Reactivity of stable heteroleptic silylene PhC(N t Bu) 2SiNPh2 toward diazobenzene and N -benzylidineaniline

Article abstract of DOI:10.1021/om300999e

The reaction of heteroleptic silylene LSiNPh₂ [L = PhC(NtBu)₂] with diazobenzene afforded product 6. This involves one o-C-H bond activation at one of the phenyl groups of diazobenzene and migration of this hydrogen atom from the phe

Full text of DOI:10.1021/om300999e

Article
pubs.acs.org/Organometallics
Reactivity of Stable Heteroleptic Silylene PhC(NtBu)2SiNPh2 toward
Diazobenzene and N‑Benzylidineaniline
Ramachandran Azhakar, Herbert W. Roesky,* Hilke Wolf, and Dietmar Stalke*
Institut fur Anorganische Chemie der Universitat Gottingen, Tammannstrasse 4, 37077 Gottingen, Germany
̈
̈
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̈
S
* Supporting Information
ABSTRACT: The reaction of heteroleptic silylene LSiNPh2 [L = PhC(NtBu)2] with
diazobenzene afforded product 6. This involves one o-C−H bond activation at one of the phenyl
groups of diazobenzene and migration of this hydrogen atom from the phenyl ring to one of the
nitrogen atoms, which leads to the formation of the new C−Si and N−Si bonds. The reaction of
benzylidineaniline with LSiNPh2 results in the oxidative addition of the three-membered
silaaziridine derivative 7. Compounds 6 and 7 were fully characterized by elemental analysis,
multinuclear NMR spectroscopy, and EI-MS spectrometry. The molecular structures of
compounds 6 and 7 were established unequivocally by single-crystal X-ray structural analysis.
single bond into the o-C−H bond of one of the phenyl rings or
INTRODUCTION
■
into the C−HMe2 bond of one of the o-isopropyl groups. We
also reported on the diverse reactivity pattern of silylenes LSiCl
(1) and LSitBu (3) with diazobenzene. In the reaction of 1, the
unsymmetrical polycycle 2 (Chart 1) was formed with o-C−H
Silylenes are compounds with divalent silicon atom and are the
silicon congeners of carbenes.1 For many decades, silylenes had
been recognized as short-lived species either in the gas phase, in
solution, or trapped in frozen matrixes.2 They feature both
nucleophilic as well as electrophilic reactive sites at the silicon
atom, which have a lone pair orbital and a vacant pπ orbital.3 In
1986, the remarkable divalent silicon ten-coordinate π complex,
(Me5C5)2Si, was synthesized by Jutzi and co-workers.4 The first
stable N-heterocyclic silylene (NHSi), the silicon analogue of
N-heterocyclic carbene (NHC), was reported by West et al. in
1994.5 The NHCs have been studied extensively and find a
wide variety of applications, such as organocatalyst reagents and
also as excellent σ-donor ligands to stabilize compounds with
unusual oxidation states.6,7 Many noteworthy reactivity studies
of stable NHSi's toward a variety of substrates have been
reported, such as insertion,8−13 addition,14 metal complex-
ation,15 and Lewis acidic behavior.16 Recently, we reported on
functionalized silylenes that were prepared by the facile
metathesis reaction of alkali metal amide, phosphide, alkoxide,
or organoalkyl reagent with LSiCl17,18 in toluene as a solvent.19
A brief literature survey reveals that the reactivity pattern of
different silylenes with diazobenzene shows unlike behavior.
This might be due to the presence of two reactive sites of
diazobenzene, the NN π bond and the two lone pairs of
electrons where each resides on a nitrogen atom. In 1991, Ando
et al. reported on the synthesis of 1,2-diaza-3,4-disilacyclobu-
tane by the photochemical treatment of tetramesityldisilene
with diazobenzene.20 Recently, So et al. described the
formation of 1,2-diaza-3,4-disilacyclobutane containing a five-
coordinate silicon center by the reaction of {PhC(NtBu)2}-
SiSi(Cl){(NtBu)2C(H)Ph} with diazobenzene.21 Weidenbruch
et al. observed the formation of a 1,3-diaza-2-silaindan or 1-aza-
2-silaindan derivative by the reaction of sterically encumbered
silylene R2Si: (R = 2,4,6-Me3C6H2, 2,4,6-iPr3C6H2) with
diazobenzene.22 The reaction might proceed via the inter-
mediate of siladiaziridine, followed by insertion of the N−N
Chart 1
bond activation of the phenyl ring.23 Treatment of 3 with
diazobenzene resulted in the siladiaziridine derivative 4 (Chart
1).24 We were curious to know the reactivity pattern of
diazobenzene with the LSiNPh2 (5). As expected, the reaction
proceeds in a different manner and the product 6 was obtained.
