Heck reaction of arenediazonium salts with N,N-diprotected allylamines. Synthesis of cinnamylamines and indoles

Article abstract of DOI:10.1039/c0ob01052a

Novel palladium-catalyzed reactions of arenediazonium tetrafluoroborates with N,N-diprotected allylamines are presented. The reaction of arenediazonium tetrafluoroborates with N,N-(Boc)₂ allylamine allows for an easy approach to cinnamylamines whereas using 2-alkynyl-N-(allyl) trifluoroacetanilides and 2-iodo-N-(allyl)trifluoroacetanilide the reaction provides a useful tool for appending indole rings to aniline fragments.

Full text of DOI:10.1039/c0ob01052a

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Heck reaction of arenediazonium salts with N,N-diprotected allylamines.
Synthesis of cinnamylamines and indoles†
Sandro Cacchi,* Giancarlo Fabrizi, Antonella Goggiamani and Alessio Sferrazza
Received 19th November 2010, Accepted 19th January 2011
DOI: 10.1039/c0ob01052a
Novel palladium-catalyzed reactions of arenediazonium
tetrafluoroborates with N,N-diprotected allylamines are
presented. The reaction of arenediazonium tetrafluoro-
borates with N,N-(Boc)2 allylamine allows for an easy ap-
proach to cinnamylamines whereas using 2-alkynyl-N-(allyl)-
trifluoroacetanilides and 2-iodo-N-(allyl)trifluoroacetanilide
Scheme 1 Synthesis of cinnamylamines from arenediazonium tetrafluo-
roborates and N,N-(Boc)2 allylamine.
the reaction provides a useful tool for appending indole rings
to aniline fragments.
Because of their importance as synthetic intermediates1 and
as functional groups in many biologically active compounds,2
considerable effort has been directed toward the development
of methodologies for the synthesis of primary allylic amines.3
Palladium catalysis has provided some of the most efficient
procedures, typically via amination of allylic esters3a,4 or alcohols,5
allylic C–H amination,6 aza-Claisen rearrangement,7 and Heck
reaction of allylamine derivatives.1f,3a,8 As to the latter, reactions
have been usually performed with aryl halides or triflates. However,
although arenediazonium salts offer significant advantages over
aryl halides and triflates (for example, they are less expensive and
more reactive, react under milder conditions, Heck arylations can
be carried out in air and omitting phosphine ligands) and despite
their broad application in the Heck reaction,9,10 no example using
arenediazonium salts has been described.
Therefore, as part of our on-going research in palladium-
catalyzed reactions of arenediazonium salts,9j,k,11 it appeared
of interest to us to investigate their involvement in a new
approach to cinnamylamines via Heck reaction with the N,N-
(Boc)2 allylamine 2 (Scheme 1). Our planning also included
the application of this chemistry to allylamines bearing func-
tional groups able to undergo a subsequent cyclization re-
action. In particular, we considered that the reaction of 2-
alkynyl-N-(allyl)trifluoroacetanilides 5 with arenediazonium salts
would lead to 2,3-disubstituted indoles 712 through a sequential
Scheme
2 Synthesis of indole derivatives from arenediazonium
Heck reaction/aminopalladation/reductive elimination process13
(Scheme 2a). Furthermore, 2-iodo-N-(allyl)trifluoroacetanilides 8
would afford alkylidene indolines 10 (Scheme 2b).
tetrafluoroborates and 2-alkynyl-N-(allyl)trifluoroacetanilides
2-iodo-N-(allyl)trifluoroacetanilides 8.
5 or
We started our study by examining the reaction of 2 with 4-
methoxydiazobenzene tetrafluoroborate 1a, a model of electron-
rich arenediazonium salts. After an initial screening of solvents,
bases, temperature, and precursors of Pd(0), we found that the use
of 0.0◦4 equiv of Pd2(dba)3 and 4.5 equiv of NaOAc, in MeCN
at 35 C produced 3a after 0.5 h in 61% yield (Table 1, entry 5)
along with a 24% yield of the g,g-diarylated derivative 11a. Further
Dipartimento di Chimica e Tecnologie del Farmaco, La Sapienza, Uni-
versita` di Roma, P.le A. Moro 5, 00185 Rome, Italy. E-mail: sandro.
