A Hydrosilylation Approach to Silicon-Bridged Functional Dipyrromethanes: Introducing Silicon to A New Arena

Article abstract of DOI:10.1002/asia.201901424

Two silylene-spaced ((E)-vinylsilyl)anthracene-dipyrromethane dyads have been designed and synthesized by RhCl(PPh₃)₃-catalyzed hydrosilylation reactions of 5-methyl-5′-(ethynylaryl)dipyrromethanes with (9-Anthryl)-dimethylsilane. Th

Full text of DOI:10.1002/asia.201901424

DOI: 10.1002/asia.201901424
Communication
A Hydrosilylation Approach to Silicon-Bridged Functional
Dipyrromethanes: Introducing Silicon to A New Arena
[a]
[b]
[c]
Bhaskar Garg,* Tanuja Bisht, and Yong-Chien Ling
Dedicated to Prof. S. M. S. Chauhan
the presence of two pyrrolic NHs as hydrogen bond donor
sites, DPMs have emerged as versatile anion receptors either
on their own or as building blocks within more complex set-
Abstract: Two silylene-spaced ((E)-vinylsilyl)anthracene-di-
pyrromethane dyads have been designed and synthesized
by RhCl(PPh ) -catalyzed hydrosilylation reactions of 5-
3
3
[3]
tings such as calix[4]pyrroles (CPs). Highly efficient organic re-
methyl-5’-(ethynylaryl)dipyrromethanes with (9-Anthryl)-di-
methylsilane. The complexation studies of dyads toward
different anions have also been performed, which reveal
that dyads exhibit a highly selective response towards
fluoride anion attributable to both hydrogen-bonding and
pentacoordination phenomena. This dual-mode fluoride
recognition event is unprecedented and may pave the
way for future developments in the areas of porphyri-
noids, organosilicon, polymer, and supramolecular chemis-
try.
actions or diverse strategies have been extensively applied for
many years as a powerful tool for structure modifications, and
[3–5]
modulation of anion-binding properties of DPMs and CPs.
Undoubtedly, these intellectual efforts have transformed one’s
ability to better understand the chemistry of these pyrrolic sys-
tems as well as the concepts in supramolecular chemistry. Nev-
ertheless, within this “tool box” of chemical functionalization
of DPMs or CPs, an efficient reaction which has never received
any attention as we think it merits is catalytic hydrosilylation
[6]
reaction. This scientific apathy has left the literature limited
to only some elegant silicon complexes of DPMs and CP bear-
[7]
ing NÀSi bond.
We initially envisioned that meso-substituted “two-wall” aryl
Since the pioneer synthesis of first compound, tetraethyl
silane, unarguably, the organosilicon chemistry has come a
long way with all ups and downs, and currently, making a pro-
found impact on both industrial and academic settings at the
interface of synthetic organic chemistry, polymer chemistry,
medicinal chemistry, materials science, and in the development
[8]
extended CP with alkyne groups at aromatic walls might
serve as an embryonic system to incorporate silylene spacer
between CP and an appropriate signaling motif, following hy-
drosilylation reaction. At this juncture, however, it would be
worthwhile to emphasize that controlling the regiochemistry
of alkyne hydrosilylation is especially challenging due to the
[1,2]
of fluorescent sensors with improved biological attributes.
[6b]
Fascinating this “aura” of organosilicon compounds and appre-
ciable as this scientific progress are, studies pertaining synthet-
ic pyrrolic systems within silicon domain are still very limited.
Consequently, there remains an urgent need to develop new
organosilicon compounds so that the ability of silicon to
bridge the gap between different disciplines could be truly re-
alized. In this context, the pyrrolic receptors such as dipyrro-
methanes (DPMs) are particularly appealing.
formation of several isomers (Upper Panel, Scheme 1). Aside
from that, the formation of stereoisomers of aryl-extended CPs
in significantly low yields is another penalty essentially encoun-
[5d,8,9]
tered in their synthesis.
With such considerations in mind,
5,5’-alkylaryl DPMs, “better half” of aryl-extended CPs appeared
quite attractive to us in order to develop a new class of sili-
con-containing synthetic pyrrolic receptors as well as to inves-
tigate the role of silicon atom on the anion-induced coordina-
tion events.
DPMs are of utmost importance in organic synthesis,
namely, porphyrinogens and related macrocycles. Owing to
Herein, we report the synthesis, characterization, and anion-
complexation properties of two novel Si-bridged anthracene-
DPM dyads, 5a and 5b, that are to the best of our knowledge
without precedent in the literature.
