An access to base-stabilized three-membered silicon heterocycles

Article abstract of DOI:10.1039/c2dt31326j

The three-membered silacyclic ring compounds LSi[N₂(Ph) ₂]tBu (1), LSi[HCN(Ph)₂]tBu (2) and LSi[C ₂(Ph)₂]tBu (3) were obtained by the treatment of base stabilized monoalkylsilylenes LSitBu (L = PhC(NtBu)₂) with PhNNPh, PhNCHPh and PhCCPh. The reaction of PhNNPh and PhCCPh with LSitBu shows a different reactivity pattern with base stabilized monochlorosilylene LSiCl. The arrangement of the three-membered ring (SiNN) in 1 is the first structurally isolated example of a siladiaziridine compound.

Full text of DOI:10.1039/c2dt31326j

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An access to base-stabilized three-membered silicon heterocycles†
Ramachandran Azhakar, Herbert W. Roesky,* Rajendra S. Ghadwal,* Julian J. Holstein and Birger Dittrich*
Received 13th June 2012, Accepted 28th June 2012
DOI: 10.1039/c2dt31326j
Compounds with silicon(IV) atoms are quite easy to access
from Si(II) by the oxidative addition reaction.9a–e The divalent
silicon species called silylenes were generated as short-lived
intermediates in numerous thermal and photochemical reactions
from organosilicon precursors.10 The existence of silylenes has
been evidenced by chemical trapping and spectroscopic methods
in the fluid phase or in low-temperature matrices. Initially the
stable silylenes were isolated as N-heterocyclic silylene (NHSi)
by West et al. in 1994.11 One can compare the reactivities of
NHSis with those of the N-heterocyclic carbenes (NHCs).
The latter find already a number of applications.12–14 In 2006,
we reported the three coordinate stable chlorosilylene LSiCl
(L = PhC(NtBu)2) stabilized by an amidinato ligand in less than
10% yield using potassium as a reducing agent.15 Subsequently
we reported the synthesis of LSiCl utilizing LiN(SiMe3)2 as a
reducing agent to get higher yields without using hazardous
reducing agents.16 Currently we are interested in the chemistry
of silylenes and utilized them as σ-donor ligands for transition
metal complexes,17 oxidative addition reactions with organic
substrates18 and Lewis bases.19 Recently we synthesized the
monoalkylsilylene LSitBu by a facile metathesis reaction treating
LSiCl with LitBu.20 We observed a quite different reactivity
pattern of LSitBu with N2O when compared with that of LSiCl.
In the case of LSitBu we obtained [LSitBu(μ-O)]2 containing a
four-membered Si2O2 ring.20 In contrast the reaction of LSiCl
proceeds to the trimer [LSi(μ-O)Cl]3 with a Si3O3 six-membered
ring.19b We presume that the difference in reactivity is due to the
presence of the bulky tBu substituent attached to the silicon
atom. This motivated us to explore the chemistry of LSitBu
further and to compare its reactivity pattern with LSiCl. We
treated LSitBu with PhNvNPh, PhNvCHPh and PhCuCPh
leading to the products LSi[N2(Ph)2]tBu (1), LSi[HCN(Ph)2]tBu
(2) and LSi[C2(Ph)2]tBu (3). These three reactions yielded the
three-membered silacyclic ring compounds. The reaction of
PhNvNPh and PhCuCPh with LSitBu shows a different reac-
tivity pattern when compared with that of LSiCl. The reaction of
LSiCl with PhNvNPh resulted in the formation of the fused
