Synthesis of Structurally Varied 1,3-Disiloxanediols and Their Activity as Anion-Binding Catalysts

Article abstract of DOI:10.1002/chem.201604103

A series of new 1,3-disiloxanediols has been synthesized, including naphthyl-substituted and unsymmetrical siloxanes, and demonstrated as a new class of anion-binding catalysts. In the absence of anions, diffusion-ordered spectroscopy (DOSY) displays self-association of 1,3-disiloxanediols through hydrogen-bonding interactions. Binding constants determined for 1,3-disiloxanediol catalysts indicate strong hydrogen-bonding and anion-binding abilities with unsymmetrical siloxanes displaying different hydrogen-bonding abilities for each silanol group.

Full text of DOI:10.1002/chem.201604103

DOI: 10.1002/chem.201604103
Communication
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Siloxanes |Hot Paper|
Synthesis of Structurally Varied 1,3-Disiloxanediols and Their
Activity as Anion-Binding Catalysts
Kayla M. Diemoz, Sean O. Wilson, and Annaliese K. Franz*[a]
plexes with both silanols serving as hydrogen-bond donors
Abstract: A series of new 1,3-disiloxanediols has been syn-
(e.g., 1a·pyrdine-HCl, Figure 1C).[10b]
thesized, including naphthyl-substituted and unsymmetri-
Although disiloxanediols are recognized for their hydrogen-
cal siloxanes, and demonstrated as a new class of anion-
bonding and anion-recognition capabilities, they have not
binding catalysts. In the absence of anions, diffusion-or-
been previously reported as anion-binding catalysts.[11] Anion-
dered spectroscopy (DOSY) displays self-association of 1,3-
binding catalysis has been identified as an important new
disiloxanediols through hydrogen-bonding interactions.
mode of catalysis that proceeds by activation of electrophilic
Binding constants determined for 1,3-disiloxanediol cata-
ionic substrates by hydrogen bonding to their counter
lysts indicate strong hydrogen-bonding and anion-binding
anions.[12] Here, we report the synthesis of structurally varied
abilities with unsymmetrical siloxanes displaying different
disiloxanediols and the first example of disiloxanediols as
hydrogen-bonding abilities for each silanol group.
a new class of anion-binding catalysts.[13] Catalytic activity with
1,3-disiloxanediols is demonstrated using the N-acyl Mannich
1
reaction of isoquinoline.[14] We also report H NMR experiments
Introduction
to confirm the mode of catalysis and determine the aggrega-
tion of disiloxanediols and association constants for binding to
chloride ions.
The SiÀO bond is one of the most common naturally occurring
bonds and the siloxane linkage (SiÀOÀSi), being the most
abundant motif in the earth’s crust, has wide application in sili-
cone polymers and silica materials. Unique structural and acid-
base properties of the SiÀO bond have been observed and al-
ternately attributed to either increased hyperconjugation (reso-
nance-type) delocalization[1] or the ionic character from the
electropositive Si-atom.[2] Organosilanols have been studied for
their interesting hydrogen-bonding capabilities[3] and are often
utilized as homogeneous models for the surface silanol groups
of silica materials.[4] Recently, they have been applied as hydro-
gen-bonding organocatalysts.[5]
Figure 1. Hydrogen-bonding interactions observed for 1,3-disiloxanediols in
the solid state; A: self-association in a head-to-head manner, B: self-associa-
tion in an off-set manner, C: anion-binding.
Disiloxane-1,3-diols contain a core siloxane linkage with two
silanol functionalities available for several modes of hydrogen
bonding.[3,6] Previous reports of the hydrogen-bonding capabil-
ities of 1,3-disiloxanediols in solution have focused primarily
on studies with 1,1,3,3-tetraphenyldisiloxane-1,3-diol (1a), in-
cluding applications in molecular assembly,[7] anion binding[8]
and organometallic ligands.[9] In the solid state, various 1,3-disi-
loxanediols have been observed to self-associate into higher-
ordered species through hydrogen bonding (e.g., Fig-
ure 1A,B).[3,10] In the presence of strong H-bond acceptors,
such as anions, 1,3-disiloxanediols form receptor–anion com-
Results and Discussion
Aiming at developing a modular synthetic route incorporating
various aromatic and alkyl groups, we have synthesized
a series of new 1,3-disiloxanediols, including electron-with-
drawing substituents to enhance silanol acidity (Scheme 1).