cacchi@uniroma1.it; Fax: +39(06)4991-2780; Tel: +39(06)4991-2795
† Electronic supplementary information (ESI) available: A complete
description of experimental details and product characterization. See DOI:
10.1039/c0ob01052a
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Table 1 Optimization of the reaction of 4-methoxydiazobenzene tetrafluoroborate 1a with N,N-(Boc)2 allylamine 2a
entry
1a : 2
[Pd] (equiv)
base (equiv)
solvent
T/◦C
time (h)
yield%b of 3a
1
2
3
4
5
6
7
2 : 1
2 : 1
2.1
1 : 1.1
1.5 : 1
1 : 1.1
1 : 1.1
Pd(OAc)2 (0.05)
Pd(OAc)2 (0.05)
Pd(OAc)2 (0.05)
Pd(OAc)2 (0.05)
Pd2(dba)3 (0.04)
Pd2(dba)3 (0.04)
Pd2(dba)3 (0.02)
—
MeOH
MeOH
MeOH
MeCN
MeCN
MeCN
MeCN
35
50
50
rt
35
rt
18
6
4
traces
traces
—
—
61d
92
90
CaCO3 (2)
EtN(i-Pr)2 (2)
DABCO (3)
NaOAc (4.5)
NaOAc (3)
NaOAc (3)
c
8
0.5
0.5
3
rt
a Reactions were carried out on a 0.5 mmol scale in 5 mL of solvent. b Yields are given for isolated products. c An almost quantitative reduction of 1a to
anisole was observed. d The g,g-diarylated derivative 11a was isolated in 24% yield.
Table 2 Palladium-catalyzed reaction of arenediazonium tetrafluorobo-
rates 1 with N,N-(Boc)2 allylamine 2a
optimization of the reaction conditions led to the isolation of 3a
in 90% yield using 0.02 equiv of Pd2(dba)3 and 3 equiv of NaOAc
at room temperature (Table 1, entry 7).‡
entry
1
proc
time (h)
yield%b of 3
1
2
3
4
5
6
7
8
4-MeOC6H4N2BF4
2-MeOC6H4N2BF4
2,4-Me2C6H3N2BF4
3,5-Me2C6H3N2BF4
2-PhC6H4N2BF4
PhN2BF4
3-CF3C6H4N2BF4
4-CNC6H4N2BF4
4-MeCOC6H4N2BF4
4-MeO2CC6H4N2BF4
2-ClC6H4N2BF4
2-BrC6H4N2BF4
4-IC6H4N2BF4
A
A
A
A
A
A
B
B
B
B
B
B
B
B
B
3
2
5
3
90
85
83
72
61
80
83
83
70
67
83
74
63
63
77
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3
2.5
3.5
1.5
5
6
4
4
4
3
4
However, no vinylic substitution product formation
was observed when these conditions were applied to 3-
trifluoromethyldiazobenzene tetrafluoroborate 1g, a model of
electron-poor arenediazonium salts. Better results, although
unsatisfactory from a synthetic point of view, were obtained
omitting the base and using MeOH as the solvent. The
corresponding vinylic substitution derivative 3g was isolated in
35% yield after 5 h at room temperature. The main reaction
product (59% yield) was the monoprotected vinylic substitution
derivative 12g, most probably formed by a partial deprotection
process due to the acidity of the reaction medium.
9
10
11
12
13
14
15
4-BrC6H4N2BF4
4-ClC6H4N2BF4
a Reactions were carried out on a 0.5 mmol scale at room temperature
using 1 equiv of 1, 1.1 equiv of 2, 0.02 equiv of Pd2(dba)3, and (procedure
A) 3 equiv of NaOAc in 5 mL of MeCN or (procedure B) 1.5 equiv of
CaCO3 in 5 mL MeOH. b Yields are given for isolated products.
We then came back to optimizing reaction conditions with
electron-poor arenediazonium salts and found that the vinylic
substitution derivative 3g could be obtained in 83% yield in the
presence of 0.02 equiv of Pd2(dba)3 and 1.5 equiv of CaCO3 in
MeOH14 (3.5 h, room temperature). Nevertheless, these conditions
proved less successful with electron-rich arenediazonium salts.
Compound 1a produced 3a only in 65% yield after 4 h.
Therefore, two protocols were used when we next explored the
scope and generality of the reaction: Pd2(dba)3, NaOAc, MeCN,
room temperature with neutral and electron-rich arenediazonium
salts (procedure A); Pd2(dba)3, CaCO3, MeOH, room temperature
with electron-poor arenediazonium salts (procedure B).
As shown by Table 2, vinylic substitution products 3 were
isolated in good to excellent yields under these conditions and
several useful functional groups are tolerated, including ortho
as well as chloro, bromo, and iodo substituents. The ability to
incorporate the latter makes this reaction particularly attractive
for increasing the molecular complexity, for example via transition
metal-catalyzed coupling reactions.
