Modulation of the catalytic activity of manganese(III) salen complexes in the epoxidation of styrene: Influence of the oxygen source

Article abstract of DOI:10.1039/b309125b

Several achiral Mn(III) salen complexes with different groups in the diimine bridge and in the aldehyde fragment were synthesised and their catalytic activity in the epoxidation of styrene was studied at room temperature, using two oxygen sources, NaOCl or PhIO, and in two solvents, CH₃CN and CH₂Cl₂. These manganese(III) salen complexes present high chemoselectivities as homogeneous catalysts in the epoxidation of styrene, using either iodosylbenzene or sodium hypochlorite as oxygen sources. In general, when iodosylbenzene is used as oxidant higher styrene epoxide yields and lower yields of by-products, other than benzaldehyde, are obtained than with aqueous sodium hypochlorite solutions. It was possible to tune the catalytic activities of [Mn(salen)X] complexes by introduction of substituents in the diimine bridge and in the aldehyde fragment. The presence of bulky substituents in the diimine bridge always increases the catalytic activity of these complexes, regardless of the oxidant, an indication of steric tuning. However, the electronic tuning of the catalytic activity by introducing substituents in the 5 and 3 positions of the aldehyde fragment has different effects depending on the oxygen source. For the one-phase system resulting from the use of PhIO, electron withdrawing groups increase (electron donating groups decrease) the catalytic activity of the complexes, which probably results from destabilisation (stabilisation) of [O=Mn(v)(salen)X], the identified active species making them more (less) reactive. However, when NaOCl is used, the observed behaviour is the opposite: electron donating groups make the complexes better catalysts. The apparent similarity between the solubility of the complexes in the organic solvent and their catalytic activity seems to suggest that solubility must play a key role in their activity.

Full text of DOI:10.1039/b309125b

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P A P E R
Modulation of the catalytic activity of manganese(III) salen
complexes in the epoxidation of styrene: influence of the
oxygen source
Ana Rosa Silva, Cristina Freire* and Baltazar de Castro
´
ˆ
REQUIMTE, Departamento de Quımica, Faculdade de Ciencias, Universidade do Porto, Rua do
Campo Alegre, 4169-007, Porto, Portugal. E-mail: acfreire@fc.up.pt; Fax: þ351 22 6082959;
Tel: þ351 22 6082890
Received (in London, UK) 1st August 2003, Accepted 14th November 2003
First published as an Advance Article on the web 6th January 2004
Several achiral Mn(III) salen complexes with different groups in the diimine bridge and in the aldehyde fragment
were synthesised and their catalytic activity in the epoxidation of styrene was studied at room temperature,
using two oxygen sources, NaOCl or PhIO, and in two solvents, CH3CN and CH2Cl2 .
These manganese(III) salen complexes present high chemoselectivities as homogeneous catalysts in the
epoxidation of styrene, using either iodosylbenzene or sodium hypochlorite as oxygen sources. In general, when
iodosylbenzene is used as oxidant higher styrene epoxide yields and lower yields of by-products, other than
benzaldehyde, are obtained than with aqueous sodium hypochlorite solutions. It was possible to tune the
catalytic activities of ‘‘[Mn(salen)X]’’ complexes by introduction of substituents in the diimine bridge and in the
aldehyde fragment. The presence of bulky substituents in the diimine bridge always increases the catalytic
activity of these complexes, regardless of the oxidant, an indication of steric tuning. However, the electronic
tuning of the catalytic activity by introducing substituents in the 5 and 3 positions of the aldehyde fragment has
different effects depending on the oxygen source. For the one-phase system resulting from the use of PhIO,
electron withdrawing groups increase (electron donating groups decrease) the catalytic activity of the
=
complexes, which probably results from destabilisation (stabilisation) of [O Mn(V)(salen)X], the identified
active species making them more (less) reactive. However, when NaOCl is used, the observed behaviour is the
opposite: electron donating groups make the complexes better catalysts. The apparent similarity between the
solubility of the complexes in the organic solvent and their catalytic activity seems to suggest that solubility
must play a key role in their activity.
manner from diamines and salicylaldehyde derivatives, and
Introduction
optimal selectivities for a given substrate can be achieved by
tunning the catalyst’s steric and electronic properties.3,6
Thus, manganese(III) salen complexes have demonstrated to
be useful laboratory and industrial homogeneous catalysts in
the epoxidation of some unfunctionalised alkenes, using iodo-
sylbenzene, sodium hypochlorite, hydrogen peroxide and alkyl
hydroperoxides as oxygen sources.3 The mechanism by which
oxygen transfer takes place from the intermediate oxo-metallic
species to the alkene, as well as the spin state of the intermedi-
ate oxo-metallic species, possibly in a higher oxidation state, is
Epoxidation of alkenes is one of the most widely methods used
in organic synthesis, as epoxides can be easily transformed into
a large variety of compounds via highly regio- and stereoselec-
tive ring opening reactions,1 and direct oxygen transfer to the
alkene is the most common method of epoxide synthesis.2
Manganese(III) complexes with Schiff base and porphyrin
ligands have been extensively used as models of a monooxi-
dase, the heme containing citochrome P-450.3 These complexes
catalyse the transfer of oxygen atoms to organic substrates and
are important in the study of bio-organic mechanisms and in
the development of efficient catalysts in organic synthesis for
laboratorial and industrial application.4
Epoxidation of alkenes catalysed by manganese(III) com-
plexes with Schiff bases of N2O2 coordination sphere, com-
monly known in the literature as salen complexes, has been
the object of numerous publications since Kochi et al.
