Reversible, complete cleavage of Si=Si double bonds by isocyanide insertion

Article abstract of DOI:10.1002/anie.201209281

There and back: An isocyanide-centered silene-disilene reversibility was observed for the insertion of isocyanides into unsymmetrically substituted disilenes. This reaction leads to the formation of silenes at room temperature; the disilene is regenerated in the presence of a Lewis acid.

Full text of DOI:10.1002/anie.201209281

Angewandte
Chemie
DOI: 10.1002/anie.201209281
Silicon Double Bonds
=
Reversible, Complete Cleavage of Si Si Double Bonds by Isocyanide
Insertion**
Moumita Majumdar, Volker Huch, Iulia Bejan, Antje Meltzer, and David Scheschkewitz*
ꢀ
Since the discovery of the first stable compound with Si Si
myne Ar’GeGeAr’ and distannyne Ar’SnSnAr’ (Ar’ = 2,6-
(2,6-iPr2C6H3)2C6H3), exhibited similar reactivity with tert-
butyl or mesityl isocyanide to afford the mono or bis
adducts,[9] in the case of distannyne in a reversible fashion.[10]
Isocyanide complexes of silylenes display two bonding
extremes. They form silylene–isocyanide Lewis adducts with
very bulky substituents;[11] however, a heterocumulene struc-
ture has been reported with less-bulky substituents.[12]
Notably, up to now all reactivity studies of the multiply
bonded heavier Group 14 elements with isocyanides have
revolved around symmetrically substituted double or triple
bonds, which is presumably due to the synthetically more
challenging access to unsymmetrical substitution patterns.[13]
Earlier, we established a facile method for the multiple
=
double bond, tetramesityldisilene (Mes2Si SiMes2, Mes =
2,4,6-trimethylphenyl) by West, Michl, and Fink,[1] numerous
disilenes have been reported and their reactivity studied.[2]
=
For instance, even in the absence of catalysts the Si Si bond
of disilenes readily adds various reagents, such as alcohols,
alkenes, alkynes, carbonyl compounds, and dienes.[2,3]
ꢀ ꢁ
Isocyanides (R N C) serve as versatile building blocks in
organic synthesis and as ligands in metal complex chemistry.[4]
They undergo facile [2+1] cycloadditions with different
dipolarophiles.[5] In view of the inherently dipolar nature of
[6]
=
the Si Si bond, the reaction of disilenes with isocyanides
appears to be particularly promising for the preparation of
extended systems incorporating unsaturated or cyclic silicon-
based repeat units. To date, however, only two reports of such
reactivity are available: West et al. reported the reaction of
tetra(2,6-dimethylphenyl)disilene with 2,6-dimethylphenyli-
socyanide at room temperature to form disilacyclopropani-
mine A (Scheme 1).[7] A similar [2+1] cycloaddition of
=
transfer of Si Si moieties to aromatic substrates and thus the
synthesis of unsymmetrical disilenes, including phenyl dis-
ilene 1 as well as phenylene-bridged tetrasiladienes.[14]
Herein, we report the surprising reactivity of isocyanides
towards unsymmetrically substituted (and thus somewhat
=
=
isocyanide with R*PhSi SiPhR* (R* = SitBu3) was reported
polar) disilenes to yield silaaziridines 2a–c with exocyclic Si
by Wiberg et al.[3b]
C double bonds (Scheme 2; Tip = 2,4,6-isopropylphenyl). The
Scheme 1. The disilacyclopropanamine A from West et al. (R=2,6-
Me2C6H3).
Scheme 2. Reaction of 1 with isocyanides, yielding silenes 2a–c
(Tip=2,4,6-iPr3C6H2; R=tBu (a), CMe2CH2tBu (b), 2,6-Me2C6H3 (c)).
