Palladium-Catalyzed Remote meta-Selective C-H Bond Silylation and Germanylation

Article abstract of DOI:10.1021/acs.organomet.7b00309

Selective meta-C-H activation of arenes to date has met with a limited number of functionalizations. Expanding the horizon of meta-C-H functionalization, herein we disclose an unprecedented meta-silylation and -germanylation protocol by employing a simple nitrile-based directing template. Longer linkers between the target site and the directing template were successfully explored for meta-silylation (sp²-? and sp²-ζ). Additionally, synthetic utility was demonstrated with several postsynthetic elaborations and with a formal synthesis of TAC101, a promising drug for the treatment of lung cancer.

Full text of DOI:10.1021/acs.organomet.7b00309

Article
pubs.acs.org/Organometallics
Palladium-Catalyzed Remote meta-Selective C−H Bond Silylation and
Germanylation
Atanu Modak, Tuhin Patra, Rajdip Chowdhury, Suman Raul, and Debabrata Maiti*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: Selective meta-C−H activation of arenes to date
has met with a limited number of functionalizations.
Expanding the horizon of meta-C−H functionalization, herein
we disclose an unprecedented meta-silylation and -germany-
lation protocol by employing a simple nitrile-based directing
template. Longer linkers between the target site and the
directing template were successfully explored for meta-silylation (sp2-ε and sp2-ζ). Additionally, synthetic utility was
demonstrated with several postsynthetic elaborations and with a formal synthesis of TAC101, a promising drug for the treatment
of lung cancer.
Scheme 1. Directing Group Assisted meta-C−H Silylation
INTRODUCTION
■
and Germanylation
Development of an efficient method for C−Si bond formation
has received substantial attention due to the immense
significance of silylated entities in synthetic,1 medicinal,2
agrochemical,3 and material chemistry.4 Organo-silicon com-
pounds are considered as valuable precursors for late-stage
modification in organic synthesis, as silicon moieties are easily
transformable to versatile organic functionalities through cross-
coupling reactions1a,d,f−h or ipso substitution.1c,e In drug
discovery, the replacement of a carbon atom by silicon in
existing marketed drugs is a vivid research area in the search for
new druglike candidates with modified biological properties:
i.e., increased lipophilicity and altered efficacy and pharmaco-
kinetic profiles.5 The high abundance, low cost, and environ-
mentally benign nature of silicon make it suitable for a readily
acceptable component for preparing advanced materials and
alternating copolymers.4a
intrigued by the importance of silylated entity, we intended to
develop directed meta-silylation with an easily attachable and
removable directing template for benzylsulfonates, since
benzylsulfonates are considered as useful synthons for several
organic transformations utilizing their α-sulfonyl carbanion
(Scheme 2).13b,e,16 Moreover, a considerable effort was made to
silylate 2-phenylethanesulfonic acid and 3-phenylpropane-1-
sulfonic acid derivatives by overcoming a consequent rise in
Scheme 2. Importance and General Overview of the Work
Conventionally, arylsilanes are prepared either from the
reaction of an organometallic intermediate with silicon
electrophiles6 or transition-metal-catalyzed coupling reactions
between aryl halide and hydrosilane/disilane.7 In this context,
directed C−H silylation could be advantageous in terms of
atom economy, mild reaction conditions, and selectivity.8 In
recent years, ortho-directed C−H silylation has been reported
in the literature through a five- or six-membered metallacyclic
intermediate.9 However, meta-C−H silylation is considered
extremely challenging, owing to the large strain energy
associated with more than 11-membered metallacyclic inter-
mediates (Scheme 1). The judicial design of directing groups
and placement of a suitable linker to connect the directing
group and the targeted C−H bond is of paramount importance
in addressing this challenge.10 In other ways, meta-C−H
activation was also achieved using electronic, steric biasness, or
transient directing group strategies.11
With the pioneering work by Yu group12a and recent
development of directed meta-C−H functionalization12−15 and
Received: April 21, 2017
© XXXX American Chemical Society
A
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a
18
,
strain energy associated with larger metallacyclic intermediates
(12-membered and 13-membered, respectively) (Scheme 2).
Though organogermaniums have been widely used as valued
organic materials,17 the formation of C−Ge bonds through C−
H activation has been rarely studied.9a In this report,
germanylation of the meta-C−H bond of benzylsulfonate has
also been developed to address the earlier synthetic short-
comings to access the organogermanium compounds.
