R. S. Ghadwal, H. W. Roesky, R. A. Mata, D. Stalke et al.
culations were carried out with the Molpro2010.1 pack-
age.[18d]
been observed. Further details about the calculations can be
found in the Supporting Information.
The computed values are DG1 =À18.5, DG2 =À20.9, and
DG3 =À51.1 kcalmolÀ1, which show that the reactions that
lead to compounds 3 and 4 are almost isoenergetic. Relative
to the reactions shown in Scheme 2, we find that compound
5 should be the thermodynamic product. The difference of
30.2 kcalmolÀ1 (at 298.15 K) in the computed free energies
is quite significant. Given the experimental observation that
4 is formed preferably, it should correspond to the kinetic
product with a lower activation barrier. The formation of 5
in solution has been observed when the temperature in-
creases, which again suggests that the latter corresponds to
the thermodynamic product.
The difficulty found in crystallizing compound 5 can also
be explained on the basis of our computational results. The
crystal structure of 4 reveals close intermolecular contacts
between 1,3-diisopropyl groups. This contact is allowed be-
cause the 4-dimethylaminophenyl group stretches along the
plane of the five-membered ring, thereby allowing for an
unhindered contact. The proton migration in 5, however,
generates an sp3 carbon at this connection, thus leading to a
bend in the 4-dimethylaminophenyl moiety (Figure 3). The
Conclusion
In summary, we report for the first time that the reaction of
stable N-heterocyclic silylenes LSiCl and L’Si with thioke-
tone 4,4’-bis(dimethylamino)thiobenzophenone leads to
CSiS three- and C3SiS five-membered rings. The CSiS three-
membered ring is stable at room temperature under an inert
atmosphere. The reaction of L’Si results in temperature-de-
pendent product formation. At low temperature, the dear-
omatized C3SiS five-membered ring is formed, whereas at
room temperature an additional migration of a proton from
the aromatic to the nonaromatic carbon occurred to yield a
mixture of compounds of 4 and 5. The experimental results
are supported by DFT calculations.
Experimental Section
All manipulations were carried out in an inert atmosphere of dinitrogen
using standard Schlenk techniques and in a dinitrogen-filled glovebox.
Solvents were purified using an MBRAUN SPS-800 solvent purification
system. All chemicals were purchased from Aldrich and used without fur-
ther purification. LSiCl[3b] and L’Si[3c] were prepared as reported in the
literature. 1H and 29Si NMR spectra were recorded using
a Bruker
Avance DRX 500 spectrometer with C6D6 as solvent. Chemical shifts (d)
are given relative to SiMe4. EIMS spectra were obtained using a Finnigan
MAT 8230 instrument. Elemental analyses were performed by the Insti-
tut fꢀr Anorganische Chemie, Universitꢁt Gçttingen.
Synthesis of 3: Toluene (60 mL) was added to a 100 mL Schlenk flask
that contained LSiCl (1; 0.29 g, 0.98 mmol) and 4,4’-bis(dimethylamino)-
thiobenzophenone (0.29 g, 1.02 mmol). The reaction mixture was stirred
at room temperature for 12 h. The solvent was reduced under vacuum to
about 20 mL and stored at À328C in a freezer for 5 d to obtain single
crystals of 3 (0.42 g, 74%). For elemental analysis, 3·0.25toluene was
treated under vacuum for six hours to remove the toluene molecules.
1H NMR (500 MHz, C6D6, 258C): d=1.12 (s, 18H; C
ACTHUNGTRNEUNG(CH3)3), 2.58 (s,
12H; NCH3), 6.71–6.93 ppm (m; ArH); 29Si{1H} NMR (99.36 MHz, C6D6,
258C): d=À104.42 ppm; EIMS: m/z: 544.0 [M+ÀCl]; elemental analysis
calcd (%) for C32H43ClN4SSi (579.31): C 66.34, H 7.48, N 9.67; found: C
66.24, H 7.45, N 9.58.
Figure 3. Superimposed structures of 4 (gray) and 5 (black). Hydrogen
atoms have been removed for clarity. Upon proton migration, the 4-di-
methylaminophenyl moiety bends towards the top of the plane formed
by the five-membered ring.
Synthesis of 4: A solution of L’Si (2; 0.24 g, 0.54 mmol) in n-hexane
(20 mL) at À788C was added to a cooled solution of 4,4’-bis(dimethyl-
amino)thiobenzophenone (0.16 g, 0.56 mmol) in n-hexane (20 mL) at
À788C. After the addition, the reaction mixture was allowed to warm
slowly to 08C and stirring was continued for a further 6 h at 08C. The sol-
vent was reduced and stored in a freezer at À328C for 12 h to obtain
yellow single crystals of 4 (0.26 g, 66%). For elemental analysis, 4·hexane
was treated under vacuum for 6 h to remove the hexane molecules.
latter builds an intermolecular contact with one of the diiso-
propyl groups, effectively shielding it from intermolecular
interactions. We have also computed the interaction energy
between two diisopropyl groups at the DF-SCS-MP2/cc-
pVTZ level of theory on the basis of the crystal-structure
coordinates. Each contact should stabilize the crystal by
about 4.6 kcalmolÀ1.
Having considered products 4 and 5 for the reaction of 2,
one could also question the possibility of forming a three-
membered ring, in analogy to compound 3. One indeed ob-
tains a stable minimum with a free reaction energy of
À12.5 kcalmolÀ1. This is somewhat higher than DG2 =
À20.9 kcalmolÀ1 and explains why such a byproduct has not
1H NMR (500 MHz, C6D6, 258C): d=1.28 (d, J=7 Hz, 6H; CH
1.31 (d, J=7 Hz, 6H; CH(CH3)2), 1.39 (d, J=7 Hz, 3H; CH(CH3)2), 1.44
(d, J=7 Hz, 6H; CH(CH3)2), 1.45 (s, 3H; NCCH3), 1.57 (d, J=7 Hz, 3H;
CH(CH3)2), 2.35 (s, 9H; NCH3), 2.38 (s, 3H; NCH3), 3.49 (s, 1H;
NCCH2), 3.54 (m, 1H; CH(CH3)2), 3.60 (m, 1H; CH(CH3)2), 3.85 (m,
2H; CH(CH3)2), 4.04 (s, 1H; NCCH2), 5.52 (s, 1H; g-CH), 5.74 (m, 1H;
ACHTUNGTRENNUNG(CH3)2),
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
CH), 6.25 (m, 2H; CH), 6.52 (m, 1H; CH), 6.99–7.01 (m; ArH), 7.10–
7.22 ppm (m; ArH); 29Si{1H} NMR (99.36 MHz, C6D6, 258C): d=
2.88 ppm; EIMS: m/z: 728.4 [M+]; elemental analysis calcd (%) for
C46H60N4SSi (729.15): C 75.77, H 8.29, N 7.68; found: C 75.85, H 8.18, N
7.64.
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4
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!
Azhakar, Ramachandran
