Organometallics 2011, 30, 1067–1072 DOI: 10.1021/om1011038
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Isomeric Dipyrrinato and Dipyrromethanato Boranes Catherine Bonnier, Warren E. Piers,* and Masood Parvez. Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4 Received November 24, 2010
Reaction of the dipyrrin ligand 1,3,7,9-tetramethyl-2,8-diethyldipyrrin with BH3 3 SMe2 initially gives a dipyrrin-BH3 adduct, which eliminates dihydrogen to form the dipyrrin-supported BH2 borane LHBH2. This species is unstable toward isomerization to its dipyrromethane isomer 1, via a (possibly BH3 catalyzed) hydride transfer from boron to the meso position of the dipyrrin core. Computations suggest that 1 is ∼4 kcal mol-1 more stable than the dipyrrin isomer. These conclusions were supported by low-temperature NMR and UV-vis/fluorescence spectroscopic experiments and labeling studies using BD3 3 SMe2. Borane 1 is susceptible to hydride abstraction to form the previously characterized borenium ion [LHBH][B(C6F5)4] and takes up hydride to form a dipyrromethane borohydride, which was characterized spectroscopically and crystallographically.
Introduction Boron difluoride complexes of the dipyrrinato ligand family (the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes, commonly known as BODIPY compounds, Chart 1) are strongly absorbing small molecules that exhibit relatively sharp emission profiles with high quantum yields.1,2 They are relatively easy to prepare, air and moisture stable, and commercially available; as such, they have been applied as fluorescent probes and labels in a wide variety of applications. Despite these attributes, improvements to the performance of these dyes are desirable. Specifically, lower energy emission profiles and greater Stoke’s shifts3 would widen their applicability to biological systems. To this end, several research groups have extensively explored the functionalization of the BODIPY core structure,4 via alteration of the dipyrrinato periphery through substitution of the 1-3/ 5-7 and 8 positions and, more recently, through replacement of the fluorine substituents on the boron 4 position with other moieties. Substitutes for fluoride that have been explored
include alkoxides (OR)5-12 and a wide variety of hydrocarbyl groups.2,13-17 Missing from these boron substituent modified dyes is the simplest example, that is, the borane derivative, where F has been replaced by H. Recently, we reported the synthesis of a borenium cation based on the BODIPY framework18 ([LHBH][B(C6F5)4], Chart 1) that incorporated an H atom substituent on boron; we envisioned that the neutral borane BODIPY complex LHBH2 might serve as a conveniently prepared precursor to this borenium ion and were mildly surprised to find that such a compound had not been reported. Herein we describe our attempts to prepare this precursor and provide an explanation for its absence from the literature. Synthesis and Characterization. Our published procedure for the synthesis of borenium ion [LHBH][B(C6F5)4] involves abstraction of fluoride from the 4,4-difluoro BODIPY dye, followed by transmetalation of the remaining fluoride with an aluminum hydride reagent.18 Seeking a more direct route, we predicted that hydride abstraction from LHBH2 could be a viable strategy, provided it could be generated readily. Accordingly, we explored the reaction of the free ligand with a source of BH3; the chemistry observed is summarized in Scheme 1. Upon addition of a toluene solution of BH3 3 SMe2 to a suspension of the free dipyrrin ligand in the same solvent at room temperature, gas evolution was observed and an orange solution that was visibly fluorescent upon irradiation
*To whom correspondence should be addressed. E-mail: wpiers@ ucalgary.ca. (1) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891–4932. (2) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008, 47, 1184–1201. (3) Ziessel, R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31, 496–501. (4) Arroyo, I. J.; Hu, R.; Merino, G.; Tang, B. Z.; Pena-Cabrera, E. J. Org. Chem. 2009, 74, 5719–5722. (5) Kim, H.; Burghart, A.; B. Welch, M.; Reibenspies, J.; Burgess, K. Chem. Commun. 1999, 1889–1890. (6) Ikeda, C.; Nabeshima, T. Chem. Commun. 2008, 721–723. (7) Wijesinghe, C. A.; El-Khouly, M. E.; Blakemore, J. D.; Zandler, M. E.; Fukuzumi, S.; D’Souza, F. Chem. Commun. 2010, 46, 3301–3303. (8) Hudnall, T. W.; Lin, T.-P.; Gabbaı¨ , F. P. J. Fluorine Chem. 2010, 131, 1182–1186. (9) Parhi, A. K.; Kung, M.-P.; Ploessl, K.; Kung, H. F. Tetrahedron Lett. 2008, 49, 3395–3399. (10) Ikeda, C.; Maruyama, T.; Nabeshima, T. Tetrahedron Lett. 2009, 50, 3349–3351. (11) Kubo, Y.; Minowa, Y.; Shoda, T.; Takeshita, K. Tetrahedron Lett. 2010, 51, 1600–1602.
