Chemoselective epoxidation of dienes using polymer-supported manganese porphyrin catalysts

Article abstract of DOI:10.1016/j.tet.2004.05.026

Manganese porphyrin catalysts supported on different polymer resins were assessed in the selective epoxidation of three dienes. The recyclability of the catalysts was examined.

Full text of DOI:10.1016/j.tet.2004.05.026

Tetrahedron 60 (2004) 5913–5918
Chemoselective epoxidation of dienes using polymer-supported
manganese porphyrin catalysts
a
Emile Brul e´ , Yolanda R. de Miguel and King Kuok (Mimi) Hii
b
c,*
a
Department of Chemistry, King’s College London, Strand, London WC2R 2LS, UK
LABEIN Technological Centre, Cuesta de Olabeaga, 16, Bilbao 48013, Bizkaia, Spain
b
c
Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, UK
Received 15 March 2004; revised 21 April 2004; accepted 13 May 2004
Available online 5 June 2004
Abstract—Manganese porphyrin catalysts supported on different polymer resins were assessed in the selective epoxidation of three dienes.
The recyclability of the catalysts was examined.
q 2004 Elsevier Ltd. All rights reserved.
1
. Introduction
which we attributed to restricted mobility of the reactive
metalloporphyrin moiety, imposed by the short spacer
between the support and the catalyst. On the other hand, the
flexible PEG spacer afforded by the Argogel-supported
catalyst was found to be unstable, and extensive leaching
was observed during catalytic recycling.
Heterogeneously-supported manganese porphyrins have
been widely investigated as catalysts in the epoxidation of
alkenes. A survey of the literature revealed that these
catalysts are most efficient in the epoxidation of conjugated
alkene substrates (most commonly styrene and indene),
followed by cyclic alkenes (cyclohexene, cyclooctene). In
contrast, they are less successful in catalysing the epoxi-
dation of less electron-rich terminal olefins such as dodec-1-
ene and 1-hexene. Hence, in principle, supported manga-
nese porphyrins may show good chemoselectivity in
epoxidation of substrates containing more than one type
of alkene. Although limonene has been widely chosen as
a substrate to probe the steric hindrance of synthetic
In this paper, we report the preparation of two new catalysts
from commercially available polymer supports with more
robust and longer linkers. These were compared with the
Merrifield-supported catalyst in the chemoselective epoxi-
dation of three types of dienes. Recyclability and selectivity
of these catalysts were compared and contrasted.
1
–3
metalloporphyrins,
rarely examined as substrates.
other dienes and polyolefins are
2. Results and discussion
2.1. Catalyst preparation
Previously, we immobilised the manganese complex of
5
-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin 1 on
4
Following previously reported procedures, porphyrin 1 was
tethered to commercially available Merrifield, bromo-Wang
and carboxy bromo-Wang resins via an ether linkage,
by treating the chlorinated/brominated resin beads with
5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin 1, in the
presence of K CO at 80 8C for three days (Scheme 1)—
Irori Kane reactors were used to protect the polymer beads
from structural damage that may arise through prolonged
mechanical stirring. Functionalized polymer supports 2–4
were thus obtained as dark purple beads. The reactions were
monitored by following the displacement of the halide (%Cl
or %Br analysis), whereas the final yields of the reactions
were calculated from %N content. With the exception of 3,
yields were typically high. Subsequent metallation of the
Merrifield and Argogel resins and examined their subse-
quent catalytic activity in the epoxidation of a wide range of
mono-alkenes (styrene, stilbene, methylstyrene, cyclo-
octene, cyclohexene, norbornene, hex-1-ene and dodec-1-
4
ene). During the study, we discovered important com-
2
3
patibility issues between the nature of the linker group and
catalyst activity and stability. The Merrifield resin-sup-
ported catalyst has been demonstrated to be robust, and may
be subjected to several successive catalytic reactions
without leaching. However, rates of turnover were slow,
Keywords: Manganese porphyrins; Alkene epoxidation; Chemoselectivity;
Polymer-supported catalysis.
*
Corresponding author. Tel.: þ44-207-594-1142; fax: þ44-207-594-5804;
e-mail address: mimi.hii@imperial.ac.uk
supported porphyrins 2–4 with MnCl at high temperature
2
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.05.026

5914
E. Brul e´ et al. / Tetrahedron 60 (2004) 5913–5918
Scheme 2. General reaction conditions employed for catalytic epoxidation.
