Rhodium-catalyzed formation of boron-nitrogen bonds: A mild route to cyclic aminoboranes and borazines

Article abstract of DOI:10.1039/b102361f

Secondary amine-borane adducts R₂NH·BH₃, which are stable to H₂ elimination below 100 °C, undergo efficient catalytic dehydrocoupling at 25-45 °C in the presence of Rh^I or Rh^III complexes to quantitatively form cyclic aminoboranes [NR₂-BH₂]₂ (1: R = Me or 2: cyclo-C₄H₈); under similarly mild conditions, the analogous adducts NH₃·BH₃ and MeNH₂·BH₃ yield borazines [RN-BH]₃ (3: R-H or 4: R = Me) in yields limited by intermolecular coupling reactions.

Full text of DOI:10.1039/b102361f

Rhodium-catalyzed formation of boron–nitrogen bonds:
a mild route to cyclic aminoboranes and borazines
Cory A. Jaska, Karen Temple, Alan J. Lough and Ian Manners*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada
M5S 3H6. E-mail: imanners@alchemy.chem.utoronto.ca
Received (in Cambridge, UK) 13th March 2001, Accepted 12th April 2001
First published as an Advance Article on the web 10th May 2001
1
Secondary amine–borane adducts R
2
NH·BH
3
, which are
centre. While H and 13C NMR spectra were also consistent
stable to H
2
elimination below 100 °C, undergo efficient
with the proposed product, single-crystal X-ray diffraction‡
provided definitive confirmation (Fig. 1). Notably, the B–N
bond lengths of 1 (1.595(4) Å) closely match those of the
2 2 3
analogous cyclic trimer [Me N–BH ] (1.61(4) Å) and
moreover, are in the range observed for amine–boranes (ca.
1.58 Å). The four-membered ring appears to be strained with
bond angles deviating significantly from the tetrahedral ideal
with values of 93.7(2) and 86.3(2)° for the N(1)–B(1)–N(1A)
and B(1)–N(1)–B(1A) angles, respectively.
I
17
catalytic dehydrocoupling at 25–45 °C in the presence of Rh
III
or Rh complexes to quantitatively form cyclic aminobor-
1
8
anes [NR
2
–BH
2
]
2
(1: R = Me or 2: cyclo-C
4
H
8
); under
·BH
3
(3: R = H or 4:
similarly mild conditions, the analogous adducts NH
and MeNH ·BH yield borazines [RN–BH]
3
3
2
3
R = Me) in yields limited by intermolecular coupling
reactions.
The application of transition metal catalysis to organic synthesis
is of enormous current importance. In contrast, the development
of analogous methods for the formation of homonuclear or
heteronuclear bonds between main group elements is relatively
unexplored. Nevertheless, the discovery of new synthetic
methods, which can complement the classical reactions used in
main group chemistry such as salt eliminations, is likely to be of
key future importance for the general development of molecular
and macromolecular p-block chemistry. Recent work has
focused on catalytic dehydrocoupling routes1 and the well-
established catalytic dehydropolymerization of silanes, ger-
manes and stannanes represents a key advance.2 More
recently, homodehydrocoupling chemistry has been extended to
–6
7
include P–P bond formation and catalytic heterodehydrocou-
pling reactions to form B–Si, Si–P,9 Si–N, and Si–O
8
10
bonds1
1,12
have also been reported. Recently, we reported the
first examples of the transition metal-catalyzed formation of P–
Fig. 1 Molecular structure of 1. Selected bond lengths (Å) and angles (°):
N(1)–B(1) 1.596(4), N(1A)–B(1) 1.595(4), N(1)–C(1) 1.478(4); N(1)–
B(1)–N(1A) 93.7(2), B(1)–N(1)–B(1A) 86.3(2).
B bonds.13 Thus, dehydrocoupling of phosphine–borane ad-
ducts at 60–130 °C in the presence of a range of precatalysts
I
(
e.g. Rh complexes) was found to provide a new route to
13,14
phosphinoborane rings, chains and macromolecules.
