Borate

Note pubs.acs.org/Organometallics

A Novel Boron Heterocycle Comprised of an Inner Boronium/Borate Adduct Günter Seidel and Alois Fürstner* Max-Planck-Institut für Kohlenforschung, D-45470 Mülheim/Ruhr, Germany S Supporting Information *

ABSTRACT: The reaction of [MeSCH2Li·(tmeda)] with 9Cl-9-BBN led to the formation of the unprecedented boron heterocycle 3, which is an inner salt formally composed of boronium and borate subunits. This product rearranges upon heating to the thermodynamically more stable Ci-symmetric dimer 4, which can also be directly prepared if 9-MeO-9-BBN is chosen as the starting material instead of 9-Cl-9-BBN. The reasons for the different outcomes are discussed, and the solid-state structures of the dimeric products 3 and 4, as well as of the derived α-functionalized organoboron species 6, 9, and 12, are presented.

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Scheme 1. Preparation of Boron/Sulfur Heterocycles by Dimerization of Transient (Methylthio)methyl-9-BBNa

he transmetalation of unsaturated boron compounds with Au+ constitutes a valuable method or the preparation of organogold species, which are relevant for mechanistic discussions of π-acid catalysis but difficult to obtain otherwise.1−3 Provided this reaction can be extended to saturated organoboranes carrying a potential leaving group at the α-position, it might open access to variously substituted gold carbenoids. Such species are commonly invoked in gold catalysis,4 but experimental data about structure and bonding in pertinent gold carbenoids remain exceedingly rare.5−7 For this very reason we became interested in boranes bearing a methylene-linked thioether moiety as a potential entry point for our mechanistic investigations in this timely field of research. Among the known examples of such functionalized boron compounds,8 the 9-BBN derivative 2 seemed particularly attractive. The dimeric nature of this compound in the solid state (4) had previously been recognized, although full characterization data are missing.9 We reasoned that the rather electrophilic boron center of transient 2 should translate into stable ate complexes on treatment with an appropriate base, as deemed necessary for a subsequent transmetalation by an electrophilic transition-metal source; at the same time, the −SMe group provides a handle for tuning the leaving group properties by S-alkylation and eventual replacement of the resulting sulfonium moiety by other heteroatom substituents.10 Because 4 had previously been prepared by a multistep procedure,9 we sought for a direct and more scalable route. To this end, a solution of 9-Cl-9-BBN (1, X = Cl)11 in hexane was reacted at low temperature with the readily accessible TMEDA adduct of [LiCH2SMe] (1 equiv)12 (Scheme 1). The resulting mixture was slowly warmed to ambient temperature, the precipitated salts were removed, and the crude product was recrystallized from toluene. Even though the expected dimer 4 had not been fully characterized in the literature,9 the data for the compound prepared in this way were clearly not matching. While the MS spectra indicated the presence of a dimer, massive line broadening rendered an accurate interpretation of © 2014 American Chemical Society

Reagents and conditions: (a) [MeSCH2Li·(tmeda)], hexane, −75 °C, 53% (3); (b) [MeSCH2Li·(tmeda)] (second equivalent), 34% (n = 1) + 26% (n = 2); (c) 1 (X = Cl), hexane, 72%; (d) toluene, 80 °C, quantitative; (e) [MeSCH2Li·(tmeda)], THF, −75 °C, 86%; ( f) BF3· (OEt2), THF, −80 °C → room temperature, 82% or TMSCl, THF, room temperature, 73%. a

the NMR spectra difficult. The spectral fingerprint did not improve much upon changing the temperature or the solvent ([D8]-THF, C6D6). Gratifyingly though, single crystals of this sensitive material suitable for X-ray diffraction could be grown. The structure in the solid state (Figure 1) shows that the obtained adduct is an inner salt formally comprised of a borate and a boronium subunit tightly held together by the thioether groups.13,14 While the heterocyclic ring of 3 adopts a regular chair conformation, Received: June 11, 2014 Published: August 4, 2014 4336