2.3. Epoxidation of 7-methyl-1,6-octadiene (Table 1,
entries 1–3)
From previous studies, we expect the more electron-rich
double bond to be epoxidised more favourably over the
terminal alkene. Indeed, this proved to be the case (Table 1,
entries 1–3). Tri-substituted epoxide was obtained exclu-
sively in high yields (91–100%) in all cases.
Table 1. Selective epoxidation of 7-methyl-1,6-octadiene and
cyclooctadiene
a
Entry
Substrate
Product
Catalyst T/h Yield/%
1
5
33
94
2
3
6
7
33
24
91
100
4
5
36
76
b
5
6
7
8
42
36
36
36
68
63
6
7
c
76
b,d
51
Reaction conditions outlined in Scheme 2.
a
GC yields with respect to starting material.
Carried out with recovered catalyst.
Bis-epoxide (9%) was observed after 6 h.
Formation of bis-epoxide was not observed in the second cycle.
b
c
Scheme 1. Preparation of manganese porphyrins supported on commer-
cially available resins: (a) Merrifield (2)/bromo-Wang (3)/carboxy bromo-
d
Wang (4), K
165 8C, 2.5 h.
2 3 2
CO , DMF, 80 8C, 3 days; (b) MnCl (50 equiv.), DMF,
Monitoring the progress of the reactions by GC (Fig. 2), the
presence of an induction period of 4 h was detected before
the Merrifield-supported catalyst 5 commenced its turnover.
Beyond this, its activity was roughly comparable to that of
Wang-supported 6, which appeared to be the least robust, as
its turnover ceased after 30 h. In contrast, the carboxy-Wang
supported catalyst 7 gave the highest and quickest turnover,
achieving 97% conversion within 12 h.
afforded the four supported manganese porphyrins 5, 6
and 7, respectively. Complexation was indicated by the
disappearance of the n(N–H) absorption band in the single-
bead transmittance FTIR spectrum. The catalytic loading of
the polymer beads, verified by %Mn analysis (ICP-AES),
was between 0.26 and 0.53 mmol/g.
2
.2. Catalytic epoxidation of dienes and recycling studies
Three diene substrates were chosen in the catalytic study
Fig. 1): 1,5-cyclooctadiene, 7-methyl-1,6-octadiene and
limonene. Using previously optimised conditions (Scheme
(
2
4
), the epoxidation reactions were performed employing
mol% of catalyst in the presence of imidazole, with
sodium periodate as the oxidant.
Figure 2. Rate of conversion of 7-methyl-1,6-octadiene catalysed by 5 (V),
6 (B) and 7 (X).
Figure 1. Chosen substrates for the chemoselective epoxidation.

E. Brul e´ et al. / Tetrahedron 60 (2004) 5913–5918
5915
2
.4. Epoxidation of cyclooctadiene (Table 1, entries 4–8)
(4-styrylmethyl)pyridinium chloride, which gave an overall
yield of 91% in a 3:1 ratio.
1
,5-Cyclooctadiene contains two identical double bonds,
from which it is possible to obtain mono- and bis-epoxide
products. Most oxidants tend to reduce the diene to a
mixture of products, including diols and ketones. Whilst the
peroxide-based oxidants (such as that catalysed by MTO)
Generally speaking, the covalently-bound manganese
porphyrins 5–7 gave fairly comparable results, with ratios
greater than 2:1 in most cases (Table 2). The selectivity of
the process appeared to be dependent on the amount of
oxidant employed. For catalysts 5 and 6, a 2:1 oxidant-to-
alkene ratio seemed to be optimal for both activity and
selectivity (entries 1–3 and 4–6). In contrast, carboxy-
Wang supported manganese porphyrin 7 appeared to be
more robust, since the amount of oxidant did not affect its
activity significantly. The highest chemoselectivity of 2.7:1
was obtained with catalysts 6 and 7 (Entries 5 and 7) with
yields of between 66 and 68%. A small amount of the bis-
epoxide was formed during the catalytic reaction employing
catalyst 7.
5
,6
tend to yield the bis-epoxide,
useful mono-epoxide may be obtained by the epoxidation of
the synthetically more
7
,8
the diene by peracids, with yields of between 40 and 72%.
