Oxidation of methyl(phenyl)silylene - Synthesis of a dioxasilirane

Article abstract of DOI:10.1021/ja000192y

The oxidation of methyl(phenyl)silylene, 3f, with molecular oxygen is investigated using matrix isolation spectroscopy in combination with DFT calculations. The UV irradiation (λ > 305 nm) of phenylsilyldiazomethane, 8, matrix-isolated in argon at 10 K, p

Full text of DOI:10.1021/ja000192y

J. Am. Chem. Soc. 2000, 122, 6727-6734
6727
Oxidation of Methyl(phenyl)silylenesSynthesis of a Dioxasilirane
Holger Bornemann and Wolfram Sander*
Contribution from the Lehrstuhl fu¨r Organische Chemie II der Ruhr-UniVersita¨t,
D-44780 Bochum, Germany
ReceiVed January 18, 2000. ReVised Manuscript ReceiVed April 28, 2000
Abstract: The oxidation of methyl(phenyl)silylene, 3f, with molecular oxygen is investigated using matrix
isolation spectroscopy in combination with DFT calculations. The UV irradiation (λ > 305 nm) of
phenylsilyldiazomethane, 8, matrix-isolated in argon at 10 K, produces silylene 3f in very high yield. In O2
doped argon matrixes, the thermal reaction of 3f with O2 could be directly monitored by IR spectroscopy. The
only reaction product is dioxasilirane 5f, presumably formed via cyclization of silanone O-oxide 4f. DFT
calculations reveal that the activation barrier for the 4f f 5f cyclization is less than 1 kcal/mol, too small to
allow the matrix isolation of 4f. Dioxasilirane 5f is photolabile, and irradiation with blue light (λ > 420 nm)
results in a [1,2]-phenyl migration to give silaester 7f. The product of the methyl migration, silaester 13, is
formed as a minor constituent.
Introduction
Scheme 1
A general route for the matrix isolation and spectroscopic
characterization of carbonyl O-oxides 1sthe “Criegee inter-
mediates”sis the reaction of carbenes with molecular oxygen.1-6
Carbonyl oxides are photolabile, and visible light irradiation
results in cyclization to the corresponding dioxiranes 2 or in
the loss of the terminal oxygen atom and formation of carbonyl
compounds (Scheme 1). This reaction sequence can be used
for the preparative-scale synthesis of carbonyl oxides or stable
dioxiranes.7-9
Scheme 2
While many examples of carbene oxidations have been
reported, only three papers on the reaction of silylenes (si-
lanediyls) 3 with molecular oxygen have been published.10-12
Ando et al. investigated the oxidation of dimesitylsilylene 3b
in solid oxygen at 16 K. During UV photolysis of the precursor
of 3b (the corresponding trisilane), a new IR absorption at 1084
* Corresponding author. FAX: (+49) 234 3214353. Email: sander@
xenon.orch.ruhr-uni-bochum.de
(1) Lorberth, J.; Krapf, H.; Noeth, H. Chem. Ber. 1967, 100, 3511-
3519.
(2) Sander, W. Angew. Chem. 1986, 98, 255-256; Sander, W. Angew.
Chem., Int. Ed. Engl. 25, 255-257.
cm-1 was observed, which was claimed to be the O-O
stretching vibration of dimesitylsilanone O-oxide 4b (Scheme
2).10,13 The assignment of the 1084 cm-1 vibration was based
on comparison with RHF/6-31G(d) calculations for the triplet
state of the parent silanone oxide 4a (no minimum for singlet
4a was located at this level of theory).10 The direct thermal
(3) Ganzer, G. A.; Sheridan, R. S.; Liu, M. T. H. J. Am. Chem. Soc.
1986, 108, 1517-1520.
(4) Sander, W. Angew. Chem. 1990, 102, 362-372; Sander, W. Angew.
Chem., Int. Ed. Engl. 1981, 29, 344-54.
(5) Sander, W. W.; Patyk, A.; Bucher, G. J. Mol. Struct. 1990, 222, 21-
31.
(6) Sander, W.; Bucher, G.; Wierlacher, S. Chem. ReV. 1993, 93, 1583-
1621.
(7) Kirschfeld, A.; Muthusamy, S.; Sander, W. Angew. Chem. 1994, 106,
2261-2263; Kirschfeld, A.; Muthusamy, S.; Sander, W. Angew. Chem.,
Int. Ed. Engl. 1985, 33, 2212.
3
reaction of 3b with O2 was not found, and other IR bands of
4b were not reported. In later communications, a singlet ground
state of 4a and a small activation barrier for the 4a f 5a
rearrangement was predicted on the basis of MP2 calcula-
tions.11,13
In our laboratory, the oxidation of dimethyl-, difluoro-, and
dichlorosilylene 3c-e was investigated in O2 doped argon
matrixes (Scheme 2).11,12,14,15 Dimethylsilylene 3c was found
to react thermally in 0.5% O2 doped argon matrixes at
temperatures as low as 40 K.11 This indicates that the activation
barrier for this oxidation must be very small or zero. The only
(8) Sander, W.; Kirschfeld, A.; Kappert, W.; Muthusamy, S.; Kiselewsky,
M. J. Am. Chem. Soc. 1996, 118, 6508-6509.
(9) Sander, W.; Schroeder, K.; Muthusamy, S.; Kirschfeld, A.; Kappert,
W.; Boese, R.; Kraka, E.; Sosa, C.; Cremer, D. J. Am. Chem. Soc. 1997,
119, 7265-7270.
