Synthesis of Air- and Moisture-Stable, Storable Chiral Oxorhenium Complexes and Their Application as Catalysts for the Enantioselective Imine Reduction

Article abstract of DOI:10.1002/chem.201501914

Air-/moisture-stable, crystalline, and storable chiral salicyloxazoline based oxorhenium(V) complexes have been synthesized and their catalytic application for the asymmetric reduction of ketimines using hydrosilane as hydride source is disclosed. A broad substrate scope, high yields, and excellent enantioselectivities (up to 99 %) are attained. Furthermore, the syntheses of enantiopure α-amino esters, γ- and δ-lactams, and isoindolinones have also been carried out using this methodology. Finally, the method has been applied to synthetic targets of pharmaceutical relevance, such as R-(+)-salsolidine and R-(+)-crispine A.

Full text of DOI:10.1002/chem.201501914

DOI: 10.1002/chem.201501914
Communication
&
Asymmetric Catalysis
Synthesis of Air- and Moisture-Stable, Storable Chiral Oxorhenium
Complexes and Their Application as Catalysts for the
Enantioselective Imine Reduction
Braja Gopal Das, Rajender Nallagonda, Dhananjay Dey, and Prasanta Ghorai*[a]
been attempted. However, the obtained enantioselectivity re-
Abstract: Air-/moisture-stable, crystalline, and storable
chiral salicyloxazoline based oxorhenium(V) complexes
have been synthesized and their catalytic application for
the asymmetric reduction of ketimines using hydrosilane
as hydride source is disclosed. A broad substrate scope,
high yields, and excellent enantioselectivities (up to 99%)
are attained. Furthermore, the syntheses of enantiopure
a-amino esters, g- and d-lactams, and isoindolinones have
also been carried out using this methodology. Finally, the
method has been applied to synthetic targets of pharma-
ceutical relevance, such as R-(+)-salsolidine and R-
(+)-crispine A.
mained very low. Only the cyanobis(oxazoline) ligand-based
oxorhenium complexes developed by Toste and co-workers
have to date found success in asymmetric imine reductions.[8]
Salicyloxazoline ligand-based oxorhenium(V) complexes
have found superior reactivity for hydrosilane activation[9a,b]
and oxidative cyanation of tertiary amines.[9c] However, their
asymmetric variants have never been developed. Nevertheless,
chiral salicyloxazoline ligands have been utilized in numerous
transition metal-catalyzed enantioselective transformations, af-
fording high levels of selectivity.[10,11] Interest in using this type
of ligand has increased because of their easy synthesis from
natural amino acids.[11a]
In this article, we have embarked on a program aimed to de-
velop non-racemic rhenium(V)–oxo complexes based on salicy-
loxazoline as a monoanionic bidentate ligand. Ligands La–Ld,
having an N,O coordination site, formed bright green metal–
ligand complexes (Scheme 1),[12] which are air/moisture stable
and crystalline at ambient temperature. Furthermore, these
complexes exhibited catalytic activity for the asymmetric re-
duction of ketimines using hydrosilane as a hydride source,
with a broad substrate scope, high yields, and excellent enan-
The development of new chiral catalysts for asymmetric cataly-
sis remains an attractive area of research because of the ever-
increasing importance of enantiopure molecules in pharma-
ceuticals, flavors, fragrances, and agrochemical industries.[1] The
use of transition metal complexes remains central to this
goal.[2] Several excellent chiral catalysts, especially low-valent
transition metal complexes, have been developed for various
asymmetric transformations. However, they are generally air
and/or moisture sensitive. Therefore, many such catalysts are
prepared in situ just before use and are not storable for ex-
tended periods. Recently, high valent oxo–metal complexes,
especially oxorhenium complexes, have gained much attention
as catalysts for various organic transformations due to their
mild and air-/moisture-stable nature.[3,4] Such complexes have
become an attractive and practical alternative to many low-
valent transition metal complexes, even for reduction reac-
tions. In this context, the unique mode of hydrosilane activa-
tion by oxorhenium catalysts emerged as a new area of cataly-
sis.[5] However, the development of suitable chiral oxorhenium
complexes for asymmetric synthesis is evidently a necessary
task to ensure the access to enantiomerically enriched prod-
ucts. To this end, the development of substituted 2-amino al-
cohol-based[6] and salen-based[7] oxorhenium complexes have
[a] B. G. Das, R. Nallagonda, D. Dey, Dr. P. Ghorai
Department of Chemistry
Indian Institute of Science Education and Research Bhopal
Bhauri, Indore By-pass Road, Bhopal-462066 (India)
E-mail: pghorai@iiserb.ac.in
Scheme 1. a) Synthesis of chiral oxorhenium catalysts; b) ORTEP representa-
tion of catalyst 3a with additional CH2Cl2 solvent molecule. Thermal ellip-
soids are drawn at 50% probability.[12]
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501914.