In the case of silicon–silicon triple-bonded systems,
Sekiguchi et al. recently reported the reaction of a disilyne
with tert-butylisocyanide or 1,1,3,3-tetramethylbutylisocya-
reaction is akin to the formation of silaaziridines with
=
exocyclic C C bond from isocyanides and inherently polar
silenes,[15] but in marked contrast to the previously reported
nide
to
produce
disilyne–isocyanide
adducts
reactivity of symmetrical disilenes.[3b,7] Notably, the process of
=
[(R’NC)2RSiSiR] (R = SiiPr[CH(SiMe3)2]2, R’ = tBu or
CMe2CH2tBu) that are stable at ꢀ308C but decompose to
the corresponding 1,2-dicyano disilenes at room temper-
ature.[8] The heavier Group 14 element homologues, diger-
complete cleavage of the Si Si bond to form 2a–c is
reversible inasmuch as the disilene starting material 1 is
regenerated upon addition of large excess of BEt3 as
isocyanide scavenging reagent. Compounds 2a–c were char-
acterized by multinuclear NMR spectroscopy and, in the case
of 2a, also by X-ray crystallography.
[*] Dr. M. Majumdar, Dr. V. Huch, Dr. I. Bejan, Dr. A. Meltzer,
Prof. Dr. D. Scheschkewitz
Reaction of the unsymmetrical phenyl disilene 1 with tert-
butyl isocyanide in 1:1 ratio in benzene at room temperature
yields compound 2a almost quantitatively,[16] which was
crystallized from pentane as bright orange crystals in 85%
yield (m.p. 65–678C). The 29Si NMR spectrum has two peaks
at d = ꢀ4.4 and ꢀ80.2 ppm. The peak at d = ꢀ4.4 ppm is
attributed to the Si = C silicon atom, at much higher field than
in case of most previously reported silenes, suggesting an
Krupp-Lehrstuhl fꢀr Allgemeine und Anorganische Chemie
Universitꢁt des Saarlandes
66125 Saarbrꢀcken (Germany)
E-mail: scheschkewitz@mx.uni-saarland.de
Homepage: http://www.uni-saarland.de/fak8/scheschkewitz/
index.html
[**] Funding by the EPSRC (EP/H048804/1) and the Alfried Krupp
Foundation is gratefully acknowledged.
inversely polarized Si C bond.[17] The 13C NMR signal of the
Si C moiety, however, is observed at relatively high field as
=
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209281.
=
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well (d = 144.15 ppm), which is a likely consequence of the
incorporation into the three-membered ring. Significantly
=
high-field shifted signals for both atoms of the Si C unit had
also been observed in case of a stable 4-silatriafulvene.[18] UV/
Vis spectroscopy of 2a in hexane revealed the longest
wavelength
absorption
at
lmax = 397 nm
(e =
9620 Lmolꢀ1 cmꢀ1).
In analogous fashion, reaction with tert-octyl isocyanide
gives 2b, which was characterized by NMR spectroscopy, in
quantitative yield. Conversely, according to the 29Si NMR
spectrum, reaction of 1 with the aromatic xylyl isocyanide
yields 2c (d = 2.5 and ꢀ75.7 ppm) only as part of a mixture
with unidentified byproducts (see the Supporting Informa-
tion).[16] In an effort to shed light on the diverging reactivity of
Figure 1. Structure of 2a in the solid state (ellipsoids set at 30%
probability). Hydrogen atoms, solvent molecules, and disorder in
isopropyl groups omitted for clarity. Selected bond lengths [ꢂ]: Si2–
C01 1.735(2), Si1–C01 1.835(2), Si1–N1 1.735(2), C01–N1 1.440(3).
isocyanides towards symmetrically substituted disilenes,[7] we
[19]
=
treated Tip2Si SiTip2
with one equivalent of tBuNC in
[D6]benzene. No reaction was observed by NMR spectrosco-
py even after heating to 758C for 16 h. Apparently, the steric
congestion in the homoleptic disilene with four Tip substitu-
ents is too high to allow for the attack of the isocyanide. In
view of the aforementioned results of West et al. regarding
tetra(2,6-dimethylphenyl)disilene,[7] however, it is reasonable
to assume that the steric discrimination in phenyl disilene 1 is
in part responsible for the observed rearranged isocyanide
addition product 2a.
=
reflecting the larger covalent radius of silicon. The Si2 C01
bond distance is 1.735(2) ꢀ, which is at the short end of values
found for donor-substituted silenes.[20] Although the presence
of p-donating nitrogen atom at the carbon center is predicted
to elongate the Si C bond,[21] the electronegative aryl groups
=
at Si can be expected to overcompensate for this effect.[22]
Nonetheless, the sum of bond angles SSi2 = 355.85(18)8 is
indicative of slight pyramidalization, as had previously only
been demonstrated in case of a cyclic Brook-type silene.[23]
To check the suitability of this new reaction mode for the
preparation of extended conjugated systems containing more
=
=
than one Si C unit, we treated para-phenylene-bridged 3 with
The twist angle around Si2 C01 with t = 14.61(12)8 confirms
two equivalents of tBuNC. As anticipated, we obtained 4 with
the distortion from the classical double-bond geometry.
