Thermal isomerisation of tris(silyl)hydroxylamines to silylaminodisiloxanes - Experimental and quantum chemical results

Article abstract of DOI:10.1002/1099-0682(200203)2002:4<876::AID-EJIC876>3.0.CO;2-2

O-Lithio-N,N-bis(trimethylsilyl)hydroxylamide, Li-O-N-(SiMe₃)₂, reacts with ClSiMe₂H to give HMe₂Si-O-N(SiMe₃)₂ (1), while LiO-N(SiMe₂CMe₃)₂ reacts with C

Full text of DOI:10.1002/1099-0682(200203)2002:4<876::AID-EJIC876>3.0.CO;2-2

FULL PAPER
Thermal Isomerisation of Tris(silyl)hydroxylamines to Silylaminodisiloxanes ؊
Experimental and Quantum Chemical Results
[
a]
[b]
[b]
[b]
Stefan Schmatz,* Friedhelm Diedrich, Christina Ebker, and Uwe Klingebiel*
Keywords: Silicon / Hydroxylamines / Isomerisation / Ab initio calculations / Transition states
O-Lithio-N,N-bis(trimethylsilyl)hydroxylamide,
SiMe reacts with ClSiMe to give HMe
N(SiMe (1), while LiO−N(SiMe CMe reacts with
SiF to give C SiF −O−N(SiMe CMe (2). 2 iso-
LiO−N-
rearrangement of tris(silyl)hydroxylamines to silylaminodisi-
loxanes for the model compounds H Si−O−N(SiMe (A),
Si−O−N(SiH )SiMe F (C), and
(D) demonstrate that N-bonded
in model compound B or SiFMe in model
(
3
)
2
,
2
H
2
Si−O−
3
3 2
)
3
)
2
2
3
)
2
Me
Me
3
Si−O−N(SiH
Si−O−N(SiMe
3
)
2
(B), Me
F)SiMeF
3
3
2
C
6
H
5
3
6
H
5
2
2
3
)
2
3
2
2
merises in solution to give N,O-bis(tert-butyldimethylsilyl)- silyl units, e.g. SiH
2
N-[difluoro(phenyl)silyl]hydroxylamine (3). Boiling of 1 leads
compound D, preferentially insert into the N−O bond. The
reaction mechanisms and the structures of the transition
states are discussed in detail.
to
the
structurally
−NH−SiMe
CSiMe −O−Si(Me)CMe
isomeric
(4), while boiling of 3 leads to
−NMe−SiF
silylaminodisiloxane,
Me
3
Si−O−SiMe
2
3
the isomeric Me
3
2
3
2
C
6
H
5
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
(5). Quantum chemical calculations (B3LYP) on the thermal 2002)
Introduction
The most dramatic difference between the chemistry of
silylhydroxylamines and that of organylhydroxylamines is
the ability of the former to undergo anionic, radical, and
(1)
[1Ϫ7]
thermal intramolecular rearrangements.
The first ex-
ample of a 1,2-anionic rearrangement of a silyl group from
an oxygen atom to a nitrogen atom was observed for
lithiated N,O-bis(silyl)hydroxylamines.[
1,2,8]
Although li-
1,2-Silyl shifts in neutral organosilylhydroxylamines have
thiobis(silyl)hydroxylamines have been successfully used in been known since 1973,[
1,2,9]
when Frainnet and Nowakow-
the synthesis of tris(silyl)hydroxylamines since the end of ski described an exchange of trialkylsilyl groups in bis(or-
the 1960s, the first crystal structures of these compounds ganosilyl)hydroxylamines.[ Later, West and co-workers
9]
were described only two years ago.[
6,7]
It was found that found a reversible rearrangement involving positional ex-
lithium derivatives of N,O-bis(silyl)hydroxylamines form O- change between the organosilicon groups on the oxygen
lithio-N,N-bis(silyl)hydroxylamides in the solid state.
and the nitrogen atom in tris(silyl)hydroxylamines, which
Depending on the bulkiness of the silyl groups and on they suggested to proceed via a dyotropic transition
four-, six-, or eight-membered state[
and dimeric, trimeric, or tetrameric
1,2,10,11]
[Equation (2)].
the solvent used, (LiϪO)
2
,3,4
rings were formed,[
6,7]
O-lithio-N,N-bis(silyl)hydroxylamides could be isolated.
This indicated that silyl group migration from the oxygen
atom to the nitrogen atom had occurred. In the reactions
of halosilanes with these lithium salts, the new silyl group
is attached to the oxygen atom in each case [Equation (1)].
(
2)
[
[
a]
b]
Institut für Physikalische Chemie der Universität Göttingen,
Tammannstraße 6, 37077 Göttingen, Germany
E-mail: sschmat@gwdg.de
Institut für Anorganische Chemie der Universität Göttingen,
Tammannstraße 4, 37077 Göttingen, Germany
E-mail: uklinge@gwdg.de
In 1998, we studied the rearrangement of O-(fluorosilyl)-
N,N-bis(trimethylsilyl)hydroxylamines to the isomeric N-
876
 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ1948/02/0404Ϫ0876 $ 17.50ϩ.50/0 Eur. J. Inorg. Chem. 2002, 876Ϫ885

Thermal Isomerisation of Tris(silyl)hydroxylamines to Silylaminodisiloxanes
FULL PAPER
(
fluorosilyl)-N,O-bis(trimethylsilyl)hydroxylamines
and
[5]
found it to be irreversible. Ab initio and density func-
tional calculations were consistent with the dyotropic
course of the isomerisation.
(
5)
(
3)
The lithium salt of N,O-bis(tert-butyldimethylsilyl)hyd-
roxylamine crystallises from THF as dimeric N,N-bis(tert-
butyldimethylsilyl)hydroxylamide[ and reacts with trifluoro-
7]
In a thermal rearrangement, which proceeds at temper-
atures up to 200 °C, tris(organosilyl)hydroxylamines un-
dergo an intramolecular isomerisation involving the inser-
tion of a silicon moiety into the bond between the nitrogen
and the oxygen atom and the transfer of an organyl group
from the silicon to the nitrogen atom, thereby forming silyl-
(
phenyl)silane at 0 °C with formation of N,N-bis(tert-bu-
tyldimethylsilyl)-O-[difluoro(phenyl)silyl]hydroxylamine (2).
[
1,2,6,12]
aminodisiloxanes.
