P.K. Bera et al. / Applied Catalysis A: General 467 (2013) 542–551
543
attack at the ligand system and protein’ [8]. Our own experience
2.3. General procedure for the synthesis of macrocyclic ligands
L1–L4
with the use of macrocyclic salen ligands with V [9], Mn [10] and
Cu [11] for the enantioselective cyanoformylation, epoxidation and
nitroaldol reactions respectively also vouched to attempt this class
of ligands with Mn for the OKR of racemic alcohols as well. Herein,
we have synthesized a new series of reusable macrocyclic Mn(III)
salen complexes (C1–C6) to catalyze OKR of secondary alcohols
with PhI(OAc)2 and NBS as co-oxidant in water-dichloromethane
solvent mixture. In the present system the separation of the cat-
alyst is easy and the recovered catalyst was reused seven times
without any loss in its activity and enantioselectivity. The stability
of the catalyst was demonstrated by UV–vis spectrophotometric
studies that showed recovery of the original spectral characteristics
on culmination of the catalytic run.
To an ice cold solution of bis-aldehyde 2a/2b/3 (1 mmol) in dry
methanol (50 mL) methanolic solution (5 mL) of (1R,2R)-(−)-1,2-
diaminocyclohexane/(1R,2R)-(+)-1,2-diphenylethelenediamine
(1 mmol) was added drop-wise and the resulting solution was
stirred for 5–6 h at room temperature (RT). After the completion
of the reaction (checked on TLC), solvent was evaporated under
reduce pressure to have the macrocyclic ligands L1–L4.
2.4. Characterization data for L1–L3
L1: Yellow solid; yield 96%, m.p. 80–83 ◦C; 1H NMR (200 MHz,
CDCl3, 25 ◦C, TMS): ı = 13.49 (br, 2H), 8.27 (s, 2H), 7.41 (s, 2H),
7.08 (s, 2H), 4.54–4.69 (m, 4H), 3.68–3.79 (m, 4H), 3.28 (br, 2H),
1.55–1.86 (m, 8H), 1.24 (s, 18H).13C NMR (50 MHz, CDCl3) 164.797,
156.477, 140.536, 129.038, 127.069, 124.818, 117.300, 72.340,
69.551, 67.549, 33.653, 33.260, 32.969, 31.102, 24.601, 24.358,
23.898. Anal. Calcd. for C32H44N2O4 C, 73.81; H, 8.52; N, 5.38;
Found C, 73.76; H, 8.50; N, 5.31. LC–MS: m/z Calcd. for [C32H44N2O4]
520.33, Found 521.85 [M + H].
2. Experimental
2.1. General methods and materials
Nuclear magnetic resonance (NMR) of ligands, substrates and
products were obtained from Bruker-Avance-DPX-200 (200 MHz)
spectrometer using TMS as internal standard. Electronic spectra
were recorded in chloroform on a Varian Cary 500 Scan UV–vis–NIR
spectrophotometer. Microanalysis of the intermediates was done
by CHN analyser. High resolution mass spectra were obtained with
a LC–MS (QTOFF)LC (Waters), MS (Micromass) instruments. The
enantiomeric excess of unreacted alcohols were determined by chi-
ral Shimadzu-HPLC with SPD-M10A-VP and SPD-M20A UV detector
and PDR-advanced Laser Polarimeter (PDR-ALP), using Daicel Chi-
ralcel OD, OD-H and AD-H chiral columns with 2-propanol/hexane
mixture as eluent of the reaction mixture after separating the
catalyst. Absolute configurations of chiral 1-phenylethanol were
determined by comparing the sign of optical rotation (obtained
from PDR-ALP) with the standard. Tetrabutylammonium bromide,
tetraethylammonium bromide, PhI(OAc)2, NBS chiral auxiliaries,
their corresponding ketones purchased from Sigma–Aldrich (USA),
with sodium borohydride. The chloromethyl salicylaldehyde (1)
[12], 3-butyl-1-methyl-1H-imidazolium bromide (O) [13], ligand
for the synthesis of complex C5 [14] and C6 [12] were synthesized
according to the previously reported procedures in the literature.
Thin layer chromatography (TLC) was carried out on silica plates
(Merck).
