Convenient direct syntheses of novel fused-ring CB4N5 systems by nitrile hydroboration

Article abstract of DOI:10.1039/a705523d

Reactions between B₂H₆ or BH₃·thf (thf = tetrahydrofuran) and nitriles RC≡N (R = Me, Et, Bu^t or CH₂F), previously known to generate borazines (RCH₂NBH)₃ have been found to generate also 25-37% yields of novel carboraza bicyclic systems related to dihydronaphthalene, H₃B₄N₅(RCH₂)₄CHR, thus affording for the first time a direct route from commercially available acyclic reagents into mixed carbon-boron-nitrogen heterocyclic chemistry.

Full text of DOI:10.1039/a705523d

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Convenient direct syntheses of novel fused-ring CB4N5 systems by
nitrile hydroboration†
Robert Coult,a Mark A. Fox,a Brian Rand,b Kenneth Wade*,a and Aidan V. K. Westwoodb
a
Chemistry Department, Durham University Science Laboratories, South Road, Durham,
UK DH1 3LE
b School of Materials, University of Leeds, Leeds, UK LS2 9JT
proton to be significantly closer to one methylene proton (Ha)
Reactions between B2H6 or BH3ؒthf (thf = tetrahydrofuran) and
than the others (Hb, Hc and Hd). The different values of Ha and
Hb are due to the restriction in the rotation of the ethyl group at
N1 by the ethyl group at N8 with an hypothetical distance of 3.5
Å between the methyl carbon at N1 and the methylene carbon at
N8 as the minimum allowed.
t
᎐
nitriles RC᎐N (R = Me, Et, Bu or CH F ), previously known to
᎐
2
generate borazines (RCH2NBH)3 have been found to generate
also 25–37% yields of novel carboraza bicyclic systems related
to dihydronaphthalene, H3B4N5(RCH2)4CHR, thus affording
for the first time a direct route from commercially available
acyclic reagents into mixed carbon–boron–nitrogen
heterocyclic chemistry.
† Syntheses of 1–4 and 7–10. In a typical experiment, B2H6 (5.5 g, 0.25
mol), generated by dripping BF3ؒOEt2 (50 cm3, 0.40 mol) into a solution
of NaBH4 (12.5 g, 0.33 mol) in dry diglyme (2-methoxyethyl ether) (80
cm3) over a period of 2 h, was swept in a stream of nitrogen into the
3
᎐
nitrile RC᎐N (51.3 cm , 1.0 mol) at 60 ЊC. Distillation through a short
᎐
Bubbling diborane in nitrogen through refluxing acetonitrile
was shown in 19681 to give the borazine (EtNBH)3 1 in 35–40%
yield, together with what appeared (from mass spectroscopic
studies) to be a complex mixture of derivatives of higher
boron–nitrogen heterocyclic systems including B5N5 naphth-
alene analogue 5 and B6N6 biphenyl analogue 6.
Re-examination of such reactions, using a wider range of
nitriles and reaction conditions, has confirmed that borazines
(RCH2NBH)3 1–4† are indeed major volatile products, but has
also revealed that in all of the systems studied a second major
product could be separated by low-pressure distillation from
involatile residues. This second product, accounting for some
25–37% of the total mass of the products, was the unexpected
novel fused-ring CB4N5 heterocyclic system 7–10.
column allowed unchanged nitrile and the borazine H3B3N3R3 1–4
to be removed separately. Subsequent distillation at <0.1 mmHg
(1 mmHg = 133.322 Pa) afforded the carboraza heterocycle 7–10 as a
colourless liquid distillate leaving a waxy involatile residue.
When BH3ؒthf [as a molar solution in tetrahydrofuran (thf)] was
used as the source of borane, the appropriate volume to generate ultim-
ately a 1:2 molar ratio of BH3 to RCN was added dropwise to the hot
nitrile during 2 h. The volatility of the thf allowed it to be distilled, with
unchanged nitrile, from the reaction mixture before the boron-
containing products.
1,3,5-Triethylborazine 12 (9.55 g, 36% based on consumed B2H6), b.p.
154–160 ЊC. 1,3,6,8-Tetraethyl-2-methyl-1,3,6,8,10-pentaaza-4,5,7,9-
tetraboradihydronaphthalene 7 (11.88 g, 36%), b.p. 70–80 ЊC at 0.05
mmHg (Found: C, 44.7; H, 10.6; N, 26.6%; Mϩ 260. C10H27B4N5
requires C, 46.1; H, 10.4; N, 26.9%; M 260); δB(164 MHz; solvent
CDCl3; standard BF3ؒEt2O) 35.0 (1 B, d, BH), 33.8 (1 B, d, BH), 32.4 (1
B, d, BH), 25.8 (1 B, s, B9); δH (500 MHz; CDCl3; SiMe4) 4.43 (1 H, s,
