NHC catalysed trimethylsilylation of terminal alkynes and indoles with Ruppert's reagent under solvent free conditions

Article abstract of DOI:10.1039/c4ra08727e

An organo-catalytic protocol for the trimethylsilylation of terminal alkynes employing Ruppert's reagent (CF₃SiMe₃) as a trimethylsilyl source has been developed under solvent and fluoride free conditions. This method was found to be very effective as a variety of terminal alkynes bearing aliphatic or aromatic substituents underwent smooth transformation to their corresponding silylated products in excellent yields within a few minutes using N-heterocyclic carbene as an organo-catalyst. This methodology was also applied to the chemospecific N-silylation of indoles. This journal is

Full text of DOI:10.1039/c4ra08727e

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NHC catalysed trimethylsilylation of terminal
alkynes and indoles with Ruppert's reagent under
Cite this: RSC Adv., 2014, 4, 49775
solvent free conditions†
Received 15th August 2014
Accepted 29th September 2014
*
Panjab Arde, Virsinha Reddy and Ramasamy Vijaya Anand
DOI: 10.1039/c4ra08727e
www.rsc.org/advances
An organo-catalytic protocol for the trimethylsilylation of terminal
Although most of the above mentioned methods show good
alkynes employing Ruppert's reagent (CF3SiMe3) as a trimethylsilyl functional group compatibility, limitations such as use of metal
source has been developed under solvent and fluoride free conditions. or a uoride source and longer reaction times render these
This method was found to be very effective as a variety of terminal methods from practicality. Hence, development of an efficient,
alkynes bearing aliphatic or aromatic substituents underwent smooth metal and uoride free method for the preparation of alky-
transformation to their corresponding silylated products in excellent nylsilyl compounds is highly desired.
yields within a few minutes using N-heterocyclic carbene as an
Being an integral part of organocatalysis, N-heterocyclic
organo-catalyst. This methodology was also applied to the chemo- carbene (NHC) catalysis is emerging as a powerful tool for
specific N-silylation of indoles.
carbon–carbon and carbon–heteroatom bond forming reac-
tions.12,13 It is well documented in the literature that NHC forms
a hypervalent complex with silicon compounds.14 This distinc-
tive reactivity of NHC towards silicon has been explored in
fundamental transformations such as cyanosilylation of
carbonyl compounds and imines,15 aziridine ring opening16 and
Mukaiyama aldol reactions.17 Recently, Song's group reported
NHC catalysed triuoromethylation of aldehydes and ketones,18
in which the CF3 anion was utilized as a nucleophile (eqn (1),
Scheme 1). While developing NHC catalysed tri-
uoromethylation of various functional groups with CF3SiMe3,
we envisioned that CF3 anion, generated in situ during the
reaction of NHC with CF3SiMe3, could be used as a traceless
base to deprotonate acetylenic proton and the resulting acety-
lide anion could be trapped with electrophilic silicon. Based on
this idea, we have developed an efficient and alternative
protocol for the preparation of alkynyl trimethylsilanes from
terminal acetylenes and CF3SiMe3 (Ruppert's reagent)19 using
NHC as a catalyst (eqn (2), Scheme 1).
Alkynylsilicon reagents are considered to be valuable synthons
due to their widespread applications in carbon–carbon bond
forming reactions such as metal catalysed cross-coupling reac-
tions,1 alkynylation reactions2 and metathesis reactions.3 Alky-
nylsilicon compounds also serve as versatile intermediates in
the synthesis of some biologically important natural products.4
Apart from the traditional method for the preparation of alky-
nylsilicon compounds,5 which involves deprotonation of
terminal acetylene with organolithium or Grignard reagent
followed by quenching with a silyl electrophile, some other
methods have also been developed which include metallic Zn6
or Zinc salts mediated7 and Zn(OTf)2 catalysed8 trimethylsily-
lation of terminal alkynes with suitable silyl electrophiles. Few
iridium complexes are also known to catalyse silylation of
terminal acetylenes.9 Another fascinating approach to alkynyl
silyl compounds encompasses metal catalyzed dehydrogenative
coupling of terminal alkynes with silyl hydrides.10 Apart from
the use of conventional silylating agents, Ishizaki's group has
demonstrated uoride catalysed trimethylsilylation of terminal
alkynes using an unconventional reagent, CF3SiMe3 (Ruppert's
reagent), as a silyl source.11
The optimisation studies were performed using phenyl
acetylene (1) and CF3SiMe3 (2). A wide range of NHCs derived
from NHC precursors 4–9 were utilised for this purpose
Department of Chemical Sciences, Indian Institute of Science Education and Research
(IISER) Mohali, Sector 81, Knowledge City, S.A.S. Nagar Manauli (PO), Mohali, Punjab
– 140306, India. E-mail: rvijayan@iisermohali.ac.in; Tel: +91 172 2293190
† Electronic supplementary information (ESI) available: Experimental procedures
and copies of NMR spectra. See DOI: 10.1039/c4ra08727e
Scheme 1 NHC catalysed trifluoromethylation and trimethylsilylation
reactions with CF3SiMe3.