(
6)
Product 2 could be isolated following the removal of LiF
in a centrifuge by virtue of the low reaction temperature
and the bulky SiMe CMe3 groups. In solution, even at
2
room temperature, the onset of the dyotropic rearrange-
ment of 2 into the isomeric N,O-bis(tert-butyldimethylsilyl)-
N-difluorophenylsilyl)hydroxylamine (3) can be observed by
means of NMR spectroscopy. The conversion of 2 to 3 is
4) found to be quantitative within a few hours in boiling THF.
(
The softer Lewis acid F SiPh migrates to the softer Lewis
2
This conversion occurs in high yields. We studied re-
arrangements of N-(fluorosilyl)-N,O-bis(organylsilyl)hyd-
roxylamines and found that the insertion of a silicon moiety
into the NϪO bond exclusively involves the silicon moiety
that was bonded to the nitrogen atom. The insertion of an
base nitrogen, while the harder Lewis acid SiMe CMe mi-
2
3
grates to the harder Lewis base oxygen.
SiF unit into the NϪO bond has not been observed.
2
For a better understanding of our results, we report
herein on quantum chemical investigations of the isomeris-
ations of tris(silyl)hydroxylamines based on several model
compounds and the thermal rearrangements of two N,N,O-
tris(silyl)hydroxylamines with formation of the isomeric si-
lylaminodisiloxanes.
(
7)
When heated under reflux for 3 d, 1 and 3 undergo a
thermal rearrangement to form the silylaminodisiloxanes 4
and 5, respectively. The conversion of 1 to 4 was monitored
Results and Discussion
Experimental
1
by H NMR spectroscopy. It was found that a structural
O-Lithio-N,N-bis(trimethylsilyl)hydroxylamide
reacts isomer of 1, namely N-(dimethylsilyl)-N,O-bis(trimethylsi-
with ClSiMe H to give O-(dimethylsilyl)-N,N-bis(trimethyl- lyl)hydroxylamine, was formed initially, which could not be
2
silyl)hydroxylamine (1) [Equation (5)].
isolated under the given reaction conditions.
Eur. J. Inorg. Chem. 2002, 876Ϫ885
877

S. Schmatz, F. Diedrich, C. Ebker, U. Klingebiel
FULL PAPER
N,N-bis(silyl)-O-(trimethylsilyl)hydroxylamine (B), N-(fluo-
rodimethylsilyl)-N-silyl-O-(trimethylsilyl)hydroxyl-
amine (C), and N-(fluorodimethylsilyl)-N-(difluoromethyl-
silyl)-O-(trimethylsilyl)hydroxylamine (D).
In all four cases, insertion of a silicon atom initially
located at the nitrogen atom into the NϪO bond occurs.
For A and B, only one product (PA and PB, respectively)
is possible for symmetry reasons. However, A and B can
undergo a dyotropic rearrangement yielding structures A1
and B1. Starting from these isomers, a silicon atom from
the nitrogen site can be inserted into the NϪO bond
yielding products PA1 and PB1, respectively.
(
(
8)
9)
The formation of 4 involves the insertion of an SiMe2
moiety into the NϪO bond and a hydrogen atom transfer
from the silicon to the nitrogen atom, whereas the forma-
tion of 5 involves the insertion of an SiMeCMe moiety
3
into the NϪO bond and the migration of a CH group from
3
the silicon to the nitrogen atom.
Quantum Chemical Study on the Isomerisation of
Tris(silyl)hydroxylamines
In order to gain more insight into the mechanisms gov-
erning the thermal isomerisation reactions of tris(silyl)hyd-
roxylamines, quantum chemical calculations have been car-
ried out.
Methodology
The program package GAUSSIAN-98[13] was employed
throughout. The geometries of the minima and first-order
saddle points on the potential energy surfaces (PES) were
calculated using the density functional variant B3LYP,
which is a hybrid method employing Becke’s exchange func-
[
14]
tional
and the Lee, Yang, and Parr correlation func-
The 6-31G* basis set was used. The calculated
(
10)
[
15]
tional.
stationary points on the PES were characterised by the Hes-
sian matrices. The obtained harmonic vibrational frequen-
cies were used to calculate the zero-point energy contribu-
tions. The saddle points were further characterised using
the TS routine in GAUSSIAN-98 (intrinsic reaction coord-
[
16]
inate method).
Using the B3LYP/6-31G* methodology, we have recently
studied various isomerisation reactions in silylhydrazine
[
17Ϫ21]
chemistry.
The calculated structures were compared
with available X-ray diffraction data and good agreement
was found throughout.
(
11)
For C and D, two different reaction pathways are pos-
Here, four different model compounds are investigated in sible, yielding products PC1/PC2 and PD1/PD2, respect-
detail: N,N-bis(trimethylsilyl)-O-(silyl)hydroxylamine (A), ively.
878
Eur. J. Inorg. Chem. 2002, 876Ϫ885

Thermal Isomerisation of Tris(silyl)hydroxylamines to Silylaminodisiloxanes
FULL PAPER
Table 1. B3LYP/6-31G(d) equilibrium geometries of the tris(si-
lyl)hydroxylamines (reactants A, A1, B, B1, C, and D); bond
lengths in pm, angles in °; bond angles are denoted by α, torsional
angles by θ; A and B exhibit C symmetry; values in parentheses
s
give the equilibrium geometry of structure B obtained at the
2
B3LYP/6-31ϩG(d) level of theory; C: SiЈ ϭ SiMe F; D: SiЈ ϭ
SiMeF
2
A
A1
B
B1
C
D
r(SiO)
r(ON)
r(NSiЈ)
r(NSiЈЈ)
α(SiON)
α(ONSiЈ)
168.5 170.8 171.3 (171.7) 169.0 170.9 172.0
149.8 148.4 147.7 (147.6) 149.2 147.6 146.6
178.8 176.0 176.1 (176.4) 178.9 176.2 173.1
178.8 178.8 176.1 (176.4) 176.1 176.6 175.7
110.2 117.0 114.8 (115.0) 108.5 116.2 117.6
105.4 107.7 108.2 (108.2) 108.6 111.2 108.4
α(SiЈNSiЈЈ) 127.5 123.4 125.0 (125.1) 126.9 122.2 128.9
θ(SiONSiЈ) 111.0 121.9 111.0 (110.9) 115.2 118.4 117.4
bond. Depending on the substituents at the silicon atom,
the SiϪO bond lengths range from 169 to 172 pm and the
SiϪN bond lengths from 173 to 179 pm. The SiϪOϪN
3
(
12) angles are in the range 109 to 118°, indicating pure sp to
2
sp hybridisation of the oxygen atom. The angles OϪNϪSi,
on the other hand, clearly show that the nitrogen atom is
3
sp -hybridised. Depending on the steric demand of the silyl
groups, the SiϪNϪSi angles are between 122 and 129°. As
can be seen from the torsional angles SiϪOϪNϪSi, all hy-
droxylamines adopt a trans conformation.