We surmise that the product of the reaction is dependent on
the substituents at the silicon center. On the one hand,
compound 6 was formed by the activation of one o-C−H bond
atom of one of the phenyl groups of diazobenzene and
migration of this hydrogen atom from the phenyl ring to one of
the nitrogen atoms. This is connected with the formation of the
new C−Si and N−Si bonds. On the other hand, the reaction of
benzylidineaniline with LSiNPh2 led to the formation of the
three-membered silaaziridine derivative 7, similar to the
silaaziridine products obtained from the reaction of LSiCl
and LSitBu with diazobenzene,24,29 respectively.
Received: October 24, 2012
Published: December 7, 2012
© 2012 American Chemical Society
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RESULTS AND DISSCUSSION
■
Compounds 6 and 7 were obtained by one-pot reactions of
LSiNPh2 (5) with PhNNPh (for 6) and PhNCHPh (for
7) in the molar ratio of 1:1 (Scheme 1). Compounds 6 and 7
Scheme 1. Synthesis of Compounds 6 and 7
Figure 1. Molecular structure of 6. The anisotropic displacement
parameters are depicted at the 50% probability level. Selected bond
lengths (Å) and bond angles (deg): N2−Si1 2.0239(18), N1−Si1
1.847(2), N3−Si1 1.777(2), N4−Si1 1.8306(18), N4−N5 1.422(2);
N1−Si1−N2 68.08(8), N3−Si1−N4 93.44(9), Si1−N4−N5
114.79(13).
lengths of the phenyl ring, which is attached to the silicon atom
(between adjacent carbon atoms C34−C39), is in the range of
1.37−1.4 Å, indicating complete delocalization of π-electron
density within the ring, retaining its aromatic character. The
Si1−N4 bond length is 1.8306(18) Å and indicates a Si−N
single-bond character.28
The silaaziridine compound 7 formed by [1 + 2]-cyclo-
addition reaction exhibits a single resonance at δ −133.1 ppm
in its 29Si NMR spectrum. Like 6, the tBu protons reside over
the nitrogen atoms of compound 7 and display a double
resonance of equal intensity in the 1H NMR spectrum (δ 0.67,
1.34 ppm). The imino proton shows a singlet at δ 4.27 ppm.
Further, compound 7 shows its molecular ion in the mass
spectrum at m/z 608.
are soluble in common organic solvents, and they are stable,
both in the solid state as well as in solution without any
decomposition under an inert gas atmosphere. Compounds 6
and 7 were fully characterized by spectroscopic methods. The
molecular structures of compounds 6 and 7 were established
unambiguously by single-crystal X-ray structural analysis.
Compound 6 shows a single resonance at δ −88.2 ppm in its
29Si NMR spectrum, which is upfield shifted when compared
with that of LSiNPh2 (δ −20.5 ppm).19b The tBu protons of
compound 6 in the 1H NMR spectrum exhibit a double
resonance of equal intensity for the tBu groups that reside over
the nitrogen atoms (δ 0.91 and 1.00 ppm). The N−H proton
shows a singlet at 5.35 ppm. Further, compound 6 displays its
molecular ion in the mass spectrum at m/z 609.
