B
A. Harinath et al.
H3CO
Ph
OCH3
Si
OCH3
MN(SiMe3)2 5 mol-%
hydrosilanes as coupling partners (Table 2, entries 9–11). In
contrast, the conversion of benzyl alcohol into benzyloxy
(triphenyl)silane, bisbenzyloxy(diphenyl)silane, andtribenzyloxy
(phenyl)silane was lower (70 %) upon reaction with the corre-
sponding hydrosilanes with appropriate stoichiometry (Table 2,
entries 12–14) at 308C. However, these reactions required 4 h to
achieve such conversion values, and no improvement in con-
version was observed even at 608C. The molecular structure of
benzyloxy(triphenyl)silane obtained from the dehydrogenative
coupling reaction between benzyl alcohol and triphenylsilane
(1 : 1) (Table 2, entry 14) was established by single-crystal X-ray
analysis and is shown in Fig. 1.[24]
3CH3OH ϩ PhSiH3
Neat or solvent, RT
M ϭ Li, Na, or K
Scheme 1. Alcoholysis of silanes under different reaction conditions. RT,
room temperature.
Table 1. Screening of catalysts for the alcoholysis of silanes
Pre-catalyst (5 mol-%) was loaded into a Schlenk tube followed by addition
of alcohol (3 mmol) and silane (1 mmol). After 1 h, the mixture was trans-
ferred into an NMR tube, and chloroform (0.6 mL) was added. Conversions
were obtained on the basis of the consumption of PhSiH3 from integration of
signals in the 1H NMR spectra
The silyl ether N crystallizes in monoclinic space group P21
having two molecules in the unit cell. The Si1–O1 distance of
1.638(4) A is similar with the literature reported value.[25] The
˚
Entry Catalyst
Time [h] Solvent
Conversion of alcohol [%]
geometry around the silicon is distorted tetrahedral with bond
angles of O1–Si1–C19 109.638, O1–Si1–C8 104.028, C19–Si1–
C8 112.408, and C20–Si1–C8 104.028.
1
2
3
4
5
6
LiN(SiMe3)2
1
1
1
1
1
1
Neat
99
95
99
99
99
99
NaN(SiMe3)2
KN(SiMe3)2
KN(SiMe3)2
KN(SiMe3)2
KN(SiMe3)2
Neat
Neat
THF
To appraise the functional-group tolerance of the alkali metal
catalyst, we employed several alcohols with different functional
groups attached to them. Allyl alcohol was used as a coupling
partner for different aliphatic and aromatic silanes, and a
satisfactory yield of up to 88 % was obtained within ,3–4 h
(entries 15–19, Table 2). This result indicated that the alkene
group remained unaffected under the reaction condition
employed. We also examined the reaction between alcohols
containing halogen functional groups, such as 2-chloroethanol
and 2-bromoethanol, and either phenyl silane or diphenyl silane
as the reaction partner at 308C. Excellent conversion (99 %) was
achieved after 4 h. Nevertheless, steric influences and the
functional groups attached to the various substrates, such as
alcohols and silanes, are relevant for them to undergo dehydro-
genative coupling to form the Si–O bonds. Similar observations
were reported by Cui and Gao when tertiary silane was treated
Toluene
Benzene
(Table 1, entries 1–3). The results from this initial assessment
indicate that the use of 5 mol-% alkali metal salts at 308C can be
considered as the optimized process for the facile synthesis of
silyl ethers. To prepare several silyl ethers, the potassium salt
[KN(SiMe3)2] was preferentially used over the other alkali salts
because of its greater availability. It is noted that the use of
solvents such THF, toluene, and C6H6 did not affect the overall
yield (Table 1, entries 4–6), and almost complete conversion of
silanes was achieved at 308C. Thus, we performed all subse-
quent reactions under neat conditions.
The initial resultsinspiredus tostudy the scope of thisprotocol
using potassium hexamethyldisilazide as the catalyst for the
reaction of a wide variety of aliphatic and aromatic alcohols,
containing different functional groups, with different aromatic
silanes, such as PhSiH3, Ph2SiH2, and PhMeSiH2, and aliphatic
silanes such as Et3SiH and Et2SiH2. We observed that
alcohols were easily converted into the corresponding mono-,
bis-, and tris-substituted alkoxy silanes at room temperature
depending on the stoichiometry of the reaction. The results of
the detailed study are summarized in Table 2. Methanol can be
completely converted into trimethoxy(phenyl)silane within 1 h
at 308C with vigorous evolution of hydrogen gas when it is
reacted with phenyl silane at a molar ratio of 3 : 1 (Table 2, entry
1). Reaction between methanol and diphenylsilane (2 : 1) or
triphenylsilane (1 : 1) afforded complete conversion within 2 h
to obtain dimethoxy(diphenyl) and methoxy(triphenyl)silane,
respectively (Table 2, entries 2 and 3). However, lower silane
conversions of 73 % and 90 % were achieved after 3 h of reaction
when aliphatic diethylsilane and triethylsilane were used as
coupling partners with methanol (Table 2, entries 4 and 5). The
gradual decrease in conversion values obtained as the silane is
changed from a phenylsilane to an alkyl silane is presumably
due to the decreasing reactivity of silanes, which is reported in
literature.[22] The reaction between ethanol and PhSiH3,
Ph2SiH2, and Ph3SiH required 3 h to attain conversion values
of 90 %, 74 %, and 60 %, respectively, at 308C. However,
complete conversion in each case can be achieved in 2 h at
608C (Table 2, entries 6–8). Secondary isobutyl alcohols under-
went smooth conversion, up to 99 %, within 3 h to give the
corresponding alkoxysilanes upon reaction with various
t
with BuOH using NHC as a catalyst.[21] Thus, potassium
hexamethyldisilazide acts as an efficient catalyst for the forma-
tion of Si–O bonds with a wide range of substrates. Moreover, to
the best of our knowledge, this is the first example of the use of
potassium salt as a catalyst for dehydrogenative coupling of
hydrosilanes with hydroxyl groups.
Kinetics Study
To explore the mechanism of the reaction, we performed
kinetics studies. In situ NMR experiments were performed.
Known amounts of the catalyst [KN(SiMe3)2] (to achieve con-
centrations of 0.025, 0.03, 0.035, 0.04, and 0.045 M) were added
to a solution containing Ph3SiH (0.130 g, 0.5 mmol), MeOH
(0.016 g, 0.5 mmol), and C6D6 (0.4 mL). The solution was set at
308C. At indicated time intervals, the solution was analyzed by
1H NMR (see Figs S4–S14, Supplementary Material). The NMR
spectra showed that with increase in time, the singlet peak signal
of ‘SiH’ at 5.01 ppm gradually decreased clearly, indicating the
formation of Si–O bond. The study showed that the KN
(SiMe3)2-catalyzed dehydrocoupling reaction of MeOH with
Ph3SiH has a first-order dependence on the concentration of
the catalyst KN(SiMe3)2. The reaction rate increased with
increase in the amount of catalyst in a linear fashion (Figs 2 and
3). The result suggests that the active catalyst is a mononuclear
intermediate in the catalytic cycle and this agrees with the
proposed mechanism shown in Scheme 2 where the alkali metal
complex reacts with alcohol to generate a metal alkoxide i via
elimination of HN(SiMe3)2. The metal alkoxide acts as the