The geometries of the two saddle points corresponding
to the dyotropic transition states TSdA and TSdB are given
in Table 2. The structure of TSdB is shown graphically in
Figure 1. The steric demand of the bicyclic structures is
considerable because both three-membered rings exhibit
acute OϪSiϪN angles of ca. 45° and thus underlie strong
ring tension. It should be noted that the first calculations
on dyotropic transition states in silylhydroxylamines were
carried out by Müller.[ Compared to the reactants A and
B, the OϪN bonds are only slightly longer by 4.6 and
5]
5
.3 pm, respectively. In both transition states, the hy-
droxylamine SiϪO bonds are considerably stretched (by
9.7 pm and 46.1 pm for TSdA and TSdB, respectively).
3
The original and the newly formed SiϪO bonds are almost
(
13)
Table 2. B3LYP/6-31G(d) geometries of the dyotropic transition
states TSdA and TSdB; bond lengths in pm, angles in °; bond
angles are denoted by α, torsional angles by θ
The transition states corresponding to these reactions are
denoted as follows: TSdA and TSdB are the dyotropic
transition states in the isomerisations A Ǟ TSdA Ǟ A1
and B Ǟ TSdB Ǟ B1. The transition states leading to the
silylaminodisiloxanes PA, PA1, PB, PB1, PC1, PC2, PD1,
and PD2 are denoted by TSA, TSA1, TSB, TSB1, TSC1, r(ON)
TSdA
TSdB
154.4
208.2
181.4
206.5
188.6
183.5
46.0
153.0
217.4
180.8
212.4
187.8
180.2
44.0
r(SiЈO)
TSC2, TSD1, and TSD2, respectively.
r(SiЈN)
r(SiЈЈO)
r(SiЈЈN)
Structures
In Table 1, the B3LYP/6-31G* equilibrium geometries for r(NSi)
the hydroxylamines A, A1, B, B1, C, and D are given. A α(OSiЈN)
α(OSiЈЈN)
45.7
44.4
and B exhibit C symmetry with the SiϪOϪN chain con-
s
α(SiЈNSi)
tained in the symmetry plane. The OϪN bond length is
θ(SiЈONSiЈЈ)
118.7
122.1
115.6
118.4
123.3
117.0
calculated to be within the range 147Ϫ150 pm. Electron-
withdrawing (e.g. fluorosilyl) groups shorten the OϪN
θ(SiЈONSi)
Eur. J. Inorg. Chem. 2002, 876Ϫ885
879

S. Schmatz, F. Diedrich, C. Ebker, U. Klingebiel
FULL PAPER
3
ral silicon atom is clearly sp -hybridised (as is the case for
the diazasilacyclopropanes[
17,18]
). The SiϪOϪSi angles,
2
however, are between 138 and 153°, and indicate sp hy-
bridisation of the oxygen atom with a tendency towards sp
hybridisation. The SiϪNϪSi angles are in the range 124 to
1
32°. The sum of the angles at the nitrogen centre amounts
2
to almost 360° throughout, clearly indicating sp hybridis-
ation.
Table 4. B3LYP/6-31G(d) equilibrium geometries of the transition
states for Si insertion into the NϪO bond (TSA, TSA1, TSB,
TSB1, TSC1, TSC2, TSD1 and TSD2); bond lengths in pm, angles
in °; bond angles are denoted by α, torsional angles by θ; R ϭ H
for TSA1, TSB and TSC2; R ϭ Me otherwise; values in paren-
theses give the equilibrium geometry of structure TSB obtained at
the B3LYP/6-31ϩG(d) level of theory
Figure 1. Structure of saddle point TSdB (bicyclic dyotropic trans-
ition state; see text for details)
TSA TSA1 TSB
TSB1 TSC1 TSC2 TSD1 TSD2
equal in length. On the contrary, the hydroxylamine SiϪN
bonds are stretched by only ca. 10 pm, and the newly
formed endocyclic SiϪN bond lengths are 7 pm longer that
the original ones. The exocyclic SiϪN bond is slightly r(SiN)
elongated in the dyotropic TS compared to the SiϪN bonds r(NSi)
r(SiO)
r(OSi)
166.3 169.7 169.9 (170.4) 166.6 169.9 170.2 170.5 169.1
192.1 183.0 182.4 (183.5) 193.2 183.6 181.8 185.3 176.7
170.5 171.6 171.5 (171.6) 170.5 170.0 170.4 168.6 169.1
178.0 178.8 177.1 (177.4) 176.7 176.4 176.5 174.8 177.9
206.4 207.1 205.2 (205.5) 204.6 205.6 206.7 203.2 211.9
208.4 160.5 161.0 (160.9) 209.8 206.1 162.2 207.3 202.0
248.0 212.7 210.5 (210.3) 245.4 248.5 208.4 245.6 247.4
r(NO)
r(SiR)
r(NR)
in hydroxylamines. The angle between the two three-mem-
bered rings is given by the torsional angle θ(Si ONSi ),
1
2
which is calculated to be 122 and 123° in TSdA and TSdB,
α(SiOSi) 135.8 134.7 141.6 (138.9) 137.7 135.3 137.7 135.9 139.9
respectively. The geometries are in very good agreement α(OSiN) 69.1 71.4 70.8 (70.7) 68.1 71.0 71.8 69.9 75.5
[
5]
α(SiNSi) 139.7 131.6 131.0 (131.9) 131.2 133.8 136.5 137.1 133.9
θ(SiOSiN) 94.2 100.8 100.8 (97.7) 87.8 99.5 92.2 95.6 111.4
θ(OSiNSi) 89.5 92.7 88.0 (89.5) 87.8 90.3 89.7 84.2 84.3
with the results for H SiOϪN(SiH ) reported in ref.
