Reaction of 9-borabicyclo[3.3.1]nonane with N-trimethylsilylamines - Cleavage of the N-Si or N-H bond

Article abstract of DOI:10.1515/znb-1998-0321

The reaction of dimeric 9-borabicyclo[3.3.1]nonane 1 with N-trimethylsilyl-aniline 2 affords 9-anilino-9-borabicyclo[3.3.1]nonane 5 by elimination of trimethylsilane. In contrast, 1 reacts with the N-trimethylsilyl-2-aminopyridines 3 and 4 selectively by

Full text of DOI:10.1515/znb-1998-0321

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The reaction of 1 with 2 was carried out as d e-
scribed for other silylam ines such as (M e3Si)2N H
or sB uN H SiM e3, and it was found to proceed in
the sam e way [2]: Form ation of trim ethylsilane
was observed, and the am inoborane 5 was ob-
tained in > 90% yield (eq. (1)).
Reaction of 9-Borabicyclo[3.3.1]nonane
with N-Trimethylsilylamines - Cleavage
of the N-Si or N-H Bond
B ernd W rackm eyer3*, G erald K ehra,
Saqib A lib
a Laboratorium für Anorganische Chemie der
Universität Bayreuth, D-95440 Bayreuth,
Germany
b Department of Chemistry, Quaid-I-Azam
University, Islamabad, Pakistan
^ V n;
(1)
Z. Naturforsch. 53b, 393-395 (1998);
received December 29, 1997
Aminoborane, Aminosilane, Boron, Pyridine,
NMR Data
In contrast to the behaviour of 2, the 2-am ino-
pyridines 3 and 4 react with 1 selectively by elim i-
nation of H 2 (eq. (2)). The n B N M R spectra of
the reaction m ixtures indicate the form ation of ad -
ducts of the type 8 (<5n B -3.6) prior to H 2 elim ina-
tion. T herefore, it appears that the interm ediacy of
the adduct 8 prevents cleavage of the N-Si bond.
Starting from 8, H 2 elim ination takes place, and
The reaction of dimeric 9-borabicyclo[3.3.1]no-
nane 1 with N-trimethylsilyl-aniline 2 affords 9-
anilino-9-borabicyclo[3.3.1]nonane 5 by elimina-
tion of trimethylsilane. In contrast, 1 reacts with
the N-trimethylsilyl-2-aminopyridines 3 and 4 se-
lectively by elimination of H2 to give the corre-
sponding N-trimethylsilyl-aminoboranes 6 and 7.
The latter reactions proceed via formation of bo-
rane-pyridine adducts.
the N -trim ethylsilyl-am inoboranes 6 and
7 are
form ed in > 90% yield (N M R data of 5 - 8 are
listed in Table I and in the experim ental part). P re-
viously, other 2-pyridylam inoboranes with an N H
or an N C H 2Ph function have been prepared either
from trialkylboranes [5] or from diorgano-thio-bo-
ranes [6],
The reactions of various am ines with borane d e-
rivatives of the type R B H 2, R 2BH , or E t3N -B H 3
proceed by elim ination of H 2, opening efficient
routes to am inoboranes [1]. Recently, we have
found that silylam ines can react w ith boranes in a
different m anner [2-4], leading also to am ino-
boranes with silane being form ed instead of H 2.
Thus, there is com petition betw een cleavage of the
N -H and the N-Si bond, and this may be related
to m ore or less significant coordinative B-N in-
teractions in the first step of the reaction. In order
to shed m ore light on this problem we have now
studied the reactions of dimeric 9-borabicyclo[3.3.-
l]n o n an e 1, [9-BBN]2, with N -trim ethylsilyl-ani-
line 2 and the N -trim ethylsilyl-2-am inopyridines 3
and 4 (see e q .(l) and (2)).
In the case of 2, the basicity of the nitrogen
atom is strongly reduced as the result of the pres-
ence of the phenyl and the M e3Si groups. O n the
o th er hand, the pyridine nitrogen atom in both 3
and 4 can act as a powerful Lewis-basic centre
tow ards an electrophilic borane.
The rotation about the B-N bond in 5 is slow on
the N M R tim e scale, indicating strong B N (pp);r
Reprint requests to Dr. B. Wrackmeyer.
