Electronic effects on the stereoselectivity of epoxidation reactions catalysed by manganese porphyrins

Article abstract of DOI:10.1002/(SICI)1099-0690(199912)1999:12<3281::AID-EJOC3281>3.0.CO;2-1

A series of Mn(III) porphyrins progressively halogenated in the β- pyrrolic positions was employed to catalyse the epoxidation of cis-stilbene by iodosylbenzene, and to study the role of the electronic effects on the stereoselectivity of this process. A gradual improvement in the stereoselectivity on increasing the number of β-halogen atoms was observed. The role of steric effects upon the epoxidation was also investigated by placing ortho-substituents in the mesophenyl rings, and it was found that steric effects are more important than electronic effects toward the stereoselectivity of this process. These results can be rationalised by proposing a competition between a nonstereoselective electrophilic pathway of addition and a stereospecific pathway of oxygen insertion, the former being disfavoured by electron-withdrawing substituents. Alternatively, the formation of an open intermediate between the Mn(V) oxene and the substrate could be suggested, where the stereoselectivity ought to be determined by the competition between closure of the epoxide ring and rotation around the C-C bond. In this case, the enhanced stereoselectivity given by our polyhalogenated porphyrins might be attributed to an acceleration of the epoxide ring closure caused by the electron-withdrawing effect of the halogen substituents.

Full text of DOI:10.1002/(SICI)1099-0690(199912)1999:12<3281::AID-EJOC3281>3.0.CO;2-1

FULL PAPER
Electronic Effects on the Stereoselectivity of Epoxidation Reactions Catalysed
by Manganese Porphyrins
[
a]
[†][c]
[a]
[a]
[b]
Enrico Baciocchi,* Tristano Boschi,
Luigi Cassioli, Carlo Galli, Laurent Jaquinod,
[
a]
[c]
[b]
[c]
Andrea Lapi,* Roberto Paolesse, Kevin M. Smith, and Pietro Tagliatesta*
Keywords: Alkenes / Epoxidation / Manganese / Porphyrins / Catalysis
A series of MnIII porphyrins progressively halogenated in the
β-pyrrolic positions was employed to catalyse the
epoxidation of cis-stilbene by iodosylbenzene, and to study
the role of the electronic effects on the stereoselectivity of
this process. A gradual improvement in the stereoselectivity
on increasing the number of β-halogen atoms was observed.
The role of steric effects upon the epoxidation was also
electrophilic pathway of addition and a stereospecific
pathway of oxygen insertion, the former being disfavoured
by electron-withdrawing substituents. Alternatively, the
V
formation of an open intermediate between the Mn oxene
and the substrate could be suggested, where the
stereoselectivity ought to be determined by the competition
between closure of the epoxide ring and rotation around the
investigated by placing ortho-substituents in the meso- C–C bond. In this case, the enhanced stereoselectivity given
phenyl rings, and it was found that steric effects are more
important than electronic effects toward the stereoselectivity
by our polyhalogenated porphyrins might be attributed to an
acceleration of the epoxide ring closure caused by the
of this process. These results can be rationalised by electron-withdrawing effect of the halogen substituents.
proposing competition between nonstereoselective
a
a
Introduction
depends upon the structure of the reactants and also upon
the oxidation potential of the alkene and of the porphyrin
catalyst, the latter being susceptible to modulation by an
appropriate choice of substituents connected to the tetra-
pyrrolic ring or to the meso aryl groups.
Manganese(III) porphyrins have been shown to be effi-
cient catalysts in epoxidation reactions of alkenes[ in asso-
ciation with a wide variety of oxygen donor species, such as
1]
[
2]
[3]
iodosylarenes,
hydrogen peroxide,
monopersulfates
alkylhydroperoxi-
or hypochlorites.
We have recently been involved in the synthesis of new
metalloporphyrins, bearing either ortho-dimethoxy substi-
des,[ peracids,
4]
[5]
[6]
[7]
The manganese(III) porphyrin is transformed into an oxo-
manganese(V) porphyrin [PMn ϭO], which is believed to
[9]
tuents in the meso-phenyl groups, or halogen atoms in the
V
[10]
four β-pyrrolic positions.
