The elpaN-salen series: Multifunctional ligands based on D-glucose for the Mn(III)-catalyzed enantioselective epoxidation of styrenes

Article abstract of DOI:10.1016/j.ica.2013.06.007

It is described the simple design of a series of N,N′,O,O′- chiral ligands (elpaN-salen-H₂), which represent a subset of the elpaN-type library of ligands based on β-1,2-D-glucodiamine. The corresponding Mn(III) complexes were examined in the asymmetric epoxidation of styrenes with satisfactory results, e.g. cis-β-methylstyrene was functionalised in ees up to 88%.

Full text of DOI:10.1016/j.ica.2013.06.007

Inorganica Chimica Acta 405 (2013) 288–294
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
The elpaN-salen series: multifunctional ligands based on D-glucose
for the Mn(III)-catalyzed enantioselective epoxidation of styrenes
a
a
Francesco Ruffo a,b, , Alessandro Bismuto , Andrea Carpentieri , Maria E. Cucciolito a,b, Matteo Lega a,b
,
⇑
Angela Tuzi a
a Dipartimento di Scienze Chimiche, Università degli Studi di Napoli ‘‘Federico II’’, Complesso Universitario di Monte S.Angelo, via Cintia, I-80126 Napoli, Italy
b Consorzio Interuniversitario di Reattività Chimica e Catalisi (CIRCC), via Celso Ulpiani 27, I-70126 Bari, Italy
a r t i c l e i n f o
a b s t r a c t
It is described the simple design of a series of N,N0,O,O0-chiral ligands (elpaN-salen-H2), which represent a
subset of the elpaN-type library of ligands based on b-1,2- -glucodiamine. The corresponding Mn(III)
complexes were examined in the asymmetric epoxidation of styrenes with satisfactory results, e.g. cis-
b-methylstyrene was functionalised in ees up to 88%.
Article history:
Received 12 April 2013
Received in revised form 3 June 2013
Accepted 5 June 2013
D
Available online 13 June 2013
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Carbohydrates
Salen
Alkene epoxidation
Manganese
Asymmetric catalysis
1. Introduction
2 and 3 are decorated with the coordinating functions that
typically characterize the privileged ligands [7] based on chiral
It is well recognized [1] that appropriate functionalization of
carbohydrates can produce effective ligands for enantioselective
catalysis.
In addition to economic and synthetic benefits, this route is par-
ticularly attractive because it allows to design flexible libraries of
modular ligands, thanks to the rich and well-developed chemistry
of sugars.
trans-1,2-diaminocyclohexane (DACH-salen, DACH-py and Trost
ligands).
Finally, the ‘‘auxiliary’’ positions 4 and 6 are generally protected
with a benzylidene ring, which stabilizes the chair conformation of
the sugar ring. This option can be advantageous for the result of the
catalysis. At the same time, the protection can be easily removed
and replaced by more specific tags [3b,3d].
This fruitful strategy becomes even more productive when car-
bohydrates are used as building blocks of ‘‘privileged’’ ligands [1h],
with the aim of amplifying their field of application. In fact, the
introduction of a privileged structure in a sugar moiety returns
unique ligand structures, which combine efficiency with conve-
nience and flexibility.
Over the years, the Naple-type library was successfully exam-
ined in several asymmetric reactions involving unsaturated sub-
strates, in both traditional and unconventional conditions [2–6].
However, in each of these enantioselective processes, only one
enantiomer of the chiral product can actually be favored. In fact,
for privileging the production of the other one, use of the enantio-
Within this context, our group recently produced the library of
ligands Naple-type illustrated in Fig. 1 [2–6].
meric ligands based on L-glucose would be necessary. This alterna-
tive is unlikely due to the high cost of this non-natural sugar.
Aiming to fill this gap, our strategy is recently focused towards
the design of the enantiomeric counterpart of the Naple-type
library, in order to perform the same catalytic processes, but with
opposite stereochemical outcome.
D
-glucose is the sugar ‘‘building block’’, and its structure was
optimized to ensure synthetic convenience and high performance
in catalysis. More precisely, C-1 is glycosylated with a benzyl
group, which can be selectively introduced in a-position. Positions
This approach is producing the pseudo-enantiomeric library of
ligands (elpaN-type), which has been designed by simply shifting
the coordinating functions from positions C2 and C3 to C1 and
C2 (Fig. 2).
⇑
Corresponding author at: Dipartimento di Scienze Chimiche, Università degli
Studi di Napoli ‘‘Federico II’’, Complesso Universitario di Monte S.Angelo, via Cintia,
I-80126 Napoli, Italy. Tel: +39 081 674543; fax: +39 081 674090.
E-mail address: ruffo@unina.it (F. Ruffo).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.06.007

F. Ruffo et al. / Inorganica Chimica Acta 405 (2013) 288–294
289
2.2. Preparation of b-1-azide-3,4,6-tri-O-acetyl-2-amino-
(2)
D-glucose
O
O
6
phase tag
O
5
1
2
4
OBz
L
3
This compound was obtained by adapting a known procedure
[9] in order to improve yield and safety: solution of 1-
bromo-3,4,6-tri-O-acetyl- -glucosamine hydrobromide 1 (4.5 g,
X
X
a
L
Naple-type
a
-D
X-L: -NHCH2Ar (cyclopropanation, Cu)
X-L: -NH(CO)C6H4-2-PPh2 (AAA, Pd and ACA, Cu)
X-L: -N=CH-2-py (AAA, Mo)
10 mmol) in THF (100 mL) was treated with TMSN3 (1.2 g,
10 mmol) and potassium t-butoxide (2.4 g, 21 mmol) in this order.
After stirring over night at RT, the solid was removed by filtration
and the solvent was removed under vacuum. The residue was dis-
solved in dichloromethane (10 mL) and filtered over a short pad of
Celite. The pure product was precipitated by slow addition of
petroleum ether (2.9 g, 86%). For spectroscopic characterization,
see Ref. [8a].
