X. Zhu et al. / Journal of Molecular Catalysis A: Chemical 329 (2010) 1–6
5
(40 m × 25 mm × 0.25 m). An Shimadzu HPLC with a UV detector
and a CHIRALCEL OD (4.6 × 250 mm, 10 mic) column for separation
was also utilized. Elemental analyses were performed by Desert
Analytics Lab (Tucson, AZ, USA).
4.1. Preparation of catalysts
The Co(salen) macrocycle (1) and the bi-Co(salen) (2) complexes
were prepared according to published literature methods [22,23].
The 1H NMR spectra and matrix assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectra obtained for 1 as well as
2 were similar to literature reports.
Elemental analysis for Co(salen) macrocycles (1): calcd (%) C
70.37, H 8.07, N 4.00, Co 8.42; found: C 70.63, H 8.04, N 3,56, Co
8.25. Elemental analysis for bi-Co(salen) (2): calcd (%) C 70.42, H
7.73, N 4.06, Co 8.53; found: C 69.70, H 7.61, N 3.92, Co 8.56. The
monomeric Co(salen) (4) was used as received from Sigma–Aldrich.
The pimelate-linked oligomeric Co(salen) (3) catalyst was pre-
pared in close analogy to the method reported by Jacobsen and
co-workers [21]. A solution of (1R,2R)-1,2-diaminocyclohexane
mono-(+)-tartrate salt (1.75 g, 6.59 mmol) in THF was mixed with
K2CO3 in H2O (8.2 mL) and the resulting solution was refluxed for
30 min. Next, 0.5 equivalent of dialdehyde, which was prepared
by condensation of 3-tert-butyl-2,5-dihydroxy benzaldehyde and
pimelic acid, was added as a solution in THF (22 mL). The reac-
tion was stirred at reflux for 2 h, cooled to room temperature and
diluted with ethyl acetate (100 mL). After separation, a yellow solid
Fig. 5. Asymmetric ring-opening of racemic 1,2-epoxyhexane with methanol cat-
alyzed by 1(OTs) with 0.2 mol% Co loading.
3. Conclusions
Upon aerobic oxidation under acidic conditions, the mixture of
Co(salen) macrocycles 1(OTs) exhibited excellent catalytic prop-
erties in the ring-opening of epoxides with phenols and alcohols,
showing substantial decreases in catalyst loading compared to
the monomeric salen catalyst 4(OTs), and displaying excellent
enantioselectivity. The high reactivity and enantioselectivity of
1(OTs) may be associated with the incorporation of salen units
into an extremely flexible cyclic framework allowing for improved
bimetallic cooperativity. Kinetic studies indicated that epoxide
ring-opening with 1(OTs) followed a first-order kinetic depen-
dence on catalyst, which was consistent with an intramolecular
bimetallic cooperative reaction of multiple centers within the
cyclic oligomeric framework. While macrocycles 1(OTs) and 3(OTs)
displayed similar activity in the HKR reaction, complex 1(OTs)
proved superior for the kinetic resolution of terminal epoxides
with aliphatic alcohols under the conditions used. Catalyst 1(OTs)
also proved to be substantially more active than other cooper-
ative Co(salen) catalysts reported in the literature for epoxide
ring-opening with phenols. Catalyst 2(OTs), which was infe-
rior in the HKR, unexpectedly showed similar activity to 1(OTs)
in ring-opening of 1,2-epoxyhexane with methanol. The recy-
cle studies showed that over three runs in the ring-opening of
1,2-epoxyhexane with methanol, the activity of 1(OTs) clearly
decreased but that catalytic selectivity remained excellent.
was obtained quantitatively, and characterized by 1H NMR and 13
C
NMR spectroscopies and MS, with the results consistent with the
literature. Metalation using cobalt(II) actetate tetrahydrate with
the (salen) macrocycles in a dichloromethane/methanol mixture
afforded the oligomeric Co(salen) (3) pre-catalyst. Elemental anal-
ysis for Co(salen) macrocycles (3): calcd (%) C 64.91, H 6.85, N 4.33,
Co 9.10; found: C 64.49, H 6.78, N 4.55, Co 8.00.
4.2. General procedure for the catalytic synthesis of ˛-alkoxy
alcohols or ˛-aryloxy alcohols
The alcohol or phenol of choice (45 mmol), epoxide
(10.00 mmol), internal standard chlorobenzene (100 L) and
CH3CN or tert-butyl methyl ether (TBME) (0.2 mL) were mixed
with activated 1 (7.2 mg, 0.01 mmol Co containing for Table 2
entry 3) at room temperature or 4 ◦C, and the solution was stirred
until GC analysis indicated complete conversion of the alcohol.
The reaction was then diluted with 5 mL Et2O and filtered through
a plug of silica gel to remove the catalyst. The plug was washed
with 20 mL Et2O. The filtrates were combined and concentrated
under reduced pressure to provide the crude product. Further
purification included flushing through a silica gel column and
distillation under vacuum for recovery of individual compounds.
The ee of the product was determined by chiral GC or HPLC.
4. Experimental section
Reagents were used as received unless otherwise noted.
Dichloromethane (DCM) was dried by passing through columns of
activated alumina. Toluene and tetrahydrofuran (THF) were dried
by passing through columns of activated copper oxide and alu-
mina successively. 1H and 13C NMR spectra were acquired with
a Varian Mercury 400 MHz spectrometer, and chemical shifts are
reported in ppm with reference to the corresponding residual
nuclei of the deuterated solvents. Mass spectra were analyzed
using a VG 7070 EQ-HF hybrid tandem mass spectrometer. Gel
permeation chromatography (GPC) analyses were performed on
American Polymer Standards columns equipped with a Waters 510
pump and UV detector, using poly(styrene) standards for calibra-
tion and THF as the mobile phase at a flow rate of 1.0 mL/min.
Enantiomeric excesses were determined by capillary gas-phase
chromatography (GC) analysis on a Shimadzu GC 14A instru-
ment equipped with a FID detector and a Chiraldex ␥-TA column
Acknowledgement
We are thankful to the Department of Energy Office of
Basic Energy Sciences through Catalysis Contract No. DEFG02-
03ER15459 for financial support of this work.
References
[1] P.G. Cozzi, Chem. Soc. Rev. 33 (2004) 410.
[2] T. Katsuki, Chem. Soc. Rev. 33 (2004) 437.
[3] L. Canali, D.C. Sherrington, Chem. Soc. Rev. 28 (1999) 85.
[4] N.E. Leadbeater, M. Marco, Chem. Rev. 102 (2002) 3217.
[5] D.J. Darensbourg, Chem. Rev. 107 (2007) 2388.
[6] N. Madhavan, C.W. Jones, M. Weck, Acc. Chem. Res. 41 (2008) 1153.
[7] R.M. Haak, S.J. Wezenberga, A.W. Kleij, Chem. Commun. 46 (2010) 2713.