[a] Dr. B. Garg
Department of Chemistry
Indian Institute of Technology Roorkee
Roorkee 247667, Uttarakhand (India)
E-mail: bhaskargarg111@gmail.com
The structures and synthesis of target compounds 5 are out-
lined in Scheme 1. Meso-substituted DPMs 1 were prepared by
[8,10]
minor modifications of known procedures.
1, in turn, could
[
b] Dr. T. Bisht
be carried on to the key-protected alkynyl derivatives 2 by ex-
posure to an excess of alkynyl alcohol in TEA-toluene at 608C
Department of Chemistry
Government Girls P. G. College
Haldwani 263139 (India)
[11]
in the presence of Pd(PPh ) Cl -CuI. The deprotection of 2
3
2
2
[
c] Prof. Dr. Y.-C. Ling
Department of Chemistry
National Tsing Hua University
with NaOH afforded moderately sensitive alkyne precursors 3a
and 3b in 72% and 64% yields, respectively. 3a and 3b were
kept at low temperature prior to use further. The synthesis of
1
01, Section 2, Kuang-Fu Road, Hsinchu 30013 (Taiwan) (ROC)
(
9-Anthryl)-dimethylsilane precursor 4 was accomplished in two
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
steps. Specifically, the lithiation of 9-bromoanthracene with n-
https://doi.org/10.1002/asia.201901424.
BuLi at À788C followed by treatment with Me SiHCl in THF af-
2
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Communication
of 3a in toluene (0.2m) in the presence of Wilkinson’s catalyst
and KI gave 5a as dark brown solid in 12% yield after column
chromatography (silica gel; DCM-hexane: 8:2, v/v). However,
5
b could be isolated in quite low yield (ca. 2.9%) under virtual-
ly identical reaction conditions. The isolation of 5 in low yields
against the reputation of standard hydrosilylation reaction, in
general, can be attributed to scrambling together with the
sensitivities of 3a and 3b, which possibly decompose during
the course of hydrosilylation reactions, resulting in the recov-
ery of 4 in ca. 15–23% yields. The structures of all intermedi-
ates and bench stable dyads 5 were characterized by different
spectroscopic techniques (Figures S1–S27 in the Supporting In-
formation).
1
The H NMR spectrum of 5a recorded in CDCl (Figure 1) dis-
3
played a single NH resonance at d 8.51 ppm, which was slight-
ly downfield shifted as compared to the DPMs 1a–3a. The
four distinctive resonances at d 8.53 (1H), 8.03 (2H), 7.81 (2H),
and 7.44 (4H) ppm could be assigned to anthryl protons with
ease. In addition, two sharp singlet resonances at d 2.04 and
0.83 ppm were corresponded to the meso- and silylene-methyl
protons, respectively (Figure S18 in the Supporting Informa-
tion). Most importantly, an unequivocal proof about the stereo-
chemistry that is, trans-configuration of 5a was accessed by
the characteristic signals at d 6.93 (H ) and 7.03 (H ) ppm (as
Scheme 1. Upper panel: A schematic for hydrosilylation reaction of alkynes
with possible stereoisomers. Lower panel: Synthesis of silylene-spaced func-
tional dipyrromethane dyads 5. Reagents: a) pyrrole, CF
3
CO
2
H;
b) Pd(PPh
lyst.
3 2 2
) Cl , CuI, 2-methyl-3-butyn-2-ol; c) NaOH; and d) Wilkinson’s cata-
a
b
highlighted in yellow background, in Figure 1a), which fur-
nished large coupling constants (J= ~19 Hz) for the vinyl
group. The absence of any resonance around d 6.5 ppm (J
ꢀ15 Hz) ruled out the possibility of any cis-vinylene bonds.
forded 4 as shiny yellow crystals in 59% yield after chromato-
graphic work up, and subsequent recrystallization with EtOH.
Once both precursors in hand, the hydrosilylation reaction be-
tween 3a and 4 was screened and optimized under a variety
1
The H NMR resonating pattern of 5a for rest of the signals
(aryl and pyrrolic) were found nearly the same relative to those
[
12]
of conditions. As such, the slow addition of 4 into a solution
of 1a–3a.
1
Figure 1. Selected regions of the H NMR (400 MHz) spectra of (a) 5a and (b) 5b acquired in CDCl
3
and [D
3
]CH
3
CN, respectively, at 298 K. Inset (in violet): the
2
9
1
corresponding Si{ H} NMR (80 MHz) spectra of 5a and 5b. The molecular structures with colored dots and circles indicate specific proton assignments.