X-L: -N=CHC6H4-2-OH (epoxydation, Mn)
Fig. 1. General structure of the Naple-type library.
p
e
h
a
s
e
t
a
g
O
O
p
h
a
s
2.3. Preparation of elpaN-salen-aH2, -bH2, -cH2
e
t
a
g
L
O
O
O
OBz
L
p
h
a
3
s
2
t
a
g
L
Azide 2 (0.66 g, 2.0 mmol) was stirred in dry toluene (8.0 mL)
under inert atmosphere at 273 K. The desired aldehyde was added
(4.2 mmol), followed by P(Me)Ph2 (0.40 g, 2.0 mmol). The mixture
was stirred over night at RT. The solvent was removed under vac-
uum and the crude product was purified by flash chromatography
on silica (eluent: EtOAc/PE = 1/3, NEt3 1%), yield: elpaN-salen-aH2,
44%; elpaN-salen-bH2, 42%; elpaN-salen-cH2, 51%. elpaN-salen-
aH2: MS(MALDI) Calc. for C34H45N2O9 [LH]+: 625.31. Found
625.40. 1H NMR(C6D6): dH = 12.62 (1 H, s, OH), 12.53 (1 H, s,
OH), 8.37 (1 H, s, N@CH) 8.03 (1 H, s, N@CH), 7.29–7.05 (5 H, m,
aromatics), 6.94 (1 H, d, J = 6.9, aromatic) 5.67 (1 H, t,
J3,4 = J3,2 = 9.7, 3-H), 5.42 (1 H, t, J4,5 = 9.7, 4-H), 4.50 (1 H, dd,
O
O
X
2
1
X
X
X
L
Fig. 2. Naple-type vs elpaN-type libraries.
0
J6,6 = 12.3, J5,6 = 4.7, 6-H), 4.46 (1 H, d, J1,2 = 8.6, 1-H), 4.22 (1 H,
dd, J5,6 = 1.2, 60-H), 3.57 (1 H, m, 5-H), 3.18 (1 H, t, 2-H), 1.80 (3
0
R3O
R3O
H, s, OAc), 1.77 (3 H, s, OAc), 1.55 (3 H, s, OAc), 1.17 (9 H, s, t-
Bu), 1.12 (9 H, s, t-Bu). 13C NMR(C6D6): dC = 170.2, 170.1, 169.6,
169.4, 165.9, 159.8, 159.7, 141.6, 141.5, 131.2, 131.1, 129.1,
128.7, 118.2, 118.1, 117.6, 117.5, 92.1, 74.8, 74.7, 74.4, 68.8, 62.4,
34.0, 33.9, 31.5 (6C), 20.5, 20.4, 20.1; IR:
(C@O), 1635 (C@N). elpaN-salen-bH2: MS(MALDI) Calc. for
34H45N2O9 [LH]+: 625.31 Found 625.43. 1H NMR(C6D6):
R3O
O
N
N
m
max/cmꢀ1 = 1751
R
OH HO
R1
C
dH = 13.45 (1 H, s, OH), 13.44 (1 H, s, OH), 8.30 (1 H, s, N@CH)
7.95 (1 H, s, N@CH), 7.29 (2 H, t, J = 8.6, aromatics), 6.94 (1 H, d,
J = 6.9, aromatic), 6.86 (1 H, d, J = 6.9, aromatic), 6.74 (1 H, t,
J = 6.7, aromatic), 6.67 (1 H, t, J = 6.7, aromatic), 5.64 (1 H, t,
J3,4 = J3,2 = 9.7, 3-H), 5.40 (1 H, t, J4,5 = 9.7, 4-H), 4.50 (1 H, dd,
R2
R2
Fig. 3. General formula of the elpaN-salen–H2 library of ligands.
The stratagem already led to successful results. In preliminary
experiments [8], an asymmetric allylic substitution catalyzed by
palladium or molybdenum yielded the chiral products in high ee,
and in opposite configuration with respect to those achieved with
the original Naple-type library.
In this report, the scope of the strategy is confirmed through the
synthesis of a subset of the elpaN-type ‘‘bouquet’’ (elpaN-salen-H2
series of Fig. 3), and its application in the manganese-catalyzed
asymmetric epoxidation of styrenes.
0
J6,6 = 12.4, J5,6 = 4.6, 6-H), 4.41 (1 H, d, J1,2 = 8.6, 1-H), 4.21 (1 H,
dd, J5,6 = 1.5, 60-H), 3.57 (1 H, m, 5-H), 3.03 (1 H, t, 2-H), 1.82 (3
0
H, s, OAc), 1.81 (3 H, s, OAc), 1.57 (3 H, s, OAc), 1.56 (9 H, s, t-
Bu), 1.49 (9 H, s, t-Bu). 13C NMR(C6D6): dC =Calc. for C<170.6,
170.2, 169.6, 169.4, 166.2, 161.0(2C), 138.0(2C), 131.1, 131.0(2C),
130.9, 118.8, 118.7(3C), 92.0, 74.6, 74.2, 74.1, 68.8, 62.3, 35.2,
35.2, 29.7(3C), 29.6(3C), 20.4, 20.3, 20.1. IR:
(C@O), 1629 (C@N). elpaN-salen-cH2: MS(MALDI) Calc. for
42H61N2O9 [LH]+: 737.44. Found 737.50. 1H NMR(C6D6):
m
max/cmꢀ1 = 1752
C
dH = 13.38 (1 H, s, OH), 13.30 (1 H, s, OH), 8.37 (1 H, s, N@CH)
8.03 (1 H, s, N@CH), 7.56 (1 H, s, aromatic), 7.54 (1 H, s, aromatic),
7.07 (1 H, s, aromatic), 7.00 (1 H, s, aromatic), 5.70 (1 H, t,
J3,4 = J3,2 = 9.7, 3-H), 5.43 (1 H, t, J4,5 = 9.7, 4-H), 4.50 (1 H, dd,
2. Experimental
2.1. General
0
J6,6 = 12.3, J5,6 = 4.6, 6-H), 4.46 (1 H, d, J1,2 = 8.6, 1-H), 4.21 (1 H,
dd, J5,6 = 1.5, 60-H), 3.59 (1 H, m, 5-H), 3.17 (1 H, t, 2-H), 1.80 (3
0
NMR spectra were recorded in CDCl3 (CHCl3, d = 7.26 ppm, and
13CDCl3d = 77 ppm, as internal standards), or C6D6 (TMS, d = 0 ppm,
and 13C6D6d = 128.62 ppm, as internal standards) with a 400 MHz
(Bruker DRX-400). J values are given in Hz. Dichloromethane was
distilled from CaH2, toluene from Na. The absolute configuration
of the enantiomers was established by NMR spectroscopy in the
presence of the chiral shift reagent Eu(hfc)3 and in comparison
with literature data [6].