BH), 4.42 [1 H, q, 3J(HH) 6, C2H], 4.38 (1 H, s, BH), 4.09 (1 H, s, BH),
In all of the cases studied (R = Me, Et, But or CH2F ) the
‘carboraza’ (carbon–boron–nitrogen) products 7–10 were
colourless liquids that decomposed slowly in moist air. They
were characterized by multinuclear NMR, mass and infrared
spectroscopy and elemental analyses. Their bicyclic structures
and the identities and sites of substituents were deduced from
their NMR spectra. Boron-11 NMR studies of 7 showed a
group of doublets at δ 35.0, 33.8 and 32.4 and a singlet at δ 25.8
(intensity ratio 3:1) that could be assigned to the three unique
BH groups and B9 (the boron atom common to both rings, with
no substituent hydrogen atom) respectively. The carbon-13 and
proton NMR spectra of 7 showed peaks for the CHMe and
non-equivalent CH2Me units as expected.
3
2
3
3.41 [2 H, q, J(HH) 7, CH2], 3.41 [1 H, dq, J(HH) 14, J(HH) 7, Hd/
Hc], 3.23 [2 H, q, J(HH) 7, CH2], 3.19 [1 H, dq, J(HH) 14, J(HH) 7,
Hc/Hd], 3.13 [1 H, dq, 2J(HH) 14, 3J(HH) 7, Hb], 2.95 [1 H, dq, 2J(HH)
14, 3J(HH) 7, Ha], 1.20 [3 H, d, 3J(HH) 6, C2CH3], 1.15 [3 H, t, 3J(HH)
7, CH3], 1.12 [9 H, t, 3J(HH) 7 Hz, 3 CH3]; δC(100 MHz; CDCl3; SiMe4)
69.1 (d, C2), 45.3 (t, CH2), 44.5 (t, CH2), 43.5 (t, CH2), 42.2 (t, CH2),
26.1 (q, C2CH3), 20.5 (q, CH3), 20.1 (q, 2 CH3), 16.6 (q, CH3); m/z
260 (Mϩ, 0.8%), 245 (M Ϫ CH3, 100). 1,3,5-Tripropylborazine 22 32%,
b.p. 40–45 ЊC at 0.05 mmHg. 2-Ethyl-1,3,6,8-tetrapropyl-1,3,6,8,10-
3
2
3
pentaaza-4,5,7,9-tetraboradihydronaphthalene
8
37%, b.p. 120–
125 ЊC at 0.1 mmHg; δB 33.8 (3 B, br, 3 BH), 26.1 (1 B, s, B9); m/z 330
(Mϩ, 1.4%), 302 (Mϩ Ϫ C2H4, 100). 1,3,5-Tri(tert-butylmethyl)borazine
3 63%, b.p. 95–105 ЊC at 0.08 mmHg; δB 34.9; m/z 276 (Mϩ Ϫ CH3, 8%),
234 (Mϩ Ϫ C4H9, 100). 2-tert-Butyl-1,3,6,8-tetra(tert-butylmethyl)-
1,3,6,8,10-pentaaza-4,5,7,9-tetraboradihydronaphthalene 9 26%, b.p.
145–155 ЊC at 0.02 mmHg; δB 34.3 (3 B, br, 3 BH), 27.8 (1 B, sh, B9); m/z
1
The H NMR spectra of 7 revealed two of the four CH2
groups to host diastereotopic protons. This clearly means that
the two CH2 groups at N1 and N3 are closer to the centre of
asymmetry, i.e. the ring carbon, than the other ones.3 This
enables us to distinguish 7 from the other possible isomer with
the ring carbon in the 4 position. The structures of 8–10 were
deduced similarly from their NMR spectra.
Further support for the structure of 7 which we have been
unable to obtain in suitable crystalline form for X-ray crystallo-
graphic characterization is shown by proton nuclear Over-
hauser effect (NOE) and boron IGLO/NMR 4 studies (Fig. 1).
The structure of 7 was optimized at the STO-3G level5 to give
the geometry shown in Fig. 2. Using the optimized geometry of
7 as the model, the minimum distances between the methine
proton and the methylene protons are 1.99 Å for Ha and 2.17 Å
for Hb, Hc and Hd in agreement with NOE showing the methine
471 (Mϩ, 0.2%), 456 (Mϩ Ϫ CH3, 2), 414 (Mϩ Ϫ C4H9, 100). 1,3,5-
1
–
Tri(2Ј-fluoroethyl)borazine 4 46%; ( scale) b.p. 55–65 ЊC at 0.04
4
mmHg; δB 34.3; m/z 218 (Mϩ, 3%), 185 (Mϩ Ϫ CH2F, 100). 2-
Fluoromethyl-1,3,6,8-tetra(2Ј-fluoroethyl)-1,3,6,8,10-pentaaza-4,5,7,9-
1
–
tetraboradihydronaphthalene 10 28%
(
scale; Caution: it can
decompose spontaneously to a non-volat4ile polymeric solid during
vacuum distillation), b.p. 120–130 ЊC at 0.04 mmHg; δB 34.6 (3 B, br,
3 BH), 25.6 (1 B, s, B9); m/z 350 (Mϩ, 0.8%), 317 (Mϩ Ϫ CH2F, 100).
Synthesis of 11. Compound 7 (2.36 g, 9 mmol) in Et2O (10 cm3)
was treated with MeLi (35 mmol in 30 cm3 hexane) and heated under
reflux for 70 h. Diethyl ether was removed. The product was filtered
and distilled to afford a fraction b.p. 140–160 ЊC at 0.05 mmHg
identified as 11. 2,4,5,7-Tetramethyl-1,3,6,8-tetraethyl-1,3,6,8,10-penta-
aza-4,5,7,9-tetraboradihydronaphthalene 11 (2.26 g, 7.5 mmol, 83%);
δB 37.8 (1 B, s, BMe), 35.9 (2 B, s, 2 BMe), 29.1 (1 B, s, B9); m/z 302 (Mϩ,
0.6%), 288 (Mϩ Ϫ CH2, 100).
J. Chem. Soc., Dalton Trans., 1997, Pages 3411–3413
3411

