Ruthenium- and rhodium-catalyzed dehydrogenative ortho-alkenylation of benzylamines via free amino group directed C-H bond cleavage

Article abstract of DOI:10.1002/adsc.201400088

The dehydrogenative direct coupling of α,α-disubstituted benzylamines with acrylates takes place efficiently at room temperature under ruthenium catalysis, accompanied by free amino group-directed ortho-alkenylation and successive cyclization to produce (isoindol-1-yl)acetic acid derivatives. The reactions using styrenes in place of acrylates proceed effectively in the presence of a rhodium catalyst rather than ruthenium.

Full text of DOI:10.1002/adsc.201400088

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DOI: 10.1002/adsc.201400088
Ruthenium- and Rhodium-Catalyzed Dehydrogenative
ortho-Alkenylation of Benzylamines via Free Amino Group
À
Directed C H Bond Cleavage
Chiharu Suzuki,a Keisuke Morimoto,a Koji Hirano,a Tetsuya Satoh,a,b,*
and Masahiro Miuraa,*
a
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
Fax : (+81)-6-6879-7362; e-mail: satoh@chem.eng.osaka-u.ac.jp or miura@chem.eng.osaka-u.ac.jp
JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
b
Received: January 23, 2014; Revised: February 13, 2014; Published online: April 15, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400088.
Abstract: The dehydrogenative direct coupling of
a,a-disubstituted benzylamines with acrylates takes
place efficiently at room temperature under ruthe-
nium catalysis, accompanied by free amino group-
directed ortho-alkenylation and successive cycliza-
tion to produce (isoindol-1-yl)acetic acid deriva-
tives. The reactions using styrenes in place of acryl-
ates proceed effectively in the presence of a rhodi-
um catalyst rather than ruthenium.
À
À
Keywords: amines; C C coupling; C H activation;
dehydrogenation; heterocycles
Scheme 1. Synthesis of (isoindol-1-yl)acetate derivatives.
lyzed direct coupling with alkynes accompanied by
[4–6]
À
Isoindoline frameworks can be seen in a variety of free amino group-directed C H bond cleavage.
In
biologically active compounds including pharmaceuti- the context of our further studies of ruthenium-[7,8] as
cals.[1] In addition, isoindoline N-oxides are known to well as rhodium-catalyzed[9,10] C H functionalization,
À
be utilizable as antioxidizing agents as well as EPR we succeeded in finding that a one-step synthesis of
spin traps.[2] Therefore, development of effective (isoindol-1-yl)acetate derivatives can be achieved by
methods for their preparation has attracted much at- rhuthenium- or rhodium-catalyzed dehydrogenative
tention in organic synthesis. Among such an impor- coupling of N-unsubstituted a,a-disubstituted benzyl-
tant class of compounds, (isoindol-1-yl)acetic acids amines with acrylates through free amino-directed
are of particular interest because of their applicability ortho-alkenylation and successive domino cyclization.
as melanocortin receptor ligands.[3] These molecules In addition, the direct coupling of the amines with
are usually synthesized from benzylamines through styrenes could be conducted under rhodium catalysis.
complicated multistep procedures including protec- The latter reaction proceeded unaccompanied by cy-
tion and deprotection steps on the amino nitrogen
ACHTUNGTRENcNGNU lization to produce the corresponding ortho-alkeny-
(for example, see Scheme 1). Unprotected amino lated benzylamines, which are structurally relevant to
functions, although they are ubiquitous in organic an antibiotic agent, fumimycin.[11] These new findings
molecules, have been recognized to be too reactive are described herein.
toward various reagents and coordinate to transition
metals too tightly to be involved in catalytic process- was treated with n-butyl acrylate (2a) (0.5 mmol) in
es. the presence of [Cp*RhCl2]2 (0.01 mmol) and
Defying this conventional wisdom, we recently Cu(OAc)2·H2O (0.5 mmol) in o-xylene under N2 at
found that 2-aminobiphenyls undergo ruthenium-cata- 808C for 2 h. As a result, an alkenylation/cyclization
In an initial attempt, cumylamine (1a) (0.25 mmol)
AHCTUNGTRENNUNG
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Chiharu Suzuki et al.