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Communication
tioselectivities. Notably, the catalysts can be stored for over
6 months in air at room temperature without loss of reactivity.
Chiral benzyl amines are among the most ubiquitous func-
tional groups in fine chemical, agrochemical, and pharmaceuti-
cal products.[13] Several well-known drugs and natural products
contain a-branched benzylamine moieties (Figure 1).[14] Various
Table 1. Optimization of reaction conditions.
Entry Catalyst (mol%) Solvent Silane t [h] Conv. [%][a] ee [%][b]
1
2
3
4
5
6
7
8
3a (10)
3b (10)
3c (10)
3d (10)
3d (5)
3d (3)
3d (1)
3d (5)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
Et3SiH 72
Et3SiH 72
Et3SiH 72
Et3SiH 72
Et3SiH 72
Et3SiH 72
Et3SiH 72
Et3SiH 36
72
78
82
75
74
33
21
78
80
66
83
92
91
91
96
98
[a] The conversion was calculated based on 1H NMR spectroscopy of the
crude reaction mixture using anisole as internal standard; [b] enantiomer-
ic excess was determined by HPLC analysis on a chiral stationary phase.
Table 2. Reduction of substituted aromatic ketimines.[a]
Figure 1. Drug molecules containing a-branched benzylamine moieties.
a-amino esters can also potentially be synthesized from a-
branched benzylamine amines.[14] The asymmetric reduction of
prochiral ketimines remains the straightforward route for such
amines.[15] In this regard, transition-metal catalyzed hydrosilyla-
tion strategies have gained considerable attention because the
silane is inexpensive and safe to handle. There are few exam-
ples of high-valent metal complexes that have been used as
catalyst compared to the low-valent transition metal com-
plexes.[16] Therefore, we investigated using our newly devel-
oped chiral oxorhenium complexes for imine reduction. We
chose to use diphenylphosphinyl-protected imines, because
such protecting groups are easy to remove, compared to aryl
or alkyl groups, and, moreover, their bulkiness increases the
E/Z ratio in imines, providing conformational restriction, a pre-
requisite for efficient asymmetric induction.[17,18]
Entry
R
t [h]
2, yield [%][b]
ee [%][c]
1
H (1a)
H (1a)
2-F (1b)
3-F (1c)
48
120
36
48
60
40
120
60
60
60
70
40
40
24
60
60
60
2a, 78
2a, 71
2b, 77
2c, 92
2d, 81
2e, 78
2 f, 48
2g, 79
2h, 88
2i, 69
2j, 83
2k, 94
2l, 82
2m, 84
2n, 82
2o, 84
2p, 85
97
97
92
99
95
93
79
96
79
63
88
91
92
95
97
95
96
2[d]
3
4
5
6
7[e]
8
4-F (1d)
4-NC (1e)
4-O2N (1 f)
4-F3C (1g)
4-MeO (1h)
3,4-(MeO)2 (1i)
4-H3C (1j)
4-Br (1k)
2-Cl (1l)
9
10
11
12
13
14
15
16
17
2,4-Cl2 (1m)
2-Br (1n)
4-I (1o)
The synthesized oxorhenium catalysts 3a–d (Scheme 1)
were screened for the reduction of N-diphenylphosphinyl
imine derived from acetophenone (Table 1). Catalyst 3d, de-
rived from l-valine, gave the highest enantioselectivity (92%)
with good yields, compared to the other catalysts (Table 1, en-
tries 1–4). The screening of silanes revealed that triethylsilane
remained the best choice with respect to yield and enantiose-
lectivity (see the Supporting Information). The decrease in cat-
alyst loading from 10 mol% to 5 mol% resulted in no change
in yield or enantioselectivity, (Table 1, entry 4 vs. entry 5). How-
ever, on further decrease in catalyst loading to 1 mol%
through 3 mol%, the yields of the reaction fell drastically
(Table 1, entries 6 and 7). Extensive solvent screening indicated
that CHCl3 was the best choice with respect to reaction time
and enantioselectivity (Table 1, entry 8; for details, see the Sup-
porting Information).