The idea behind the synthesis
be the actual oxidizing species (Equation 1, where SO is the
of these substituted porphyrins was to build more robust
catalysts for the epoxidation of alkenes and the hydroxyla-
tion of alkanes. Additionally, we wanted to investigate how
the catalytic activity of porphyrins toward the said reactions
was influenced by the electronic effect of the halogen substi-
tuents in the β-pyrrolic positions.
oxygen atom donor).[
1]
Herein we report on the influence of electronic effects on
the stereoselectivity of the epoxidation reaction catalysed
by an extended series of manganese(III) porphyrins, which
are substituted in the β-pyrrolic positions by halogen atoms
increasing in number from 2 to 8. Since the steric hindrance
is kept constant within these series, it should be possible to
distinguish electronic from steric effects toward the stere-
oselectivity of the epoxidation process. This information
can be very useful for a better understanding of the mecha-
nism of the epoxidations induced by manganese porphyrins,
and particularly of the relative importance of the reaction
paths shown in Scheme 1.
A unified mechanism (Scheme 1) for the oxygen transfer
from the oxo-metalloporphyrin to the alkenes was proposed
[
8]
by Bruice. It involves the rate-determining formation of a
charge transfer complex, which can be converted into the
epoxide by a variety of pathways: a one-electron oxidation
followed by reaction of the radical cation of the alkene with
the reduced form of the manganese-oxo complex (path a),
an electrophilic addition leading to a carbocation which
then undergoes ring closure (path b), or a concerted oxene
insertion (path c). The choice between these three pathways
[
a]
Dipartimento di Chimica, Universit a` “La Sapienza”,
Piazzale A. Moro 5, I-00185 Roma, Italy
Fax: (internat.) ϩ 39-06/490421
E-mail: baciocchi@axcasp.caspur.it
Department of Chemistry, University of California,
Davis, CA 95616, USA
[
[
b]
c]
Results and Discussion
Dipartimento di Scienze e Tecnologie Chimiche,
Universit a` di Tor Vergata,
The β-halo-substituted MnIII porphyrins employed in
this investigation are depicted in Chart 1. Three series were
developed: the more extended series has unsubstituted meso
Via della Ricerca Scientifica, I-00133 Roma, Italy
Deceased
[
†]
Eur. J. Org. Chem. 1999, 3281Ϫ3286
 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ193X/99/1212Ϫ3281 $ 17.50ϩ.50/0
3281

E. Baciocchi, A. Lapi, P. Tagliasesta et al.
FULL PAPER
Scheme 1. Unified mechanism for the oxygen transfer from oxo-metalloporphyrins to alkenes[8]
phenyl groups (MnTPPCl) and bears an increasing number, Scheme 1, where the steric hindrance of the porphyrin ring,
from 2 to 8, of bromo substituents in the β-pyrrolic posi- as it approaches the substrate, would matter more or less
tions; the second series comprises three members, featuring strongly (vide infra).
ortho-dimethoxy substitution at the meso phenyls (TDMPP)
The epoxidation reactions were carried with cis-stilbene
and 0, 4 or 8 chloro atoms in the β-pyrrolic positions. The as the substrate, in aerobic conditions at 25°C in dichloro-
last series features ortho-dichloro substitution at the meso methane, with iodosylbenzene (PhIO) as the oxygen atom
phenyls (TDCPP) and 0 or 4 bromo atoms in the β-pyrrolic donor (Equation 2). No supplemetary axial ligands were
positions. Even the last shorter series was worth investiga- added in view of the fact that they can influence the stereo-
ting as in a previous paper we reported that the electronic selectivity of the epoxidations induced by manganese por-
effect of the halogen substituents reaches its maximum ef- phyrins, thereby masking the substituent effects. In all cases
fect upon the catalytic activity of the porphyrin with 4 β- cis-stilbene oxide and trans-stilbene oxide were the only
[
10]
halogens.