Table 1. Rhodium-catalyzed reaction of a,a-disubstituted
benzylamines 1 with alkenes 2.[a]
(entry 9). 3-(2-Naphthyl)pentan-3-amine (1e) coupled
with 2a to afford a tricyclic product, 3i, in a moderate
yield (entry 10).
Meanwhile, the reaction of 1a with 2a could also be
carried out efficiently in the presence of a ruthenium
catalyst in place of rhodium. Thus, treatment of 1a
(0.25 mmol) with 2a (0.5 mmol) in the presence of
[Ru (0.0125 mmol),
G
AgSbF6
(0.05 mmol), and CuACHTUNGTREN(NUNG OAc)2·H2O (0.5 mmol) in diox-
ane under N2 at 808C for 3 h gave 3a in 97% yield
(entry 1, Table 2). In other solvents such as o-xylene
(entry 2), t-AmOH (entry 3), and DMF (entry 4), the
product yield decreased slightly. In the absence of
AgSbF6, the reaction was sluggish (entry 5). It should
be noted that the reaction proceed smoothly even at
room temperature. Although an elongation of the re-
action time (6 h) was needed, 3a was obtained in
quantitative yield (entry 6). In the case of decreasing
the catalyst amount by half, the high product yield
was maintained by further extension of the reaction
time to 24 h (entry 7). Doubling the reaction scale
with 1a (0.5 mmol) did not affect the reaction efficien-
cy (entry 8).
The coupling of various benzylamines and alkenes
was next examined under ruthenium catalysis
(Table 3). The reactions of 1a using 2b–e proceeded
efficiently at room temperature to produce 3b–e in
slightly better yields, in comparison with those under
rhodium catalysis (entries 1–4 in Table 2 versus en-
Table 2. Ruthenium-catalyzed reaction of cumylamine (1a)
with n-butyl acrylate (2a).[a]
[a]
Reaction conditions: 1a (0.5 mmol), 2a (2 equiv.),
[Cp*RhCl2]2 (2 mol%), CuACHTNUGRTNEUNG(OAc)2·H2O (2 equiv.) in o-
xylene (3 mL) at 808C under N2.
Isolated yield.
In dioxane (3 mL).
GC yield.
[b]
[c]
[d]
Entry Solvent
Temp. [oC] Time [h] Yield of 3a [%][b]
1
dioxane
80
3
97
2
3
o-xylene 80
t-AmOH 80
3
96
3
92
4
DMF
80
80
r. t.
r. t.
r. t.
3
3
6
24
6
87
52
98
97
product, butyl 2-(3,3-dimethylisoindolin-1-yl)acetate
(3a), was formed in 88% yield (entry 1, Table 1). In
dioxane, the 3a yield significantly decreased (entry 2).
In o-xylene, the reactions using isobutyl (2b), tert-
butyl (2c), cyclohexyl (2d), and ethyl (2e) acrylates in
place of 2a also proceeded efficiently to produce the
corresponding (isoindol-1-yl)acetates 3b–e in 81–91%
yields (entries 3–6). In contrast to reactive 1a, trityl-
5[c]
6
dioxane
dioxane
dioxane
dioxane
7[d]
8[e]
98 (92)
[a]
Reaction conditions: 1a (0.25 mmol), 2a (2 equiv.),
[Ru(p-cymene)Cl2]2 (5 mol%), AgSbF6 (20 mol%),
Cu(OAc)2·H2O (2 equiv.), solvent (3 mL) under N2.
GC yield based on the amount of 1a used. Value in pa-
rentheses indicates yield after purification.
Without AgSbF6.
With [Ru
(10 mol%).
With 1a (0.5 mmol).