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In contrast to 5a, the overlapping and significant broaden-
ing of most proton signals of 5b hampered its characterization
produced by H while the initially merged signals (H and H )
b
a
6
were well resolved during the course of the titration. In this
milieu, it would be worthwhile to mention that the naked-eye
in CDCl . However, all proton signals were well resolved in
3
À
1
[
D ]CH CN and could be assigned with ease as shown in Figur-
recognition of F was also found quite striking in the H NMR
titration study, which was not observed, otherwise, at relatively
low concentration of 5a (UV/Vis), manifested in a color change
from yellow to orange, to red in the NMR tube (Inset: Fig-
ure S28). Aside from those of aromatic and pyrrolic protons,
the upfield shifts of Dd 0.21 and Dd 0.024 ppm were also rec-
ognized for meso-methyl and silylene-methyl protons, respec-
tively, together with the appearance of two new resonances of
relatively high and very poor intensity in the alkyl region of
the spectra (Figure 2B). On careful examination, the newly ap-
peared resonances at d 0.03 and 0.12 ppm were found identi-
cal to those of pentacoordinated and hexacoordinated fluoride
3
3
es 1b and S19.
1
The H NMR structural assignments of 5a and 5b were
13
29
found in agreement with those of C and Si NMR spectra of
a and 5b (Figures S20-S23 in the Supporting Information).
5
29
1
Specifically, the Si{ H} NMR spectra of 5a and 5b in CDCl ex-
3
hibited a single resonance at dÀ11.76 and À11.56 ppm, re-
spectively (Insets, Figure 1), which registered a significant
downfield shift relative to that of silane precursor 4 (d=
À25.98 ppm).
1
H NMR titrations were performed to probe the complexa-
À
À
À
tion behavior of 5a towards different anions (F , Cl , AcO ,
H PO4 and CN ) as their tetrabutylammonium (TBA) salts. Typ-
À
À
À1
on the silylene moiety (BDE; Si-F=135 kcalmol ) as reported
2
ically, aliquots of the putative anionic guests (0.2m) were
previously with conjugated silicon polymers in the presence of
[16]
added to a solution of 5a (0.02m) in [D ]DMSO, and the shift
TBAF. Based on these results, the plausible models for overall
6
À
of the NHs along with other protons recognized after each ad-
dition.
complexation of F to 5a may be epitomized as shown in the
upper panel of Figure 2B. Nevertheless, taking into account of
very low intensity of hexacoordinated fluoride resonance in
À
As shown in Figure 2A, upon gradual addition of F , the
1
À
pyrrolic NHs originally appearing at d 10.3 ppm constantly
conjunction with H NMR titration data such as F -bound 5a
complex and the corresponding fitted stoichiometry of 1:2
(H:G) led us to suggest that the observed signal at d 0.12 ppm
could be an artefact, and therefore, the pentacoordination of
fluoride appeared to be the most likely.
19
shifted downfield and finally split by interaction with F nu-
cleus (Ddꢀ4.25 ppm; J =23.5 Hz), a hallmark of strong bind-
HF
[
13]
ing. Furthermore, the absence of any resonance around d
1
6 ppm (Jꢀ117 Hz) completely ruled out the possibility of any
[14,5c]
À
deprotonation.
Besides NHs, interestingly, a noticeable
In contrast to what observed with F , both either the
1
À
À
downfield shift for one set of b-pyrrolic protons (H ; Dd=
H NMR (500 MHz) titration spectra of 5a with CN , H PO
Cl and AcO or in the presence of excess of Br , I , and
,
4
2
À
4
À
À
À
0.21 ppm) was also observed, and that could be attributed to
À
the ring current effect of the phenyl group associated with in-
HSO4 (5 equiv each) did not cause any significant changes,
À
creasing rigidity of the molecule upon F complexation (Fig-
either in the NH region, or in the spectrum as a whole, sug-
À
ure S28 in the Supporting Information). Nevertheless,
through-bond” propagation effect was evident for protons H2,
H H and H which all experienced a shielding effect; signifi-
a
gesting a remarkable selectivity of 5a towards F (Figures S30–
“
S34 in the Supporting Information).
À
To obtain more clarity about F -complexation to dyad 5a,
3,
5
6,
19
cantly higher for H and H (Dd=0.18–0.32 ppm) than those of
F NMR spectrum of TBAF was recorded in [D ]DMSO solution
6
5
6
19
H and H (Dd=0.01–0.11 ppm), a typical behavior of anion-
in the presence of 5a (Figure 3A). Originally, F NMR spectrum
of TBAF displayed a free fluoride resonance at dÀ104.9 ppm.