H, s, OAc), 1.77 (3 H, s, OAc), 1.61 (9 H, s, t-Bu), 1.58 (3 H, s,
OAc), 1.54 (9 H, s, t-Bu), 1.31 (9 H, s, t-Bu), 1.23 (9 H, s, t-Bu). 13C
NMR(C6D6): dC = 171.0, 170.2, 169.7, 169.5, 166.8, 159.0, 158.9,
140.8(2C), 137.5, 137.4, 127.5(2C), 127.0(2C), 118.3, 118.2, 92.1,
74.7, 74.5, 74.3, 68.9, 62.4, 35.5(2C), 34.4, 34.3, 31.7(3C),
31.6(3C), 29.8(6C), 20.5, 20.4, 20.2. IR: m
max/cmꢀ1 = 1752 (C@O),
1628 (C@N).

290
F. Ruffo et al. / Inorganica Chimica Acta 405 (2013) 288–294
2.4. Preparation of 3 and 30
29.7(3C), 20.5, 20.4, 20.1. IR:
(C@N).
m
max/cmꢀ1 = 1751 (C@O), 1617
Azide 2 (0.66 g, 2.0 mmol) was stirred in dry toluene (8.0 mL)
under inert atmosphere. The desired aldehyde was added
(2.0 mmol), and the mixture was left to react over night. The sol-
vent was removed under vacuum and the crude product was puri-
fied by flash chromatography on silica gel (eluent: EtOAc/PE = 1/2,
NEt3 1%), yield: 3, 46%; 30, 51%. 3: 1H NMR(C6D6): dH = 13.22 (1 H,
s, OH), 8.03 (1 H, s, N@CH), 7.62 (1 H, d, J = 2.3, aromatic), 7.06 (1
H, d, J = 2.2, aromatic), 5.44 (1 H, t, J3,4 = J3,2 = 9.7, 3-H), 5.31 (1 H, t,
2.6. Preparation of elpaN-salen-c0H2
elpaN-salen-cH2 (0.50 mmol) was dissolved in methanol
(15 mL), and a catalytic amount (5% respect to the ligand) of
KOH was added. The reaction was controlled by TLC, and after
2 h was complete. The solvent was removed under vacuum, and
the residue was dissolved in THF and filtrated over a short pad
of Celite in order to remove the residual base. Evaporation of
the solvent afforded the product in quantitative yield. 1H NMR(C6-
D6): dH = 13.77 (1 H, s, OH), 13.51 (1 H, s, OH), 8.36 (1 H, s, N@CH)
8.18 (1 H, s, N@CH), 7.54 (1 H, s, aromatic), 7.50 (1 H, s, aromatic),
7.07 (1 H, s, aromatic), 7.02 (1 H, s, aromatic), 4.58 (1 H, d,
J1,2 = 8.5, 1-H), 3.99–3.90 (2 H, m, 6-H + 60-H), 3.84 (1 H, t,
J3,4 = J3,2 = 8.9, 3-H), 3.74 (1 H, t, J4,5 = J3,4, 4-H), 3.47 (1 H, m, 5-
H), 3.17 (1 H, t, 2-H), 1.64 (9 H, s, t-Bu), 1.50 (9 H, s, t-Bu), 1.26
(9 H, s, t-Bu), 1.22 (9 H, s, t-Bu). 13C NMR(C6D6): dC = 166.8,
158.9, 140.7(2C), 137.4, 137.1, 127.4, 127.3, 118.6, 118.2, 93.4,
78.1, 76.5, 75.9, 70.7, 62.7, 35.5(2C), 34.4(2C), 31.7(3C), 31.6(3C),
29.9(3C), 29.8(3C).
0
J4,5 = 9.7, 4-H), 4.40 (1 H, dd, J6,6 = 12.4, J5,6 = 4.4, 6-H), 4.13 (1 H, d,
J1,2 = 8.7, 1-H), 4.06 (1 H, dd, J5,6 = 1.9, 60-H), 3.40 (1 H, m, 5-H),
0
3.07 (1 H, t, 2-H), 1.81 (3 H, s, OAc), 1.78 (3 H, s, OAc), 1.63 (9 H,
s, t-Bu), 1.51 (3 H, s, OAc), 1.32 (9 H, s, t-Bu). 13C NMR(C6D6):
dC = 166.4(2C), 166.1, 165.6, 165.3, 157.2, 134.3, 127.2, 127.0,
114.9, 85.2, 70.5, 69.6, 68.8, 64.4, 57.9, 31.3, 25.8(3C), 16.4, 16.3,
16.0. 30: 1H NMR(C6D6): dH = 13.28 (1 H, s, OH), 7.90 (1 H, s,
N@CH), 7.30 (1 H, d, J = 7.7, aromatic), 6.85 (1 H, d, J = 7.7, aro-
matic), 6.72 (1 H, t, J = 7.7, aromatic), 5.34 (1 H, t, J3,4 = J3,2 = 9.7,
0
3-H), 5.24 (1 H, t, J4,5 = 9.7, 4-H), 4.34 (1 H, dd, J6,6 = 12.4,
J5,6 = 4.5, 6-H), 4.06 (1 H, d, J1,2 = 8.7, 1-H), 4.00 (1 H, dd,
J5,6 = 2.0, 60-H), 3.33 (1 H, m, 5-H), 2.91 (1 H, t, 2-H), 1.74 (3 H,
0
s, OAc), 1.69 (3 H, s, OAc), 1.52 (9 H, s, t-Bu), 1.45(3 H, s, OAc).