In this
The formation of the cyclic dimer 1 via metal-catalyzed
dehydrocoupling of a secondary amine–borane adduct repre-
sents a new, mild route to boron–nitrogen rings. We found that
the strategy can be extended to other adducts such as
pyrrolidine–borane, (CH ) NH·BH . Thus, the addition of
preliminary communication we report well-characterized exam-
ples of the use of catalytic dehydrocoupling to form new boron–
nitrogen bonds under mild conditions.15
Amine–borane adducts undergo thermally-induced dehy-
drocoupling at elevated temperatures to afford mixtures of small
rings such as cyclic aminoboranes and borazines. For example,
2
4
3
catalytic (ca. 0.5 mol%) amounts of [Rh(1,5-cod)(m-Cl)] or
2
RhCl ·3H O to a solution of the adduct (toluene, 25 °C, 24 h)
3
2
dimethylamine–borane, Me
2
NH·BH
3
, eliminates hydrogen at
N–BH
in toluene was
(1,5-cod =
O at 25 °C for 48–60 h, the
quantitative formation of the cyclic aminoborane 1 was detected
similarly led to the quantitative formation of the cyclic
aminoborane 2 (Scheme 1). Cyclic 2 was also characterized by
multinuclear NMR spectroscopy† and by single-crystal X-ray
diffraction.19
The catalytic dehydrocoupling methodology is not restricted
to secondary amine–borane adducts. Thus, in the presence of
1
1
30 °C to quantitatively yield the cyclic dimer [Me
2
2 2
]
16
.
2 3
However, when a solution of Me NH·BH
treated with ca. 0.5 mol% of [Rh(1,5-cod)(m-Cl)]
2
1,5-cyclooctadiene) or RhCl ·3H
3
2
11
by B NMR spectroscopy. The reaction time was reduced to
4 h when the temperature was raised to ca. 45 °C or the amount
of catalyst increased to 5 mol%. The iridium complex [Ir(1,5-
also catalyzed the dehydrocoupling reaction but
[Rh(1,5-cod)(m-Cl)] (1.5 mol%), the parent ammonia–borane
2
2
adduct NH ·BH was found to eliminate two equivalents of
3
3
hydrogen to form borazine 3 (diglyme or tetraglyme, 45 °C,
72 h) by 11B NMR (Scheme 2). However, isolation from the
reaction mixture by vacuum fractionation proved difficult; pure
20
cod)(m-Cl)]
2
this precatalyst (ca. 0.5 mol%) proved less active with the
reaction time for quantitative conversion to 1 increasing to 72 h
2 3
at 45 °C. Significantly, prolonged heating of neat Me NH·BH
in the absence of catalyst over a period of 7 days at 45 °C
resulted in the quantitative recovery of unreacted adduct, which
clearly demonstrated the catalytic effect of the Rh and Ir
complexes.
Cyclic aminoborane 1 was isolated as a colorless, extremely
1
11
volatile yet air-stable crystalline solid.† The H-coupled
B
1
NMR spectrum showed a triplet at d 4.75 ( JBH 110 Hz), which
was consistent with the coupling of two hydrogens to the boron
Scheme 1
962
Chem. Commun., 2001, 962–963
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102361f

3
8
m
1.533(10)°, g = 65.942(8)°, U = 196.80(5) Å , Z = 1, D
c
= 0.960 Mg
23
21
, m = 0.055 mm , F(000) 64, T = 103(1) K, l(Mo-Ka) = 0.71070
Å, crystal size 0.30 3 0.30 3 0.25 mm, 895 independent reflections, 3782
collected. Goodness-of-fit on F2 = 1.162, final R indices [I > 2s(I)], R
.0963, wR = 0.2866. Refinement was by full-matrix least squares on F
1
=
2
0
2
using all data. All hydrogens were included in calculated positions and
refined isotropically.
CCDC 160965. See http://www.rsc.org/suppdata/cc/b1/b102361f/ for
crystallographic data in .cif or other electronic format.
Scheme 2
1
2
For a recent review, see: F. Gauvin, J. F. Harrod and H.-G. Woo, Adv.
Organomet. Chem., 1998, 42, 363.
C. Aitken, J. F. Harrod and E. Samuel, J. Am. Chem. Soc., 1986, 108,
4059.
3
was isolated in only ca. 10 % yield† with the major products
21,22
being non-volatile, oligomeric species.
Current preparations of borazine (e.g. from NaBH
4
and
3 T. D. Tilley, Acc. Chem. Res., 1993, 26, 22.
4 J. A. Reichl and D. H. Berry, Adv. Organomet. Chem., 1998, 43, 197.
5 T. Imori, V. Lu, H. Cai and T. D. Tilley, J. Am. Chem. Soc., 1995, 117,
(
NH SO ) require rather forcing conditions (elevated tem-
4
)
2
4
23
peratures of 140–160 °C), and, in our hands, are aggravated by
similar difficulties in isolation. If optimized, the metal-
catalyzed route may be advantageous, which is potentially
significant as borazine and borazine oligomers have been shown
to be useful precursors to cyclolinear polymers, boron nitride
ceramics and nanotubes.2
9
931.