(10) Akasaka, T.; Nagase, S.; Yabe, A.; Ando, W. J. Am. Chem. Soc.
1988, 110, 6270-6272.
(11) Patyk, A.; Sander, W.; Gauss, J.; Cremer, D. Angew. Chem. 1989,
101, 920-922; Patyk, A.; Sander, W.; Gauss, J.; Cremer, D. Angew. Chem.,
Int. Ed. Engl. 1980, 28, 898-900.
(12) Patyk, A.; Sander, W.; Gauss, J.; Cremer, D. Chem. Ber. 1990, 123,
89-90.
(13) Nagase, S.; Kudo, T.; Akasaka, T.; Ando, W. Chem. Phys. Lett.
1989, 163, 23-28.
10.1021/ja000192y CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/30/2000

6728 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Bornemann and Sander
product is dimethyldioxasilirane, 5c, while dimethylsilanone
O-oxide, 4c, the proposed precursor of 5c, is not observed. This
is in contrast to carbene oxidations, where carbonyl oxides 1
are the primary thermal products and dioxiranes 2 are formed
on secondary photolysis of 1.4 Under the assumption that the
thermal reaction of silylenes 3 with molecular oxygen leads to
silanone oxides 4 as the primary products, it follows that the
barrier of the 4 f 5 rearrangement must be considerably smaller
than that of the 1 f 2 rearrangement. This is in agreement with
MP2 calculations, which predict a barrier of 22.8 kcal/mol for
the carbonyl oxide, but only 6.5 kcal/mol for the silanone oxide
rearrangement (parent compounds, R ) R′ ) H).11
Despite the high affinity of silicon toward oxygen, the
halogenated silylenes 3d and e can be matrix-isolated in neat
solid oxygen at 10 K, and even annealing of the oxygen matrix
at 40 K does not result in any thermal reaction.12 Irradiation
with UV or visible light is required to induce the reaction, and
the only products formed are the dioxasiliranes 5d and e,
respectively.
Scheme 3
silylenes are of similar energy30 and, in several cases, can be
interconverted photochemically. Thus, visible irradiation of
dimethylsilylene results in a [1,2]-H shift to 2-silapropene, which
on UV irradiation rearranges back to the silylene.20,23,31
Visible-light irradiation (λ > 400 nm) of phenylsilyldiazo-
methane, 8, matrix-isolated in argon at 10 K, results in the
complete disappearance of all IR absorptions assigned to 8.
Newly formed bands in the range 2192-2147 cm-1 indicate
Si-H bonds (SiH stretching vibrations), and a weak absorption
at 1640 is typical of a diazirine (NN stretching vibration).
Several minutes of UV irradiation (λ > 305 nm) of the product
mixture results in the partial recovery of 8, which strongly
indicates that phenylsilyldiazirine, 9, is one of the products of
the 400-nm photolysis of 8 (Scheme 3). This type of photo-
chemical interconversion of diazo compounds with diazirines
has been frequently observed with silyldiazomethanes.25-28
A second constituent of the 400-nm irradiation of 8 is
1-phenylsilene, 11. This silene was independently synthesized
by Maier et al. via a thermal retro Diels-Alder reaction in the
gas phase with subsequent trapping of the products in solid
argon.33 Comparison of our IR data with that reported by Maier
et al.33b allows attribution of absorptions at 2192, 1433, 1122,
965, 961, 852, and 738 cm-1 to silene 11. The assignment of
IR absorptions to diazirine 9 and silene 11 is in qualitative
agreement with calculations at the B3LYP/6-311++G(d,p) level
of theory.
The photolysis of 8 was also followed by UV spectroscopy,
which shows the disappearance of the 220- and 228-nm
absorptions of diazo precursor 8 and simultaneously the
formation of a broad absorption with λmax ) 308 nm assigned
to the mixture of 9 and 11 (Figure 1). For silene 11, an
absorption maximum at 300 nm was reported,33a again in
agreement with our finding.
Prolonged UV irradiation leads to the complete decomposition
of 8, 9, and 11 and formation of a compound with strong IR
absorptions at 1429, 1092, 735, and 695 cm-1. In the UV-vis
spectrum, an intense absorption with a maximum at 260 nm
and a broad absorption with a maximum at 482 nm are growing
in. The visible absorption results in an orange-red coloring of
the matrix. Such broad absorptions in the visible range are
characteristic of silylenes, while alkyl- and monoaryl-substituted
silenes absorb around 250 and 300 nm, respectively.31-34 DFT
calculations of the IR spectrum of methyl(phenyl)silylene, 3f,
The limited number of experimental studies on the oxygen-
ation of silylenes is mainly due to the lack of suitable
precursors.16 The photolysis of matrix-isolated trisilanes10,17-19
produces silylenes in close proximity to disilenes or other
products of the precursor decomposition rather than matrix-
isolated silylenes. Gas-phase thermolysis of disilanes and other
thermal precursors20-22 requires very high temperatures, and the
photolysis of diazidosilanes23,24 short-wavelength UV irradiation.
In all of these cases, the yields of silylenes are rather poor. Here
we describe a route to the matrix isolation of a silylene in high
yield which allows study of the oxygenation reaction in detail.