13C NMR(C6D6): dC = 166.9(2C), 166.1, 165.6, 165.3, 155.2, 133.8,
124.7, 123.2, 114.3, 85.3, 70.6, 69.7, 69.0, 64.5, 58.0, 31.6, 30.4,
27.8(3C), 25.8(3C), 16.3, 16.1(2C).
2.7. Preparation of [Mn(elpaN-salen)]PF6 complexes
The proper ligand (0.30 mmol) and Mn(OAc)2ꢂ4H2O
(0.30 mmol) were dissolved in absolute ethanol (7 mL). The mix-
ture was stirred bubbling air through the solution. After 8 h, a solu-
tion of NaPF6 (3.0 mmol) in 1.5 mL of water was added. After
stirring further 18 h, water (50 mL) was added, obtaining a brown
precipitate which was filtered and washed with water. The solid
was dissolved in DCM, anhydrified with sodium sulphate, concen-
trated and precipitated in hexane, obtaining the title products in
75–80% yield. [Mn(elpaN-salen-a)]PF6: Anal Calc. for C34H42F6-
MnN2O9P: C, 49.64; H, 5.15; N, 3.41; Mn, 6.68. Found: C, 49.05;
2.5. Preparation of elpaN-salen-dH2 and elpaN-salen-eH2
A solution of 3 or 30 (1.0 mmol) in dry dichloromethane
(5.0 mL) under inert atmosphere was added to a solution of
P(Me)Ph2 (0.20 g, 1.0 mmol) in dry dichloromethane (5.0 mL) at
0 °C. The mixture was stirred 2 h at RT, and then the desired alde-
hyde was added (1.0 mmol). After 20 h, the solvent was removed
under vacuum and the crude product was purified by flash chro-
matography on silica gel (eluent: EtOAc/PE = 1/3, NEt3 1%). Yield:
elpaN-salen-dH2, 48%; elpaN-salen-eH2, 46%. elpaN-salen-dH2:
MS(MALDI) Calc. for C38H53N2O9 [LH]+: 681.38. Found 681.65. 1H
NMR(C6D6): dH = 13.40 (1 H, s, OH), 13.32 (1 H, s, OH), 8.28 (1
H, s, N@CH) 7.97 (1 H, s, N@CH), 7.55 (1 H, d, J = 2.2, aromatic),
7.22 (1 H, d, J = 7.7, aromatic), 7.04 (1 H, d, J = 2.2, aromatic),
6.81 (1 H, d, J = 7.7, aromatic), 6.62 (1 H, t, J = 7.7, aromatic),
5.66 (1 H, t, J3,4 = J3,2 = 9.7, 3-H), 5.38 (1 H, t, J4,5 = 9.7, 4-H), 4.45
H, 5.22; N, 3.27; Mn, 6.69%. IR:
m
max/cmꢀ1 = 1751 (C@O), 1617
(C@N), 845 (PF6ꢀ); MS(ESI): 676.77 [M]+, 144.91 [PF6]ꢀ. [Mn(el-
paN-salen-b)]PF6: Anal. Calc. for C34H42F6MnN2O9P: C, 49.64; H,
5.15; N, 3.41; Mn, 6.68. Found: C, 50.79; H, 5.14; N, 3.56; Mn,
6.73%. IR: m
max/cmꢀ1 = 1752 (C@O), 1617 (C@N), 845 (PF6ꢀ); MS(E-
SI): 677.14 [M]+, 144.91 [PF6]ꢀ.[Mn(elpaN-salen-c)]PF6: Anal. Calc.
for C42H58F6MnN2O9P: C, 53.96; H, 6.25; N, 3.00; Mn, 5.88. Found:
C, 53.39; H, 6.45; N, 2.94; Mn, 5.64%. IR: m
max/cmꢀ1 = 1752 (C@O),
1610 (C@N), 843 (PF6ꢀ); MS(ESI): 789.35 [M]+, 144.91 [PF6]ꢀ.
[Mn(elpaN-salen-d)]PF6: Anal. Calc. for C38H50F6MnN2O9P: C,
51.94; H, 5.74; N, 3.19; Mn, 6.25. Found: C, 52.12; H, 5.90; N,
0
(1 H, dd, J6,6 = 12.4, J5,6 = 4.6, 6-H), 4.37 (1 H, d, J1,2 = 7.9, 1-H),
4.17 (1 H, dd, J5,6 = 1.8, 60-H), 3.52 (1 H, m, 5-H), 3.08 (1 H, t, 2-
0
H), 1.76 (6 H, s, OAc ꢁ 2), 1.58 (9 H, s, t-Bu), 1.54 (3 H, s, OAc),
1.44 (9 H, s, t-Bu), 1.29 (9 H, s, t-Bu). 13C NMR(C6D6):
dC = 167.1(2C), 166.2, 165.7, 165.6, 162.2(2C), 157.0, 155.0,
136.8, 133.9, 133.6, 127.2, 127.0, 123.1, 114.8, 114.3, 87.8, 70.7,
70.5, 70.2, 64.9, 58.4, 31.6, 31.2, 30.4, 27.8(3C), 25.9(3C),
3.27; Mn, 6.48%. IR:
m
max/cmꢀ1 = 1752 (C@O), 1613 (C@N), 843
(PF6ꢀ); MS(ESI): 732.90 [M]+, 144.91 [PF6]ꢀ. [Mn(elpaN-salen-
e)]PF6: Anal. Calc. for C38H50F6MnN2O9P: C, 51.94; H, 5.74; N,
3.19; Mn, 6.25%. Found: C, 51.46; H, 5.88; N, 3.07; Mn, 5.99%. IR:
max/cmꢀ1 = 1752 (C@O), 1613 (C@N), 843 (PF6ꢀ); MS(ESI):
25.8(3C), 16.5, 16.4, 16.2. IR:
m
max/cmꢀ1 = 1751 (C@O), 1617
m
732.90 [M]+, 144.91 [PF6]ꢀ.
