Co(III) catalysed asymmetric ring-opening of epichlorohydrin by salicylaldehyde derivatives: Reversal of enantioselectivity and rate acceleration on addition of AlCl3

Article abstract of DOI:10.3906/kim-1005-607

Using asymmetric Cobalt(III) salen catalysts, the ring-opening of epichlorohydrin by 2, 3-dihydroxybenzal- dehyde and 2, 4- dihydroxybenzaldehyde was found to occur at the phenolic groups most distant from the aldehydic group. Switching catalysts afforded a reversal in enantioselectivity. For 2, 3-dihydroxybenzaldehyde and salicylaldehyde, addition of AlCl₃ to the reaction mixture led to an increase in reaction rate without any decrease in product enantiopurity. TUeBITAK.

Full text of DOI:10.3906/kim-1005-607

Turk J Chem
34 (2010) , 711 – 718.
¨
˙
c
ꢀ TUBITAK
doi:10.3906/kim-1005-607
Co(III) catalysed asymmetric ring-opening of
epichlorohydrin by salicylaldehyde derivatives: reversal
of enantioselectivity and rate acceleration on addition of
AlCl3
∗
˙
Leman KARADENIZ, Gamze KOZ, Kadriye AYDIN, Stephen T. ASTLEY
Department of Chemistry, Faculty of Science, Ege University,
˙
35100, Bornova, Izmir-TURKEY
Received 18.05.2010
Using asymmetric Cobalt(III) salen catalysts, the ring-opening of epichlorohydrin by 2,3-dihydroxybenzal-
dehyde and 2,4- dihydroxybenzaldehyde was found to occur at the phenolic groups most distant from the
aldehydic group. Switching catalysts afforded a reversal in enantioselectivity. For 2,3-dihydroxybenzaldehyde
and salicylaldehyde, addition of AlCl3 to the reaction mixture led to an increase in reaction rate without
any decrease in product enantiopurity.
Key Words: Asymmetric catalysis; salicylaldehyde; Co(III) salen; aryloxy alcohols; epoxide ring-opening
Introduction
The reaction between phenols and epoxides is an attractive route to synthetically useful α-aryloxy alcohols.1,2
However, by comparison with other substrates such as alcohols,3 amines,4 thiols,5 or carboxylic acids6 this ring-
opening reaction does not proceed readily and traditionally harsher reaction conditions have been used which
involve formation of phenoxide anions.7−10 Nonetheless, asymmetric ring-opening of epoxides by phenols was
first reported in the late 1990s. These initial reports involved ring-opening of meso epoxides by p-methoxyphenol
catalysed by heterobimetallic gallium bis(napthoxide) complexes11 and kinetic resolution of racemic epoxides
using Jacobsen’s asymmetric cobalt(III) catalyst (1a),12 which is depicted in Figure. The latter reaction could
readily be applied to unsubstituted or para substituted phenols.13 However, the reaction with less active ortho-
substituted phenols required more active oligomeric catalysts14,15 which required multi-step synthesis.
∗Corresponding author
711

˙
Co(III) catalysed asymmetric ring-opening of epichlorohydrin by..., L. KARADENIZ, et al.,
Figure. The catalysts and substrates used in this study.
More recently Kim and coworkers reported that a heterobimetallic catalyst prepared from 5-t-bu substi-
tuted catalyst (1b, X = 4-nitrobenzenesulfonate) and AlCl3 could afford high enantiomeric excesses of meta
substituted phenols and that this type of catalyst was more active than a similar catalyst prepared from 1a.16
Although the exact structure of these catalysts may not be completely clear, it appears reasonable that catalyst
1b would on steric grounds allow more rapid reactivity than the 3,5-di-t-butyl catalyst (1a) and therefore cata-
lyst 1b seems as though it may be a good candidate for reactions with less active ortho-substituted phenols. In
addition, it also seems reasonable that addition of Lewis acids such as AlCl3 may also be useful for increasing
the reactivity of less active substrates. As salicylaldehyde based compounds are ubiquitous in metal-ligand
chemistry and have many applications, we thought that it would be interesting to attempt the ring-opening
reaction of epoxides by salicylaldehyde derivatives using catalysts 1a and 1b and to observe the effect of added
AlCl3 in these reactions. We report here our findings that catalyst 1b can indeed impart considerably greater
reactivity than the 3,5-di-t-butyl catalyst for sterically hindered OH groups. However, an unexpected reversal
of enantioselectivity was observed. Furthermore, addition of AlCl3 to the reaction mixture showed no adverse
effect on enantioselectivity but in some cases showed significant rate acceleration.
Experimental
All 1 H-NMR and 13 C-NMR spectra were recorded using a Varian AS 400+ Mercury FT NMR spectrometer
at ambient temperature. IR spectra were recorded as either KBr discs or thin films between NaCl plates on a
Perkin Elmer 1600 FTIR spectrometer. Molecular sieves were powdered and dried in a vacuum oven at 130 o C.
HPLC analyses were performed using a chiralcel OD-H column. Solvents were used as received from commercial
suppliers. Catalysts 1a and 1b were prepared according to the literature method.17
Reactions of aldehydes with epichlorohydrin
The aldehyde (1.0 equiv.) and epichlorohydrin (2.1 equiv.) were added to 1a or 1b (0.05 equiv for 2a and
2b; 0.1 equiv for 2c) and molecular sieves (3A, 100 mg). In some instances, an amount of AlCl3 (equimolar
712