It is also important to stress that all the por- products formed. No carbonyl compounds were detected.
phyrins reported in Chart 1 are pure compounds, featuring Product yields and cis/trans epoxide ratios are reported in
only those particular halogen atoms in the indicated posi- Table 1.
tions, and are not mixtures of various regioisomers. Besides
Inspection of Table 1 shows a regular increase in the ste-
that, another relevant feature is that the steric hindrance at reoselectivity for an increase in the number of halogen
the meso phenyl positions is kept constant within the same atoms in the macrocycle β-positions. Within the family of
series of porphyrins so that, as already mentioned, a clear the simple tetraphenylporphyrins (entries 1Ϫ7) the cis/trans
dissection of electronic effects from steric effects is possible. epoxide ratio ranges from 0.6 for the nonhalogenated por-
This is particularly relevant if a decision about the interve- phyrin (entry 1) to 8.0 when all the eight β-positions are
ning mechanism of epoxidation has to be made with respect substituted with bromine atoms (entry 7). For the meso 2,6-
to the possible competition of the paths delineated in dimethoxyphenyl porphyrins (TDMPP) the cis/trans ratio
3282
Eur. J. Org. Chem. 1999, 3281Ϫ3286

Epoxidation Reactions Catalysed by Manganese Porphyrins
FULL PAPER
Chart 1. MnIII porphyrins employed in this work
approaches 20, when eight chloro atoms substitute the β- unless there are substituents in the ortho-position of the
[
2b,7c]
positions of the catalyst (entry 10), while it soars to the meso-phenyl rings,
or neutral axial ligands, such as py-
remarkable value of 61 for the meso 2,6-dichlorophenyl por- ridine or imidazole,[
7b,7c,11]
are added to the solution. A
phyrin (TDCPP) substituted by four β-bromo atoms (en- number of interpretations have been offered to account for
tries 11Ϫ12). The increase in stereoselectivity appears re- such a complex situation. Thus, the generally low stereose-
III
markably regular, as a good linear correlation is observed lectivity of the epoxidation catalysed by Mn porphyrins
between the logarithm of the cis/trans epoxide ratio and the may be attributed to the formation of an open intermediate
sum of HammetЈs σ of the substituents in β-positions (Fi- such as 3, where rotation along the CϪC bond is possible
p
2
[12]
gure 1), for both the MnTPPCl (r ϭ 0.95, ρ ϭ 0.58) and before closure of the epoxide ring.
On this basis, the ef-
2
the MnTDMPPCl (r ϭ 0.99, ρ ϭ 0.30) series. Thus, in fect of ortho-substituents would be that of hindering such a
view of the constant steric effects, it appears firmly rotation.[
2b,7c]
Alternatively ortho substituents may favour
established that the presence of electron-withdrawing sub- the concerted (stereoselective) oxygen insertion by preven-
stituents significantly enhances the stereoselectivity of the ting the interaction of the alkene with the π-electron system
epoxidation.
of the porphyrin. This interaction would stabilise the deve-
The epoxidation of olefins catalysed by manganese por- lopment of the positive charge on the alkene and promote
phyrins is generally endowed with a low stereoselectivity, a nonstereoselective electrophilic mechanism involving the
Eur. J. Org. Chem. 1999, 3281Ϫ3286
3283

E. Baciocchi, A. Lapi, P. Tagliasesta et al.
FULL PAPER
Table 1. Product yields in the epoxidation of cis-stilbene with PhIO
III
catalysed by three series of increasingly halogenated Mn porphy-
rins
Entry[a]
Porphyrin
cis-epoxide trans-epoxide cis/trans[c]
b] [b]
yield (%)[
yield (%)
1
2
3
4
5
6
7
8
9
MnTPP
22
33
48
49
57
57
12
9.0
50
52
76
78
39
0.6
0.8
1.9
2.5
3.7
4.1
8.0
4.7
8.8
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
2
3
4
6
7
8
40
25
20
16
14
1.6
1.9
5.7
3.1
3.3
1.3
MnTDMPP
MnTDMCl
MnTDMCl
MnTDCPP
8
PPCl
PPCl
4
8
1
1
1
0
1
2
8
17
23
61
MnTDCPPBr
4
[
a]
Reactions were carried out in dichloromethane at 25°C with a
[b]
molar ratio Substrate/Oxidant/Porphyrin ϭ 50:5:1. Ϫ Yields are
referred to the oxidant; errors, based on duplicated experiments,
[
c]
are ± 5%. Ϫ cis/trans epoxide molar ratio.