naphthyl (1p)
[a] Reaction conditions, unless otherwise stated: 1 (0.20 mmol), Et3SiH
(0.30 mmol), 3d (5 mol%), CHCl3 (2 mL), RT; [b]yield of product isolated
by column chromatography; [c] enantiomeric excesses were determined
by HPLC analysis on a chiral stationary phase; [d] reaction was carried out
on 1.0 g scale; [e] reaction was carried out at 608C.
the aryl ring, such as fluoro (1b–d), 4-NC (1e), 4-O2N (1 f), and
4-F3C (1g), provided good yields and excellent enantioselectivi-
ties (up to 99% ee; Table 2, entries 3–8). Interestingly, changing
the position of substituent on aryl ring (i.e., ortho, meta, or
para) brought about little change in the yield or enantioselec-
tivity (Table 2, entries 3–5). Electron-rich substituents on the
aryl ring, such as 4-MeO (1h), 3,4-(MeO)2 (1i), and 4-Me (1j),
gave rise to slight decreases in ee (Table 2, entries 9–11). How-
ever, the reactions proceeded smoothly. Halogen substituents,
such as Cl, Br and I, on the aryl ring remained had similar ef-
fects on the current process (Table 2, entries 12–16). The naph-
With the optimized reaction conditions in hand, the scope
of substitutions on the aryl ring was briefly investigated
(Table 2). Substrates bearing electron-withdrawing groups on
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thyl substrate also gave a good yield, (85%) and enantioselec-
tivity (96%) of the corresponding amine 2p (Table 2, entry 17).
To elaborate the substrate scope, we turned our focus to the
alkyl ketimines. Imines having ethyl, n-propyl, isopropyl, n-
butyl, or n-pentyl alkyl chains provided the desired chiral sec-
ondary amines 2q–u with good yields (75–90%) and high
enantioselectivities (up to 98% ee; Scheme 2). Cyclic alkyl sys-
tems, such as imines derived from a-tetralone (1v) and inda-
ed the desired a-amino esters 5a–d in good yields and enan-
tioselectivities. Notably, the corresponding enantiopure l-
phenyl glycine derivatives could easily be obtained upon the
acid hydrolysis of these a-amino esters.
Next, the asymmetric reduction of g-, d-, and e-imino esters
(6a–c) was demonstrated (Table 4). All substrates were reduced
Table 4. Synthesis of enantioenriched lactams.[a,d]
Entry
n
t [h] 7, yield [%][b] ee [%][c] 8, yield [%][b] ee [%][c]
1
2
3
2 (6a) 72
3 (6b) 48
4 (6c) 36
7a, 71
7b, 80
7c, 69
98
96
95
8a, 92
8b, 91
–
98
96
–
[a] Reaction conditions for step 1 were as for Table 2; [b] yield of isolated
product; [c] enantiomeric excesses were determined by HPLC analysis on
a chiral stationary phase; [d] reaction conditions for step 2: HCl (5 equiv),
MeOH (1 mL), then NaOH (7 equiv).
efficiently to afford secondary amines 7a, 7b, and 7c in up to
80% yield and up to 98% ee. The subsequent deprotection fol-
lowed by cyclization of these protected amines gave the corre-
sponding enantioenriched substituted lactams 8a and 8b in
good yields. Notably, no loss of the enantioselectivity was ob-
served during the course of the deprotection–cyclization pro-
cess.
