Syntheses of [6, 6]-Fused-Ring 1, 2-Azaborines

Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109-1055, .... Aaron M. Pejlovas , Adam M. Daly , Arthur J. Ashe , Stephen G. ...
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Syntheses of [6,6]-Fused-Ring 1,2-Azaborines Ahleah D. Rohr, Jeff W. Kampf, and Arthur J. Ashe, III* Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109-1055, United States S Supporting Information *

ABSTRACT: BN-naphthalene (2) and BN-tetralin (7) have been prepared by a short synthesis from allyltributylstannane (3). The reaction of 3 with BCl3 followed by diallylamine and Et3N afforded (diallylamino)diallylborane (5), which gave 1,4,5,8-tetrahydro[1,2]azaborino[1,2-a][1,2]azaborine (6), on treatment with Grubbs (I) catalyst. The reaction of 6 with Pd gave 2 and 7. The reaction of 7 with Cr(CO)6 gave the corresponding Cr(CO)3 adduct 8, for which a crystal structure has been obtained. The isomerization of 6 to 7 has been examined using DFT calculations.

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INTRODUCTION 1,2-Dihydro-1,2-azaborine 1 is an aromatic heterocycle that is isoelectronic with benzene. Syntheses of monocyclic derivatives of 1 were reported in the 1960s by Dewar1 and White.2 Dewar and co-workers subsequently reported on the preparation of several fused-ring azaborines3 including the parent BNnaphthalene (2).4,5 The next three decades saw little sustained activity in this research area. In 2000−2001 we reported two general syntheses of 1,2-dihydro-1,2-azaborines that have made the ring system more readily available.6,7 We subsequently developed an improved synthesis of 2.8 Recent work on 1,2-azaborines includes the synthesis of the parent 1a by the Liu group9 and the elegant demonstration of its aromaticity.10,11 The Liu group has also explored the various BN-isosters of biologically active aromatics12 and the use of similar BN compounds for hydrogen storage.13 The Piers group14 and others3b,c have further developed novel syntheses of BN-heterocycles that may have uses in organic electronics. However, no further experimental work on BN-naphthalene has appeared. We report here on a more efficient synthesis of 2 and on the synthesis of the novel BN-tetralin (7).

The reaction of allyltributylstannane (3) with BCl3 in pentane at −78 °C affords diallylboron chloride (4). It is convenient not to attempt to isolate the labile 415 but to treat it directly with diallylamine followed by triethylamine. In this manner 5 may be isolated by distillation as a mildly air-sensitive liquid in 66% yield. Upon treatment of 5 with 3 mol % of Grubbs catalyst at 25 °C in methylene chloride followed by reflux, the doubly cyclized product 6 was obtained in 70% yield. No other products were noted. In an effort to dehydrogenate 6, it was heated to 110 °C with 1.6 mol % of 5% Pd on charcoal in excess cyclohexene for 18 h. On distillation, a 1:2 mixture of BN-naphthalene (2) and the novel BN-analogue of tetralin (7) was obtained in 60% yield. Separation of 2 and 7 can readily be achieved by gas−liquid partition chromatography (GLPC). The BN-naphthalene was identical in all respects to a sample of previously obtained material.8 The spectra of the major product are consistent with the assigned structure 7. In particular the low-field portion of the 1H NMR spectrum shows a four-proton pattern, which is highly characteristic of a C-unsubstituted 1,2-dihydro-1,2azaborine ring,7 while the four higher field signals are those expected for a (CH2)4-bridging group. In order to better establish the structure of 7, it was of interest to explore its coordination chemistry. A mixture of 2 and 7 was heated to 140 °C in THF with Cr(CO)6, affording the Cr(CO)3 complex 8 in 38% yield as the sole product. No Cr(CO)3 complex of BN-naphthalene could be detected. Moreover the 11B NMR spectrum of the reaction mixture indicated that the relative concentration of 2 had increased. Therefore, it seems likely that 7 is a better ligand. Reduction of the π-ligand strength upon annulation of aromatic rings has