(12) Crawford, S. M.; Thompson, A. Org. Lett. 2010, 12, 1424–1427. (13) Goze, C.; Ulrich, G.; Mallon, L. J.; Allen, B. D.; Harriman, A.; Ziessel, R. J. Am. Chem. Soc. 2006, 128, 10231–10239. (14) Bonnier, C.; Piers, W. E.; Al-Sheikh Ali, A.; Thompson, A.; Parvez, M. Organometallics 2009, 28, 4845–4851. (15) Ziessel, R.; Bura, T.; Olivier, J.-H. Synlett 2010, 15, 2304–2310. (16) Haefele, A.; Zedde, C.; Retailleau, P.; Ulrich, G.; Ziessel, R. Org. Lett. 2010, 12, 1672–1675. (17) Kumaresan, D.; Thummel, R. P.; Bura, T.; Ulrich, G.; Ziessel, R. Chem.;Eur. J. 2009, 15, 6335–6339. (18) Bonnier, C.; Piers, W. E.; Parvez, M.; Sorensen, T. S. Chem. Commun. 2008, 4593–4595.
r 2011 American Chemical Society
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Chart 1
Scheme 1 Figure 1. Thermal ellipsoid diagram (50%) of 1-DMAP. Selected bond distances (A˚): B(1)-N(1), 1.512(4); B(1)-N(2), 1.524(4); B(1)-N(3), 1.623(4). Selected bond angles (deg): N(1)-B(1)-(N2), 108.2(2); N(1)-B(1)-N(3), 107.2(2); N(2)B(1)-N(3), 111.2(2).
with a hand-held UV lamp resulted. However, NMR analysis of an aliquot indicated the formation of multiple products. Heating the orange solution at 60 °C for ∼3 h resulted in the fading of the orange color and loss of the fluorescent character of the solution under UV irradiation. From this solution, a white crystalline material was isolated in ∼70% yield by sublimation of the residue and recrystallization of the sublimate from hexanes at -35 °C. By NMR spectroscopy and derivatization with 4-dimethylaminopyridine (DMAP), the product was identified as the dipyrromethane-coordinated borane 1 (Scheme 1). The 1H NMR spectrum of 1 (Figure S1) is consistent with a C2v symmetric structure and was noteworthy for the absence of the typical one-proton singlet in the aromatic region associated with the meso proton of the 8 position (Chart 1). Instead a new, slightly broad signal integrating for two protons was found at 3.76 ppm in C6D12 solvent; a 13C DEPT experiment established this as a methylene group, and an HMQC experiment showed that it is correlated to a carbon resonance at 22.2 ppm. Both of these resonances are consistent with chemical shifts observed for the CH2 groups of unsubstitued dipyrromethanato ligands.19 In addition, the multiplicity of the signal at 1.90 ppm associated with the 3,7-methyl protons was observed to be a triplet with a small coupling of 1.1 Hz. 1 H-1H correlation spectroscopy indicated that the 3.76/1.90 ppm resonances are coupled, so this small coupling is assigned as the 5JHH between these two protons, and the broadness in the resonance at 3.76 ppm is due to its unresolved septet structure. A broad signal hidden in the baseline at 5.17 ppm sharpened in the {11B} decoupled spectrum and integrates to one proton; this is assigned as the resonance for the BH proton. The 11B NMR spectrum showed a broad doublet at 26 ppm (333 K) with a coupling constant of 148 Hz,20 indicative of a tricoordinated21 B-H moiety. Taken together, these data imply that borane 1 is monomeric in solution. The infrared spectra recorded both in a (19) Wang, Q. M.; Bruce, D. W. Synlett 1995, 1267–1268. (20) Hermanek, S. Chem. Rev. 1992, 92, 325–362. (21) Kennedy, J. D. Multinuclear NMR; Plenum Press: New York, 1987.
