1,2-azaboretidine formation in the reactions of (boryl)(silyl)iminomethanes via possible generation of (amino)(boryl)carbene species

Article abstract of DOI:10.1016/S0022-328X(01)01321-3

[Bis(diisopropylamino)boryl](organosilyl)(N-arylimino)methanes, which was formed in the reaction of aryl isocyanides with (organosilyl)bis(diisopropylamino)borane at room temperature, underwent new skeletal rearrangement reaction to provide aryl(1,2-azaboretidin-3-yl)(organosilyl)amines at 110 °C. Reaction of 1,2-diisocyanobenzene with the silylborane afforded 2-(organosilyl)-3-(1,2-azaboretidin-3-yl)-4,5-benzoimidazole, which was isolated as the corresponding borane complexes. The reactions may proceed through (amino)(boryl)carbene intermediates, which is formed via aza-Brook-type 1,2-silyl migration, giving the 1,2-azaboretidines by intramolecular insertion into the C-H bonds α to the nitrogen atoms.

Full text of DOI:10.1016/S0022-328X(01)01321-3

Journal of Organometallic Chemistry 643–644 (2002) 508–511
www.elsevier.com/locate/jorganchem
Note
1,2-Azaboretidine formation in the reactions of
(boryl)(silyl)iminomethanes via possible generation of
(amino)(boryl)carbene species
Michinori Suginome *, Takeshi Fukuda, Yoshihiko Ito *
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto Uni6ersity, Sakyo-ku,
Kyoto 606-8501, Japan
Received 16 July 2001; accepted 26 September 2001
This paper is dedicated to Professor Franc¸ois Mathey on the occasion of his 60th birthday in recognition of his outstanding contribution to
organometallic chemistry
Abstract
[Bis(diisopropylamino)boryl](organosilyl)(N-arylimino)methanes, which was formed in the reaction of aryl isocyanides with
(organosilyl)bis(diisopropylamino)borane at room temperature, underwent new skeletal rearrangement reaction to provide
aryl(1,2-azaboretidin-3-yl)(organosilyl)amines at 110 °C. Reaction of 1,2-diisocyanobenzene with the silylborane afforded 2-
(organosilyl)-3-(1,2-azaboretidin-3-yl)-4,5-benzoimidazole, which was isolated as the corresponding borane complexes. The
reactions may proceed through (amino)(boryl)carbene intermediates, which is formed via aza-Brook-type 1,2-silyl migration,
giving the 1,2-azaboretidines by intramolecular insertion into the CꢀH bonds h to the nitrogen atoms. © 2002 Elsevier Science
B.V. All rights reserved.
Keywords: Silyl migration; Azaboretidine; Isocyanide; (Amino)(boryl)carbene; Quinoxaline; Silylborane
Iminomethanes carrying boryl groups at the imino
carbon have hardly been isolated in monomeric forms
[1], although there have been known many reactions
involving the boryliminomethanes as reactive interme-
diates [2]. Recently, we reported a ready synthetic ac-
cess to monomeric (boryl)(silyl)(N-arylimino)methanes
via insertion of isonitriles into the siliconꢀboron bond
of silylboranes 1 [3]. The reaction proceeded under mild
reaction conditions below 50 °C in the absence of
catalysts such as transition–metal complexes [4].
found that the imines underwent the new thermal rear-
rangement reaction. Herein, we disclose the skeletal
rearrangement of (boryl)(silyl)iminomethanes, which
presumably proceeds through generation of (bo-
ryl)(amino)carbene species by 1,2-shift of the silyl
group from carbon to nitrogen. Furthermore, related
reactions of (boryl)(silyl)[N-(2-isocyanophenyl)imino]-
methanes, which are derived from 1,2-diisocyanoben-
zenes via similar generation of carbene intermediates
are also described.
3,5-Dimethylphenyl isocyanide (2a) was reacted with
silylborane 1a bearing diisopropylamino groups on the
boron atom (Scheme 1). As reported previously, the
reaction
afforded
(boryl)(silyl)(N-3,5-xylylimino)-
methane (3a) in high yield at room temperature in
toluene. To our surprise, however, an attempted reac-
tion of 1a with 2a under toluene reflux (110 °C) re-
sulted in clean formation of an unexpected product 4a
which might be derived via a skeletal rearrangement of
3a. The product 4a isolated by silica gel column chro-
matography (pretreated with Et3N; eluent: hexane) ex-
In the course of our further investigation on the
reactivity of the new (boryl)(silyl)iminomethanes, we
* Corresponding authors. Fax: +81-75-7535668.
E-mail address: suginome@sbchem.kyoto-u.ac.jp (M. Suginome).
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01321-3

