Bifunctional silanol-based HBD catalysts for CO2 fixation into cyclic carbonates

Article abstract of DOI:10.1039/c9nj04840e

First examples of unprecedented silanol-based bifunctional HBD catalysts [(^tBuO)₂{(N(CH₂CH₂)₃N)CH₂CH₂O}Si(OH)]⁺I^- and (Rac)- and (R)-[(^tBuO)

Full text of DOI:10.1039/c9nj04840e

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Bifunctional silanol-based HBD catalysts for CO2
fixation into cyclic carbonates†
Cite this: New J. Chem., 2019,
43, 18525
Jovana Perez-Perez,ab Uvaldo Hernandez-Balderas,ab Diego Martınez-Oteroab and
´
´
´
´
ab
Vojtech Jancik
*
First examples of unprecedented silanol-based bifunctional HBD catalysts [(tBuO)2{(N(CH2CH2)3N)-
CH2CH2O}Si(OH)]+IÀ and (Rac)- and (R)-[(tBuO)2{(N(CH2CH2)3N)CH2(Et)CHO)}Si(OH)]+IÀ with tetraalkyl-
ammonium units directly incorporated into their structures were prepared from tailor-made silanols.
These bifunctional silanols were used together with other mixed alkoxysilanols of general formula
i
(tBuO)2(RO)SiOH (R = Me, Et, Pr, –CH2CH2I and –CH(Et)CH2I) in a systematic study of their catalytic
properties for the preparation of cyclic carbonates using a library of epoxides and industrial-grade carbon
dioxide. With 4 mol% catalyst loading in the absence of a solvent and an external nucleophile source, the
bifunctional catalysts showed good to very good conversion of epoxides to the corresponding cyclic
carbonates within 10 h at 70 1C and 75 psi of CO2. Furthermore, the developed synthetic approach used
in the preparation of these mixed alkyl silanols via the hydrolysis of the corresponding acetoxysilyl alkoxides
(tBuO)2(RO)Si(OAc) (R = organic moiety) allowed a straightforward route to the modification of the steric bulk
around the silicon atom and the introduction of functional groups for further derivatization.
Received 24th September 2019,
Accepted 4th November 2019
DOI: 10.1039/c9nj04840e
rsc.li/njc
Introduction
Carbon dioxide is an ideal C1 building block in organic synthesis
because it is abundant, inexpensive and nontoxic. In this sense,
the development of chemical transformations involving CO2 as a
starting material has attracted attention in green and sustainable
chemistry.1–5 Particularly, the synthesis of cyclic carbonates
through cycloaddition of CO2 with epoxides is considered as
one of the most promising processes due to its 100% atom
utilization ratio.6,7 Furthermore, cyclic carbonates find high
Scheme 1 Synthesis of cyclic carbonates from epoxides and the catalytic
reduction of the carbonate to methanol.
utility as polar aprotic solvents, electrolytes in lithium batteries, attracts wide attention both in academia and industry.7,17,18
starting materials for different polymeric materials, and as A dynamic field in this area is the use of metal-free catalysts since
chemical precursors.8–12 Moreover, the presence of five-membered they are usually cheaper, readily available, less toxic than metal-
cyclic carbonate fragments in the structure of some natural products containing species and do not tend to initiate the polymerization
has been described.13 Additionally, cyclic carbonates can be of the cyclic carbonates.17 A variety of hydrogen bond donors
catalytically reduced, producing methanol and 1,2-diols,14,15 (HBD) have been identified as metal-free catalysts to facilitate the
where the latter can be converted back to the epoxide (Scheme 1).16 synthesis of cyclic carbonates, as they are capable of activating the
Therefore, the development of new and inexpensive catalytic epoxide moiety via the formation of a hydrogen bond with the
systems capable of selectively producing these cyclic carbonates oxygen atom as an acceptor.19–22 However, these HBDs need to
under mild reaction conditions without the formation of polymers be accompanied by suitable nucleophiles such as quaternary
ammonium salts acting as cocatalysts, resulting in two-
a
component catalytic systems. These systems allow the synthesis
of cyclic carbonates at low temperature and low pressure due to the
synergistic effect of hydrogen bonding and nucleophilic anions,
making the ring-opening of the epoxide more efficient.6,23 In this
regard, HBDs such as phenols,24 alcohols,25–27 carboxylic28 or
boronic acids29 and ionic liquids30 have been used to facilitate
the synthesis of cyclic carbonates usually in combination with
´
´
´
Universidad Nacional Autonoma de Mexico, Instituto de Quımica,
´
´
Ciudad Universitaria, Ciudad de Mexico, 04510, Mexico
b
´
´
Centro Conjunto de Investigacion en Quımica Sustentable UAEM-UNAM,
´
´
Carr. Toluca-Atlacomulco km 14.5, 50200 Toluca, Estado de Mexico, Mexico.