Compound 6 crystallizes in the monoclinic space group P21/
n, and the molecular structure is shown in Figure 1. The
tentative mechanism for the formation of 6 has been proposed
in Scheme 2. The silicon atom is five-coordinate, and the
coordination environment is made up from four nitrogen atoms
and a carbon atom from the phenyl group. The structural index
τ, which defines the extent of deviation from trigonal-
bipyramidal to square-pyramidal geometry (τ = 1 for perfect
trigonal-bipyramidal; τ = 0 for perfect square-based pyramid25),
is 0.78, indicating a closer arrangement to the regular trigonal-
bipyramidal geometry. The N1−Si1−N2 bite angle between
the silicon atom with the backbone ligand is 68.08(8)°. The
bond length between the Si and nitrogen atom of the NPh2
moiety is 1.777(2) Å. Here, the azo compound is converted
into the hydrazine derivative. The position of the hydrogen
atom at N5 was taken from the difference Fourier map and
refined freely. The bond length between N4−N5 is 1.422(2) Å
and indicates a N−N bond length that is quite comparable to
the N−N bond lengths of other hydrazine or hydrazone
derivatives that are reported in the literature.26,27 The bond
The molecular structure of compound 7 is shown in Figure 2.
It crystallizes in the monoclinic space group P21/c. Like 6, the
silicon atom in 7 is also five-coordinate (four nitrogen atoms
and one carbon atom) with τ = 0.45. The N1−Si1−N2 bite
angle is 69.61(6)°. Again, the position of the hydrogen atom at
C16 was taken from the difference Fourier map and refined
freely. The angles of the newly formed NSiC three-membered
ring are N3−Si1−C16 (48.95(7)°), Si1−N3−C16 (69.91(9)°),
and N3−C16−Si1 (61.13(9)°). Likewise, the bond lengths
within the ring are Si1−N3 (1.7410(16) Å), C16−N3
(1.499(2) Å), and Si1−C16 (1.8672(19) Å). These values
are quite comparable to those of Si−N,24,29 C−N,24,29 and Si−
C24,29 distances reported in the literature. The Si1−N4 bond
length is 1.7356(16) Å.
Moreover, it is worth mentioning that Woerpel et al.30
reported on the three-membered ring expansion of silaazir-
idines with organic substrates. The presence of an aromatic
group either at the carbon or at the nitrogen atom of the
silaaziridine moiety undergoes dearomatization upon treatment
with benzaldehyde with the relief of strain in the three-
membered ring.
CONCLUSION
In conclusion, we have shown a unique reactivity pattern of
diazobenzene with heteroleptic silylene LSiNPh2, leading to
■
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Scheme 2. Proposed Mechanism for the Formation of 6
(0.21 g, 1.15 mmol) at room temperature. The reaction mixture was
stirred for 14 h. Then the reaction mixture was filtered, and the solvent
of the filtrate was reduced in vacuo to 20 mL. The concentrate was
stored at −27 °C in a freezer for a week, and finally, maroon-colored
crystals of 6 were obtained (0.52 g, 76%). Elemental analysis (%) calcd
for C39H43N5Si (609.88): C, 76.81; H, 7.11; N, 11.48. Found: C,
1
76.76; H, 7.02; N, 11.37. H NMR (500 MHz, C6D6, 25 °C): δ 0.91
(s, 9H, C(CH3)3), 1.00 (s, 9H, C(CH3)3), 5.35 (s, 1H, NH), 6.70−
8.39 (m, ArH) ppm. 29Si{1H} NMR (99.36 MHz, C6D6, 25 °C): δ
−88.2 ppm. EI−MS: m/z 609 [M+].
Synthesis of 7. Toluene (60 mL) was added to a 100 mL Schlenk
flask containing LSiNPh2 (5) (0.45 g, 1.05 mmol) and PhNCHPh
(0.20 g, 1.10 mmol) at room temperature. The reaction mixture was
stirred for 14 h. The reaction mixture was then filtered, and the solvent
of the filtrate was reduced in vacuo to 20 mL. The concentrate was
stored at −27 °C in a freezer for 3 days to obtain the colorless crystals
of 7 (0.55 g, 86%). Elemental analysis (%) calcd for C40H44N4Si
(608.89): C, 78.90; H, 7.28; N, 9.20. Found: C, 78.77; H, 7.16; N,
1
9.12. H NMR (500 MHz, C6D6, 25 °C): δ 0.67 (s, 9H, C(CH3)3),
1.34 (s, 9H, C(CH3)3), 4.27 (s, 1H, CHPh), 6.53−8.13 (m, ArH)
ppm. 29Si{1H} NMR (59.63 MHz, C6D6, 25 °C): δ −133.1 ppm. EI−
MS: m/z 608 [M+].