KBr pellet and in hexanes solution showed a weak to medium absorption at ∼2610 cm-1, which shifted to 1943 cm-1 upon deuterium labeling, further supporting the monomeric nature of the compound.22 Although 1 is a crystalline solid, attempts to grow crystals suitable for X-ray diffraction analysis were unsuccessful. To confirm the structure of 1, a Lewis base adduct with DMAP was prepared. The 11B NMR spectrum of 1-DMAP exhibits a doublet at -1.3 ppm (characteristic for a tetracoordinated borane moiety21) with 1JBH = 144 Hz at 333 K in 1,1,2,2tetrachloroethane (TCE-d2) solvent. Crystallographic analysis of single crystals of 1-DMAP confirmed its structure; a thermal ellipsoid diagram is shown in Figure 1 along with selected metrical parameters. The compound crystallizes with a solvent molecule (benzene, not shown), which perturbs the N(1)-B(1)-N(3) and N(2)B(1)-N(3) angles slightly since the solvent of crystallization appears to π stack with the coordinated DMAP ligand. The borane hydrogen was not located, but the pyramidalized geometry about boron clearly indicates its presence. As in other tetracoordinated metal dipyrromethanato complexes, 23 1-DMAP exhibits slight curvature in the ligand framework due to the sp3-hybridized methylene carbon and boron centers, resulting in puckering of the central six-membered ring. The B-N bond distances in this formally dianionic dipyrromethanato ligand are shorter than in a typical boron dipyrrinato complex. The B(1)-N(3) bond distance of 1.623(4) A˚ is longer than comparable DMAP adducts,24,25 indicating that boron is weakly Lewis acidic. The lability of the DMAP ligand is confirmed by variable-temperature NMR spectroscopy; the diastereotopic CH2-meso protons (2JHH = 18 Hz) are distinct at room temperature but coalesce at 316 K upon warming 1-DMAP in TCE-d2 solvent (ΔG‡ = 14.7(5) kcal/mol, Figure S2). Likely, this exchange is commuted by a dissociation/ reassociation of DMAP. Accordingly, when 2,6-dideuterioDMAP was added to solutions of 1-DMAP, the deuteriumlabeled DMAP slowly washed into the coordinated DMAP ligand as unlabeled DMAP was released. (22) Socrates, G., Infrared and Raman Characteristic Group Frequencies, 3rd ed.; Wiley: New York, 2001. (23) Ballmann, J.; Sun, X.; Dechert, S.; Bill, E.; Meyer, F. J. Inorg. Biochem. 2007, 101, 305–312. (24) Hudnall, T. W.; Gabbaı¨ , F. P. Chem. Commun. 2008, 4596–4597. (25) Lesley, M. J. G.; Woodward, A.; Taylor, N. J.; Marder, T. B.; Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D. W.; Kakkar, A. K. Chem. Mater. 1998, 10, 1355–1365.