Disiloxanediols 1b–f were synthesized in overall yields of 39–
68% starting from either 1-bromonaphthalene or 1-bromo-4-
fluoronaphthalene via silanol 5. Silanol 5 was combined with
chlorosilane 4 to obtain siloxane 6, which was converted to
the disiloxanediol using a palladium-mediated hydrolysis.[15] Al-
ternative routes to obtain silanol 5 (e.g., hydrolysis of the silyl
chloride) were also compared (see the Supporting Informa-
tion), but reduction to diarylsilane 3 was selected because it
minimizes purification (i.e., column chromatography) and pro-
[a] K. M. Diemoz, S. O. Wilson, Prof. Dr. A. K. Franz
Department of Chemistry
University of California, Davis
One Shields Avenue, Davis, Ca (USA)
E-mail: akfranz@ucdavis.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201604103.
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Figure 2. 1H NMR analysis of concentration-dependent behavior of disiloxa-
nediols 1b and 1 f in C6D6. [1b]=0.01 vs 0.2m, Dd=0.89 ppm; [1 f]=0.01
vs 0.2m, Dd: SiOHA =1.59 ppm and SiOHB =1.13 ppm.
concentration was increased to 0.2m, the measured MWDOSY in-
creased to 537.35 gmolÀ1, suggesting that self-assembly into
dimers or oligomers is occurring at these concentrations. Upon
further increasing the concentration to 0.4m, the measured
MWDOSY of 743.7 gmolÀ1 approaches the calculated MW of the
dimer (MWcalc =829.2 gmolÀ1). The increasing MWDOSY value
supports the occurrence of self-association of disiloxanediols at
higher concentrations.
Scheme 1. Synthesis of 1,3-disiloxanediols (TCCA=trichloroisocyanuric acid).
ceeds with overall higher yields. Chlorosilanes 4 were commer-
cially available or synthesized in one step by chlorination of
silane precursor 3, which could also be obtained from the
naphthyl Grignard reagent.[16] Disiloxanediols 1b–f were easily
isolated after purification by column chromatography and are
bench-top stable.
Analysis of these newly synthesized 1,3-disiloxanediols using
1H NMR spectroscopy demonstrates interesting details regard-
ing their hydrogen-bonding properties. In the case of unsym-
metrical disiloxanediols, distinct shifts for each silanol hydroxy
1
group were observed.[17] Using H NMR, we observed that the
SiOH hydroxyl peaks of 1,3-disiloxanediols exhibit a concentra-
tion-dependent shift and broadening at higher concentrations,
providing evidence that self-association occurs at higher con-
centrations.
1,1,3,3-Tetranaphthyldisiloxane-1,3-diol
(1b)
showed a downfield chemical shift and broadening of the sila-
nol signal (Dd=0.89 ppm) when the concentration was in-
creased from 0.01 to 0.2m (Figure 2). In the case of unsymmet-
rical disiloxanediol 1 f, containing alkyl groups and fluoro-sub-
stituted naphthyl groups, the silanol groups were not equiva-
lent and two distinct SiOH signals (SiOHA and SiOHB) were ob-
served. With disiloxanediol 1 f, enhanced downfield shifting
(Dd: SiOHA =1.59 and SiOHB =1.13 ppm) was observed when
the concentration was increased from 0.01 to 0.2m, with the
larger shift assigned to the more acidic silanol.
Figure 3. 1H NMR DOSY studies to determine averaged MW of disiloxanediol
species in solution based on self-association at various concentrations.
Based on the anion-recognition capabilities of disiloxane-
diols,[8] we proceeded to demonstrate that 1,3-disiloxanediols
exhibit activity as a new class of anion-binding catalysts.[13] We
investigated 1,3-disiloxanediols as catalysts for the N-acyl Man-
nich reaction of substituted isoquinolines, also known as a Reis-
sert-type dearomatization reaction.[12f, 14] The nucleophilic addi-
tion of silyl ketene acetal 8 to N-acylisoquinolinium ion 10 af-
forded dearomatized product 9 in 83 and 84% yield using
20 mol% of either the 1,1,3,3-tetraphenyldisiloxane-1,3-diol
catalyst (1a) or 1,1,3,3-tetranaphthyldisiloxane-1,3-diol catalyst
(1b), respectively (Table 1, entries 2 and 3 vs entry 1). When
the catalyst loading was reduced to 10 mol%, naphthyl cata-
lyst 1b demonstrated enhanced activity relative to phenyl cat-
alyst 1a (71 vs 15% yield, entries 5 vs 4). Further modification
of the disiloxanediol catalyst with electron-withdrawing groups
To further investigate the self-association of these 1,3-disilox-
anediols, 1H NMR diffusion-ordered spectroscopy (DOSY) ex-