Crystal Structure Determination. Suitable single crystals for X-
ray structural analysis of 6 and 7 were mounted at low temperature in
inert oil under an argon atmosphere by applying the X-Temp 2
device.31 The diffraction data were collected at 100 K on a Bruker D8
three circle diffractometer equipped with a SMART APEX II CCD
detector and a microfocus source with INCOATEC Quazar mirror
optics (λ = 0.71073 Å).32 The data were integrated with SAINT,33 and
an empirical absorption correction with SADABS34 was applied. The
structures were solved by direct methods (SHELXS-97) and refined
against all data by full-matrix least-squares methods on F2 (SHELXL-
97).35 All non-hydrogen atoms were refined with anisotropic
displacement parameters. The hydrogen atoms were refined isotropi-
cally on calculated positions using a riding model with their Uiso values
constrained to 1.5Ueq of their pivot atoms for terminal sp3 carbon
atoms and 1.2 times for all other carbon atoms. In 6, H1 (attached to
N5) and, in 7, H16 (attached to C16) were refined freely from the Fo
− Fc density map.
Figure 2. Molecular structure of 7. The anisotropic displacement
parameters are depicted at the 50% probability level. Selected bond
lengths (Å) and bond angles (deg): N1−Si1 1.8241(16), N2−Si1
1.9299(16), N3−Si1 1.7410(16), N4−Si1 1.7356(16), C16−N3
1.499(2), Si1−C16 1.8672(19); N1−Si1−N2 69.61(6), N1−Si1−N3
105.84(7), N2−Si1−N3 149.08(7), N3−Si1−N4 108.13(7), C16−
Si1−N3 48.95(7), Si1−N3−C16 69.91(9), N3−C16−Si1 61.13(9).
product 6. This was formed by the activation of the o-C−H
bond of one of the phenyl groups and migration of this
hydrogen atom to one of the nitrogen atoms of diazobenzene.
The reaction of benzylidineaniline with LSiNPh2 led to the
formation of the three-membered silaaziridine ring 7.
EXPERIMENTAL SECTION
■
Syntheses were carried out under an inert atmosphere of dinitrogen in
oven-dried glassware using standard Schlenk techniques. All other
manipulations were accomplished in a dinitrogen-filled glovebox.
Solvents were purified by the MBRAUN solvent purification system
MB SPS-800. All chemicals were purchased from Aldrich and used
without further purification. LSiNPh219b (5) was prepared as reported
in the literature. 1H and 29Si NMR spectra were recorded with a
Bruker Avance DPX 300 or a Bruker Avance DRX 500 spectrometer,
using C6D6 as a solvent. Chemical shifts δ are given relative to SiMe4.
EI−MS spectra were obtained using a Finnigan MAT 8230
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystal and structure refinement parameters for compounds 6
and 7 (Table S1). CIF files for 6 and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
spectrometer. Elemental analyses were performed at the Institut fur
̈
Anorganische Chemie, Universitat Gottingen. For elemental analysis,
̈
̈
*E-mail: hroesky@gwdg.de (H.W.R.), dstalke@chemie.uni-
goettingen.de (D.S.).
compounds were dried under vacuum for 6 h to remove the solvent
molecules.
Notes
Synthesis of 6. Toluene (60 mL) was added to a 100 mL Schlenk
flask containing LSiNPh2 (5) (0.48 g, 1.12 mmol) and PhNNPh
The authors declare no competing financial interest.
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̈
ACKNOWLEDGMENTS
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Soc. 2010, 132, 3038−3046. (b) Jana, A.; Schulzke, C.; Roesky, H. W.
J. Am. Chem. Soc. 2009, 131, 4600−4601. (c) Jana, A.; Schulzke, C.;
Roesky, H. W.; Samuel, P. P. Organometallics 2009, 28, 6574−6577.
We are thankful to the Deutsche Forschungsgemeinschaft
(DFG RO 224/60-1) and Prohama, Ludwigshafen, for
supporting this work. R.A. is thankful to the Alexander von
Humboldt Stiftung for a research fellowship. D.S. and H.W. are
grateful to the DNRF funded Center for Materials Crystallog-
raphy (CMC) for support, and the Land Niedersachsen for
providing a fellowship in the Catalysis of Sustainable Synthesis
(CaSuS) Ph.D. program.
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