Ru-Catalyzed asymmetric transfer hydrogenation of substituted dibenzo[b,f] [1,4]oxazepines in water

Article abstract of DOI:10.1039/c7ob01229b

This is the first report on the asymmetric transfer hydrogenation (ATH) of dibenzo[b,f][1,4]oxazepine compounds in the presence of an (R,R)-Ru-Ts-DPEN complex. The developed catalytic asymmetric protocol provides biologically active 11-substituted-10,11-d

Full text of DOI:10.1039/c7ob01229b

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Ru-Catalyzed asymmetric transfer hydrogenation
of substituted dibenzo[b,f][1,4]oxazepines in
water†
Cite this: DOI: 10.1039/c7ob01229b
Received 18th May 2017,
Accepted 6th June 2017
Ganesh V. More and Bhalchandra M. Bhanage
*
DOI: 10.1039/c7ob01229b
rsc.li/obc
This is the first report on the asymmetric transfer hydrogenation heteroaromatic compounds is well studied,7 but studies on the
(ATH) of dibenzo[b,f][1,4]oxazepine compounds in the presence of asymmetric hydrogenation of seven-membered cyclic imines
an (R,R)-Ru-Ts-DPEN complex. The developed catalytic asym- are very limited.8 Recently, Zhou et al. reported the enantio-
metric protocol provides biologically active 11-substituted-10,11- selective hydrogenation of seven-membered dibenzo[b,f][1,4]
dihydrodibenzo[b,f][1,4]oxazepines with excellent conversion oxazepines using the [Ir(COD)Cl]2/(S)-Xyl-C3*-TunePhos
(up to >99%) and high enantioselectivity (up to 93% ee) in water as complex as the catalyst in the presence of morpholine-HCl
an environmentally benign solvent.
under 700 psi H2 in dichloromethane (Scheme 1, eqn (a)).9
The reported protocol achieved high enantioselectivity, but
there is a chance to develop the green protocol by avoiding
phosphine based ligands, additives, Ir(I) metal precursors,
organic solvents and molecular hydrogen in a high pressure
autoclave.
Introduction
The 11-substituted-10,11-dihydrodibenzo[b,f][1,4]oxazepine
moiety is important in synthetic organic chemistry and in
pharmaceutical intermediates due to its presence in many
physiologically active compounds.1 Molecules containing a
dibenzo[b,f][1,4]oxazepine motif are part of antidepressants,
antipsychotics, and progesterone receptor agonists,2,3 and
hence this motivated us to develop new strategies for their syn-
thesis. Considerable work has been done on the synthesis of
11-substituted-10,11-dihydrodibenzo[b,f][1,4]oxazepine deriva-
tives via microwave-assisted one-pot Ugi-four component reac-
tion and Pd-catalyzed intramolecular ortho-arylation,4a domino
elimination–rearrangement–addition reaction of N-alkoxy(aryl-
methyl)amines,4b intramolecular π-cationic cyclization of the
N-acyliminium ions4c and 1,3-dipolar cycloaddition reactions
of dibenzoxazepinium ylides.4d However, the practical syn-
thesis of chiral 11-substituted-10,11-dihydrodibenzo[b,f][1,4]
oxazepines is still a challenge.
ATH reaction is a green, cost-effective and operationally
simpler approach than molecular hydrogenation. ATH
methods have additional advantages such as there is no need
to maintain an inert atmosphere, excellent selectivity for pro-
ducts and wide substrate capacity. Due to these specialities,
the ATH protocol is widely used in industry as well as
academic
science.10
The
bidentate
chiral
ligand
N-monosulfonylated 1,2-diamine with a transition metal cata-
lyst such as [Ru(p-cymene)Cl2]2 or [(Cp*MCl2)2] (M = Rh, Ir)
has been regularly used for ATH reactions.10h To the best of
our literature knowledge, there is no report on the ATH reac-
tion of dibenzo[b,f][1,4]oxazepine derivatives. In continuation
of our interest in asymmetric work,11 we herein document our
results on the (R,R)-Ru-TsDPEN catalyzed ATH reaction of
Asymmetric hydrogenation of imines is one of the most
prominent methods for directly accessing enantiopure amines.
In the past decade, various methods have been developed for
the asymmetric hydrogenation of imines by a transition metal
chiral catalyst.5,6 In addition, the asymmetric hydrogenation of
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai-
400019, India. E-mail: bm.bhanage@gmail.com, bm.bhanage@ictmumbai.edu.in;
Fax: +91-22-33611020; Tel: +91-22-33612601
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7ob01229b
Scheme 1 ATH of dibenzo[b,f][1,4]oxazepines in water.