Results and Discussion
Photochemistry of Phenylsilyldiazomethane, 8. The pho-
tolysis of matrix-isolated silyldiazomethanes is a convenient way
for the synthesis of silenes.25-29 Silenes and their isomeric
(14) Sander, W.; Trommer, M.; Patyk, A. Oxidation of silenes and
silylenes: matrix isolation of unusual silicon species. Organosilicon
Chemistry III, [Muench. Silicontage], 3rd ed. 1996, 86-94; Auner, N., Weis,
J., Eds.; Wiley-VCH Verlag GmbH: Weinheim, Germany, 1998.
CODEN: 65UHAS.
(15) Sander, W.; Kirschfeld, A. Matrix-Isolation of Strained Three-
Membered Ring Systems; Halton, B., Ed.; JAI Press: London, U.K., 1995;
Vol. 4, Chapter 2; pp 1-80.
(16) Nefedov, O. M.; Egorov, M. P.; Ioffe, A. I.; Menchikov, L. G.;
Zuev, P. S.; Minkin, V. I.; Simkin, B. Y.; Glukhovtsev, M. N. Pure Appl.
Chem. 1992, 64, 266-314.
(17) Vancik, H.; Raabe, G.; Michalczyk, M. J.; West, R.; Michl, J. J.
Am. Chem. Soc. 1985, 107, 4097-4098.
(18) Bott, S. G.; Marshall, P.; Wagenseller, P. E.; Wang, Y.; Conlin, R.
T. J. Organomet. Chem. 1995, 499, 11-16.
(19) Miyazawa, T.; Koshihara, S.-y.; Liu, C.; Sakurai, H.; Kira, M. J.
Am. Chem. Soc. 1999, 121, 3651-3656.
(20) Reisenauer, H. P.; Mihm, G.; Maier, G. Angew. Chem. 1982, 94,
864-865; Reisenauer, H. P.; Mihm, G.; Maier, G. Angew. Chem., Int. Ed.
Engl. 1973, 21.
(21) Maier, G.; Reisenauer, H. P.; Pacl, H. C2H2Si isomers: Generation
by pulsed flash pyrolysis and matrix-spectroscopic identification/Organo-
silicon Chem. II, [Muench. Silicontage]; Auner, N., Weis, J., Eds.; VCH:
Weinheim, 1996; Vol. II.; pp 303-307.
(22) Maier, G.; Reisenauer, H. P.; Meudt, A. Eur. J. Org. Chem. 1998,
1285-1290.
(28) Trommer, M.; Sander, W.; Patyk, A. J. Am. Chem. Soc. 1993, 115,
11775-11783.
(23) Raabe, G.; Vancik, H.; West, R.; Michl, J. J. Am. Chem. Soc. 1986,
108, 671-677.
(29) Trommer, M.; Sander, W.; Ottosson, C.-H.; Cremer, D. Angew.
Chem., Int. Ed. Engl. 1995, 34, 929-931.
(24) Welsh, K. M.; Michl, J.; West, R. J. Am. Chem. Soc. 1988, 110,
6689-6696.
(30) Boatz, J. A.; Gordon, M. S. J. Phys. Chem. 1990, 94, 7331-7333.
(31) Maier, G.; Mihm, G.; Reisenauer, H. P.; Littmann, D. Chem. Ber.
1984, 117, 2369-2381.
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Barton, T. J.; Tumey, M. L. J. Am. Chem. Soc. 1976, 98, 7844-7846.
(26) Chedekel, M. R.; Skoglund, M.; Kreeger, R. L.; Shechter, H. J.
Am. Chem. Soc. 1976, 98, 7846-7848.
(32) West, R. Pure Appl. Chem. 1984, 56, 163-173.
(33) (a) Maier, G.; Mihm, G.; Reisenauer, H. P. Chem. Ber. 1984, 117,
2351-2368. (b) Maier, G.; Reisenauer, H. P., private communication.
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Oxidation of Methyl(phenyl)silylene
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6729
Table 1. IR Spectroscopic Data of Methyl(phenyl)silylene, 3f
argon, 10 K
no ν, cm-1 Ia
B3LYP/6-311++G(2d,p)
Ia assignmentb
sym
ν, cm-1
42 3085.7
41 3075.1
37 3055.3
36 3023.6
35 2964.9
34 1585.5
32 1496.2
31 1429.5
29 1378.4
28 1332.0
27 1260.5
26 1217.7
25 1187.8
24 1159.3
2
15
4
6
6
33
3
48
1
5
4
3184
3174
3095
3054
2995
1620
1515
1462
1439
1357
1299
1257
1212
1185
1105
1095
1016
792
26 A′
34 A′
17 A′
C-H str
C-H str
CH3 str
11 A′′ CH3 str
6
A′
32 A′
A′
19 A′
CH3 str
ring str
1
skel. ring, in pl. C-H
skel. ring, in pl. C-H
HCH bend
skel. ring, in pl. C-H
skel. ring, in pl. C-H
HCH bend
skel. ring, in pl. C-H
skel. ring, in pl. C-H
skel. ring, in pl. C-H
skel. ring, in pl. C-H
ring breathing
2
8
7
A′
A′
A′
16
10
2
26 A′
11 A′
Figure 1. UV-vis spectra showing the photochemistry of phenyl-
silyldiazomethane 8. (a) Spectrum of 8, matrix-isolated in argon at 10
K. (b) Same spectrum after 400-nm irradiation. The main products are
now diazirine 9 and silene 11. (c) Same spectrum after 305-nm
irradiation. The major constituent is now silylene 3f.