described in 1986 that they were highly effective, chemoselec-
tive and stereoselective catalysts.5 At the beginning of the last
decade, they were also found, independently by the Jacobsen
and Katsuki groups, to be highly enantioselective catalysts
when the Schiff base ligands have chiral centres in the diimine
bridge, eventually also with bulky chiral groups near the metal
centre.3,6
still a matter of debate.1,2,6 Nevertheless, the [O Mn(V)-
=
salenCl] species was identified directly by mass spectrometry7
and indirectly by nuclear magnetic resonance8 when iodosyl-
benzene was used as the oxygen source. Recent reports have
shown that the mechanism, and consequently the products,
depend on several factors: alkene,6 oxidant, counter-ion,
structure of the salen ligands and added Lewis base.2,9
In contrast, with the structurally varied but extensively stu-
died metalloporphyrins, the tuning of the catalytic activity of
the manganese(III) salen complexes by the easy introduction
of substituents on the structure remains a area of untapped
potential,10 since reports have been principally focused on
the tuning of the asymmetric induction of the chiral counter-
parts.3,6
The synthetic accessibility of the manganese(III) Schiff base
complexes from readily available and inexpensive precursors
constitutes one of their vital and more attractive features.
The salen ligand system is easily constructed in a convergent
We have been interested in the covalent attachment of tran-
sition metal Schiff base complexes onto activated carbon,11,12
especially of manganese(III) salen complexes13 in order to pre-
pare efficient heterogeneous catalysts making use of activated
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 5 3 – 2 6 0
253

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the catalytic experiments were from Aldrich and used as
received; dichloromethane and acetonitrile were of HPLC
grade (Romil).
In the synthesis of H2saltMe, 2,3-diamino-2,3-dimethylbu-
tane was synthesised by reduction of 2,3-dimethyl-2,3-dinitro-
butane with tin in acid media, using a procedure described in
the literature.16 Iodosylbenzene was synthesised using reported
procedures,17 by hydrolysis of iodobenzene diacetate with a
solution of sodium hydroxide.
carbon as a support. However, due to the variety of experi-
mental conditions reported for the epoxidation of alkenes,
we chose to do an initial systematic study of their catalytic
activity in homogeneous phase varying the oxidant and the
solvent in order to apply to the heterogeneous case.
Herein we present a systematic study of the epoxidation of
styrene catalysed by achiral manganese(III) salen complexes
(Scheme 1) with different diimine bridges, using two common
oxidants, sodium hypochlorite and iodosylbenzene, and two
of the most commonly used solvents with these oxidants,
dichloromethane and acetonitrile. Styrene was chosen as a
model substrate because it is one of the most important
pro-chiral alkenes.14 Experimental conditions described by
Jacobsen et al.15 were chosen for the reaction with sodium
hypochlorite as oxygen source, and adapted to the reactions
with iodosylbenzene. Some conclusions concerning the effect
of the groups in the diimine bridge and aldehyde fragment,
the type oxygen source and solvent used are presented.
Synthesis of the manganese(III) complexes
The Schiff base ligands were prepared by the standard proce-
dure of refluxing ethanolic solutions of the corresponding dia-
mine and salicylaldehyde derivative in a 1 : 2 molar ratio.18
Typically, 0.037 mol of the diamine were refluxed with 0.075
mol of the salicylaldehyde in 100 cm3 of ethanol for 1–2 hours.
The solutions were kept in the refrigerator overnight, and the
precipitated solids were collected by filtration and dried under
vacuum. Yields were in the range 75–90%. The following Schiff
base ligands were prepared (Scheme 1): N,N0-bis(salicyli-
dene)ethylenediamine, H2salen; N,N0-bis(salicylidene)tetra-
methylethylenediamine, H2saltMe; N,N0-bis(salicylidene)-1,2-
diphenylethylenediamine, H2saldPh; N,N0-bis(salicylidene)-
1,2-cyclohexanediamine, H2salhd; N,N0-bis(3,5-di-tert-butylsa-
licylidene)-1,2-cyclohexanediamine, H2(3,5-dtButsalhd); and
N,N0-bis(3,5-dichlorosalicylidene)-1,2-cyclohexanediamine,
H2(3,5-dClsalhd).
Experimental
Materials and solvents
The reagents used in the synthesis of Schiff base ligands and
their complexes were used as received, except for ethylenedia-
mine and 1,2-diaminocyclohexane which were distilled prior to
use. Ethylenediamine and 2-hydroxybenzaldehyde were from
Merck; 1,2-diaminocyclohexane (mixture of cis/trans isomers),
meso-1,2-diphenylethylenediamine, 2,3-dimethyl-2,3-dinitro-
butane, tin (granulate), manganese(II) acetate tetrahydrate
and iodobenzene diacetate from Aldrich; and manganese(II)
chloride tetrahydrate from Sigma.
As the synthesis and characterisation of these ligands have
already been reported in the literature,19 their purity was only
checked by H NMR and infrared spectroscopy (FTIR).
