Efficient asymmetric Baeyer-Villiger oxidation of prochiral cyclobutanones using new polymer-supported and unsupported chiral co(salen) complexes

Article abstract of DOI:10.1007/s12039-012-0277-6

Two heterogeneous catalysts 4 and 5 and three homogenous complexes 1-3 were prepared and used for enantioselective Baeyer-Villiger (B-V) oxidation of prochiral cyclobutanones. Among the prepared catalysts, the supported ones showed better enantioselectivi

Full text of DOI:10.1007/s12039-012-0277-6

c
J. Chem. Sci. Vol. 124, No. 4, July 2012, pp. 871–876. ꢀ Indian Academy of Sciences.
Efficient asymmetric Baeyer–Villiger oxidation of prochiral
cyclobutanones using new polymer-supported and unsupported
chiral co(salen) complexes
REZA SANDAROOSa,∗, MOHAMMAD TAGHI GOLDANIb,∗, SAMAN DAMAVANDIc and
ALI MOHAMMADId
aDepartment of Chemistry, Faculty of Science, University of Birjand, Birjand, Iran
bDepartment of Chemistry, Islamic Azad University, Birjand Branch, Birjand, Iran
cDepartment of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
dDepartment of Chemistry, Sarvestan Branch, Islamic Azad University, Sarvestan, Iran
e-mail: R_Sandaroos@yahoo.com; MT.Goldani@yahoo.com
MS received 31 January 2012; accepted 8 March 2012
Abstract. Two heterogeneous catalysts 4 and 5 and three homogenous complexes 1–3 were prepared and
used for enantioselective Baeyer–Villiger (B–V) oxidation of prochiral cyclobutanones. Among the prepared
catalysts, the supported ones showed better enantioselectivity, good thermal stability and negligible loss of
activity over consecutive recycling offset by lower chemical yield.
Keywords. Polymer-supported; enantioselective; Baeyer–Villiger oxidation; cyclobutanones;
Co(Salen) Complexes.
1. Introduction
σ-bond interacts with the σ*-bond topos-selectively. It
was also considered that the topos-selective interaction
would be realized if the Criegee adduct makes a chelate
(Criegee adduct B), and the chelate conformation is reg-
ulated appropriately. Furthermore, a metallosalen com-
plex with cis-structure was considered to be a suitable
catalyst for this purpose, because it provides two neigh-
bouring coordinating sites for chelate formation and its
metal center is chiral.4
As discussed above, each factor shifting the equi-
librium between intermediates A and B toward inter-
mediate B, must increase enatioselectivitiy of the reac-
tion. Accordingly, unfavourable steric hindrance next to
the center of reaction introduced by ligand, substrate
or oxidant proceeds reaction through the less sterically
congested transition state A.
B–V oxidation is a very old and valuable chemi-
cal transformation that was first discovered in 18991
and subsequently converted into a catalytic process
in 1993.2 Despite its long history, its enantioselec-
tive version using transition metal catalysts is relatively
recent.3–12
B–V oxidation is a two-step reaction: (i) nucleophilic
addition of an oxidant giving the Criegee adduct and
(ii) rearrangement of the adduct to ester (or lactone).
If the starting carbonyl compound is a pro-chiral cyclic
one, the products are a pair of enantiomeric lactones
(figure 1). The stereochemistry of the B–V reaction is
dictated by two factors: face selectivity in oxidant addi-
tion and enantiotopos selectivity in migration. How-
ever, as Criegee adduct formation is a reversible step
and its migration to lactone is an irreversible and rate-
determining one, topos-selection in the migration step is
considered to strongly influence the stereochemistry of
B–V reaction. Interaction of the σ-orbital of the migrat-
ing C–C bond and the σ*-orbital of the O–O bond is
crucial for the migration.13 Therefore, it was expected
that high enantioselectivity would be achieved if the
[Co(salen)]s show high asymmetric induction14,15
and high Lewis acidity.15 In addition, it has been reported
that iron, manganese, titanium and some of the cobalt
complexes of Schiff base took cis-β structure.4,16,17
On the other hand, it has been reported that electron-
withdrawing groups on the salen ligand of the Cu(III)
complexes increase enatioselectivitiy of such a reaction.5
Keeping these considerations in our mind and in
order to investigate electronic and steric effects of lig-
ands on the enantioselectivity and the chemical yield
of B–V oxidations, we have synthesized three chairal
complexes 1–3, bearing three different salen ligands.
∗For correspondence
871

