Relay catalysis by a metal-complex/bronsted acid binary system in a tandem isomerization/carbon-carbon bond forming sequence

Article abstract of DOI:10.1021/ja807591m

A one-pot tandem isomerization/carbon-carbon bond forming sequence catalyzed by a ruthenium complex/Bronsted acid binary system is demonstrated. The method enables the use of readily available allylamides to deliver reactive imines under relay catalysis using a binary catalytic system. Subsequent Bronsted acid catalyzed carbon-carbon bond forming reactions of the generated imines with nucleophilic components afforded Friedel-Crafts and Mannich products in good yields. Copyright

Full text of DOI:10.1021/ja807591m

Published on Web 10/14/2008
Relay Catalysis by a Metal-Complex/Brønsted Acid Binary System in a
Tandem Isomerization/Carbon-Carbon Bond Forming Sequence
Keiichi Sorimachi and Masahiro Terada*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan
Received September 24, 2008; E-mail: mterada@mail.tains.tohoku.ac.jp
Catalysis by transition metal complexes has been applied to a
broad range of organic transformations and occupies a privileged
position in synthetic organic chemistry. Much research on catalysis
has centered on the use of metal complexes to activate a variety of
chemical bonds. In the past decade, however, tremendous progress
has been made in catalysis using small organic molecules, namely
organocatalysis,1 in which fundamental organic transformations are
efficiently accelerated using a catalytic amount of organic mol-
ecules. In recent years, with the idea of taking advantage of both
of these catalytic approaches, researchers have combined metal
complexes and organic molecules in cooperative catalysis, which
has attracted much attention as it could potentially enable unprec-
edented transformations.2 Indeed, several excellent approaches have
been established using cooperative catalysis.3,4 In most cases, each
reactant is activated by one type of catalyst;3 for example, a metal
complex is used to activate the electrophile while an organic
molecule is used to activate the nucleophile. In this communication,
we describe an unprecedented relay catalysis5 for a tandem
isomerization/carbon-carbon bond formation sequence using a
binary catalytic system consisting of a ruthenium hydride complex
(1) and a Brønsted acid (2) (Scheme 1). The proposed sequential
transformation involves a three step relayed catalysis, where (i)
isomerization of the allylamide (3) to the enamide (4) is catalyzed
by 1;6 (ii) subsequent isomerization of 4 to an imine intermediate
(5) is relayed by 2;7 and (iii) the catalytic sequence is terminated
by a carbon-carbon bond forming reaction of 5 with a nucleophilic
component (6: Nu-H) under the influence of the Brønsted acid
catalyst (2). The advantage of the present relay catalysis is that the
method would enable the generation of reactive imines (5) from
readily available allylamides (3) in a one-pot reaction via tandem
isomerization.
chemical yield (Table 1, entry 1). Screening of Brønsted acids (2)
identified phosphoric acid (2d) as the better catalyst (entries 2-4).
Further modification of the phosphoric acid catalyst (2e) by the
addition of phenyl substituents at the 3,3′-position exhibited a
marked effect on the catalytic activity, affording 7aa in excellent
yield (entry 5). Control experiments revealed that both of the
catalysts, 1 and 2, are indispensable in allowing the sequential
processes to progress and finally give rise to the F-C product (7aa);
the reaction did not proceed at all in the absence of 1, while, under
acid-free conditions, isomerization of 3a to enamide (4a) took place
but the desired F-C product was not obtained.8 Further investiga-
tion of the protecting group on the allylamine showed that for the
N-Boc allylamine (3b) a substituted phosphoric acid catalyst (2e)
was suitable (entry 6). Whereas, for the N-Bz-protected allylamine
(3c), stronger acids such as TfOH (2a) or Tf2NH (2f) were more
effective than the phosphoric acid (2e) (entries 7-9).
Table 1. Relay Catalysis for Tandem Isomerization/Friedel-Crafts
Sequential Transformation of 3 with 6a (Eq 1)a
entry
2
3
time (h)
7
yield (%)b
1
TfOH (2a)
TFA (2b)
CSA (2c)
2d
2e
2e
3a
3a
3a
3a
3a
3b
3c
3c
3c
14
24
24
6
3
5
12
2
2
7aa: R1 ) Cbz
33
56
15
74
98
92
89
88
84
Scheme 1. Relay Catalysis by Metal-Complex/Brønsted Acid
Binary System for Sequential Transformations
2c
3c
4
5
6
7
8
7ba: R1 ) Boc
7ca: R1 ) Bz
2e
TfOH (2a)
Tf2NH (2f)
9
a Unless otherwise noted, all reactions were carried out with 0.50
mmol of 3, 0.75 mmol of 6a (1.5 equiv), 0.005 mmol of 1 (1 mol%),
and 0.025 mmol of 2 (5 mol%) in 1.0 mL of toluene at 40 °C. b Isolated
yield. c At 50 °C.