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Organometallics, Vol. 30, No. 5, 2011 Scheme 2
Mechanistic Studies. On the basis of the outcome of the chemistry described above, it appears that target dipyrrinato borane LHBH2 is not stable toward isomerization to the observed thermodynamic product, dipyrromethanato borane 1. The qualitative observation of fluorescence during the reaction, however, suggests that dipyrrinato complexes may be important as kinetic products on the reaction path. In order to get more information on these species and the mechanism of formation of 1, various spectroscopic experiments were undertaken. Low-temperature NMR spectroscopic experiments were performed first. After mixing equimolar amounts of dipyrrin ligand and D3B 3 SMe2 in C7D8 or C7H8 at 195 K, the 1H and 2 H NMR spectra indicated that both starting materials remained unreacted at this temperature. Upon slowly increasing the temperature of the probe, a new species emerged at a temperature of 243(2) K; complete conversion was achieved at 288(2) K. The spectrum displayed the usual signals for a dipyrrin moiety, but with inequivalent alkyl groups; in the 2H NMR spectrum, a signal at 3.21 ppm was observed (Figure S3). A resonance at 1.37 ppm for free dimethylsulfide was also detected. These observations are consistent with the displacement of the SMe2 Lewis base by the dipyrrin ligand to form a new Lewis acid-base adduct (LH 3 BD3, Scheme 2). This species was reasonably stable at this temperature, although experiments using H3B 3 SMe2 were not as spectroscopically clean, suggesting an isotope effect in the next step may stabilize this intermediate in the d3-isotopomer. As the temperature of the probe was raised further, a complex spectrum was observed from which signals associated with d2-1 emerged. The nature of the intermediate species formed from LH 3 BD3 en route to d2-1 were not discernible from the spectra obtained, but UV-vis and fluorescence spectroscopy proved informative, since at least two of the intermediates were fluorescent. Due to the lower concentrations necessary for the fluorescence experiments, the use of an excess of H3B 3 SMe2 was hard to avoid; in any case, use of an excess was necessary to have the reaction proceed to completion. Under these conditions, shortly after the addition of an approximately 3-fold excess of BH3 3 SMe2 to a toluene solution of free dipyrrin ligand LHH at 195 K, and warming slowly, the absorption spectrum associated with the ligand underwent a bathochromic shift from 448 to 486 nm (Figure S4). The similar wavelength and appearance
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(i.e., the absence of vibrational fine structure2) of the new absorption is in agreement with the formation of LH 3 BH3, as determined by NMR spectroscopy. This new species is not emissive upon selective excitation, in keeping with the notion that it is an unchelated dipyrrin. Upon further temperature increase and elapsed time, the intensity of the 486 nm absorption slowly diminished, to afford a new absorption band at 519 nm (Figures S5-S7). The appearance of this absorption band is more similar to that of a typical BODIPY dye, with well-defined vibrational fine structure.2 Interestingly, close inspection of the isosbestic point shows that it is not stationary, but migrates toward lower wavelengths with the growth of the absorption at 519 nm. This behavior is indicative of more than one species being associated with this absorption.26,27 Unlike the free ligand and intermediate LH 3 BH3, these species are strongly fluorescent, and excitation at two different wavelengths (λ = 522 and 531 nm) was each associated with a distinct emission maximum (λ = 530 and 538 nm), again suggesting the presence of two emissive intermediates (Figures S8, S9). As the reaction progressed at 333 K, the intensity of the 519 nm absorption decreased and a new absorption emerged at higher energy (Figure S10). In toluene, the solvent absorption window obscures the λmax for this species, but comparison to an isolated sample of 1 indicates that this spectrum arises from the final borane product, which is nonemissive and colorless (Figure S11). We interpret these results according to Scheme 2. It is most logical that the first emissive species (the kinetic product) is the targeted dipyrrinato BX2 complex LHBX2 in the scheme formed upon elimination of H2 (or HD) from LH 3 BX3; indeed, HD is observed to form as LH 3 BD3 decomposes. The suprafacial transannular hydrogen shift necessary to convert the kinetic product LHBX2 to 1 is symmetry allowed upon examination of the HOMO of LHBX2, but may be facilitated by the presence of excess BX3, which is certainly present in the more dilute solutions necessary for the fluorescence monitoring. Partial abstraction of hydride (or deuteride) from LHBX2 by BX3 would form a borenium-like species (speculatively assigned to the second observable emissive compound in the sequence) that is essentially the BX4- salt of [LHBH][B(C6F5)4] (Chart 1) we have previously characterized.18 This anion is likely to be substantially more coordinating, accounting perhaps for the lower emission wavelengths observed for this species in comparison to [LHBH][B(C6F5)4]. This notion is further supported by the observation that [LHBH][B(C6F5)4] transforms readily to 1 upon treatment with NaBH4 (see Scheme 3). The conversion of the BODIPY analogue LHBH2 suggests that it is thermodynamically less stable than the borane isomer 1. Singlepoint energy DFT computations at the B3LYP/6-31G(d,p) level corroborate this; minimized structures for both isomers indicate that 1 is 4 kcal mol-1 more stable than LHBH2 (Tables S1-S3). Reactivity. Initially, compound LHBH2 was conceived as a convenient precursor to the borenium ion [LHBH][B(C6F5)4]; as it happens, the borane isomer of LHBH2 prepared here is an excellent precursor to this borenium ion, which forms quantitatively upon treatment of 1 with [Ph3C]þ[B(C6F5)4](Scheme 3). Conversion of [LHBH][B(C6F5)4] back to 1 is readily accomplished by addition of hydride using NaBH4. (26) Croce, A. E. Can. J. Chem. 2008, 86, 918–924. (27) Berlett, B. S.; Levine, R. L.; Stadtman, E. R. Anal. Biochem. 2000, 287, 329–333.
