Free radical reactions of stable silylenes and germylenes

Article abstract of DOI:10.1021/om049233f

Compound 1 reacts with TEMPO to yield the disiloxane 5 as the major product. Stable germylenes 9 and 10 both reacted with TEMPO to yield the 1:2 adducts. The reaction of 1 with di-tert-butyl peroxide gave the 1:2 adduct 7 in addition to 15% of an unexpect

Full text of DOI:10.1021/om049233f

6330
Organometallics 2004, 23, 6330-6332
Free Radical Reactions of Stable Silylenes and
Germylenes
Akinobu Naka,† Nicholas J. Hill,‡ and Robert West*,‡
Department of Chemistry and Bioscience, Kurashiki University of Science and the Arts,
Kurashiki, Okayama, 712-8505, Japan, and Organosilicon Research Center, University of
Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706
Received October 5, 2004
Summary: Compound 1 reacts with TEMPO to yield the
disiloxane 5 as the major product. Stable germylenes 9
and 10 both reacted with TEMPO to yield the 1:2
adducts. The reaction of 1 with di-tert-butyl peroxide
gave the 1:2 adduct 7 in addition to 15% of an unex-
pected trisilane, 8.
Although many reactions of stable silylenes have now
been described,1 little is yet known about their free-
radical chemistry. The EPR spectra of several adducts
of silylene 1 with small amounts of free radicals and
radical precursors have been studied (Scheme 1),2 and
Scheme 1
recently Kira and co-workers have reported the reaction
of a highly hindered silylene, 2,3 with 2,2,6,6-tetrameth-
ylpiperidine N-oxide (TEMPO), resulting in the disi-
ladioxetane 3.4,5 The formation of 3 was explained as
arising from dimerization of an intermediate silanone
(Scheme 2).
Figure 1. Molecular structure of 5. Thermal ellipsoids are
given at the 40% probability level, and hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles
(deg): Si(1)-O(1), 1.6242(4); Si(1)-O(2), 1.6540(9); Si(1)-
N(1), 1.7239(11); Si(1)-N(2), 1.7268(10); O(1)-Si(1)-O(2),
102.11(5); Si(1)-O(1)-Si(1A), 151.71(8); N(1)-Si(1)-N(2),
93.78(5); Si(1)-O(2)-N(3), 125.14(7).
Scheme 2
peroxide). The products are rather different from those
found by Kira.
Results and Discussion
Compound 1 reacts with 2 equiv of TEMPO to yield
the disiloxane 5 as the major (80% by NMR) product.
Even when 1 equiv of TEMPO was used, compound 5
was the major product. The crystal structure of 5 is
shown in Figure 1, and a possible route to 5 is shown
in Scheme 3. The free radical 4, postulated as an
intermediate in Scheme 3, has been identified by EPR
spectroscopy in the reaction of 1 with TEMPO.2
An unusual product is also found in the reaction of 1
with di-tert-butyl peroxide. The major product is the 1:2
adduct 7, but this is accompanied by 15% of the quite
unexpected trisilane 8. The crystal structure of 8 is
Here we report the reaction of stable silylene 1 with
TEMPO and with tert-butoxy radical (from di-tert-butyl
† Kurashiki University of Science and the Arts.
‡ University of Wisconsin.
(1) For reviews see: (a) Gaspar, P. P.; West, R. In The Chemistry of
Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley:
New York, 1999; Vol. 2, Part 3, pp 2463-2568. (b) Haaf, M.;
Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704. (c) Gehrhus,
B.; Lappert, M. F. J. Organomet. Chem. 2001, 617, 209.
(2) Tumanskii, B.; Pine, P.; Apeloig, Y.; Hill, N. J.; West, R. J. Am.
Chem. Soc. 2004, 126, 7786.
(4) Iwamoto, T.; Masuda, H.; Ishida, S.; Kabuto, C.; Kira, M. J. Am.
Chem. Soc. 2003, 125, 9300.
(5) Iwamoto, T.; Masuda, H.; Ishida, S.; Kabuto, C.; Kira, M. J.
Organomet. Chem. 2004, 689, 1337.
(3) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc.
1999, 121, 9722.
10.1021/om049233f CCC: $27.50 © 2004 American Chemical Society
Publication on Web 11/24/2004