0932-0776/98/0300-0393 $06.00 © 1998 Verlag der Zeitschrift für Naturforschung. All rights reserved.
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Table I. n B, 13C, 15N and 29Si NMR data[a] of the aminoboranes 5-7, and of the adduct 8 (R = H).
No.
<513C
C(5)
<5n B
<515N
<329Si
other
C(6)
C(2)
C(3)
C(4)
148.2
6
7
8
5
161.7
120.8
137.2
118.9
120.5
112.4
-2.1 (MeSi)
25.5 (CB)lbl
51.4
55.5
-3.6
51.5
7.8
7.5
6.0
-78.4 (N=)
-246.8 (NB)
160.8
157.2
129.0
112.1
122.7
138.0
138.4
129.1
146.7
144.2
1.9 (MeSi), 19.0 (Me)
24.7, 26.1 (CB)M
-69.1 (N=)
-249.1 (NB)
-0.9 (MeSi)
22.0 (C B )^
n.m.
143.7
(lPso)
123.2
(para)
27.8, 22.8 (CB)
34.0, 33.3 (CH2-C H )
23.8 (-C H 2)
—251
(ortho) (meta)
[a] Measured in C6D6 at 25 ± 1 °C (ca. 20%); assignments are based on 2D 'H /'H COSY and 13C/!H HETCOR
experiments; n.m. means not measured;
[b] other d13C data of the 9-BBN moiety: 33.4 (CH2-CB), 23.7 (-CH2-);
[c] other <513C data of the 9-BBN moiety: 34.0, 33.8, 33.2, 32.8 (CH2-CB), 24.2. 22.8 (-CH2-);
[d] Other 613C data of the 9-BBN moiety: 37.0, 30.2 (CH2-CB), 21.6 (-CH2-);
[e] 14N NMR measurement.
bonding. H ow ever, the d 13C(o, p ) data of the p h e-
nyl ring (<313C (o) = 122.7; (3l3C (p) = 123.2) suggest
that there are still significant interactions [al-
though less than in aniline [7]: (3l3C (o) = 115.1 and
<5t3C(p) = 118.5] betw een the lone pair of electrons
at the nitrogen atom and the arom atic it system.
In the case of 6, the room tem perature N M R
data are in agreem ent with free rotation about the
B-N bond. The deshielding of the 13C(3,5) nuclei
in 6 in com parison with 3 [8] is sim ilar to that o b -
served for 5 with respect to aniline. The ‘-^ (p y ri-
dine) nucleus in 6 (<515N = -78.4) is m uch less
shielded than in 3 (c515N = -107.4 [8]), in accor-
dance with reduced jt interactions betw een the
am ino nitrogen atom and the heteroarom atic ring.
The shielding of the 15N (am ino) nuclei in 6 as well
as in 7 is typical of am inoboranes [9].
the deshielding of the 15N (pyridine) nucleus in 7
((315N = -69.1) with respect to 4 (d15N= -108.1 [8])
indicates negligible bonding
n interactions be-
tw een the am ino nitrogen atom and the pyridine
ring.
The adduct 8 (R = H ) could be characterised in
the reaction m ixture by n B and 13C N M R. The
appearance of two 13C N M R signals for the dia-
stereotopic m ethylene carbon atom s C H 2CHB
proves that there is a strong coordinative N-B
bond. Exchange of ligands with an excess of 1 or
3 is slow on the N M R tim e scale. Together with
the n B N M R signal at (31'B = -3.6 , typical of such
adducts [10], which becom es significantly sharper
by !H decoupling [1J ( 11B,1H) ca. 115 Hz], there
can be no doubt about the presence of pyridine-9-
BBN adducts in the reaction mixtures.
The N M R spectra of the am inoborane 7 reflect
steric crow ding causing restricted rotation about
both the C (2)-N (am ino) and the N-B bond. The
BNSi plane is tw isted against the plane of the pyri-
dine ring; this gives rise to six different 13C signals
for the C H 2 and two 13C signals for the B C H
groups of the 9-BBN moiety. The 13C(3) nucleus
in 7 ((313C = 129.0) is m uch deshielded with respect
Experimental
All com pounds w ere handled in an atm osphere
of dry argon, and carefully dried solvents were
used for syntheses and preparation of the sam ples
for N M R m easurem ents. Starting m aterials were
prepared as described: 1 [11], 2 [12], 3 and 4 [8].