formation of a carbocationic intermediate.[8a] However, ac-
cording to more recent suggestions, the low stereoselectivity
might also arise from a contamination of the Mn -oxo
Figure 1. Correlation diagram of the logarithm of the cis/trans epo-
V
p
xides ratio vs. the sum of HammettЈs σ for the epoxidation of cis-
complex by a nonstereoselective oxo-manganese(IV) species stilbene catalysed by a series of progressively β-halogenated
IV
[8][13]
MnTPPCl (filled circles) and MnTDMPPCl (open circles)
(
i.e. PMn ϭO)
and any beneficial effect from axial
ligands would be that of preventing the conversion of
PMn ϭO to PMn ϭO.
V
IV
[13b]
reoselective electrophilic attack of the oxygen to the alkene,
Against this background we can now try to rationalise to give a carbocationic intermediate (Scheme 1). As already
the observed regular enhancement of the stereoselectivity mentioned, the competition between these two pathways
on increasing the number of electron-withdrawing (EWG) may depend upon the strength of the interaction of the al-
substituents at the pyrrolyl β-positions. Certainly, it seems kene with the π-electron system of the porphyrin, which fa-
very difficult to attribute some share of this role to the pre- vours the non stereoselective path b.[ The presence of
8a]
IV
sence of a nonstereoselective PMn ϭO contaminant, un- EWG substituents may reduce the electron density of the
IV
less we suggest that the amount of PMn ϭO regularly dec- π-system of porphyrins, and make the carbocation pathway
reases on increasing the number of β-halogens atoms. This less favoured than the concerted insertion. Thus, increasing
IV
appears unlikely as PMn ϭO should be formed by a one- the number of electron-withdrawing substituents on the
V
[13]
electron reduction of the PMn ϭO species (Equation 3),
pyrrole rings ought to increasingly disfavour the formation
and therefore its formation should be made easier the gre- of the carbocation, and the concerted stereoselective path-
ater the number of EWG halogens, as they accordingly raise way should progressively predominate.[ Consistent with
the reduction potential of the porphyrin.[ Therefore, the this view is the reduced sensitivity of the ortho-dimethoxy-
16]
14]
IV
amount of the nonstereoselective PMn ϭO complex substituted MnTDMPPCl porphyrin to the electronic ef-
should increase along the series, and the stereoselectivity fects of the β-halogen substituents, relative to the simple
decrease, which is contrary to what is actually observed.
MnTPPCl, which is evident from the above-reported ρ va-
lues. The bulky ortho-methoxy groups may hinder the inte-
raction between the alkene and the π-electron system of the
porphyrin, thus lowering the effect of the β-substituents.
Alternatively, one might assume that the epoxidation pro-
Another strong indication against a significant role for ceeds mainly by the electrophilic attack of the oxygen to the
IV
PMn ϭO is the absence of any detectable amount of car- alkene to give the carbocation intermediate (Scheme 1, path
bonyl compounds in the reaction mixture, which would be b), and that the cis/trans stereoselectivity is determined by
expected from the reaction of this species under aerobic the competition of two processes: the rotation around the
[
8b,10b]
[17]
conditions.
It should be noted here that, in the pre- alkene CϪC bond and the epoxide ring closure.
The ob-
V
sence of an olefin substrate, the Mn oxene reacts directly served regular increase in stereoselectivity upon increasing
to give the epoxide, rather than undergoing a preliminary the number of β electron-withdrawing substituents might be
IV
[15]
reduction to the Mn -oxo porphyrin.