Notes
Organometallics, Vol. 23, No. 26, 2004 6331
Scheme 3
Scheme 4
Scheme 5
3 shows the structure of 11). The average Ge-O bond
distance in 11 is 1.7923 Å, and the O(1)-Ge-O(2) angle
is 102.07(6)°; the equivalent values for 12 are 1.7992 Å
and 103.22(3)°. In comparison, the average Ge-O length
in 13 (the 1:2 adduct obtained by Kira and co-workers
shown in Figure 2. Plausible pathways leading to 7 and
8 are presented in Scheme 4.
The reaction of germylenes 9 and 10 with TEMPO
yielded the 1:2 adducts 11 and 12 (Scheme 5). No other
products were detected in the reaction mixture. Both
adducts were crystallographically characterized (Figure
Figure 2. Molecular structure of 8. Thermal ellipsoids are
given at the 40% probability level, and hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles
(deg): Si(1)-O(1), 1.6458(9); Si(1)-Si(2), 2.5051(4); Si(1)-
N(1), 1.7671(11); Si(1)-N(2), 1.7676(11); O(1)-Si(1)-Si(2),
103.42(3); Si(1)-Si(2)-Si(1A), 124.34(1); N(1)-Si(1)-N(2),
90.98(5).
Figure 3. Molecular structure of 11. Thermal ellipsoids
are given at the 40% probability level, and hydrogen
atoms are omitted for clarity. Selected bond distances (Å)
and angles (deg): Ge(1)-O(1), 1.7889(10); Ge(1)-O(2),
1.7958(12); Ge(1)-N(1), 1.8252(15); Ge(1)-N(2), 1.8276(14);
O(1)-Ge(1)-O(2), 102.07(6); N(1)-Ge(1)-N(2), 91.77(6);
Ge(1)-O(1)-N(3), 119.55(10); Ge(1)-O(2)-N(4), 119.48(9).