Table 2. Ligand Optimization
RESULTS AND DISCUSSION
■
During initial exploration, we attempted the silylation of
benzylsulfonate with 2-cyanophenol as directing template and
hexamethyldisilane as a silylating agent. A combination of
Pd(OAc)2, Ac-Gly-OH, and Ag2CO3 at 100 °C in hexafluor-
oisopropyl alcohol (HFIP) as solvent gave only 11%
monosilylated product (conversion 13%) with exclusive meta
selectivity.18 Such a low conversion is likely due to the
deactivation of Pd(II) catalyst to Pd(0) and full reduction of
the silver(I) salt to Ag(0) at 100 °C by hexamethyldisilane. To
minimize the catalyst waste through this sacrificial reduction,
we decreased the reaction temperature and, with a gradient
temperature of 45 °C (for first 12 h) and then at 65 °C (for
next 24 h) along with portionwise addition of hexamethyldi-
silane, provided a better yield of the desired meta-silylated
product. After an extensive optimization of reaction conditions,
a total yield of 51% of the meta-silylated product (40% mono,
11% di) was achieved.
a
n.d.: no desired product detected.
arenes, we tested this silylation protocol for different
monofluoro-, difluoro-, and trifluoro-substituted benzylsulfo-
nates. Interestingly, mono- and difluorinated substrates, with
two available meta-C−H bonds, provided the corresponding
silylated products in 65−76% yield (2b−e). As expected,
difluoro-substituted substrates with two unsymmetrical meta
positions yielded both of the monosilylated products in unequal
ratios (2d−e). 2-Methylbenzylsulfonate afforded only a single
mono product due to the possible steric restraint at the other
meta position by a methyl group (2f). Similarly, a 10:1 ratio for
meta:meta′ (2g) was observed for the 2-chlorobenzylsulfonate
derivative. Furthermore, tolerance of the chloro substituents
further demonstrated the versatility of this method, as a chloro
group can be utilized for late-stage organic functionalization. As
anticipated, both electron-donating and electron-withdrawing
substitution at the 3-position of the aromatic ring provided
moderate to good yields of the monosilylated products (2h−l).
Next, we planned to functionalize the meta-C−H of 2-
phenylethanesulfonic acid (sp2-ε) and 3-phenylpropane-1-
sulfonic acid (sp2-ζ) derivatives. The increment of distance
between the targeted C−H bond and the metal directing site is
expected to cause an elevation in the consequential strain
energy and disfavor the entropy factor for the formation of the
critical macrocyclic transition state. Encouragingly, the desired
silylated products were achieved successfully. Moderate to good
yields of silylated product were observed for 2-phenyl-
ethanesulfonic acid derivatives with electron-deficient (3b,d,
Table 4) and electron-donating (3c) substituents in the
aromatic ring (Table 4). In the case of 3-phenylpropane-1-
sulfonic acid (4a), an excellent meta selectivity with useful yield
was observed (Table 4).
Furthermore, we envisioned that modification of the
directing template may play a key role in improving the nitrile
coordination for effective meta-C−H palladation. Accordingly,
by tuning the electronic properties of the 2-hydroxybenzonitrile
moiety, it was found that 2-hydroxy-4-methoxybenzonitrile led
to the best result with 61% of silylated product (Table 1, 45%
Table 1. Directing Template Optimization18
a
reaction was done at 45 °C; 30 h.
mono, 16% di). With 2-hydroxy-4-methoxybenzonitrile as the
directing scaffold, further ligand optimization proved that Ac-
Gly-OH is an appropriate ligand among others for better
reactivity and selectivity (Table 2).
Proceeding further with this optimized condition, we
explored the meta silylation for a broad range of benzylsulfonate
ester moieties. Expectedly, the reaction was found to be
compatible with fluoro, trifluoromethyl, methyl, chloro, and
trimethylsilyl substituents present in the benzene ring (Table
3). Intrigued by the medicinal relevance of fluoro-substituted
In order to test the generality of our protocol, we tested
other disilane reagents. Unfortunately, under our reaction
conditions aryldisilanes failed to generate meta-silylated product
(Table 5).
In synthetic chemistry, organogermanium compounds have
immense applications but there exist limited methods to
prepare this class of compounds. In this context, we planned to
prepare meta-germanylated arenes by developing a protocol
B
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Table 3. Scope of Silylation on Benzylsulfonic Acid
Derivatives
Table 4. Silylation on 2-Phenylethanesulfonic Acids (sp2-ε)
b
18
,
a18
,
and 3-Phenylpropane-1-sulfonic Acid (sp2-ζ)
a
Reaction conditions: Pd(OAc)2 (10 mol %), Ac-Gly-OH (20 mol %),
Ag2CO3 (3 equiv). dry HFIP (1.8 mL), anhydrous Na2SO4 (200 mg),
hexamethyldisilane (4 equiv). A sequential addition of Pd(OAc)2 (5
mol %) and hexamethyldisilane (2 equiv) was carried out after 24 h.