E-mail: vjancik@unam.mx; Fax: +52 55 56162217; Tel: +52 722 2766610
† Electronic supplementary information (ESI) available: Copies of 1H, 13C and
29Si NMR spectra of products. CCDC 1917027–1917029. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c9nj04840e
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tetraalkylammonium salts. Therefore, one way to improve the Results and discussion
HBD catalytic systems for the synthesis of carbonates is the
specific design of new bifunctional catalysts that would contain
Synthesis and characterization
The traditional synthetic methods for silanols starting from
SiCl438–42 or alkoxysilanes43,44 present significant disadvantages
as especially the use of chlorosilanes requires the use of large
quantities of dry solvents and bases such as aliphatic amines,45
anilines,46 or pyridine38 to capture the HCl produced during the
reaction and the subsequent hydrolysis of the trialkoxychloro-
silane to the final silanol. Furthermore, any trace of acid or base
in the final product catalyzes the condensation of the Si–OH
groups and this tendency increases with the number of Si–OH
groups attached to the same silicon atom due to their higher
acidity.47 In addition, silanols with at least one alkoxy group
with a lower steric bulk are oils further hampering their
purification.42 In this regard, we have recently reported a green
method for the synthesis of alkoxysilanetriols from triacet-
oxysilylalkoxides37 where the only byproduct is acetic acid or
acetamide that helps stabilize the target silanols. Therefore, to
obtain the silanols (tBuO)2(RO)Si(OH) we have designed a two-
step synthesis from (tBuO)2Si(OAc)2 (1) (obtained from Si(OAc)4
in their molecule the required nucleophile and a few of such
systems have been reported recently, but they usually need high
pressures and/or temperatures.28,31–35
Silanols are compounds with at least one Si–OH group and
are an emerging class of HBD catalysts. In this regard, a bis(1-
naphthyl)silanediol/TBAI system was used for the cycloaddition
of CO2 with different epoxides under mild reaction conditions,36
and our research group evaluated the catalytic activity of
the more acidic but less stable alkoxysilanetriols and alkoxy-
bis(silanetriols) in the synthesis of styrene carbonate.37
A quantitative conversion was achieved after 15 h at 60 1C
and 1 atm of CO2, using tetrabutylammonium iodide (TBAI) as
the nucleophile source. In both cases, the use of a cocatalyst
was necessary to facilitate the opening of the epoxide. Despite
their good catalytic activity, more extensive use of silanols is
hampered by difficulties during their synthesis and purification
and especially the propensity of the silanols to self-condense,
which increases with the number of OH groups attached to the
same silicon atom.36,37
We envisioned the use of trialkoxysilanols with a general
formula (tBuO)2(RO)SiOH, where the SiO4 unit increases the
acidity of the Si–OH group when compared to the analogue with
Si–C bonds, while the two bulky tBu groups help to increase the
stability of the silanols against hydrolysis and condensation.
The variation of the third alkoxy group allows fine-tuning of the
steric properties and incorporation of further functional groups
into the molecule. However, the synthesis of such mixed-alkyl
trialkoxysilanols is a synthetic challenge due to poor control
over the degree of substitutions with two different alkoxy
groups. Herein we report a straightforward synthesis of such
mixed-alkyl (tBuO)2(RO)Si(OH) silanols from silicon tetraacetate
and simple alcohols. Furthermore, silanols with –O–CH2CH2I
and –O–CH(Et)CH2I groups were used in the synthesis of the first
examples of ionic silanol-based HBD catalysts [(tBuO)2{(N(CH2-
CH2)3N)CH2CH2O}Si(OH)]+IÀ and (Rac)- and (R)-[(tBuO)2{(N-
(CH2CH2)3N)CH2(Et)CHO}Si(OH)]+IÀ (Fig. 1) that contain in the
same molecule the acidic Si–OH HBD group and a quaternary
ammonium ion with an IÀ anion. Finally, the tailor-made silanols
were evaluated as HBD catalysts in the synthesis of cyclic
carbonates using a library of epoxides and industrial-grade
carbon dioxide.
t
and 2 equivalents of BuOH)48 and the corresponding alcohol
(methanol, ethanol, iodoethanol, isopropanol or 1-iodo-2-
butanol) yielded the monoacetoxysilylalkoxides (tBuO)2(RO)-
i
Si(OAc) (2, R = Me; 3, R = Et; 4, R = CH2CH2I; 5, R = Pr; 6,
R = CH(Et)CH2I) with increasing steric bulk around the silicon
atom (Schemes 2 and 3). As anticipated, the higher volume of
the alcohol increases the difficulty in substituting one of the
acetate groups. However, it is possible to obtain compounds 2–6
in a pure form in nearly quantitative yields varying the concentration
of the alcohol and reaction times without any purification. Stirring of
Scheme 2 General method for the preparation of (a) monoacetoxysilyl-
alkoxides and silanols, (b) silanol-based bifunctional HBD catalysts.
Scheme 3 Prepared (a) monoacetoxysilylalkoxides 2–6; (b) the corres-
ponding silanols 7–11 and (c) silanol-based bifunctional HBD catalysts 12
and 13.
Fig. 1 Bifunctional silanol-based HBD catalysts proposed in this work.
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compounds 2–6 for 1 to 6 hours in an aqueous ammonia
solution at ambient temperature (see the Experimental section)
yielded the corresponding monosilanols 7–11.
The hydrolysis proceeds via a nucleophilic attack of ammonia
on the CQO group of the acetate substituent as evidenced by the
formation of acetamide. Again, higher steric bulk of the R group
requires longer reaction times and a higher concentration of the
ammonia solution to achieve complete conversion. Under these
conditions, no evidence of condensation of the silanols or
hydrolysis of the alkoxide groups was observed. It is noteworthy
that both the acetoxysilanes and the silanols, even those with
small R substituents, are air- and moisture-stable facilitating
their handling and storage. Furthermore, the exceptional stability
of these silanols was demonstrated by using compounds 9 and
11 in reactions with 1,4-diazabicyclo[2.2.2]octane (DABCO) in
boiling toluene to synthesize the first examples of unprecedented
silanol-based bifunctional HBD catalysts [(tBuO)2{(N(CH2CH2)3N)-
Fig. 2 Molecular structures of compounds 12, (Rac)-13 and (R)-13, with
CH2CH2O}Si(OH)]+IÀ (12) and [(tBuO)2{(N(CH2CH2)3N)CH2(Et)- thermal ellipsoids set at the 50% probability level. Carbon-bound hydrogen
CHO}Si(OH)]+IÀ (13), respectively (Schemes 2 and 3). Compounds
atoms (except in the chiral carbon of 13) were eliminated for the sake of clarity.
11 and 13 were prepared both as racemic mixtures and as pure R
enantiomers that are labeled here as (Rac)- and (R)-, respectively.
Table 1 Selected bond distances (Å) and angles (1) for compounds 12,
(Rac)-13 and (R)-13
All compounds were characterized by 29Si, 13C, and 1H NMR
and IR spectroscopy, elemental analysis, and mass spectrometry.
12
(Rac)-13
(R)-13
Additionally, the molecular structures of the ionic silanols 12 and
13 were determined by single crystal X-ray diffraction studies. The
bands associated with the CQO stretching vibration of the
acetate group in the (tBuO)2(RO)Si(OAc) compounds are shifted
Si–OH
1.628(7)
1.615(4)
1.620(3)
Si–OtBu
Si–OR
1.598(5)–1.618(6) 1.593(5)–1.611(4) 1.609(3)–1.611(3)
1.625(5)
3.404(8)
2.62(4)
1.645(4)
3.392(4)
2.67(3)
1.637(3)
3.403(3)
2.584(19)
OÁ Á ÁI
to lower wavenumbers (v 1736–1742 cmÀ1) when compared with
OHÁ Á ÁI
O–C (range)
N–Cexo
˜
À1
t
those for Si(OAc)4 (v 1760 cm ) or ( BuO)2Si(OAc)2 (v 1744 cmÀ1).
