Asymmetric Baeyer-Villiger oxidation with Co(Salen) and H2O 2 in water: Striking supramolecular micelles effect on catalysis

Article abstract of DOI:10.1039/b916262n

A micellar environment enables catalytic, diastereoselective and enantioselective Baeyer-Villiger oxidation of cyclobutanones (ee up to 90%) with H₂O₂ as oxidant using Co(Salen) catalyst 1, while the same catalytic system is inactive in organic solvents. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b916262n

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www.rsc.org/greenchem | Green Chemistry
Asymmetric Baeyer–Villiger oxidation with Co(Salen) and H2O2 in water:
striking supramolecular micelles effect on catalysis
Giulio Bianchini, Alessandra Cavarzan, Alessandro Scarso* and Giorgio Strukul*
Received 18th May 2009, Accepted 6th August 2009
First published as an Advance Article on the web 14th August 2009
DOI: 10.1039/b916262n
A micellar environment enables catalytic, diastereoselec-
tive and enantioselective Baeyer–Villiger oxidation of cy-
clobutanones (ee up to 90%) with H2O2 as oxidant using
Co(Salen) catalyst 1, while the same catalytic system is
inactive in organic solvents.
to ligands derived by trans-1,2-diaminocyclohexane, (ii) electron
poor ligands provided active and enantioselective catalysts while
electron rich ones did not induce asymmetry in the lactones
and (iii) polar organic solvents increased both catalytic activity
and ee’s.
Herein we report the remarkable effect induced by the
employment of different surfactants in the asymmetric BV oxi-
dation reaction14 of four membered ring ketones with hydrogen
peroxide as terminal oxidant in water mediated by Co(Salen) 1
catalyst bearing an electron rich chiral ligand characterized by
a small spacer between N donors, as reported in Scheme 1.
Nowadays catalysis, as the discipline devoted to the development
of new species able to steer reactions activity and selectivity,
has to face new challenges, including the ability to meet
sustainability criteria which, once translated into chemical
words, means the implementation of the twelve green chemistry
principles.1 Switching from chlorinated organic solvents to
water2 as reaction medium is a challenging task enabling
avoidance of volatile toxic species, decreasing the overall E
factor of the process3 and increasing the selectivity thanks to
the hydrophobic effect.4 Despite limitations imparted by low
solubility of organic substrates, synthesis and catalysis in water
is a highly investigated field of research.5,6
Micellar catalysis represents a viable solution to solubiliza-
tion problems; in fact, surfactant aggregation driven by the
hydrophobic effect ensures the formation of apolar nanoen-
vironments where both substrates as well as catalysts can be
efficiently dissolved and interact with high local concentrations.
Moreover, this strategy allows the direct employment of catalysts
developed for organic media without the need of chemical
modification with water soluble tags that often negatively
alter the performance of the catalysts.7 Running asymmetric
catalytic reactions in micelles often turns out to have positive
effects on enantioselectivity due to stronger substrate–catalyst
interactions arising once embedded in the palisade provided
by the alignment of the alkyl chains of surfactant.8,9a This can
be reasonably compared to performing chemical reactions in
liquid crystals,9,10 where enantioselective reactions were recently
demonstrated to occur with higher enantioselectivity due to the
highly anisotropically ordered medium.11
Oxidation reactions with hydrogen peroxide are intrinsically
predisposed to be extended in pure water, and recent investiga-
tions underlined the positive effect of micellar catalysis in asym-
metric oxidation reactions catalyzed by chiral PtII complexes.12
In 2001 Katsuki reported CoIII(Salen) asymmetric Baeyer–
Villiger (BV) oxidation of meso cyclobutanones13 observing
that (i) atropoisomeric Salen ligands bearing four carbon atom
between N donors led to much more active catalysts compared
Scheme 1 Enantioselective BV oxidation of cyclic ketones with H2O2
mediated by Co(Salen) catalyst 1 in water with surfactants.