To ascertain the viability of the proposed relay catalysis, we
attempted the reaction of trimethoxybenzene (6a) with N-Cbz
protected allylamine (3a) under the binary catalytic system to
provide the Friedel-Crafts (F-C) product (7aa) (eq 1). An initial
experiment was performed using 1 mol% of [RuClH(CO)(PPh3)3]
(1)6c,d and 5 mol% of TfOH (2a) in toluene at 40 °C. To our delight,
the corresponding F-C product (7aa) was obtained, albeit in low
Table 2 summarizes experiments carried out to probe the scope
of the present sequential transformation. Having determined the
optimal acid catalyst for different protecting groups on the
allylamines (Method A: 2e/N-Cbz or N-Boc allylamines, Method
B: 2f/N-Bz allylamines), we employed the phosphoric acid catalyst
(2e) for a series of substituted allylamines (3) bearing a N-Cbz
protecting group (entries 1-4). F-C products were obtained in
9
14452 J. AM. CHEM. SOC. 2008, 130, 14452–14453
10.1021/ja807591m CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
good yields for linear and branched alkyl substituents as well as
for an aromatic substituent (entries 1-3). N-Cbz methallylamine
(3g) was also applicable to the present transformation, although
the reaction gave a modest chemical yield (entry 4). We next
investigated a series of aromatic compounds (6). The reaction of
2-methoxyfuran (6b) with N-Boc allylamine (3b) proceeded
smoothly using Method A (entry 5). The reaction of 1,3-dimethoxy-
benzene (6c) was conducted by Method B (entries 6,7), that is,
using a combination of 2f and N-Bz allylamines, because the F-C
product could not be obtained by Method A. Method B is also
applicable to other aromatic compounds (entries 8-10), such as
1-methoxynaphthalene (6d), 2-methylfuran (6e), and N,N-dimethy-
laniline (6f), affording the corresponding F-C products in good
yield.
In conclusion, we have successfully demonstrated a one-pot tandem
isomerization/C-C bond forming sequence using a ruthenium complex/
Brønsted acid binary catalytic system. The present relay catalysis
enables the use of readily available allylamides to generate imines for
further successive transformations. In future studies we will continue
to develop related catalytic sequences, including an enantioselective
version of the methodology described herein.
Table 2. Substrate Scope of Tandem Isomerization/Friedel-Crafts
Sequential Transformationsa,b
Acknowledgment. This work was supported by JSPS for a Grant-
in-Aid for Scientific Research (A) (Grant No. 20245021). We also
acknowledge the JSPS Research Fellowship for Young Scientists (K.S.)
from the Japan Society for the Promotion of Sciences.
Supporting Information Available: Representative experimental
procedure including details of control experiments and spectroscopic
data for the reaction products (7). This material is available free of
charge via the Internet at http://pubs.acs.org.
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a Method A: The reactions were carried out with 0.50 mmol of 3,
0.75 mmol of 6, 0.005 mmol of 1 (1 mol%), and 0.025 mmol of 2e (5
mol%) in 1.0 mL of toluene. b Method B: The reactions were carried out
with 0.50 mmol of 3, 5.0 mmol of 6, 0.005 mmol of 1 (1 mol%), and
0.050 mmol of Tf2NH (2f, 10 mol%) under solvent-free conditions.
c Isolated yield. d 2 mol% of 1. e 3.0 equiv of 6e in 1 mL of toluene.
f 1.5 equiv of 6f in 1 mL of toluene. g 4-Addition product/2-addition
product ) 94:6.
The distinct advantage of the present relay catalysis is highlighted
in comparison with a control experiment using enamide (4) (eq 2
vs 3). The reaction of 6d with enamide (4c) in the presence of 2f
gave an oligomer of the enamide (4c) as the major product, and
the F-C product (7cd) was obtained in low yield (eq 3). This result
clearly shows that enamide (4c) on its own functions as a
nucleophilic component and reacts predominantly with the inter-
mediary imine (5c) delivered from isomerization of 4c. In contrast,
the present sequential transformation provided the desired F-C
product (7cd) in good yield (eq 2). It is considered that the
concentration of 4c is maintained at low levels under the relay
catalysis, and hence the oligomerization of 4c was effectively
suppressed. The present relay catalysis is also applicable to the
reaction of a 1,3-dicarbonyl compound (6i) as a nucleophilic
component. The corresponding Mannich product (7bi) was obtained
in an acceptable yield using 2 mol% of 1 and 5 mol% of 2e (eq 4).
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Synlett 2008, 1661–1664. Also see: (d) Kobayashi, S.; Gustafsson, T.;
Shimizu, Y.; Kiyohara, R. Org. Lett. 2006, 8, 4923–4925.
(8) 1:1 E/Z mixture of N-Cbz protected enecarbamate (4a) was obtained.
JA807591M
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