2
3
[
15]
bound NH complexes. Similar, albeit smaller, changes were
1
À
Figure 2. A) Selected region (only NHs; labelled as red dots) of H NMR (400 MHz) spectral changes during the titration of 5a (0.02m) with F (0.2m) in
À
À
À
À
À
6
[D ]DMSO at 298 K. a) 5a only, b) 5a+0.33 equiv F , c) 5a+0.66 equiv F , d) 5a+1.0 equiv F , e) 5a+1.33 equiv F , f) 5a+1.66 equiv F , g) 5a+2.0 equiv
À
À
À
1
À
F , h) 5a+2.66 equiv F , and i) 5a+4.0 equiv F . B) Selected alkyl regions of H NMR (400 MHz) spectral changes during the titration of 5a (0.02m) with F
À
(0.2m) in [D
6
]DMSO at 298 K. a) 5a only and b) 5a+4.0 equiv F . The resonances of TBA are omitted for clarity.
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1
9
6
Figure 3. A) Partial F NMR (376 MHz, [D ]DMSO, 298 K) spectra of (a) TBAF
2
9
only and (b) TBAF+0.25 equiv 5a. B) Partial Si NMR (80 MHz, CDCl
spectra of (a) 5a only and (b) 5a+5.0 equiv TBAF.
3
, 298 K)
Figure 4. Fluorescence emission (titration) spectra of dyad 5a in DCM (1 mm)
À
upon incremental addition of TBAF. From bottom to top trace, [F ]=0.0,
0
4
.476, 0.91, 1.30, 1.67, 2.0, 2.31, 2.6, 2.86, 3.10, 3.33, 3.55, 3.75, 3.94, 4.12,
.29, 4.44 and 4.6 mm. Inset (right): changes in the emission intensity of 5a
À
vs. F concentration (lex =371 nm and lem =426 nm).
The addition of 5a (0.25 equiv) to the [D ]DMSO solution of
6
TBAF resulted in the appearance of two new broad resonances
at dÀ93.44 and À123.8 ppm in the downfield and upfield re-
gions, respectively. Whereas a significant downfield shift of Dd
[
22]
planar leading to an enhanced fluorescence response and/or
À
1
1.5 ppm w.r.t. free TBAF resonance clearly indicates the partic-
(ii), and most obviously, pentaccordination of F to silicon sig-
À
[23]
ipation of F in hydrogen bonding with NHs of dyad 5a, an
nificantly contributes toward fluorescence enhancement.
upfield shift of Dd 18.9 ppm supports the generation of penta-
The fluorescence titrations data of 5a and 5b were used to
estimate quantitative binding constants and stoichiometry. In
particular, the binding constants were obtained using OpenDa-
coordinated fluoride species. In order to get an unequivocal
29
proof about later, Si NMR spectrum of 5a was also recorded
in the presence of TBAF. As shown in Figure 3B(b), mixing 5a
[24]
taFit (Bind Fit) software. As shown in Table 1, the best fit
À
and TBAF in CDCl resulted in the appearance of a doublet at
were obtained for 1:2 (5:F ) non-cooperative model using
3
dÀ28.68 ppm having J =225.6 Hz. This observation is in
both Nelder Mead and L-BFGS-B fit methods. In case of 5a,
SiF
agreement with those of previously reported organosilicon
both fit methods produced nearly comparable values of K and
1
[
17,18]
compounds exhibiting pentaccordination phenomena.
K2 and covariance of the fit (Cov ). However, Nelder Mead
fit
À
Having established the multisite F coordination (NHs and
method was found superior over L-BFGS-B for 5b based on
1
À
[25]
Si) to dyad 5a, the H NMR titration result of 5a with F was
the Cov . Furthermore, 5b exhibited a higher binding affini-
fit
À
also examined by fluorescence spectroscopy (Figure 4). Inter-
ty for F , which is ca. 3 order of magnitude greater than 5a.
À
estingly, the incremental addition of F into a DCM solution of
To conclude, herein, we introduced a hydrosilylation method
that allowed a one-step synthesis of silicon-bridged functional
DPMs from appropriate silane and alkynyl precursors. Notably,
precursors 3 bear a structural or functional group resemblance
5
a resulted in a significant enhancement of the emission in-
À
tensity at lem =426 nm. When 4.6 equiv of F was added, the
emission intensity reached its maximum with ca. 32-fold en-
hancement factor. Notably, there was a linear dependence of
[8]
to known CPs, —a feature that we hope will reinstate hydrosi-
lylation method as a viable avenue for the development of a
new class of promising chemodosimeteric, macrocyclic and
polymeric systems having a rich molecular recognition chemis-
À
the fluorescence intensity on the F concentration in the range
2
from 1.5 to 4.0 mM (R =0.9960) with a detection limit of 1.97ꢁ
À7
.