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Table 1 Optimisation of reaction conditions
Table 2 Substrate scope
S. no
Catalyst
Base
Solvent
Time [min]
Yielda [%]
1
2
3
4
5
6
7
8
5
5
5
5
5
5
4
6
7
8
9
—
5
5
NaH
NaH
NaH
NaH
DBU
—
NaH
NaH
NaH
NaH
NaH
NaH
NaNH2
KOtBu
THF
DCM
Et2O
—
—
—
—
—
—
—
20
20
20
7
360
360
60
20
30
120
120
360
20
50
76
30
98
72
—
50
81
50
23
7
9
10
11
12
13
14
—
—
—
—
Trace
85
92
15
a
Isolated yield; RT ¼ 31–33 ꢀC.
(Table 1). When the experiment was conducted using 2 mol% of
NHC, derived from 5 and NaH in THF, and 2 as a silyl source the
expected product 3 was obtained in 50% yield. Encouraged by
this observation, further optimisation experiments were carried
out under different conditions with or without solvent (Table 1).
Surprisingly, the reaction rate and the yield of the product 3
were found to be higher when the reaction was performed
under solvent free conditions. Out of several conditions
screened, the best result was obtained (98% yield in 7 min)
using NHC derived from 5 under neat conditions, thus chosen
as standard condition (Entry 4). No product or only traces of
product was observed if the reaction was conducted without
base (Entry 6) or without NHC (Entry 12, Table 1). We also tried
the silylation of phenyl acetylene (1) with other silyl pro-
nucleophiles (see ESI† for more information) including
Et3SiH20 using 5 mol% of NHC derived from 5 as a catalyst
under various conditions. Unfortunately, the desired silylated
product was not observed in any of the cases.
a
2 equiv. of CF3SiMe3 was used. b 3 equiv. of CF3SiMe3 was used; RT ¼
31–33 ꢀC.
electron rich aromatic groups (10a–10i). Alkynes substituted
with aliphatic groups also underwent smooth conversion to the
corresponding silylated alkynes (10n–10u) under the standard
conditions. This method was found to be efficient for the tri-
methysilylation of alkynes bearing hetero-aromatic substituents
as well (10v and 10w).
At this stage, our attention was shied towards elucidating a
suitable mechanism for this transformation. Careful moni-
toring of the reaction revealed that the reaction was exothermic
and some gas evolution was taking place in the reaction vessel
when CF3SiMe3 was added to NHC and phenyl acetylene. Since
To generalise this protocol, a variety of terminal alkynes
bearing aromatic and aliphatic substituents were subjected to
silylation reaction under optimised reaction conditions and the
results are summarised in Table 2. It is apparent from Table 2
that, irrespective of the nature of the substituents, most alkynes
gave the corresponding trimethylsilylated products in quanti-
tative yields. Almost in all the cases, the reaction was completed
within
a few minutes. Phenyl acetylenes with electron-
withdrawing substituents on the aromatic ring (10j–10m) reac-
ted at relatively faster rates than the alkynes attached with
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While developing the silylation of terminal acetylenes, we
were also interested to utilise this methodology for N-silylation
of indoles, which is considered to be an important trans-
formation, because N-silylated indoles serve as prevalent
intermediates for the synthesis of indole based natural prod-
ucts.25 Besides, chemoselective N-silylation of indoles still
remains as a challenge. The conventional method for the
preparation of N-silyl indoles involves deprotonation of indole
using a bit excess of a strong base followed by treatment with
Me3SiCl.26 The drawback associated with this method was the
formation of equimolar quantities of salt and the requirement
of strong base such as nBuLi or NaH. In order to over come these
issues, few metal catalyzed dehydrogenative Si–N coupling
methods have been recently developed.27 Hartwig's group
reported silyl directed site selective borylation of N-silyl indoles,
where N-silyl indoles were prepared in situ through Ruthenium
catalyzed dehydrogenative silylation.28 Very recently, Oestreich
group has also developed ruthenium catalysed dehydrogenative
Si–N coupling of amines including indoles using hydrosilanes
under base free conditions.29 But, so far, organo-catalytic
version for the synthesis of N-silylated indoles is not reported
in the literature. It is highly preferred to develop an organo-
catalytic N-silylation of indoles to avoid the use of expensive
or toxic metal catalysts.