1.419(8)–1.464(9) 1.424(6)–1.440(7) 1.432(4)–1.443(3)
1.518(8) 1.507(6) 1.513(4)
˜
˜
These bands are absent in silanols 7–11 that contain broad bands
O–Si–O (range) 103.9(4)–109.3(4) 103.5(2)–116.1(3) 103.4(1)–115.6(2)
À1
Si–O–H
O–HÁ Á ÁI
125(6)
0.841(11)
119(3)
0.828(14)
125(3)
0.840(14)
˜
at v 3384–3403 cm
corresponding to the OH groups. The
complete conversion of the acetates into the silanols has also
C–O–Si (range) 123.4(6)–134.9(5) 122.8(3)– 136.6(4) 122.7(2)–134.4(2)
1
been corroborated by H NMR spectra of silanols 7–11 that are
C–N–Cexo
108.8(6)–111.1(5) 106.8(4)–113.1(4) 106.5(3)–112.7(3)
devoid of the signals for the acetate group but contain a broad
signal for the hydrogen of the OH moiety. Furthermore, 1H NMR
spectra of 12 and 13 contain also the signals corresponding to the
monoalkylated DABCO fragment. The IR spectra of 12, (Rac)-13
P
of H and I ( rVdW (I,H) 3.16 Å) and explain the rather large shift
in the frequency of the stretching vibration for the OH group in
the IR spectra (vide supra).49 In all structures, the SiO4 unit has a
distorted tetrahedral geometry with angles ranging from
101.6(8) to 114.0(4)1. The Si–OtBu (1.598(5)–1.618(6) Å), Si–OR
(1.625(5)–1.647(6) Å) and Si–O(H) (1.614(6)–1.628(7) Å) bond
lengths are only slightly influenced by the different alkyl sub-
stituents. Selected bond lengths and angles for compounds 12,
(Rac)-13 and (R)-13 are listed in Table 1.
and (R)-13 contain the broad bands (v 3275–3300 cmÀ1) of the OH
˜
group, that are shifted to lower wavenumbers compared to
À1
those of precursors 9 (v 3396 cm ), (Rac)-11 (v 3398 cmÀ1) or
˜
˜
(R)-11 (v 3403 cmÀ1).
˜
Description of crystal structures
The molecular structures of the ionic silanols 12, (Rac)-13 and
(R)-13 were determined by single crystal X-ray diffraction
studies. Single crystals were obtained from a saturated THF/
acetonitrile solution (12, (Rac)-13) and a THF/toluene ((R)-13)
Catalytic studies
Initial screening. The bifunctional HBD catalysts 12 and 13,
solution at À30 1C. Compound 12 crystallizes in the monoclinic together with silanols 7–11 and (tBuO)3SiOH, were evaluated as
space group P21/c, while compound 13 ((Rac)- and (R)-) was catalysts in the cycloaddition of CO2 with styrene oxide (SO) to
solved in the monoclinic P21 space group (Fig. 2 and Table S2 in produce styrene carbonate (SC) (Fig. 3). Tetrabutylammonium
the ESI†). The molecular structures confirm the incorporation iodide (TBAI) was used as a cocatalyst in the case of silanols
of the DABCO moiety into the molecule and the formation of 7–11, while ionic silanols 12 and 13 were used without any
the ionic species. The main feature of these compounds is additional iodide source. The single crystal analysis of 12 and
the presence of an O–HÁ Á ÁI hydrogen bond with distances of 13 revealed a strong interaction between the silanol group and
2.62(4) Å (12), 2.74(2) ((Rac)-13) and 2.58(2) ((R)-13) Å, respec- the iodide anion, suggesting that higher temperature will be
tively, that are shorter than the sum of the van der Waals radii necessary to increase the competitivity of the epoxides with the
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Fig. 3 Silanols, including the bifunctional HBD catalysts 12 and 13, used in
the cycloaddition of CO2 with styrene oxide.
Table 2 Hydrogen-bond donor catalyst screening
Fig. 4 The effects of different parameters on the yield of SC catalyzed by 12.
Entry
Catalyst
Catalyst (mol%)
TBAI (mol%)
Yield
(82 and 77%, respectively; Fig. 4a). A nearly linear dependence
(R2 = 0.9815) was observed between the CO2 pressure and the
yield of SC due to the increased interaction of CO2 with the
epoxide and the catalyst (Fig. 4b). Increasing temperature led to
a higher yield, reaching a maximum at 70 1C (Fig. 4c). However,
a further increase in temperature caused a decrease in the yield,
possibly due to side reactions.50 The influence of reaction time
on the yield was also evaluated (Fig. 4d), indicating that a
reaction time of 10 h was necessary for the maximum conversion of
styrene oxide under the selected conditions. Longer reaction time
resulted in a decrease in the yield. Therefore, the optimized reaction
conditions for the reaction are 70 1C, 75 psi, 4 mol% of 12, and 10 h.
1
2
3
4
5
6
7
8
7
8
9
10
5
5
5
5
5
5
5
5
5
5
—
5
5
5
5
5
5
5
—
—
—
5
77
60
81
63
86
85
70
82
75
72
38
(Rac)-11
(R)-11
(tBuO)3SiOH
12
(Rac)-13
(R)-13
9
10
11
iodide anions for the coordination to the Si–OH group. Therefore,
the reaction conditions for the initial screening were set to 70 1C, 60
psi CO2, 5 mol% catalyst loading, and 15 hour reaction time.
Under these conditions, a synergistic effect between the
ammonium salt and the silanol is observed, as the reaction
does not proceed in the absence of TBAI, while the absence of
silanol leads to only 38% conversion (Table 2). On the other
hand, monosilanols 7, 8 and 10 (7, R = Me; 8, R = Et; 10, R = iPr)
present moderate catalytic activity (60–77%) similar to the
bulkier monosilanol (tBuO)3SiOH (70%), while compounds 9,
(Rac)-11 and (R)-11 that contain a covalently bound iodine atom
show conversions of 81–86%. Finally, the use of the functionalized
silanols 12, (Rac)-13 and (R)-13 (based on 9 and 11) led under the
same conditions to conversions of 72–82% even in the absence
of TBAI (Table 2).