1
The manganese(III) Schiff base complexes (Scheme 1),
[Mn(salen)Cl] (1), [Mn(saltMe)Cl] (2), [Mn(saldPh)Cl] (3),
[Mn(salhd)Cl] (4), [Mn(3,5-dtButsalhd)Cl] (5) and [Mn(3,5–
dClsalhd)CH3COO] (6), were prepared using procedures
adapted from those described in the literature,15 by refluxing
equimolar quantities of an ethanolic solution of ligand and a
methanolic solution of manganese(II) chloride tetrahydrate;
in the synthesis of complex 6, manganese(II) acetate tetrahy-
drate was used. Typically, 3.30 mmol of manganese(II) chloride
tetrahydrate and 3.00 mmol of ligand in 100 cm3 of ethanol
were refluxed for 1–2 hours, during which the yellow-orange
colour of the solution changed to dark brown. The solutions
were concentrated by evaporation, yielding dark brown solids,
which were re-crystalised from acetonitrile or ethanol, and
dried under vacuum. Yields were in the range 30–75%.
Styrene, decane, chlorobenzene, sodium hypochlorite solu-
tion, benzaldehyde, styrene epoxide and iodobenzene used in
[Mn(salen)Cl] (1), chloro-[N,N0-bis(salicylidene)ethylenedi-
aminato]manganese(III): MnC16H14N2O2Cl. FAB-HRMS, m/
z: calculated (MnC16H14N2O2 ) 321.0436, experimental
þ
321.0438. UV–Vis, lmax/nm: 230, ꢀ293, ꢀ329, 363, 417,
ꢀ500(sh), ꢀ600(sh). FTIR, n/cmꢁ1: 1624 vs, 1599 vs, 1541 s,
1468 m, 1444 s, 1386 m, 1332 m, 1325 m, 1292 vs, 1254 f,
1213 (sh), 1201 m, 1151 m, 1130 m, 1084 w, 1049 w, 1034 w,
978 w, 902 m, 866 w, 854 w, 825 w, 802 s, 773 s, 756 s, 688
w, 631 s, 594 m, 488 w, 467 s.
[Mn(saltMe)Cl] (2), chloro-[N,N0-bis(salicylidene)tetra-
methylethylenediaminato]manganese(III): MnC20H22N2O2Cl.
þ
EI-HRMS, m/z: calculated (MnC20H22N2O2
) 377.1062,
experimental 377.1060. UV–Vis, lmax/nm: 277, 308, 352,
399, 480, ꢂ690 (sh). FTIR, n/cmꢁ1: 2986 w, 2964 w, 1601
vs, 1539 m, 1466 w, 1443 m, 1396 m, 1380 vw, 1292 s, 1207
w, 1143 m, 1123 w, 1031 w, 905 w, 847 w, 798 w, 755 m, 740
w, 625 m, 534 m, 456 w, 427 w.
[Mn(saldPh)Cl] (3), chloro-[N,N0-bis(salicylidene)-1,2-diphe-
nylethylenediaminato] manganese(III): MnC28H22N2O2Cl.
EI-HRMS, m/z: calculated (MnC28H22N2O2 ) 473.1062,
þ
experimental 473.1047. UV–Vis, lmax/nm: 231, 281, 308,
ꢀ343 (sh), 410, ꢀ470 (sh), ꢀ689 (sh). FTIR, n/cmꢁ1: 1618
(sh), 1603 vs, 1537 s, 1495 w, 1468 m, 1441 s, 1383 m, 1338 w,
1308 s, 1300 s, 1280 m, 1248 w, 1221 (sh), 1207 m, 1149 m,
Scheme 1 Molecular formula of the Mn(III) salen complexes.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
254
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 5 3 – 2 6 0

View Article Online
complex the reaction time for maximum epoxide yield was
1126 m, 1101 w, 1074 w, 1045 w, 1032 w, 1016 vw, 995 m, 972
m, 945 w, 930 w, 908 m, 877 vw, 860 m, 831 vw, 816 m, 793 vw,
769 (sh), 760 s, 723 vw, 704 m, 660 mf, 644 m, 631 vw, 621 m,
590 w, 575 m, 559 vw, 530 m, 498 vw, 476 vw, 455 m.
[Mn(salhd)Cl] (4), chloro-N,N0-bis(salicylidene)-1,2-cyclo-
determined in CH2Cl2 , by withdrawing periodically 0.1 cm3
aliquots from the reaction mixture, and this time was used to
monitor the efficiency of the catalyst, performing at least two
independent experiments in each of the two solvents, CH2Cl2
and CH3CN. The composition of reaction media was deter-
mined by GC with styrene, styrene epoxide and benzaldehyde
being quantified by the internal standard method (decane in
CH2Cl2 and chlorobenzene in CH3CN). All other products
detected by GC were designated as others. Blank experiments
with each oxidant and using the same experimental conditions
but without catalyst were also performed.
hexanodiaminato]manganese(III):
MnC20H20N2O2Cl.
FAB-HRMS, m/z: calculated (MnC20H20N2O2þ) 375.0905,
experimental 375.0920. UV–Vis, lmax/nm: 212, ꢀ276 (sh),
298, 352, 395, ꢀ455 (sh), ꢀ686 (sh). FTIR, n/cmꢁ1: 2933 m,
2926 m, 2870 (sh), 2858 m, 1622 vs, 1599 vs, 1543 s, 1470 m,
1446 s, 1394 m, 1350 m, 1336 (sh), 1319 s, 1309 s, 1277 s,
1246 w, 1217 m, 1200 m, 1149 s, 1124 w, 1051 w, 1030 w,
1010 w, 906 m, 862 w, 848 w, 808 m, 750 s, 686 w, 623 m,
605 w, 569 m, 515 w, 459 w, 426 m.