A prior study by Jørgensen deployed an iron(II) phthalo-
cyanine/iodosylbenzene system in the epoxidation of
1
,5-cyclooctadiene, which led to the formation of a mixture
9
of oxidised products in low yields. In light of this, it was
somewhat surprising to find that highly selective mono-
epoxidation may be achieved using supported manganese
porphyrins. Catalysts 5 and 6 gave exclusive formation of
the mono-epoxide (Table 1, entries 4–6), whereas a small
formation of bis-epoxide was observed with catalyst 7
Table 2. Epoxidation of (R)-limonene
a
Entry Catalyst Oxidant:diene ratio Time (h) Yields 8:9 Ratio 8:9
(entry 7). In the latter case, the presence of the bis-epoxide
was detected after 6 h.
1
2
3
4
5
6
7
8
9
5
5
5
6
6
6
7
7
7
3:1
2:1
1:1
3:1
2:1
1:1
3:1
2:1
1:1
24
24
24
24
24
24
24
24
24
12:9
1.3:1
2.3:1
1.9:1
1.8:1
2.7:1
1.8:1
2.7:1
2.3:1
1.9:1
52:23
38:20
43:24
48:18
42:24
50:18
Since catalysts 5 and 7 exhibited similar turnovers (entries 4
and 7), these were recovered by filtration. UV analysis of the
reaction mixture (filtrate) did not exhibit any discernable
absorption peaks that may be attributed to free or
manganese porphyrin, thus indicating that no leaching of
the catalyst occurred; the %Mn content of the recovered
beads was also similar to the original value. The recovered
catalysts were thus subjected to a second catalytic run. Both
of the recycled catalysts showed much reduced catalytic
activity (Table 1, entries 4 vs. 5, 7 vs. 8). In the reaction
catalysed by recovered carboxy-Wang supported catalyst 7,
the slower turnover evidently suppressed the formation of
the bis-epoxide.
b
51:22
30:16
Reaction conditions as outlined in Scheme 2, except for the amount of
oxidant.
a
GC yield with respect to starting material. No diastereomeric excess was
observed in either product.
% Bis-epoxide.
b
6
Once again, catalyst 7 was found to be the most active,
compared to the other two catalysts. Under these conditions,
the maximum yield of the 1,2-epoxide was reached within
12 h, whereupon the formation of the bis-epoxide became
apparent, which coincides with the drop in yield (Fig. 3).
2
.5. Epoxidation of limonene
The monoterpene limonene contains an internal trisubsti-
tuted double bond and a disubstituted terminal double bond,
and has been widely studied as a substrate in porphyrin
epoxidation chemistry. Bis-epoxide may be obtained from
simultaneous epoxidations, and the formation of 1, 2- and/or
8
used to denote the selectivity of the process, generally taken
, 9-limonene oxides 8 and 9 (Scheme 3) was commonly
2
to reflect the steric environment of the metalloporphyrin.
Figure 3. Rate of formation of 1,2-limonene epoxide 8 with catalyst 7.
Scheme 3. Epoxidation of limonene.
The diastereomeric ratio of products 8 and 9 was found to be
1:1 in each case, i.e. the inherent chirality of the starting
material did not induce any facial selectivity in the
epoxidation of either of the prochiral alkene moieties.
Immobilisation of manganese porphyrin catalysts typically
leads to a catastrophic effect on their selectivity towards the
epoxidation of limonene, lowering the ratio from 9:1 in
favour of the 1,2-epoxide 8, to typically less than 1.7:1.
The most selective supported manganese catalyst reported
to date is a Mn(III) porphyrin ionically bound to poly-
1
0
Finally, recyclability of the immobilised catalysts was
tested by subjecting 5 and 7 to four successive catalytic

5916
E. Brul e´ et al. / Tetrahedron 60 (2004) 5913–5918
Table 3. Recycling catalysts 5 and 7 for the epoxidation of limonene
purchased from Aldrich, Avocado, BDH, Fluka or
Lancaster, and were used as received, unless otherwise
stated. Manganese dichloride was purchased from Aldrich
a
Entry
Catalyst (mol%)
Run
Time (h)
Yields 8:9
Ratio 8:9
(
99.999% purity).
