Syntheses of [6,6]-fused-ring 1,2-azaborines

Article abstract of DOI:10.1021/om401077k

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.

Full text of DOI:10.1021/om401077k

Note
pubs.acs.org/Organometallics
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.
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.
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 BN-
naphthalene (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).
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
1
the H NMR spectrum shows a four-proton pattern, which is
highly characteristic of a C-unsubstituted 1,2-dihydro-1,2-
azaborine 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
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).
Received: November 7, 2013
Published: February 27, 2014
© 2014 American Chemical Society
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Note
Scheme 1
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
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 BN-
naphthalene more available for experimental study.
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 cyclohexene-
like 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
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Note
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.
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
1
700 MHz spectrometers. The H 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 allyltributylstan-
nane (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
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
1
distillation, bp = 35−39 °C at 0.025 Torr. H 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
1
on 7). H 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 micro-
focus 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
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Organometallics
Note
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.
(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.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving X-ray characterization of 8 and figures giving
1
the H, 11B, and 13C NMR spectra of new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for A. J. Ashe: ajashe@umich.edu.
Notes
The authors declare no competing financial interest.
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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