Cycloaddition of CO2 with various epoxides
To study the general applicability of the neutral and ionic
bifunctional silanol catalysts, the cycloaddition reactions of
CO2 with a broad range of different epoxides were investigated,
using silanols 9, 11–13 as catalysts (Fig. 5 and Table 4). Epoxides
with electron-withdrawing and electron-donating substituents can
be converted to the respective carbonates in good to very good yields
at 70 1C, 75 psi within 10 h, using neutral silanols 9 and 11 (with
TBAI as cocatalyst) or in the case of the bifunctional catalysts 12 and
13, even in the absence of an external nucleophile source. The very
good conversions of 1,2-epoxy-3-phenoxypropane can be explained
by the electron-withdrawing substituent, which facilitates the
nucleophilic attack of the iodine during the ring opening of the
epoxide. Cyclohexene oxide shows none to a very low conversion
Optimization of the reaction parameters
Based on the initial screening, optimization of the reaction to the corresponding cyclic carbonate, due to its high steric
conditions for the preparation of styrene carbonate was under- hindrance.30,31
taken. The bifunctional HBD catalyst 12 was used to evaluate its
Furthermore, to confirm that the activity is related to the
potential because it showed one of the highest conversions combination of silanol and the iodide nucleophile, a control
under the screening conditions. Fig. 4 shows the effect of the experiment using only 1,4-diazabicyclo[2.2.2]octane was carried out
amount of catalyst, CO2 pressure, reaction temperature, and using the optimized conditions (4% DABCO loading, 70 1C, 75 psi,
reaction time on the SC yield, while the different reaction 10 h). However, DABCO was found to be inactive in the synthesis of
conditions for this optimization, including the final optimized styrene carbonate (o1% conversion) and even epichlorohydrin
reaction parameters are tabulated in Table 3. Increasing the (highly reactive epoxide with an electron-withdrawing substituent)
amount of 12 from 2 to 4 mol% led to a significant increase of was converted to the corresponding carbonate only from 8%.
the SC formed (60 to 87%). However, a further increase of the Therefore, both the acidic Si–OH HBD and the quaternary ammo-
catalyst loading to 5 and 10 mol% led to lower SC conversions nium ion with IÀ anions are necessary for the catalytic activity.
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Table 3 All conditions used in the optimization of the reaction conditions for the preparation of styrene carbonate. Numbers in bold gave the best
results
Optimized condition
Catalyst loading (mol%)
Temperature (1C)
Pressure of CO2 (psi)
Time (h)
Initial screening
Catalyst loading
CO2 pressure
Temperature
Time
5
70
70
70
60
60
15
15
15
15
2, 3, 4, 5, 10
4
4
4
4
15, 45, 60, 75, 90
25, 40, 60, 70, 80
70
70
75
75
75
5, 10, 15, 20
10
Best conditions
Fig. 6 Comparative yields of SC obtained after three consecutive runs
using recycled catalyst 12.
Fig. 5 Library of epoxides used in the study.
Conclusions
The enantiopure (R)-1,2-butylene oxide was used as a sub-
strate with the (R)-13 catalyst under the optimized conditions
to determine the degree of retention of the chirality of the
epoxide. (R)-1,2-Butylene carbonate was isolated with 99%
enantiomeric excess (ee) confirming nucleophilic attack of
the iodide ion at the less hindered C1-carbon of the epoxide.
The reaction of (Rac)-1,2-butylene with CO2 using the chiral
catalyst (R)-13 produced a racemic mixture of the corres-
ponding carbonate probably due to the use of elevated temperature
(70 1C) and longer distance of the chiral center from the Si–OH
group that hamper the control of enantioselectivity.51–53
Furthermore, the recyclability of catalyst 12 was examined
using SO as the substrate under the optimized reaction conditions.
After the reaction, catalyst 12 was precipitated from the reaction
mixture using diethyl ether. A gradual loss of the catalytic activity
after the first run was observed (Fig. 6) probably due to catalyst
inactivation.
We described the first bifunctional silanol-based HBD catalysts
12 and 13 that can be synthesized using an easy methodology
via the (tBuO)2(RO)Si(OAc) and (tBuO)2(RO)Si(OH) intermediates.
Furthermore, this synthetic method opens the possibility for the
introduction of a wide range of functional groups (even chiral
ones) and therefore easy access to tailor-made silanol-based
HBD catalysts. Compounds 12 and 13 proved to be efficient
catalysts for the cycloaddition of various epoxides and CO2
under mild reaction conditions (70 1C, 75 psi bar CO2, 10 h)
without a solvent and in the absence of any metal or external
nucleophile source. Good to very good conversion was observed,
even if the epoxide needs to compete for the Si–OHÁ Á ÁO(epoxide)
interaction with the iodide anion. Therefore, these systems
represent an exciting starting point for a new class of bifunctional
HBD catalysts based on silanols. This work is currently underway
in our laboratory.
Table 4 Cycloaddition of CO2 with various epoxides catalyzed by silanols 9, 11–13
Cyclic carbonate (conversion)
Catalyst
Cat. (mol%)
TBAI (mol%)
15a (%)
15b (%)
15c (%)
15d (%)
15e (%)
15f (%)
15g (%)
15h (%)
9
4
4
4
4
4
4
4
4
4
—
—
—
74
87
81
87
76
79
83
92
95
72
73
72
—
92
91
—
75
85
97
499
499
98
98
98
96
89
84
98
95
92
91
85
84
82
87
85
92
93
95
76
75
72
12
12
10
4
0
0
11-(Rac)
11-(R)
12
13-(Rac)
13-(R)
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Experimental section
General information
solution of the corresponding alcohol in THF was added to
(tBuO)2Si(OAc)2 1. The reaction mixture was stirred at ambient
temperature (20 1C) for 16 to 72 h. Afterward, all volatiles were
removed under reduced pressure, and the product was isolated
as a colorless oil.