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Borane 1 is a new monomeric borane related to commonly employed and commercially available29 hydroboration reagents such as catechol borane (HBcat) and pinacol borane (HBpin). The latter two reagents are utilized in a variety of transition metal catalyzed hydroborations30,31 but do not undergo uncatalyzed reactions readily due to the π-donating catecholate and pinacolate ligands. Similarly, 1 is unreactive toward, for example, terminal alkynes. Furthermore, attempts to catalyze a hydroboration between phenyl acetylene and 1 using Wilkinson’s catalyst were unsuccessful.
Conclusions
Figure 2. Thermal ellipsoid diagram (50%) of 2-Na. Selected bond distances (A˚): B(1)-N(1), 1.531(3); B(1)-N(2), 1.530(3); Na(1)-N(1), 2.531(2); Na(1)-N(2), 2.600(2); Na(1)-C(9), 2.639(2); Na(1)-C(4), 2.639(2); Na(1)-C(1), 2.650(2); Na(1)C(6), 2.676(2); Na(1)-C(8), 2.738(2); Na(1)-C(7), 2.756(2); Na(1)-C(3), 2.814(2); Na(1)-C(2), 2.822(2). Selected bond angle (deg): N(1)-B(1)-(N2), 108.52(16).
In addition to being susceptible to hydride abstraction, borane 1 also reacts with sodium hydride to form a hydrido borate product 2-Na, which can be isolated as a white solid and fully characterized. In the 1H NMR spectrum a broad signal integrating to two protons at 4.2 ppm correlates to a resonance at 8.0 ppm in the 11B NMR spectrum. X-ray quality crystals were obtainable via evaporation of solvent from a hexanes solution, but analysis of the crystal showed that the unit cell was comprised of 88% 2-Na and 12% of a hydroxy hydrido species presumably formed due to the high oxygen or water sensitivity of this salt. In any case, the two species cocrystallize, and the structure was solved using the above fractional occupancies. The molecular structure of 2-Na is shown in Figure 2, along with selected metrical data; a depiction of the cocrystallized hydroxo molecular structure is given in Figure S12. In the structure, two monomeric units are arranged in a head-to-tail fashion and bridged through the alkali cation, which bonds to the flanking pyrrole rings in a η5-coordination mode.28 The closest nonbonding contact between the ring systems is between B(1) and C(5) at 4.504 A˚; these two units are tilted toward each other, causing a slight deviation from planarity in the C3N2B central ring. Other metrical parameters are unremarkable. Compound 2-Na is easily converted back to borane 1 via treatment with Ph3CCl, completing a series of hydride addition/ abstraction protocols centered around borane 1, as depicted in Scheme 3. (28) Ghesner, I.; Piers, W. E.; Parvez, M.; McDonald, R. Can. J. Chem. 2006, 84, 81–92.
In this study we have shown that the putative BODIPY derivative incorporating hydride ligands at boron instead of the usual fluoride substituents is thermodynamically unstable relative to its borane isomer 1, although it may be fleetingly observed in the reaction of a dipyrrin ligand with BH3. The borane is an effective precursor to the borenium ion [LHBH][B(C6F5)4], Chart 1, via hydride abstraction with a trityl borate reagent. The borane also reversibly adds hydride to form hydridoborate salt 2-Na; this latter reaction suggests that addition of other nucleophiles, followed by two hydride abstractions, would serve as a convenient route to a variety of other borenium ions, a prospect we are actively pursuing in current research.
