ 39.04 (CH3), 122.96, 123.64, 123.97, 124.97, 125.02, 125.29, 125.49, 126.56, 126.58, 127.02 (br), 130.99, 131.98, 132.02 (br), 132.27 (br), 132.56, 135.97 (CH), 137.67, 143.20, 143.94, 144.85, 146.73, 151.35 (br), 157.39 (br), 171.56 (br) (C). Anal. Calcd (%) for C53H43BN2: C 88.57, H 6.03, N 3.90. Found: C 88.49, H 6.12, N 3.87. Synthesis of [11(IMe)]. A solution of IMe (63.0 mg, 655 μmol) in toluene (3.5 mL) was added at rt to a solution of 2,3-Me2-10 and 1,2Me2-10 (ratio of isomers: 1:2.4, 350 mg, 648 μmol) in toluene (2.5 mL). The yellow solution was heated to 105 °C for 6 h, resulting in formation of a dark red mixture. After filtration, the solvent was reduced to about 2.5 mL, and hexane (5 mL) was added. Within a time frame of 12 h pale yellow crystals precipitated. The solution was decanted, and the crystals were washed with hexane (3 × 1.5 mL) and

dried under vacuum to yield a mixture of [11(IMe)eq] and [11(IMe)ax] (276 mg, 434 μmol, 67%) as a pale yellow crystalline solid (ratio of isomers: 11:1). Single crystals of [11(IMe)eq] suitable for X-ray diffraction were obtained by diffusion of hexane into a saturated solution of [11(IMe)eq] and [11(IMe)ax] in benzene. [11(IMe)eq]: 1H NMR (500 MHz, CD2Cl2, 296 K): δ 1.28 (s, 3H, NCH3), 1.54 (s, 6H, CH3-borepine), 2.28 (s, 3H, p-CH3-Mes), 2.78 (s, 3H, NCH3), 2.95 (s, 6H, o-CH3-Mes), 5.31 (d, 3JH−H = 1.86 Hz, 1H, NCH), 5.58 (d, 3JH−H = 1.86 Hz, 1H, NCH), 6.61−6.64 (m, 2H, C6H5), 6.72−6.76 (m, 2H, C6H5), 6.80−6.83 (m, 2H, C6H5), 6.86− 6.88 (m, 4H, C6H5 and m-CH-Mes), 6.92−7.00 (m, 6H, C6H5), 7.11− 7.13 (m, 2H, C6H5), 7.30−7.34 (m, 2H, C6H5), 7.59−7.61 (m, 2H, C6H5). 11B NMR (160 MHz, CD2Cl2, 296 K): δ −11.6. 13C{1H} NMR (126 MHz, CD2Cl2, 296 K): δ 19.76, 20.92, 27.56, 35.55, 39.42 (CH3), 120.72, 123.91, 124.06, 124.91, 126.41, 127.26, 127.28, 127.94, 129.27, 130.21, 132.22, 132.53, 136.78 (CH), 132.63, 137.40, 139.43, 142.59, 146.38, 149.28, 150.37 (br), 153.92 (br), 167.06 (br) (C). [11(IMe)ax]: 11B NMR (160 MHz, CD2Cl2, 296 K): δ −13.3. Anal. Calcd (%) for C46H45BN2: C 86.78, H 7.12, N 4.40. Found: C 87.06, H 7.15, N 4.40.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details for the synthesis of 2,3-Me2-10 and 1,2Me2-10 and X-ray crystallographic data (CIF files). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+49) 93131-84623. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Christine Werner for experimental assistance, and we gratefully acknowledge financial support provided by the Deutsche Forschungsgemeinschaft (DFG).

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REFERENCES

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