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RESULTS AND DISCUSSION The syntheses of derivatives of 1,2-azaborines using the Grubbs ring-closing metathesis (RCM) are particularly efficient.6,8 For this reason we have chosen to further explore the syntheses of [6,6]-fused-ring 1,2-azaborines using RCM on appropriate allylboranes (see Scheme 1). © 2014 American Chemical Society

Received: November 7, 2013 Published: February 27, 2014 1318

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Scheme 1

be isolated in 71% by distillation. In a similar manner heating a mixture of 2 and 7 with palladium on carbon affords only 7. These data imply that 7 is not an intermediate on the path to 2. Palladium black is a better catalyst than palladium on carbon for the conversion of 6 to 2. Thus heating 6 in cyclohexene with palladium black to 87 °C for 2 days affords a mixture of 2 and 7 in a ratio of 65:35. The yield of 2 is 30%. Unfortunately lowering the temperature further makes the conversion impractically slow. Obviously a major product of treatment of 6 by Pd was its isomerization to 7. The isomerization may involve reversible hydrogen transfers from 6 to Pd, which ultimately give the thermodynamically favored 7. Dixon, Liu, and co-workers have previously calculated that 1a (R = R′ = H) has a resonance stabilization energy (RSE) of 21 kcal/mol.9 Except for the (presumably small) substituent effects 7 might have a similar RSE. However, one expects the nonconjugated 6 to have little RSE. Thus, the observed conversion of 6 to 7 should be highly exothermic. In order to verify these qualitative arguments, we have performed density function calculations on 6 and 7. The ground electronic state energies of 6 and 7 at the G2(MP2) level were used to calculate the heats of formation and isomerization. The experimental enthalpies were calculated from the JANAF complilations18 at the G2(MP2) level of optimization. The calculated ΔH isomerization (298 K) from the ground electronic energies at the G2(MP2) level of 6 to 7 is −17.4 kcal/mol, which is in reasonable agreement with the qualitative arguments given above. The nucleus-independent chemical shift [NICS(1)] values have become important magnetic criteria of aromaticity.19 For comparison we find the NICS(1) value of 6 calculated at the B3LYP/TZVP level to be 0.2, which is well outside the aromatic region. The NICS(1) value of 7 is −5.2, a value similar to the −7.3 found by Liu and Dixon for 1a.9 In summary we developed a short and efficient synthesis of the [6,6]-fused-ring 1,2-azaborines 2 and 7. BN-naphthalene (2) has been prepared in three steps and an overall yield of 14% from commercially available 3. The best prior synthesis of 2 uses six steps from 3 with an overall yield of 6%.8 The new synthesis should make the theoretically interesting BNnaphthalene more available for experimental study.

precedent in the observation that naphthalene is a poorer ligand than benzene toward Cr(CO)3.16 Recrystallization of 8 from hexane gave red-orange crystals that were suitable for an X-ray study. The molecular structure of 8 is illustrated in Figure 1, and selected bond distances are

Figure 1. Molecular structure of 8 (ORTEP). Thermal ellipsoids are set at the 50% level. Selected distances (Å): B−N, 1.458(3); B−C(7), 1.520(4); C(6)−C(7), 1.395(4); C(5)−C(6), 1.421(4); C(4)−C(5), 1.381(4); C(4)−N, 1.398(3).

listed in the figure caption. The structure shows the molecule has a near-planar C4H4BN ring that is η6-bound to the Cr(CO)3 group in a typical piano stool fashion. The noncoordinated C4H8BN ring has a half-chair cyclohexenelike conformation.17 It seems useful to compare the bond distances of 8 with those of related 1,2-dihydro-1,2-azaborine-Cr(CO)3 complexes. For example the corresponding intraring C4H4BN bond distances of 8 and 97 differ by an average of only ±0.006 Å. The corresponding C4H4BN−Cr distances differ by an average of only ±0.016 Å. By these criteria the bonding is likely to be analogous. The ratio of the products 2 and 7 can be considerably altered by adjusting the reaction conditions. Heating 6 to 173 °C for 3 h with palladium black converted it exclusively to 7, which can 1319