1
A′
23 1091.7 100
22 1065.1
100 A′
18 A′
6
19
13
66
61
11
7
3
3
5
20
15
14
13
12
11
10
953.6
777.7
735.2
695.5
686.8
666.1
657.9
649.7
575.4
8
A′
32 A′
CH3 rock
754
709
700
641
38 A′′ C-H wag
47 A′′ C-H wag
17 A′
33 A′
28 A′
Si-CH3 str
Si-CH3 str
629
9
575
3
A′′ C-H wag
a Relative intensity based on the strongest peak. b The assignment
of experimental and calculated IR absorption is based on peak positions
and peak intensities and for the weak absorptions is only tentative.
Figure 2. Bottom: IR spectrum (in absorbance) of methyl(phenyl)-
silylene 3f, matrix-isolated in argon at 10 K. The silylene was generated
by 305-nm irradiation of 8. Top: IR spectrum of 3f, calculated at the
B3LYP/6-311++G(2d,p) level of theory.
demonstrate that this silylene is the major photoproduct (Figure
2, Table 1). West reported a UV-vis maximum of 3f in a
hydrocarbon glass at 490 nm,32 close to the 482-nm absorption
in argon.
The structures and relative energies of phenylsilylcarbene 10
and several of its rearranged products were calculated with the
B3LYP method and a large triple-ú basis set (6-311++G(d,p),
all relative energies include the zero-point energy ZPE) (Figure
4). At this level of theory, the singlet-triplet splitting ∆EST of
methylene is reproduced within 3 kcal/mol,35 and for the singlet
closed-shell molecules involved in this study the error should
be even smaller. As expected, for carbene 10 a triplet ground
state is calculated. The singlet close shell structure S-10 could
not be located; instead, a transition-state TS-10 (20.2 kcal/mol
above T-10), connecting silenes 11 and 12, was found (Scheme
4). This transition state is 68.3 and 65.4 kcal/mol higher in
energy than 11 and 12, respectively. Thus, the photolysis of 8
on the singlet potential energy surface produces a structure close
to TS-10. At this point, neither the intersystem crossing to triplet
carbene T-10 nor the [1,2]-Ph migration to 12 can compete with
the [1,2]-H migration to silene 11, which is the precursor of
silylene 3f.
Figure 3. Center: Difference IR spectrum showing the photochemistry
(420-nm irradiation) of methyl(phenyl)dioxirane 5f. Bands pointing
downward are disappearing during the irradiation and assigned to 5f;
bands pointing upward are appearing and mainly assigned to silaester
7f. Bottom: IR spectrum of 5f, calculated at the B3LYP/6-311++G-
(d,p) level of theory. Top: IR spectrum of 7f, calculated at the B3LYP/
6-311++G(d,p) level of theory.
The barrier for the [1,2]-H migration of 11 to give 3f is 37.9
kcal/mol, and the reaction energy is almost zero. Although 11
is formed with a huge excess energy of 68.3 kcal/mol, the
activation barrier should be large enough to efficiently prevent
the 11 f 3f rearrangement. Most likely this rearrangement is a
photochemical secondary reaction during the UV photolysis of
8/9.
Oxygenation of Methyl(phenyl)silylene, 3f. Irradiation
(λ > 305 nm) of diazo compound 8 in a 0.5% O2 doped argon
matrix at 10 K produces mainly silylene 3f and small amounts
of oxidation products. As long as the matrix is kept at 10 K, no
(35) Schreiner, P. R.; Karney, W. L.; Schleyer, P. v. R.; Borden, W. T.;
Hamilton, T. P.; Schaefer, H. F. I. J. Org. Chem. 1996, 61, 7030-7039.

6730 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Bornemann and Sander
Figure 4. Geometries and some structural data of 3f and several of its isomers calculated at the B3LYP/6-311++G(d,p) level of theory.
Scheme 4
Scheme 5
further reaction is observed; however, annealing the matrix at
38 K for 1 h results in the almost complete reaction of 8 and
formation of a new compound, A, with strong IR absorptions
at 1136, 1002, and 793 cm-1 (Figure 3). Compound A is
photolabile, and on irradiation with λ > 420 nm readily
rearranges to a compound B. This reaction sequence, thermal
reaction with molecular oxygen on annealing in an O2 doped
argon matrix and photochemical rearrangement of the primary
thermal product, was also observed with dimethylsilylene 3c.11
It is thus tempting to assign A to methyl(phenyl)dioxasilirane,
5f, and B to either methyl(phenoxy)silanone, 7f, or methoxy-
(phenyl)silanone, 13 (Scheme 5). Again, the comparison of the
experimental and DFT-calculated IR data allows identification
of the species formed in the matrix and confirmation of this
assignment (Tables 2 and 3).
The three-membered ring in 5f shows two characteristic
vibrations: the strong Si-O stretching mode at 1002 cm-1
(calculated, 1005 cm-1) and the weak O-O stretching mode at
577 cm-1 (calculated, 608 cm-1). In 5c, these two absorptions
are found at 1013 cm-1 (strongest peak in the spectrum) and
554 cm-1 (weak),11 which is an additional confirmation of the
assignment of A to 5f. On 18O2 labeling, the Si-O stretching
vibration is red-shifted by 25.3 cm-1 (calculated, 28 cm-1) and
the O-O stretching vibration by 23.5 cm-1 (calculated, 21
cm-1). Other products, and in particular the silanone oxide 4f,
are not found as thermal products of the oxygenation of silylene
3f.