6
7
For a related example of demethanative coupling of germanes, see:
S. M. Katz, J. A. Reichl and D. H. Berry, J. Am. Chem. Soc., 1998, 120,
9
844.
N. Etkin, M. C. Fermin and D. W. Stephan, J. Am. Chem. Soc., 1997,
19, 2954.
4–27
1
In an attempt to minimize the intermolecular dehydrocou-
pling reactions, we also investigated the catalytic elimination of
8 Q. Jiang, P. J. Carroll and D. H. Berry, Organometallics, 1993, 12,
177.
9 R. Shu, L. Hao, J. F. Harrod, H. G. Woo and E. Samuel, J. Am. Chem.
Soc., 1998, 120, 12988.
H
2
from N-methylamine–borane adduct. Indeed, we found that
MeNH ·BH undergoes dehydrocoupling in the presence of
Rh(1,5-cod)(m-Cl)] (5 mol%), under similarly mild conditions
monoglyme or diglyme, 45 °C, 60 h) to afford N-trimethylbor-
2
3
1
0 J. He, H. Q. Lui, J. F. Harrod and R. Hynes, Organometallics, 1994, 13,
[
(
2
3
36.
1
1
1 Y. Li and Y. Kawakami, Macromolecules, 1999, 32, 6871.
2 R. Zhang, J. E. Mark and A. R. Pinhas, Macromolecules, 2000, 33,
11
azine 4 by B NMR. Pure 4 was isolated† in moderate yield
1,22
(
ca. 35–40%) by vacuum fractionation.2
oped routes to 4 typically involve high temperature dehydroge-
nation reactions such as the thermolysis of MeNH ·BH at
00 °C to give the cyclic trimer (MeNH–BH , followed by
further pyrolysis at 200 °C. Interestingly, through monitoring
Previously devel-
3
508.
1
3 H. Dorn, R. A. Singh, J. A. Massey, A. J. Lough and I. Manners, Angew.
Chem., Int. Ed. Engl., 1999, 38, 3321.
2
3
1
2
)
3
14 H. Dorn, R. A. Singh, J. A. Massey, J. M. Nelson, C. A. Jaska, A. J.
Lough and I. Manners, J. Am. Chem. Soc., 2000, 122, 6669.
28
of the Rh catalyzed reaction by 1 B NMR, dehydrocoupling to
1
15 To our knowledge, no examples of transition metal catalyzed dehy-
drocoupling routes to B–N bonds have appeared in the open literature.
A patent (see Y. Blum and R. M. Laine, US Pat., 4,801,439 1989) claims
the catalytic formation of partially characterized mixtures of B–N
compounds (including borazines) and insoluble polymeric products by
form the cyclic aminoborane (MeNH–BH was found to occur
2 3
)
1
1
29
first (d 25.1, JBH 108 Hz; lit. d 25.4, JBH 107 Hz), followed
by further loss of hydrogen to yield 4. The isolation of a ca. 50%
yield of non-volatile residue, indicates that intermolecular
coupling also occurs in this case.22 This process may involve
3 3
the dehydrocoupling of mixtures of primary amines and Me N–BH
over extended periods of time at elevated temperatures. The conditions
for the examples cited, typically 30–80 h, ca. 60 °C, in the presence of
catalytic dehydrocoupling of the intermediate (MeNH–BH
2
)
3
.
In summary, amine–borane adducts undergo B–N bond
formation reactions under mild conditions in the presence of
rhodium dehydrocoupling precatalysts (Schemes 1 and 2).
Future work will involve an expansion of the scope of this new
chemistry, which offers the prospect of improved routes to B–N
compounds, and mechanistic investigations.
This research was supported by the Natural Science and
Engineering Research Council of Canada (NSERC) and the
Petroleum Research Fund (PRF) administered by the ACS.
C. A. J. would like to thank the University of Toronto for a U of
T Open Fellowship and K. T. thanks NSERC for a Postgraduate
Scholarship (1997–2001). We would also like to acknowledge
Professor William E. Buhro and Carolyn R. Jones from the
Department of Chemistry, Washington University for many
helpful discussions.
Ru (CO)12, are also more forcing than with the Rh catalysts used in this
communication.