3
3 2
In Table 3, the equilibrium structures of the eight silyl-
aminodisiloxanes PA, PA1, PB, PB1, PC1, PC2, PD1, and
PD2 are given. These structures may be compared with the
structures of the diazasilacyclopropanes discussed in
Table 4 gives the geometrical parameters of the saddle
[
17,18]
refs.
The triangular OϪSiϪN structural motif is ana- points corresponding to transition states TSA, TSA1, TSB,
logous to the SiN ring; however, the NϪO bond is broken TSB1, TSC1, TSC2, TSD1, and TSD2. As an example, the
2
(
distance 272 to 286 pm) and the nitrogen atom bears an structure TSC1 is shown graphically in Figure 2. The OϪN
additional substituent. The OϪSi bonds are considerably bonds (between 203 and 212 pm) are markedly elongated
shortened compared to those in the hydroxylamines, on av- (by more than 30 pm) compared with the corresponding
erage by 4 pm. The shortest OϪSi bond length is calculated bonds in the reactant structures. The exocyclic SiϪO bond
to be only 162 pm. The SiϪN bonds are also slightly length is only slightly greater than the corresponding bond
shorter in the disiloxanes, but the effect is less pronounced length in the products. The newly formed OϪSi bond is
compared to the situation with the SiϪO bonds. The cent- relatively long (up to 20 pm longer than that in the silylami-
nodisiloxanes). The exocyclic NϪSi bond length is slightly
greater than the corresponding bond length in the product.
Table 3. B3LYP/6-31G(d) equilibrium geometries of the silylamino-
The SiϪN bond bridging to the oxygen atom is only slightly
disiloxanes (products PA, PA1, PB, PB1, PC1, PC2, PD1, and
shorter than that in the silylaminodisiloxanes, and much
α, torsional angles by θ; R ϭ H for PA1, PB and PC2; R ϭ Me shorter than that in the hydroxylamines. The acute central
PD2); bond lengths in pm, angles in °; bond angles are denoted by
otherwise values in parentheses give the equilibrium geometry of
structure PB obtained at the B3LYP/6-31ϩG(d) level of theory
PA PA1 PB
PB1 PC1 PC2 PD1 PD2
r(SiO)
r(OSi)
r(SiN)
r(NSi)
r(NO)
r(NR)
164.4 166.7 166.8 (166.3) 164.9 167.9 167.0 168.0 169.1
167.6 165.1 164.4 (163.9) 167.2 163.9 164.5 163.8 162.1
174.3 173.1 173.9 (173.9) 174.9 172.8 174.0 174.0 170.3
176.9 176.3 174.4 (174.7) 174.3 174.6 173.7 171.8 175.0
278.7 285.8 285.6 (283.9) 280.6 272.4 283.6 271.7 275.7
147.8 101.7 101.6 (101.7) 148.0 147.9 101.8 148.4 148.4
α(SiOSi) 148.9 150.3 153.3 (178.4) 144.3 142.5 150.6 142.2 138.3
α(OSiN) 109.2 115.3 115.1 (114.3) 110.2 107.9 113.8 107.0 112.1
α(SiNSi) 126.0 131.8 129.1 (129.2) 128.5 124.6 130.8 123.8 126.0
θ(SiOSiN) 179.2 65.8 40.3 (174.8) 49.5 148.2 58.4 146.5 154.1
θ(OSiNSi) 55.8 78.1 83.8 (84.1) 113.0 170.1 115.0 175.6 35.3
Figure 2. Structure of the saddle point TSC1 (SiH insertion into
3
the OϪN bond; see text for details)
880
Eur. J. Inorg. Chem. 2002, 876Ϫ885

Thermal Isomerisation of Tris(silyl)hydroxylamines to Silylaminodisiloxanes
FULL PAPER
OϪSiϪN angle is between 68 and 76°. The SiϪOϪSi and Energetics
SiϪNϪSi angles are between 131 and 142°.
In Table 5, the energetics of the dyotropic transition state
The transition states are bicyclic structures with the first
ring formed by the NO moiety and the inserting silicon
atom, while the second ring is formed by the central silicon
atom, the nitrogen atom, and the group R transferred from
the silicon atom to the nitrogen atom (see Figure 2). The
transferred group (R) is either a hydrogen atom or a methyl
group. The silicon, the oxygen, the nitrogen, and the hydro-
gen atom (or the methyl carbon atom) are almost coplanar.
At the saddle point, the reacting SiϪR bonds are consider-
ably stretched. Thus, the reacting SiϪC bond in TSA is
TSdA and silylhydroxylamine A1 relative to species A are
given. A1 is more stable than A by ca. 3 kcal·mol , and
Ϫ1
zero-point energy effects enhance its stability. Energetically,
Ϫ1
the dyotropic TS is located at 33 kcal·mol above the re-
actant. This is of the same order of magnitude as reported
[5]
by Wolfgramm et al. The dyotropic TSdB is calculated to
Ϫ1
be 36 kcal·mol above the reactant B (Table 6). The isomer
Ϫ1
B1 is 1.5 kcal·mol higher in energy than B. This indicates
that configurations with silyl groups at the nitrogen atom
and trimethylsilyl groups at the oxygen site are energetic-
ally favoured.
1
9.2 pm longer than the other two SiϪC bonds (198.2 pm)
at the same silicon atom. The SiϪH bond in TSA1 is
1.7 pm longer compared to the other two SiϪH bond
In Figures 3Ϫ6, the potential energy profiles of the iso-
merising tris(silyl)hydroxylamines A, B, C, and D are dis-
played graphically as functions of the reaction coordinate.