The major constituent of the photolysis products of dioxa-
silirane 5f was identified as silaester 7f. The intense absorption
at 963 cm-1 (calculated: 953 cm-1) shows an isotopic shift of
-24.6 cm-1 (calculated: -25 cm-1) in 18O2-7f and is assigned
to the Si-O(Ph) stretching vibration. Two other strong absorp-
tions at 1250 and 1244 cm-1 with pronounced 18O-isotopic shifts
of -26.2 and -44.2 cm-1 are assigned to the C-O and the
SidO stretching vibration of 7f, respectively (Table 3). Again,
band positions, intensities, and isotopic shifts are in excellent
agreement with the results of the DFT calculations. In dimeth-
ylsilanone, the SidO stretching vibration is found at 1210
cm-1 36,37
of 7f.
,
and in dichlorosilanone at 1240 cm-1 38
, close to that
In addition to the absorptions of 7f, several other medium-
intensity bands (1484, 1229, 1121, 689, and 528 cm-1) are found
in the IR spectrum of the photolysis products of 5f, which

Oxidation of Methyl(phenyl)silylene
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6731
Table 2. IR Spectroscopic Data for Methyl(phenyl)dioxasilirane, 5f
argon, 10 K
B3LYP/6-311++G(d,p)
no
ν, cm-1
Ia
νi/νb
ν, cm-1
Ia
νi/νb
sym
assignmentc
48
47
46
44
43
42
40
37
36
35
34
33
32
31
29
27
25
24
3086.3
3075.2
3071.6
3058.7
3024.7
3015.1
1596.8
1435.0
1432.5
1429.5
1336.9
1307.5
1260.3
1218.0
1135.6
1033.7
1013.1
1002.1
998.2
2
1
3
2
1
1.000
3191
3183
3175
3156
3122
3099
1629
1461
1456
1455
1357
1311
1301
1211
1139
1049
1013
1005
11
13
5
3
1
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
0.999
0.999
1.000
0.972
A′
A′
A′
A′
A′
A′′
A′
A′
A′
A′′
A′
A′
A′
A′
A′
A′
A′
A′
C-H str
C-H str
1.000
1.000
C-H str
C-H str
CH3 str
3
1.000
1.000
1.000
1.000
1.000
1.000
1.000
0.998
2
CH3 str
ring str
skel. ring, in pl. C-H
HCH bend
17
30
2
7
3
5
10
4
92
4
1
10
13
3
6
4
10
20
2
97
1
3
100
HCH bend
skel. ring, in pl. C-H
HCH bend, ring str
HCH bend
in pl. C-H bend
skel. ring, in pl. C-H
skel. ring, in pl. C-H
ring breathing
SiO str
0.999
0.999
81
28
100
4
0.975
20
19
792.7
783.4
0.999
0.994
820
800
72
15
0.998
0.998
A′
A′′
CH3 rock
CH2 twist
777.0
737.5
729.9
717.6
694.5
678.8
576.7
456.3
4
18
17
16
15
14
12
11
29
17
3
39
8
28
11
0.999
1.004
0.952
1.000
0.991
0.959
0.999
750
747
731
710
695
608
466
40
38
3
31
8
25
9
1.000
0.996
0.956
1.000
0.986
0.965
1.000
A′′
A′
A′′
A′′
A′
A′
A′′
C-H wag
SiC str, CH3 def
SiO def, CH3 def
C-H wag
O2 str, CH3 def
C-H wag
a Relative intensity based on the strongest peak. b Ratio of the frequencies of the 18O vs 16O isotopomers. c The assignment of experimental and
calculated IR absorption is based on peak positions and peak intensities and for the weak absorptions is only tentative.
Table 3. IR Spectroscopic Data for Methyl(phenoxy)Silanone, 7f
argon, 10 K B3LYP/6-311++G(d,p)
no ν, cm-1 Ia νi/νb ν, cm-1 Ia νi/νb
with calculated IR spectra, this minor product is tentatively
assigned to methoxy(phenyl)silanone, 13, while several other
possible products (e. g., methyl(phenyl)silanone) were ruled out.
The formation of the silaesters 7f and 13 requires the cleavage
of the O-O bond in 5f and migration of the phenyl or methyl
group, respectively. This rearrangement might proceed via a
dioxydiradical as an intermediate or, if the diradical is not a
minimum on the singlet potential energy surface, be a concerted
reaction. The preference of the phenyl migration compared with
the methyl migration reflects different migratory aptitudes of
phenyl and methyl; however, our experiments do not reveal
details of the mechanism of the rearrangement.
The relative stability of the products of the oxygenation of
3f was calculated using DFT methods (B3LYP/6-311++G(d,p)
+ ZPE) (Scheme 6). At this level of theory, the experimental
IR39,40 and NMR41 spectra of carbonyl oxides are nicely
reproduced; we therefore used B3LYP theory for the calculation
of ground-state properties of 4f and its isomers.