Despite repeated efforts, only relatively poor quality
crystals of one diastereomer of 4 could be obtained by
crystallization from benzene solution at room temperature.
Although the X-ray diffraction study leaves no doubt about
the constitution as the R,R- and S,S-racemic mixture
=
two silaaziridine units comprising exocyclic Si C bonds
(Scheme 3).[16] The 29Si NMR signals of the reaction mixture
(Figure 2),[16] the bond precision is limited. The Si2 C50 and
ꢀ
ꢀ
=
Si4 C103 distances of the two Si C bonds are 1.743(4) and
1.726(4) ꢀ, respectively, as obtained after the less-than-ideal
final refinement, and hence similar to the bond parameters
found in 2a.
Scheme 3. Reaction of para-phenylene-bridged tetrasiladiene 3 with
tert-butylisocyanide.
Silenes 2a–c are stable at room temperature in the
absence of air and moisture. Heating of 2a in C6D6 for 12
hours in a sealed NMR tube at 658C, however, led to the
at d = ꢀ1.3, ꢀ6.9, ꢀ80.3, and ꢀ80.6 ppm suggest the forma-
tion of a pair of diastereomers in an approximate 1.2:1 ratio.
13
=
The C NMR signals of the two Si C units appears at d =
144.27 and 143.78 ppm at very similar field as in the case of 2a
confirming the expected formation of 4. Crystallization at
room temperature from benzene led to the isolation of the
racemic mixture with R,R- and S,S-configuration in 56% yield
29
13
=
(d Si NMR ꢀ6.9 and ꢀ80.3 ppm; d C = 144.27 ppm (Si C)).
UV/Vis spectra in hexane shows an absorption at lmax
=
398 nm (e = 15,700 Lmolꢀ1 cmꢀ1) and a shoulder at 477 nm
(e = 5410 Lmolꢀ1 cmꢀ1).
The structure of 2a in the solid state was determined by X-
ray diffraction on single crystal obtained from pentane at
room temperature (Figure 1).[16] The exocyclic silicon atom,
Si2, is substituted with two Tip groups, while the silicon atom
of the silaaziridine ring, Si1, features one phenyl and one Tip
group as substituents. The endocyclic ring angles at Si1, C01,
and N1 are 47.48(9), 62.61(12), and 69.91(11)8, respectively,
Figure 2. Structure of 4 in the solid state (ellipsoids set at 30%
probability). Hydrogen atoms, solvent molecules, and isopropyl groups
of the Tip substituents omitted for clarity. Selected bond lengths [ꢂ]:
Si2–C50 1.743(4), Si1–C50 1.820(4), N1–C50 1.419(5), Si1–N1
1.707(3), Si1–C51 1.839(4), Si4–C103 1.726(4), Si3–C103 1.808(4),
N2–C103 1.425(5), Si3–N2 1.705(4), Si3–C54 1.856(4).
2
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=
regeneration of phenyl disilene 1 in low yield (18%), along
with the formation of unidentified products. The principal
reversibility of the reaction thus demonstrated, we speculated
that the side-products arise from the reaction of liberated
isocyanide with remaining 2a under these conditions. This
prompted us to investigate whether Lewis acids could trap
isocyanide and thus shift the equilibrium to the disilene side at
lower temperatures. Indeed, addition of excess triethylborane
as Lewis acid to 2a at room temperature leads to slow
conversion to phenyl disilene 1 in 91% NMR yield in 10 days.
The regeneration is accelerated upon heating overnight at
658C and hence phenyl disilene was cleanly obtained in 96%
yield (according to NMR spectroscopy). Appearance of peaks
at d = 7.51 and 13.27 ppm in the 11B NMR spectrum strongly
value of Si C at d = 158.8 ppm is, however, only a qualitative
match with the experimental value of d = 144.15 ppm.