Extra dry industrial grade carbon dioxide with 99.8% purity was
purchased from INFRA, S.A. de C.V. and used without further
purification. The synthesis of compounds 1–6 was performed
under a dried dinitrogen atmosphere using Schlenk and glove-
box techniques. On the other hand, the synthesis of silanols
7–13 did not require the use of an inert atmosphere. Solvents
were purchased from Sigma-Aldrich and where necessary dried
before use with an MBraun SPS solvent purification system
(tBuO)2(MeO)Si(OAc) (2): methanol (1.5 M in THF, 4.6 mL,
6.84 mmol); 1 (2.00 g, 6.84 mmol); stirring for 16 h. Yield: 1.65 g,
91%. Elemental analysis (%) calcd for C11H24O5Si (264.39 g molÀ1):
C 49.97, H 9.15; found: C 49.38, H 8.72. FT-IR (ATR) (cmÀ1) 2977,
1
2935 (w, C–H, CH3), 1739 (m, CQO), 1067 (s, Si–O–C). H NMR
(300.53 MHz, CDCl3): d (ppm) 3.60 (s, 3H, CH3), 2.10 (s, 3H, OCCH3),
1.33 (s, 18H, C(CH3)3). 13C{1H} NMR (75.57 MHz, CDCl3): d (ppm)
168.9 (OCCH3), 74.2 C(CH3)3, 51.3 (CH3), 31.2 (C(CH3)3), 23.0
(OCCH3). 29Si NMR (59.63 MHz, CDCl3): d (ppm) À95.2. EI-MS:
m/z (%) 249 (13) [M À Me]+, 191 (21) [M À OtBu]+, 135 (100)
t
using Grubs’ columns. BuOH was dried with metallic sodium
and distilled before use; tetrabutylammonium iodide (TBAI)
was purchased from Sigma-Aldrich and recrystallized from a
CH2Cl2/diethyl ether mixture. 1,4-Diazabicyclo[2.2.2]octane
(DABCO), 2-iodoethanol, (Rac)- and (R)-1,2-epoxybutane, epichloro-
hydrin, styrene oxide, cyclohexene oxide, 2-(4-fluorophenyl)-
oxirane, 2-(4-chlorophenyl)oxirane and 1,2-epoxy-3-phenoxy-
propane were purchased from Sigma-Aldrich and used without
further purification, whereas (tBuO)2Si(OAc)2 1 was prepared
according to the literature procedure from Si(OAc)4 and tBuOH.48
1-iodobutan-2-ol was prepared using a modified methodology for
1-iodopropan-2-ol.54 CDCl3 was used without further purification.
NMR spectroscopic data were recorded on a Bruker Avance III
300 MHz or a Varian Inova 500 MHz spectrometer. FT-IR spectra
t
[M À 2 Bu À Me]+.
(tBuO)2(EtO)Si(OAc) (3): ethanol (1.5 M in THF, 4.6 mL,
6.84 mmol); 1 (2.00 g, 6.84 mmol); stirring for 48 h. Yield: 1.75 g,
95%. Elemental analysis (%) calcd for C12H26O5Si (278.42 g molÀ1):
C 51.77, H 9.41; found: C 50.42, H 9.00. FT-IR (ATR) (cmÀ1) 2976,
2932 (w, C–H, CH3, CH2), 1742 (w, CQO), 1060 (s, Si–O–C). 1H NMR
(300.53 MHz, CDCl3): d (ppm) 3.90 (q, 2H, 3JH–H = 7.0 Hz, CH2CH3,
3
2.10 (s, 3H, OCCH3), 1.33 (s, 18H, C(CH3)3), 1.23 (t, 3H, JH–H
=
7.0 Hz, CH2CH3). 13C{1H} NMR (75.57 MHz, CDCl3): d (ppm) 168.6
(OCCH3), 74.0 C(CH3)3, 59.4 (CH2CH3), 31.0 (C(CH3)3), 22.8
(OCCH3), 17.7 (CH2CH3). 29Si NMR (59.63 MHz, CDCl3): d (ppm)
À96.6. EI-MS: m/z (%) 263 (21) [M À Me]+, 219 (2) [M À OAc]+,
were measured on a Bruker Tensor 27 using the ATR technique
with a diamond window in the range of v 500–4000 cm
À1
˜
.
Electron impact mass spectrometry (EI-MS) measurements were
carried on a Shimadzu GCMS-QP2010 Plus using direct insertion
in the detection range of m/z 20–1090. Elemental analyses
(C, H, N) were performed on an Elemental vario MICRO Cube
analyzer. It is noteworthy that the carbon content in most of the
compounds is low, probably due to the formation of silicon
carbide, which is difficult to pyrolyze. Melting points were
measured on a Bu¨chi B-540 apparatus.
Cycloaddition reactions were conducted in Q-tube systems
using the neat epoxide and the catalyst. Before the reaction, the
reactor was purged with CO2 and subsequently pressurized
with CO2 to the selected pressure. The reactor was heated to
the desired temperature for a preset time, whereupon it was
cooled to room temperature. THF (10 mL) was added, and the
reaction mixture was filtered. The obtained solution was analyzed
using gas chromatography on an Agilent 7890B instrument with
an FID using an HP-5 capillary column (30 m, 0.320 mm,
0.25 mm) with nitrogen as the carrier gas. The analysis conditions
for most of the carbonates were as follows: initial temperature
50 1C, kept for 5 minutes, then increased at a ramp rate of
10 1C minÀ1 to 250 1C, and held at the final temperature for
5 minutes. Cyclic carbonates were purified by distillation or
crystallization and the isolated carbonate products were charac-
terized by 1H NMR and matched with previously reported
data34,36 (see the ESI†).
t
205 (36) [M À OtBu]+, 149 (100) [M À 2 Bu À Me]+.
(tBuO)2(ICH2CH2O)Si(OAc) (4): 2-iodoethanol (1.3 M in THF,
5.3 mL, 6.84 mmol); 1 (2.00 g, 6.84 mmol); stirring for 72 h.
