[3+2]-Annulation of platinum-bound azomethine ylides with distal C=C bonds of N-allenamides

Article abstract of DOI:10.1039/c6cc07874e

A Pt-catalyzed, highly regioselective reaction between N-allenamides and imino-alkynes leading to pyrrolo[1,2-a]indoles is described. This represents the first example of [3+2]-annulation of Pt-bound azomethine ylides with the distal C=C bond of N-allenam

Full text of DOI:10.1039/c6cc07874e

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[3+2]‐Annulation of Platinum‐Bound Azomethine Ylides with
Distal C=C Bonds of N‐Allenamides
Received 00th January 20xx,
Accepted 00th January 20xx
Indradweep Chakrabarty,ab Suleman M. Inamdar,a Manjur O. Akram,ab Amol B. Gade,ab Subhrashis
Banerjee,c Saibal Berac and Nitin T. Patil*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
A Pt‐catalyzed, highly regioselective, reaction between N‐ catalyzed cascade annulation, leading to synthetically important
allenamides and imino‐alkynes leading to pyrrolo[1,2‐a]indoles is carbocyclic or heterocyclic scaffolds (Scheme 1). The research group
described. This represents the first example of [3+2]‐annulation of of González,9 Chen,10 Mascareñas/López11 and Bandini12 reported
Pt‐bound azomethine ylides with the distal C=C bond of N‐ gold‐catalyzed intermolecular [2+2]‐annulation with N‐allenamide
allenamides. The mechanism of the reaction was established by to obtain cyclobutanes (Scheme 1a, path a). Mascareñas/López,13
computational studies.
Vicente14 and Zhang15 successfully established gold‐catalyzed
intermolecular [4+2]‐annulation of various diene systems (path b)
and alkene‐tethered ketones16 (path c) with N‐allenamides.
Similarly, Chen co‐workers reported [3+2]‐annulation of N‐
allenamides with nitrones and azomethine imines (path d).17 Very
recently, gold catalyzed regioselective annulation reaction of 2‐(1‐
alkynyl)‐2‐alken‐1‐ones with proximal C=C bond of N‐allenamides
was developed by Zhang and co‐workers (Scheme 1b).18 Be noted
that Iwasawa and co‐workers, in their pioneering work, reported a
series of [3+2] cycloaddition reactions of metal bound azomethine
ylides with alkenes.19 Inspired by the above reports and our own
The pyrrolo[1,2‐a]indoles and their structural analogues are
important structural motifs commonly found in numerous bioactive
natural products.1 The most promising natural products belonging
to this catagories are yuremamine,2 mitomycins1b‐d and flinderoles3
(Figure 1). As
a consequence, this scaffold has received
considerable attention over the years and many methods for their
preparation have been developed in the literature.4 Accordingly, a
general catalytic method5 to access functionalized pyrrolo[1,2‐
a]indoles is in high demand and would serve to provide a great
opportunity to accelerate biological studies of these valuable
compounds.6
Figure 1 Some natural products containing pyrrolo[1,2‐a]indole
motif
Over the past decade, gold/platinum‐catalyzed annulation
reactions have emerged as a powerful technique for rapid access to
various ring systems in an extremely efficient and stereoselective
manner.7 In this context, due to unique reactivity and ease of
accessibility, N‐allenamides8 have gained much interest in gold
a
Division of Organic Chemistry, CSIR‐National Chemical Laboratory, Dr. Homi
Bhabha Road, Pune – 411008, India.
b Academy of Scientific and Innovative Research (AcSIR), New Delhi – 110025, India
c Physical and Materials Chemistry Division, CSIR‐National Chemical Laboratory, Dr.
Homi Bhabha Road, Pune – 411008, India.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Scheme 1 Annulation reactions of N‐allenamides with various
partners
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interest in the field of π‐acid catalysis, we envisioned a scenario also proceded well when phenyl ring of N‐allenamide was replaced
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wherein annulation of metal‐bound azomethine ylides with N‐ by ‐Bn group to afford the corresponding prDoOdIu:c1t0.(130k3)9/inC67C0C%07y8ie7l4dE.
allenamides would lead to pyrrolo[1,2‐a]indoles A or B via a series Interestingly, 2‐oxazolidinone allenamides which are frequently
of cascade events (Scheme 2). Especially, we became interested to used in related cycloaddition reactions (Scheme 1) are found to be
know whether the distal bond (path a) or proximal bond (path b) of inert under the present reaction conditions.
N‐allenamide would participate in the annulation reactions. Herein, Table 1 Optimization studiesa
we report, for the first time, catalytic [3+2]‐annulation of metal‐
bound azomethine ylides with distal C=C bonds of N‐allenamides.