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Co(III) catalysed asymmetric ring-opening of epichlorohydrin by..., L. KARADENIZ, et al.,
to the amount of catalyst) was also added. Tert-butylmethyl ether (TBME) (0.15 mL) was added to the
stirred mixture and stirring was continued at the specified temperature until TLC monitoring showed complete
disappearance of the aldehyde. The solvent was then removed and the pure products were obtained by
subjecting the obtained residues to Si gel column chromatography eluting with a n-hexane/ethyl acetate (2:1)
solvent system. Absolute configurations of the products were determined by repeating the experiments using
epichlorohydrin that had been resolved by hydrolytic kinetic resolution.18 The products were characterised
by standard techniques and their structures were further confirmed by derivatization as oximes. For products
3a and 3b, cyclisation of the resultant oximes to afford benzisoxazoles was also carried out. 4-(3-chloro-2-
hydroxypropoxy)-2-hydroxybenzaldehyde (3a); mp 93.1-94.8 o C; 1 H-NMR (CDCl3)δ(ppm): 11.36 (s, 1H),
9.66 (s, 1H), 7.38 (d, J= 8.8 Hz, 1H), 6.49 (dd, J= 8.4, 2.4 Hz, 1H), 6.38 (d, J= 2.4 Hz, 1H), 4.18 (m, 1H), 4.07
(d, J= 5.6 Hz, 2H), 3.68 (m, 2H), 2.52 (br, 1H); 13 C-NMR (CDCl3)δ: 194.74, 165.50, 164.58, 135.67, 115.87,
108.58, 101.80, 69.75, 69.14, 46.01 ppm. IR (KBr): 3381, 1672, 1601 cm−1 . Anal. Calcd. for C10 H10 ClO4 :
C, 52.05; H, 4.81. Found: C, 51.75; H, 4.00%. 3-(3-chloro-2-hydroxypropoxy)-2-hydroxybenzaldehyde (3b);
1 H-NMR (CDCl3)δ(ppm): 11.12 (s, 1H), 9.92 (s, 1H), 7.24 (dd, J= 7.6, 1.6 Hz, 1H), 7.18 (dd, J= 8, 1.6 Hz,
1H), 6.96 (t, J= 7.6 Hz, 1H), 4.24 (m, 1H), 4.17 (d, J= 4.8 Hz, 2H), 3. 77 (m, 2H); 13 C-NMR (CDCl3)δ:
196.68, 152.13, 147.27, 126.14, 120.03, 121.56, 121.51, 71.22, 70.11, 45.52 ppm. IR (NaCl): 3424, 2961, 2932,
1726 cm−1 . Anal. Calcd. for C10 H10 ClO4 : C, 52.05; H, 4.81. Found: C, 51.75; H, 5.05%. 2-(3-chloro-2-
hydroxypropoxy)benzaldehyde (3c). 1 H-NMR (CDCl3)δ(ppm): 10.38 (s, 1H), 7.80 (dd, J= 7.6, 2.8 Hz, 1H),
7.54 (m, 1H), 7.06 (dd, J= 7.6, 1.5 Hz, 1H), 7.02 (d, J= 8.0 Hz, 1H), 3.77 (t, J= 4.8 Hz, 1H), 3.71 (d, J= 5.2 Hz,
2H), 3. 21 (s, 1H), 1.28 (m, 1H); 13 C-NMR (CDCl3)δ: 190.09, 160.50, 136.30, 130.35, 125.35, 121.77, 113.32,
69.89, 69.69, 45.85 ppm. IR (NaCl): 3416, 2879, 1688, 1030 cm−1 . MS (ESI) calcd. for C10 H10 ClO3 Na
(M+Na+): 237. Found: 237.
Preparation of oximes
The oximes were prepared using a procedure similar to reported methods.19 A 5 mL solution of NH2 OH.HCl
(0.34 mmol) and CH3 COONa (0.34 mmol) in water was added dropwise to a solution of the appropriate
aldehyde (0.33 mmol) in 2 mL of ethanol. The resulting solution was stirred at room temperature for 3 h.
Ethanol was evaporated and the product was extracted with TBME (3 × 5 mL). The combined organic phases
were dried (Na2 SO4) and concentrated to afford the crude product. The pure products were crystallized from
dichloromethane/hexane to afford white solids. 4-(3-chloro-2-hydroxypropoxy)-2-hydroxybenzaldoxime (4a)
(79% yield); mp 102-105.2 o C; 1 H-NMR (DMSO) δ(ppm): 8.23 (s, 1H), 7.35 (d, J= 8.8 Hz, 1H), 6.47 (dd,
J= 8.8, 2.8 Hz, 1H), 6.43 (d, J= 2.4 Hz, 1H), 4.00 (m, 1H), 3.95 (m, 2H), 3.67 (m, 2H); 13 C-NMR (DMSO)
δ: 161.03, 158.26, 148.61, 130.06, 112.20, 107.20, 69.75, 69.23, 47.31 ppm. IR (KBr): 3379, 1621 cm−1 . Anal.
Calcd. for C10 H12 ClNO4 : C, 48.89; H, 4.92; N, 5.70. Found: C, 48.46; H, 4.82; N, 5.76%. 3-(3-chloro-2-
hydroxypropoxy)-2-hydroxybenzaldoxime (4b) (91% yield); mp 100-103 o C; 1 H-NMR (CDCl3)δ(ppm): 10.3
(bs, 1H), 8.22 (s, 1H), 7.01 (dd, J= 7.2, 2.4 Hz, 1H), 6.88 (m, 2H), 4.23 (m, 1H), 4.18 (d, J= 4.8 Hz, 2H), 3.75
(m, 2H); 13 C-NMR (CDCl3) δ: 152.45, 147.66, 147.05, 124.14, 120.17, 117.68, 117.50, 71.89, 70.35, 45.20 ppm.
IR (KBr): 3395, 3256, 1579, 1479 cm−1 . Anal. Calcd. for C10 H12 ClNO4 : C, 48.89; H, 4.92; N, 5.70. Found:
C, 48.87; H, 4.83; N, 5.85%. (4c) (88% yield); mp 100-103 o C; 1 H-NMR (CDCl3)δ(ppm): 8.36 (s, 1H), 7.65
713