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complex 5 (Scheme 1).17 This borate was in fact obtained in good yield; as expected, it furnished product 3 on reaction with 9-Cl-9-BBN.18 On the basis of this information about the relative electrophilicity of different 9-X-9-BBN derivatives vis-à-vis organolithium reagents, a rational approach to the targeted complex 4 became fairly obvious. Owing to the stronger boron−oxygen bond, the use of 9-MeO-9-BBN (1, X = OMe)19 as the electrophilic partner should afford the persistent ate complex 6 in the first place, which prevents a second equivalent of [MeSCH2Li·(tmeda)] from reacting with the boron center. Once the organolithium reagent has been consumed, the methoxide substituent of 6 can be removed with the help of an adequate second electrophile; the released borane 2 is expected to dimerize to the Ci-symmetric product 4. This simple route indeed worked very well. The borate complex 6 can be isolated in pure form; its structure in the solid state (Figure 2) shows the expected tight contacts between the

Figure 1. Structure of complex 3 in the solid state. Selected bond lengths (Å): B1−C11, 1.6258(18); B1−C15, 1.6081(18); B1−S1, 1.9534(8); S1−C1, 1.8018(9); S1−C2, 1.8196(9); B2−C2, 1.6526(12); B2−C21, 1.6447(18); B2−C25, 1.6498(18).15

both S−Me groups are axially oriented. This conformation allows for an antiperiplanar orientation of the sulfur lone pairs and the low-lying σ*(B1−S) orbitals, resulting in strong hyperconjugation; at the same time, both S−Me σ bonds are antiperiplanar to one of the σ* B−C bonds of the BBN cage, which likely explains why this anti-disposed B1−C11 bond (1.6258(18) Å) is notably longer than the corresponding B1− C15 bond (1.6081(18) Å). As one might expect for a formally positively charged boronium subunit,13 these B−C bonds are shorter than the corresponding B2−C21 and B2−C25 bonds within the anionic borate subunit. The situation in solution is obviously more complex. 1D and 2D 11B NMR spectra show the presence of two equilibrating conformers in a ∼2:1 ratio. As one might expect, the formal borate B atom (B2) resonates fairly sharply in the normal range for tetraalkylborate entities (δB −16.4, −19.8 ppm). Strikingly though, the other B atom (B1) of 3 gives rise to two broad signals for the two conformers at +2.4 and −1.5 ppm. Although B1 is formally a boronium center,13 these high-field shifts indicate that little positive charge resides at this boron center; rather, the sulfur atoms must have taken much of it over. This interpretation is fully consistent with DFT calculations at the PBE0/6-31+G(d,p) level, which reproduce the X-ray structure and the 11B NMR shifts for C6D6 very well. The computations indicate the presence of a second low-energy conformer in solution with one axial and one equatorial S−Me group (ΔH298 ≈ −0.2 kcal mol−1) and suggest significant charge delocalization over all heteroatoms; for details see the Supporting Information. The unusual dimer 3 is not only a novel boron heterocyclic ring system but also a very rare example of a formally cationic boron entity contained within a neutral (rather than charged) compound.13,16 However, it is unlikely that 3 is thermodynamically more favorable than the Ci-symmetric product 4, which we had originally targeted. The observed strong endotherm in the DSC thermogram, with an onset at 144 °C, supports this notion. This transition is irreversible and actually converts 3 into 4, as later confirmed by comparison with an authentic sample. The same transformation could also be enforced in solution upon heating of a sample of 3 in [D8]-toluene at 80 °C for 4 h. The formation of 3 as the only detectable product from the reaction of 1 equiv of 9-Cl-9-BBN (1, X = Cl) and [MeSCH2Li· (tmeda)] implies that the incipient product 2 traps the remaining lithium reagent in solution much faster than 9-Cl9-BBN itself. If this is the case, then the use of 2 equiv of [MeSCH2Li·(tmeda)] should afford the corresponding ate

Figure 2. Structure of complex 6 in the solid state. Selected bond lengths (Å): B1−C1, 1.6326(8); B1−C5, 1.6389(7); B1−C9, 1.6600(8); C9−S1, 1.8171(5); B1−O1, 1.5380(7); O1−Li1, 1.9049(11); S1−Li1, 2.5388(11).15

heteroatoms and the lithium counterion. Treatment of this compound with either BF3·(OEt2) or TMSCl in THF afforded the expected dimer 4 in high yield on a multigram scale. The structure of 4 in the solid state is unusual in that the central heterocyclic ring adopts a slightly skewed boat conformation without a clear-cut distinction between equatorial or axial −SMe substituents (Figure 3). In this regard, 4 is markedly different from the known dimer 7, the regular chair conformation of which holds the large B−Ph as well as the S− Me groups in an equatorial orientation.20 Likewise, the dimeric borane 8 adopts a chair conformation in the solid state with