Results and discussion
[Mn(3,5-dtButsalhd)Cl] (5), chloro-N,N0-bis(3,5-di-tert-
butylsalicylidene)-1,2-cyclohexanodiaminato]manganese(III):
Characterisation of the Mn(III) salen complexes
MnC36H52N2O2Cl.
([MnC36H52N2O2-Cl]þ) 599.3409, experimental 599.3417.
UV–Vis,
FAB-HRMS,
m/z:
calculated
All manganese complexes synthesised are dark brown, as
expected for manganese(III) Schiff base complexes,20 but their
solubility is strongly dependent on solvent polarity: complexes
2, 3 and 5 (Scheme 1) are very soluble in CH2Cl2 , CH3CN,
dmf and dmso, whereas complexes 1, 4 and 6 are only moder-
ately soluble in the less polar solvents, although very soluble in
dmf and dmso.
HRMS data indicate that the manganese(III) salen com-
plexes have the expected stoichiometry (the diference between
the experimental and calculated values are less than 5 ppm).
These complexes ionise mainly as [M ꢁ X]þ, where M repre-
sents the manganese(III) salen complex without the axial anion
coordinated to the metal centre, and X the axial anion. Low
intensity peaks due to traces of dimeric species [M þ M ꢁ H]þ
and [M þ M þ X]þ are also observed. Formation of traces of
these dimeric species in solution is common for manganese(III)
salen complexes, and prevents the attainment of acceptable
errors (<1%) for elemental analyses.20,21
l
max/nm: 228, 282, 319, ꢀ354(sh), 415, 510,
ꢀ642(sh). FTIR, n/cmꢁ1: 2954 ff, 2906 m, 2867 m, 1612 vs,
1550 m, 1535 s, 1462 m, 1433 m, 1419 w, 1388 m, 1362 m,
1342 m, 1313 s, 1271 m, 1252 s, 1200 m, 1174 s, 1136 vw,
1092 vw, 1030 w, 987 vw, 972 vw, 953 vw, 930 vw, 895 vw,
870 vw, 839 m, 814 w, 781 m, 750 m, 640 vw, 567 m, 544 m,
515 vw, 486 w, 415 w.
[Mn(3,5-dClsalhd)CH3COO] (6), carboxylate-N,N0-bis(3,5-
di-chlorosalicylidene)-1,2-cyclohexanodiaminato]manganese(III):
MnC22H19N2O4Cl4 .
FAB-HRMS,
m/z:
calculated
([MnC20H16N2O2Cl4–CH3COO]þ) 512.9317, experimental
512.9320. UV–Vis, lmax/nm: 227, ꢀ273(sh), ꢀ346(sh), 419,
ꢀ472(sh), ꢀ612(sh). FTIR, n/cmꢁ1: 1626 vs, 1589 w, 1529 s,
1439 s, 1410 m, 1385 m, 1342 w, 1317 m, 1286 w, 1223 w,
1209 w, 1180 m, 1107 vw, 1022 w, 966 vw, 922 vw, 870 w,
823 w, 769 m, 750 m, 723 vw, 667 vw, 654 vw, 611 vw, 582
vw, 548 m, 438 w.
=
When the energies of the C N stretching vibration of the
ligands are compared with those of their respective mangane-
se(III) complexes, a shift of 4 to 26 cmꢁ1 to lower energies is
observed in all complexes (Table 1), a consequence of metal
binding to the imine nitrogen atoms, sustaining the coordina-
tion of the metal to the Schiff base ligands.21
Physical measurements
1H NMR spectra were recorded with a Bruker AC 200 spectro-
meter at 397 K, using tetramethylsilane as internal reference.
FTIR spectra were obtained as KBr pellets in a Biorad
FTS 155 in the range 400–4000 cmꢁ1, and ultraviolet diffuse
reflectance spectra on a Shimadzu UV-3101 PC in the range
1600–200 nm and using barium sulfate as reference.
FAB-HRMS and IE-HRMS were performed at ‘Unidade
´
de Espectometrıa de Masas’, Universidade de Santiago de
Compostela (Spain).
The electronic spectra of these compounds are dominated by
=
intense bands in the 200–490 nm region due to: p* (C N) d
=
(charge transfer), p* (C N) p and p* (phenolic oxygen) n
(intraligand) transitions;22 but show also several low intensity
asymmetric bands (that may appear as shoulders) between 500
and 700 nm assigned to the three allowed d–d transitions
expected for complexes with a square pyramidal geometry,
Gas chromatography experiments (GC) were performed with
a Varian Star 3400CX chromatograph equipped with a TCD
detector and using helium as carrier gas and a fused silica Var-
ian Chrompack capillary column CP-Sil 8CB (30 m ꢃ 0.53 mm
id; 1.50 mm film thickness). The chromatographic conditions
2
2
2
(dx2ꢁy dxz), (dyz , dx2ꢁy dxy) and (dx2ꢁy dz2) (decreasing
energy), which may or may not be resolved.22
Styrene epoxidation catalysed by Mn(III) salen complexes
were: 60 ꢄC (3 min), 5 ꢄC minꢁ1, 170 ꢄC (2 min), 20 ꢄC minꢁ1
,
The brown colour of solutions containing the Mn(III) salen
complex and the substrate was intensified by addition of
any oxidant, indicating the formation of oxo-metallic
250 ꢄC (10 min); injector temperature, 200 ꢄC; detector tem-
perature, 270 ꢄC; filament temperature, 290 ꢄC. Aliquots of
0.1 cm3 were withdrawn from the reaction mixture with a hypo-
dermic syringe, filtered through PP 0.22 mm syringe filters and
injected directly into the injector using a 1 ml Hamilton syringe.