Experimental Section General Procedures and Equipment. All operations were performed under a purified argon atmosphere using glovebox or vacuum line techniques. Toluene, hexanes, and THF solvents were dried and purified by passing through activated alumina and vacuum distilled from Na/benzophenone. CH2Cl2, CD2Cl2, C6H12, TCE, and TCE-d2 were dried over and distilled from CaH2, while C6D12 was dried over molecular sieves. All NMR spectra were recorded on Bruker AMX-300 MHz and DRX-400 MHz spectrometers, operating at 300 and 400 MHz (1H), 46 MHz (2H), 96 and 128 MHz (11B), 100 MHz (13C), and 282 or 376 MHz (19F) at 25 °C, unless indicated. Chemical shifts are reported in ppm relative to residual solvent signal (1H and 13 C{1H}), BF3 3 OEt2 (11B), and C6F6 (19F) standards. Note that IUPAC numbering for the BODIPY ring system (shown in Chart 1) is different from that for the dipyrromethane ligands in the new compounds reported here;1 NMR assignments below reflect the appropriate numbering system for these ligands. Low-resolution mass spectra were obtained using a Bruker Esquire 3000 spectrometer operating in electrospray ionization (ESI) mode or using a Finnigan MAT SSQ7000 operating at 70 eV in electron impact (EI) mode. High-resolution mass (29) Hayashi, T.; Matsumoto, Y.; Ito, Y. J. Am. Chem. Soc. 1989, 111, 3426–3428. (30) Vogels, C. M.; Westcott, S. A. Curr. Org. Chem. 2005, 9, 687–699. (31) Lata, C. J.; Crudden, C. M. J. Am. Chem. Soc. 2009, 132, 131–137.
Article spectra were obtained on a Waters GCT Premier operating at 70 eV in electron impact (EI) mode. Fluorescence spectra were obtained using a PTI equipped with a Quantum Northwest TC125 temperature controller or Yvon-Jobin spectrophotometers with excitation and emission set to 1.0 nm bandpass, and UV-visible spectra were obtained using a Cary 50 Bio equipped with a Cary Peltier temperature controller or Cary 100 Bio or Cary 5000 spectrophotometers operating in single- or double-beam mode. X-ray crystallographic analyses were performed on suitable crystals coated in paratone oil and mounted on a Nonius Kappa CCD diffractometer. Infrared spectra were obtained on a Nicolet Nexus 470. Elemental analyses were performed using a Perkin-Elmer model 2400 series II analyzer by Johnson Li (University of Calgary). Materials. BH3 3 SMe2 (2 M in toluene and neat) and NaBH4 were purchased from Aldrich and used as received. DMAP and Ph3CCl were obtained from Aldrich and sublimed under dynamic vacuum before use. All NMR solvents and BD3 3 SMe2 were purchased from Cambridge Isotope Laboratories Inc. and dried according to the procedures outlined above or used as received. 1,3,7,9-tetramethyl-2,8-diethyldipyrrin (LHH),32 [LHBH][B(C6F5)4],18 and d2-DMAP33 were prepared according to literature procedures. Synthesis of 1. LHH (540 mg, 2.1 mmol) was dissolved in toluene (15 mL), and BH3 3 SMe2 (2 M in toluene, 1.0 mL) was added via syringe at room temperature. The solution was first stirred 15 min at room temperature and 4 h at 60 °C. Volatiles were removed in vacuo, and the red, oily residue was dried under high vacuum at 40 °C and sublimed under dynamic vacuum at 70 °C. The pink solid was recrystallized from hexanes at -40 °C to afford colorless crystals (394 mg, 1.5 mmol) of 1. Yield: 70%. 