Figure 3. Structure of complex 4 in the solid state. Selected bond lengths (Å): B1−C2, 1.632(4); B1−C11, 1.628(4); B1−C15, 1.632(4); B1−S1, 2.050(3); S1−C1, 1.803(3).15 4337

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equatorial −SMe groups, although this compound is known to be dynamic in solution.21

2.44−2.24 (m), 2.24−1.81 (m), 1.81−1.42 (m) (∑34 H), 0.99 (br s, 1.6 H), 0.79 (br s, 1.6 H), 0.56 (br s, 0.4 H), 0.41 (br s, 0.4 H); 13C NMR (100 MHz, [D8]-THF, −80 °C) (characteristic signals) δ 33.6, 32.7, 32.6, 32.5, 32.3, 32.2, 31.8, 31.3, 30.9, 26.7, 26.5, 26.4, 25.9, 24.1, 20.9 (br), 17.8, 15.0; 11B NMR (96 MHz, C6D6) δ 2.4 (34%), −1.5 (16%), −16.4 (34%), −19.8 (16%); MS (70 eV) m/z (%) 364 (67) [M+], 303 (89), 253 (18), 196 (41), 182 (32), 181 (31), 168 (62), 148 (58), 134 (92), 120 (100), 106 (87), 93 (93), 92 (82), 79 (70), 67 (64), 53 (54), 41 (63). Due to the sensitivity of the compound, satisfactory elemental analysis could not be obtained. The compound was characterized by X-ray diffraction; cf. the Supporting Information. Complex 6. TMEDA (7.44 g, 64.0 mmol) was added over 30 min to nBuLi (1.6 M in hexane, 40.0 mL) in hexane (60 mL). Me2S (3.97 g, 64.0 mmol) was introduced and the mixture stirred for 20 h before it was cooled to −75 °C. A solution of 9-MeO-9-BBN (9.73 g, 64.0 mmol) in THF (100 mL) was added over the course of 1 h. The cooling bath was removed, and the suspension was warmed to ambient temperature, causing the formation of a clear solution. Volatile materials were distilled off in vacuo (15 mbar), and the residue was suspended in pentane (120 mL). After the suspension was stirred for 30 min, the solid material was collected, washed with pentane, and dried in vacuo to give the title complex as a colorless crystalline material (18.55 g, 86%): 1H NMR (400 MHz, [D8]-THF) δ 3.12 (s, 3H), 2.31 (s, 4H), 2.15 (s, 12H), 1.98 (s, 3H), 1.84−1.80 (m, 8H), 1.54 (br s, 2H), 1.46−1.37 (m, 4H), 0.47 (br s, 2H); 13C NMR (100 MHz, [D8]-THF) δ 58.8, 48.2, 46.2, 34.2, 32.6, 31 (br), 27.1, 26.8, ∼25 (br, BCH2), 19.8; 11B NMR (96 MHz, [D8]-THF) δ −1.4. Anal. Calcd for C17H38BLiN2OS (336.3): C, 60.72; H, 11.39. Found: C, 60.68; H, 11.81. Complex 4. Method A. BF3·(OEt2) (7.62 g, 53.7 mL) was slowly added to a solution of complex 6 (13.96 g, 41.5 mmol) in THF (80 mL) at −80 °C, and the mixture was stirred for 30 min before the cooling was discontinued. All volatile materials were distilled off in vacuo (15 mbar) at ambient temperature. The residue was extracted with hot (80 °C) toluene (3 × 60 mL), and the combined toluene phases were reduced to about half of the original volume, causing partial precipitation of the product. This suspension was warmed to 80 °C until a clear solution had formed, which was then cooled to ambient temperature. The resulting crystals were collected, washed with cold toluene, and dried in vacuo (6.14 g, 82%). Method B. TMSCl (456 mg, 4.2 mmol) was added to a solution of complex 6 (1.315 g, 3.9 mmol) in THF (10 mL), causing a slight exotherm. After the mixture was stirred for 1 h at ambient temperature, all volatile materials were evaporated in vacuo and the residue was processed as described above, giving product 4 in the form of colorless crystals (419 mg, 73%): DSC 189.3° (onset of endotherm); 1H NMR (400 MHz, C6D6) δ 2.2−2.03 (m, 6H), 2.03−1.82 (m, 16H), 1.82−1.70 (m, 6H), 1.47 (s, 6H), 1.05−0.7 (br m, 4H); 13C NMR (100 MHz, [D8]-toluene, 193 K) δ 32.7, 32.4, 32.0, 30.6, 29.6, 25.1, 24.7, 23.2, 21.9, 18.1; 11B NMR (96 MHz, C6D6) δ 0.2; MS (70 eV) m/z (%) 182 (52) [M/2], 134 (100), 119 (20), 106 (55), 93 (36), 92 (48), 91 (37), 79 (21), 65 (17), 53 (16), 41 (18). Anal. Calcd for C20H38B2S2 (364.3): C, 65.95, ;H, 10.52. Found: C, 65.79; H, 10.24. For the crystal structure analysis, see the Supporting Information. Sulfonium Salt 9a (X = I). A suspension of 4 (599 mg, 1.64 mmol), NaI (735 mg, 4.9 mmol), and MeI (2.1 mL, 33 mmol) in MeCN (20 mL) was stirred for 2 days. During this time, a clear solution was temporarily formed, from which a yellowish solid started to precipitate. Volatile materials were distilled off, the residue was suspended in CH2Cl2 (30 mL), and the insoluble material was filtered off and washed with CH2Cl2 (2 × 5 mL). The combined filtrates were evaporated, and the residue recrystallized upon carefully layering a solution in the minimum amount of CH2Cl2 with pentane. After 1 day, the precipitated material was collected, washed with cold pentane, and dried to give 9a in the form of colorless crystals (1.03 g, 86%):23 1H NMR (400 MHz, CD2Cl2) δ 3.11 (s, 4.4 H), 2.91 (s, 1.5 H), 2.88 (s, 0.5 H), 2.49 (s, 3 H), 2.09 (s, 1.6 H) [1.99−1.61 (m), 1.53−1.40 (m); 12 H], 1.28 (s, br, 0.7 H), 0.77 (s, br, 1.3 H); 13C NMR (100 MHz, CD2Cl2) δ 116.7, 36.8 (br t, BCH2), 31.2 (t), 28.1 (q), 24.4 (t), 23.5