Scheme 2. Synthesis of chiral secondary amines. Reaction conditions are the
same as Table 2. Yields are for products isolated by column chromatography.
Enantiomeric excesses were determined by HPLC analysis on a chiral station-
ary phase.
These reaction conditions were also successfully applied for
the reduction of o-imino benzoates 9a–c (Table 5). The resul-
tant enantioenriched amines were then utilized for the synthe-
sis of isoindolinones 11 a–c with high enantioselectivities (up
to 97% ee).
none (1w), also smoothly provided the desired products, 2v
and 2w, respectively. An imine incorporating a heteroaromatic
group, thiophene (1x), was also reduced with good yield and
high enantioselectivity (2x). However, the reduction of purely
aliphatic imines 1y–za proceeded with very low enantioselec-
tivities (2y–za).
Table 5. Isoindolinone synthesis.[a,d]
We next explored the reductions of a-imino ester substrates
(Table 3). Electron-neutral groups such as phenyl (4a), electron-
withdrawing groups such as p-FC6H4 (4b) and p-(F3C)C6H4 (4c),
and electron-donating group such as p-MeC6H4 (4d) all provid-
Entry
R
t [h] 10, yield [%][b] ee [%][c] 11, yield [%][b] ee [%][c]
1
2
3
H (9a) 36
Me (9b) 72
Cl (9c) 60
10a, 68
10b, 82
10c, 78
97
96
97
11 a, 92
11 b, 92
11c, 88
97
96
97
Table 3. Substrate scope: a-imino esters.[a]
[a] Reaction conditions for step 1 were as for Table 2; [b] yield of isolated
product; [c] enantiomeric excesses were determined by HPLC analysis on
a chiral stationary phase; [d] reaction conditions for step 2: HCl (5 equiv),
MeOH (1 mL) then NaOH (7 equiv).
Entry
R
t [h]
5, yield [%][b]
ee [%][c]
1
2
3
4
H (4a)
48
60
48
60
5a, 71
5b, 68
5c, 74
5d, 72
94
94
94
97
Finally, the current methodology was demonstrated for the
synthesis of natural product (+)-salsolidine and antitumor drug
crispine A (Scheme 3). The reduction of imine 12 gave the cor-
responding amine 13 with 82% yield and 94% ee. Subsequent
deprotection–cyclization provided the intermediate isoquinoli-
none, which, upon borohydride reduction, led to the formation
4-F (4b)
4-F3C (4c)
4-H3C (4d)
[a] Reaction conditions were as for Table 2; [b] yield of isolated product;
[c] enantiomeric excesses were determined by HPLC analysis on a chiral
stationary phase.
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Acknowledgements
The authors are grateful for financial support from DST, New
Delhi, India and IISER Bhopal, India. We are thankful to Dr.
Deepak Chopra, Asst. Professor, IISER Bhopal, for his assistance.
B.D. and R.N. acknowledge CSIR, New Delhi, India for fellow-
ship.
Keywords: asymmetric catalysis
· hydrosilylation · ligand
effects · oxorhenium · natural products
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source. The corresponding amines were obtained in high
yields and with excellent enantioselectivities. Chiral amino
esters have also been synthesized and subsequently cyclized
to give g- and d-lactams and isoindolinones in high enantiose-
lectivities. Finally, the methodology has been extended to the
synthesis of natural products R-(+)-salsolidine and R-(+)-crispi-
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Received: May 15, 2015
Published online on July 21, 2015
Chem. Eur. J. 2015, 21, 12601 – 12605
www.chemeurj.org
12605
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