Silica gel-mediated hydroamination/semipinacol rearrangement of 2-alkylaminophenylprop-1-yn-3-ols: Synthesis of 2-oxindoles from alkynes and 1-(2-aminophenyl) ketones

Article abstract of DOI:10.1002/adsc.201300911

2-Alkylaminophenylprop-1-yn-3-ols, prepared from the lithium diisopropylamide (LDA)- mediated 1,2-addition of alkynes to 1-(2-aminophenyl) ketones, can be converted to 3,3-disubstituted 2-oxindoles by using silica gel in n-hexane/ ethyl acetate (20:1 v/v) as the reaction medium. The utility of the approach as a potential scale-up strategy for the synthesis of 2-oxindoles was exemplified by the large-scale synthesis of one adduct in excellent yield. The synthetic applicability of this chemistry was also demonstrated by the recycling of the silica gel up to 8 times with no apparent loss of activity being observed for the same example.

Full text of DOI:10.1002/adsc.201300911

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DOI: 10.1002/adsc.201300911
Silica Gel-Mediated Hydroamination/Semipinacol Rearrangement
of 2-Alkylaminophenylprop-1-yn-3-ols: Synthesis of 2-Oxindoles
from Alkynes and 1-(2-Aminophenyl) Ketones
Dewi Susanti,a Linda Li Ru Ng,a and Philip Wai Hong Chana,*
a
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, Singapore 637371, Singapore
Fax : (+65)-6791-1961; e-mail: waihong@ntu.edu.sg
Received: October 11, 2013; Published online: February 7, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300911.
Abstract: 2-Alkylaminophenylprop-1-yn-3-ols, pre-
pared from the lithium diisopropylamide (LDA)-
mediated 1,2-addition of alkynes to 1-(2-amino-
phenyl) ketones, can be converted to 3,3-disubsti-
tuted 2-oxindoles by using silica gel in n-hexane/
ethyl acetate (20:1 v/v) as the reaction medium.
The utility of the approach as a potential scale-up
strategy for the synthesis of 2-oxindoles was exem-
plified by the large-scale synthesis of one adduct in
excellent yield. The synthetic applicability of this
chemistry was also demonstrated by the recycling
of the silica gel up to 8 times with no apparent loss
of activity being observed for the same example.
bioactive natural products,[8] in good to excellent
yields (Scheme 1). The nitrogen-containing ring form-
ing process relies on base-mediated 1,2-addition of an
alkyne to a 1-(2-aminophenyl) ketone followed by
silica gel-catalyzed hydroamination/semipinacol rear-
rangement[9,10] of the resulting propargylic alco-
hol.[11,12] Added to this, the reaction features opera-
tional simplicity under conditions that do not require
the exclusion of air or moisture at room temperature.
The synthetic method also tolerates a broad range of
functional groups that allows for the efficient and
atom-economical assembly of a variety of 2-oxindoles
from the readily available substrates.
A demonstrative example is the large-scale addition
of phenylacetylene (3.5 equiv.) to 2-(methylamino)-
benzophenone 1a (1 g, 4.73 mmol) in the presence of
LDA (3.5 equiv.) shown in Eq. (1) in Table 1. By di-
rectly treating a 5% EtOAc/n-hexane solution in an
open round-bottom flask containing the aqueous
worked-up crude mixture of this reaction with silica
Keywords: alkynes; carbonyl compounds; heteroge-
neous catalysis; 2-oxindoles; silica gel
The various chemical and physical properties of silica gel (100 equiv.) under ambient conditions at room
gel have established it to be an indispensible function- temperature for 2.5 h, the 2-oxindole product 3a was
al material in chemistry.[1–5] This is reflected by its nu- obtained in 89% yield. The structure of the 1,2-addi-
merous applications, ranging from its use as the sta-
tionary phase in the separation of organic compounds
by flash column chromatography[2] to serving as the
solid phase support for the metal catalyst in heteroge-
neous catalysis.[3] In the case of the former activity,
the marginally acidic nature of the native form of
silica gel, it has a pH value that is close to neutral,
can sometimes make the isolation of compounds con-
taining acid-labile functional groups a challenge.[4] On
the flip side, harnessing this weak Brønsted acid prop-
erty can also provide the opportunity to devise new
silica gel-mediated functional group transformations
to various synthetically valuable products.[5] Herein,
we report our discovery of a scalable and recyclable
heterogeneous synthetic approach to 3,3-disubstituted
Scheme 1. Synthesis of 2-oxindoles from silica gel-mediated
2-oxindoles,[6,7] a structural motif found in a myriad of cycloisomerization of 2-alkylaminophenylprop-1-yn-3-ols.