872
Reza Sandaroos et al.
R
R
R
ROOH
O
R
O
M
O
O O
Rout a
low enantioselective
M
M
A
O
O
ROOH
R
ROOH
R
*
R
R
O
high enantioselective
O
M
O
O
O OR
M
O
Rout b
R
M
O OR
B
Figure 1. Schematic representation of M-adduct complexes.
Despite high efficiency of catalysts 1 and 2 for asym- and Shimadzu-IR 470 spectrometer, respectively. Ele-
metric B–V reactions, separation of these from the reac- mental analysis for CHN was carried out by the CHNO
tion mixture was a serious problem. Additionally, in order type from the Helaus Co.
to investigate probable supporting effects, we prepared
polymer-supported catalysts 4 and 5, which were nearly
heterogeneous types of catalysts 1 and 2.
2.2 General procedures
Complex 2 was synthesized according to Katsuki’s pro-
cedure.7c [Co(salen)] complexes 1 and 3 were also
synthesized similarly (figure 2).
2. Experimental
2.1 General
All solvents and commercially available starting mate- 2.2a [Co(salen)] complex 1: Brown solid. IR (KBr):
rials were supplied by Merk Chemical Co. (Darmstadt, 3090, 3005, 2912, 2910, 1612, 1554, 1500, 1486, 1436,
Germany) and Fluka Co. and were used as received. 1387, 1315, 1210, 1182, 1124, 1076, 1036, 989, 927,
The 3-substituted cyclobutanones were prepared 871, 822, 802, 742, 692, 572, 522, 492. Anal. calc. for
according to the literatures.18,19 1HNMR and IR spec- C34H18CoIN6O10: C, 47.68; H, 2.12; N, 9.81; found: C,
trums were recorded on a Bruker BRX-100 AVANCE 47.11; H, 2.01; N, 9.76.
(R)
(R)
(R)
N
O
N
O
N
O
N
N
N
O
O
O
Co
I
Co
I
X
OH
X
O
O
HO
X
O
X
O
X
X
X
X
X
X
[Co(salen)] 1: X = NO2
[Co(salen)] 2: X = F
[Co(salen)] 3: X = t-Bu
[Co(salen)]-PSI 4 and immobilized form of ligand A: X = F
[Co(salen)]-PSI 5 and immobilized form of ligand B: X = NO2
= 90 µm Hydroxymethylpolystyrene
Figure 2. Structures of Co(III)(salen) complexes 1–5 and ligands A and B.