M. Suginome et al. / Journal of Organometallic Chemistry 643–644 (2002) 508–511
509
Scheme 1.
Fig. 1. Crystal structure of 4c. The thermal ellipsoids are plotted at
30% probability level. Hydrogen atoms are omitted for clarity. Se-
,
lected bond distance (A) and angles (°) are as follows: Si1ꢀN1=
1.738(2), N1ꢀC1=1.494(3), C1ꢀB1=1.617(4), B1ꢀN3=1.423(4),
N3ꢀC2=1.490(4),
C2ꢀC1ꢀB1=84.5(2), C1ꢀB1ꢀN3=90.6(2), B1ꢀN3ꢀC2=95.6(2),
N3ꢀC2ꢀC1=89.2(2).
C2ꢀC1=1.593(4),
B1ꢀN2=1.408(4);
Scheme 2.
azaboretidine 4b in 86% yield, in which the alkynyl
moiety was left intact. Similarly, (triphenylsi-
lyl)bis(diisopropylamino)borane 1b provided the corre-
sponding azaboretidine 4c in good yield [8]. tert-Butyl-
substituted alkynylisocyanide (2d) also afforded 4d in
good yield in the reaction with 1a. We also examined
the reaction of 2c with silylborane 1c, which carried a
pinacol ligand on the boron instead of the diisopropyl-
amino groups. Although the corresponding imine was
formed in high yield, no further reaction took place at
110 °C at all. The triphenyl derivative 4c isolated as
single crystals was subjected to X-ray crystallographic
analysis, establishing the formation of the azaboretidine
ring, in which the four angular atoms were in one plane
without puckering, as evidenced by the sum of the four
interior angles (360.0°) (Fig. 1) [9].
Although there is no further information for the
mechanistic elucidation of the azaboretidine formation,
we may propose the following mechanism which in-
volves 1,2-carbon-to-nitrogen silyl migration as the key
step, resulting in the reversible generation of
(amino)(boryl)carbene species A (Scheme 4). The car-
bene may undergo insertion into the CꢀH bond of the
isopropylamino group, thus forming the azaboretidine
ring.
Scheme 3.
1
hibited H-NMR spectrum indicating the presence of
only three of the four expected isopropyl groups on the
nitrogen atoms [5]. The NMR data as well as single
crystal X-ray studies for the related compounds (see
below) convinced us that the product has 1,2-azabore-
tidine structure, a four-membered ring containing a
BꢀN bond [6]. It was proven that the azaboretidine 4a
was formed via (boryl)(silyl)iminomethane (3a) on the
basis of an experiment in which a solution of 1a and 2a
was stirred at room temperature to the complete con-
sumption of both the reactants followed by raising the
reaction temperature to 110 °C. Initially formed 3a was
completely converted into 4a at 110 °C. In sharp con-
trast to the reactivity of the 3,5-xylyl isocyanide (2a),
2,6-xylyl isocyanide (2b) only afforded the correspond-
ing imine 3b even at 110 °C (Scheme 2). The imine 3b
hardly underwent the further rearrangement, probably
due to the increased steric hindrance.
We then became interested in the reaction of 1,2-di-
isocyanobenzenes with silylboranes, since the two iso-
cyano groups in close proximity may participate in the
insertion reaction in a concerted or stepwise manner
[10], thus leading to the formation of heterocyclic prod-
ucts functionalized by silyl and boryl groups. 1,2-Diiso-
cyanobenzene (5a) was reacted with an equimolar
amount of 1a at room temperature (Scheme 5). 1H-
The formation of the azaboretidine ring was also
found in the reactions of o-alkynylisocyanobenzene [7]
with 1a under the reaction conditions identical to those
for 2a (Scheme 3). Thus, isocyanide 2c afforded the