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Co(III) catalysed asymmetric ring-opening of epichlorohydrin by..., L. KARADENIZ, et al.,
(dd, J= 7.6, 1.6 Hz, 1H), 7.33 (m, 1H), 7.05 (d, J= 8.0 Hz, 1H), 6.95 (d, J= 7.2 Hz, 1H), 4.03 (m, 4H), 3.70
(m, 2H); 13 C-NMR (CDCl3)δ: 165.68, 144.29, 131.41, 126.10, 122.07, 121.63, 113.52, 70.34, 69.34, 47.27 ppm.
IR (KBr): 3390, 3251, 1570 cm−1 . Anal. Calcd. for C10 H12 ClNO3 : C, 52.30; H, 5.27; N, 6.10. Found: C,
51.96; H, 5.18; N, 5.69%.
Preparation of benzisoxazoles
Cyclisation of the oximes was carried out using the literature procedure.20 A mixture of triphenylphosphine
(0.49 mmol) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) (0.49 mmol) in dry dichloromethane (5 mL)
was stirred at room temperature for 1 min. Salicylaldoxime (0.33 mmol) was then added. TLC monitoring
showed completion of the reaction. The solvent was evaporated. Column chromatography of the crude mixture
on silica gel using n-hexane and ethyl acetate (3:1) as eluent gave the desired products as white solids. 6-(3-
chloro-2-hydroxypropoxy)-1,2-benzisoxazole (5a) (86% yield); mp 79.4-82 o C; 1 H-NMR (CDCl3)δ(ppm): 8.59
(s, 1H), 7.59 (d, J= 8 Hz, 1H), 7.08 (s, 1H), 6.97 (dd, J= 8.8, 2.4 Hz, 1H), 4.28 (m, 1H), 4.18 (m, 2H), 3.79
(m, 2H); 13 C-NMR (CDCl3)δ: 164.12, 161.04, 146.03, 122.58, 115.51, 114.88, 93.76, 69.89, 69.36, 46.10 ppm.
Anal. Calcd. for C10 H10 ClNO3 : C, 52.76; H, 4.43; N, 6.15. Found: C, 52.39; H, 4.41; N, 6.22%. IR (KBr):
3371, 1618 cm−1 . 7-(3-chloro-2-hydroxypropoxy)-1,2-benzisoxazole (5b) (39% yield); mp 63-65 o C; 1 H-NMR
(CDCl3)δ(ppm): 8.70 (s, 1H), 7.34 (d, J= 8 Hz, 1H), 7.24 (t, J= 7.6 Hz, 1H), 7.05 (d, J= 7.6 Hz, 1H), 4.39 (s,
1H), 4.37 (d, J= 4 Hz, 2H), 4.34 (m, 1H), 3.84 (m, 2H); 13 C-NMR (CDCl3)δ: 153.06, 146.62, 143.31, 125.17,
123.73, 114.68, 113.29, 70.31, 70.02, 45.98 ppm. Anal. Calcd. for C10 H10 ClNO3 : C, 52.76; H, 4.43; N, 6.15.
Found: C, 52.32; H, 4.55; N, 6.03%. IR (KBr): 3399, 2927, 1607, 1532, 1467 cm−1
.
Results and discussion
The ring-opening reactions of epichlorohydrin that were carried out can be seen in Scheme 1. Initial reactions
were carried out using aldehyde (2a) and these results are given in Table 1. In these experiments, the rates
of the reaction for both catalysts (1a) and (1b) were very similar. Thus, the reactions were completed within
approximately 24 h at room temperature to give a single product arising from selective reaction at the 4-OH
group. This was determined from 1 H-NMR spectra of the product where the characteristic low field signal of
the 2-OH group could be observed at δ 9.0 ppm. It was further confirmed by conversion of the product to
the corresponding 1,2-benzisoxazole (5a) (Scheme 2). Surprisingly, very poor enantioselectivity was observed
for catalyst 1a. When using catalyst 1b, enantioselectivity was increased, but, in a further surprise, the major
enantiomer in this case was the S enantiomer, not the expected R enantiomer. This surprising enantioselectivity
clearly suggests that substrate 2a is not activated by catalysts 1 in the same way that simple phenols are.21
Decreasing the reaction temperature rapidly decreased rates of reaction but afforded only a small increase in
enantiomeric excess. Addition of AlCl3 to the reaction mixture caused no difference in reaction rates. This
observation is consistent with the results of Song et al., who have suggested that Lewis acids such as AlCl3
do not form bimetallic catalysts with Co(III)salen complexes under these conditions and have no effect on the
hydrolytic kinetic resolution of epoxides.22
714