(C@N); MS(ESI) m/z = 681.65 (LH+) elpaN-salen-eH2: MS(MALDI)
Calc. for C38H53N2O9 [MH]+: 681.38. Found 681.53. 1H NMR(C6D6):
dH = 13.47 (1 H, s, OH), 13.29 (1 H, s, OH), 8.29 (1 H, s, N@CH)
7.90 (1 H, s, N@CH), 7.53 (1 H, d, J = 1.9, aromatic), 7.25 (1 H, d,
J = 7.4, aromatic), 6.97 (1 H, d, J = 2.1, aromatic), 6.87 (1 H, d,
J = 6.7, aromatic), 6.68 (1 H, t, J = 7.7, aromatic), 5.62 (1 H, t,
J3,4 = J3,2 = 9.7, 3-H), 5.39 (1 H, t, J4,5 = 9.7, 4-H), 4.48 (1 H, dd,
2.8. Preparation of complex [Mn(OH)(elpaN-salen-c0)]
Ligand elpaN-salen-c0H2 (0.30 mmol) and Mn(OAc)2ꢂ4H2O
(0.30 mmol) were dissolved in absolute ethanol (7 mL), and the
mixture was stirred for 8 h bubbling air through the solution. After
18 h, 50 mL of water were added, and the solution was extracted
three times with dichloromethane (15 mL). The organic phase
was dried with sodium sulphate, concentrated and precipitated
in hexane, obtaining the title product in 74% yield. Anal. Calc. for
0
J6,6 = 12.4, J5,6 = 4.5, 6-H), 4.40 (1 H, d, J1,2 = 8.5, 1-H), 4.16 (1 H,
dd, J5,6 = 1.5, 60-H), 3.53 (1 H, m, 5-H), 3.03 (1 H, t, 2-H), 1.76 (3
0
H, s, OAc), 1.75 (3 H, s, OAc), 1.52 (3 H, s, OAc), 1.51 (9 H, s, t-
Bu), 1.44 (9 H, s, t-Bu), 1.21 (9 H, s, t-Bu). 13C NMR(C6D6):
dC = 170.6(2C), 170.2, 169.7, 169.5, 166.8, 161.0, 158.9, 140.8,
138.0, 137.5, 131.0(2C), 127.4, 118.8, 118.7, 118.1, 92.4, 74.7,
74.3, 74.1, 68.7, 62.3, 35.5, 35.2, 34.3, 31.6(3C), 29.7(3C),
C
36H53MnN2O7: C, 63.52; H, 7.85; N, 4.12; Mn, 8.07. Found: C,
63.22; H, 7.70; N, 4.27; Mn, 8.28%. IR: m
max/cmꢀ1 = 1618 (C@N);
MS(MALDI): 663.20 [MꢀOH]+.

F. Ruffo et al. / Inorganica Chimica Acta 405 (2013) 288–294
291
2.9. Typical procedure for the asymmetric epoxidation using m-
chloroperbenzoic acid as oxidant
(SIR97 program [11]) and refined by the full matrix least-squares
method on F2 using SHELXL-97 program [12] with the aid of the pro-
gram WINGX [13]. Non-hydrogen atoms were refined anisotropically;
H atoms of hydroxyl groups were found in difference Fourier maps,
the positions of the other H atoms were determined stereochemically.
H atoms were refined by the riding model with Uiso = 1.2 Ueq of the
carrier atom (1.5 Ueq for H atoms of methyl groups).
The olefin substrate (0.48 mmol) and N-methylmorpholine-N-
oxide (2.4 mmol) were added to a solution of the appropriate Mn
complex (0.024 mmol) in dichloromethane (4 mL). The mixture
was cooled at 196 K, then m-chloroperbenzoic acid (0.96 mmol)
was added as a solid over a 2 min period. After an established per-
iod of time, 5 mL of 1 M NaOH were added, and the organic phase
separated, washed with brine (10 mL) and dried with sodium sul-
phate. The resulting solution was purified by filtration through a
small pad of silica gel. The solvent was removed under vacuum,
and the yield was determined by 1H NMR. Eu(hfc)3was used for
the valutation of the ee%.
3. Results and discussion
3.1. Synthesis of the ligands
Scheme 1 reports the simple synthesis of the elpaN-salen-H2
ligands along with their labels.
2.10. Typical procedure for the asymmetric epoxidation using NaClO as
oxidant
The key-intermediate is the known 1-azido-2-aminoglucose
derivative 2 [9], whose synthesis from bromide 1 (step i) was
greatly improved within this study. In ꢀfact, the dangerous silver
azide, originally used for providing N3 and capturing Brꢀ ions,
has been safely substituted by the couple trimethylsilylazide/t-
BuOK, the latter playing as base, activator of the organic azide
and source of K+.
Compound 2 could be reacted with the appropriate salicylalde-
hyde ArCHO (step ii), affording the 2-iminoderivatives 3 and 30.
Condensation with a second equivalent of aldehyde (via Staudinger
reaction, step iii) provided the target ligands. Steps ii and iii could
be conveniently performed in one pot, although the syntheses of
elpaN-salen-dH2 and -eH2 required isolation in the solid state of
the intermediates 3 and 30.
cis-b-methylstyrene
(0.48 mmol)
and
pyridine-N-oxide
(0.072 mmol) were added to a solution of [Mn(elpaN-salen-
b)]PF6 (0.024 mmol) in dichloromethane (1 mL). After the addition
of 2 mL of buffered NaClO solution (1.0 mmol, pH 11.3) as the oxi-
dant at 273 K, the resulting mixture was stirred vigorously. After
4.5 h, the mixture was diluted with dichloromethane (2.5 mL).