Yield: 2.57 g, 93%. Elemental analysis (%) calcd for C12H25O5SiI
(404.31 g molÀ1): C 35.65, H 6.23; found: C 34.73, H 6.13. FT-IR
(ATR) (cmÀ1) 2976, 2934 (w, C–H, CH3, CH2), 1736 (m, CQO),
1065 (s, Si–O–C). 1H NMR (300.53 MHz, CDCl3): d (ppm) 4.06 (t,
3
3
2H, JH–H = 7.0 Hz, CH2CH2I), 3.26 (t, 2H, JH–H = 7.0 Hz,
CH2CH2I), 2.09 (s, 3H, OCCH3), 1.34 (s, 18H, C(CH3)3). 13C{1H}
NMR (75.57 MHz, CDCl3): d (ppm) 169.1 (OCCH3), 74.6 C(CH3)3,
64.8 (CH2CH2I), 31.3 (C(CH3)3), 23.1 (OCCH3), 5.8 (CH2CH3I).
29Si NMR (59.63 MHz, CDCl3): d (ppm) À97.0. EI-MS: m/z (%)
389 (16) [M À Me]+, 331 (36) [M À OtBu]+, 345 (3) [M À OAc]+,
t
275 (100) [M À 2 Bu À Me]+.
(tBuO)2(iPrO)Si(OAc) (5): isopropanol (2.6 M in THF, 2.7 mL,
6.84 mmol); 1 (2.0 g, 6.84 mmol); stirring for 48 h. Yield: 1.85 g,
92%. Elemental analysis (%) calcd for C13H28O5Si (292.44 g
molÀ1): C 53.39, H 9.65; found: C 52.03, H 9.21. FT-IR (ATR)
(cmÀ1) 2976, 2935 (w, C–H, CH3), 1739 (m, CQO), 1068 (s,
Si–O–C).1H NMR (300.53 MHz, CDCl3): d (ppm) 4.34 (sept, 1H,
3JH–H = 6.1 Hz, CH(CH3)2, 2.09 (s, 3H, OCCH3, 1.33 (s, 18H,
3
C(CH3)3), 1.21 (d, 6H, JH–H = 6.1 Hz, CH(CH3)2), 13C{1H} NMR
(75.57 MHz, CDCl3): d (ppm) 169.0 (OCCH3), 74.2 C(CH3)3
66.7 (CH(CH3)2), 31.3 (C(CH3)3), 25.2 (CH(CH3)2), 23.3 (OCCH3).
29Si NMR (59.63 MHz, CDCl3): d (ppm) À97.9. EI-MS: m/z (%)
277 (18) [M À Me]+, 233 (20) [M À OAc]+, 219 (47) [M À OtBu]+,
163 (100) [M À 2 Bu À Me]+.
General synthetic route for monoacetoxysilylalkoxides 2–6
t
Monoacetoxysilylalkoxides (tBuO)2(RO)Si(OAc) (R = Me, Et,
(tBuO)2{(ICH2(Et)CHO)}Si(OAc) (Rac)-6: (Rac)-1-iodobutan-2-
i
t
ICH2CH2, Pr, ICH2(Et)CH, Bu) were prepared as follows: a ol (1.37 g, 6.84 mmol); 1 (2.00 g, 6.84 mmol); no solvent; stirring
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Paper
for 72 h. Yield: 2.65 g, 90%. Elemental analysis (%) calcd for
(tBuO)2(ICH2CH2O)Si(OH) (9): 4 (2.57 g, 6.34 mol); aqueous
C
14H29O5SiI (432.37 g molÀ1): C 38.89, H 6.76; found: C 37.15, H ammonia (7.5 M, 10 mL); stirring for 4 h. Yield: 2.21 g, 96%.
6.50. FT-IR (ATR) (cmÀ1) 2975, 2935 (w, C–H, CH3, CH2), 1736 Elemental analysis (%) calcd for C10H23O4SiI (362.28 g molÀ1):
(m, CQO), 1068 (s, Si–O–C). 1H NMR (300.53 MHz, CDCl3): C 33.15, H 6.40; found: C 33.17, H 6.58. FT-IR (ATR) (cmÀ1) 3396
3
d (ppm) 3.83 (pent, 1H, JH–H = 7.0 Hz, CHO), 3.34 (m, 2H, (w, br, O–H), 2974, 2933 (w, C–H, CH3, CH2), 1056 (s, Si–O–C).
3
3
CHCH2I), 2.09 (s, 3H, OCCH3), 1.67 (pent, 2H, JH–H = 7.3 Hz, 1H NMR (300.53 MHz, CDCl3): d (ppm) 3.99 (t, 2H, JH–H
=
3
3
CH2CH3), 1.34 (s, 18H, C(CH3)3), 0.90 (t, 3H, JH–H = 7.3 Hz, 7.0 Hz, CH2CH2I, 3.28 (t, 2H, JH–H = 7.0 Hz, CH2CH2I), 2.51
CH2CH3). 13C{1H} NMR (75.57 MHz, CDCl3): d (ppm) 168.9 (s, br, 1H, OH), 1.34 (s, 18H, C(CH3)3). 13C{1H} NMR
(OCCH3), 74.6 C(CH3)3, 73.4 CHO, 31.3 (C(CH3)3), 29.3 (75.57 MHz, CDCl3): d (ppm) 73.7 C(CH3)3, 64.3 (CH2CH2I),
(CHCH2I), 23.2 (OCCH3), 13.0 (CH2CH3), 9.2 (CH2CH3). 29Si 31.4 (C(CH3)3), 6.1 (CH2CH2I). 29Si NMR (59.63 MHz, CDCl3):
NMR (59.63 MHz, CDCl3): d (ppm) À98.2. EI-MS: m/z (%) 417 d (ppm) À87.4. EI-MS: m/z (%) 437 (100) [M À Me]+, 289 (38)
(10) [M À Me]+, 359 (40) [M À OtBu]+, 303 (58) [M À 2 tBu À Me]+. [M À OtBu]+, 233 (62) [M À C4H8 À Me]+.
(tBuO)2{(ICH2(Et)CHO)}Si(OAc) (R)-6: (R)-1-iodobutan-2-ol
(tBuO)2(iPrO)Si(OH) (10): 5 (1.85 g, 6.31 mmol); aqueous
(1.37 g, 6.84 mmol); 1 (2.00 g, 6.84 mmol); no solvent; stirring ammonia (7.5 M, 10 mL); stirring for 6 h. Yield: 1.45 g, 92%.
for 72 h. Yield: 2.70 g, 91%. Elemental analysis (%) calcd for Elemental analysis (%) calcd for C11H26O4Si (250.16 g molÀ1): C
C
14H29O5SiI (432.37 g molÀ1): C 38.89, H 6.76; found: C 37.15, H 52.76, H 10.47; found: C 51.38, H 10.33. FT-IR (ATR) (cmÀ1
)
6.50. FT-IR (ATR) (cmÀ1) 2974, 2935 (w, C–H, CH3, CH2), 1736 3393 (w, br, O–H), 2974, 2933 (m, C–H, CH3), 1057 (s, Si–O–C).