=
Table 1 Energy of the C N vibration of the manganese(III) salen
complexes
Complex
n/cmꢁ1
Dn/cmꢁ1 a
Catalysis experiments
The reactions were carried out in CH2Cl2 or CH3CN, at room
temperature with constant stirring, and the composition of the
reaction medium was 2.50 mmol of styrene, 2.50 mmol
of decane or chlorobenzene (internal standard), 0.10 mmol of
Mn(III) salen complex (catalyst) and using 6.25 mmol
of sodium hypochlorite or iodosylbenzene (oxidant), in 5.00
cm3 of solvent. When the oxidant was sodium hypochlorite,
the solution was buffered to pH 11 (NaH2PO4 þ NaHO), to
minimise formation of chlorinated products.15 For each
[Mn(salen)Cl] (1)
[Mn(saltMe)Cl] (2)
1624
1601
1603
1622
1612
1626
11
26
20
4
[Mn(saldPh)Cl] (3)
[Mn(salhd)Cl] (4)
[Mn(3,5-dtButsalhd)Cl] (5)
[Mn(3,5-dClsalhd)CH3COO] (6)
18
5
a
Values refer to the difference in energy of the ligand and the
respective complex.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 5 3 – 2 6 0
255

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and chlorinated products. Formation of the latter deserves
an explanation, because the reactions were carried out at pH
11, a value at which the quantity of chlorinated by-products
is minimised;15 these are formed because the oxygen transfer
from the oxo-metallic active species to the alkene takes place
at the inter-phase of CH2Cl2 (containing the organic substrate
and the catalyst) with the aqueous NaOCl solution.23
With the exception of complexes 5 and 3, all catalysts show a
decrease in styrene epoxide selectivity with reaction time, and a
concomitant increase in the amount of benzaldehyde and
others formed. This behaviour has been attributed to catalyst
deactivation15 through formation of dimeric m-oxo-Mn(IV)
species,24 which are inactive towards styrene epoxidation.15,24
The decrease in styrene epoxide selectivity that is observed
after a certain reaction time for the less efficient catalysts, 1,
4, 2 and 6, must result from catalyst deactivation that makes
the rate of styrene epoxidation smaller than the rate of the par-
allel reaction of degradation of styrene epoxide into alcohols
and chlorinated products.
intermediates of the catalyst.6 After complete consumption of
the alkene the solution regains its initial colour, suggesting that
the catalyst was regenerated.
The reaction profiles at room temperature for the epoxida-
tion of styrene catalysed by the several manganese(III) salen
complexes were assessed in CH2Cl2 , using NaOCl or PhIO
as oxygen sources. For the reaction time at which the styrene
epoxide yield reaches its maximum, at least duplicate experi-
ments were subsequently performed in CH2Cl2 and CH3CN.
The values of styrene conversion, and the selectivities in
styrene epoxide, benzaldehyde, others, and styrene epoxide
yields for each complex, both in CH2Cl2 and CH3CN and
for the reaction time that maximises the epoxide yield in
CH2Cl2 , are collected in Table 2 (NaOCl as oxidant) and in
Table 3 (PhIO). Finally, it must be stressed that in all cases
with no manganese(III) salen complexes in the reaction media
(blank experiment), no styrene conversion was observed.
Epoxidation with sodium hypochlorite
Reaction profiles. The styrene epoxidation profiles in CH2Cl2
catalysed by the manganese(III) salen complexes and using
NaOCl as oxidant are depicted in Fig. 1: (a) styrene conver-
sion, (b) styrene epoxide selectivity, (c) styrene epoxide yield,
(d) benzaldehyde selectivity, and (e) selectivity in others.
Analysis of Fig. 1 shows that under the conditions used only
complexes 5 and 3 convert styrene efficiently. Nevertheless, all
the catalysts are selective towards the formation of styrene
epoxide, despite formation of by-products such as benzalde-
hyde and other reaction products (others), which have been
identified by GC-MS as styrene epoxide derivatives: alcohols
Catalytic activity. When the reactions are carried out in
CH2Cl2 , styrene conversions were of 91% and 100% using 3
and 5 as catalysts, respectively, whereas for the other com-
plexes ranged between 9 and 38%. In CH3CN, styrene conver-
sion dropped to 62% for 3 and to 98% for 5, whereas better
conversions (34–45%) were generally obtained with the other
complexes.