The UV/Vis spectrum of 2aDip simulated by TD-DFT
shows the most intense absorption peak at l = 422 nm
(HOMO!LUMO+1, f = 0.1086) to correspond to the pSi
=
C
to p*Si transition, which is reasonably close to the exper-
=
C
imental value of 397 nm. The pronounced red-shift in
comparison to most silenes[20] is attributed to the antibonding
=
interaction of the Si C p-bond with the nitrogen p-type lone
pair, which appears to be more prominent in the HOMO than
in the LUMO+1 and therefore accounts for the decreased
energy gap between these orbitals (Figure 3). In marked
contrast to previously reported silenes,[20] TD-DFTreveals an
additional transition at longer wavelength (486 nm,
HOMO!LUMO, f = 0.0477). The LUMO is related to
LUMO+1 inasmuch that it also features a certain p*Si
character, but is mainly located on the phenyl group.
Furthermore, it shows a significant stabilizing contribution
by antibonding Walsh-type orbitals at the silaaziridine moiety
(Figure 4), suggesting an unusual case of the conjugative
interaction that is common in cyclopropane chemistry.[26]
ꢀ
suggests the formation of Et3B CNtBu and its decomposition
products.[24] Among silenes 2a–c, compound 2c is more prone
to the elimination of isocyanide. When 2c is heated overnight
at 658C, free phenyl disilene 1 is formed in 30% yield (NMR
spectroscopy; 2a: 18%; see the Supporting Information).[16]
The aniline-like conjugation of the nitrogen lone pair with the
aromatic xylyl substituent might be responsible for this
increased reversibility.
=
C
To elucidate the potential of silenes of type 2 to engage in
extended conjugation, DFT calculations were performed on
a truncated model system 2aDip (Dip = 2,6-iPr2C6H3 instead
of Tip in 2a) at the B3LYP/6-31G(d,p) level of theory.[25]
Figure 3 shows the optimized geometry, which reproduces the
Figure 4. Contour plots of the HOMOꢀ9 and the LUMO of 2aDip in
an orientation showing interaction of Walsh-type with p-type orbitals
(isovalue 0.04 au).
The shape of HOMOꢀ9 demonstrates that this interac-
tion is not restricted to the frontier orbitals. In the exper-
imental absorption spectrum, the peak corresponding to the
HOMO!LUMO transition apparently merges with the more
intense band at lmax = 397 nm, but at lower concentrations of
2a it can be perceived as a barely discernible shoulder at l
ꢂ 450 nm.
TD-DFT calculations on 4Ph (a truncated version of the
experimental structure of 4 with Ph instead of Tip) yield
numerous states with absorption wavelengths between l =
Figure 3. Relevant contour plots of 2aDip at an isovalue of 0.04 au.
425 and 386 nm that comprise contributions from the pSi
!
=
C
p*Si
transitions. More importantly, longer wavelength
=
C
experimental data of 2a reasonably well. Mulliken charge
distribution analysis of 2aDip shows charges of + 0.28 at the
absorptions at l = 557 and 538 nm (HOMO!LUMO and
HOMOꢀ1!LUMO, f = 0.1065 and 0.0266) are predicted
that correspond to transitions from the pSi bonds to the
LUMO, which is almost exclusively formed from quinoid p-
orbitals of the phenylene bridge and Walsh-type orbitals at
the silicon atoms of the silaaziridine rings (see the Supporting
Information). Compared to 2aDip, this constitutes a red-shift
of at least Dl = 52 nm and as such provides an illustrative
example of the conjugative interaction of Walsh-type orbitals
=
silicon and ꢀ0.10 at the carbon of the Si C moiety, indicating
=
C
slight reverse polarization as asserted on the basis of the solid-
state structure of 2a. The experimental 29Si NMR shifts of 2a
agree satisfactorily with those calculated for the model
compound 2aDip (exp. d = ꢀ4.4 and ꢀ80.2 ppm; d = 18.1
and ꢀ76.8 ppm at GIAO/6-311 + G(2df) for Si and 6-31G-
(d,p) for C, N and H). The calculated 13C NMR chemical shift
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via a bridging p-system. In the experimental case, the red-shift
of the longest-wavelength transition is slightly less pro-
nounced: phenylene-bridged 4 features a prominent shoulder
at l ꢂ 477 nm in accordance with the frequently observed
underestimation[14a,27] of the HOMO–LUMO gap in conju-
gated systems of silicon by the B3LYP functional.