Adv. Synth. Catal. 2014, 356, 353 – 358
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Dewi Susanti et al.
Table 1. Large-scale and recyclable silica gel for cycloisomerization of 2a to 3a.[a]
Run
1
2
3
4
5
6
7
8
Yield [%][b]
94 (91)[c,d]
94
94
94
94
90
93
94
[a]
All reactions were performed with 0.5 mmol of 1a and 1.75 mmol of phenylacetylene with 3.5 equiv. of LDA in THF at
À788C to room temperature for 3 h followed by treatment with 3 g of silica in 10 mL of n-hexane/EtOAc (20:1 v/v) at
room temperature for 18 h.
[b]
[c]
[d]
Yield determined by 1H NMR with CH2Br2 as the internal standard.
Reaction time=2.5 h.
Value in parenthesis denotes isolated yield.
tion adduct was determined by NMR analysis while
that of the nitrogen-containing heterocycle was con-
firmed by X-ray crystallography.[13] A second illustra-
tive example is the establishment of a recyclable
system for the second step of this reaction involving
silica gel-mediated cycloisomerization of the 2-alkyl-
Table 2. Cycloisomerization of 2a mediated by various acidic
reaction conditions.[a]
ACHTUNGTRENNUNGaminophenylprop-1-yn-3-ol 2a (Table 1). Addition of
silica gel (100 equiv.) to the aqueous worked-up crude
mixture obtained from the base-mediated reaction of
0.5 mmol of 1a and 3.5 equiv. of phenylacetylene in
5% EtOAc/n-hexane, contained in an open round-
bottom flask at room temperature for 2.5 h gave 3a in
91% yield. The silica gel was removed by filtration
and used directly for 7 further consecutive transfor-
mations under the same above-mentioned conditions.
As shown in cycles 2–8, product yields of 90–94%
were achieved for each consecutive conversion with
no apparent loss of activity observed albeit longer re-
action times of 18 h were required for these latter
runs.
The discovery of this unprecedented and yet simple
2-oxindole forming process was serendipitously ob-
served during an attempt to purify the crude mixture
obtained from LDA-mediated reaction of 1a with
phenylacetylene by silica gel flash column chromatog-
raphy. Our initial intentions for preparing the 2-alkyl-
Entry Acid
Solvent
Yield [%][b]
1
MCM-41
EtOAc/n-hexane[c] 40
[d]
2
3
Mont-K10
SiO2
EtOAc/n-hexane[c]
n-hexane
PhMe
–
82
4
SiO2
80[e]
90[e]
89
5
SiO2
CH2Cl2
CH3NO2
EtOAc
acetone
THF
6
SiO2
[f]
7
SiO2
–
–
–
–
[f]
8
SiO2
[f]
9
10
SiO2
p-TsOH·H2O[g] EtOAc/n-hexane[c]
[h]
[a]
All reactions were performed with 0.5 mmol of 1a and
1.75 mmol of phenylacetylene with 3.5 equiv. of LDA in
THF at À788C to room temperature for 3 h followed by
the solid acid (3 g) in 10 mL of solvent at room tempera-
ture for 24 h.
ACHTUNGTRENNUNGaminophenylprop-1-yn-3-ol substrate had been for
a study focused on defining the generality of the
silver-catalyzed hydroamination of the N-protected p-
rich alcohols to the corresponding indolin-3-ols.[12b]
The unexpected formation of the 2-oxindole product
via a mechanistically interesting redox rearrangement
prompted us to further examine and gain a better un-
derstanding of the cycloisomerization process
(Table 2). With this in mind, we first surveyed the
analogous transformations mediated by other solid
acids in place of silica gel (entries 1 and 2). This re-
vealed that a markedly lower product yield of 40%
was obtained when the reaction was repeated with
[b]
1
Yield determined by H NMR with CH2Br2 as the inter-
nal standard.
[c]
[d]
Ratio of n-hexane/EtOAc=20:1.