A more reasona- explained by assuming that this electronic effect gradually
ble explanation would be that the stereochemistry of the speeds up the ring-closure, while the rate of the CϪC bond
process is determined by a competition between the rate of rotation should reasonably remain unaffected throughout
a stereoselective (concerted) oxygen insertion and a nonste- the same series of porphyrins. Accordingly, the cis/trans
3284
Eur. J. Org. Chem. 1999, 3281Ϫ3286

Epoxidation Reactions Catalysed by Manganese Porphyrins
FULL PAPER
epoxide ratio should become larger, as it is, in fact, obser- (480 mg, 2.64 mmol) was added to the solution, which was then
maintained at 90°C for 24 h in air, while being protected from moi-
ved.
sture with a CaCl
using silica gel TLC plates, eluting with CHCl
2
valve. The reaction was continuously monitored
, and checked by
Finally the data in Table 1 also clearly confirm that the
effect upon the epoxidation stereoselectivity of substituents
at the ortho-positions of the meso-phenyl rings is mainly of
steric origin. Accordingly, both MnTDMPPCl (ortho-meth-
oxy phenyl substituents) and MnTDCPPCl (ortho-chloro
phenyl substituents) exhibit a higher stereoselectivity than
3
FAB-MS. The solvent was evaporated under vacuum and the resi-
due was purified twice on a silica gel column by elution with
CHCl
rated and the residue recrystallised from CHCl
ving 107 mg of product (39% yield). UV/Vis (CHCl
3
.The fraction containing the desired porphyrins was evapo-
/hexane (1:3), gi-
): λmax ϭ 428,
3
3
1
MnTPPCl (no ortho-substituents in the phenyl rings), in 523, 600, 656. Ϫ H NMR (400 MHz, CDCl , TMS): δ ϭ 8.7 (s, 4
3
spite of the fact that methoxy and chloro exert opposite H β-H pyrrole), (m, 8 H, phenyl), 7.65Ϫ7.83 (m, 12 H, phenyl). Ϫ
electronic effects.[
18]
ϩ
However, the stereoselectivity of FAB-MS: m/z ϭ 1206 [M] . Ϫ C44
H 1.50, N 4.65; found C 43.79, H 1.44, N 4.97.
is probably due to the superposition of steric and electronic Manganese(III) Derivatives: The manganese derivatives of
4 8 4
H18Br Cl N (1206): C 43.83,
MnTDCPPCl is greater than that of MnTDMPPCl, which
effects, since chlorine is an electron-withdrawing group.
H
H
2
(TPPBr
x
),
(x
ϭ
1Ϫ8), (TDMPP),
H
2
2
H (TDCPP),
2
(TDMCl
8
PPCl
4
) and H
2
(TDCPPBr ) were synthesised following
4
25]
[
a method described in the literature.
free porphyrin base (100 mg) was dissolved in degassed DMF
50 mL) and MnCl ·4H O (100 mg) was added under nitrogen. The
solution was refluxed for 4 h under nitrogen and the solvent evapo-
2 5
P O . 1,2-Dichloroethane and 1,2-dibromoethane were dried over rated under vacuum. The residue was taken up in CHCl , washed
In a typical synthesis the
Experimental Section
(
2
2
2 2
General: Dry CH Cl was obtained by refluxing and distilling over
3
anhydrous K
2
CO
3
. Methanol (HPLC grade) was purchased from
with water, and then dried over NaCl. The solvent was evaporated
and the residue purified by column chromatography on silica gel
Carlo Erba. cis-Stilbene was purchased from Aldrich and used as
received. cis-Stilbene oxide and trans-stilbene oxide were purchased
eluting with CHCl to isolate the starting free base and then with
3
from Fluka. Iodosylbenzene was prepared by basic hydrolysis of
CHCl /CH OH (97:3). The fraction containing the metal complex
3
3
iodosylbenzene diacetate as previously reported,[
19]
stored at
was evaporated and the residue crystallised from a CHCl /hexane
3
Ϫ20°C and titrated every 3 months. NMR spectra were performed
(1:3) solvent mixture. Yields were between 60 and 70%. The UV
on a Bruker AM 400. HPLC analysis were performed on a HP-
and FAB-mass spectra and the elemental analysis of the new com-
pounds are reported in Table 2, Table 3 and Table 4, respectively.