The reaction of silylene 3f with 3O2 to give the syn silanone
oxide 4f in its singlet ground state is calculated to be exothermic
by 27.8 kcal/mol. The syn conformer of 4f is clearly a minimum
on the potential energy surface, although the activation barrier
for the cyclization to 5f is less than 1 kcal/mol. The anti isomer
is even less stable, and the very shallow minimum could only
be located with the large triple-ú, but not with a double-ú basis
set. The singlet-triplet splitting of syn 4f is calculated to 8.2
kcal/mol, and thus, the spin-allowed reaction of singlet silylene
3f with 3O2 to give triplet silanone oxide T-4f is still exothermic
assignmentc
47 3061.8
46 3038.0
40 1606.3 13 0.998 1634
39 1598.3 3 1.000 1628
38 1498.6 51 0.999 1520
36 1454
32 1259.1 24 0.998 1299
31 1250.0 89 0.979 1263
30 1243.7 30 0.964 1246
29 1170.1
27 1075.8
26 1031.2 14 0.999 1044
22 962.8 100 0.975
21 907.5
19 817.6
18
2 1.000 3190
1 1.000 3183
1
1.000 C-H str
1.000 C-H str
1.000 ring str
1.000 ring str
2
9
1
17 0.999 skel. ring, in pl. C-H
1
4
1.000 HCH bend
0.999 HCH bend
100 0.982 C-O str, SidO str
1
2
1
1
0.974 C-O str, SidO str
0.999 in pl. C-H bend
1.000 skel. ring, in pl. C-H
0.999 skel. ring, in pl. C-H
7 0.999 1187
6 1.000 1095
953
911
834
782
768
746
688
480
414
29 0.974 Si-O str, ring breath.
2 1.000
2 1.000
1
3
2
0.998 C-H wag
0.998 CH3 rock
0.999 CH3 def
17 756.4 30 0.999
16 739.9
15
11 473.0 14 0.978
9
16 0.997 C-H wag
1 1.001
1
2
5
1
0.996 C-O def, C-H wag
1.000 C-H wag
0.983
0.976
a Relative intensity based on the strongest peak. b Ratio of the
frequencies of the 18O vs 16O isotopomers. c The assignment of
experimental and calculated IR absorption is based on peak positions
and peak intensities and for the weak absorptions is only tentative.
indicates the formation of a second, minor photoproduct. Since
only the strongest peaks of this minor product are observed,
the identification is difficult. On the basis of the comparison
(39) Cremer, D.; Gauss, J.; Kraka, E.; Stanton, J. F.; Bartlett, R. J. Chem.
Phys. Lett. 1993, 209, 547-556.
(40) Cremer, D.; Kraka, E.; Szalay, P. G. Chem. Phys. Lett. 1998, 292,
(36) Khabashesku, V. N.; Kerzina, Z. A.; Mal'tsev, A. K.; Nefedov, O.
M. IzV. Akad. Nauk Ser. Khim. 1986, 1215.
(37) Khabashesku, V. N.; Kerzina, Z. A.; Baskir, E. G.; Maltsev, A. K.;
Nefedov, O. M. J. Organomet. Chem. 1988, 347, 277-293.
(38) Schnoeckel, H. Z. Anorg. Allg. Chem. 1980, 460, 37-50.
97-109.
(41) Kraka, E.; Sosa, C. P.; Cremer, D. Chem. Phys. Lett. 1996, 260,
43-50.

6732 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Bornemann and Sander
Figure 5. Geometries and some structural data of 5f and several of its isomers calculated at the B3LYP/6-311++G(d,p) level of theory.
Scheme 6
distance in formaldehyde oxide 1 (R ) R′ ) H) is calculated
to 1.257 Å, the O-O distance to 1.352 Å, and the C-O-O
bond angle to 119.1°. At 1.407 Å, the O-O bond distance in
the parent silanone oxide 4a is considerably longer, and the
Si-O-O bond angle of 115.9° is smaller than the C-O-O
bond angle in formaldehyde oxide. The substituents in syn-4f
result in a further lengthening of the O-O bond distance to
1.445 Å (Figure 5). The Si-O distance in 4f is calculated to be
1.598 Å. While the parent silanone oxide 4a, like many silyl
radicals, is pyramidalized at the silicon atom, the coordination
at the silicon atom of 4f is trigonal planar and the molecule
shows Cs symmetry. Obviously, the energy required for the
planarization is more than compensated for by the formation
of a delocalized π system between the phenyl group and the
Si-O-O moiety.
Similar to the carbonyl oxides 1, diradicaloid (I) and ionic
(II and III) resonance structures of silanone oxides 4 can be
formulated. Resonance structure II seems to be most appealing
since the charges are located at the electropositive silicon and
electronegative terminal oxygen atom, respectively. The NPA
atomic charges of 4f, calculated at the B3LYP/6-31 g(d,p) level
of theory, reveal a different picture. With -0.61 au, the central
oxygen atom in 4f is more negatively charged than the terminal
oxygen atom (-0.47)sin contradiction to resonance structures
II and IIIsand the negative charges at the carbon atoms attached
to silicon are even higher (phenyl ipso carbon, -0.62; methyl,
-1.25).
by 19.6 kcal/mol. The cyclization of S-4f to dioxasilirane 5f is
strongly exothermic by another 49.6 kcal/mol.
These results indicate that silanone oxide 4f is a very labile
species lying in a shallow minimum. Even at low temperature
the rapid rearrangement to dioxasilirane 5f is expected, in
particular if the excess energy released on the primary reaction
3
of 3f with O2 is considered.