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Hz) 1H, 6.18 (t, J = 6.7 Hz) 1H, 3.79 (t, J = 5.9 Hz) 2H, 1.86 (m) 2H, 1.71 (m) 2H, 1.35 (t, J = 7.0 Hz) 2H. 13C NMR (CDCl3, 125.7 MHz): δ 141.7, 139.1, 130 (br), 110.0, 53.1, 27.7, 21.65, 14 (br). 11B NMR (CDCl3, 128.2 MHz): δ 36.86. HRMS (EI, m/z): calcd for C8H1211BN (M+), 133.1063; found, 133.106. (b) In a similar manner a sample of 6 (0.42 g, 3.2 mmol), palladium-black (0.114 g, 1.1 mmol), and 4.0 mL of cyclohexene were heated to 87 °C for 42 h. Pot-to-pot distillation at 0.1 Torr, bp ∼45 °C, gave oily crystals. Analysis by 11B NMR spectroscopy showed it to be 3% 6, 34% 7, and 63% 2. The yield of 2 was 30%. 5,6,7,8-Tetrahydro[1,2]azaborino[1,2a][1,2]azaborine (7). (a) A sample of 6 (0.380 g, 2.86 mmol), palladium black (0.101 g, 0.95 mmol), and 2.0 mL of cyclohexene were degassed, flushed with nitrogen gas, and sealed in a thick-walled tube with a magnetic spin bar. The mixture was heated with stirring to 173 °C for 3 h. After filtration and removal of the solvent the residue was pot-to-pot distilled at room temperature at 0.5 Torr to afford 0.270 g (71% yield) of pure 7 as a colorless, air-sensitive liquid. No other products were noted. (b) In a similar manner a sample of 6 (0.256 g, 1.92 mmol), 10% palladium on carbon (0.106 g, 0.10 mmol), and 2.0 mL of cyclohexene were heated to 88 °C for 19 h. 11B NMR of an aliquot indicated that 2 and 7 were present in the ratio of 35:65. The bulk of the mixture was heated to 172 °C for 3 h. On pot-to-pot distillation, pure 7 (0.136 g, 53% yield) was isolated. Tricarbonyl[5,6,7,8-tetrahydro[1,2]azaborino[1,2-a][1,2]azaborine]chromium (8). A 60 mg sample of 7 containing 35% 2 (0.29 mmol of 7) was taken up in 2.5 mL of THF and was added to Cr(CO)6 (64 mg, 0.29 mmol). The mixture was degassed and heated in a thick-walled tube to 140 °C for 19 h, after which the color had turned to dark red. The 11B NMR spectrum now showed the ratio of 2/7 to be 50:50. The solvent was removed under reduced pressure, which left a red tar. Hot hexane (5 mL) was added, and the resultant solution was decanted from the green residue. On cooling to −20 °C red-orange crystals formed, which were collected, washed with cold pentane, and dried under vacuum. The yield was 30 mg (38%, based on 7). 