As expected, on the basis of Katsuki’s findings,13 catalyst 1 was
almost inactive and non enantioselective in the BV oxidation
of meso cyclobutanones in chlorinated solvents, as reported
in Table 1. Similar results were also obtained in methanol.
Surprisingly, simply switching to water with surfactants as the
reaction medium enabled the formation of the corresponding
lactones, albeit in low yields but with a certain degree of
enantioselectivity. In particular, SDS turned out to be the best
surfactant for meso substrates.
The enantioselective induction in BV reactions arises from the
control of which C atom next to the carbonyl unit of the Criegee
intermediate migrates to the oxygen. Small cyclobutanones are
usually more reactive but less enantioselective compared to
larger cyclic ketones.12c While 1 turned out to be inactive with
meso cyclohexanones under micellar conditions, it performed
much better with asymmetric chiral cyclobutanones.
In Table 2 are reported the results for the BV oxidation of
( )-4 where two lactone products are possible, namely normal
lactone 5 (NL) when migration involves the most substituted
Dipartimento di Chimica, Universita` Ca’ Foscari di Venezia, Dorsoduro
2137, I-30123, Venezia, Italy. E-mail: alesca@unive.it, strukul@unive.it;
Fax: +39-041-2348517; Tel: +39-041-2348569
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Table 1 Asymmetric BV oxidation of meso 3-substituted-cyclobuta-
nones in water with H2O2 catalyzed by 1 in the presence of different
surfactants
Yields were limited by the low solubility of the apolar
substrate in the medium, in fact by switching to a more
polar substrate like 7 bearing the same cyclobutanone moiety,
much better results were observed (Scheme 3). Under identical
experimental conditions, substrate 7 turned out to be much more
reactive leading to formation of the corresponding lactones in
moderate to good yields, with rather high diastereoselectivity
and moderate to good ee’s.
#
R
Medium
Yield (%)a
ee (%)b
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Bu
Bu
Bu
Bu
Cy
Cy
Cy
Cy
CH2Cl2
2
20
12
15
1
12
6
5
2
24
12
2
0
13
13
11
0
H2O/SDSc
H2O/TritonX100d
H2O/TritonX114e
CH2Cl2
H2O/SDSc
26
20
15
0
H2O/TritonX100d
H2O/TritonX114e
CH2Cl2
10
11
12
H2O/SDSc
14f
3f
H2O/TritonX100d
H2O/TritonX114e
4f
Experimental conditions: [Sub]0 = 0.5 M; [H2O2] = 0.5 M; [1] = 1% mol,
◦
H2O: 0.5 mL, T = 5 C, 24 h.a Yield determined by GC analysis on
HP-5 column. b ee determined on chiral GC column Lipodex B. c Sodium
dodecylsulfate (75 mM). d Polyoxyethylene(10)isooctylphenyl ether
(150 mM). e Polyoxyethylene(8)isooctyl cyclohexyl ether (150 mM).
f ee determined on chiral GC column ASTEC g -TA.
Scheme 3 Enantioselective BV oxidation of ( )-cis-bicyclo(3.2.0)hept-
2-en-6-one 7 with H2O2 mediated by Co(Salen) catalyst 1 in water with
surfactants.
Table 2 Asymmetric BV oxidation of 4 in water with H2O2 catalyzed
by 1 in the presence of different surfactants
#
Medium
Yield (%)a dr 5/6a ee 5 (%)b ee 6 (%)b
It is worth noting that a remarkable surfactant effect was
observed comparing anionic with neutral surfactants, in fact the
latter enabled much higher preference for the NL with dr 8/9 up
to 86 : 1. This suggested the use of half the equivalent of oxidant
to ensure high ee of product 8.
1
2
3
4
5
H2O/SDSc
24
42
37
13
31
28
11
16
6
22
3
—
20
—
—
46
H2O/SDBSd
H2O/TritonX100e 44
H2O/TritonX114f 30
H2O/SPAN60g
26
0
In Table 3 (entries 7–14) are reported the results of these
experiments together with the effect of surfactant concentration.