Although the photochemical rearrangement of 5f produces
silaester 7f with an unfavorable SidO bond, this rearrangement
is calculated to be exothermic by 61.7 kcal/mol. The energy
required to form a SidO bond is more than compensated for
by the loss of the strain energy in the three-membered ring,
substitution of a Si-C bond by a Si-O bond, and formation of
an additional C-O bond. The migration of a methyl group and
formation of silaester 13 is only slightly less exothermic (-59.4
kcal/mol), and even the migration of both the methyl and the
phenyl substituent in 5f, producing silylene 14, is strongly
exothermic (Scheme 6).
Electronic Structure of Silanone Oxide 4f and Dioxa-
silirane 5f. At the B3LYP/cc-pVTZ level of theory, the C-O

Oxidation of Methyl(phenyl)silylene
J. Am. Chem. Soc., Vol. 122, No. 28, 2000 6733
In dioxasilirane 5f, the oxygen atoms are charged by -0.61
and the silicon atom by +2.1 au. The charges at the carbon
atoms attached to silicon are very similar to that in 4f. From
that we conclude that the charge distribution in 4f reflects the
differences in the electronegativities of the atoms rather than a
special weight of resonance structure II or III. The small
singlet-triplet splitting, on the other hand, indicates a large
diradicaloid character of silanone O-oxides 4.
The transition state for the 4f f 5f rearrangement is only
0.8 kcal/mol above 4f, and its structure is very close to that of
4f. The C(Ph)-Si-O-O dihedral angle in the TS is now 12.6°
(0° in 4f), and the Si-O-O bond angle reduced from 119.5°
in 4f to 102.9° (Figure 5). The O-O bond is slightly longer
and the Si-O bond slightly shorter than in 4f. The out-of-plane
twisting of the Si-O-O group results in a less efficient
conjugation with the phenyl π system and buildup of charge at
the terminal oxygen atom.
At 1.59 Å, the calculated O-O bond distance in dioxasilirane
5f is significantly longer than the O-O distances in the parent
dioxirane 2 (R ) R′ ) H, R(O-O) ) 1.516 Å)42 and
dimesityldioxirane (R ) R′ ) mesityl, R(O-O) ) 1.503 Å).9
The Si-O bond lengths of 1.85-1.86 Å are within the normal
range of Si-O single bonds. As expected, the O-Si-O bond
angle of 56.8° is smaller than the O-C-O angles of dioxiranes
(66° in the parent system and 60° in the dimesityl derivative).
theory with 6-31G(d,p) and 6-311++G(d,p) basis sets. For the
triplet species, UB3LYP was used with the same basis set.
Materials and General Methods. 1H and 13C NMR spectra
were taken at 200.1 and 50.3 MHz, respectively (Bruker AM
400), in CDCl3 as the solvent. Mass spectra (EI, 70 eV) were
taken on a Varian MAT CH7 instrument. All reactions involving
moisture-sensitive silicon reactants were performed under an
atmosphere of dry argon. Precursors for matrix experiments were
purified by preparative-scale GLC using a Siemens RGC 202
gas chromatograph.
Phenylsilyl Trifluoromethanesulfonate. The triflate was
prepared in a way analogous to a procedure by Bassindale and
Stout44 for the synthesis of dimethylsilyl triflate. Trifluoro-
methanesulfonic acid (36 mmol, 3.2 mL) was added dropwise
to neat diphenylsilane (36 mmol, 6.6 g) at room temperature.
After complete addition, benzene (formed as coproduct) was
slowly removed under vacuum. The yield of phenylsilyl triflate
1
was quantitatively obtained. H NMR (CDCl3, 200 MHz): δ
7.32-7.67 (m, 5 H, Ph), 5.25 (s, 2 H, SiH2). 13C NMR (CDCl3,
50 MHz): δ 135.79, 133.32, 129.21, 128.74, 126.52, 121.90,
1J(C,F) 314 Hz.
Phenylsilyldiazomethane (8). 8 was synthesized in a way
analogous to a procedure by Martin45 for the synthesis of
(trimethylsilyl)diazomethane. Freshly prepared phenylsilyl tri-
fluoromethanesulfonate (36 mmol, 9.3 g) was added dropwise
to equimolar quantities of diazomethane (36 mmol, 1.5 g in
100 mL of diethyl ether) and ethyldiisopropylamine (40 mmol,
5.2 g) at -65 °C. After complete addition, the mixture was
slowly warmed to 0 °C and the white precipitate was removed
by rapid filtration through a frit. The yellow solution was
concentrated in a first step at -50 °C in vacuo in order to
remove excess diazomethane. In a second step, 8 (0.8 g, 15%)
was obtained after distillation of the residue under reduced
pressure (∼30 °C/0.2 Torr). The yellow distillate was finally
purified by preparative GLC (OV 101, 0.5 m, 60 °C, detector/
Conclusion
Silylene 3f is easily accessible in high yields by matrix
photolysis of phenylsilyldiazomethane 8. The combination of
matrix IR spectroscopy and DFT calculations allows the detailed
investigation of reactions of 3f with small molecules trapped
3
in the matrix. The reaction of silylene 3f with O2 (Scheme 2)
parallels that of the well-established oxidation of carbenes
(Scheme 1).4 The major difference is the larger exothermicity
and smaller activation barrier of the 4 f 5 compared with the
1 f 2 rearrangement. Although the calculated barrier depends
on the method used for the calculation (MP2 predicts a barrier
of about 6 kcal/mol,11,13 B3LYP ∼1 kcal/mol), it is safe to argue
that the activation barrier can be overcome by the thermal excess
energy of the oxygenation of silylene 3. The only reactions
where the oxygenation of a silylene could be directly monitored
in low-temperature matrixes are that of dimethyl-11 and methyl-
(phenyl)silylene, 3c and f, respectively. In both cases, the
dioxasiliranes 5 are the only detectable products, although the
reaction temperature of 30-45 K should allow isolation of very
labile intermediates. We thus conclude that silanone oxides 4
are too labile to be isolated as intermediates in silylene
oxidations.