(m, CQO), 1068 (s, Si–O–C). 1H NMR (300.53 MHz, CDCl3): 1H NMR (300.53 MHz, CDCl3): d (ppm) 4.24 (sept, 1H, JH–H
=
3
3
d (ppm) 3.83 (pent, 1H, JH–H = 7.3 Hz, CHO, 3.34 (m, 2H, 6.1 Hz, CH(CH3)2, 2.62 (s, br, 1H, OH), 1.33 (s, 18H, C(CH3)3),
CHCH2I), 2.10 (s, 3H, OCCH3), 1.66 (pent, 2H, JH–H = 7.3 Hz, 1.21 (d, 6H, JH–H = 6.1 Hz, CH(CH3)2), 13C{1H} NMR (75.57
3
3
3
CH2CH3), 1.34 (s, 18H, C(CH3)3), 0.90 (t, 3H, JH–H = 7.3 Hz, MHz, CDCl3): d (ppm) 73.1 (C(CH3)3), 65.9 (CH(CH3)2, 31.3
CH2CH3). 13C{1H} NMR (75.57 MHz, CDCl3): d (ppm) 169.2 (C(CH3)3), 25.2 (CH(CH3)2) 29Si NMR (59.63 MHz, CDCl3):
(OCCH3), 74.6 (C(CH3)3), 73.4 CHO, 31.3 (C(CH3)3), 29.4 d (ppm) À87.6. EI-MS: m/z (%) 235 (100) [M À Me]+, 179 (100)
(CHCH2I), 23.2 (OCCH3), 13.2 (CH2CH3), 9.2 (CH2CH3). 29Si [M À C4H8 À Me]+.
NMR (59.63 MHz, CDCl3): d (ppm) À98.0. EI-MS: m/z (%) 417 (8)
(tBuO)2{(ICH2(Et)CHO)}Si(OH) (Rac)-11: 6 (2.65 g, 6.13 mmol)
[M À Me]+, 359 (37) [M À OtBu]+, 303 (48) [M À 2 Bu À Me]+. was suspended in a solution of aqueous ammonia (7.5 M,
t
10 mL) for 4 hours. Yield: 2.22 g, 93%. Elemental analysis (%)
calcd for C12H27O4SiI (390.33 g molÀ1): C 36.92, H 6.97; found:
C 36.16, H 6.73. FT-IR (ATR) (cmÀ1) 3398 (w, br, O–H), 2974,
General synthetic route for monosilanols 7–11
The corresponding monoacetoxysilylalkoxide was suspended in 2934 (w, C–H, CH3, CH2), 1057 (s, Si–O–C). 1H NMR (300.53
a solution of aqueous ammonia. The reaction mixture was MHz, CDCl3): d (ppm) 3.74 (m, CHO), 3.33 (m, 2H, CHCH2I),
vigorously stirred at room temperature for 1 to 6 h. Subsequently, 2.44 (s, br, 1H, OH), 1.66 (m, 2H, CH2CH3), 1.34 (s, 18H,
the silanol was extracted with ethyl acetate and dried with Na2SO4. C(CH3)3), 0.92 (t, 3H, JH–H = 7.3 Hz, CH2CH3). 13C{1H} NMR
3
Finally, all volatiles were removed under reduced pressure, and the (75.57 MHz, CDCl3): d (ppm) 73.5 C(CH3)3, 73.2 (CHO), 31.5
product was isolated as a colorless oil.
(C(CH3)3), 29.5 ICH2(CH2CH3), 13.4 (CH2CH3), 9.4 (CH2CH3). 29Si
(tBuO)2(MeO)Si(OH) (7): 2 (1.65 g, 6.22 mmol); aqueous NMR (59.63 MHz, CDCl3): d (ppm) À88.2 EI-MS: m/z (%) 375 (62)
ammonia (3.8 M, 10 mL); stirring for 1 h. Yield: 1.24 g, 90%. [M À Me]+, 317 (45) [M À OtBu]+, 261 (70) [M À C4H8 À Me]+.
Elemental analysis (%) calcd for C9H22O4Si (222.35 g molÀ1): C
(tBuO)2{(ICH2(Et)CHO)}Si(OH) (R)-11: (R)-6 (2.70 g, 6.24 mol)
48.61, H 9.97; found: C 47.24, H 9.26. FT-IR (ATR) (cmÀ1) 3392 was suspended in a solution of aqueous ammonia (7.5 M,
(w, br, O–H), 2975, 2932 (w, C–H, CH3), 1056 (s, Si–O–C). 10 mL) for 4 hours. Yield: 2.33 g, 96%. Elemental analysis (%)
1H NMR (300.53 MHz, CDCl3): d (ppm) 4.32 (s, br, 1H, OH), calcd. for C12H27O4SiI (390.33 g molÀ1): C 36.92, H 6.97; found:
3.53 (s, 3H, CH3), 1.33 (s, 18H, C(CH3)3). 13C{1H} NMR (75.57 C 36.43, H 6.97. FT-IR (ATR) (cmÀ1) 3403 (w, br, O–H), 2973,
MHz, CDCl3): d (ppm) 73.2 C(CH3)3, 50.7 (CH3), 31.3 (C(CH3)3). 2934 (w, C–H, CH3, CH2), 1058 (s, Si–O–C). 1H NMR (300.53
29Si NMR (59.63 MHz, CDCl3): d (ppm) À85.9. EI-MS: m/z (%) MHz, CDCl3): d (ppm) 3.74 (m, CHO), 3.34 (m, 2H, CHCH2I),
207 (94) [M À Me]+, 151 (100) [M À C4H8 À Me]+.