Yields in parentheses are based on recovered starting materials.
Table 5. Silylation on Benzylsulfonic Acid with Other
Disilanes18
a
18
,
Table 6. Scope of meta Germanylation
a
b
72 h, 45 °C. Reaction conditions: Pd(OAc)2 (10 mol %), Ac-Gly-
OH (20 mol %), Ag2CO3 (3 equiv), dry HFIP (1.8 mL), anhydrous
Na2SO4 (200 mg), hexamethyldisilane (sequential addition). Yields in
parentheses are based on recovered starting materials;
similar to that of silylation. Despite the higher reducing
property of the digermanium reagent and less stability of the
C−Ge bond, we succeeded in generating the meta-germany-
lated product with moderate to good yields (5a−d, Table 6).
The synthetic utility of the current meta silylation was further
demonstrated through three-way modification of the mono-
silylated product. First, employing the trimethylsilyl entity as a
transformable group, C−I,19 C−O,20 and C−C21 bonds were
a
Yields in parentheses are based on recovered starting materials.
formed (Scheme 3, 7a−c). As a direct synthetic route for
preparing meta-heteroarylated arenes has yet to be reported, a
two-step pathway to access these organic moieties through
C
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Scheme 3. Synthetic Diversification of meta-Silylated
Product
Scheme 6. Proposed Mechanistic Cycle for meta Silylation
a
18
,
a
Conditions: (a) AgNO3/I2; (b) Pd(OAc)2/PhI(OCOCF3)2/
EtCO2H; (c) Pd(MeCN)2Cl2/CuCl2; (d) PhCHO/LDA; (e) Pd-
(OAc)2/Ac-Gly-OH/Ag2CO3.
(CMD) reaction at the meta-C−H bond with the help of an
amino acid ligand and HFIP solvent. Then hexamethyldisilane
undergoes palladation through σ-bond metathesis or trans-
metalation. Successive reductive elimination results in the meta-
silylated product with concomitant formation of palladium(0).
Next, palladium oxidizes to palladium(II) in the presence of the
silver(I) salt. A detailed mechanistic study is currently
underway in our laboratory.
meta-silylation followed by C−C coupling has immense
significance (7c). Second, the synthetic applicability of the
benzylsulfonate moiety was established by utilizing a Julia-type
olefination reaction to afford the meta-silylated olefin (7d).
Finally, the residual meta-C−H bond of the monosilylated
benzylsulfonate was further olefinated with a trimethylsilyl
entity present at the other meta position (7e,f).
The usefulness of this protocol was further revealed by
applying our protocol judiciously for the formal synthesis of the
anticancer drug molecule TAC101. A synthetic outline was
proposed to achieve the target molecule starting from benzyl
chloride, employing our silylation strategy as the key step
(Scheme 4).
CONCLUSION
■
In conclusion, we developed the first directing group assisted
meta-C−H silylation protocol using hexamethyldisilane as the
silylating agent. This silylation protocol was applied on
benzylsulfonate esters along with 2-phenylethanesulfonic esters
by overcoming the consequential strain energy related to a
larger metallacycle. meta-Germanylation has also been achieved
with synthetically useful yields and excellent selectivity for
benzylsulfonate entities. The late-stage modification of a
monosilylated entity was carried out to show the synthetic
applicability. Additionally, a formal synthetic route was outlined
for TAC101, a potential drug for lung cancer. These
operationally simple reaction conditions with easily available
silylating reagents are expected to have significance for broader
applications in synthetic chemistry.22
Scheme 4. Formal Synthesis of TAC101 from Benzyl
Chloride
EXPERIMENTAL SECTION
■
General Procedure A for Silylation through Remote meta-
C−H Activation of Benzylsulfonic Acid Derivatives. In a clean,
oven-dried screw-cap reaction tube, with previously placed magnetic
stir bar, were placed substrate (0.2 mmol); Pd(OAc)2 (0.1 equiv, 0.02
mmol, 4.5 mg); N-acetylglycine (0.4 equiv, 0.04 mmol, 4.5 mg),
Ag2CO3 (3 equiv, 0.6 mmol, 166 mg), and anhydrous Na2SO4 (200
mg). Then hexafluoroisopropyl alcohol (1.8 mL) which was previously
distilled and collected over activated 4 Å molecular sieves was added
by syringe. Next, hexamethyldisilane (5 equiv, 1 mmol, 200 μL) was
added to the mixture by syringe. The tube was tightly closed with the
screw cap and placed in a preheated oil bath at 45 °C. After 12 h of
vigorous stirring the reaction temperature was increased to 65 °C and
2 equiv of hexamethyldisilane (0.4 mmol, 80 μL) was added to the
rection mixture. At time t = 24 h, another 1 equiv of
hexamethyldisilane (0.2 mmol, 40 μL) was added to the reaction
mixture and the mixture was stirred for another 12 h at 65 °C. The
reaction mixture was cooled to room temperature and filtered through
Moreover, the removal and quantitative recovery of the
directing template was performed through the hydrolysis of the
meta-silylated benzylsulfonate substrate (Scheme 5).