16 A. B. Burg and C. L. Randolph Jr., J. Am. Chem. Soc., 1949, 71,
3
3
451.
1
7 The crystal structure of 1 has been performed previously, but was only
published as part of a dissertation by P. J. Schapiro from Cornell
University (Dissertation Abstr., 1962, 22, 2607). This structure was
found to have a different space group (C2/m) and unit cell dimensions
than the one presented here.
18 L. M. Trefonas and W. N. Lipscomb, J. Am. Chem. Soc., 1959, 79,
4435.
19 C. A. Jaska, K. Temple, A. J. Lough and I. Manners, unpublished
results.
2
0 Blank reactions performed in the absence of catalyst showed no
dehydrocoupling products under the same conditions. Pure 3 and 4 were
obtained through vacuum fractionation using traps at 245, 278 and
2196 °C.
2
1 For an example of the trap-to-trap vacuum fractionation set-up for
product purification, see: T. Wideman, P. J. Fazen, A. T. Lynch, K. Su,
Notes and references
E. E. Remsen and L. G. Sneddon, Inorg. Synth., 1998, 32, 232.
Selected spectroscopic data: for 1: yield (isolated) (0.18 g, 62%). 1H
22 The non-volatile residue was analyzed by B NMR spectroscopy.
Several distinct resonances were observed in the region d = 26–36
suggesting further coupling. See, for comparison: P. J. Fazen, E. E.
Remsen, P. J. Carroll, J. S. Beck, A. R. McGhie and L. G. Sneddon,
Chem. Mater., 1995, 7, 1942.
11
†
13
1
NMR (300 MHz, CDCl
NMR (75 MHz, CDCl
3
): d 3.2–2.0 (q, br, BH
2
), 2.42 (s, CH
3
). C{ H}
): d
11
3
3
): d 52.0 (s, CH ). B NMR (160 MHz, CDCl
3
1
1
4
.75 (t, JBH 110 Hz, BH
2
). For 2: yield (isolated) (1.22 g, 73%). H NMR
), 2.84 (t, br, NCH CH ), 1.70 (m,
). C{ H} NMR (75 MHz, CDCl ): d 60.1 (s, NCH CH ), 23.7
): d 2.56 (t, JBH 110 Hz, BH ).
For 3: yield (isolated) (0.57 g, ca. 10%). H NMR (300 MHz, C ): d 5.54
(300 MHz, CDCl ): d 3.2–2.0 (q, br, BH
3
2
2
2
1
3
1
NCH
2
CH
CH
2
3
2
2
23 T. Wideman and L. G. Sneddon, Inorg. Chem., 1995, 34, 1002.
24 R. T. Paine and L. G. Sneddon, Chemtech, 1994, 24, 29.
25 P. J. Fazen, J. S. Beck, A. T. Lynch, E. E. Remsen and L. G. Sneddon,
Chem. Mater., 1990, 2, 96.
26 T. Wideman and L. G. Sneddon, Chem. Mater., 1996, 8, 3.
27 O. R. Lourie, C. R. Jones, B. M. Bartlett, P. C. Gibbons, R. S. Ruoff and
W. E. Buhro, Chem. Mater., 2000, 12, 1808.
28 T. C. Bissot and R. W. Parry, J. Am. Chem. Soc., 1955, 77, 3481.
29 C. K. Narula, J. F. Janik, E. N. Duesler, R. T. Paine and R. Schaeffer,
Inorg. Chem., 1986, 25, 3346.
11
1
(s, NCH
2
2
). B NMR (160 MHz, CDCl
3
2
1
6 6
D
11
1
6 6
(t, br, NH), 4.45 (q, br, BH). B NMR (160 MHz, C D ): d 30.2 (d, JBH
1
141 Hz, BH). For 4: yield (isolated) (0.39 g, 40%). H NMR (300 MHz,
1
3
1
C
6 6
D
): d 4.65 (q, br, BH), 2.96 (s, CH
3
). C{ H} NMR (75 MHz, C
6 6
D ):
11
1
d 38.5 (s, CH
EI-MS (70 eV): 122 (M 2 H, 100%).
Crystal data for 1: C , M = 113.81, triclinic, space group P1, a
5.8330(7), b = 6.0290(10), c = 6.2400(10) Å, a = 80.372(8)°, b =
). B NMR (160 MHz, C D ): d 33.2 (d, JBH 132 Hz, BH).
3 6 6
+
¯
‡
=
4 16 2 2
H B N
Chem. Commun., 2001, 962–963
963