As one might anticipate, the S−B contacts in 4 are significantly longer than those in 3, which comprises a more electrophilic formal boronium center. This difference explains why the MS spectrum of 4 shows the molecular mass of the monomeric unit, whereas 3 gets ionized as an intact dimeric array. With sufficient material in hand, the S-methylation of 4 and the exchange of the resulting sulfonium moiety for an iodo substituent on treatment with BI3 were accomplished (Scheme 2). During the latter reaction, however, the carbenoid reactivity Scheme 2. α-Functionalization of Compound 4a

a

Reagents and conditions: (a) MeI, NaI, MeCN, 86% (X = I) or MeOTf, MeCN, 49% (X = OTf); (b) BI3, CH2Cl2, 78% (10:11 ≈ 70:30); (c) pyridine, CH2Cl2, 28%.

of the intermediate surfaced in competing ring expansion to give appreciable amounts of 11 as a byproduct.22 As expected, compounds 9 and 10 and the derived adduct 12 are monomeric in the solid state (their X-ray structures are contained in the Supporting Information). Applications of this now readily accessible new family of α-functionalized organoboron reagents in organometallic synthesis will be reported in due course.

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EXPERIMENTAL SECTION

General Considerations. All reactions were carried out in flamedried glassware under Ar, using carefully dried solvents. For details, see the Supporting Information. Detailed procedures for the preparation of 5 (n = 1, 2) and 9c are also given in the Supporting Information. Complex 3. TMEDA (2.54 g, 21.9 mmol) was added over 15 min to a solution of nBuLi (1.6 M in hexane, 13.7 mL) in hexane (30 mL). Me2S (1.36 g, 21.9 mmol) was introduced, and the mixture was stirred for 20 h at ambient temperature before it was cooled to −75 °C. A solution of 9-Cl-9-BBN (3.42 g, 21.9 mmol) in hexane (50 mL) was slowly added over the course of 1 h, leading to the formation of a voluminous suspension. Once the addition was complete, the mixture was warmed to ambient temperature and stirring was continued for 4 h. The precipitate was filtered off under Ar and washed with pentanes. Evaporation of all volatile materials afforded a solid residue, which was suspended in toluene (60 mL). The suspension was briefly warmed to 80 °C, and the insoluble material was removed and resuspended in warm toluene (20 mL). After all remaining undissolved material was filtered off, the combined filtrates were cooled to ambient temperature, causing the product to crystallize. The mother liquor was siphoned off, and the remaining solid material was triturated with cold (0 °C) toluene and dried in vacuo. The mother liquor was reduced to about half of its volume, leading to the precipitation of a second crop of product, which was equally washed and dried. Compound 3 was thus obtained in the form of colorless crystals (2.08 g, 53%): DSC 144 °C (onset of endotherm, see text); 1H NMR (400 MHz, C6D6) (due to massive line broadening, only characteristic regions can be given) δ 4338