˙
Co(III) catalysed asymmetric ring-opening of epichlorohydrin by..., L. KARADENIZ, et al.,
Scheme 1. Ring-opening of epichlorohydrin by salicylaldehyde derivatives using catalysts 1a and 1b.
Table 1. Yields and optical activities obtained in the catalytic asymmetric ring-opening of epichlorohydrin by 2aa .
Substrate Catalyst (5 mol%) AlCl3 (mol%) Temp. Time Yieldb eec (%)
2a
2a
2a
2a
2a
2a
2a
2a
1a
1a
1a
1a
1b
1b
1b
1b
-
-
RT
22h
11d
11d
43h
24h
11d
11d
43h
87
33
41
69
80
52
50
66
5(R)
3(R)
4(R)
4(R)
30(S)
38(S)
40(S)
15(S)
o
0 C
o
5
5
-
0 C
RT
RT
o
-
0 C
o
5
0 C
5
RT
areactions run in TBME.
bisolated yields after column chromatography based on 2a.
cdetermined by chiral HPLC analysis. Major enantiomer in parentheses.
Using 2,3-dihydroxybenzaldehyde, a different set of results was obtained (Table 2). For these reactions no
significant amount of product was obtained after 30 days using only catalyst 1a or 1b. This suggests that both
phenolic groups may be too sterically encumbered to be sufficiently activated by either Co(III) salen catalyst.
However, if a catalytic amount of AlCl3 was added, good conversion to product was obtained within 1 to 4
days. This represents a dramatic rate acceleration. Confirmation that reaction occurred cleanly at the 3-OH
position was again obtained from 1 H-NMR and conversion of the product to the corresponding benzisoxazole.
715