Note that the stationary points are connected by a smooth
curve; the ‘‘reaction coordinate’’ in Figures 3Ϫ6 does not
have any quantitative physical significance. The influence of
zero-point vibrational energy (at the harmonic level; reac-
tion coordinate at saddle points not included) is given in
Tables 5Ϫ8. The effect of finite temperature (298 K) is also
1
lengths (148.8 pm). However, the new bond to the nitrogen
atom has yet to be formed at the saddle point. The NϪC
distance in TSA is 248.0 pm and the NϪH distance in
TSA1 amounts to 212.7 pm; these are much longer than the
corresponding distances in the silylaminodisiloxanes
(Table 3). Thus, the saddle points reported in Table 4 may
be classified as early transition states. The NϪO bond is
not yet fully broken and the interaction between the nitro-
gen atom and the transferred moiety is only weak. The bi-
cyclic first-order saddle point structures for silicon insertion
into the NϪO bond are analogous to the dyotropic trans-
ition states TSdA and TSdB (the bond between the central
silicon atom and the nitrogen atom adopts the role of the
NϪO bond in the dyotropic TS). The B3LYP/6-31G(d)
structures of all compounds described in this quantum
chemical study can be obtained from one of the authors
highlighted in these tables (reaction enthalpies ∆ H°). As
R
can be seen from Figure 3, the insertion of an SiH group
3
into the NϪO bond is favoured compared to SiMe inser-
2
tion. In addition to the thermodynamic control (|∆EPA1| Ͼ
|
∆EPA|), the reaction is also kinetically controlled (by the
barriers to isomerisation ∆ETSA1 Ͻ ∆ETSA). Silylhydroxyl-
amine A first has to undergo isomerisation via the dyotropic
transition state TSdA, for which the activation energy (bar-
rier height ∆ETSdA) is very similar to the corresponding
value for the silyl group insertion (∆ETSA1). The larger reac-
tion enthalpy and the lower isomerisation barrier determine
the course of the reaction (A Ǟ PA1).
(sschmat@gwdg.de).
The influence of diffuse basis functions [6-31ϩG(d) basis
set] on the geometries of the stationary points was investig-
ated for structures B, PB, and TSB (see Tables 1, 3, and 4).
The only remarkable deviation from the B3LYP/6-31G(d)
results was found for the SiϪOϪSi angle in the silylamino-
disiloxane PB, which is widened by 25°. The SiϪOϪSi unit
becomes almost linear (178.4°) when the additional diffuse
functions are taken into account. However, the energetics
are hardly affected. The saddle point TSB is found at
A similar conclusion can be drawn from Figure 4. Here,
reaction B Ǟ PB (SiH insertion) is favoured both thermo-
3
dynamically and kinetically. The barrier ∆ETSdB for the dy-
otropic rearrangement B Ǟ B1 is smaller than the barrier
∆
ETSB1 for SiMe insertion B1 Ǟ PB1.
3
In Figure 5, insertion of a silyl group is compared with
Ϫ1
Ϫ1
3
8.3 kcal·mol above B [38.2 kcal·mol with 6-31G(d)], that of a fluorodimethylsilyl group. Again, SiH insertion
3
Ϫ1
whereas the product PB is located at 84.9 kcal·mol below (C Ǟ PC1) is favoured both thermodynamically and kin-
B [83.8 kcal·mol with 6-31G(d)].
Ϫ1
etically (Table 7).
Table 5. Energetics of the reactions involving tris(silyl)hydroxylamine A (energies in kcal·molϪ1 with respect to the starting material); Ezp
denotes the zero-point energy contribution
A
TSdA
A1
TSA1
PA1
TSA
PA
∆
∆
∆
∆
∆
∆
E
B3LYP/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-311ϩG(2d,p)
0.0
0.0
0.0
0.0
0.0
0.0
31.2
30.7
Ϫ2.6
Ϫ2.9
Ϫ2.8
Ϫ2.6
Ϫ1.1
Ϫ0.7
31.7
29.2
Ϫ87.7
Ϫ86.1
Ϫ85.7
Ϫ91.7
Ϫ90.0
Ϫ90.9
41.2
39.1
Ϫ65.9
Ϫ64.5
Ϫ64.3
Ϫ66.1
Ϫ69.6
Ϫ69.2
(EϩEzp
)
R
H°(298 K)
[a]
E
E
E
32.8
26.5
26.6
30.9
48.4
50.2
40.0
55.9
55.3
[a]
MP2/6-31G(d)
MP2/6-31ϩG(d,p)[
a]
[
a]
At B3LYP/6-31G(d) geometry.
Eur. J. Inorg. Chem. 2002, 876Ϫ885
881

S. Schmatz, F. Diedrich, C. Ebker, U. Klingebiel
FULL PAPER
Table 6. Energetics of the reactions involving tris(silyl)hydroxylamine B (energies in kcal·molϪ1 with respect to the starting material); Ezp
denotes the zero-point energy contribution
B
TSdB
B1
TSB1
PB1
TSB
PB
∆
∆
∆
∆
∆
∆
E
B3LYP/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-311ϩG(2d,p)
0.0
0.0
0.0
0.0
0.0
0.0
35.7
35.3
1.5
1.5
1.5
1.3
0.4
0.2
46.4
44.5
Ϫ65.1
Ϫ63.5
Ϫ63.3
Ϫ65.5
Ϫ70.2
Ϫ70.0
38.2
35.9
Ϫ83.8
Ϫ81.8
Ϫ81.5
Ϫ87.6
Ϫ88.5
Ϫ89.8
(EϩEzp
)
R
H°(298 K)
[a]
E
E
E
35.7
30.8
30.4
44.6
58.3
57.4
36.9
53.1
53.8
[a]
MP2/6-31G(d)
MP2/6-31ϩG(d,p)[
a]
[
a]
At B3LYP/6-31G(d) geometry.
Figure 3. Potential energy diagram for reaction A (schematic)
Figure 5. Potential energy diagram for reaction C (schematic)
It should be noted that the theoretical results are in nice
agreement with the experimental finding that during the
conversion of 1 to 4 a structural isomer of 1 was monitored.
This isomer is formed by a dyotropic rearrangement. After
this step, the SiHMe group inserts instead of the SiMe3
2
group.
It remains to be considered how well hybrid DFT
methods, e.g. B3LYP, can predict transition-state geomet-
ries and barrier heights. Hybrid DFT involves mixing vari-
ous amounts of the HartreeϪFock non-local exchange op-
erator with DFT exchange-correlation functionals, and it
has been observed that the fraction needed for an accurate
prediction of thermochemical data differs from the optimal
fraction for predicting accurate barrier heights. Recently,
Lynch and Truhlar[ reported DFT calculations of barrier
22]
Figure 4. Potential energy diagram for reaction B (schematic)
heights in 22 reactions and found a mean unsigned error of
Ϫ1
3
.4Ϫ4.2 kcal·mol (depending on the basis set) in compar-
Figure 6 shows the thermal rearrangement of N-(di- ison with best estimates derived from experimental data.
fluoromethyl)-N-(fluorodimethylsilyl)-O-(trimethylsilyl)- MøllerϪPlesset second-order perturbation theory (MP2)
hydroxylamine. Insertion of the difluorosilyl group has a performed slightly less well. Lynch and Truhlar recommend
slightly smaller associated barrier height ∆ETSD1 than inser- their own hybrid method MPW1K,[ which gave a mean
23]
Ϫ1
tion of the monofluorosilyl group. However, the product unsigned error of only 1.5 kcal·mol . However, all of the
Ϫ1
PD2 is energetically favoured by 1.3 kcal·mol (Table 8). 22 reactions involve free radicals and the transfer of a hy-
The reaction is clearly subject to thermodynamic control. drogen atom, and all but one are bimolecular reactions.
