Silylene and germylene intermediates in the reactions of silole and germole dianions with N,N′-Di-tert-butylethylenediimine

Article abstract of DOI:10.1002/ejic.200800083

Spiro-diazasilole (germole) adducts were observed in reactions of dilithium salts of silole and germole dianions with N,N′-di-tert- butylethylenediimine in THF at room temperature. A proposed mechanism of the reaction includes metallation of N,N′-di-tert-butylethylenediimine and the formation of an intermediate silylene or germylene. This reaction mechanism was supported by experimental data and theoretical calculations. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800083

FULL PAPER
DOI: 10.1002/ejic.200800083
Silylene and Germylene Intermediates in the Reactions of Silole and Germole
Dianions with N,NЈ-Di-tert-butylethylenediimine
Irina S. Toulokhonova,[a] Vitaly I. Timokhin,[a] David N. Bunck,[a] Ilia Guzei,[a]
Robert West,*[a] and Thomas Müller*[b]
Keywords: Silole dianion / Germole dianion / N,NЈ-Di-tert-butylethylenediimine / Silylene / Germylene / Metallation
Spiro-diazasilole (germole) adducts were observed in reac-
tions of dilithium salts of silole and germole dianions with
N,NЈ-di-tert-butylethylenediimine in THF at room tempera-
ture. A proposed mechanism of the reaction includes metal-
lation of N,NЈ-di-tert-butylethylenediimine and the forma-
tion of an intermediate silylene or germylene. This reaction
mechanism was supported by experimental data and theoret-
ical calculations.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Here we report that the reaction of the dilithiosilole 1a
and the dilithiogermole 1b with N,NЈ-di-tert-butylethylene-
diimine (2) in THF at room temperature also leads to the
formation of novel spiro adducts 3a and 3b (Scheme 2).
Introduction
The silole[1] and germole[2] (metallole) dianions I (Fig-
ure 1) are interesting compounds both from theoretical and
practical points of view due to their aromaticity[3,4] and
unique reactivity. Metallole dianions act as strong reducing
agents[5] and nucleophiles.[1]
Scheme 2. Reaction of dilithio compounds 1a,b with N,NЈ-di-tert-
butylethylenediimine (2) (a, E = Si; b, E = Ge).
Figure 1. Metallole dianions I (E = Si, Ge).
1
These products were characterized by 29Si, 13C, and H
The tetraphenylsilole dianion is a precursor of a stable
diradical[6] and a silene.[7] Recently, we found that silole di-
anions undergo novel oxidative cyclization reactions with
1,4-butadienes with formation of a spiro adduct and release
of free lithium metal[8] (Scheme 1).
NMR spectroscopy, and the formation of 3a,b was con-
firmed by synthesis via an independent route. The reaction
of the dichlorosilole 4a or the dichlorogermole 4b with the
dilithium salt of the diimine 5 in THF at room temperature
also yielded 3a,b (Scheme 3).
Scheme 1. Oxidative cyclization of 1,3-dimethyl-1,4-butadiene with
1,1-dilithio-2,3,4,5-tetraphenylsilole.
Scheme 3. Synthesis of 3a,b from the dichlorosilole 4a and the
dichlorogermole 4b with N,NЈ-dilithio-N,NЈ-di-tert-butylethene (5)
(a, E = Si; b, E = Ge).
[a] Department of Chemistry, University of Wisconsin-Madison,
1101 University Ave.,
Madison, WI 53706, USA
[b] Institut für Reine und Angewandte Chemie der Carl von Ossi-
etzky Universität Oldenburg,
Although the cycloaddition reactions in Schemes 1 and
2 are formally similar, they appear to proceed by quite dif-
ferent mechanistic pathways. The reaction of Scheme 1 is
believed to take place through nucleophilic addition of the
silole to the diene, assisted by complexing of the diene to
Carl von Ossietzky Str. 9-11, 26129 Oldenburg, Federal Repub-
lic of Germany
Fax: +49-441-798 3352
E-mail: thomas.mueller@uni-oldenburg.de
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Silylene and Germylene Intermediates
Scheme 4. Proposed mechanism of the reaction of the dianions 1a,b with the diimine 2 (a, E = Si; b, E = Ge).
lithium.[8] In contrast, the cycloaddition of Scheme 2 ap-
pears to involve lithium transfer to the diimine to give 5,
generating the silylene or germylene 6a,b, which then adds
to the excess diimine (Scheme 4).[9]
The dilithiodiamine 5 was identified in the reaction mix-
7
ture by Li NMR spectroscopy, as well as by derivatization
with D2O to give the dideuteriodiamine 7 (Scheme 5).