calculated the 29Si NMR shifts of both isomers (syn-6aDip:
d = ꢀ41.4 and ꢀ56.6 ppm; anti-6aDip d = ꢀ46.4 and
ꢀ81.76 ppm at GIAO/6-311 + G(2df) for Si and 6-31G(d,p)
for C, N and H). On this basis, the thermally least stable
signals at ꢀ53.9 and ꢀ79.9 ppm could be assigned to anti-6a,
featuring a stereochemistry that is unsuitable for isomer-
ization to 2a. The two pairs of signals at d = ꢀ54.6 and
ꢀ61.9 ppm, ꢀ57.9 and ꢀ62.8 ppm are tentatively attributed to
two unspecified rotational isomers of syn-6a due to the
similarity of chemical shifts with the computed values for syn-
6aDip. Finally, as the interconversion between rotational
isomers becomes fast on the NMR timescale at 273 K, the
aforementioned two pairs of signals collapse and isomer-
ization sets in to 2a as the only detectable product at room
temperature.
In view of the fundamentally different behavior of
1 compared with the simple [2+1] cycloaddition of an
isocyanide to a disilene,[7] we attempted to detect possible
primary addition products at low temperature. The 13C NMR
spectrum of a 1:1 mixture of phenyl disilene 1 and tBuNC in
[D8]toluene at 213 K shows the appearance of at least three
distinct peaks around d = 200 ppm and thus much closer to
that reported for A than to the corresponding signal of 2a (A:
d = 214.0 ppm;[7] 2a: d = 144.15 ppm; see the Supporting
Information). These low-field signals suggested the initial
formation of isomers of disilaaziridines akin to A. Accord-
ingly, in the 29Si NMR, three pairs of signals appear that show
some further splitting, which is presumably due to the
presence of a large number of discernible rotamers at
213 K. Upon warming to 243 K, the 29Si spectrum simplifies
to one major product with sharp signals at d = ꢀ53.9 and
ꢀ79.9 ppm, and two minor products in an approximate 2:1
ratio at d = ꢀ54.6 and ꢀ61.9 ppm as well as d = ꢀ57.9 and
ꢀ62.8 ppm, respectively. The major product is the least stable
in view of the complete disappearance of the signals at the
low- and high-field ends of the 29Si NMR spectrum upon an
increase in the temperature to 273 K.
The formation of cyclic silenes from the reaction of
disilenides with vinyl bromides or carboxylic acid chlorides
was previously reported by us and Sekiguchiꢁs group in
a collaborative effort.[23,30] Independently, Sekiguchi et al.
have demonstrated the formation of silenes from a disilenide
and ketones in a Brook-type rearrangement.[31] In this work,
=
we presented the formation of an Si C moiety from a neutral
disilene by reaction with isocyanides. The complete, rever-
=
sible rupture of the Si Si moiety is instructive for the
development of new synthetic strategies towards double
=
bonds between heavier elements in general and Si Si bonds
in particular. The analogous clean conversion of conjugated
phenylene-bridged tetrasiladienes with tert-butyl isocyanide
reveals significant potential in polymer chemistry concerning
the use of silaaziridine repeat units with pronounced con-
jugative interactions of Walsh-type orbitals.
In accordance with theoretical predictions,[28,29] we assume
that a donor–acceptor complex such as 5a (or the related 5a’)
is probably not a detectable intermediate in the reaction of 2a
with tBuNC, but rather a transition state (Scheme 4). This is
Received: November 20, 2012
Published online: && &&, &&&&
Keywords: disilenes · insertion reactions · isocyanides ·
.
reversibility · silenes
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assignment of the experimentally observed species, we
4
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Supporting Information. CCDC 907680 (2a) and 907681 (4)
contain the supplementary crystallographic data for this paper.
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Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Silicon Double Bonds
There and back: An isocyanide-centered
silene–disilene reversibility was observed
for the insertion of isocyanides into
unsymmetrically substituted disilenes.
This reaction leads to the formation of
silenes at room temperature; the disilene
is regenerated in the presence of a Lewis
acid.
M. Majumdar, V. Huch, I. Bejan,
A. Meltzer,
D. Scheschkewitz*
&&&& — &&&&
=
Reversible, Complete Cleavage of Si Si
Double Bonds by Isocyanide Insertion
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!