6332 Organometallics, Vol. 23, No. 26, 2004
Notes
analysis. 7: MS (M+) 342; 1H NMR (δ, C6D6) 1.35 (s, 18H, tBu),
1.36 (s, 18H, tBu), 5.73 (s, 2H, dCH); 13C NMR (δ, C6D6) 31.0,
32.1 (Me3C), 51.3 (N-CMe3), 73.8 (O-CMe3), 109.9 (dC); 29Si
NMR (δ, C6D6) -73.1; HRMS exact mass calcd for C18H38O2-
SiN2 m/z 342.2703, found 342.2697. Anal. Calcd for C18H38N2O2-
Si: C, 63.11; H, 11.18; N, 8.18. Found: C, 63.13; H, 10.92; N,
from reaction of the germanium analogue of 2 with
TEMPO) is 1.8253(3) Å and the O(1)-Ge-O(2) angle is
115.71(12)°.4
Compound 13 underwent thermolysis at 50 °C to
produce the 1,3-digermadioxetane, whose structure is
analogous to that of silicon compound 3. Our less
hindered compounds 11 and 12, however, do not un-
dergo this thermal rearrangement but instead persist
unchanged to ∼150 °C, where they decompose com-
pletely.6
1
8.42. 8: MS (M+) 734; H NMR (δ, C6D6) 1.41 (s, 18H, tBu),
1.43 (s, 18H, tBu), 1.50 (s, 18H, tBu), 1.52 (s, 18H, tBu), 5.77
(s, 2H, dCH), 5.80 (d, 2H, HCd, J ) 4 Hz), 5.86 (d, 2H, HCd,
J ) 4 Hz); 13C NMR (δ, C6D6) 32.0, 32.3, 32.8, 33.0 (Me3C),
51.8, 52.1, 52.4 (N-CMe3), 74.3 (O-CMe3), 112.1, 113.8, 115.5
(dC); 29Si NMR (δ, C6D6) -53.1, -20.1. Anal. Calcd for
C38H78N6O2Si3: C, 62.07; H, 10.69; N, 11.43. Found: C, 61.99;
H, 10.48; N, 11.59.
Experimental Section
All reactions and manipulations were conducted under an
argon atmosphere with standard Schlenk techniques. 1H, 13C,
and 29Si NMR spectra were recorded on a Varian INOVA-500
or UNITY-500 spectrometer working at 500 MHz for 1H,
125.75 MHz for 13C, and 99.38 MHz for 29Si in C6D6. The
chemical shifts are expressed in ppm using TMS as an internal
standard. Hexane and toluene used as a solvent were dried
over sodium/benzophenone under a nitrogen atmosphere and
distilled just before use.
Reaction of Silylene 1 with 2 Equiv of TEMPO. To a
mixture of 1 (0.200 g, 1.02 mmol) and TEMPO (0.325 g, 2.08
mmol) was added dry hexane (1 mL). After the mixture was
stirred for 15 min at 0 °C using an ice-water bath, the solvent
and unreacted TEMPO were removed in vacuo. The residue
was washed with 2 mL of dry cold MeOH, followed by
recrystallization from hexane to give pure 5 (0.285 g, 0.560
mmol, 55%): MS (M+) 720; 1H NMR (δ, C6D6) 1.23 (s, 18H,
tBu), 1.45-1.55 (m, 54H, tBu, CH3, CH2), 5.75 (s, 4H, dCH);
13C NMR (δ, C6D6) 17.3 (CH2), 20.8, 21.6 (CH3), 31.4, 31.8
(Me3C), 40.9, 41.1 (CH2), 51.5, 51.8 (CMe3), 61.0, 61.2 (NC),
110.2, 111.2 (dC); 29Si NMR (δ, C6D6) -64.3. Anal. Calcd for
C38H76N6O3Si2: C, 63.28; H, 10.62; N, 11.65. Found: C, 63.03;
H, 10.92; N, 11.42.
Reaction of Silylene 1 with tert-Butyl Peroxide. To a
mixture of silylene (0.880 g, 4.48 mmol) and tert-butyl peroxide
(0.83 mL, 4.52 mmol) was added dry toluene (5 mL). After the
mixture was heated to reflux for 30 min, the solvent was
removed in vacuo. The NMR spectrum showed one major
product, 7, in addition to a small amount of minor product 8.
The oil mixture was vacuum-distilled at 130 °C and 0.1 Torr,
and the colorless oil 7 was isolated. Crystals of 8 were obtained
from a solution in hexane after several days. The ratio of 7 to
8 was determined to be 5.7:1 by 1H NMR spectrometric
Reaction of Germylene 9 with 2 Equiv of TEMPO. A
mixture of 9 (0.300 g, 1.25 mmol) and TEMPO (0.398 g, 2.55
mmol) was dissolved in dry hexane (1 mL). After the mixture
was stirred for 30 min, the solvent and unreacted TEMPO
were removed in vacuo. Recrystallization from hexane gave
1
pure 11 (0.600 g, 0.560 mmol, 87%): MS (M+) 554 (M+); H
NMR (δ, C6D6) 1.22 (s, 12H, CH3), 1.30-1.43 (m, 12H, CH2),
1.46 (s, 12H, CH3), 1.50 (s, 18H, tBu), 5.79 (s, 4H, dCH); 13C
NMR (δ, C6D6) 17.4 (CH2), 20.5 (CH3), 31.4 (Me3C), 33.0 (CH3),
41.1 (CH2), 52.5 (CMe3), 61.1 (NC), 110.4 (dC). Anal. Calcd
for C28H56N4O2Ge: C, 60.77; H, 10.20; N, 10.12. Found: C,
60.46; H, 10.55; N, 9.89.
Reaction of Germylene 10 with 2 Equiv of TEMPO.
To a mixture of 10 (0.550 g, 2.26 mmol) and TEMPO (0.708 g,
4.53 mmol) was added dry hexane (3 mL). After the mixture
was stirred for 30 min at room temperature, the solvent and
unreacted TEMPO were removed in vacuo. Recrystallization
from hexane gave pure 12 (0.995 g, 1.79 mmol, 79%): MS (M+)
556 (M+); 1H NMR (δ, C6D6) 1.24 (s, 12H, CH3), 1.40-1.59 (m,
12H, CH2), 1.42 (s, 18H, tBu), 1.46 (s, 12H, Me), 2.92 (s, 4H,
CH2); 13C NMR (δ, C6D6) 17.4 (CH2), 20.4 (CH3), 30.2 (Me3C),
33.7 (CH3), 41.0 (CH2), 43.0 (CH2), 51.8 (CMe3), 60.7 (NC).
Anal. Calcd for C28H58N4O2Ge: C, 60.55; H, 10.53; N, 10.09.
Found: C, 60.31; H, 10.89; N, 10.48.
Acknowledgment. This research was supported by
a grant from the National Science Foundation. We
thank Dr. Ilia A. Guzei (UW-Madison) for crystal-
lographic advice.
Supporting Information Available: Tables detailing the
X-ray structure determination, bond lengths, and bond angles
of 5, 8, 11, and 12. This material is available free of charge
via the Internet at http://pubs.acs.org.
(6) The silylene analogous to the saturated germylene 10 also reacts
with TEMPO, to give a complex reaction mixture which has so far
proved intractable.
OM049233F