1 H{11B} NMR (400 MHz, C6D12): 5.17 (br, 1H, BH), 3.76 (s, br, 2H, 5-H), 2.36 (q, 4H, 3JHH = 7.6 Hz, 2-CH2CH3), 2.22 (s, 6H, 1-CH3), 1.90 (t, 6H, 5JHH = 1.1 Hz, 3-CH3), 1.04 (t, 6H, 2-CH2CH3). 13C{1H} NMR (100 MHz, C6D12): 127.0 (2-C), 126.3 (1-C), 125.2 (4-C), 117.3 (3-C), 22.19 (5-C), 18.49 (2-CH2CH3), 15.69 (2-CH2CH3), 10.40 (1-CH3), 9.09 (3-CH3). 11 B NMR (128 MHz, C7D8, 333 K): 25.5 (d, 1JBH = 148 Hz). FTIR (KBr pellet) ν cm-1 (intensity): 2612 (m, ν (B-H)). UVvis (C6H12) λ nm (ε, M-1 cm-1): 273 (35733). CI-MS m/z (nature of peak, relative intensity): 269.0 ([M þ H]þ, 100). Anal. Calcd for C17H25N2B: C, 76.13; H, 9.39; N, 10.44. Found: C, 76.10; H, 9.48; N, 10.32. Synthesis of 1-d2. This was performed according to the procedure for synthesizing 1 by adding neat BD3 3 SMe2 dropwise, via syringe. 1H NMR (400 MHz, C6D12): 3.72 (s, 1H, 5-H), 2.36 (q, 4H, 3JHH = 7.6 Hz, 2-CH2CH3), 2.22 (s, 6H, 1-CH3), 1.90 (s, 6H, 3-CH3), 1.04 (t, 6H, 2-CH2CH3). 2H{1H} NMR (46 MHz, C6H12): 5.12 (br, 1D, BD), 3.68 (d, 1D, 2JHD = 1.7 Hz, 5-D). 13C{1H} NMR (100 MHz, C6D12): 127.0 (2-C), 126.2 (1C), 125.2 (4-C), 117.3 (3-C), 21.85 (t, 1JCD = 18.9 Hz, 5-C), 18.50 (2-CH2CH3), 15.70 (2-CH2CH3), 10.37 (1-CH3), 9.08 (3-CH3). 11B NMR (96 MHz, C6D12): 26.3 (br). FTIR (KBr pellet) ν cm-1 (intensity): 1943 (m, ν (B-D)). CI-MS m/z (nature of peak, relative intensity): 271.0 ([M þ H]þ, 100). Synthesis of 1-DMAP. 1 (366 mg, 1.4 mmol) and DMAP (167 mg, 1.4 mmol) were placed in a flask, and CH2Cl2 (20 mL) was added at room temperature. The solution was stirred 15 min, and volatiles were removed in vacuo. The pink solid was recrystallized from CH2Cl2/hexanes to afford white crystals (512 mg, 1.3 mmol). Yield: 96%. X-ray quality crystals were obtained from a CH2Cl2/C6H6 mixture. 1H NMR (400 MHz, TCE-d2): 7.71 (d, 2H, 3 JHH = 7.5 Hz, R-H), 6.51 (d, 2H, 3JHH = 7.4 Hz, β-H), 4.44 (br, 1H, BH), 3.70 (d, 1H, 2JHH = 17.9 Hz, 5-H), 3.46 (d, 1H, 5-H), 3.05 (s, 6H, N(CH3)2), 2.40 (q, 4H, 3JHH = 7.4 Hz, 2-CH2CH3), 2.20 (s, 6H, 1-CH3), 1.99 (s, 6H, 3-CH3), 1.08 (t, 6H, 2-CH2CH3). 13C{1H} (32) Cipot-Wechsler, J.; Ali, A. A.-S.; Chapman, E. E.; Cameron, T. S.; Thompson, A. Inorg. Chem. 2007, 46, 10947–10949. (33) Rubottom, G. M.; Evain, E. J. Tetrahedron 1990, 46, 5055–5064.
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NMR (100 MHz, TCE-d2): 155.4 (γ-C), 143.5 (R-C), 124.8 (1-C or 4-C), 124.6 (1-C or 4-C), 120.6 (2-C), 110.6 (3-C), 106.7 (β-C), 39.38 (N(CH3)2), 21.67 (5-C), 18.13 (2-CH2CH3), 16.55 (2-CH2CH3), 11.10 (1-C), 9.27 (3-C). 11B NMR (128 MHz, C7D8, 333 K): -1.3 (d, 1JBH = 144 Hz). FTIR (KBr pellet) ν cm-1 (intensity): 2432 (s, νs (B-H)), 2411 (s, νas (B-H)). UV-vis (C6H12) λ nm (ε, M-1 cm-1): 274 (37435). ESI-MS m/z (nature of peak, relative intensity): 389.2 ([M - H]þ,