2.44 (s, br, 1H, OH), 1.67 (m, 2H, CH2CH3), 1.34 (s, 18H,
(tBuO)2(EtO)Si(OH) (8): 3 (1.75 g, 6.30 mmol); aqueous C(CH3)3), 0.92 (t, 3H, JH–H = 7.3 Hz, CH2CH3). 13C{1H} NMR
ammonia (3.8 M, 10 mL); stirring for 2 h. Yield: 1.37 g, 92%. (75.57 MHz, CDCl3): d (ppm) 73.5 C(CH3)3, 73.2 (CHO), 31.5
Elemental analysis (%) calcd for C10H24O4Si (236.38 g molÀ1): C (C(CH3)3), 29.5 ICH2(CH2CH3), 13.4 (CH2CH3), 9.4 (CH2CH3). 29Si
50.81, H 10.23; found: C 48.80, H 9.99. FT-IR (ATR) (cmÀ1) 3384 NMR (59.63 MHz, CDCl3): d (ppm) À88.2 EI-MS: m/z (%) 375 (98)
(w, br, O–H), 2975, 2931 (w, C–H, CH3, CH2), 1056 (s, Si–O–C). [M À Me]+, 317 (58) [M À OtBu]+, 261 (100) [M À C4H8 À Me]+.
1H NMR (300.53 MHz, CDCl3): d (ppm) 4.22 (s, br, 1H, OH), 3.82
3
3
Preparation of bifunctional catalysts 12, (Rac)-13 and (R)-13
(q, 2H, JH–H = 7.0 Hz, CH2CH3), 1.33 (s, 18H, C(CH3)3), 1.22 (t,
3H, 3JH–H = 7.0 Hz, CH2CH3).13C{1H} NMR (75.57 MHz, CDCl3): [(tBuO)2{(N(CH2CH2)3N)CH2CH2O}Si(OH)]+IÀ 12: to a mixture
d (ppm) 73.2 C(CH3)3, 59.0 (CH2CH3), 31.3 (C(CH3)3), 17.9 of monosilanol 9 (1.00 g, 2.76 mmol) and DABCO (0.31 g,
(CH2CH3). 29Si NMR (59.63 MHz, CDCl3): d (ppm) À86.7. 2.76 mmol) was added toluene (3 mL). The reaction mixture
EI-MS: m/z (%) 221 (94) [M À Me]+, 165 (100) [M À C4H8 À Me]+. was heated to 100 1C over a period of 3 h. Afterward, the solvent
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was removed under reduced pressure and 12 was washed with Acknowledgements
diethyl ether (2 Â 3 mL) and hexane (2 Â 3 mL). Yield: 1.09 g,
´
˜
N. Zavala-Segovia, L. Triana-Cruz, C. Martınez-Soto, A. Nu´nez-
83%. M.p. 190–191 1C. Elemental analysis (%) calcd for
Pineda, L. Rios-Ruiz and C. Marquez-Alonso are acknowledged for
their technical assistance. This work was supported by Direccion
General de Asuntos del Personal Academico from the UNAM
(PAPIIT Grant IN206218) and CONACyT (Grant 285054). J. P.-P. is
grateful to CONACyT for her PhD fellowship (305746).
C
16H35O4N2SiI (474.45 g molÀ1): C 40.50, H 7.44, N 5.90; found:
´
C 40.07, H 7.51, N 6.11. FT-IR (ATR) (cmÀ1) 3275 (w, br, O–H),
2970, 2934 (w, C–H, CH3, CH2), 1055 (s, Si–O–C). 1H NMR
(300.53 MHz, CDCl3): d (ppm) 5.25 (s, br, 1H, OH), 4.22 (m, 2H,
´
OCH2CH2), 3.77 (m, 8H, OCH2CH2, CH2N+), 3.22 (t, 6H, 3JH–H
=
7.1 Hz, CH2Nterciary), 1.28 (s, 18H, C(CH3)3). 13C{1H} NMR
(75.57 MHz, CDCl3): d (ppm) 73.5 (C(CH3)3), 66.0 (OCH2CH2),
56.6 (OCH2CH2), 53.4 (CH2N+), 45.3 CH2Nterciary, 31.6 C(CH3).
29Si NMR (59.63 MHz, CDCl3): d (ppm) À87.7. EI-MS: m/z (%)
347 (30) [M À I]+, 112 (15) [C6H12N2]+, 56 (100) [C4H8]+.
References
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C 43.02, H 7.82, N 5.57; found: C 42.29, H 7.80, N 5.22. FT-IR
(ATR) (cmÀ1) 3300 (w, br, O–H), 2971, 2934 (w, C–H, CH3, CH2),
1056 (s, Si–O–C). 1H NMR (300.53 MHz, CDCl3): d (ppm)
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3.65 (m, 1H, CH2N+), 3.38 (m, 1H, CH2N+), 3.26 (m, 6H,
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´
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3
1.00 (t, 3H, JH–H = 7.3 Hz, CH2CH3). 13C{1H} NMR (75.57 MHz,
CDCl3): d (ppm) 73.8 C(CH3)3, 69.4 (OCH), 67.8 (CHCH2N+),
53.6 (CH2N+), 45.5 (CH2Nterciary), 31.7 (C(CH3)3), 28.7 (CH2CH3),
9.1 (CH2CH3). 29Si NMR (59.63 MHz, CDCl3): d (ppm) À88.9. EI-MS: 10 V. Besse, F. Camara, C. Voirin, R. Auvergne, S. Caillol and
m/z (%) 375 (5) [M À I]+, 112 (12) [C6H12N2]+, 56 (80) [C4H8]+.
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¨
¨
¨
(%) calcd. for C18H39O4NSiI (502.50 g molÀ1): C 43.02, H 7.82, N
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5.57; found: C 42.20, H 7.77, N 5.52. FT-IR (ATR) (cmÀ1) 3296 (w, 15 Y. Li, K. Junge and M. Beller, ChemCatChem, 2013, 5, 1072–1074.
br, O–H), 2971, 2934 (w, C–H, CH3, CH2), 1058 (s, Si–O–C). 1H NMR 16 A. Kaithal, M. Holscher and W. Leitner, Angew. Chem., Int.
¨
(300.53 MHz, CDCl3): 5.04 (s, br, 1H, OH), 4.66 (m, 1H, OCH), 3.80
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Conflicts of interest
There are no conflicts to declare.
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