1
injector 80 °C). H NMR (CDCl3, 200 MHz): δ 7.60-7.65
(m, 2 H, Ph), 7.40-7.42 (m, 3 H, Ph), 4.77 (d, 2 H, SiH2), 2.87
(t, 1 H, CHN2).13C NMR (CDCl3, 50 MHz): δ 135.11, 130.40,
128.26, 15.47. EI-MS (m/e (%)): 148 (26) [M+], 120 (18)
[M+ - N2], 119 (38) [M+ - (N2 + H)], 105 (100), 93 (28), 91
(19). HRMS calcd for C7H8N2Si 148.0457, obsd 148.0456. IR
(argon, 10 K): 3063.1 (1), 2195.3 (2), 2180.0 (2), 2154.6 (3),
2134.0 (7), 2093.0 (14), 2088.2 (100), 2082.9 (51), 1433.0 (5),
1274.0 (2), 1265.8 (8), 1158.0 (2), 1119.3 (11), 951.5 (2), 940.4
(9), 862.2 (100), 860.3 (5), 856.8 (87), 853.7 (9), 851.1 (11),
847.7 (1), 779.1 (3), 746.4 (2), 709.8 (6), 699.0 (2), 587.0 (4),
507.0 (1), 495.6 (3), 451.9 (1) cm-1 (relative intensity). UV
(argon, 15 K): λmax ) 220, 228 nm.
Matrix Spectroscopy. Matrix isolation experiments were
performed by standard techniques with an APD CSW-202
Displex closed cycle helium cryostat. Matrixes were produced
by deposition of argon (Messer Griesheim, 99.9999%) or
mixtures of argon and oxygen (Messer Griesheim, 99.998%)
on top of a CsI (IR) or sapphire (UV-vis) window at a rate of
approximately 0.15 mmol/min at 30 K. Infrared spectra were
recorded by using either a Bruker IFS66 FTIR or an Equinox
55 FTIR spectrometer with a standard resolution of 0.5 cm-1
in the range 400-4000 cm-1. UV-vis spectra were recorded
Experimental Part
Calculations. The ab initio and density functional theory
calculations were carried out using the Gaussian 98 suite of
programs43 and standard basis sets on an IBM RS/6000
workstation. Geometry optimization and frequency calculations
of the singlet species were performed at the B3LYP level of
(42) Suenram, R. D.; Lovas, F. J. J. Am. Chem. Soc. 1978, 100, 5117-
5122.
(43) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A. J.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, ReVision A.3;
Gaussian: Pittsburgh, PA, 1998.
(44) Bassindale, A. R.; Stout, T. J. Organomet. Chem. 1984, 271, C1-
C3.
(45) Martin, M. Synth. Commun. 1983, 13, 809-811.

6734 J. Am. Chem. Soc., Vol. 122, No. 28, 2000
Bornemann and Sander
on a Hewlett-Packard 8452A diode array spectrophotometer with
a resolution of 2 nm. Irradiations were carried out using Osram
HBO 500 W/2 mercury high-pressure arc lamps in Oriel
housings equipped with quartz optics. IR irradiation from the
lamps was absorbed by a 10-cm path of water. For wavelength
selection, dichronic mirrors (“cold mirrors”) in combination with
Schott cutoff filters (50% transmission at the wavelength
specified) were used.
1-Phenylsilene (11). Irradiation of matrix-isolated phenyl-
silyldiazomethane, 8, with light of the wavelength λ > 400 nm
produced also 1-phenylsilene, 11. IR (argon, 10 K): 2192.3 (13),
1433.4 (56), 1317.9 (3), 1282.5 (3), 1191.3 (3), 1122.4 (100),
964.5 (28), 960.9 (19), 917.7 (7), 851.7 (38), 737.6 (42), 656.7
(10), 646.2 (6), 450.7 (9) cm-1 (relative intensity).
Phenylsilyldiazirine (9). Irradiation of matrix-isolated phen-
ylsilyldiazomethane, 8, with light of the wavelength λ > 400
nm produced phenylsilyldiazirine, 9. IR (argon, 10 K): 2174.8
(13), 2171.7 (7), 2162.0 (5), 2151.7 (31), 2149.9 (6), 2147.1
(7), 1638.7 (4), 1433.4 (18), 1294.9 (5), 1191.3 (1), 1121.4 (20),
948.7 (4), 936.8 (35), 858.6 (100), 855.7 (3), 848.1 (4), 844.3
(5), 840.5 (3), 838.9 (9), 701.0 (28), 590.2 (3), 582.1 (10), 470.4
(9) cm-1 (relative intensity).
Acknowledgment. This work was supported by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen Indus-
trie. We thank Prof. G. Maier and Dr. H. P. Reisenauer for
helpful discussions and for providing IR data for phenylsilene
11.
JA000192Y