This is in keeping with the conditions of preparation of the Since the empirical data are as yet only limited, and since,
silylaminodisiloxane, i.e. heating of a fluoro-substituted in the present work, we focus on the energetic differences
tris(silyl)hydroxylamine up to 200 °C and keeping it at this between several reaction pathways, we are confident that
temperature for several hours.
the B3LYP method yields reliable results.
882
Eur. J. Inorg. Chem. 2002, 876Ϫ885

Thermal Isomerisation of Tris(silyl)hydroxylamines to Silylaminodisiloxanes
FULL PAPER
Table 7. Energetics of the reactions involving tris(silyl)hydroxylamine C (energies in kcal·molϪ1 with respect to the starting material); Ezp
denotes the zero-point energy contribution
C
TSC1
PC1
TSC2
PC2
∆
∆
∆
∆
∆
∆
E
B3LYP/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-311ϩG(2d,p)
MP2/6-31G(d)[
0.0
0.0
0.0
0.0
0.0
0.0
36.4
34.0
Ϫ85.9
Ϫ83.6
Ϫ83.5
Ϫ90.1
Ϫ90.3
Ϫ91.6
41.7
39.5
Ϫ70.4
Ϫ68.8
Ϫ68.7
Ϫ70.8
Ϫ74.6
Ϫ73.8
(EϩEzp
)
R
H°(298 K)
[a]
E
E
E
35.0
51.1
51.6
41.6
55.7
56.0
a]
MP2/6-31ϩG(d,p)[
a]
[
a]
At B3LYP/6-31G(d) geometry.
Table 8. Energetics of the reactions involving tris(silyl)hydroxylamine D (energies in kcal·molϪ1 with respect to the starting material); Ezp
denotes the zero-point energy contribution
D
TSD1
PD1
TSD2
PD2
∆
∆
∆
∆
∆
∆
E
B3LYP/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-311ϩG(2d,p)
MP2/6-31G(d)[
0.0
0.0
0.0
0.0
0.0
0.0
43.5
41.6
Ϫ68.6
Ϫ66.7
Ϫ66.8
Ϫ69.2
Ϫ71.9
Ϫ71.0
45.6
43.7
Ϫ69.9
Ϫ68.0
Ϫ68.1
Ϫ71.1
Ϫ74.1
Ϫ73.3
(EϩEzp
)
R
H°(298 K)
[a]
E
E
E
43.0
59.6
60.0
45.2
57.6
57.6
a]
MP2/6-31ϩG(d,p)[
a]
[
a]
At B3LYP/6-31G(d) geometry.
states are slightly lower in energy than the corresponding
saddle points from the MP2 calculations. However, all other
MP2 saddle points are much higher in energy. This can be
explained by the fact that the dyotropic transition states
(
(
TS) can be classified as ‘‘tight’’ TS, whereas the other TS
Table 4) are ‘‘loose’’ TS with considerably stretched bonds.
MP2 does not include a major fraction of the non-dynam-
ical correlation energy and is not capable of describing such
loose structures. In particular, the order of the two saddle
points in reaction according to eq. 13 is reversed at the MP2
level of theory.
Normal Modes of Transition States
The harmonic vibrational frequencies ωTS of the reactive
normal modes QTS at the various saddle points are given in
Table 9. The two dyotropic transition states have rather sim-
ilar imaginary frequencies and corresponding reduced
Figure 6. Potential energy diagram for reaction D (schematic)
masses µ . In both cases, a silyl group and a trimethylsilyl
TS
group undergo simultaneous migration. However, the mo-
tion along QTS is governed by the oxygen atom, which is
much lighter than the heavy silyl groups (see Figure 7).
From the viewpoint of reaction dynamics, the term ‘‘silyl
group migration’’ is somewhat misleading.
In order to study the influence of polarization and diffuse
basis functions on the energetics of the four systems, we
performed B3LYP/6-311ϩG(2d,p) calculations on the
B3LYP/6-31G(d) geometries (see Tables 5Ϫ8). Overall, the
results are in good agreement with those obtained em-
ploying the smaller basis set. The products PA1, PB, and
PC1 are lowered in energy because the additional diffuse
Table 9. Harmonic vibrational frequencies in cmϪ1 of the reactive
and polarization functions allow for a better description of normal coordinates QTS of the transition states; the corresponding
reduced mass in a.m.u. is denoted by µTS
the SiH ϪNH unit in these cases.
2
We also carried out MP2 calculations with the 6-31G(d)
and 6-31ϩG(d,p) basis sets at the optimized B3LYP geo-
TSdA TSA TSA1 TSdB TSB TSB1 TSC1 TSC2 TSD1 TSD2
metries (see Tables 5Ϫ8). The energetic differences between
reactants and products are very close to the DFT results.
The two saddle points describing the dyotropic transition
ω
TS 278i 558i 505i 267i 559i 617i 613i 527i 509i 546i
µ
TS 8.96 2.78 8.46 8.04 7.04 2.48 2.40 7.04 8.99 8.80
Eur. J. Inorg. Chem. 2002, 876Ϫ885
883

S. Schmatz, F. Diedrich, C. Ebker, U. Klingebiel
FULL PAPER
droxylamines, whereas the transition state for insertion of
Ϫ1
SiMe , SiMeF, and SiF is ca. 10 kcal·mol
higher
2
2
Ϫ1
(
44Ϯ3 kcal·mol ).
Experimental Section
General: All experiments were performed in oven-dried glassware
using standard inert gas and vacuum-line techniques. NMR spectra
1
were recorded from samples in CDCl
C, F, Si) as internal and MeNO
3
with SiMe
4
and C
F
6 6
( H,
1
3
19
29
15
2
( N) as external references.
Mass spectral data are reported in mass-to-charge units (m/z). The
1
9
progress of the reactions was monitored by F NMR spectroscopy.