1050 equipped with a LC-18-DB column and connected to a HP-
1047 UV/Vis detector. UV/Vis spectra were performed on a HP-
8453 diode array spectrophotometer. FAB mass spectra were per-
3
Table 2. UV characterisation of the new compounds (in CHCl )
formed with a multiple quadrupole instrument (VG Quattro); sam-
ples were bombarded with a 8Ϫ10 kV caesium beam from a cae-
sium gun anode.
Compound
λ (nm)
ϩ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
[
MnTPPBr
2
] Cl
377
378
380
389
384
420
426
380
482
487
477
501
499
501
498
475
624
ϩ
Syntheses of the Porphyrins:The meso-tetraphenylporphyrin β-bro-
[MnTPPBr ] Cl
593
580
652
654
660
646
576
3
ϩ
minated bases, H
ding to literature procedures.
and H (TDCPP) were synthesised as reported in the litera-
2
(TPPBr
x
), (x ϭ 3, 4, 7, 8) were synthesised accor-
[MnTPPBr
[MnTPPBr
[
[
4
6
7
8
] Cl
ϩ
[20]
] Cl
2 2 8 8
H (TDMPP), H (TDMCl PPCl )
ϩ
MnTPPBr
MnTPPBr
] Cl
ϩ
2
] Cl
ture.[
NO TPP) followed by denitration and rearomatisation of the chlo-
rin. H
NO TPP) followed by denitration. Both
(TPPBr ) have been characterised by X-ray crystallography. De-
tails of the synthesis of (TPPBr )H and (TPPBr )H are given el-
21Ϫ23]
H
2
(TPPBr
) was prepared by bromination of H
(2- [MnTDMCl PPCl ] Cl
ϩ
Ϫ
2
2
8
4
ϩ
Ϫ
4
[MnTDCPPBr ] Cl
2
II
2
(TPPBr
6
) was prepared by perbromination of Cu (2-
(TPPBr and
2
H
2
2
)
H
2
6
Table 3. The FAB-Mass spectra of the new compounds
2
2
6
2
[
24]
sewhere.
(TDMPPCl12): H
in dry 1,2-dichloroethane (100 mL) and N-chlorosuccinimide
Compound
calculated
found
H
2
2
(TDMPP) (200 mg, 0.23 mmol) was dissolved
ϩ
ϩ
ϩ
ϩ
ϩ
ϩ
[
MnTPPBr
MnTPPBr
2
3
]
]
825.4
904.3
983.2
1141.0
1219.9
1298.8
1321.2
1258.8
827
904
985
1142
1220
1297
1322
1260
[
(436 mg, 3.22 mmol) was added to the solution, which was then [MnTPPBr ]
4
refluxed for 4 h in air, while being protected from moisture with a
CaCl valve. The reaction was continuously monitored on silica gel
TLC plates, eluting with CHCl , and checked by FAB-MS. The
solvent was evaporated under vacuum and the residue was purified
twice on a silica gel column by elution with CHCl . The fraction
containing the desired porphyrins was evaporated and the residue
was recrystallised from CHCl /hexane (1:3), giving 124 mg of pro-
duct (42% yield). UV/Vis (CHCl ): λmax ϭ 428, 523, 596, 651. Ϫ
, TMS): δ ϭ 8.8 (s, 4 H, β-H pyrrole),
.83 (s, 4 H, p-phenyl), 3.2 (s, 24 H, OCH ). Ϫ FAB-MS: m/z ϭ
(1268): C 49.25, H 2.70, N
[MnTPPBr
6
7
8
]
]
]
[
[
[
MnTPPBr
2
MnTPPBr
3
MnTDMCl PPCl ]
ϩ
8 4
ϩ
[MnTDCPPBr ]
4
3
2 8 8
The manganese derivative of H (TDMCl PPCl ) was obtained
3
using a method reported by us,[ and modified as follows: the
22]
3
porphyrin base (100 mg) was dissolved in degassed propionic acid
1
H NMR (400 MHz, CDCl
3
·
(
50 mL) and then MnCl
2
4H
2
O (100 mg) was added under nitro-
7
1
4
3
gen. The solution was refluxed for 4 h and then the solvent was
evaporated under vacuum. The residue was taken up in CHCl
washed with 10% NaHCO solution, water and then dried over
anhydrous Na SO . The solvent was evaporated and the residue
was chromatographed on silica gel with CHCl
ϩ
266 [M Ϫ 2H] . Ϫ C52
4 8
H34Cl12N O
3
,
.42; found: C 49.29, H 2.60, N 4.47.