Scheme 6.
Scheme 5. Treatment of 5 with D2O.
7Li NMR for the reaction mixtures of the dianions 1a
and 1b with the diimine 2 was similar to that obtained for
the dilithiodiamine salt 5, prepared independently from the
Scheme 7.
diimine 2 and metallic lithium,[10] and different from that
for the silole dianion 1a.
According to our model computations the initial step of
This mechanistic proposal is consistent with the results
of density functional computations at the B3LYP/6-
311+G(d,p) level (see Computational Methods for details).
Note that the electron affinity EA which is computed for
1,4-diazabutadiene is significant [EA = –0.24 eV at B3LYP/
6-311+G(d,p)]. This suggests that for the reactivity of di-
anions 1a,b towards diazabutadienes, electron transfer is a
plausible reaction mode. For the calculation of the reaction
path we used as model compounds the unsubstituted η5,η5-
dilithium complex of the silole or germole dianion, 8a,b,[3,8]
and the methylated diazabutadiene 9, to reduce the com-
plexity of the theoretical problem. As solvation is important
for the reactivity and stability of organolithium compounds
in solution[11] we used dimethyl ether as a model for the
THF used in the synthesis, to complete the coordination
sphere of the lithium ions in 8a,b. The computed structures
of reactants, products, and intermediates are given in the
case of the silicon compounds in Figure 3 and the complete
data set can be found in the Supporting Information. The
reaction course is summarized in Schemes 6 and 7 and the
computed reaction profiles for the complete transformation
8a,b/9Ǟ14a,b are shown in Figure 4.
the reaction cascade is the reduction of the diazabutadiene
9 by the lithiated metalloles 8a,b which yields the cyclic 1-
metallacyclopenta-2,4-dienylidenes 10a,b.[9] Under the reac-
tion conditions carbene analogues such as 10a,b (Figure 2)
will form Lewis acid-base complexes either with the solvent
THF or with the reactant diazabutadiene 2. In the case of
the silicon compound 10a, the computations predict that
the complexation with diazabutadiene 9 to form compound
11a is favored compared to the reaction with dimethyl ether
by 30.6 kJmol–1 and complex 11a is stabilized compared to
its two independent components by 96.3 kJmol–1 (Fig-
ure 3). Encounter complex 11a undergoes an intramolecu-
lar [2+1] addition giving the silaaziridine 13a as the product
(Scheme 7). In a subsequent reaction step the silaaziridine
13a is transformed by facile nucleophilic ring extension into
Figure 2. Carbene analogues 10a,b.
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R. West, T. Müller et al.
FULL PAPER
Figure 3. Calculated molecular structures for compounds 10a–14a and for the transition states TS1a and TS2a [at B3LYP/6-311+G(d,p)].
Selected bond lengths in pm; color code: C white, large; Si black; N grey; H, white, small.
Figure 4. (left and right). Calculated reaction coordinates for the reactions of the dianions 8a,b with the diazabutadiene 9. The energetic
relation between 8a,b/9 and 11a,b is calculated from Scheme 6 using the absolute energies of reactants and products.
the spirocyclic compound 14a. Both barriers, that for the tion and activation energies for the individual reaction steps
[2+1] addition 11aǞ13a and that for the consecutive ring differ significantly (see Figure 4, right). As a result of the
extension reaction 13aǞ14a are relatively small generally higher relative stability of dicoordinated states in
(45.7 kJmol–1 and 56.0 kJmol–1, see Figure 4, left). This germanium chemistry the dilithiogermole 8b as well as the
suggests that both intermediates, the encounter complex germylene complex 11b are lower in energy compared to
11a and the silaaziridine 13a, would be difficult to detect in the tetravalent compounds 13b and 14b. Consequently, the
the experiments which were conducted at room tempera- computed overall reaction energy is much smaller in the
ture. The overall reaction 8a + 2 9Ǟ14a + 12a is strongly germanium case (39.5 kJmol–1) than in the silicon case and
exothermic (185.3 kJmol–1). Therefore, the computation for also the barrier predicted for the cycloaddition to form ger-
the model compounds 8a and 9 suggests that the reaction maaziridine 13b is significantly higher (87.9 kJmol–1 vs.