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[b]
ACHTUNGTRENNUNGamine (1b) exhibited low reactivity in the reaction
[c]
[d]
with 2a to give 3f in a low yield (entry 7). While the
reaction of 3-(4-methylphenyl)pentan-3-amine (1c)
gave 3g in 63% yield (entry 8), 3-(4-chlorophenyl)-
pentan-3-amine (1d) was found to be poorly reactive
ACHTUGNERTN(NUNG p-cymene)Cl2]2 (2.5 mol%) and AgSbF6
[e]
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Ruthenium- and Rhodium-Catalyzed Dehydrogenative ortho-Alkenylation
Table 3. Ruthenium-catalyzed reaction of a,a-disubstituted
benzylamines 1 with alkenes 2.[a]
tries 3-6 in Table 1). N-tert-Butylacrylamide (2f) also
reacted with 1a, although the product 3j was contami-
nated by a minor amount of (isoindolin-1-ylidene)ace-
tate 4j (entry 5). In the case of using acrylonitrile
(2g), nucleophilic addition by the NH of 1a was faster
than the desired alkenylation to form 5k predomi-
nantly along with a minor amount of 3k (entry 6). In
contrast to these reactive monosubstituted alkenes,
disubstituted methyl crotonate did not react with 1a
at all. Meanwhile, 1b–d coupled with 2a effectively to
form 3f–h in 78–89% yields after 24–48 h (entries 7–
9). It should be emphasized that the reaction efficien-
cies were dramatically improved by using the rutheni-
um catalyst, especially in cases with 1b and 1d. In the
reaction of 1e, the yield of tricyclic 3i was also en-
hanced up to 80% (entry 10). In addition, an a-mono-
substituted benzylamine, 1-phenylethan-1-amine (1f),
underwent the coupling with 2a to produce 3l as
a
mixture of diastereomers (cis:trans=2.2:1)
(entry 11). However, in the case with a sterically less
hindered amine, the a-unsubstituted benzylamine
(1g), nucleophilic addition by its NH took place prior
to alkenylation to give 5m (entry 12).
In contrast to acrylates 2, styrene (6a) did not react
with 1a at all under ruthenium catalysis (entry 1 in
Table 4). It was, however, observed that 6a could be
coupled under rhodium catalysis. Thus, treatment of
1a (0.5 mmol) with 6a (1 mmol) in the presence of
[Cp*RhCl2]2 (0.01 mmol), AgSbF6 (0.04 mmol), and
CuACHTNURGTENUNG(OAc)2·H2O (1 mmol) in dioxane under N2 at 808C
for 4 h gave ortho-strylated product 7a in 24% yield
(entry 2). In t-AmOH, the reaction proceeded more
smoothly to produce 7a in 51% yield (entry 3). The
yield of 7a decreased in other solvents including n-
BuOH, EtOH, DMF, o-xylene, DCE, PhCl, and
PhCF3 (entries 4–10). When the reaction was conduct-
ed in t-AmOH at 1008C, the product yield was slight-
ly reduced (entry 11). Even in t-AmOH at 808C, the
reaction did not proceeded at all in the absence of
AgSbF6 (entry 12). Therefore, formation of a cationic
Rh active species in situ seems to be essential for the
reaction. Possible active species are probably
[Ru
(OAc)]
E
and
+A[SbF6]À,
[Cp*RhACTHUNGTRENNU(G OAc)] CHTUNGTRENNUNG
which could be formed from anion exchange between
the Rh catalyst precursor, [Cp*RhCl2]2, and AgSbF6.
Under optimized conditions (entry 3), the reactions of
methyl- (6b), methoxy- (6c), chloro- (6d), and
trifluoroACTHNUTRGNEUGmN ethyl- (6e) substituted styrenes with 1a pro-
[a]
Reaction conditions:
[Ru(p-cymene)Cl2]2 (5 mol%), AgSbF6 (20 mol%),
Cu(OAc)2·H2O (2 equiv.) in dioxane (3 mL) at room
1
(0.5 mmol),
2
(2 equiv.),
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
temperature under N2 for 6 h.
Isolated yield.
For 24 h.
For 48 h.
cis/trans=2.2:1.
[b]
[c]
[d]
[e]
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Chiharu Suzuki et al.