3
H
2
(TDCPPBr ): H (TDCPP) (200 mg, 0.22 mmol) was dissolved
4
2
2
4
in dry 1,2-dibromoethane (100 mL) and N-bromosuccinimide
3
to elute the starting
3285
Eur. J. Org. Chem. 1999, 3281Ϫ3286

E. Baciocchi, A. Lapi, P. Tagliasesta et al.
FULL PAPER
tanari, S. Quici, J. Chem. Soc., Chem Commun. 1981, 1206. Ϫ
Table 4. Elemental analysis of the new compounds (calcd./found)
[3d]
P. Battioni, J. P. Renaud, J. F. Bartoli, M. Reina-Artiles, M.
Fort, D. Mansuy, J. Am. Chem. Soc. 1988, 110, 8462.
D. Mansuy, P. Battioni, J. P. Renaud, J. Chem. Soc., Chem.
Commun. 1984, 1255.
[
[
[
4]
Compound
C
H
N
5] [5a]
ϩ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
L. C. Yuan, T.C. Bruice, J. Am. Chem. Soc. 1986, 108, 1643.
[
[
[
[
[
[
[
[
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
MnTPPBr
2
3
4
6
7
8
] Cl
61.39/61.08 3.04/3.20
56.23/55.99 2.68/2.55
51.88/51.66 2.37/2.30
44.92/45.10 1.88/1.92
42.10/42.22 1.69/1.77
39.61/39.94 1.51/1.60
46.04/45.87 2.38/2.44
40.83/42.12 1.25/1.33
6.51/6.67
5.96/6.08
5.50/5.40
4.76/4.55
4.46/4.59
4.20/4.12
4.13/4.15
4.33/4.29
Ϫ [ L. C. Yuan, T.C. Bruice, Inorg. Chem. 1985, 24, 987.
5b]
ϩ
] Cl
6] [6a]
ϩ
B. de Poorter, B. Meunier, Nouv. J. Chim. 1985, 9, 393. Ϫ
B. de Poorter, B. Meunier, J. Chem. Soc., Perkin Trans. 2
] Cl
[6b]
ϩ
] Cl
ϩ
1985, 1735.
] Cl
[
7] [7a]
[7b]
ϩ
E. Guilmet, B. Meunier, Tetrahedron Lett. 1980, 4449. Ϫ
B. Meunier, E. Guilmet, M. E. De Carvalho, R. Poilblanc, J.
] Cl
ϩ
Ϫ
MnTDMCl PPCl ] Cl
8 4
[7c]
ϩ Ϫ
Am. Chem. Soc. 1984, 106, 6668. Ϫ
J. Chem. Soc., Perkin Trans. 2 1984, 1967. Ϫ
Made, R. J. M. Nolte, J. Mol. Catal. 1984, 26, 333. Ϫ
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O. Bortolini, B. Meunier,
4
MnTDCPPBr ] Cl
[7d]
A. W. Van der
[
7e]
F.
[7f]
5
0, 4888. Ϫ
A. W. Van der Made, J. M. C. Bax, R. J. M.
free base and then with CHCl
ning the metal complex was evaporated and the residue was recry-
stallised from a CHCl /hexane (1:3) solvent mixture (yield 75%).