When the solvent is changed from CH2Cl2 to CH3CN and
for all catalysts the styrene epoxide selectivity drops and those
of benzaldehyde and other by-products increase (Table 2 and
Table 2 Styrene epoxidation with NaOCl catalysed by manganese(III) salen complexes, in CH2Cl2 or CH3CN (average of at least three
experiments)
Selectivity (%)
Catalyst
Time/h
Solvent
Conversion (%)
Epoxide yield (%)
Epoxide
Benzaldehyde
Other
[Mn(salen)Cl] (1)
1.5
2
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
34.3 ꢅ 1.0
44.7 ꢅ 0.7
34.7 ꢅ 0.6
45.1 ꢅ 0.2
90.8 ꢅ 0.3
61.5 ꢅ 5.3
37.5 ꢅ 1.8
34.2 ꢅ 0.9
100.0 ꢅ 0.0
98.0 ꢅ 0.9
9.1 ꢅ 0.4
26.4
22.5
32.3
40.6
83.9
51.5
35.2
21.2
95.7
85.7
7.8
77.0 ꢅ 0.6
50.3 ꢅ 0.4
93.1 ꢅ 2.1
90.1 ꢅ 0.9
92.4 ꢅ 0.3
83.8 ꢅ 3.5
93.8 ꢅ 1.9
62.1 ꢅ 3.4
95.7 ꢅ 0.6
87.4 ꢅ 1.0
86.2 ꢅ 0.3
74.7 ꢅ 2.1
3.2 ꢅ 0.2
7.8 ꢅ 0.0
5.5 ꢅ 0.3
5.2 ꢅ 0.8
2.2 ꢅ 0.3
5.5 ꢅ 1.1
3.3 ꢅ 0.9
15.9 ꢅ 4.5
1.0 ꢅ 0.4
5.2 ꢅ 1.0
8.3 ꢅ 0.2
7.1 ꢅ 0.7
19.8 ꢅ 0.8
41.9 ꢅ 0.4
1.4 ꢅ 1.8
4.7 ꢅ 0.3
5.3 ꢅ 0.0
10.7 ꢅ 2.7
2.9 ꢅ 1.2
22.0 ꢅ 6.8
3.0 ꢅ 1.0
7.2 ꢅ 0.1
5.5 ꢅ 0.1
18.2 ꢅ 1.4
[Mn(saltMe)Cl] (2)
[Mn(saldPh)Cl] (3)
[Mn(salhd)Cl] (4)
5
3
[Mn(3,5-dtButsalhd)Cl] (5)
[Mn(3,5-dClsalhd)CH3COO] (6)
2
5
2
37.7 ꢅ 1.8
28.2
Table 3 Styrene epoxidation with PhIO catalysed by manganese(III) salen complexes, in CH2Cl2 or CH3CN (average of at least three experiments)
Selectivity (%)
Catalyst
Time/h
Solvent
Conversion (%)
Epoxide yield (%)
Epoxide
Benzaldehyde
Other
[Mn(salen)Cl] (1)
3
2
2
3
2
2
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
CH2Cl2
CH3CN
91.9 ꢅ 0.2
77.9 ꢅ 0.9
100.0 ꢅ 0.0
100.0 ꢅ 0.0
99.3 ꢅ 0.2
100.0 ꢅ 0.0
100.0 ꢅ 0.0
93.6 ꢅ 0.2
61.3 ꢅ 1.1
81.5 ꢅ 2.7
100.0 ꢅ 0.0
98.3 ꢅ 0.1
85.7
71.2
85.7
98.3
87.5
98.2
86.9
88.5
60.0
77.3
95.5
94.7
93.3 ꢅ 0.1
91.4 ꢅ 0.6
85.7 ꢅ 0.5
98.3 ꢅ 1.1
88.1 ꢅ 0.5
98.2 ꢅ 0.3
86.9 ꢅ 0.4
94.6 ꢅ 0.1
97.9 ꢅ 0.8
94.9 ꢅ 0.1
95.5 ꢅ 0.0
96.3 ꢅ 0.0
5.1 ꢅ 0.2
7.7 ꢅ 0.4
1.2 ꢅ 0.1
0.1 ꢅ 0.1
3.4 ꢅ 0.1
1.7 ꢅ 0.1
2.5 ꢅ 0.0
5.4 ꢅ 0.1
1.4 ꢅ 0.1
5.1 ꢅ 0.1
2.9 ꢅ 0.2
3.1 ꢅ 0.6
1.6 ꢅ 0.1
0.8 ꢅ 0.2
12.9 ꢅ 0.4
1.7 ꢅ 1.0
8.5 ꢅ 0.5
0.2 ꢅ 0.2
10.4 ꢅ 0.4
0.0 ꢅ 0.0
1.3 ꢅ 0.7
0.0 ꢅ 0.0
1.6 ꢅ 0.3
0.6 ꢅ 0.6
[Mn(saltMe)Cl] (2)
[Mn(saldPh)Cl] (3)
[Mn(salhd)Cl] (4)
[Mn(3,5-dtButsalhd)Cl] (5)
[Mn(3,5-dClsalhd)CH3COO] (6)
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
256
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 5 3 – 2 6 0

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Fig. 1 Reaction profiles of the styrene epoxidation with NaOCl catalysed by Mn(III) salen complexes in CH2Cl2 , at room temperature: (a) styrene
conversion, (b) styrene epoxide selectivity, (c) styrene epoxide yield, (d) benzaldehyde selectivity and (e) others selectivity.
Fig. 2). These results may be attributed to the larger miscibility
between CH3CN and the sodium hypochlorite aqueous solu-
tion, leading to more extensive hydrolysis of the styrene epox-
ide and formation of chlorinated by-products. In summary, the
use of NaOCl as oxygen source in CH2Cl2 led to lower yields
of by-products and, for the better catalysts, to larger styrene
conversions than in CH3CN.
as when NaOCl is used as the oxygen source, formation of
benzaldehyde and of other by-products, although different
from those of the reaction with NaOCl, is also observed.