The organic layer was separated, washed with brine, and then
dried with sodium sulphate. The resulting solution was purified
by filtration through a small pad of silica gel. The solvent was re-
moved and yield was determined by 1H NMR. Eu(hfc)3was used
for the valutation of the ee%.
The five ligands differ for the position and the number of t-bu-
tyls in positions 30 and 50 of the aromatic portions. All of them, with
the exception of elpaN-salen-aH2, show tactical t-Bu groups in 30,
i.e. ortho to the alcoholic oxygen. This steric motif is acknowledged
as essential requirement for promoting high ees in the asymmetric
epoxidation of styrenes promoted by the corresponding Mn(III)
complexes.
ElpaN-salen-bH2 and -cH2 are symmetrically substituted: the
former shows only the just mentioned t-butyls in 30, the latter dis-
plays subsidiary t-butyls in the 50-positions of both imino arms.
Instead, elpaN-salen-dH2 and -eH2 do present only one addi-
tional t-butyl group in 50, respectively in the imino function at C2
(elpaN-salen-dH2) and C1 (elpaN-salen-eH2).
2.11. Typical procedure for the asymmetric epoxidation using n-
Bu4HSO5 as oxidant
cis-b-methylstyrene (0.48 mmol) and N-methylmorpholine-N-
oxide (0.48 mmol) were added to a solution of [Mn(elpaN-salen-
b)]PF6 (0.024 mmol) in 3 mL of acetonitrile. The mixture was
cooled at 255 K, then the oxidant (0.75 mmol) was added as a solid
over a 15 min period. After an established period of time, the reac-
tion was quenched by adding dimethyl sulfide (ca. 1 mmol). Excess
solid K2CO3 was added, the mixture was allowed to reach room
temperature and then filtered. The filtrate was concentrated and
the residue was purified through a small pad of silica gel (eluent
n-hexane/ethylacetate = 9/1). The solvent was removed and yield
was determined by 1H NMR.Eu(hfc)3was used for the valutation
of the ee%.
This set of ligands was selected and prepared for assessing pri-
mary and secondary effects of the steric hindrance around the me-
tal center, as well for disclosing possible enhancement of
enantioselectivity due to selective crowding at only one side of
the ligand.
2.12. X-ray crystallography of elpaN-salen-bH2
A deprotected ligand (elpaN-salen-c0H2) was also prepared, by
treatment of the parent compound with catalytic potassium
hydroxide in methanol (step iv). This action returned the hydroxyl
functions available for further functionalization, and such a possi-
bility was exploited for assessing the effect of other coordinating
functions on the outcome of the catalysis.
Single crystal suitable for X-ray analysis were obtained by slow
evaporation of a methanol solution at room temperature.1 The qual-
ity of the crystals was rather poor as witnessed by the fact that the dif-
fraction pattern gave few high h reflections. Data were collected on a
Bruker-Nonius Kappa-CCD diffractometer using graphite monochro-
mated Mo K
a
radiation (k = 0.71073 Å) at ambient temperature. Unit
The characterisation of the ligands was carried out through ele-
mental analysis, IR and NMR spectroscopy. Deuterobenzene was
used as solvent for NMR measurements, in order to inhibit hydro-
lysis of the imino functionalities, which takes place more easily in
the acidic chlorinated solvents. As expected, the CH@N protons
resonated at high frequency, as well as the –OH nuclei which were
detected in the range 12–14 ppm. The absorptions at ca. 1630 cmꢀ1
in the IR spectra recorded in Nujol mull were attributed to the C@N
stretchings.
The X-ray structure of elpaN-salen-bH2 was also determined
(Fig. 4).
All bond distances fall in the normal range [14]. In the molecule
the sugar ring adopts the expected chair conformation and the
cell parameters were determined by least squares refinement of the h
angles of 70 strong reflections in the range 3.089° < h < 12.966°. Data
reduction and semiempirical absorption correction were done using
SADABS program [10]. The structure was solved by direct methods
1
Empirical formula: C34H44N2O7; Formula weight: 624.71; Crystal system: mono-
clinic; Space group: C2; a (Å): 21.835(4); b (Å): 9.207(2); c (Å): 18.641(3); b (°):
106.804(8); V (Å3): 3513(1); Z: 4; Dcalc (g/cm3): 1.181;
l
(mmꢀ1: 0.085; h Range for
data collection (°): 3.13–25.00; Reflections collected/unique: 9950/3244
[Rint = 0.1138]; Data/restraints /parameters: 3244/1/414; R indices: [I > 2 (I)]; R1:
r
0.0740; wR2: 0.1234; R indices (all data) R1: 0.2084; wR2: 0.1573; Largest difference
peak and hole (electron/Å3) 0.251, ꢀ0.177.

292
F. Ruffo et al. / Inorganica Chimica Acta 405 (2013) 288–294
AcO
AcO
AcO
AcO
AcO
AcO
AcO
AcO
AcO
H2N
O
3: R=R2= t-Bu
(i)
(ii)
R
3': R= H, R2= t-Bu
O
O
N
N3
-
Br H3N+
Br
N3
2
1
OH
(iii)
R2
HO
AcO
HO
N
AcO
AcO
HO
O
(iv)
O
N
N
N
t-Bu
OH HO
t-Bu
R
OH HO
R1
t-Bu
t-Bu
R2
R2
elpaN-salen-c'H2
elpaN-salen-aH2: R= R1= t-Bu, R2= H
elpaN-salen-bH2: R= R1= H, R2= t-Bu
elpaN-salen-cH2: R= R1= R2= t-Bu
elpaN-salen-dH2: R1= H, R= R2= t-Bu
elpaN-salen-eH2: R1= R2= t-Bu, R= H
(i) THF, Me3SiN3, t-BuOK, -KBr, -t-BuOSiMe3
(ii) toluene, ArCHO, -H2O;
(iii) toluene, PPh2Me, ArCHO, -N2, -OPPh2Me;
(iv) MeOH, KOH
Scheme 1. Synthesis of the elpaN-salen-H2 library of ligands.