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Co(III) catalysed asymmetric ring-opening of epichlorohydrin by..., L. KARADENIZ, et al.,
Enantioselectivities were greater than in the case of 2,4-dihydroxybenzaldehyde. Again, catalyst 1b afforded
the S enantiomer as the major steroisomer whereas catalyst 1a afforded the R enantiomer as the major isomer.
Upon cooling the temperature, reaction rates decreased significantly but the optical purities of the product
increased. Thus, in this case it appears as though the phenolic groups may be activated by the Lewis acidic
AlCl3 but not by the Co(III) salen catalyst. Indeed, considering the well-known ability of AlCl3 to coordinate
to aromatic diol groups such as catechol23 or binol,24 it seems likely that AlCl3 may coordinate to the diol part
of the substrate aldehyde. Although the related catechol borates are capable of ring-opening epoxides in the
absence of transition-metal catalysts,25 in this case it is clear that the chiral Co(III) salen catalyst also must
have a role to play; otherwise the product would be obtained in racemic form. Therefore, it seems likely that
when 2,3-dihydroxybenzaldehyde is the substrate, the epoxide group is activated by coordination to the Co(III)
salen catalyst whereas the phenol group is activated by coordination to AlCl3 . This proposal is consistent with
the generally accepted mechanism for Co(III) salen catalysed reactions in which the attacking nucleophile and
the epoxide are thought to be activated by separate metal centres.26
Scheme 2. Synthesis of oximes and benzisoxazoles. Reagents and conditions: (i)NH2 OH.HCl, CH3 COONa, EtOH,
RT; (ii) PPh3 , DDQ, CH2 Cl2 , RT.
Table 2. Yields and optical activities obtained in the catalytic asymmetric ring-opening of epichlorohydrin by 2ba .
Substrate Catalyst (5 mol%) AlCl3 (mol%) Temp. Time Yieldb eec (%)
2b
2b
2b
2b
1a
1a
1b
1b
5
5
5
5
RT
4d
11d
2d
39
29
58
31
35(R)
50(R)
47(S)
70(S)
o
0 C
RT
o
0 C
9d
a
reactions run in TBME.
bisolated yields after column chromatography based on 2b.
cdetermined by chiral HPLC analysis. Major enantiomer in parentheses.
Using salicylaldehyde, in the absence of AlCl3 , the reaction using catalyst 1a was so slow that no isolable
amount of product could be obtained after 1 month at RT (Table 3). In contrast, using catalyst 1b, a moderate
yield of product could be obtained after 3 days. When AlCl3 was added, a significant rate acceleration was
observed for catalyst 1a. Thus, within 6 days a moderate yield of product was obtained. However, no increase
in reaction rate was observed when catalyst 1b was used. As with aldehydes 2b and 2c, when catalyst 1a was
716

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Co(III) catalysed asymmetric ring-opening of epichlorohydrin by..., L. KARADENIZ, et al.,
employed the R enantiomer was the major stereoisomer formed, whereas using catalyst 1b afforded a greater
proportion of the S enantiomer.
Table 3. Yields and optical activities obtained in the catalytic asymmetric ring-opening of epichlorohydrin by 2ca .
Substrate Catalyst (10 mol%) AlCl3 (mol%) Temp. Time Yieldb eec (%)
2c
2c
2c
2c
1a
1a
1b
1b
-
RT
RT
RT
RT
32d
6d
-
-
10
-
61
23
60
30(R)
11(S)
18(S)
3d
10
3d
a
reactions run in TBME.
bisolated yields after column chromatography based on 2c.
cdetermined by chiral HPLC analysis. Major enantiomer in parentheses.
Conclusion
Reactions of all 3 salicylaldehydes 2a-c with epichlorohydrin led to ring-opened products as a mixture of
enantiomers. It is thought that this mixture arises from the lack of a single enantiomeric version of the attacking
phenol group. For reactions which are particularly slow, it has been found that addition of alternative reagents
which may be able to activate the phenolic group, such as AlCl3 , can dramatically increase the reaction rate
and afford products in moderate to good enantiomeric excess. This procedure therefore opens a route for the
preparation of certain α-aryloxy alcohols which may be difficult to synthesise using alternative methods.
Acknowledgements
¨
˙
We are grateful to TUBITAK (The Scientific and Technological Research Council of Turkey) for research funds
¨
˙
(project numbers 107T254 and 107T778). Gamze Koz thanks TUBITAK for their bursary support.
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