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seven-membered cyclic imines containing dibenzo[b,f][1,4] buffers gave >99% conversion. We observed a marginal
oxazepines in water. decrease in the enantioselectivity in the presence of potassium
The reaction was carried out at 40 °C for 12 h and afforded hydrogen phthalate buffer (Table 1, entry 8) and an aqueous
the enantioenriched product in excellent conversion up to 1 M solution of HCOOH–HCOONa (FA + SF) gave the same
>99% and high enantiomeric excess up to 93% ee (Scheme 1). enantioselectivity with HCOONa as the hydrogen source
Initially, we chose imine 1a as a model substrate, the Noyori (Table 1, entry 9). Then, a reaction was carried out only in the
complex (R,R)-Ru-TsDPEN{RuCl(p-cymene)[(R,R)-TsDPEN]} as buffer solution of HCOOH–HCOONa and the same results
a chiral catalyst and sodium formate as the hydrogen source in were observed. This confirms that the mixture of HCOOH–
water. Disappointingly, only 7% conversion was observed at HCOONa plays a dual role, wherein it acts as a buffer solution
40 °C in 20 h. However, the enantioselectivity of product 2a as well as a hydrogen source (Table 1, entry 10). Subsequently,
was 88% ee (Table 1, entry 1). Previous reports show that the we studied the effect of pH values on the conversion by varying
pH value of the solution plays a crucial role in the ATH reac- the ratio of HCOOH–HCOONa and found that the conversion
tion,12 so we focussed our attention to check the effect of the of 1a depends on the pH value of the buffered solution
solvent pH on the ATH of 1a. In this context, we used a 1 M (Table 1, entries 11–15). The best catalytic activity was observed
buffer aqueous solution of CH3COOH–HCOONa (AA + SF) and at pH 4 with >99% conversion and 93% ee, at a reaction temp-
observed that the conversion increases drastically up to >99% erature of 40 °C (Table 1, entry 12).
and the enantioselectivity of 2a also increases to 93% ee
Encouraged by these exciting results, we examined the
(Table 1, entry 2). We also studied other organic solvents like buffer concentration for the ATH of 1a. The reaction was
toluene, DMF, ACN, EtOH, and DCM; all these solvents failed carried out at different buffer concentrations ranging from
to give conversion (Table 1, entries 3–7). The conversion along 0.1 M to 2 M (Table 1, entries 16–18). The 1 M buffer solution
with the enantioselectivity of the desired product was higher is the best for high conversion and enantioselectivity of the
in the buffer solution of CH3COOH–HCOONa (93% ee), and, desired product (Table 1, entry 12). From these results, we
therefore, we screened various buffers and found that all could infer that the conversion of 1a also depends on the con-
centration of the buffered solution. To further optimize the
reaction conditions, we checked the effect of catalyst loading,
temperature and time on the conversion and enantioselectivity
of the desired product 2a. The catalyst loading was screened
Table 1 Effect of solvents and various buffers on the ATH of 1aa
from 0.25 mol% to 2.5 mol% (Table 2, entries 1–4) and it
showed that 1 mol% of the catalyst provides a complete con-
version of 1a with 93% ee. Furthermore, we studied the effect
of temperature and observed that the conversion was depen-
dent on the temperature and it decreases with lowering the
reaction temperature (Table 2, entry 5). Finally, the reaction
time was also optimized (Table 2, entries 8–11) and a
maximum conversion of >99% and 93% ee were obtained after
12 h. Thus, the optimized reaction conditions are substrate 1a
Sr. no.
Solvent
pH
Conv.b (%)
eeb (%)
Effect of solvent
1
2c
3
4
5
6
7
H2O
—
4
—
—
—
—
—
7
88
93
—
—
94
—
98
AA + SF
Toluene
DMF
ACN
EtOH
DCM
>99
NR
NR
5
NR
5
Table 2 Effect of the reaction parameters on the ATH of 1aa
Conv.b
(%)
eeb
(%)
Effect of various buffers
8
Catalyst
loading (mol%)
Temp.
(°C)
Time
(h)
KHPd + NaOH
FA + SF
FA + SF
4
4
4
>99
>99
>99
91
93
93
Sr. no.