While for TritonX100 the increase of concentration favored
catalytic activity and ee but decreased slightly the dr, with
TritonX114 the lower concentration employed was the best,
ensuring a dr up to 70 : 1 and ee up to 90%.
Experimental conditions: [Sub]0 = 0.5 M; [H2O2] = 0.5 M; [1] =
◦
1% mol, H2O: 0.5 mL, T = 5 C, 24 h.a Yield and dr determined
by GC analysis on HP-5 column. b ee determined on chiral HPLC
using Chiralcel OD-H column. ee% of 6 not reported if dr > 15.
c Sodium dodecylsulfate (75 mM). d Sodium dodecylbenzensulfonate
(75 mM). e X100 Polyoxyethylene(10)isooctylphenyl ether (150 mM).
f X114 Polyoxyethylene(8)isooctyl cyclohexyl ether (150 mM). g Sorbitan
monostearate (50 mM).
In conclusion, we have demonstrated that it is possible to teach
old catalysts new tricks; in fact, commercially available chiral
C atom, and abnormal lactone 6 (AL) when the less substituted
C atom is the migrating one. Overall the reaction is a double
parallel kinetic resolution BV reaction, and the concomitant
competition for NL and AL allows high ee’s of products also at
high conversions (Scheme 2). Micellar conditions in this case
allowed catalyst 1 to oxidize the chiral substrate with good
diastereoselectivity and with enantioselective induction up to
46% when the neutral surfactant SPAN60 was employed.
Table 3 Asymmetric BV oxidation of 7 in water with H2O2 catalyzed
by 1 in the presence of different surfactants
ee 8 ee 9
#
Medium
Yield (%)a dr 8/9a (%)b (%)b
1
CH2Cl2
15
75
83
68
34
14
15
52
86
73
17
10
74
58
56
70
41
55
0
4
8
—
45
—
—
—
—
—
13
—
—
—
—
—
—
2
3
4
5
6
7
8
9
H2O/SDS (75 mM)
H2O/SDBS (75 mM)
H2O/CTABrc (75 mM)
H2O/TritonX100 (150 mM) 65
H2O/TritonX114 (150 mM) 81
H2O/SDS (75 mM)
H2O/SDBS (75 mM)
H2O/TritonX100 (75 mM)
15
45
68
13
6
51
52
60
90
74
88
60d
52d
48d
10 H2O/TritonX100 (125 mM) 54d
11 H2O/TritonX100 (190 mM) 68d
12 H2O/TritonX114 (75 mM)
60d
13 H2O/TritonX114 (125 mM) 98d
14 H2O/TritonX114 (190 mM) 80d
Experimental conditions: [Sub]0 = 0.5 M; [H2O2] = 0.5 M; [1] =1% mol,
H2O: 0.5 mL, T = 5 ◦C, 24 h.a Yield and dr determined by GC analysis
on HP-5 column. b ee determined on chiral HPLC using Chiralcel OD-H
column, absolute configuration not assigned, ee% of 9 not reported if
dr > 15. c Hexadecyltrimethyl ammonium bromide. d [H2O2] = 0.25 M.
Scheme 2 Enantioselective BV oxidation of chiral racemic cyclobu-
tanone ( )-1,2A,7,7A-tetrahydro-cyclobuta(a)inden-2-one 4 with H2O2
mediated by Co(Salen) catalyst 1 in water with surfactants.
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Co(Salen) 1 complex is almost inactive and non enantioselective
in the BV oxidation of cyclobutanones in organic media but,
once dissolved in water with the aid of surfactants, moderate
catalytic activity, high diastereoselectivity and enantioselectivity
were generally observed, along with excellent results with one
chiral substrate. Micellar media represent viable environments
for enantioselective reactions; surfactant type and concentra-
tion being further variables deserving optimization since each
substrate/catalyst/surfactant combination is a unique catalytic
system.12c Despite this extra work, favorable effects on catalytic
activity and selectivity are often possible.