two five-membered unsymmetric rings formed by C–H bond
activation and LSiHCl2 elimination.21 The reaction of LSiCl
with PhCuCPh proceeds in a 2 : 1 ratio to yield the four-
membered ring compound16 while LSitBu yielded the silacyclo-
propene 3. It is of interest to mention that silylenes react with
alkynes to form a variety of small membered silacycles.9,22
The formation of the SiNN three-membered ring in 1 is the first
structurally isolated example of a siladiaziridine compound.
Compounds 1, 2 and 3 were obtained in good yields when
LSitBu was treated with PhNvNPh, PhNvCHPh and
The three-membered silacyclic ring compounds LSi[N2(Ph)2]-
tBu (1), LSi[HCN(Ph)2]tBu (2) and LSi[C2(Ph)2]tBu (3) were
obtained by the treatment of base stabilized monoalkyl-
silylenes LSitBu (L
= PhC(NtBu)2) with PhNvNPh,
PhNvCHPh and PhCuCPh. The reaction of PhNvNPh
and PhCuCPh with LSitBu shows a different reactivity
pattern with base stabilized monochlorosilylene LSiCl. The
arrangement of the three-membered ring (SiNN) in 1 is the
first structurally isolated example of
a siladiaziridine
compound.
Three-membered ring compounds possessing higher coordinate
group 14 elements especially silicon at the position adjacent to
the heteroatom have received considerable attention because of
their unique structures and also their wide application in syn-
thetic chemistry.1 These three-membered ring compounds
bearing a silicon atom are interesting for chemists, due to their
high strain and novel bonding arrangement within the ring.2
Moreover, these systems can be compared with epoxides and
aziridines which have become an important class of com-
pounds.3,4 Epoxides as well as aziridines are natural products
and show a great number of transformations.4 Related reactions
are expected with silicon containing three-membered rings. In
the case of Brook rearrangement, the transition state has been
proposed as a three-membered ring compound bearing a penta-
coordinate silicon at the position adjacent to an oxygen.5 Further
Nevárez and Woerpel reported on the dearomatization reaction
which involves treatment of benzaldehyde with silaziridine with
an aryl group either on carbon or nitrogen atoms, which is con-
sidered to be driven by the relief of the ring strain of the three-
membered ring.6 Recently there has been emerging interest in
the replacement of carbon atoms with silicons for developing
new innovative drugs.7 Several bioactive silacycles have been
reported. One example is sila-haloperidol which shows a differ-
ent metabolism pattern when compared to that of the carbon ana-
logue.8 However, due to the high ring strain of the three-
membered ring system only a few silicon compounds with
heteroatoms have been reported so far.9
Institut für Anorganische Chemie, Universität Göttingen,
Tammannstrasse 4, 37077 Göttingen, Germany.
E-mail: hroesky@gwdg.de, rghadwal@uni-goettingen.de,
bdittri@gwdg.de
†Electronic supplementary information (ESI) available: Experimental
section. CCDC 883101, 883102, 883103 for 1, 2 and 3 respectively. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt31326j
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9601–9603 | 9601