1
2
3
4
5
6
7
8
9
5 (4)
5 (4)
5 (8)
5 (8)
5 (8)
7 (4)
7 (4)
7 (8)
7 (8)
7 (8)
1
2
2
3
4
1
2
2
3
4
24
36
24
36
36
24
24
36
36
36
52:23
40:20
54:19
60:26
48:17
2.3:1
2.0:1
2.8:1
2.3:1
2.8:1
2.3:1
2.1:1
2.0:1
2.2:1
2.4:1
The MacroKane used in the immobilisation of porphyrins
is made of polypropylene with an internal volume of
2.4 mL, purchased from Irori Europe Limited (Tarporley
Business Centre, Cheshire CW6 9UT, UK).
b
51:22 (6 )
48:23
50:20
57:26
53:22
4.2. Instrumentation
10
Infrared spectra were recorded on a Perkin Elmer Spectrum
One spectrometer. Spectra of porphyrin and metallo-
porphyrin samples were recorded as solutions in CCl and
Oxidant/limonene (2:1).
a
GC yields with respect to limonene. No diastereomeric excess was
observed in either product.
Bis-epoxide.
b
4
CHCl , respectively, within a sealed cell with a path length
3
of 0.1 mm with NaCl windows. Single bead FT-IR spectra
(transmittance) were recorded with a beam-condensing
accessory (BCA), using a diamond compressor to flatten the
bead. UV spectra were recorded on a Perkin Elmer Lambda
reactions (Table 3). For the Merrifield-supported porphyrin
, the recovered catalyst became increasingly less active—
requiring either longer reaction times (entries 1 vs. 2 and 2
vs. 3) or higher catalytic loading (entries 2 vs. 3) to achieve
comparable yields. In sharp contrast, catalyst 7 may be
reused with no significant loss in activity in all four cycles
5
18 spectrometer and performed in a thermostated (25 8C)
cell of 10 mm path length. The samples were analysed in
dichloromethane or in a solution of 20% piperidine in DMF.
(entries 8–10). Also, competitive formation of the bis-
epoxide was not observed after the first catalytic run.
1
13
H and C NMR spectra were recorded using Bruker AM
60 and AVANCE 400 spectrometers NMR spectra of
3
Remarkably, the selectivity was broadly maintained
between the different cycles, and both catalysts may be
subjected to four consecutive runs without significant
changes in the $2:1 ratio.
polymers were recorded using a Bruker AVANCE 400
spectrometer fitted with a special HR-MAS probe, with the
resin placed in a 4 mm rotor. Chemical shifts were recorded
in parts per million (d: ppm) referenced to TMS (d: 0) as an
internal standard.
3. Conclusion
Mass spectra (MS) and high-resolution mass spectra
(HRMS) were recorded using the EPSRC MS Services at
To conclude, we have prepared and examined the
comparative catalytic activity, selectivity and reusability
of three polymer-supported manganese porphyrins 5–7.
Good chemoselectivity was observed in the epoxidation of
three cyclic and acyclic dienes. Catalyst 7 consistently
excelled in its catalytic activity and selectivity. As the
length of the linkers was identical between catalysts 6 and 7,
we attribute its superiority to the higher reactive surface area
of its beads (200–400 mesh), compared to the other two
catalysts (100–200 mesh). All three resins may be recycled
with varying successes, but encouragingly, no decline in
selectivity was observed.
University of Swansea, Wales or the Mass Spectrometry
Service within the Department of Chemistry, King’s College.
FAB MS was run on a KRATOS ‘MS89OMS’ spectrometer.
ES MS was run on a Micromass ‘Q-TOF’. GC-MS spectra
were recorded on a Varian Saturn 220 spectrometer equipped
with an autosampler using a CP-Sil 8CB column.
4.3. Services
Elemental analyses were carried out by the Elemental
Analysis Services at the University of North London (C, H,
N) or University College London (halogens). %Mn was
determined using inductively coupled plasma atomic
emission spectroscopy (ICP-AES) by Medac Ltd.
This work has led us to develop a new class of supported
manganese porphyrin catalysts with improved selectivity
and catalyst activity, which will be reported in due course.