organic phase. 5 and 6: chiral HPLC analysis on a Chiralcel
OD-H column, hexane/iPrOH 99/1 from 0 to 15 min, then up
to hexane/iPrOH 96.5/3.5 at 20 min with constant composition
until 60 min, flow 1.2 mL min-1, (+)-(1R,5S)-5a tR=29.1 min,
(-)-(1S,5R)-5b tR = 34.3 min; (-)-(1R,5R)-6a tR = 28.0 min,
(+)-(1S,5S)-6b tR = 31.1 min. 8 and 9: chiral HPLC analysis on a
Chiralcel OD-H column, hexane/iPrOH 99/1 from 0 to 30 min,
then up to hexane/iPrOH 98.5/1.5 at 35 min with constant
composition until 60 min, flow 1.0 mL min-1, (+)-(1R,5S)-8a
tR=23.8 min, (-)-(1S,5R)-8b tR = 26.4 min; (-)-(1R,5R)-9a tR =
21.7 min, (+)-(1S,5S)-9b tR=22.4 min.
Acknowledgements
Experimental
This work was supported by MIUR, Universita` Ca’ Foscari di
Venezia and Consorzio INSTM.
Reagents and materials
General. The chiral catalyst (R,R)-N,N¢-bis(3,5-di-tert-
butylsalicylidene)-1,2-cyclohexanediamino-cobalt(II) 1 and cy-
clobutanone ( )-cis-bicyclo[3.2.0]hept-2-en-6-one 7, hydrogen
peroxide (35%) and all surfactants employed are all commercial
products (Aldrich) and were used as received. meso Cyclobu-
tanones 2 and ( )-1,2A,7,7A-tetrahydro-cyclobuta(a)inden-2-
one 4 were prepared following procedures reported in the
literature.15 Lactone products were confirmed by 1H NMR
analysis in comparison with reported assignments.12,14 GLC
measurements were taken on a Hewlett-Packard 5890A. 1H
NMR spectra were recorded at 298 K, unless otherwise
stated, on a Bruker AVANCE 300 spectrometer operating at
300.15 MHz, d values in ppm are relative to SiMe4. GLC
measurement were taken on a Hewlett-Packard 5890A gas
chromatograph equipped with a FID detector (carrier gas He).
All reactions progress were monitored by GC.
Notes and references
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Catalytic studies. General procedure for asymmetric BV
oxidation of cyclobutanones with H2O2 catalyzed by 1: in a
vial equipped with a screw capped septum the cyclic ketones
(0.25 mmol) were dissolved in water (0.5 mL) with the aid of the
proper amount of surfactant. Complex 1 was added (1% mol) to
this solution and the vial was thermostatted at 5 ◦C. 35% H2O2
(1 eq. or 0.5 eq., checked iodometrically prior to use) was then
added in one portion and the resulting mixture was vigorously
stirred. After 24 h reaction time 2 ml of H2O and 2 ml of AcOEt
were added, the mixture stirred for 10 min, followed by phase
separation. The organic layer was concentrated under vacuum,
and yield and dr were determined by GC analysis on a HP-5
column 1.0 mL min-1 carrier He.
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five membered ring lactones derived by meso cyclobutanones
was analyzed by GC: 4-phenyldihydrofuran-2(3H)-one 3a:
chiral GC column Lipodex B, 170 ◦C isotherm analysis,
carrier He 1.0 mL min-1, tR = 24.8 min, 25.5 min; 4-
butyldihydrofuran-2(3H)-one 3b: chiral GC c◦olumn ASTEC
g -TA, He 1.0 mL min-1, 120 ◦C X 5 min, 5 C min-1 up to
200 ◦C, tR = 19.3 min, 19.6 min; 4-cyclohexyldihydrofuran-
◦
2(3H)-one 3c: chiral GC column Lipodex B, 170 C isotherm
analysis, carrier He 1.0 mL min-1, tR = 25.6 min, 26.6 min.
Enantiomeric excess of five membered ring lactones from chiral
cyclobutanones was determined after preparative TLC chro-
matography with hexane–AcOEt 4 : 6 on the AcOEt extracted
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