The compounds were obtained in analytically pure form.
Figure 7. The reaction coordinate QTS at the saddle point TSdB, i.e.
the normal coordinate of the mode with the imaginary vibrational
frequency ωTS
O-(Dimethylsilyl)-N,N-bis(trimethylsilyl)hydroxylamine (1): To a so-
lution of N,N-bis(trimethylsilyl)hydroxylamine (8.8 g, 0.05 mol) in
n-hexane (70 mL), nBuLi (1.6  solution in n-hexane, 0.05 mol) was
slowly added. After heating under reflux for 2 h, the reaction mix-
2
ture was cooled to room temperature, whereupon ClSiMe H
The transition states for SiH group insertion into the (0.05 mol, 4.7 g) was added. LiCl was removed on a frit and 1 was
3
1
NϪO bond (TSA, TSB1, and TSC1) exhibit very similar purified by distillation; yield 73%; b.p. 63 °C/0.01 mbar. H NMR
characteristic ωTS and µTS data for the reactive normal
modes Q . Inspection of the remaining saddle points
TSA1, TSB, TSC2, TSD1, and TSD2) reveals that substi-
tution of an SiMe group by SiMe F or SiMeF has only a
small influence on the nuclear dynamics in the saddle-point
region of the PES (the curvature of the saddle point, given
TS, is relatively similar). The reactive motion is de-
scribed by the fission of the NϪO bond and the insertion
of the silicon atom (see Figure 8). The bond between the
transferred group (H or CH ) is considerably stretched. The
nitrogen atom moves towards the transferred group and
forms a new bond.
3
(
CDCl
H, Si(CH
CDCl ): δ ϭ Ϫ1.22 (SiC
.27 [Si(CH ], 13.43 (SiH). MS (EI): m/z (%) ϭ 235(21) [M] ,
20(8) [M Ϫ CH
3
): δ ϭ 0.11 [s, 18 H, Si(CH
3
)
3
], 0.22 [d, JH,H ϭ 2.9 Hz, 6
3
13
3 2
) ], 4.7 (sept, JH,H ϭ 2.9 Hz, 1 H, SiH). C NMR
TS
2
9
(
8
2
3
3 2 3
), 0.76 (SiC ). Si NMR (CDCl ): δ ϭ
(
ϩ
3 3
)
3
2
2
ϩ
Ϫ1
] . IR: ν˜ ϭ 2131.6 cm (SiH). C
H25NOSi
8 3
3
(
235.55): calcd. C 40.79, H 10.70; found C 40.53, H 10.32.
2
N,N-Bis(tert-butyldimethylsilyl)-O-(difluorophenylsilyl)hydroxyl-
amine (2): F SiPh (8.1 g, 0.05 mol) was added to N,N-bis(tert-butyl-
dimethylsilyl)-O-lithiohydroxylamine (13.0 g, 0.5 mol) at 0 °C and
the mixture was slowly allowed to warm to room temperature. 2
was freed of LiF by centrifugation; yield 76%; b.p. 100 °C/
by µTS
ω
3
[
6]
3
1
6
0
.05 mbar. H NMR (CDCl
NSiCH
CDCl
7.89 (NSiCC
3
): δ ϭ 0.23 (t, JHF ϭ 0.56 Hz, 12 H,
13
3
3 3 6 5
), 1.01 [s, 18 H, NSiC(CH ) ], 7.0Ϫ7.2 (C H ). C NMR
5
(
3
): δ ϭ Ϫ2.91 (t, JC,F ϭ 1.51 Hz, NSiC), 20.03 (NSiCC
3
),
2
2
3
), 125.04 (t, JC,F ϭ 26.7 Hz, C-1 of Ph), 128.47 (t,
4
3
J
C,F ϭ 0.7 Hz, C-3,5 of Ph), 132.25 (C-4 of Ph), 134.72 (t, JC,F
ϭ
1
9
): δ ϭ 29.5. 29Si
1
19 Hz, C-2,6 of Ph). F NMR (CDCl
3
/C
6
F
6
3
NMR (CDCl ): δ ϭ Ϫ65.02 (t, JSiF ϭ 278.7 Hz, OSiF), 14.85
ϩ
(
3
NSi). MS (EI): m/z (%) ϭ 388(8) [M Ϫ CH ] , 346(100) [M Ϫ
ϩ
3 3
C(CH ) ] .
N,O-Bis(tert-butyldimethylsilyl)-N-(difluorophenylsilyl)hydroxyl-
amine (3): A solution of 2 (20.2 g, 0.05 mol) in THF (50 mL) was
1
9
refluxed for 3 d ( F NMR control). Thereafter, the 3 obtained was
1
distilled in vacuo. yield 67%. H NMR (CDCl
OSiCH ), 0.16 (s, 6 H, NSiCH ), 0.28 [s, 9 H, OSiC(CH
s, 9 H, NSiC(CH ], 7.2Ϫ7.8 (m, 5 H, C
δ ϭ Ϫ6.37 (NSiCH ), Ϫ5.49 (OSiCH ), 17.82 [NSiC(CH
[OSiC(CH ], 25.84 [NSiC(CH ], 26.39 [OSiC(CH
CDCl ): δ ϭ 27.19 (OSiF). Si NMR (CDCl ): δ ϭ Ϫ51.02 (t,
SiF ϭ 271.19 Hz, NSiF), 15.49 (NSi), 24.56 (OSi). MS (EI): m/z
3
): δ ϭ 0.12 (s, 6 H,
], 0.88
). C NMR (CDCl ):
], 18.12
3
3
3 3
)
1
3
[
3
)
3
6
H
5
3
3
3
3 3
)
1
9
Figure 8. The reaction coordinate QTS at the saddle point TSC2,
i.e. the normal coordinate of the mode with the imaginary vibra-
tional frequency ωTS
3
)
3
3
)
3
3 3
) ]. F NMR
29
(
3
3
1
J
ϩ
ϩ
ϩ
(
%) ϭ 403(2) [M] , 388(8) [M Ϫ CH
3 3
] , 346(100) [M Ϫ CMe ] .