4
.3.1. Synthesis of 4-(10,15,20-triphenyl-porphyrin-5-yl)-
1
1
phenol 1. To a refluxing solution of propionic acid
500 mL), were added pyrrole (13.9 mL, 0.2 mol), benz-
aldehyde (15.3 mL, 0.15 mol) and p-hydroxybenzaldehyde
6.1 g, 0.05 mol). The reaction mixture was stirred at reflux
(
4. Experimental
(
4
.1. Materials
for 40 min. Propionic acid was then removed under vacuum.
The black residue was first purified by column chroma-
tography on silica, eluting with chloroform. The fraction
All resins were purchased from Novabiochem Wang bromo
polystyrene and Merrifield resins are 1% cross-linked with
bead sizes between 100 and 200 mesh. Brominated Wang
resin was 1% cross-linked with bead sizes between 200 and
containing TPPOH was suspended in CH
washed with CH OH to give 1.6 g (2.53 mmol, 5%) of the
desired porphyrin as a purple solid, which was used in the
subsequent steps without further purification. R 0.36
, 20 8C):
-porphyrin), 8.23 (d, 6H,
OH, filtered and
3
3
4
00 mesh. Anhydrous DMF was purchased from Aldrich
f
1
and dichloromethane was freshly distilled from CaH2
under nitrogen. Commercially available chemicals were
(CHCl
3
); mp (220 8C. H NMR (360 MHz, CDCl
and H
3
d 8.91–8.86 (m, 8H, H
7
8

E. Brul e´ et al. / Tetrahedron 60 (2004) 5913–5918
5917
3
3
J¼7.6 Hz; H -Ph), 8.10 (d, 2H, J¼8.3 Hz; H -Ar), 7.80–
temperature, the beads were transferred to a sintered tube
and washed with CH Cl (5 mL£10) and HPLC-grade
2
2
3
7
H -Ar), 5.01 (s, 1H; OH), 22.76 (s, 2H; NH). C NMR
.75 (m, 9H; H -Ph, H -Ph), 7.22 (d, 2H, J¼8.3 Hz;
3
4
2
2
1
3
pentane (5 mL£10). The resin was dried under vacuum at
50 8C for 2 h to give dark purple beads.
3
(
90.5 MHz, CDCl , 20 8C): d 155.8, 142.6, 136.1, 135.0,
3
1
32.0–131.0, 128.1, 127.1, 120.5, 114.1. UV–Vis
CH Cl ): l (1): 415 (103 557), 515 (8461), 550
(
(
4.5.1. Merrifield 4-(10,15,20-triphenyl-porphyrin-5-yl)-
phenol manganese chloride 5. 100% metallation, based
on Mn%; elemental analysis found for a loading of
0.52 mmol g2 , Mn 2.86%.
2
2
max/nm
3680), 589 (2152), 647 (2077) nm. HR-MS (FAB) (m/z):
þ
21
calcd for 630.2420, found: 630.2412 [M ]. IR (CCl , cm
4
)
1
nNH 3313, nOH 2962.
4
.4. General procedure for the synthesis of Merrifield
4.5.2. Wang 4-(10,15,20-triphenyl-porphyrin-5-yl)-
phenol manganese chloride 6. 100% metallation, based
and Wang supported 4-(10,15,20-triphenyl-porphyrin-5-
yl)-phenols
on Mn%; elemental analysis found for a loading of
1
0
.32 mmol g2 , Mn 1.76%.
A Macro Irori Kane containing the appropriate resin
(0.45 mmol) was placed in a N purged three-necked round
bottom flask with K CO (0.34 g, 2.47 mmol), KI (0.164 g,
4.5.3. Carboxy Wang 4-(10,15,20-triphenyl-porphyrin-5-
yl)-phenol manganese chloride 7. 100% metallation, based
on Mn%; elemental analysis found for a loading of
2
2
3
0
.99 mmol) and p-4-(10,15,20-triphenyl-porphyrin-5-yl)-
0.53 mmol g2 , Mn 2.92%. FT-IR (cm ): nCO 1695.
1
21
phenol 1 (0.47 g, 0.742 mmol). Anhydrous DMF (20 mL)
was added via syringe and the mixture was magnetically
stirred at 80 8C for 3 days. After cooling to room temperature,
the beads were transferred to a sintered tube and washed
4.6. General epoxidation procedure
successively with acetone/CH OH (1:1) (5 mL£5), acetone/
In a round-bottomed flask or in a Radley’s carousel reaction
tube, the appropriate catalyst (0.01 mmol), alkene
(0.23 mmol) and axial ligand (0.1 mmol) were stirred in
3
CH OH/H O (1:1:1) (5 mL£5), acetone/CH OH (1:1)
3
2
3
(
5 mL£5), ethyl acetate (5 mL£5), CH Cl (3 mL£5) and
2
2
HPLC-grade pentane (5 mL£5). The resin was dried under
vacuum at 50 8C for 2 h to give dark purple beads.