L2: Yellow solid; yield 92%, m.p. 98–101 ◦C; 1H NMR (200 MHz,
CDCl3, 25 ◦C, TMS): 8.26 (s, 2H), 7.39 (d, 2H, J = 2 Hz), 7.07 (d,
2H, J = 2 Hz), 4.53–4.69 (m, 4H), 3.68 (s, 12H), 3.27–3.30 (m,
2H), 1.58–1.92 (m, 8H), 1.59–1.62 (m, 4H), 1.23 (s, 18H).13
C
NMR (50 MHz, CDCl3)165.056, 156.740, 140.797, 129.357, 129.282,
127.387, 125.000, 117.587, 72.658, 70.614, 69.750, 35.362, 33.945,
33.256, 31.404, 24.148, 23.560. Anal. Calcd. for C36H52N2O6 C,
71.02; H, 8.61; N, 4.60; Found C, 71.10; H, 8.58; N, 4.54. LC–MS:
m/z Calcd. for [C36H52N2O6] 608.38, Found 609.84 [M + H].
L3: Yellow solid; yield 95%, m.p. 104–107 ◦C; 1H NMR (200 MHz,
CDCl3, 25 ◦C, TMS): ı = 13.31 (br, 2H), 8.37 (s, 2H), 7.07–7.25 (br,
12H), 4.63–4.71 (m, 4H), 3.69 (s, 12H), 1.22 (s, 18H).13C NMR
(50 MHz, CDCl3) 140.777, 139.202, 129.234, 128.028, 127.946,
127.653, 127.393, 127.230, 70.369, 69.525, 67.359, 33.665, 31.106.
Anal. Calcd. for C44H54N2O6 C, 74.76; H, 7.70; N, 3.96; Found C,
74.69; H, 7.66; N, 3.90. LC–MS: m/z Calcd. for [C44H54N2O6] 706.40,
Found 707.55 [M + H].
2.5. General procedure for the synthesis of macrocyclic Mn(III)
salen complexes C1–C4
To a solution of above synthesized macrocyclic ligands (L1–L4,
1 mmol) in 20 mL dry methanol, solid Mn(OAc)2·4H2O (2 mmol)
was added under N2 atm. and the resulting solution was refluxed for
about 6–8 h. Then the reaction mixture was cooled to RT, followed
by the addition of LiCl (4 mmol) and the stirring was continued for
another 5 h under air for the aerial oxidation of Mn(II) to Mn(III).
Subsequently, the solvent was evaporated and the residue was
extracted with CH2Cl2. The organic layer was washed three times
with water, two times with brine and dried over anhydrous Na2SO4.
Finally the solvent was evaporated under reduce pressure to get the
macrocyclic Mn(III) salen complexes.
2.2. General procedure for the synthesis of bis-aldehyde 2a, 2b
and 3
To a dry two necked round bottom flask (RBF) containing
Teflon coated magnetic bead, NaH (8 mmol) was added and washed
three times with freshly dried tetrahydrofuran (THF) (5 mL × 3)
under N2 atmosphere. Subsequently 50 mL of freshly dried THF
was added to the RBF. To the suspension of NaH in THF, ethyl-
ene glycol/trigol/1,1ꢀ-bi-2-naphthol (BINOL) (2 mmol) was added
slowly and stirred for 30 min. Then to the reaction mixture the
aldehyde 1 (4 mmol) was added and stirred for another 8–10 h
(monitor on TLC), followed by complete removal of solvent. The
crude product was extracted with CH2Cl2 and the organic layer
was washed sequentially with dilute HCl (5 mL × 3), water and
brine. The organic layer was dried on anhydrous Na2SO4 and
evaporated and subjected to flash chromatography with hexane
and ethyl acetate as eluent to give the desired product in high
purity.
2.6. Characterization data for C1–C3
C1: Brown solid; yield 90%, m.p. 172–175 ◦C; Anal. Calcd. for
C32H42MnN2O4Cl: C, 63.10; H, 6.95; N, 4.60; Found C, 63.14; H, 6.90;
N, 4.55. LC–MS: m/z Calcd. for [C32H42MnN2O4Cl] 608.22, Found
573.34 [M–Cl].
C2: Brown solid; yield 93%, m.p. 168–171 ◦C; Anal. Calcd. for
C36H50MnN2O6Cl: C, 62.02; H, 7.23; N, 4.02; Found C, 61.70; H, 7.19;
N, 4.08. LC–MS: m/z Calcd. for [C36H50MnN2O6Cl] 794.29, Found
759.32 [M–Cl].