9
10e
Effect of catalyst loading
Effect of pH
11
12
13
1
2
3
4
2.5
1
0.5
0.25
40
40
40
40
20
20
20
20
>99
>99
>99
53
93
93
91
89
FA + SF
FA + SF
FA + SF
FA + SF
FA + SF
FA + SF
FA + SF
FA + SF
3
4
5
6
7
4
4
4
>99
>99
85
22
13
12
56
>99
91
93
92
90
89
91
93
90
14
Effect of temperature
15
5
6
7
1
1
1
RT
40
60
20
20
20
79
>99
>99
93
93
93
16f
17g
18h
Effect of time
8
9
10
11
1
1
1
1
40
40
40
40
4
8
12
16
90
96
>99
>99
93
93
93
93
a Reaction conditions: 1a (0.2 mmol), (R,R)-Ru-Ts-DPEN (2 mol%),
HCOONa (1 mmol), solvent: 2 mL, temperature: 40 °C. b Determined
by chiral HPLC analysis on a Chiralcel OD-H column. c Aqueous 1 M
buffer solution of AA + SF (CH3COOH + HCOONa). d Potassium hydro-
gen phthalate (KHP). e Without adding 1 mmol HCOONa (only in a a Reaction conditions: 1a (0.2 mmol), catalyst: (R,R)-Ru-Ts-DPEN, a 1 M
1 M buffer solution of FA + SF). f 0.1 M aq. FA + SF. g 0.5 M aq. FA + SF. aqueous solution of FA + SF (pH = 4): 2 mL. b Determined by chiral
h 2 M aq. FA + SF.
HPLC analysis on a Chiralcel OD-H column.
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Table 3 ATH of substituted dibenzo[b,f][1,4]oxazepine derivativesa
Table 3 (Contd.)
Entry
14
Substrate
Product
Yieldb (%)
eec (%)
88
Entry
1
Substrate
Product
Yieldb (%)
98
eec (%)
93
91
2
3
98
93
91
80
15
58
80
a Reaction conditions: 1a (0.5 mmol), (R,R)-Ru-TsDPEN (1 mol%), 1 M
aqueous HCOOH–HCOONa solution (pH = 4): 5 mL, temperature:
40 °C. b Isolated yield. c Determined by chiral HPLC analysis on
Chiralcel OD-H and Chiralpak AD-H columns.
4
5
6
96
97
92
85
87
90
(0.2 mmol), (R,R)-Ru-TsDPEN (1 mol%), and 1 M HCOOH–
HCOONa aqueous solution (2 ml) at 40 °C for 12 h.
With the optimized reaction conditions in hand, a range of
dibenzo[b,f][1,4]oxazepine derivatives were screened for the
ATH reaction with the (R,R)-Ru-TsDPEN complex (Table 3,
entries 1–15). We screened the ortho and para substituted moi-
eties; in all the cases, the desired cyclic amines were obtained
in good to excellent yields and enantioselectivities. When R1
was a methyl group with an ortho (R4) substituted substrate, it
gave high yield (up to 93%) and enantioselectivity as –Cl
(80% ee), –Br (90% ee), –CH3 (83% ee), and –Ph (91% ee).
A para-substituent (R2) substrate with R1 as a methyl group
also gave high yield and enantioselectivity as –CH3 (93% ee),
Br (85% ee), –F (87% ee), –Ph (82% ee), and –OMe (71% ee).
The ortho and para-chloro substituted compounds gave good
enantioselectivities (up to 77% ee). By changing the R1 from
the methyl to the ethyl group, a slight decrease in the yield of
the desired compound with high enantioselectivity was
observed (up to 82% ee, Table 3: entries 10 and 11). It is
notable that changing the alkyl group to the phenyl group at
the R1 position provides good enantioselectivity with a moder-
ate yield (up to 58%, Table 3: entry 15).
7
8
91
90
83
91
9
93
88
86
82
80
82
10
11
Conclusions
In conclusion, we have developed a phosphine free, additive
free catalytic ATH protocol for dibenzo[b,f][1,4]oxazepine and
its derivatives with an unmodified (R,R)-Ru-TsDPEN complex.
The desired cyclic amines were obtained in good yields (up to
>99%) with excellent enantioselectivities (up to 93% ee) at
40 °C in 12 h. Our methodology has advantages in the form of
low catalyst loading (1 mol%), the use of HCOOH–HCOONa as
a green hydrogen source, atom-economical synthesis and the
use of water as an environmentally benign solvent.
12
13
89
85
71
77
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Acknowledgements
G. V. More is very much thankful to the Council of Scientific
and Industrial Research (CSIR), New Delhi, India, for providing
the Senior Research Fellowship (SRF). We also thank the
Department of Science and Technology (DST)-SERB, India,
(Project file No. SR/S1/OC-09-2012) for financial support.
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