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(br, BCH), 5.1 (q); 11B NMR (128 MHz, CDCl3) δ −3.8; MS (70 eV) m/z (%) 262 (9-BBNCH2I, 25), 135 (100), 107 (36), 93 (52), 79 (36), 67 (26), 62 (SMe2, 28), 41 (MeCN, 60). Anal. Calcd for C13H25BINS (365.1): C, 42.76; H, 6.90. Found: C, 42.85; H, 6.88. Iodide 10 and Adduct 12. BI3 (4.57 g, 11.7 mmol) was added to a solution of 9a (X = I) (2.131 g, 5.8 mmol) in CH2Cl2 (50 mL), causing the spontaneous formation of a precipitate. The suspension was stirred for 1 h before all volatile materials were evaporated. The residue was suspended in pentane (40 mL) and the mixture stirred for 1 h. The solid was filtered off, the filtrate was evaporated, and the residue was purified by distillation (50−55 °C, 10−3 mbar) to give a mixture comprising 10 [11B NMR: δ 70.1 ppm (70%)] and the ringexpanded product 11 [11B NMR: δ 81.1 ppm (30%)] (1.19 g, 78%). A small sample of analytically pure 10 was obtained by fractional distillation (10−4 mbar, bp 33−35 °C): 1H NMR (400 MHz, CDCl3) δ 2.96 (s, 2H), 2.07−1.97 (m, 4H), 1.97−1.75 (m, 6H), 1.53−1.46 (br s, 2H), 1.46−1.38 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 34.6, 29.7 (br), 23.2, 8.1 (br); 11B NMR (128 MHz, CDCl3) δ 70.1; MS (70 eV) m/z (%) 262 (58) [M+], 179 (4), 165 (5), 153 (9), 135 (100), 107 (25), 93 (35), 79 (23), 67 (20), 53 (14), 41 (19). Anal. Calcd for C9H16BI (261.9): C, 41.27; H, 6.16. Found: C, 41.40; H, 6.21. Pyridine (0.6 mL) was added to a stirred solution of this mixture (929 mg) in CH2Cl2 (10 mL). After 30 min, the solvent was evaporated, the residue was suspended in hexane (20 mL), and the solid was filtered off. Continuous extraction of this material with pentane afforded pure 12 in the form of colorless crystals (334 mg, 28%): 1H NMR (400 MHz, CD2Cl2) δ 8.55 (d, J = 6.7 Hz, 2H), 8.04 (t, J = 7.6 Hz, 1H), 7.65 (t, J = 6.7 Hz, 2H), 2.67 (s, 2H) [1.96−1.70 (m), 1.66−1.48 (m), 1.43−1.30 (m), 1.23−1.13 (m), 1.13−1.06 (m), ∑14H]; 13C NMR (75 MHz, CD2Cl2, 193 K) δ 144.8, 140.1, 125.2, 30.9, 29.5, 24.2, 24.1 (br, t), 23.8, 23.3 (br, d); 11B NMR (128 MHz, CD2Cl2) δ 0.0; MS (70 eV) m/z (%) 341 (≤1) [M+], 262 (23), 200 (4), 167 (12), 153 (8), 135 (100), 107 (39), 93 (62), 79 (88), 67 (38), 52 (51), 41 (39). For the crystal structure, see the Supporting Information.