1
10
M Unlike H NMR, the fact that a relatively much less
[26,27]
amount of compound is required in fluorescence studies also
try.
In accord with this latter suggestion are the findings
À
allowed screening of 5b with F . In particular, 5b exhibited a
that the introduction of silicon-bridge in the functional DPMs
À
similar fluorescence response as that of 5a with ca. 12-fold en-
offered an opportunity for multi-site F recognition following
hancement factor (Figure S35 in the Supporting Information),
both hydrogen bonding and pentacoordination.
À7
2
and a detection limit of 2.57ꢁ10 m (R =0.9906). Interestingly,
Current efforts are devoted to exploring the hydrosilylation
method to the more complex systems such as CPs to develop
multifunctional materials with exciting applications, and the
findings of these work(s) will be reported soon.
À
the detection limits of dyads 5 for F were found superior
[
19,20]
than those of recently reported chemosensors.
Though ap-
preciable, these “Off-On” type fluorescence responses of dyads
5
are in marked contrast to CP anion sensors, which potentially
[
21]
function by fluorescence quenching.
On the basis of the
“
Turn-On“ fluorescence responses of dyads 5, it is reasonable
À
to speculate that (i) upon binding to F , 5 become more
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Communication
[
a]
Table 1. Comparison of various models (fit method: Nelder–Mead) for binding of fluoride anion to dyads 5a and 5b.
Binding Model
Dyad 5a
Dyad 5b
À1 [b]
À1 [b]
[b]
À1 [b]
À1 [b]
[b]
K
1
[M
]
Æ% error
K
2
[M
]
Covfit
K
1
[M
]
Æ% error
K
2
[M
]
Covfit
1
1
1
1
1
1
:1
Fit failed
Fit failed
6
5
5
5
:2 (full)
1.08ꢁ10 Æ82
2.77ꢁ10 Æ9.7
0.0117
0.0117
0.0117
0.0207
3.60ꢁ10 Æ79
3.60ꢁ10 Æ79
0.0116
0.0119
0.0188
0.0175
[
[
c]
6
5
6
5
:2 (non-coop.)
:2 (non-coop.)
:2 (additive)
:2 (statistical)
1.09ꢁ10 Æ3.8
2.73ꢁ10
2.50ꢁ10
2.90ꢁ10 Æ6.7
7.25ꢁ10
c,d]
6
5
6
5
1.00ꢁ10 Æ3.7
1.00ꢁ10 Æ5.8
2.50ꢁ10
8
8
65.6Æ51082
3.34ꢁ10 Æ51100
487Æ29521
3.48ꢁ10 Æ29523
Fit failed
Fit failed
[a] The fluorescence titration experiments were performed in DCM solvent at 208C. [b] Binding constants and covariance of the fit were obtained using
OpenDataFit (Bind Fit) at http://supramolecular.org. [c] For non-cooperative binding K
Method used was L-BFGS-B.
2 2 1 1
was calculated as K =K /4 from the K value obtained. [d] Fit
Koutnik, P. Y. Savechenkov, Y. Liu, P. Anzenbacher, Jr., J. Am. Chem. Soc.
014, 136, 1520–1525; f) A. S. F. Farinha, A. C. TomØ, J. A. S. Cavaleiro,
Acknowledgements
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This work was supported by SERB New Delhi, Government of
India (Grant YSS/2015/002036 to B. G.) under Young Scientist
Scheme. B. G. is speechless to express his gratitude towards
his little darling doll who made him entirely comfortable
during this work with all love and support.
Conflict of interest
[
[
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[
[
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Communication
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Revised manuscript received: November 18, 2019
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Version of record online: && &&, 0000
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
In hyperspace: Silylene-spaced func-
tional dipyrromethanes are synthesized
using a hydrosilylation approach and
discovered as selective fluoride sensors
following hydrogen-bonding and hyper-
coordination phenomena.
Bhaskar Garg,* Tanuja Bisht,
Yong-Chien Ling
&& – &&
A Hydrosilylation Approach to Silicon-
Bridged Functional Dipyrromethanes:
Introducing Silicon to A New Arena
Chem. Asian J. 2019, 00, 0 – 0
www.chemasianj.org
7
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