510
M. Suginome et al. / Journal of Organometallic Chemistry 643–644 (2002) 508–511
NMR monitoring of the reaction in C6D6 indicated an
initial formation of an unidentifiable product, followed
by clean conversion to a single final product 6a, which
showed NMR signals characteristic of azaboritidine
derivatives. Although the product was distillable, fur-
ther purification with silica gel column chromatography
resulted in decomposition. Treatment of the reaction
mixture with borane–THF at room temperature re-
sulted in the formation of a chromatographically stable
borane adduct 7a [11]. When the reaction was carried
out in a preparative scale in toluene, 7a was isolated in
71% overall yield by silica gel column chromatography.
4,5-Dimethylphenyl-1,2-diisocyanobenzene (5b) simi-
larly afforded the corresponding borane adduct 7b.
A single crystal X-ray analysis of the borane adduct
7a established the 4,5-benzoimidazole core bearing a
silyl group and an 1,2-azaboritidin-3-yl group at 2- and
3-positions, respectively, with coordination to BH3 at
the 1-nitrogen [12] (Fig. 2).
We also carried out the reaction of 3,6-disubstituted
1,2-diisocyanobenzenes. Although 3,6-dimethyl-1,2-di-
isocyanobenzene (5c) gave the corresponding benzimi-
dazole 6c with the azaboretidine ring formation in high
Fig. 2. Crystal structure of 7a. The thermal ellipsoids are plotted at
30% probability level. Hydrogen atoms are omitted for clarity. Se-
,
lected bond distance (A) and angles (°) are as follows: Si1ꢀC1=
1.920(5), N1ꢀB1=1.613(9), N2ꢀC2=1.468(7), C2ꢀB2=1.618(8),
B2ꢀN4=1.418(8), N4ꢀC3=1.492(7), C3ꢀC2=1.566(8), B2ꢀN3=
1.379(9); B2ꢀC2ꢀC3=85.5(4), C2ꢀB2ꢀN4=89.5(5), B2ꢀN4ꢀC3=
96.0(4), N4ꢀC3ꢀC2=89.0(4).
Scheme 6.
Scheme 4.
NMR yield, 3,6-di-p-tolyl-1,2-diisocyanobenzene (5d)
afforded 2-silyl-3-borylquinoxaline derivative 8 in high
yield without any appreciable formation of the corre-
sponding imidazole product (Scheme 6) [13].
The formation of the azaboretidine 6 suggests that
the reaction of 5a with 1a may proceed via the (imida-
zolyl)(boryl)carbene species B, which was derived from
the primary adduct, N-(2-isocyanophenyl)imine C. On
the other hand, the quinoxaline derivative 8 may be
obtained through 1,2-shift of the silyl-substituted 2-car-
bon in B to the carbene. Presumably, steric repulsion
between the bulky tolyl group and the bis(diisopropy-
lamino)boryl group may make the CꢀH insertion pro-
cess unfavorable (Scheme 7).
In summary, we reported some new skeletal rear-
rangement reactions of (boryl)(silyl)iminomethanes,
in which the intermediary formation of (amino)-
(boryl)carbene species may be crucially involved. Our
research program will be focused on the mechan-
Scheme 5.