In summary, it has been shown that the insertion of a
silyl group into the NϪO bond is energetically favoured if
it occurs from the nitrogen atom. Furthermore, a mono-
C
18
H
35
F
2
NOSi (403.20): calcd. C 53.62, H 8.75; found C 53.16,
3
H 8.49.
fluorosilyl group inserts into the NϪO bond whereas the Compounds 4 and 5: The respective tris(silyl)hydroxylamine 1 or
3
(0.03 mol) was heated under reflux without a solvent for 3 d.
difluorosilyl group does not. Insertion of SiH stabilises the
2
Ϫ1
Compounds 4 and 5 produced were purified by distillation in va-
cuo.
product by 86 kcal·mol whereas insertion of SiMe , Si-
2
MeF, or SiF2 yields a stabilisation of 66, 70, and
Ϫ1
6
9 kcal·mol , respectively. The transition state for SiH in- 1,1,1,3,3-Pentamethyl-3-[(trimethylsilyl)amino]-1,3-disiloxane (4):
2
Ϫ1
1
sertion is located at 35Ϯ3 kcal·mol above the tris(silyl)hy- Yield 82%; b.p. 63 °C/10 mbar. H NMR (CDCl
3
): δ ϭ 0.04
884 Eur. J. Inorg. Chem. 2002, 876Ϫ885

Thermal Isomerisation of Tris(silyl)hydroxylamines to Silylaminodisiloxanes
FULL PAPER
[
Si(CH
3
)
2
], 0.06 [NSi(CH
3
)
3
], 0.07 [OSi(CH
3
)
3
], 0.42 (NH). 13C
], 2.46
): δ ϭ Ϫ344.71 (NH). Si NMR
3
): δ ϭ Ϫ11.24 (NSi), 2.12 (OSiN), 6.08 (SiO). MS (EI): m/
[5]
R. Wolfgramm, T. Müller, U. Klingebiel, Organometallics 1998,
1
7, 3222.
NMR (CDCl
3
): δ ϭ 1.97 [OSi(CH
3
)
3
], 2.30 [NSi(CH )
3 3
29
[
[
6]
7]
15
F. Diedrich, U. Klingebiel, M. Schäfer, J. Organomet. Chem.
999, 588, 242.
[
Si(CH
CDCl
z (%) ϭ 235(4) [M] , 220(100) [M Ϫ CH
calcd. C 40.79, H 10.70; found C 40.31, H 10.35.
3 2 3
) ]. N NMR (CDCl
1
(
F. Diedrich, U. Klingebiel, F. DallЈAntonia, C. Lehmann, M.
Noltemeyer, T. R. Schneider, Organometallics 2000, 19, 5376.
R. West, P. Boudjouk, T. A. Matuszko, J. Am. Chem. Soc. 1969,
ϩ
ϩ
3
8 3
] . C H25NOSi (235.55):
[8]
9
1, 5184.
1
,3-Di-tert-butyl-3-[(difluorophenylsilyl)methylamino]-1,1,3-tri-
[9]
E. Frainnet, F. Duboudin, F. Dabescat, G. C. Vincon, C. R.
Acad. Sci., Ser. C 1973, 276, 1469.
C. Trindle, D. D. Schillady, J. Am. Chem. Soc. 1973, 95, 703.
M. T. Reetz, Adv. Organomet. Chem. 1977, 16, 33Ϫ65.
R. Wolfgramm, U. Klingebiel, Z. Anorg. Allg. Chem. 1998,
624, 1035Ϫ1040.
1
methyl-1,3-disiloxane (5): Yield 67%; b.p. 101 °C/0.05 mbar.
NMR (CDCl ): δ ϭ 0.14 (3 H, OSiCH ), 0.15 (3 H, OSiCH ), 0.36
3 H, SiCH ), 0.97 [9 H, SiC(CH ], 1.04 [9 H, SiC(CH ], 2.68 (t,
HF ϭ 1.15 Hz, 9 H, NCH ), 7.4Ϫ7.8 (m, 5 H, C
CDCl ): δ ϭ Ϫ2.92 (OSiCH ), Ϫ2.90 (OSiCH ), Ϫ5.66 (t, JHF
.53 Hz, SiCH ), 18.27 [SiC(CH ], 18.37 [SiC(CH ], 25.84
SiC(CH ], 26.67 [SiC(CH ], 33.72 (t, JC,F ϭ 2.45 Hz, NCH ),
28.04 (C-1 of Ph), 128.27 (C-3,5 of Ph), 131.57 (C-4 of Ph), 134.72
H
3
3
3
[10]
[11]
[12]
(
3
)
3 3
3 3
)
4
13
J
3
6 5
H ). C NMR
4
(
2
[
1
(
2
2
(
3
3
3
ϭ
[13]
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M.
A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgom-
ery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam,
A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi,
V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala,
Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G.
Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomberts, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzales,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W.
Wong, J. L. Andres, C. Gonzales, M. Head-Gordon, E. S. Re-
plogle, J. A. Pople, GAUSSIAN-98, Revision A.7, Gaussian,
Inc., Pittsburgh PA, 1998.
3
3
)
3
)
3 3
3
3
)
3
3
)
3
3
3
19
t,
3
JC,F ϭ 1.26 Hz, C-2,6 of Ph). F NMR (CDCl ): δ ϭ
29
1
4.71Ϫ26.55 (NSiF). Si NMR (CDCl
68.35 Hz, NSiF), 8.95 (t, JSiF ϭ 1.53 Hz, NSiO), 11.62 (OSi). MS
3
): δ ϭ Ϫ47.22 (t, JSiF
ϭ
3
ϩ
ϩ
EI): m/z (%) ϭ 403(2) [M] , 388(8) [M Ϫ CH
3
] , 346(100) [M Ϫ
ϩ
CMe
C 53.74, H 8.92.
3 35 2 3
] . C18H F NOSi (403.20): calcd. C 53.62, H 8.75; found
Acknowledgments
S. S. is indebted to Professor Peter Botschwina for continuous sup-
port. We would like to thank Dr. Rainer Oswald for his help with
Figures 7 and 8. Helpful discussions with Dr. Thomas Müller (Uni-
versity of Frankfurt/Main) are gratefully acknowledged. Most of
the calculations were carried out on workstations of the Gesellsch-
aft für wissenschaftliche Datenverarbeitung Göttingen (GWDG).
Support through Sonderforschungsbereich 357 ‘‘Molekulare Mech-
anismen unimolekularer Prozesse’’ is gratefully acknowledged. We
are grateful to the Deutsche Forschungsgemeinschaft and Fonds
der Chemischen Industrie for support of this work.
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