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Fig. 2 Molecular structure of 2. Selected bond lengths [Å] and angles
[°]: Si1–N1 1.9718(11), Si1–N3 1.8226(10), Si1–N4 1.7795(11), Si1–
N1 1.9718(11), Si1–C6 1.9160(13), Si1–C5 1.8622(13), C5–N4 1.4878(15);
N1–Si1–N3 68.97(5), N3–Si1–C5 124.85(5), N1–Si1–C6 102.45(6),
N1–Si1–N4 148.28(5), N4–Si1–C5 48.16(5), C5–Si1–C6 120.80(6),
N3–Si1–C5 124.85(5), Si1–N4–C5 68.83(7), N4–C5–Si1 63.01(6).
Scheme 1 Synthesis of 1–3.
bipyramidal; τ = 0 for perfect square based pyramidal23) is 0.28,
indicating strong deviation from the regular trigonal bipyramidal
geometry and closer to square pyramidal geometry. The
N1–Si1–N2 bite angle between the silicon atom with the back-
bone ligand is 69.83(6)°. The distance between the Si and the
tBu carbon atom is 1.8933(17) Å. The angles of the newly
formed NSiN three-membered ring are N3–Si1–N4 (50.97(6)°),
Si1–N3–N4 (66.12(8)°) and N3–N4–Si1 (62.91(8)°). The bond
distances within the ring are Si1–N3 (1.7466(15) Å), N3–N4
(1.5240(18) Å) and Si1–N4 (1.7938(15) Å).
Fig. 1 Molecular structure of 1. Selected bond lengths [Å] and angles
[°]: Si1–N1 1.8207(14), Si1–N2 1.9368(15), Si1–N3 1.7466(15), Si1–
N4 1.7938(15), Si1–C28 1.8933(17), N3–N4 1.5240(18); N1–Si1–N2
69.83(6), N3–Si1–N4 50.97(6), N2–Si1–C28 104.83(7), N1–Si1–N3
127.20(7), N2–Si1–N4 144.22(7), N2–Si1–C28 104.83(7), N3–Si1–C28
117.72(7), Si1–N3–N4 66.12(8), N3–N4–Si1 62.91(8).
The silaaziridine
2
exhibits
a
single resonance at
δ −104.6 ppm in its 29Si NMR spectrum. Similar to compound
1, the tBu protons attached to the nitrogen atoms of compound 2
show a double resonance of equal intensity in the 1H NMR spec-
trum (δ 0.76, 1.24 ppm). The tBu protons that reside at the
silicon atom resonate at 1.35 ppm. The imino proton resonates at
δ 3.81 ppm. Further compound 2 displays its molecular ion in
the mass spectrum at m/z 497.
PhCuCPh in a 1 : 1 ratio as shown in Scheme 1. Compounds
1–3 are soluble in common organic solvents. They are stable
both in the solid state as well as in solution without any
decomposition under an inert gas atmosphere. The molecular
structures of 1–3 were unequivocally established by single
crystal X-ray structural analyses. Furthermore they were fully
characterized by NMR spectroscopy, EI-MS and elemental
analysis.
The molecular structure of compound 2 is shown in Fig. 2. It
ˉ
crystallizes in the triclinic space group P1. The silicon atom is
five coordinate comprising three nitrogen atoms and two carbon
atoms and displays a distorted square pyramidal geometry with
τ = 0.39. The N1–Si–N3 bite angle is 68.97(5)°. The angles of
the newly formed NSiC three-membered ring are N4–Si–C5
(48.16(5)°), Si1–N4–C5 (68.83(7)°) and N4–C5–Si1 (63.01(6)°).
Correspondingly the bond distances within the ring are Si1–N4
(1.7795(11) Å), C5–N4 (1.4878(15) Å) and Si1–C5 (1.8622(13)
Å). These values are quite comparable to those of Si–N,24
C–N24b and Si–C24b distances reported in the literature.
The 29Si NMR spectrum of 1 shows a single resonance at
δ −99.6 ppm, which is upfield shifted when compared with that
of LSitBu (δ 61.5 ppm).20 The tBu protons of compound 1 in
the 1H NMR spectrum exhibit a double resonance of equal inten-
sity for the tBu groups that reside on nitrogen atoms (δ 0.98 and
1.23 ppm). The tBu protons attached to the silicon atom resonate
at 1.31 ppm. In addition 1 shows its molecular ion in the mass
spectrum at m/z 498.
The 29Si NMR spectrum of silacyclopropene 3 displays a
single resonance at δ −117.8 ppm. Contrary to 1 and 2, the tBu
1
protons attached to the nitrogen atoms of compound 3 in its H
NMR spectrum show a singlet at δ 1.09 ppm. The tBu protons
bound to the silicon atom show a resonance at 1.50 ppm. Com-
pound 3 exhibits its molecular ion in the mass spectrum at m/z
494.
The siladiaziridine product 1 crystallizes in the triclinic space
ˉ
group P1 and the molecular structure is shown in Fig. 1. We
propose that the reaction of LSitBu with PhNvNPh followed
the [1 + 2]-cycloaddition reaction. The silicon atom is five co-
ordinate and the coordination environment is made up from four
nitrogen atoms and a carbon atom. The structural index τ which
defines the extent of deviation from trigonal bipyramidal to
The molecular structure of compound 3 is shown in Fig. 3.
ˉ
Compound 3 crystallizes in the triclinic space group P1. The
silicon atom is five coordinate and nearly in a square pyramidal
geometry with τ = 0.05. The silicon coordination environment
is derived from two nitrogen atoms and three carbon atoms.
square pyramidal geometry (τ
= 1 for perfect trigonal
9602 | Dalton Trans., 2012, 41, 9601–9603
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Conclusions
In summary, three-membered silicon heterocycles can be
accessed by the reaction of base stabilized monoalkylsilylene
LSitBu with PhNvNPh, PhNvCHPh and PhCuCPh. Due to
the presence of the bulky tBu groups at the silicon(II) centers, the
reactions of LSitBu with PhNvNPh and PhCuCPh show a
different reactivity pattern compared to that of LSiCl. Compound
1 is the first structurally isolated example of a three-membered
siladiaziridine compound.
We thank the Deutsche Forschungsgemeinschaft for support-
ing this work. R. A. is thankful to the Alexander von Humboldt
Stiftung for a research fellowship.
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