A plausible mechanism has been outlined in Scheme 6. At
first palladium undergoes concerted metalation−deprotonation
Scheme 5. Removal and Recovery of Directing Template
D
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Celite. The reaction tube was washed with 10 mL of dichloromethane.
The total organic portion was concentrated and purified via column
chromatography through silica gel using petroleum ether and ethyl
acetate as eluent.
Notes
The authors declare the following competing financial
interest(s): A provisional patent on this work has been filed.
General Procedure B for Silylation through Remote meta-
C−H Activation of 2-Phenylethanesulfonic Acid Derivatives. In
a clean, oven-dried screw-cap reaction tube, with a previously placed
magnetic stir bar, were placed substrate (0.2 mmol), Pd(OAc)2 (0.1
equiv, 0.02 mmol, 4.5 mg), N-acetylglycine (0.4 equiv, 0.04 mmol, 4.5
mg), Ag2CO3 (3 equiv, 0.6 mmol, 166 mg), and anhydrous Na2SO4
(200 mg). Then hexafluoroisopropyl alcohol (1.8 mL) which was
previously distilled and collected over activated 4 Å molecular sieves
was added by syringe. Next, hexamethyldisilane (5 equiv, 1 mmol, 200
μL) was added to mixture by syringe. The tube was tightly closed with
the screw cap and placed in a preheated oil bath at 70 °C. At time t =
24 h, another 2 equiv of hexamethyldisilane (0.4 mmol, 80 μL) and
Pd(OAc)2 (0.05 equiv, 0.01 mmol, 2.2 mg) were added to the reaction
mixture and the mixture was stirred for another 24 h at 45 °C. The
reaction mixture was cooled to room temperature and filtered through
Celite. The reaction tube was washed with 10 mL of dichloromethane.
The total organic portion was concentrated and purified via column
chromatography through silica gel using petroleum ether and ethyl
acetate as eluent.
General Procedure C for Germanylation through Remote
meta-C−H Activation of Benzylsulfonic Acid Derivatives. In a
clean, oven-dried screw-cap reaction tube, with previously placed
magnetic stir bar, were placed substrate (0.1 mmol), Pd(OAc)2 (0.1
equiv, 0.01 mmol, 2.2 mg), N-acetylglycine (0.2 equiv, 0.02 mmol, 2.2
mg), Ag2CO3 (3 equiv, 0.3 mmol, 83 mg), and anhydrous Na2SO4
(100 mg). Then hexafluoroisopropyl alcohol (0.9 mL) which was
previously distilled and collected over activated 4 Å molecular sieves
was added by syringe. Next, hexamethyldigermane (4 equiv, 0.4
mmol) was added to mixture by syringe. The tube was tightly closed
with the screw cap and placed in a preheated oil bath at 45 °C. At time
t = 24 h, another 2 equiv of hexamethyldigermane (0.2 mmol) and
Pd(OAc)2 (0.05 equiv, 0.005 mmol, 1.1 mg) were added to the
reaction mixture and the mixture was stirred for 12 h at 70 °C. The
reaction mixture was cooled to room temperature and filtered through
Celite. The reaction tube was washed with 10 mL of dichloromethane.
The total organic portion was concentrated and purified via column
chromatography through silica gel using petroleum ether and ethyl
acetate as eluent.
ACKNOWLEDGMENTS
■
This activity is supported by Science and Engineering Research
Board, India. Financial support received from the CSIR (A.M.),
UGC-INDIA (T.P.), and IIT Bombay (R.C. and S.R.) is
gratefully acknowledged.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00309.
Optimization of reaction conditions, synthesis of the
substrates, application, deprotection of acid moiety,
characterization data, and 1H NMR and 13C NMR
spectra of the compounds (PDF)
Accession Codes
CCDC 1528685 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
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Corresponding Author
*E-mail for D.M.: dmaiti@chem.iitb.ac.in.
ORCID
Debabrata Maiti: 0000-0001-8353-1306
E
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DOI: 10.1021/acs.organomet.7b00309
Organometallics XXXX, XXX, XXX−XXX