1H NMR (CD2Cl2, 500 MHz): δ 5.85 (d, J = 5 Hz) 1H, 5.77 (dd, J = 9.5, 5 Hz) 1H, 5.27 (t, J ≈ 5 Hz) 1H, 4.52 (d, J = 9.6 Hz) 1H, 3.52 (m) 1H, 2.95 (m) 1H, 1.78 (m) 2H, 1.56 (m) 2H, 1.42 (m) 1H, 1.30 (m) 1H. 13C NMR (CD2Cl2, 175.9 MHz): δ 230.7, 107.3, 104.6, 84.8 (br), 81.9, 55.5, 26.6, 20.0, 10.3 (br). 11B NMR (CD2Cl2, 128.2 MHz): δ 23.9. HRMS (EI, m/z): calcd for C11H1211BCrNO3 (M+), 269.0315; found, 269.0315. IR (hexane): 1976, 1908, 1895 cm−1. Single-Crystal X-ray Crystallography. Orange plates of 8 were grown from a hexane solution of the compound at −20 °C. A crystal of dimensions 0.12 × 0.12 × 0.06 mm was mounted on a Rigaku AFC10K Saturn 944+ CCD-based X-ray diffractometer equipped with a low-temperature device and a Micromax-007HF Cu-target microfocus rotating anode (λ = 1.54187 Å) operated at 1.2 kW power (40 kV, 30 mA). The X-ray intensities were measured at 85(1) K with the detector placed at a distance 42.00 mm from the crystal. A total of 3868 images were collected with an oscillation width of 1.0° in ω. The exposure time was 5 s for the low-angle images and 15 s for high angle. The integration of the data yielded a total of 29 547 reflections to a maximum 2θ value of 136.30°, of which 2097 were independent and 2024 were greater than 2σ(I). The final cell constants were based on the xyz centroids 17 086 reflections above 10σ(I). Analysis of the data showed negligible decay during data collection; the data were processed with CrystalClear 2.021 and corrected for absorption. The structure was solved and refined with the Bruker SHELXTL (version 2008/4)20 software package, using the space group P2(1)/n with Z = 4 for the formula C11H12BNO3Cr. The unit cell dimensions were a = 8.006(2) Å, b = 11.063(3) Å, β = 103.983(7)°, c = 13.311(4) Å. Full matrix least-squares refinement based on F2 converged at R1 = 0.0380 and wR2 = 0.1018 [based on I > 2σ(I)], R1 = 0.0397 and wR2 = 0.1042 for all data. Additional details are given as Supporting Information in a CIF file. DFT Calculations. All structures were optimized using the B3LYP functional and Ahrichs-TZVP basis set. Geometry optimizations were performed with the program package Gaussian 09. The NMR chemical shift calculations were obtained at the DFT B3LYP level with the

EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under an atmosphere of nitrogen using standard Schlenk techniques. Samples of palladium-black were purchased from Strem, Aldrich, and Alfa Aesar. It was observed that catalytic activity varied from lot to lot. All other chemicals were purchased form Adlrich and used as received. Solvents were dried using standard procedures. High-resolution mass spectra were recorded on a VG-2505 spectrometer with electron impact at 70 eV. The NMR spectra were obtained with Varian Inova 400, 500, or 700 MHz spectrometers. The 1H NMR and 13C NMR spectra were calibrated from signals from solvents referenced to Me4Si. The 11B NMR spectra were referenced to external BF3-OEt2. (Diallylamino)diallylborane (5). A solution of allyltributylstannane (66.2 g, 0.20 mol) in 120 mL of hexane was added dropwise with stirring to a solution of boron trichloride (12.0 g, 0.10 mol) in 120 mL of hexane at −78 °C. The mixture was stirred at −78 °C for 1 h and then allowed to warm to 25 °C for 2.5 h with stirring. After recooling to −78 °C diallylamine (9.7 g, 0.10 mol) in 10 mL of hexane was added dropwise over 10 min followed by triethylamine (10.1 g, 0.10 mol) in 10 mL of hexane. A large precipitate formed. The mixture was allowed to warm to 25 °C and stirred for 14 h. The mixture was filtered, and the salts were washed with 2 × 50 mL of hexane. The solvent was removed from the combined organic residue using rotary evaporation. The product (12.5 g, 66%) was obtained by vacuum distillation, bp = 35−39 °C at 0.025 Torr. 1H NMR (CDCl3, 400 MHz): δ 1.82 (d, J = 7 Hz, 4H), 3.64 (d, J = 6 Hz, 4H), 4.85 (dm, J = 9.6 Hz, 2H), 4.91 (dm, J = 16.6 Hz, 2H), 5.06 m, 4H, 5.71 m, 2H, 5.86 m, 2H. 13C NMR (CDCl3, 125.7 MHz): δ 26.4 (br), 51.0, 113.8, 115.5, 136.3, 136.9. 11B NMR (CDCl3, 128.2 MHz): δ 43.9. HRMS (EI, m/z): calcd for C12H1911BN (M+ −1), 188.16106; found, 188.16108. 1,4,5,8-Tetrahydro[1,2]azaborino[1,2-a][1,2]azaborine (6). A solution of 5 (11.63 g, 62 mmol) in 10 mL of methylene chloride was added dropwise to a solution of bis(tricyclohexylphosphine)benzylideneruthenium(IV) dichloride (1.5 g, 1.8 mmol) in 50 mL of methylene chloride at 25 °C. Bubbles formed intensely, and the color changed to dark brown. The reaction mixture was then heated to reflux for 20 h. Volatile components were removed under reduced pressure at 0 °C, and the residue was subject to pot-to-pot distillation at 0.1 Torr, pot heated to 55 °C. The product was obtained as a pale yellow liquid (5.74 g, 70%), which solidified on cooling to 0 °C (mp ∼14 °C). 1H NMR (CDCl3, 500 MHz): δ 5.81 (dm, J = 10.7 Hz) 2H, 5.61 (dm, J = 10.7 Hz) 2H, 3.52 (d, J = 1.5 Hz) 4H, 1.39 (d, J = 1.7 Hz) 4H. 13C NMR (CDCl3, 125.7 MHz): δ 16.6 (br), 50.4, 124.5, 126.3. 11B NMR (CDCl3, 128.2 MHz): δ 41.2. HRMS (EI, m/z): calcd for C8H1211BN (M+), 133.1063; found, 133.1065. 5,6,7,8-Tetrahydro[1,2]azaborino[1,2-a][1,2]azaborine (7) and [1,2]Azaborino[1,2-a][1,2]azaborine (2). (a) A sample of 6 (2.0 g, 15 mmol) and 500 mg (0.24 mmol) of 5% Pd on charcoal in 10 mL of freshly distilled cyclohexene were degassed, flushed with nitrogen, and sealed in a thick-walled tube. The mixture was heated to 110 °C with stirring for 18.5 h. The mixture was allowed to cool to 25 °C and then filtered to remove the solid, which was washed with 5 mL of cyclohexene. The combined organic fractions were distilled at atmospheric pressure to remove the solvent, and the residue was subjected to pot-to-pot distillation at 0.05 Torr. Most of the material (1.18 g, 59%) boiled at 50−60 °C. The colorless distillate solidified to ice-like crystals when cooled to −78 °C. The product could be separated into two components by preparative GLPC on a Varian Aerograph 920 instrument equipped with a thermal conductivity detector. A 10′ × 1/4″ column containing 10% Carbowax 20 M on Chromosorb W operated under isothermal conditions at 150 °C with 10 lbs of pressure of He elution was used. Compound 2 (retention time 9.0 min, 35%) was isolated as a crystalline solid, identical in all respects to an authentic sample of [1,2]azaborino[1,2-a][1,2]azaborine. The yield of 2 was 20%. Compound 7 (retention time = 5.0 min, 65%) was isolated as a colorless mobile liquid that rapidly turned brown on exposure to air. 1H NMR (CDCl3, 500 MHz): δ 7.45 (dd, J = 11.0, 6.5 Hz) 1H, 7.07 (d, J = 6.7 Hz) 1H, 6.61 (d, J = 11.0 1320