Mixture of unknown decomposition products obtained
1
based on H NMR and TLC analysis of the crude reac-
tion mixture.
Reaction time=1 h.
[e]
[f]
Starting materials recovered in near quantitative yield.
Reaction was performed with 5 mol% of pTsOH·H2O.
Compound 4a was obtained in 38% yield.
[g]
[h]
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Silica Gel-Mediated Hydroamination/Semipinacol Rearrangement
MCM-41 (entry 1).[14] In contrast, the analogous reac-
tion mediated by Mont-K10,[15,16] which has a higher
acidity than silica gel,[16c] was found to give a mixture
of unidentifiable decomposition products based on
TLC and 1H NMR analysis (entry 2). These initial
findings led us to surmise that the trigger for the pres-
ent nitrogen-ring forming process could be due to the
mildly acidic conditions provided by silica gel. This
was further supported by the outcome of control ex-
periments in other solvents or in the presence of
a strong Brønsted acid such as pTsOH·H2O (en-
tries 3–10). Slightly lower product yields of 80–90%
were afforded on employing n-hexane, toluene, di-
chloromethane or mildly acidic MeNO2 instead of 5%
EtOAc/n-hexane as the solvent system (entries 3–6).
On the other hand, the propargylic alcohol was only
Table 3. Cycloisomerization of 2-alkylaminophenylprop-1-
yn-3-ols 2b–v mediated by silica gel.[a]
1
detected by H NMR analysis of the crude mixture
when polar solvents, such as EtOAc, acetone and
THF, were used as the reaction medium (entries 7–9).
With 5 mol% of p-TsOH·H2O as the catalyst in 5%
EtOAc/n-hexane, the reaction was found to furnish
the (1H-indol-2-yl)methanol 4a in 38% yield along
with a mixture of decomposition products that could
not be identified by NMR analysis or mass spectrom-
etry (entry 10).
We next explored the scope of the present 1,2-addi-
tion and cycloisomerization reaction with the LDA/
silica gel system (Table 3). These experiments showed
that a series of 3,3-disubstituted 2-oxindoles could be
afforded in good to excellent yields from the corre-
sponding 1-(2-aminophenyl) ketones 1a–o and al-
kynes. Reactions of 1-(2-aminophenyl) ketones con-
taining an electron-withdrawing (1b, 1c) or electron-
donating group (1d) on the aniline ring with phenyl-
ACHTUNGTRENNUNGacetylene were found to proceed well, giving the cor-
responding 3,3-disubstituted 2-oxindole derivatives in
84–96% yield. The presence of other aryl (1e–h), 2-
thienyl (1i), alkyl (1j) or cycloalkyl (1k–m) motifs at
the carbonyl carbon center of the ketone was found
to have no influence on the course of the reaction
and on treating with phenylacetylene, furnished 3e–m
in 59–91% yield. Likewise, 1-(2-aminophenyl) ketones
with an N-benzyl (1n) or N-allyl (1o) instead of an N-
methyl protecting group was found to be well tolerat-
ed and reaction with phenylacetylene afforded 3n and
3o in 88 and 83% yields, respectively. The analogous
reactions of 1a with different alkynes bearing
a phenyl group with an electron-donating or electron-
withdrawing group at the para position or a sterically
demanding 1-naphthalenyl, or 3-thienyl moiety were
also found to provide the corresponding 2-oxindole
adducts 3p–3t in 86–98% yields. The only exceptions
were the 1,2-addition/cycloisomerization reactions of
[a]
All reactions were performed with 0.5 mmol of 1 and
1.75 mmol of the alkyne with 3.5 equiv. of LDA in THF
at À788C to room temperature for 3 h followed by silica
gel (100 equiv.) in 10 mL of n-hexane/EtOAc (20:1 v/v)
at room temperature for 1–18 h. Values in parentheses
denote isolated product yields.
[b]
Reaction conducted at 408C for 18 h.
While the above results implicate a cyclization
1a with ethynylcyclopropane or hex-1-yne, which gave pathway involving hydroamination followed by semi-
3u and 3v in lower yields of 26 and 35%, respectively. pinacol rearrangement, to demonstrate this to be the
The structures of 3g, 3h, 3j, 3p were determined by case, the following control experiments were per-
X-ray crystal structure measurements.[17]
formed (Scheme 2). As it was anticipated that the
Adv. Synth. Catal. 2014, 356, 353 – 358
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Dewi Susanti et al.