Kochi et al. have shown that the by-products of alkene
epoxidation with PhIO are mostly formed during the course
of epoxidation and are not due to subsequent rearrangements
of the styrene epoxide.5 In fact, these authors noted that addi-
tion of PhIO to a solution of complex 1 in CH2Cl2 with no
alkene present led to an increase of intensity of the brown col-
our of the solution, taken as an indication of formation of oxo-
metallic species, and to catalytic conversion of iodosylbenzene
to iodobenzene;5 the other reaction products formed must thus
be degradation products from PhIO. These observations imply
that in the present study neither benzaldehyde nor the other
by-products originate from reactions of styrene epoxide, but
from degradation reactions of PhIO that take place in parallel
to styrene epoxidation. Furthermore, in blank experiments (no
catalyst) no iodobenzene was detected in the reaction media.
Epoxidation with iodosylbenzene
Reaction profiles. Analysis of the styrene epoxidation pro-
files in CH2Cl2 catalysed by the manganese(III) salen complexes
when PhIO is the oxidant (Fig. 3) show that in contrast with
the results obtained with NaOCl as oxidant, all complexes
are very efficient catalysts in the conversion of styrene and
present very high selectivities in styrene epoxide.
Despite the high styrene epoxide selectivities obtained with
PhIO, which do not decrease so drastically with reaction time
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 5 3 – 2 6 0
257

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Fig. 2 Styrene epoxide selectivity and % yield with NaOCl or PhIO of the Mn(III) salen catalysts in CH2Cl2 or CH3CN.
Fig. 3 Reaction profiles of the styrene epoxidation with PhIO catalysed by Mn(III) salen complexes in CH2Cl2 , at room temperature: (a) styrene
conversion, (b) styrene epoxide selectivity, (c) styrene epoxide yield, (d) benzaldehyde selectivity and (e) others selectivity.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 5 3 – 2 6 0
258
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4

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(Fig. 3 and Table 3). The introduction of electron withdrawing
The catalytic activity of the manganese(III) salen complexes
in the decomposition of iodosylbenzene to iodobenzene, and
despite of the rate of oxygen transfer from the oxo-metallic
intermediate species to the alkene being higher than the rate
of homolytic degradation of the oxidant, requires the use of
excess PhIO to obtain 100% conversion of styrene.12
groups (chloro) in the 3 and 5 positions of the aldehyde frag-
ment of the Schiff base (Scheme 1) increases always the styrene
epoxide yield, while the introduction of electron donating sub-
stituents (tert-butyl) decreases this yield. These results are in
agreement with the studies of the epoxidation of alkenes with
PhIO catalysed by manganese(III) salen complexes reported by
Kochi et al., in which they concluded that the introduction of
electron withdrawing groups (chloro and nitro) in the 5 posi-
tions of the aldehyde fragment of the Schiff base makes the
corresponding manganese(III) salen complexes more efficient
Catalytic activity. From Table 3 it can be gathered that in
CH2Cl2 , with the sole exception of complex 5, which converts
61% of styrene, the other catalysts exhibit styrene conversions
between 92 and 100% after 2–3 hours of reaction; in CH3CN
conversions range between 78 and 100% (Table 3).
The selectivity in styrene epoxide is high both in CH2Cl2 and
in CH3CN, with a general tendency to be larger in CH3CN
(Table 3 and Fig. 3). The selectivity in others is lower than
those obtained when the oxygen source is NaOCl, and is smal-
ler in CH3CN, in contrast to what is observed using NaOCl as
oxidant.
catalysts.5 In this system [O Mn(V)salenCl] was identified
=
directly by mass spectrometry7 and indirectly by nuclear
magnetic resonance8 as being the active oxidant species of the
catalyst. The introduction of electron withdrawing groups in
position 5 of the aldehyde fragment destabilises the
=
[O Mn(V)salenCl] species, making it more reactive, hence
increasing the catalytic activity of the complex.
When NaOCl is used as oxygen source, and in contrast with
what is observed with PhIO as oxidant, the order of styrene
epoxide yield obtained with the ‘‘[Mn(salhd)X]’’ catalysts is
solvent dependent; in CH2Cl2 the order is 6 < 4 ꢆ 5, whereas
in CH3CN is 4 < 6 ꢆ 5 (Fig. 1 and Table 2). But more striking
is the observation that the introduction of electron donating
groups (tert-butyl) in the 3 and 5 positions of the aldehyde
fragment of the Schiff base increases in both solvents signifi-
cantly the yield in styrene epoxide (Scheme 1). On the other
hand, the introduction of electron withdrawing groups
(chloro) decreases this yield, although only in CH2Cl2 .
It has been demonstrated that the introduction of substitu-
ents in the 3 and 5 positions of the aldehyde fragment (Scheme
1) might have direct interactions in the electronic properties of
the manganese(III) salen complexes, although the 3 position
has been reported to be associated with stereochemical
effects.6,26 Electrochemical studies of the Mn(II)/Mn(III) pro-
cess of manganese(III) salen complexes have shown that the
oxidised species are stabilised by the introduction of electron
donating substituents in the 5 position of the aldehyde frag-
ment (Scheme 1), and destabilised by electron withdrawing
groups in the same position, and the same effects have been
assumed to be operative in the reactive intermediate species.6,27
These observations may be used to support the results
obtained in the present study with the PhIO, but fails to
explain the results obtained with NaOCl.
As can be seen from Fig. 2, with the exception of 5, larger
styrene epoxide yields and selectivities are obtained using PhIO
as oxidant than with NaOCl, which may be correlated with the
facile deactivation of the manganese(III) salen complexes using
the sodium hypochlorite aqueous solutions as oxidant. More-
over, the use of PhIO as oxidant lowers the quantities of other
by-products, mainly because no styrene epoxide degradation
products are observed (i.e. alcohols and chlorinated products).