3
/CH
3
OH (97:3). The fraction contai-
Nolte, W. Drenth, Recl. Trav. Chim. Pays-Bas 1989, 108, 185.
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8] [8a]
7g]
3
[
[
D. Ostovic, T. C. Bruice, Acc. Chem. Res. 1992, 25, 314Ϫ320.
Ϫ [ R. D. Arasasingham, G. X. He, T. C. Bruice, J. Am.
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8b]
Oxidation of cis-Stilbene with PhIO: The reactions were carried out
in a thermostated bath at 25°C. PhIO (4.4 mg, 20 mmol) was added
to a stirred solution of the metalloporphyrin (4 mmol) and cis-stil-
bene (37.2 mL, 200 mmol) in dry CH
tube. The mixture was stirred for 5 min at room temperature; the
9]
E. Baciocchi, T. Boschi, C. Galli, A. Lapi, P. Tagliatesta, Tetra-
hedron 1997, 53, 4497Ϫ4502.
[10]
2
Cl
2
(1 mL) in a 5 mL Schlenk
E. Baciocchi, T. Boschi, L. Cassioli, C. Galli, A. Lapi, P. Tagli-
atesta, Tetrahedron Lett. 1997, 38, 7283Ϫ7286.
11] [11a]
[
E. Guilmet, B. Meunier, J. Mol. Catal. 1984, 23, 115. Ϫ
E. Guilmet, B. Meunier, Nouv. J. Chim. 1982, 6, 511.
internal standard (0.250 mL, 0.1  acetophenone in methanol) was
then added, followed by the addition of sodium metabisulfite
[
11b]
[12]
It should be noted that a similar suggestion has been previously
made by considering the formation of a radical intermedi-
ate.
13] [13a]
Ϫ2
(2 mL, 5 ϫ 10 ) in order to quench the residual oxidant. The
[
2b,7c]
solvent was removed under an argon stream, the residue diluted in
2 mL of methanol/water (70:30), and then analysed by HPLC on
[
J. T. Groves, M. K. Stern, J. Am. Chem. Soc. 1987, 109,
1
[
13b]
3
1
812Ϫ3814. Ϫ
J. T. Groves, M. K. Stern, J. Am. Chem. Soc.
an LC-18-DB column with methanol/water (70:30) as eluent. Yields
were determined using a calibration curve of response factors vs.
peak area. Product characterisation was performed by comparison
with authentic specimens using HPLC and 1H NMR spectros-
copic analysis.
988, 110, 8628Ϫ8638.
[
[
14]
15]
K. M. Kadish, P. Tagliatesta, unpublished data.
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[
[
16]
Actually, we cannot exclude that steric hindrance at the ortho-
positions could also disfavour the concerted oxygen insertion
to some extent, as suggested by a referee.
17] [17a]
This suggestion was also made by Kochi and co-wor-
kers[
17b]
in order to rationalise the effect of electron-withdra-
Acknowledgments
wing substituents upon the stereoselectivity of epoxidation reac-
III
tions catalysed by salen Mn complexes. In this case, however,
[
17b]
Financial support from the Ministry of University and the Scien-
tific Research (MURST) and the National Institute of Health (HC-
the formation of a radical intermediate was proposed. Ϫ
K. Srinivasan, P. Michaud, J. K. Kochi, J. Am. Chem. Soc. 1986,
08, 2309Ϫ2320.
1
22252) is gratefully acknowledged. We also thank Mr. Giuseppe
[18]
Although the methoxy groups cannot conjugate with the mac-
rocycle, due to the perpendicularity of the meso-aryl groups,
they also exert an electron donating effect, as clearly shown by
DЈ Arcangelo for mass spectral measurements and Mr. Alessandro
Leoni for technical support.
III
comparing the oxidation potentials of Fe TPPCl,
III
III
Fe TDMPPCl and Fe TDCPPCl. For a more detailed discus-
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