Scheme 2. Control experiments with 1w and 1a-18O mediated by silica gel.
propargylic alcohol would lead to the formation of an cess beginning with addition of the alcohol group to
enammonium cationic species, we reasoned that trap- the iminium carbon center to give the oxiranium
ping of this intermediate in an intramolecular manner adduct IIa. This is followed by formation of a carbo-
might be possible on introducing a Michael acceptor, cation at the C-3 position to give IIIa which, upon ox-
such as an allyl group, as a substituent in the nitrogen idative 1,2-migration of the alkyl group from the C-2
center. Thus, we first examined the reaction of N,N- to C-3 position, would furnish IVa. Subsequent depro-
diallyl-protected ketone 1w with phenylacetylene tonation of the resultant oxonium species IVa would
under the conditions described in Scheme 2, Eq. (2). then provide the 2-oxindole product. The lower prod-
This test afforded the 3,3-disubstituted 2-oxindole de- uct yields afforded for reactions with alkyl-substituted
rivative 3w in 53% yield and corroboration that for- alkynes such as ethynylcyclopropane and hex-1-yne
mation of the nitrogen-containing ring occurs via the would be consistent with the lower ability of the
posited hydroamination step.[5c,18] In a second control pendant group to stabilize a partial positive charge in
experiment, the LDA-mediated 1,2-addition of phe-
nylacetylene to 1a-18O with an 18O content of 55%
followed by treatment of the resulting crude mixture
with 3 g of silica gel in 5% EtOAc/n-hexane under
the conditions described in Scheme 2, Eq. (3) was in-
vestigated. The test reaction gave 3a-18O in 71% yield
and with retention of the 18O content based on LC-
MS measurements. This led us to surmise the appar-
ent migration of the carbonyl group in the ketone
substrate to the C-2 position in the product to occur
in an intramolecular manner. It also hinted that the
semipinacol rearrangement of the presumed formed
enammonium cationic intermediate proceeds via an
epoxide adduct.
A tentative mechanism for the present LDA-medi-
ated/silica gel-promoted reaction to form 3,3-disubsti-
tuted 2-oxindoles is put forward in Scheme 3. Using
the LDA-mediated 1,2-addition of phenylacetylene to
1a to form the propargylic alcohol 2a as a representa-
tive example, this could initially involve the activation
of the alkyne moiety of this newly formed adduct on
exposure to silica gel. As a consequence hydroamina-
tion involving nucleophilic attack by the pendant al-
kylamido group in a 5-exo-dig manner might occur to
produce the cationic enammonium cycloadduct Ia
and its iminium isomer Ia’. This is the active species
that undergoes the semipinacol rearrangement pro- 2a.
Scheme 3. Proposed mechanism for the silica gel-mediated
cycloisomerization of propargylic alcohols represented by
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Experimental Section
General Procedure
To a stirred solution of in situ formed LDA (3.5 equiv.) in
THF (3 mL) at À788C was added the appropriate alkyne
(1.75 mmol). The resulting reaction mixture was allowed to
stir for a further 1 h before the ketone 1 (0.5 mmol) in THF
(2 mL) was added to the reaction mixture and stirred for 1 h
at the same temperature and at room temperature for 1 h.
Upon completion, the reaction mixture was quenched with
saturated NH4Cl (15 mL) and extracted with EtOAc (2ꢁ
15 mL). The combined organic layers were washed with
brine (20 mL), dried over MgSO4 and concentrated under
reduced pressure. The crude mixture obtained was dissolved
with n-hexane/EtOAc (10 mL, 20:1 v/v) contained in an
open round-bottom flask and silica gel (3 g) was added. The
reaction mixture was stirred and monitored by TLC analysis
until completion. The reaction mixture was filtered, washed
with EtOAc (20 mL), concentrated under reduced pressure
and purified by flash column chromatography on silica gel
(eluent: n-hexane/EtOAc=10:1) to give the product 3.
Acknowledgements
This work is supported by a College of Science Start-Up
Grant from Nanyang Technological University and an
A*STAR-MSHE Joint Grant (122 070 3062) from A*STAR,
Singapore. We thank Drs. Yongxin Li and Rakesh Ganguly
of this Division for providing the X-ray crystallographic data
reported in this work.
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