Compounds 3 can be readily converted into the corresponding
cinnamylamines 4 as shown by the preparation of cinnamylamine
4h from 3h in almost quantitative yield (Scheme 3).
The synthesis of 2,3-disubstituted indoles 7 from the N,N-
diprotected allylamine 5 involves a vinylic substitution step to
give 6 followed by an intramolecular aminopalladation/reductive
Scheme 3 Typical procedure for the preparation of cinnamylamines from
the corresponding N,N-(Boc)2 precursors.
elimination step. It would help immensely if the Heck reac-
tion/aminopalladation/reductive elimination sequence could be
done without isolating the intermediates. Herein we report that
this process can be performed through a one-pot protocol adding
PPh3 and K2CO3 to the crude mixture formed in the Heck reaction
(when carried out in MeCN) or to the crude mixture formed in
the first step after evaporating the volatile material (when carried
out in MeOH) and increasing the temperature to 100 ◦C. Our
preparative results are summarized in Table 3.
The mechanism of the aminopalladation/reductive elimination
reaction most probably involves the basic steps outlined in
Scheme 4. The reaction of Pd(0) with 6 affords the p-allylpalladium
complex A, from which B is formed via coordination of the
C–C triple bond to palladium. A subsequent intramolecular
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Table 3 Synthesis of 2,3-disubstituted indoles 7 from arenediazonium
tetrafluoroborates 1 and 2-alkynyl-N-(allyl)trifluoroacetanilides 5a
entry
1 Ar1
5 Ar2
time (h)b
yield%c of 7
1
2
3
4
5
6
7
8
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
Ph
4-CNC6H4
4-CNC6H4
4-CNC6H4
4-CNC6H4
Ph
2 + 8
68
79
88
83
69
72
73
73
7a
7b
7c
7d
7e
7f
4-MeCOC6H4
2-ClC6H4
Ph
4-MeOC6H4
4-ClC6H4
Ph
1 + 12
1 + 12
0.5 + 12
2 + 12
1 + 6
1 + 10
1 + 12
7g
7h
4-MeCOC6H4
a Reactions were carried out on a 0.25 mmol scale under the following
conditions: first step: room temperature, 1.1 equiv of 1, 1 equiv of 5, 0.02
equiv of Pd2(dba)3, and (procedure A) 3 equiv of NaOAc in 2.5 mL of
MeCN or (procedure B) 1.5 equiv of CaCO3 in 2.5 mL of MeOH; second
step: 100 ◦C after the addition of 0.16 equiv of PPh3 and 2 equiv of K2CO3
to the crude mixture formed in the first step (when carried out in MeCN) or
to the crude mixture formed in the first step after evaporating the volatile
material (when carried out in MeOH). b Reaction times refer to the first
and to the second step, respectively. c Yields are given for isolated products.
Scheme 5 Typical procedure for the preparation of alkylidene indoline
from o-iodo-N-allyl-trifluoroacetanilide.
reagents required for the cyclization step to the crude mixture
formed in the first step after evaporating the volatile material.
In conclusion, novel palladium-catalyzed reactions of arenedia-
zonium tetrafluoroborates with N,N-diprotected allylamines have
been developed. The reaction of arenediazonium tetrafluorobo-
rates with N,N-(Boc)2 allylamine allows for an easy approach to
cinnamylamines. The new method tolerates a variety of useful
functional groups, including ortho as well as chloro, bromo, and
iodo substituents. With 2-alkynyl-N-(allyl)trifluoroacetanilides
and 2-iodo-N-(allyl)trifluoroacetanilide the reaction provides an
exceedingly useful tool for appending indole rings to aniline
fragments.
Acknowledgements
We gratefully acknowledge GlaxoSmithKline for their generous
financial support and a fellowship position and Dr Alcide Perboni
and Paolo Stabile of GlaxoSmithKline for valuable discussions.
We are also indebted to MURST and the University “La
Sapienza” for financial support.
Notes and references
‡ No vinylic substitution derivative was formed after 8 h when the same
reaction was carried out with free NH allylamine. Using N-Boc allylamine
produced after 4 h a 57% yield of a regioisomeric mixture of monoarylated
derivatives (g-arylated : b-arylated = 3 : 1).
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2-arylindoles.
aminopalladation reaction gives rise to the regioselective forma-
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and regenerates the active palladium catalyst.
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with N,N-disubstituted allylamines in appending indole moi-
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synthesis has been performed as a one-pot process adding the
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