CH CN (3.7 mL) at room temperature. In a separate flask,
3
NaIO (0.46 mmol) was dissolved in H O (1.85 mL). This
4
2
aqueous solution of NaIO was transferred to the catalytic
4
4
.4.1. Merrifield 4-(10,15,20-triphenyl-porphyrin-5-yl)-
mixture. The progress of the reaction was monitored at
regular intervals by analysing extracted aliquots by GC-MS.
The yields of epoxides were based on the starting material
consumed.
phenol 2. Yield¼93%, based on %N. After two treatments,
2
1
elemental analysis found for a loading of 0.52 mmol g Cl
1
0, N 2.90%; HR-MAS H NMR (400 MHz, CDCl , 20 8C):
3
d 8.80 (br s, 8H, H and H -porphyrin), 8.14 (br s, 6H,
7
8
H -Ph), 7.59 (br s, 9H, H -Ph, H -Ph), 7.30–6.50 (br s, PS,
4
2
3
H -Ar, H -Ar), 4.95 (br s, 2H, OCH ), 1.8 (br s, PS), 1.39 (br
3
s, PS), 22.75 (br s, 2H, NH); FT-IR (cm ): nNH 3317.
2
2
5. General procedure for the recovery and reuse of
the catalyst
2
1
4
.4.2. Wang 4-(10,15,20-triphenyl-porphyrin-5-yl)-
At the end of the epoxidation, the polymer-supported
catalyst was filtered off via a filter syringe. The filtrate was
extracted with CH Cl , washed with water, dried over
phenol 3. Yield¼37%, based on %N. After two treatments,
2
1
elemental analysis found for a loading of 0.32 mmol g Br
6
2
2
2
1
21
, N 1.76%; FT-IR (cm ): nNH 3317 cm
.
Na SO and evaporated. A small amount of the residue
2
4
obtained was dissolved in dichloromethane and analysed by
UV spectroscopy to determine if leaching of the porphyrin
from the support had occurred. The catalyst beads in the
syringe were washed with water, CH Cl and HPLC-grade
pentane and then dried under vacuum at 50 8C for 2 h. The
recovered catalyst was then subjected to another catalytic
cycle as described above.
4
5
.4.3. Carboxy-Wang 4-(10,15,20-triphenyl-porphyrin-
-yl)-phenol 4. Yield¼85%, based on %N. After one
treatment, elemental analysis found for a loading of
0
2
2
.53 mmol g2 Br 0, N 2.93%. HR-MAS H NMR
1
1
(400 MHz, CDCl , 20 8C): d 8.85 (br s, 8H, H7 and
H -porphyrin), 8.21 (br s, 6H, H -Ph), 7.71 (br s, 9H, H -Ph,
H -Ph), 7.08–6.62 (br s, PS, H -Ar, H -Ar), 5.36 (br s, 2H,
4
3
8
2
3
3
2
OCH ), 1.59–1.33 (br s, PS), 22.78 (br s, 2H, NH); FT-IR
2
2
1
(
cm ): nNH 3319, nCO 1695.
Acknowledgements
4
.5. General procedure for the synthesis of the supported
The work was conducted at the Department of Chemistry,
King’s College London. We acknowledge King’s College
London for studentship support (EB) and The Royal Society
for a Dorothy Hodgkin Fellowship (YdM).
manganese 4-(10,15,20-triphenyl-porphyrin-5-yl)-
phenols
In an oven-dried three-neck flask equipped with an overhead
stirrer and purged with N , the appropriate supported p-4-
2
(10,15,20-triphenyl-porphyrin-5-yl)-phenol (0.5 mmol) was
stirred in anhydrous DMF (30 mL). The flask was heated to
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1
2
58 8C for 5 min before manganese chloride (3.13 g,
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5918
E. Brul e´ et al. / Tetrahedron 60 (2004) 5913–5918
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