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G. Organometallics 2009, 28, 1666. (d) Heckler, J. E.; Zeller, M.; Hunter, A. D.; Gray, T. G. Angew. Chem., Int. Ed. 2012, 51, 5924. (3) Seidel, G.; Lehmann, C. W.; Fürstner, A. Angew. Chem., Int. Ed. 2010, 49, 8466. (4) (a) Fürstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46, 3410. (b) Gorin, D. J.; Toste, F. D. Nature 2007, 446, 395. (c) Obradors, C.; Echavarren, A. M. Chem. Commun. 2014, 50, 16. (d) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2010, 49, 5232. (e) Fürstner, A. Acc. Chem. Res. 2014, 47, 925. (5) (a) Seidel, G.; Fürstner, A. Angew. Chem., Int. Ed. 2014, 53, 4807. (b) Seidel, G.; Gabor, B.; Goddard, R.; Heggen, B.; Thiel, W.; Fürstner, A. Angew. Chem., Int. Ed. 2014, 53, 879. (c) Seidel, G.; Mynott, R.; Fürstner, A. Angew. Chem., Int. Ed. 2009, 48, 2510. (6) (a) Brooner, R. E. M.; Brown, T. J.; Widenhoefer, R. A. Angew. Chem., Int. Ed. 2013, 52, 6259. (b) Hussong, M. W.; Rominger, F.; Krämer, P.; Straub, B. Angew. Chem., Int. Ed. 2014, DOI: 10.1002/ anie.201404032. (c) Harris, R. J.; Widenhoefer, R. A. Angew. Chem., Int. Ed. 2014, DOI: 10.1002/anie.201404882. (7) For the structure of the carbenoid [(Ph3P)AuCH2Cl], see: Steinborn, D.; Becke, S.; Herzog, R.; Günther, M.; Kircheisen, R.; Stoeckli-Evans, H.; Bruhn, C. Z. Anorg. Allg. Chem. 1998, 624, 1303. (8) Matteson, D. S. Sci. Synth. 2005, 6, 607. (9) Bestmann, H. J.; Röder, T.; Sühs, K. Chem. Ber. 1988, 121, 1509. The dimeric nature was inferred from the high-field shift in the 11B NMR spectrum (+0.64 ppm, CDCl3), whereas the MS spectrum showed the molecular peak of the monomeric unit. (10) Badet, B.; Julia, M.; Lefebvre, C. Bull. Soc. Chim. Fr. 1984, 2, 431. (11) Brown, H. C.; Dhar, R. K.; Ganesan, K.; Singaram, B. J. Org. Chem. 1992, 57, 499. (12) Peterson, D. J. J. Org. Chem. 1967, 32, 1717. (13) For a review of formally cationic boron species and their nomenclature, see: De Vries, T. S.; Prokofjevs, A.; Vedejs, E. Chem. Rev. 2012, 112, 4246. (14) For the use of borates of the type [R2B(CH2SR)2]− as ligands to various cationic centers, see: Riordan, C. G. Coord. Chem. Rev. 2010, 254, 1815. (15) Anisotropic displacement parameters are drawn at the 50% probability level, and hydrogen atoms are omitted for clarity; for details, see the Supporting Information. (16) (a) For a pioneering study, see: Young, D. E.; Shore, S. G. J. Am. Chem. Soc. 1969, 91, 3497. (b) For the possible involvement of such species in certain electrophilic borylation reactions, see: Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767. (c) Bis(pyrazolyl)borate complexes may also be seen as formal inner borate/boronium salts; cf.: Trofimenko, S. J. Am. Chem. Soc. 1966, 88, 1842. (17) For the use of such ate complexes as donors in Suzuki reactions, see: (a) Seidel, G.; Fürstner, A. Chem. Commun. 2012, 48, 2055. (b) Fürstner, A.; Seidel, G. Tetrahedron 1995, 51, 11165. (18) These data render formation of 3 via “self-ionization” of 2 less likely; for such a pathway, see: Tsai, J.-H.; Lin, S.-T.; Yang, R. B.-G.; Yap, G. P. A.; Ong, T.-G. Organometallics 2010, 29, 4004. (19) Brown, H. C.; Knights, E. F.; Scouten, C. G. J. Am. Chem. Soc. 1974, 96, 7765. (20) Ruth, K.; Tüllmann, S.; Vitze, H.; Bolte, M.; Lerner, H.-W.; Holthausen, M. C.; Wagner, M. Chem. Eur. J. 2008, 14, 6754. (21) Nöth, H.; Sedlak, D. Chem. Ber. 1983, 116, 1479. (22) For analogous ring expansions of the 9-BBN cage on treatment with diazoalkanes, see: (a) Burgos, C. H.; Canales, E.; Matos, K.; Soderquist, J. A. J. Am. Chem. Soc. 2005, 127, 8044. (b) Canales, E.; Prasad, K. G.; Soderquist, J. A. J. Am. Chem. Soc. 2005, 127, 11572. (23) Trace amounts (