of equimolar amounts of 1a and 2 will give exclusively spi- 45.7 kJmol–1).
rocycle 3a in a theoretical yield of 50% yield, in qualitative
A crucial point in the computations is a good prediction
agreement with the experimental results.
of the structure of the dilithiated diazabutadiene 12. Al-
A qualitatively similar reaction course is predicted by the though it is only a byproduct, 12 influences the thermody-
computations for the dilithiogermole 8b, although the reac- namics of the initial electron-transfer process significantly.
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Silylene and Germylene Intermediates
The solution structures of organolithium compounds in ge-
neral, and lithiated imines in particular, are complex and in
many cases not known.[11,12] For the lithium and potassium
salt of the radical monoanion of 2, ESR and structural
studies indicate metalladiazacyclopentene structures.[13] The
molecular structure of a related dilithiated diazabutadiene
featured a s-cis-configured diazadiene dianion which coor-
dinates to each of the two lithium ions in a chelating fash-
ion.[14] The molecular structure of dilithiodiamine 5 as ob-
tained from a X-ray structure analysis, is that of an unsym-
metrical dimer with four lithium atoms in two different
binding modes.[10] 7Li NMR spectroscopy did not provide
significant information on the structure of 5 in solution.
Therefore several possible structures 12a–d of dilithiated di-
azabutadiene were optimized and the computed energies
were compared. This study included the bis-chelate complex
12a, the (E)-configured complex 12b, a mixed chelate/η4
complex 12c and a dimeric structure 12d. The structures
12a and 12d are reminiscent of solid states structures of
strongly related compounds,[10,14] while 12b and 12c
emerged as possible structures from test calculations. The
Figure 5. Molecular structure of 1,4-di-tert-butyl-6,7,8,9-tet-
raphenyl-1,4-diaza-5-silaspiro[4,4]nona-6,8-diene-2-one (15) shown
with 50% probability thermal ellipsoids.
Table 1. Crystal data and structure refinement for 15.
optimized structures of 12a–d are given in the Supporting Empirical formula
C38H40N2OSi
568.81
Formula weight
Information. The results of DFT computations for the gas
Temperature
phase and in solution (modelled by a PCM computation
Wavelength
100(2) K
0.71073 Å
triclinic
using as solvent THF) identified uniformly the bis-chelate
Crystal system
¯
complex 12a as the energetically preferred structure, and
the energy of this structure was used to compute the heat
of reaction for Scheme 6. However, in view of the crude
model and the complex problem arising from aggregation
of the lithiated species in solution, the thermodynamics of
the initial electron transfer step should be regarded only as
an estimate.
Space group
Unit cell dimensions
P1
a = 9.832(3) Å
b = 10.127(4) Å
c = 17.651(6) Å
α = 76.411(6)°
β = 78.750(7)°
γ = 73.232(6)°
1620.4(10) Å3
2
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Consistent with the proposed mechanism involving
metallation of the diimine 2 is the fact that a polar solvent
is required. When THF as the solvent was replaced by tolu-
ene, no reaction was observed. We did not observe products
of addition of silole or germole dianions to the double car-
bon–nitrogen bond (C=N) of diimine 2, as is observed in
the reaction of methyllithium with N,NЈ-di-tert-butyl-
1.166 Mg/m3
0.104 mm–1
608
Crystal size
0.49ϫ0.46ϫ0.40 mm3
Theta range for data collection 2.14 to 26.40°
Index ranges
–12ՅhՅ12
–12ՅkՅ12
–20ՅlՅ21
11097
6143
[R(int) = 0.0864]
92.4%
multi-scan with SADABS
0.9595 and 0.9507
full-matrix least-squares on F2
6143/0/385
ethylenediamine.[15,16] The addition of the dilithio com- Reflections collected
pounds 1a and 1b to 2 is sterically hindered and instead
Independent reflections
metallation of the diimine 2 takes place. Addition of more
than one equivalent of the diimine 2 in the reaction with
Completeness to θ = 26.40°
Absorption correction
the dilithiosilole 1a resulted in decreased yields of spiro-
compound 3a. This is most probably due to increased
formation of the radical anion [22Li]–· by the reaction of
the initially formed dilithio compound 5 with diimine
2[13c] and subsequent follow-up reactions of the radical
anion.