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(17) Anet, F. A. L.; Freedberg, D. I.; Storer, J. W.; Houk, K. N. J. Am. Chem. Soc. 1992, 114, 10969. (18) Enthalpy values were taken from JANAF Thermochemical Tables: Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; MacDonald, R. A.; Syrerud, A. N. J. Phys. Ref. Data 1985, 14, Suppl. No 1. (19) Chen, Z.; Wannere, C. S.; Corminboef, C.; Puchta, R.; Schleyer, P. v. R. Chem. Rev. 2005, 105, 3842. (20) Sheldrick, G. M. SHELXTL, v.2008/4; Bruker Analytical X-ray: Madison, WI, 2008. (21) Rigaku Americas and Rigaku Corp. CystalClear Expert 2.0 r12; Rigaku Americas, 2011.

Ahrichs-TZVP basis set using the GIAO formalism to treat the gauge invariance problem. The nucleus-independent chemical shifts were calculated at 1 Å above the ring on the axis perpendicular to the ring and passing through the approximate center of the ring. The heats of formation and isomerization were calculated at the G2MP2 level.

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ASSOCIATED CONTENT

* Supporting Information S

CIF files giving X-ray characterization of 8 and figures giving the 1H, 11B, and 13C NMR spectra of new compounds. 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 for A. J. Ashe: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Mr. Michael D. Carr for performing some preliminary experiments. Acknowledgement is made for funding from NSF grant CHE-0840456 for the X-ray instrumentation.

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REFERENCES

(1) Dewar, M. J. S.; Marr, P. A. J. Am. Chem. Soc. 1962, 84, 3782. (2) White, D. G. J. Am. Chem. Soc. 1963, 85, 3634. (3) For reviews, see: (a) Fritsch, A. J. Chem. Heterocycl. Compd. 1977, 30, 381. (b) Bosdet, M. J. D.; Piers, W. E. Can. J. Chem. 2009, 87, 8. (c) Campbell, P. G.; Marwitz, A. J. V.; Liu, S.-Y. Angew. Chem., Int. Ed. 2012, 51, 6074. (4) Dewar, M. J. S.; Jones, R. J. Am. Chem. Soc. 1968, 90, 2137. (5) Chemical Abstracts name for 2: [1,2]azaborino[1,2-a][1,2] azaborine. (6) Ashe, A. J., III; Fang, X. D. Org. Lett. 2000, 2, 2089. (7) Ashe, A. J., III; Fang, X. D.; Fang, X. G.; Kampf, J. W. Organometallics 2001, 20, 5413. (8) Fang, X. D.; Yang, H.; Kampf, J. W.; Banaszak Holl, M. M.; Ashe, A. J., III. Organometallics 2006, 25, 513. (9) Marwitz, A. J. V.; Matus, M. H.; Zakharov, L. N.; Dixon, D. A.; Liu, S.-Y. Angew. Chem., Int. Ed. 2009, 48, 973. (10) Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2008, 130, 7250. (11) Campbell, P. G.; Abbey, E. R.; Neiner, D.; Grant, D. J.; Dixon, D. A.; Liu, S.-Y. J. Am. Chem. Soc. 2010, 132, 18048. (12) (a) Marwitz, A. J. V.; Abbey, E. R.; Jenkins, J. T.; Zakharov, L. V.; Liu, S.-Y. Org. Lett. 2007, 9, 4905. (b) Liu, L.; Marwitz, A. J. V.; Matthews, B. W.; Liu, S.-Y. Angew. Chem., Int. Ed. 2009, 48, 6817. (c) Knack, D. H.; Marshall, J. L.; Harlow, G. P.; Dudzik, A.; Szaleniec, M.; Liu, S.-Y.; Heider, J. Angew. Chem., Int. Ed. 2013, 52, 2599. (13) Campbell, P. G.; Zakharov, L. N.; Grant, D. J.; Dixon, D. A.; Liu, S.-Y. J. Am. Chem. Soc. 2010, 132, 3289. (14) (a) Bosdet, M. J. D.; Jaska, C. A.; Piers, W. E.; Sorensen, T. S.; Parvez, M. Org. Lett. 2007, 9, 1395. (b) Bosdet, M. J. D.; Piers, W. E.; Sorensen, T. S.; Parvez, M. Angew. Chem., Int. Ed. 2007, 46, 4940. (15) Bubnov, Y. N.; Kuznetsov, N. Y.; Pastukhov, F. V.; Kublitsky. Eur. J. Org. Chem. 2005, 4633. (16) (a) Mukerjee, S. L.; Lang, R. F.; Ju, T.; Kiss, G.; Hoff, C. D. Inorg. Chem. 1992, 31, 4885. (b) Zhang, S.; Shen, J. K.; Basolo, F.; Ju, T. D.; Lang, R. F.; Kiss, G.; Hoff, C. D. Organometallics 1994, 13, 3692. (c) An analogous effect is observed in the Cr(CO)3coordination chemistry of 1,2-dihydro-2-phenyl-1,2-benzazaborine; see: Pan, J.; Kampf, J. W.; Ashe, A. J., III. Organometallics 2009, 28, 506. 1321

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