R3O
R3O
R3O
O
N
O
N
Mn+
R
O
R1
R2
R2
Fig. 5. General formula of Mn(III) complexes containing elpaN-salen ligands.
2½MnðAcOÞ2ꢃ þ 1=2O2 þ 2elpaN-salen-H2 þ 2NaPF6
¼ 2½MnðelpaN-salenÞꢃPF6 þ 2NaOAc þ 2HOAc þ H2O
Only in the case of elpaN-salen-c0H2 we found more convenient
the isolation of the neutral hydroxospecies [Mn(OH)(elpan-salen-
c0)], which is expected [6] to show a very similar catalytic
behaviour.
Fig. 4. Ortep view of elpaN-salen-bH2. Thermal ellipsoids are drawn at 30%
probability level. Intramolecular hydrogen bonds are drawn as dashed lines.
The dark brown compounds are soluble in chlorinated solvents
and aromatic hydrocarbons, and were characterised through
elemental analysis, conductivity measurements, MS and IR spec-
troscopy. The molar conductivity of the cationic species in dichlo-
substituents at C2, C3, C4, C5, C6 occupy the equatorial positions. A
planar conformation of both the phenoxyimine groups is stabilized
by intramolecular OHꢂ ꢂ ꢂN hydrogen bonds (O8–Hꢂ ꢂ ꢂN1 0.968(7),
1.821(6) Å, 130.8(4)°; O9–Hꢂ ꢂ ꢂN2 1.023(6), 1.610(6) Å, 155.2(4)°).
An angle of 71.4(2)° is observed between their mean planes. In
the crystal packing weak C–Hꢂ ꢂ ꢂO interactions favour the sugar
rings to pile up along b axis, forming layers of molecules perpen-
dicular to c axis. The layers of molecules face each to the other
through hydrophobic t-butylic contacts.
romethane is within the range 20–25 Sꢀ1cm2 molꢀ1
confirms that they are 1:1 electrolytes [15].
, which
The IR CH@N stretchings are at lower frequencies with respect
to those observed for the parent ligands. The PF6ꢀ anion gives rises
to the expected broad band at ca. 850 cmꢀ1
.
3.3. Epoxidation reactions
3.2. Synthesis and characterisation of the Mn(III) complexes
The asymmetric epoxidation of styrenes promoted by Mn(III)
complexes is of great interest because the resulting optically active
epoxides are valuable intermediates for a wide variety of pharma-
ceutically relevant compounds [16].
Thus, we examined the ability of the new sugar based Mn com-
plexes to promote the asymmetric epoxidation of styrenes, also in
comparison with the Naple-type counterpart (Fig. 6) [6].
Cationic manganese(III) complexes (Fig. 5) were prepared
from Mn(AcO)2 through a known established procedure [6] and
according to the following illustrative stoichiometry, where
elpaN-salen indicates the doubly deprotonated form of elpaN-
salen-H2:

F. Ruffo et al. / Inorganica Chimica Acta 405 (2013) 288–294
293
Table 2
O
O
-
6
Epoxidation of cis-b-methylstyrenea by using [Mn(elpaN-salen)]PF6 catalysts.
PF6
OBz
Ph
O
5
4
1
2
3
O
ox
N
N
(R)
(S)
MnL
Mn+
O
O
t-Bu
t-Bu
[Mn(Naple-salen-b)]PF6
Fig. 6. Structure of [Mn(Naple-salen-b)]PF6 catalyst .
No.
Ligand
T (h)
Conversionb
Cis/transb
Ee(%)c
–
84(1R,2S)
88(1R,2S)
82(1R,2S)
86(1R,2S)
83(1R,2S)
1
2
3
4
5
6
elpaN-salen-a
elpaN-salen-b
elpaN-salen-c
elpaN-salen-d
elpaN-salen-e
elpaN-salen-c0
0.5
0.5
0.5
0.5
0.5
0.5
>99
>99
>99
>99
>99
>99
50/50
92/8
92/8
90/10
92/8
88/12
Table 1
Epoxidation of cis-b-methylstyrene by using [Mn(elpaN-salen-b)]PF6.
d
O
ox
(R)
(S)
a
MnL
At 196 K, in dichloromethane (4 mL). Ingredients (mmol): Mn/alkene/m-CPBA/
NMO = 0.024/0.48/0.96/2.40.
b
Estimated by NMR spectroscopy by integraton of the following suitable signals:
reagent: d 6.48 (1H), cis-product: d 4.10 (1H), trans-product: d 3.59 (1H).
c
The determination of ees was carried out by 1H NMR analysis in CDCl3 in the
No.
Oxidant
T (h)
Conversiona
Cis/transa
Ee(%)b
presence of Eu(hfc)3 as shift reagent, referring to the signal at d 4.10. The ees refer to
the cis-epoxide.
1
2
3
4
m-CPBAc
0.5
4.5
4.5
4.5
>99
>99
>99
-
92/8
88/12
71/29
–
84(1R,2S)
68(1R,2S)
50(1R,2S)
–
NaClOd
d
The catalyst was the neutral hydroxospecies [Mn(OH)(elpaN-salen-c0)].
e
n-Bu4NHSO5
f
H2O2
a
Estimated by NMR spectroscopy by integraton of the following suitable signals:
reasonable selectivity and good ees (up to 88% ee), with little dif-
ferences due to the diverse distributions of the subsidiary t-butyls
in 50. Good levels of enantioselectivity were observed also with the
deprotected ligand elpaN-salen-c0, which demonstrates that deco-
ration of the ligand does not affect its performance. Furthermore,
the ees are nearly identical to that obtained by using the Jacobsen
catalyst based on chiral trans-1,2-diaminocyclohexanecontaining
the same substituents on the aryl rings [18].
reagent: d 6.48 (1H), cis-product: d 4.10 (1H), trans-product: d 3.59 (1H).