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
1.027
R1 = 0.0506, wR2 = 0.1351
R1 = 0.0672, wR2 = 0.1436
0.663 and –0.233 eÅ–3
Largest diff. peak and hole
Attempts to obtain crystals of 3a suitable for X-ray crys-
tal analysis were unsuccessful. However, upon column
chromatography 15, a product of oxidation of 3a, was sepa-
rated and characterized. The crystal structure of 15 is
The germanium–nitrogen bond in 3b (E = Ge) is less
shown in Figure 5 and the crystallographic data in Table 1. stable than a silicon–nitrogen bond and rapidly decomposes
Evidently 15 arises via C-oxidation of spirocyclo-enedi- in the presence of air to give the cyclo-trigermoletrioxane
1
amine 3 (Scheme 8). Similar oxidation of enamines, en- 16 (Scheme 8), identified by X-ray crystallography and H,
hanced in presence of molecular sieves was described pre- and 13C NMR analysis. The data for 16 are consistent with
viously by Blau et al.[17–19]
those reported in the literature.[20]
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R. West, T. Müller et al.
FULL PAPER
15 was obtained as result of oxidation of 3 on silica gel, in 50%
yield after column chromatography [eluent: hexane/Et2O (1:1)]. 15:
1H NMR (CDCl3): δ = 1.10 (s, 9 H, tBu), 1.39 (s, 9 H, tBu), 3.71
(s, 2 H), 6.76–6.78 (m, 4 H, Ar), 7.01–7.10 (m, 16 H, Ar) ppm. 13
C
NMR (CDCl3): δ = 177.18 (C=O), 154.61, 140.12, 138.83, 137.62,
136.10, 129.47, 129.23, 128.41, 128.19, 127.16, 126.84 (m, Ar),
54.95, 51.72, 50.67, (N–CMe3), 28.40, 27.99, [C(CH3)3] ppm. 29Si
NMR (CDCl3): δ = –7.80 ppm. C34H40N2Si (504.79): calcd. C
80.90, H 7.99; found C 80.64, H 8.32. Yellow crystals of 15 suitable
for X-ray analysis were obtained by crystallization from hexane.
7Li NMR for the reaction mixtures of dilithiometalloles 1a and 1b
with diimine 2 (broad multiple peaks at –1.9 to –0.8 ) were identical
to that for the dilithiodiamine 5, prepared independently from di-
imine 2 and metallic lithium,[10] and different from that for the
silole dianion 1a (–7.3 ppm, broad intense peak and broad shoulder
with low intensity at –1.1 to –3.2 ppm).
Treatment of the Reaction Mixture with D2O: The reaction mixture
was cooled to 0 °C and D2O (2.2 mmol, 0.044 g) in 10 mL of THF
was added to the mixture. After raising the temperature of the mix-
ture to 20 °C the reaction mixture was dried with MgSO4, the sol-
vent was removed, and the reaction mixture was analyzed. 7: 2H
NMR (C6D6): δ = 5.39 (D–N) ppm. Similar chemical shift at
5.4 ppm was observed when the dilithiodiamine 5, which was syn-
thesized from the diimine 2 and metallic lithium in THF at room
temperature, was treated with deuterated water.
Scheme 8. Oxidation of 3a,b.
Computational Details
All calculations were done using the Gaussian03 pro-
gram.[21] The structures of all compounds were optimized
at the hybrid density functional B3LYP level of theory[22] in
connection with 6-311+G(d,p) basis set. Every stationary
point was identified either as intermediate or transition
state (TS) by a subsequent frequency calculation. Intrinsic
reaction coordinate (IRC) computations were used to con-
nect the TS with the appropriate minima. Solvent effects
were included by using the PCM model.[23] All calculated
data are presented in the Supporting Information.