Table 4. Reaction of cumylamine (1a) with styrenes 6.[a]
Entry
R
6
Solvent Time [h] Product, Yield [%][b]
1[c]
2
3
4
5
6
7
8
9
10
11[d]
12[e]
13
14
15
16
H
H
H
H
H
H
H
H
H
H
H
H
Me
6a dioxane
6a dioxane
6a t-AmOH
6a n-BuOH
6a EtOH
6a DMF
6a o-xylene
6a DCE
3
4
4
4
4
4
4
8
4
8
2
4
6
6
6
6
7a, 0
7a, 24
7a, 51 (50)
7a, 10
7a, 17
7a, 3
7a, 9
7a, 39
7a, 43
7a, 35
7a, 43
7a, 0
7b, (46)
7c, (41)
7d, (44)
7e, (48)
Scheme 2. A plausible pathway for the ortho-alkenylation of
1.
6a PhCl
6a PhCF3
6a t-AmOH
6a t-AmOH
6b t-AmOH
ciently under ruthenium or rhodium catalysis. This is
one of the rare examples for catalytic processes utiliz-
ing a free amino group as a directing group.
OMe 6c t-AmOH
Cl 6d t-AmOH
CF3 6e t-AmOH
[a]
Reaction conditions: 1a (0.5 mmol), 2a (2 equiv.),
[Cp*RhCl2]2 (2 mol%), AgSbF6 (8 mol%),
Cu(OAc)2·H2O (2 equiv.), solvent (3 mL) at 808C under
N2.
GC yield based on the amount of 1a used. Value in pa-
rentheses indicates yield after purification.
Experimental Section
ACHTUNGTRENNUNG
General Procedure for the Ruthenium-Catalyzed
Reaction of a,a-Disubstituted Benzylamines 1 with
Alkenes 2
[b]
[c]
With 1a (0.25 mmol), 2a (2 equiv.), [Ru
N
A mixture of a,a-disubstituted benzylamine 1 (0.5 mmol),
(5 mol%), AgSbF6 (20 mol%), and Cu
T
alkene
2
(1 mmol), [RuAHCTUNGTRENNUNG
(2 equiv.).
At 1008C.
Without AgSbF6.
15 mg), AgSbF6 (0.1 mmol, 35 mg), CuACTHUNGTRENNNUG
[d]
[e]
(1.0 mmol, 200 mg), and 1-methylnaphthalene (ca. 30 mg) as
internal standard was stirred in dioxane (3 mL) under N2 at
room temperature for 6–48 h. GC and GC-MS analyses of
the mixtures confirmed formation of coupling product 3.
After cooling, the reaction mixture was poured into aqueous
NaHCO3 (40 mL) containing ethylenediamine (1 mL), ex-
tracted with ethyl acetate (40 mL), and washed with aque-
ous NaHCO3 (40 mL, two times). The organic layer was
dried over Na2SO4, and concentrated under vacuum. The
desired product 3 was isolated by column chromatography
on silica gel using hexane-ethyl acetate (2:1 v/v) as eluent.
Characterization data of products are summarized in the
Supporting Information.
ceeded to produce the corresponding stilbenes 7b–
e (entries 13–16). In contrast, an aliphatic alkene, 1-
octene, did not undergo the coupling smoothly. Only
trace amounts of unidentified products were formed.
A plausible mechanism for the dehydrogenative
ortho-alkenylation of 1 is illustrated in Scheme 2. Co-
ordination of the amino nitrogen of 1 toward the Ru
or Rh center appears to be the key for the regioselec-
À
tive C H bond cleavage at the ortho-position to form
a five-membered metallacycle intermediate A. Then,
alkene insertion into the resulting aryl-metal bond to
form B and subsequent b-hydrogen elimination may
take place to produce an ortho-alkenylated product.
Acknowledgements
À
The resulting [M] H species seems to be oxidized by
a copper salt to regenerate an active [M] X species.
À
This work was supported by Grants-in-Aid from MEXT,
JSPS, and JST, Japan.
In the case of using an alkene possessing an electron-
withdrawing group, the ortho-alkenylated product un-
dergoes successive nucleophilic cyclization to con-
struct an isoindoline framework.
In summary, we have demonstrated that the dehy-
drogenative ortho-alkenylation of a,a-disubstituted
benzylamines with alkenes can be performed effi-
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