Entry
1
Cat. M
AuCl
Solvent
toluene
Yield (%)b
‐
2
3
4
5
AuCl3
PPh3AuOTf
PPh3AuNTf2
PtI2
toluene
toluene
toluene
toluene
‐
‐
‐
51
54
66
40
52
55
43
47
16
38c
36d
10e
6
7
8
9
10
11
12
13
14
15
16
PtBr2
PtCl2
PtCl4
PtCl2
PtCl2
PtCl2
PtCl2
PtCl2
toluene
toluene
toluene
Scheme 2 [3+2]‐Annulation of N‐allenamides with metal‐bound
azomethine ylides
To explore our hypothesis, initial efforts were directed towards
finding an appropriate metal catalyst for the proposed reaction
using (E)‐N‐benzylidene‐2‐(phenylethynyl)aniline (1a) and N‐
allenamide (2a) as model substrates (Table 1). Unfortunately, the
reaction did not occur when AuCl, AuCl3, Ph3PAuOTf and
Ph3PAuNTf2 were used as catalysts (entries 1‐4) despite of their
proven abilities to activate alkynes. Next, the reaction was
conducted using PtI2 as catalyst (10 mol%) in toluene at 80 °C.
Gratifyingly, product 3a was obtained in 51% yield (entry 5). The
product 3a was found to be the result of [3+2]‐annulation reaction
between Pt‐bound azomethine ylides with distal C=C bond of N‐
allenamides. The use of PtBr2 catalyst gave slightly better result
affording 3a in 54% yield (entry 6). The yield was further improved
to 66% with the use of PtCl2 (entry 7). A Pt(IV) catalyst e.g. PtCl4
proved to be inferior and 3a was isolated in 40% yield (entry 8). The
reaction was found to be dependent on the solvent used (Table 1,
entries 9‐13). For instance, in the case of non‐polar, non‐
coordinating solvents such as benzene and m‐xylene (entries 9 and
10), yields of 3a were not much affected; while, in chlorobenzene
and fluorobenzene 3a was obtained in low yields (entries 11 and
12). A drastic reduction in yield was observed when nitrobenzene
was used as solvent (entry 13). Lowering the catalyst loading to 5
mol% resulted in incomplete conversion leading to the isolation of
3a in 38% yield (entry 14). When the temperature of the reaction
was lowered to 50 °C, the reaction was not completed and 3a was
isolated in 36% yield (entry 15). The use of 5Å MS was found to be
necessary; with its absence 3a was obtained in only 10% yield (entry
16). The overall optimization studies revealed the best condition to
obtain 3a in acceptable yield is the use of 10 mol% PtCl2 in the
presence of 5Å MS (50 mg) (entry 7).
benzene
m‐xylene
chlorobenzene
fluorobenzene
nitrobenzene
toluene
PtCl2
PtCl2
PtCl2
toluene
toluene
a
Reaction conditions: 0.2 mmol 1a, 0.4 mmol 2a, 10 mol% metal
b
catalyst, 5Å MS (50 mg), toluene (2 mL), 80 °C, 12 h. Isolated
yields. c 5 mol% catalyst loading. d Reaction performed at 50 °C for
24 h. e Reaction was carried without 5Å MS.
Table 2 Scope of the reaction with various N‐allenamidesa,b
With the optimized conditions in hand (Table 1, entry 7), we
explored the scope of the reaction using imino‐alkyne 1a with
various N‐allenamides 2 bearing a variety of electron‐deficient as
well as electron‐rich substituents at the aryl rings (Table 2). The
reactions proceeded well to give annulated products 3 in good to
excellent yields (61‐73%). Presence of electron‐rich substituents
such as 4‐Et, 4‐OMe, 2‐OPh, 3,4‐methylene‐di‐oxy, 2,4,6‐trimethyl
in the phenyl ring of N‐allenamide gave the corresponding
annulated products 3b, 3c, 3g, 3j and 3l in good yields (65‐73%).
Even the substrates bearing electron‐deficient substituents; e.g. 4‐
Cl, 4‐Br, 4‐NO2, 3‐CF3 and 3‐Br‐4‐Me reacted smoothly to give the
respective products (3d‐f, 3h and 3i) in 61‐71% yields. The reaction
a Reaction conditions: 0.2 mmol 1a, 0.4 mmol 2, 10 mol% PtCl2, 5Å
MS (50 mg), toluene (2 mL), 80 °C, 12 h. b Isolated yields.
Next, we turned our attention to explore the generality of the
[3+2]‐annulation reaction of various imino‐alkynes 1 with N‐
2 | J. Name., 2012, 00, 1‐3
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allenamide 2a (Table 3). A variety of substrates with electron‐
donating, electron‐withdrawing as well as heterocyclic substituents
at R1, R2 and R3 positions were examined to understand the scope
and limitations. This results showed that the iminoalkyne
possessing electron donating aryl groups as R1 afforded the product
3m and 3n in 75 and 76% yields; whereas, slightly lower yields were
observed in case of electron withdrawing aryl groups giving 3o and
3p in 60 and 63% yields. The heteroaromatic scaffolds were also
well tolerated to give 3q and 3r in 70 and 72% yields, respectively.