b
The determination of ees was carried out by 1H NMR analysis in CDCl3 in the
presence of Eu(hfc)3 as shift reagent, referring to the signal at d 4.10. The ees refer to
the cis-epoxide.
c
At 195 K, in dichloromethane (4 mL). Ingredients (mmol): Mn/alkene/m-CPBA/
NMO = 0.024/0.48/0.96/2.40.
d
At 273 K in dichloromethane/water(pH 11.5) 1:2 (3 mL). Ingredients (mmol):
Mn/alkene/NaClO(0.4 M)/py-N-oxide = 0.024/0.48/1.00/0.072.
e
At 255 K in acetonitrile (3 mL). Ingredients (mmol): Mn/alkene/(n-Bu4N)HSO5/
The discrepancies are very small, and therefore not sufficient to
establish secondary effectson the stereochemical outcome of the
reaction. However, this screening at least allowed to identify the
most efficient catalyst, namely [Mn(elpaN-salen-c)]PF6 (Table 2).
This compound was therefore also used for promoting epoxida-
tion of other styrenes in the same conditions: styrene and b-meth-
ylstyrene were functionalised respectively in 45% and 50% ees.
These findings are in agreement with literature data, because the
oxidation of terminal olefins promoted by Mn salen complexes
generally proceeds with lower enantioselectivity. For instance, un-
der the same experimental conditions the corresponding Jacobsen
catalyst promotes the epoxidation of styrene in 56% ee [18].
Finally, the reaction on cis- and trans-stilbene provided the
trans-oxide as a racemate. While these latter findings are not as
satisfactory, the ensemble of results herein described significantly
upgrade those obtained with the other rare examples of Mn(III)/
salen complexes derived from carbohydrates [19].
NMO = 0.024/0.48/0.75/0.48.
At 273 K in acetonitrile/acetic acid 16:1 (8 mL). Ingredients (mmol): Mn/alkene/
H2O2(35%) = 0.024/0.48/1.00.
f
First, epoxidation of cis-b-methylstyrene was carried out with
diverse oxidants under elsewhere optimized conditions (m-chloro-
perbenzoic acid [6], sodium hypochlorite [17a], n-butylammonium
persulfate [17b] and hydrogen peroxide [17c]) by using [Mn(el-
paN-salen-b)]PF6 as catalyst. This screening was necessary in or-
der to find out the best conditions for the catalysis, and the
results are shown in Table 1.
Satisfactory results were obtained in dichloromethane at 195 K,
by using m-chloroperbenzoic acid (m-CPBA) as the oxidant in the
presence of N-methylmorpholine-N-oxide (NMO). In these condi-
tions, the catalyst (entry 1) afforded the cis epoxide (cis/trans 92/
8) as major product, with high conversions (>99% within 30 min)
and good ee (84%, 1R,2S). An important outcome of this reaction
is that the product was obtained with similar enantiomeric excess
but opposite configuration with respect to what found by using the
corresponding Naple-type derivative6 of Fig. 6 (86%, 1S,2R).
On the other hand, use of NaClO or n-Bu4NHSO5 (entries 2 and
3) was accompanied by reduced cis/trans ratios (respectively 88/12
and 71/29) and moderate enantioselectivities (respectively 68%
and 50%).
4. Conclusions
This work substantiates the original idea on the possibility to
prepare libraries of pseudo-enantiomeric ligands based on D-glu-
cose by simply switching the positions of coordination from C2
and C3 (Naple-type) to C1 and C2 (elpaN-type). Thus, the series
of elpaN-salen-H2 ligands was prepared through a straightforward
synthesis starting from inexpensive sources. The Mn(III) complexes
of the corresponding O,O0-deprotonated chelates were found to be
active in the asymmetric epoxidation of styrenes. A comparison
with the enantiomeric Naple-type counterpart, returns a gratifying
view about the performance of the new complexes. The next appli-
cation will concern the extension to multiphase catalysis [20–22],
Hydrogen peroxide failed to promote the reaction.
Once found the optimal reaction conditions, cis-b-methylsty-
rene was oxidised with m-CPBA by using the entire set of ligands
(Table 2).
As expected, complex [Mn(elpaN-salen-a)]PF6, which lacks of
the strategic t-butyl groups in 30, failed to promote any enantiose-
lectivity. All the other complexes afforded the cis product in

294
F. Ruffo et al. / Inorganica Chimica Acta 405 (2013) 288–294
[7] (a) T.P. Yoon, E.N. Jacobsen, Science 299 (2003) 1691;
(b) ChemFiles, Asymmetric Catalysis, Privileged Ligands and Complexes, 8,
Aldrich Chemicals, (2008).;
using appropriate phase-tags on the auxiliary sugar positions at C3,
C4 and C6.
(c)Q.-L. Zhou (Ed.), Privileged Chiral Ligands and Catalysts, Wiley-VCH Verlag
GmbH & co., Weinheim, 2011.
[8] (a) V. Benessere, A. De Roma, R. Del Litto, M. Lega, F. Ruffo, Eur. J. Org. Chem.
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Acknowledgements
The authors thank the MIUR (Programmi di Ricerca Scientifica
di Rilevante Interesse Nazionale, Cofinanziamento 2009) for finan-
cial support, and the Centro Interdipartimentale di Metodologie
Chimico-Fisiche, Università di Napoli ’’Federico II’’ for NMR
facilities.
(b) R. Figliolia, M. Lega, C. Moberg, F. Ruffo, Tetrahedron 69 (2013) 4061.
[9] A. Bertho, A. Révész, Justus Liebigs Ann. Chem. 581 (1953) 11.
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(b) P.G. Cozzi, Chem. Soc. Rev. 33 (2004) 410;
CCDC 929467 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.ca-
m.ac.uk/data_request/cif.
(c) E.N. Jacobsen, in: G. Wilkinson, F.G.A. Stone, E.W. Abel, L.S. Hegedus (Eds.),
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