Asymmetric Baeyer-Villiger oxidation of prochiral cyclobutanones
873
2.2b [Co(salen)] complex 3: Brown solid. IR (KBr): the ligand A) in the IR spectra indicated the high con-
3059, 2957, 2916, 2870, 1593, 1470, 1419, 1250, 1200, version of activated resin to the suitable polymer-bound
1173, 1115, 1076, 1028, 984, 922, 809, 798, 739, ligands.
681, 561, 509, 484. Anal. calc. for C50H54CoIN2O2:
C, 66.67; H, 6.04; N, 3.11; found: C, 66.59; H, 6.00; was accomplished by adding solution of Co(OAc)2
N, 3.02. (0.4 mmol), which was obtained by heating Co(OAc)2.
Cobalt insertion into the polystyrene-bound ligands
Polymer-bound ligands A and B were prepared accord- 4H2O at 70–80◦C under vacuum until its colour turned
ing to Jacobsen’s procedure15b (figure 2). To a solution from pink to purple, in 5 ml of degassed MeOH/Toluen
of 2-hydroxy-3,5-dinitrobenzaldehyde (0.7 mmol) 1:1 to the polymer-bound ligand (0.4 mmol) with gentle
and 2,6-dihydroxy-3-nitrobenzaldehyde (0.7 mmol) in stirring. The mixture was heated at 60◦C for 24 h and
EtOH was added (1R)-1,1ꢁ-binaphthalene-2,2ꢁ-diamine then evaporated and the residue suspended in CH2Cl2
(0.7 mmol) and the mixture was refluxed for 4 h. After (15 ml). To this mixture, I2 (0.2 mmol) was added.
cooling to room temperature (rt), the resulting yellow After stirring for 4 h, the mixtures was evaporated and
foam was filtered off and dried in vacuo. Dissymmetric the residue was rinsed with toluene and CH2Cl2 several
ligand B was separated from the mixture by column times to obtain complexes 4 and 5.
chromatography (eluent: diethyl ether/hexanes, 1:20).
Dissymmetric ligand A was prepared similarly.
Complexing of cobalt ions by polymer-bound ligand
resulted in shifting of the imine absorption band loca-
tion towards lower frequency (about 15 cm−1). Elemen-
tal analysis of complexes 4 and 5 indicated 1.81 and
1.86% Co, respectively, corresponding to a final load-
ing of almost 0.30 mmol/g of Co. Procedure of B–V
oxidation was followed from Katsuki’s procedure.4
2.2c Dissymmetric ligand B: Yellow solid. 1HNMR
(CDCl3, 100 MHz): δ 7.1–7.9 (m, 12H), 8.5(s, 1H),
8.8 (s, 1H), 9.3 (s, 1H), 13.0 (bs, 1H). Anal. calc. for
C34H21N5O9: C, 63.45; H, 3.29; N, 10.88; found: C,
63.21; H, 3.21; N, 10.80.
Table 1. Effects of oxidant, substituent and supporting on
enantioselectivity and chemical yield of B–V oxidation.
O
O
2.2d Dissymmetric ligand A: Yellow solid. 1HNMR
(CDCl3, 100 MHz): δ 6.9–7.8 (m, 16H), 8.4(s, 1H), 13.2
(bs, 1H). Anal. calc. for C34H21F3N2O3: C, 72.59; H,
3.76; N, 4.98; found: C, 72.36; H, 3.71; N, 4.91.
Co(III)(salen) (5 mol%), oxidant (1.3 eq)
O
CH2Cl2 (5 mml), rt, 18 h
*
Ph
Ph
For immobilization of ligads A and B, hydrox-
ymethyl polystyrene resin (Advanced Chemtech, 2%
cross-linked 90 μm beads, 0.8 mmol/g) were firstly
activated by 4-nitrophenyl chloroformate according
to the Jacobsen’s work.15b An excess amount of 4-
nitrophenyl chloroformate reagent was used to obtain
maximum conversion of OH. IR spectra of products
showed no absorption at 3460 cm−1 (OH). Moreover,
the introduction of 4-nitrophenyl carbonate into poly-
mer matrix resulted strong bands at 1770 (C=O) (for
the both ligands), 1535, 1345 and 870 cm−1 (NO2) (for
ligand A).
To a suspension of activated resin (0.4 mmol)
in anhydrous DMF (5.0 mL) was added ligand
A (0.59 mmol), DMAP (0.4 mmol), and DIPEA
(0.8 mmol). The resulting orange suspension was
shaken at room temperature for 1.5 h, then filtered and
rinsed sequentially with DMF, MeOH and CH2Cl2 and
dried in vacuo to yield orange beads as Polymer-bound
ligand A. Polymer-bound ligand B were synthesized
in the same manner. Strong bands observed at 1630–
1642 cm−1 (C=N) (for the both ligands) with disappear-
ance of bands at 1530, 1350 and 860 cm−1 (NO2) (for
Entry
Catalyst
Oxidant
Yield%
ee%c
1a
2
3
1
2
3
4
5
1
2
3
4
5
1
2
3
1
2
3
H2Od2
H2Od2
54
39
25
21
51
53
40
22
27
46
65
69
78
53
48
64
58
55
18
59
61
62
61
24
65
67
0
H2Od2
4
H2Od2
5a
H2Od2
6a
UHP
UHP
UHP
UHP
7
8
9
10a
11b
12b
13b
16a
17
18
UHP
mCPBA
mCPBA
mCPBA
TBHP
TBHP
TBHP
0
0
0
0
0
aReaction was stirred for 30 minutes. Reaction was car-
b
ried out at −78◦C in the presence of 1 equiv. of N-
c
methylmorpholine N-oxide. Determined by GC column
LipodexB. dAqueous hydrogen peroxide (30%)