periments were performed to determine the diffusion coeffi-
cients and molecular weights[17d,18] of the species present at
different concentrations.[19] DOSY experiments were performed
using disiloxanediol 1a with concentrations between 0.01–
0.4m. At concentrations of 0.01 and 0.05m, averaged MWDOSY
values of 421.6 and 426.1 gmolÀ1 were measured, providing
support that the monomer (MWcalc =414.6 gmolÀ1) is the
major species at these concentrations (Figure 3). When the
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nBu4NCl, a significant downfield silanol chemical shift was ob-
served for all disiloxanediols (see the Supporting Information),
providing evidence for a favorable formation of a receptor–
anion complex between the silanol and chloride anion in solu-
tion. Broadening of the peak shape was also observed, sug-
Table 1. Addition of silyl ketene acetal to isoquinoline.[a]
1
gesting that a more dynamic equilibrium exists. The H NMR ti-
tration of disiloxanediol 1b with nBu4NCl demonstrated a stron-
ger hydrogen-bonding interaction (Dd=4.18 ppm) compared
to disiloxanediol 1a (Dd=3.79 ppm), correlating to binding
constants of 1000Æ8 and 337Æ17mÀ1, respectively.[21] The in-
creased strength in bonding of disiloxanediol 1b demonstrates
a significant effect of the naphthyl variant and correlates to
the increased activity observed as a catalyst in the N-acyl Man-
nich reaction. Binding strength to other silicon diols was also
examined to directly compare with 1,3-disiloxanediols. Di(1-
naphthyl)silanediol exhibited a hydrogen-bonding interaction
(Dd=2.67 ppm) with a binding constant of 209Æ8mÀ1 (vs
1000Æ8mÀ1 for the corresponding naphthyl disiloxanediol
1b). This data demonstrates that disiloxanediols exhibit signifi-
cantly stronger binding to chloride ions compared to other sili-
con diols, such as silanediols.[20a] The simple phenyl disiloxane-
diol 1a showed a similar binding strength as compared to
N,N’-diphenylthiourea, thus suggesting that 1,3-disiloxanediols
are promising for wide applications as hydrogen-bonding and
anion-binding catalysts.[22]
Entry
Catalyst
Loading
[mol%]
Yield
[%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13[d]
14[e]
–
–
20
20
10
10
20
20
20
10
20
20
20
20
20
10
1a
1b
1a
1b
1c
1e
1 f
1 f
11
12
13
1a
1a
83 (72)[c]
84 (76)[c]
15
71
73
76
78
68
64
18
44
76
45
[a] Reaction run at 25 mm. [b] Yields determined using 1H NMR spectros-
copy with PhSiMe3 as an internal standard. [c] Isolated yields determined
after column chromatography. [d] Reaction run at 50 mm. [e] Reaction
run at 500 mm.
Table 2. Binding constants.
Receptor
Conc.
Lewis
base
Solvent
Dd
Ka
[mÀ1
[a]
[mm]
[ppm]
]
1a
1b
10
10
10
10
10
10
10
200
200
200
nBu4NCl
nBu4NCl
nBu4NCl
nBu4NCl
nBu4NCl
nBu4NCl
DMF
DMF
DMF
DMF
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
C6D6
C6D6
C6D6
C6D6
3.79
4.18
2.67
2.86
3.86
3.52
2.52
3.44
3.40
2.93
337Æ17
1000Æ8
209Æ8
[b]
or the incorporation of alkyl groups did not significantly
impact the catalytic activity (Table 1, entries 6–8).
Np2Si(OH)2
thiourea[c]
1 f, SiOHA
1 f, SiOHB
1a
345Æ19
4850Æ203
4360Æ193
56Æ3
Monosilanols and silanediols (e.g., 11–13) exhibited lower
catalytic activity compared to 1,3-disiloxanediol catalysts
(Table 1, entries 9–11), indicating the importance of the ar-
rangement of the two silanol groups of 1,3-disiloxanediols for
effective anion binding. The low concentration utilized in this
reaction (i.e., 5.0 mm) is consistent with previous reports for
anion-binding applications of 1,3-disiloxanediols (5.0–
0.625 mm).[8] Furthermore, the low concentration utilized in
this reaction suggests that supramolecular assembly is not
a key factor required for substrate activation, and activation
proceeds through a monomeric disiloxanediol. Increasing the
reaction concentration to 50 mm, in which the catalyst still
exists as a monomer, had little impact on the observed NMR
yield (76 vs 83%, entries 1 vs 13). However, when the reaction
was performed using catalyst 1a at a higher concentration
(500 mm), a lower yield of 45% was observed (entry 14). Based
on this data, we propose that activation proceeds through
anion-binding of the disiloxanediol to promote the chloride-
stabilized iminium ion intermediate 10.