Reaction of the 1,1-Dichlorosilole 4a with N,NЈ-Dilithio-N,NЈ-di-
tert-butylethenediamine (5): A solution of N,NЈ-dilithio-N,NЈ-di-
tert-butylethenediamine 5 (1.1 mmol) in 10 mL of THF was added
to 1,1-dichlorosilole 4 at room temperature and the dark red reac-
tion mixture was stirred during 3.5 h. Analysis by 1H NMR showed
3a (63%).
Reaction of Dilithiogermole 1b with N,NЈ-Di-tert-butylethylenedi-
imine (2): This reaction was carried out similarly to the procedure
for dilithiosilole 1a and diimine 2. The product 3b (spectroscopic
yield 45%) was determined by 1H NMR, and 13C NMR spec-
troscopy. 1H NMR (C6D6): δ = 1.27 (s, 18 H, tBu), 6.04 [s, 2 H,
N–C(H)=C], 6.87–7.47 (m, 20 H, Ar) ppm. 13C NMR (C6D6): δ =
148.68–122.99 (m, Ar), 112.62 (N–C=C–N), 52.67 (N–CMe3),
30.48 [C(CH3)3] ppm.
Experimental Section
General: All procedures with air- and moisture-sensitive com-
pounds were carried out using a Schlenk line under nitrogen and
argon. Solvents were distilled from sodium benzophenone ketyl.
NMR spectroscopic data were recorded with a Varian UNITY-500
spectrometer at 499.89 MHz for 1H, 125.71 MHz for 13C,
99.31 MHz for 29Si, 194.28 MHz for 7Li NMR and 76.71 MHz for
2H NMR spectroscopy.
Reaction of 1,1-Dichlorogermole 4b with N,NЈ-Dilithio-N,NЈ-di-tert-
butylethenediamine (5): This was conducted and analyzed as for the
reaction of 4a with 5. By 1H NMR, 58% of 3b was found. Product
16 was obtained after attempts to crystallize 3b from hexane/Et2O
(10:4) mixture.
1,1-Dilithio-2,3,4,5-tetraphenylsilole (1a) and 1,1-Dichloro-2,3,4,5-
tetraphenylsilole (4a): These compounds were synthesized as de-
scribed in the literature.[3] 1,1-Dilithio-2,3,4,5-tetraphenylgermole
(1b) and 1,1-dichloro-2,3,4,5-tetraphenylgermole (4b) were ob-
tained as described by Boudjouk et al.[2] N,NЈ-Di-tert-butyl-
ethylenediimine (2) and N,NЈ-dilithio-N,NЈ-di-tert-butylethene-
diamine (5) were prepared as described previously.[10]
CCDC-673921 contains the supplementary crystallographic data
for compound 15. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see also the footnote on the first page of
this article): Calculated structures in Cartesian coordinates and ab-
solute energies of all computed compounds. Selected details of the
single-crystal structure analysis of compound 15.
Reaction of Dilithiosilole 1a with N,NЈ-Di-tert-butylethylenediimine
(2): The dilithiosilole 1a (1.1 mmol, 0.44 g) in 10 mL of THF was
added to 2 (1.1 mmol, 0.18 g) at room temperature. The reaction
mixture has a dark red color; after 1 h solvent was evaporated and
the product 3a (spectroscopic yield 55%) was characterized by
NMR spectroscopy. 1H NMR (C6D5CD3): δ = 1.26 (s, 18 H, tBu),
5.98 [s, 2 H, N–C(H)=C], 6.85–6.98 (m, 20 H, Ar) ppm. 13C NMR
(C6D5CD3): δ = 153.22, 140.12, 138.63, 137.38, 137.33, 130.2–126.5
(m, Ar), 113.88 (N–C=C–N, J1 = 9.1, J2 = 179.8 Hz), 51.89 (N–
Acknowledgments
Research at the University of Wisconsin was supported by the
National Science Foundation under grant no. CHE-0451536. The
CMe3), 30.05 [C(CH3)3] ppm. 29Si NMR (C6D5CD3): δ = –16.20 Cluster for Scientific Computation (CSC) at the Carl von Ossietzky
(JSi–CH = 6 Hz) ppm.
University Oldenburg is thanked for computer time.
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Silylene and Germylene Intermediates
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Received: January 24, 2008
Published Online: April 4, 2008
Eur. J. Inorg. Chem. 2008, 2344–2349
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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