Next, we were curious to know whether the present strategy is
applicable for variation in R2. To this end, appropriate precursors
consisting both electron‐donating as well as electron‐withdrawing
substituents were subjected to the standard reaction conditions. It
was observed that the substituents bearing electron‐donating
groups (4‐Me‐Ph and 4‐OMe‐Ph) as R2 performed well without
hampering yields of products 3s and 3t (65 and 68%). While in
presence of electron‐withdrawing acetyl group reaction shuts down
without formation of 3u. Furthermore, variation of substituents at
R3 of iminoalkynes did not affect the efficiency of the reaction
affording the desired product 3v and 3w in 67 and 70% yields,
respectively. The X‐ray crystallography data for 3a, 3l and 3s have
also been obtained which unambiguously confirmed the
structure.20 Be noted that the substrate bearing –H or –alkyl as R2
are not viable substrates for the present reactions.
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led to the formation of aminomethyl‐substituted pyrrolo[1,2‐
a]indole derivative 4 in 83% yield (5:1 dr). NDexOtI,: t1h0e.10tr3e9a/Ctm6CeCn0t7o8f734Ea
with 12 M HCl in EtOAc as a solvent was performed, the
corresponding aldehyde 5 was obtained in 41% yield (9:1 dr). The
diastereomer ratio was determined by H NMR analysis.20 Finally
1
the compund 3a was subjected to m‐CPBA oxidation to furnish
dicarbonyl compund 6 in 68% yield.
A plausible mechanism is depicted in Scheme 4. At first, Pt‐
catalyst would activate the alkyne moiety of 1a thereby triggering
the attack of imine nitrogen to generate azomethine ylide I. A direct
attack of 2a to azomethine intermediate I19c would then occur to
form intermediate II. Next, Pt(II) serves to donate electron density
into an electron‐deficient iminium ion in 1,4‐fashion (cf. II) to
generate III. In short, Pt(II)‐catalyst serves both the role i.e. to
activate the alkyne moiety in 1a to form I and also to donate
electron density back into an electron‐deficient iminium ion (cf II).
Once the intermediate III has been generated, 1,2‐aryl migration21
would occur to form intermediate IV which after subsequently
would produce 3a with the regeneration of Pt‐catalyst. Another
possibility is that the alkenyl‐Pt intermediate V,22 generated in situ
from 2a and PtCl2 would combine with I to form intermediate II
(showed in dotted lines) which would follow similar sequence of
events to form 3a. Be noted that such a kind of dual catalysis
employing gold as a catalysts are known.23 However, this possibility
was ruled out based on computational studies.20
Table 2 Scope of the reaction with various imino‐alkynesa,b
aReaction conditions: 0.2 mmol 1, 0.4 mmol 2a, 10 mol% PtCl2, 5Å
MS (50 mg), toluene (2 mL), 80 °C, 12 h. b Isolated yields. c Desired
product was not obtained.
Scheme 4 A plausible mechanism (Cl atoms on Pt‐center are
omitted for clarity)
Finally, to probe the synthetic utility of the reaction,
trasformations of 3a were performed (Scheme 3). Initialy the
catalytic hydrogenation of 3a with H2 in the presence of 10% Pd/C
In summary, we have demonstrated the first example of
intermolecular [3+2]‐annulation of Pt‐bound azomethine ylides
with distal C=C bond of an N‐allenamides. The ready availability of
the starting materials and the great importance of the pyrrolo[1,2‐
a]indoles make the current methodology particularly interesting.
Mechanistic insights gained through density functional theory
suggest that the reaction follows a single substrate activation
pathway rather than dual activation.20 Further studies addressing
the enantioselective version are currently under investigation.
Generous financial support by the Department of Science and
Technology (DST), New Delhi (Grant No. SB/S1/OC‐17/2013) and
the Council of Scientific and Industrial Research (CSIR), New Delhi
(Grant Nos. CSC0108 and CSC0130) is gratefully acknowledged. We
thank Dr. Kumar Vanka and Dr. Rahul Banerjee for theoretical
Ph
Ph
12 M HCl, EtOAc
rt, 3 h
10% Pd/C, H2
EtOH, rt, 36 h
Ph
N
3a
N
N
Ts
CHO
5, 41% (9:1 dr)
Ph
4, 83% (5:1 dr)
Ph
m-CPBA, DCM
rt, 24 h
Ph
Ph
O
Ph
N
N
Ts
O
6, 68%
Scheme 3 Transformation of 3a
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studies and x‐ray crystallographic structure determination,
respectively. I.C. thanks UGC for the award of Junior Research
Fellowship (JRF).
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DOI: 10.1039/C6CC07874E
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