1a
97Æ12
1 f, SiOHA
1 f, SiOHB
442Æ15
379Æ21
[a] Calculated using 1H NMR titration data (see the Supporting Informa-
tion); error for binding constants is reported as a 95% confidence interval
based on the non-linear regression model. [b] Di(1-naphthyl)silanediol.
[c] N,N’-Diphenylthiourea.
Binding constants with chloride ions were also determined
with unsymmetrical disiloxanediol 1 f, in which each silanol ex-
hibits different binding strength (Table 2). Upon titration of 1 f
with nBu4NCl, the chemical shifts of the two silanol peaks
(SiOHA and SiOHB) remained distinct and demonstrated un-
equal shifts (Dd=3.86 and 3.52 ppm) (Figure 4). Due to the
unsymmetrical disiloxane, each silanol has different H-bond do-
nating abilities. The larger chemical shift is observed for the
downfield silanol peak, which is attributed to the more acidic
silanol (SiOHA). From this data, the binding constants were cal-
culated individually for SiOHA and SiOHB, and determined to be
4850Æ203 and 4360Æ193mÀ1, respectively. The increased
To gain evidence for anion-binding, we measured binding
1
constants (Ka) for disiloxanediols 1a, 1b, and 1 f using H NMR
titration with tetrabutylammonium chloride (nBu4NCl) (Table 2),
following previously reported procedures.[8,20] Upon addition of
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the process of synthesizing chiral disiloxane-1,3-diols to evalu-
ate as novel enantioselective catalysts.
Acknowledgements
We acknowledge the National Science Foundation (CHE-
0847358 and CHE-1363375) for support of this research. S.O.W.
was a recipient of the UC Davis Borge Fellowship.
Keywords: anion-binding · hydrogen bonds · organocatalysis ·
self-assembly · silanol
Figure 4. 1H NMR titration of disiloxanediol 1 f (10 mm in CDCl3) with
nBu4NCl. Differential shifts for each silanol OH peak are observed, Dd: SiO-
HA =3.86 ppm and SiOHB =3.52 ppm.
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the catalyst loading is decreased to 10 mol%.
For comparison to chloride binding, the binding constants
for disiloxanediols were also measured with DMF, which indi-
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[21] The binding constant for 1,1,3,3-tetraphenyldisiloxane-1,3-diol (1a) with
chloride ions was previously determined by Kondo and co-workers to
be 377Æ15mÀ1 at 5 mm in CDCl3 and. 670Æ40mÀ1 at 5 mm in CD3CN;
see ref. 8.
[14] M. Ahamed, M. H. Todd, Eur. J. Org. Chem. 2010, 5935–5942.
[15] a) L. H. Sommer, J. E. Lyons, J. Am. Chem. Soc. 1969, 91, 7061–7067;
b) M. Jeon, J. Han, J. Park, ACS Catal. 2012, 2, 1539–1549.
[16] S. Varaprath, D. H. Stutts, J. Organomet. Chem. 2007, 692, 1892–1897.
[17] There is precedent for aggregation of organocatalysts such as silane-
diols (ref. 5e) and thioureas: a) M. C. Etter, Z. Urbanczyk-Lipkowska, M.
Zia-Ebrahimi, T. W. Panunto, J. Am. Chem. Soc. 1990, 112, 8415–8426;
b) H. S. Rho, S. H. Oh, J. W. Lee, J. Y. Lee, J. Chin, C. E. Song, Chem.
Commun. 2008, 1208–1210; c) G. Tꢅrkꢅnyi, P. Kirꢅly, S. Varga, B. Vakulya,
T. Soꢃs, Chem. Eur. J. 2008, 14, 6078–6086; d) R. Custelcean, Chem.
Commun. 2008, 295–307; e) O. V. Kulikov, D. A. Siriwardane, G. T.
[22] The binding constant for 1-methyl-3-(4-nitrophenyl)thioure with chlo-
ride ions was reported as 3162mÀ1 at 0.05 mm in CH3CN as measured
by UV/Vis spectroscopy: S. Nishizawa, R. Kato, T. Hayashita, N. Teramae,
Anal. Sci. 1998, 14, 595–597.
Received: August 30, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Anion activation: Disiloxanediols are
demonstrated as a new class of anion-
binding catalysts with NMR investiga-
tions into hydrogen bonding, self-asso-
ciation, and anion-binding constants.
&
Siloxanes
K. M. Diemoz, S. O. Wilson, A. K. Franz*
&& – &&
Synthesis of Structurally Varied 1,3-
Disiloxanediols and Their Activity as
Anion-Binding Catalysts
&
&
Chem. Eur. J. 2016, 22, 1 – 6
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!