E lectron im pact (E l) m ass spectra: Finnigan M AT
8500 with direct inlet. NM R m easurem ents:
B ruker A R X 250, p H , n B, 13C, 15N (refocused IN -
E PT [13] based on 2J ( 15N ,1H ) = 10 Hz for the pyri-
dine nitrogen atom , and 3J ( 15N,!H ) = 1.5 Hz for
the N SiM e3 group), 29Si N M R (refocused IN EPT
[13] based on 2J(29Si,'H ) = 7 Hz)]. Chem ical shifts
are given w ith respect to M e4Si [<31H (C^D^H) =
7.14. (C 6D 5C D 2H ) = 2.03; (313C (C6D 6) = 128.0,
to that in 4 (d 13C
= 116.9 [8]). The effect on
<313C(5) is m uch sm aller (7: (313C = 120.5 and 4:
<513C = 113.8 [8]). This m eans that changes in the
13C nuclear shielding of the carbon atom s in para-
positions to the am ino group is affected (shielding
of ca. 3 ppm ) not only by changes in the jt electron
density but also by other influences, such as
electric field effects exerted by the am ino substitu-
ent. Sim ilar as in 6, but even m ore pronounced.
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395
(C 6D 5C D 3)
19.867184 M Hz], neat M eN O z [dl5N
H(15N) = 10.136767 M Hz; Ö14N = 0 for S ( 14N) =
7.226455 MHz],
=
20.4; <329Si
=
0
for ~(29Si)
=
for
only traces of M e3SiH w ere trapped. n B N M R
spectra w ere m easured every 30 min, showing first
the form ation of the adducts 8, followed by de-
com position into the am inoboranes. Fractional
distillation gave the pure am inoboranes in 92%
yield as colourless oils (6: b.p. 8 7 -9 0 °C / 10-2
Torr; 7: b.p. 98-101 °C / IO"2 Torr). E l MS (70
eV ) of 6: m /e (% ) = 286 [M+] (1); 166 (18); 151
[M e3SiN C4H 4N +] (100); E l MS (70 eV ) of 7: m/e
(% ) = 300 [M+] (5); 285 [M -M e+] (17); 180 (15);
165 [M e3SiN C5H 6N +] (100). !H N M R (250 M Hz,
C6D 6): ^ H (6) = 0.15 s, 9H, M e3Si; 1.24-1.90 m,
14H, 9-BBN; 8.15 m, 1H, H -C (6); 7.14 m 1H, H-
C(4); 6.40-6.55 m 2H , H -C(3,5); d lH (7) = 0.09 s,
9H, M e3Si; 1.3-1.93 m, 14H, 9-BBN; 1.97 s 3H,
M e; 8.10 m, 1H, H -C (6); 7.03 m, 1H, H -C (4); 6.57
m, 1H, H -C (5).
=
0
9-Anilino-9-borabicyclo[3.3.1Jnonane 5
A m ixture of 1 (3.00 g; 12.3 m m ol) and 2 (2.64 g;
16 m m ol) was heated at 110-120 °C for 5h. Tri-
m ethylsilane was collected in a trap cooled to -
78 °C. The excess of 2 was recovered by fractional
distillation as a first fraction. The second fraction
(b.p. 9 5 -9 8 °C
/ 10“ 1 Torr) contained pure 5
(2.44 g; 93% ) as a colourless oil. *H N M R (250
M H z, in C6D 6): d'H = 1.30-1.85 m, 14H, 9BBN;
5.61 s (broad), 1H, NH; 6.76-6.84 m, 3H, Ph (m,
p); 7.00-7.10 m, 2H, Ph (o).
A cknow ledgem ent
N -Trim ethylsilyl-9-(2-pyndylam ino)-9-
We gratefully acknow ledge support of this work
by the D eutsche Forschungsgem einschaft and the
Fonds der C hem ischen Industrie. S. A. thanks the
E C for a M arie-C urie fellowship.
borabicyclo[3.3.1]nonanes 6 and 7
The com pounds 6 and 7 w ere prepared as d e-
scribed for 5. Elim ination of H 2 was observed, and
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