874
Reza Sandaroos et al.
3. Results and discussions
Reactions performed by H2O2 in the presence of sup-
ported catalysts 4 and 5 showed slightly better enentio-
Complexes 1–5 were examined for asymmetric B–V selectivities (59% and 61%, respectively) offset by
oxidation of 3-phenylcyclobutanone by different oxi- lower chemical yields (table 1, entries 1, 2, 4, 5).
dants (table 1). Just for comparison, we used catalyst 2
Furthermore, the use of UHP (urea.H2O2 adduct)
instead of aqueous H2O2, for all the reactions, slightly
reported already by Katsuki et al.7c
The order of enentioselectivity and chemical yield improved enentioselectivities (table 1, entries 6–10).
of reactions using catalyst 1–3 was revealed as 1>2>3 Additionally, the reactions performed with a combination
(table 1, entries 1–3). Among the examined reactions, of catalyst 1–3 and mCPBA (meta-chloroperbenzoic
the one was performed with a combination of H2O2 and acid) or TBHP (t-butyl hydroperoxide) as oxidants
catalyst 1, bearing a ligand with four strong withdraw- showed no enentioselectivities (table 1, entries 11–
ing substituents of NO2, showed the moderate chemical 18). This probably suggested that the oxidants such as
yield of 54% and good enentioselectivity of 58% at a TBHP and m-CPBA attacked the carbonyl group in
short time of 30 minutes (table 1, entry 1). Catalyst 2 an intermolecular fashion giving non-chelated Criegee
catalysed such a reaction with comparable enentioselec- adduct A, while hydrogen peroxide was coordinated to
tivity, but with lower yield and longer time of reaction the metal ion and attacked the carbonyl group to give
(table 1, entry 2). On the other hand, four steric sub- chelated Criegee adduct B (figure 1).
stituents of t-butyl, on the catalyst 3 were considered to
The effects of solvent and temperature on enantiose-
decrease enentioselectivity to the extent of 18% (table 1, lectivities and chemical yields of the above reactions by
entry 3). The unfavourable steric effect introduced by UHP in the presence of catalysts 1–5 were also exam-
four t-Butyl groups lead reaction to proceed mainly ined (table 2). Al reactions performed in polar solvents
through the non-chelated Criegee adduct A, resulting in such as AcOEt, CH3CN, THF and alcohols showed
low enentioselectivity.
slightly better enantioselectivities and chemical yields
Table 2. Effects of solvent and temperature on enantioselectivity and chemical yield of B–V oxidation.
O
O
Co(III)(salen) (5 mol%), UHP (1.3 eq)
O
solvent (5 mml),18 h
*
Ph
Ph
Entry
Solvent
Temperature
Yield%
ee%a
Catalyst NO.
Catalyst NO.
1b
2
3
4
5b
1b
2
3
4
5b
1
EtOH
EtOH
EtOH
−10◦C
0◦C
0◦C
RT
RT
RT
RT
RT
RT
RT
75
72
73
62
55
39
41
53
64
59
59
54
46
56
54
51
48
39
21
19
40
47
44
48
39
28
36
33
35
31
19
15
16
22
27
26
25
27
18
53
46
48
43
25
19
17
27
34
32
36
41
41
67
64
63
53
45
40
37
46
51
48
51
52
53
80
77
76
73
61
68
60
62
71
70
71
64
51
75
71
72
70
59
65
58
61
68
69
67
60
46
35
32
34
30
19
23
21
24
28
27
28
21
11
77
75
76
74
65
67
64
65
72
71
70
73
71
83
81
80
79
68
70
67
67
75
76
73
79
77
2
3c
4
EtOH
5
6
7
8
Hexan
Toluene
Benzene
CH2Cl2
AcOEt
CH3CN
THF
9
10
11
12
13
RT
EtOH
EtOH
30◦C
40◦C
aDetermined by GC column Lipodex B, absolute configuration not assigned. bReaction was stirred for 30 minutes. cAqueous
hydrogen peroxide (30%) was used as terminal oxidant

Asymmetric Baeyer-Villiger oxidation of prochiral cyclobutanones
4. Conclusion
875
Table 3. Asymmetric
B–V
oxidation
of
3-
arylphenylcyclobutanone in the presence of fresh and
recycled [Co(salen)]-PSI 5.
This study demonstrates the synthetic applicabil-
ity of homogeneous [Co(salen)] and heterogeneous
[Co(salen)]-PSI complexes in B–V oxidation of 3-
arylcyclobutanones. Our studies revealed that electron
deficiency of catalyst increases enantioselectivity and
chemical yield of B–V reactions as long as these cata-
lysts possess enough space for formation of the chelated
Criegee adduct B. The attachment of [Co(salen)] com-
plexes on the functionalized polystyrene provides cata-
lysts 4 and 5 which can be reused several times without
significant loss of their activities. We have also demon-
strated that supported catalysts 4 and 5 are featured
by: (i) no significant loss of enantioselectivity in the
catalytic systems with increasing of temperature up to
40◦C; (ii) higher enantioselectivity in comparison with
their homogeneous analogues.
O
O
[Co(salen)]-PSI 5 (5 mol%), H2O2 30% (1.3 eq)
EtOH (5 mml), 0 oC, 30 minutes
O
*
R
R
Entry
R
Yield%
ee%a
1
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeC6H4
Ph
4-BrC6H4
4-ClC6H4
4-FC6H4
66
65
63
64
65
59
50
47
39
32
79
79
78
80
78
77
75
66
61
54
2b
3c
4d
5e
6
7
8
9
10
aDetermined by GC column Lipodex B. b−eReusability of the
recovered catalyst in new runs
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