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H
B
H
B
H
B
RCH2N
HB
NCH2R
BH
EtN
HB
N
B
NEt
BH
N
RCH2
N
N
Et
Et
R
5
H
B
1
2
3
4
Me
Et
But
EtN
B
NEt
BH
H
B
CH2F
EtN
HB
N
N
Et
BH
N
Et
6
RCH2
N
CH2R
N
Et
N
Et
N
HB
B
N
CHR
MeB
EtN
B
N
CHMe
NEt
Fig. 1 Proton and NOE spectra of complex 7
RCH2N
NCH2R
B
B
B
B
H
H
Me
Me
11
R
7
8
9
Me
Et
H2
B
But
Et2N
H
B
10
CH2F
H2
12
Fig. 2 The STO-3G optimized geometry of the fused-ring CB4N5
system 7
Boron–nitrogen heterocycles incorporating some carbon
atoms are also accessible from reactions of organoboranes with
nitriles11 or hydrogen cyanide,12 though they require higher
temperatures or hazardous materials (HCN). Other mixed
carbon–boron–nitrogen ring systems are known, including
borazarenes containing even numbers of carbon atoms in their
rings, though these have normally required multi-step syn-
theses.13 The reactions we describe here are the first to provide
direct easy access to fused-ring ‘carboraza’ systems from
commercially available acyclic precursors.
Calculated boron NMR chemical shifts at the IGLO (or
GIAO) level from static ab initio or experimentally determined
geometries of boron compounds are known to fit well with the
observed shifts in solution.6,7 The IGLO(DZ)8 calculated shifts
are δ 32.0 (35.0) for B5, 31.8 (33.8) B7, 30.2 (32.4) B4 and 23.8
(25.8) B9 for the static STO-3G optimized geometry of 7. These
values are some 2–3 ppm upfield compared to the experimental
data shown in parentheses.‡
Compound 7 rapidly forms NHEt2, NH2Et and NH3 with
water at ambient temperature. With an excess of methyllithium
in diethyl ether, it gives the B-methylated derivative, Me3B4-
Acknowledgements
N5Et4(CHMe) 11.
It was already known that adducts RC᎐NؒBH can be iso-
We are grateful to Dr. A. M. Kenwright, I. H. McKeag
and J. M. Say for the high-field NMR spectra and EPSRC for
financial support. D. L. Ormsby and Dr. R. Greatrex (School
of Chemistry, Leeds) are thanked for the IGLO computations
and Dr. C. v. Wüllen (Bochum) for the provision of DIGLO
software.
᎐
᎐
3
lated by reactions between nitriles and diborane at low temper-
atures.9 Such adducts spontaneously rearrange at or above
ambient temperature to borazines (RCH2NBH)3 presumably
via aldiminoboranes (RCH᎐NBH ) which are known 10 as
᎐
2
n
dimers (RCH᎐NBRЈ ) when alkylated or arylated on boron.
᎐
2
2
Formation of the carboraza heterocycles 7–10 in these reac-
tions [and in similar reactions at 40–50 ЊC, or using BH3ؒthf as
the source of borane instead of B2H6] is intriguing. Nucleo-
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᎐
᎐
3
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3412
J. Chem. Soc., Dalton Trans., 1997, Pages 3411–3413

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Received 30th July 1997; Communication 7/05523D
J. Chem. Soc., Dalton Trans., 1997, Pages 3411–3413
3413