Effect of the diimine bridge substituents
Next the catalytic activities of the manganese(III) salen
complexes derived from the same aldehyde fragment but with
different diimine bridges are going to be compared. Using
NaOCl, the styrene epoxide yield for the manganese(III) salen
complexes increases in following order: in CH2Cl2 ,
1 < 2 < 4 < < 3; in acetonitrile, 1 ꢀ 4 < 2 < 3.
With PhIO, the yields in styrene epoxide for the mangane-
se(III) salen complexes in CH2Cl2 are insensitive to the presence
of substituents in the diimine bridge, whereas in CH3CN they
increase in following order: 1 < 4 < 2 ꢀ 3.
Under all experimental conditions used, 3 is always one of
the most efficient catalyst in the epoxidation of styrene and 1
is among the less efficient. With the exception of using PhIO
in CH2Cl2 , the introduction of methyl or other bulky groups
in the diimine bridge improves epoxide yield (Scheme 1, Fig.
2), implying that steric modulation may be involved in the
activity of the manganese catalysts, as has been claimed in
the literature.5 This observation deserves further elaboration,
as the accepted mechanism for the catalytic role of the
‘‘[Mn(salen)Cl]’’ complexes involves the formation of an
adduct with the oxidant and the presence of bulky substituents
would pose a steric barrier to its formation. However, the con-
clusions of a recent computational study on oxomanganese(V)-
salen complexes suggest that a side-on approach of the olefin
might more favourably occur from the salicylaldehyde ring
than from the imine portion.25 The existence of bulky groups
Adam et al.2 in an elegant study have proposed recently,
based on the cis/trans ratios obtained for the epoxidation of
cis-stilbene, that the mechanism by which epoxidation of
alkenes catalysed by manganese(III) salen complexes takes
place might involve two different active intermediate species:
(i) an active [L–O–Mn(III)salenCl] species, where L represents
the leaving group of the oxidant, that transfers the oxygen to
the alkene by a concerted step, and that was found to be
mainly associated with sodium hypochlorite as oxygen source;
=
and (ii) and active [O Mn(V)salenCl] species, that transfers the
oxygen to the alkene by a stepwise radical mechanism, and
that was observed with iodosylbenzene.
=
on the ethylene bridge would force the M O bond to bend
Data in Fig. 1 suggest that electron donating groups acceler-
ate the reaction when the oxidant is NaOCl, what may be
taken as indirect evidence to support the concerted mechanism
involving [L–O–Mn(III)salenCl], as these groups would ease
the cleavage of the O–L bond (L is cleaved as an anion).
The opposite behaviour is observed in CH2Cl2 when electron
withdrawing groups are present, as the O–L bond would be
more difficult to cleave. However, in CH3CN the presence of
electron withdrawing groups does not make the complex a
worse catalyst.
away from the bridge, thus easing the approach of the olefin
in the proposed pathway. Nevertheless, a possible role of bulky
groups in diimine bridge in stabilising the manganese(III) salen
complexes cannot be ruled out, as it would reduce catalyst
desactivation, and thus result in higher catalyst efficiency.
Effect of substituents in the aldehyde fragment
The effect of substituents in the aldehyde moiety can be judged
by comparing the catalytic behaviour of the complexes that
share the same diimine bridge.
Using PhIO as oxygen source, the styrene epoxide yields for
the different ‘‘[Mn(salhd)X]’’ type complexes used as catalysts
follow the order: 5 < 4 < 6, either in CH2Cl2 or in CH3CN
Nevertheless, our data seem to suggest the existence of the
two mechanisms outlined above: a concerted mechanism (i)
with NaOCl, and a stepwise radical mechanism (ii) with PhIO.
The anomalous behaviour in CH3CN when NaOCl is the
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 2 5 3 – 2 6 0
259

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oxidant may reside in the simplification of picture outlined
above, since the concerted mechanism requires simultaneous
O–C bond formation, which can make the prediction of
electronic effects a difficult task but, on the other hand, may
also be related to the solubility of the complexes.
Acknowledgements
ˆ
ARS thanks the Fundac¸a˜o para a Ciencia e Tecnologia
(Lisboa) and the European Social Fund for a fellowship. This
work was financed by FCT and FEDER through Proj. no.
POCTI/QUI/42931/2001.
In fact, we recall that the oxygen transfer takes place at the
water/organic solvent interphase,23 and that the catalyst must
exist in the latter. By noting that complex 5 is completely solu-
ble in CH2Cl2 and in CH3CN, whereas complexes 4 and 6 are
only partially soluble (at least for the concentrations used in
this study), and that the latter is roughly ten times less soluble
than 4, it is also possible to correlate epoxide yield with solu-
bility in CH2Cl2 . The lack of such a correlation in CH3CN
results probably from a subtle balance between solubility,
electronic effects and counter-ion. The large solubility, and
the presence of electron donating groups in conjunction with
a concerted mechanism, may explain why the Jacobsen catalyst
is so effective with NaOCl.
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It was possible to tune the catalytic activities of ‘‘[Mn
(salen)X]’’ complexes by introduction of substituents in the
diimine bridge and in the aldehyde fragment. The presence of
bulky substituents in the diimine bridge always increases the
catalytic activity of these complexes, regardless of the oxidant,
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=
from destabilisation (stabilisation) of [O Mn(V)(salen)X], the
identified active species,7,8 making them more (less) reactive.
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