M. Suginome et al. / Journal of Organometallic Chemistry 643–644 (2002) 508–511
511
(b) J.D. Rainier, A.R. Kennedy, E. Chase, Tetrahedron Lett. 40
(1999) 6325;
(c) M. Suginome, T. Fukuda, Y. Ito, Org. Lett. 1 (1999) 1977.
[8] Spectral data for 4d: 1H-NMR (CDCl3) l −0.13 (s, 9H),
0.60–1.00 (br, 6H), 1.07 (d, J=7.2 Hz, 6H), 1.04–1.14 (m, 6H),
1.28 (s, 3H), 1.30 (s, 3H), 3.17–3.36 (brm, 2H), 3.35 (s, 1H), 3.45
(sept, J=7.2 Hz, 1H), 6.79 (t, J=7.2 Hz, 1H), 6.99 (d, J=6.3
Hz, 1H), 7.18 (t, J=8.7 Hz, 1H), 7.25 (t, J=6.6 Hz, 6H),
7.28–7.38 (m, 3H), 7.56 (d, J=6.9 Hz, 6H), 8.10 (d, J=8.7 Hz,
1H); 13C-NMR (CDCl3) l 0.58, 22.2 (br), 24.3, 24.5, 24.7, 24.9,
31.2, 45.2, 46.2 (br), 60.1 (br), 67.7, 98.7, 106.8, 121.5, 122.2,
126.9, 127.9, 128.1, 128.8, 135.1, 136.2, 137.6, 152.1.
(
[9] Crystal data for 4c (C42H54BN3Si2): triclinic, space group P1
,
(No. 2); Z=2; a=10.886(1), b=11.555(1), c=17.226(2) A;
3
,
h=103.322(4), i=104.661(3), k=89.789(1)°; V=2036.5(4) A ;
zcalc=1.089 g cm−3; v=0.118 cm−1. Intensity data were mea-
sured on a Rigaku/MSC Mercury CCD diffractometer with
Scheme 7.
,
graphite monochromated Mo–Ka radiation (u=0.7107 A, T=
293 K); 14 012 reflections measured, 8244 independent, 5997
included in the refinement, 433 parameters, R=0.068, Rw=
0.085.
istic clarification and synthetic application of the in-
triguing rearrangement reactions.
[10] For the chemistry of 1,2-diisocyanobenzenes, see: (a) Y. Ito, E.
Ihara, M. Hirai, H. Ohsaki, A. Ohnishi, M. Murakami, J. Chem.
Soc. Chem. Commun. (1990) 403;
Acknowledgements
(b) Y. Ito, E. Ihara, M. Murakami, M. Shiro, J. Am. Chem. Soc.
112 (1990) 6446;
We thank Professors Mitsuru Kondo (Shizuoka Uni-
versity) and Susumu Kitagawa (Kyoto University) for
the X-ray analysis of 4c.
(c) Y. Ito, E. Ihara, M. Murakami, Angew. Chem. Int. Ed. Engl.
31 (1992) 1509;
(d) Y. Ito, T. Miyake, S. Hatano, R. Shima, T. Ohara, M.
Suginome, J. Am. Chem. Soc. 120 (1998) 11880;
(e) Y. Ito, T. Miyake, M. Suginome, Macromolecules 33 (2000)
4034 and references therein.
[11] Spectral data for 7a: 1H-NMR (CDCl3) l 0.52 (s, 3H), 0.70 (s,
3H), 0.6–1.0 (broad, 12H), 0.92 (d, J=6.9 Hz, 3H), 0.99 (d,
J=6.9 Hz, 3H), 1.04 (s, 3H), 1.09 (s, 3H), 2.6–4.0 (br, 2H), 2.98
(sept, J=6.9 Hz, 1H), 4.11 (s, 1H), 7.00–7.18 (m, 5H), 7.24–
7.58 (m, 2H), 7.88–8.00 (m, 1H), 8.52–8.61 (m, 1H) (Three
protons for the BH3 group could not be detected.); 11B-NMR
(C6D6; standard: BF3OEt2) l −19.3 (BH3), 27.6 (ring B); 29Si-
NMR (C6D6) l −11.6.
References
[1] (a) A. Meller, H. Batka, Monatsh. Chem. 101 (1970) 648;
(b) U.S. Sicker, A. Meller, W. Maringgele, J. Organomet. Chem.
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[3] M. Suginome, T. Fukuda, H. Nakamura, Y. Ito, Organometal-
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[4] For transition–metal catalyzed reactions of silylboranes, see: M.
Suginome, Y. Ito, Chem. Rev. 100 (2000) 3221.
(
[12] Crystal data for 7a (C28H46B2N4Si): triclinic, space group P1
,
(No. 2); Z=2; a=11.095(4), b=11.16(1), c=13.602(8) A; h=
3
,
87.00(6), i=85.77(4), k=66.34(5)°; V=1538.410034(2) A ;
zcalc=1.054 g cm−3; v=8.027 cm−1. Intensity data were mea-
[5] Spectral data for 4a: 1H-NMR (CDCl3) l 0.51 (s, 6H), 0.85–1.20
(brm, 6H), 1.00 (d, J=7.2 Hz, 6H), 1.10 (s, 3H), 1.20 (s, 3H),
1.22 (d, J=6.8 Hz, 3H), 1.24 (d, J=7.0 Hz, 3H), 2.11 (s, 6H),
3.10 (s, 1H), 3.25–3.60 (m, 3H), 6.31 (s, 1H), 6.88 (s, 2H),
7.28–7.38 (m, 3H), 7.54–7.65 (m, 2H); 13C-NMR (CDCl3) l
0.19, 0.92, 21.46, 23.12, 23.62, 24.50, 30.75, 44.89, 45.80, 58.2
(br), 66.46, 119.00, 119.73, 127.76, 128.81, 133.85, 136.51,
140.71, 151.13.
[6] For the precedent azaboretidine synthesis, see: S. Channareddy,
B. Glaser, E.P. Mayer, H. No¨th, S.W. Helm, Chem. Ber. 126
(1993) 1119.
[7] For reactions of o-alkynylisocyanobenzenes, see: (a) K. Onit-
suka, M. Segawa, S. Takahashi, Organometallics 17 (1998) 4335;
sured on a Mac Science MXC3 diffractometer with graphite
,
monochromated Cu–Ka radiation (u=1.54178 A, T=293 K);
5431 reflections measured, 4799 independent, 4187 included in
the refinement, 362 parameters, R=0.089, Rw=0.103.
[13] Spectral data for 8: 1H-NMR (CDCl3) l 0.81 (s, 6H), 0.83 (d,
J=6.9 Hz, 12H), 1.04 (d, J=6.9 Hz, 12H), 2.198 (s, 3H), 2.203
(s, 3H), 3.47 (sept, J=6.9 Hz, 4H), 7.08–7.21 (m, 7H), 7.56–
7.66 (m, 4H), 7.73 (d, J=8.1 Hz, 2H), 7.82 (d, J=8.1 Hz, 2H);
13C-NMR (C6D6) 1.2, 21.1, 24.2, 24.7, 48.3, 127.2, 128.7, 128.7,
128.9, 129.1, 129.8, 131.3, 131.5, 135.0, 136.6, 136.8, 136.9, 138.3,
138.7, 138.9, 140.6, 140.7, 166.0, 170.1 (br); IR (KBr) 3036, 2928,
1510, 1451, 1142, 1052, 812 cm−1
.