Scheme 2 Plausible mechanism for silylation of acetylenes.
uoroform is the only by-product possible in this methodology
and also it is a gas, we presumed that the evolved gas was CHF3.
To conrm this, the reaction mixture was analyzed by 19F NMR
spectroscopy. The spectrum showed a peak at ꢁ78.6 ppm
(please see ESI† for 19F spectrum), which clearly conrms the
presence of CHF3 in the reaction mixture.21
Based on the above experimental investigations, a plausible
mechanism has been proposed (Scheme 2). We presume that
NHC(I) reacts with CF3SiMe3 and generates a penta co-ordinated
silicon complex II, which immediately deprotonates the
terminal alkyne to give the intermediate III along with acetylide
anion IV and CHF3. Nucleophillic attack of IV on the electro-
phillic silicon center of III gives the silylated product V with the
expulsion of NHC (Scheme 2).
Although uoroform is considered to be a potential green
house gas,22 it has been widely employed as a triuoromethyl
source to introduce CF3 group in organic molecules.23,24 Since
CHF3 is the by-product in our methodology, in order to make
our process sustainable, we thought of utilising it for the
regeneration of CF3SiMe3 using a protocol reported by Surya
Prakash and co-workers.24 Scheme 3 portrays the experimental
set up adapted for the synthesis of CF3SiMe3. The uoroform
(2.1 equiv.) generated during the reaction was purged in to
another reaction ask containing KHMDS (1 equiv.) and Me3-
SiCl (1 equiv.) in toluene at ꢁ85 ꢀC. The reaction proceeded
smoothly and the product CF3SiMe3 was obtained in 58% yield.
It was conrmed by 19F and 29Si NMR spectroscopy. The yield of
the product was assigned based on 19F NMR using an internal
standard (please refer ESI† for more details).
Since we have demonstrated that the NHC–CF3SiMe3
combination was very effective for the silylation of terminal
alkynes, we thought of utilising the same recipe for the N-
silylation of indoles under solvent free conditions. To our
surprise, when indole was treated with 2 equiv. of CF3SiMe3 and
a NHC (2 mol%) derived from 5, the silylated product 12a was
obtained in 93% isolated yield aer 3 h under solvent free
condition at room temperature (Entry 12a, Table 3). A couple of
Table 3 NHC catalysed N-silylation of indoles
a
CF3SiMe3 was added at 0 ꢀC.
Scheme 3 Regeneration of Me3SiCF3 from CHF3.
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optimisation experiments were carried out using other NHCs,
but the yield of the N-silylated indole was inferior when
compared to the above mentioned condition (please refer ESI†
for more details). To demostrate the scope of this methodology,
a wide range of substituted indoles were subjected to N-
silylation reaction under the above mentioned reaction condi-
tions and the results are presented in Table 3.
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went smooth conversion to their corresponding N-silylated
products in moderate to good yields at room temperature.
Surprisingly, 2- and 3-methyl substituted indoles (12b and 12c)
reacted at faster rate when compared to indole. In the case of 5-
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In conclusion, an efficient and metal free process for the
synthesis of trimethylsilyl acetylenes has been developed using
NHC as a catalyst under solvent free conditions. We have shown
that the by-product, uoroform, can be effectively utilised for
the regeneration of CF3SiMe3. We have also demonstrated the
rst organocatalytic N-silylation of indoles using NHC as a
9 I. Kownacki, B. Marciniec, B. Dudziec and M. Kubicki,
Organometallics, 2011, 30, 2539–2545.
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Acknowledgements
The authors sincerely acknowledge the Department of Science
and Technology (DST), New Delhi for nancial support and 11 M. Ishizaki and O. Hoshino, Tetrahedron, 2000, 56, 8813–
IISER Mohali for providing infrastructure. PA and VR thank
8819.
CSIR for a research fellowship. The NMR and HRMS facilities of 12 (a) For reviews, please see: D. Enders, O. Niemeier and
IISER Mohali are gratefully acknowledged.
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