Electronic and steric effects on the stereoselectivity of cyclopropanation reactions catalysed by rhodium meso-tetraphenylporphyrins

Article abstract of DOI:10.1016/S1381-1169(02)00080-8

The rhodium meso-tetraphenylporphyrins catalysed cyclopropanation reactions of styrene, cyclohexene and norbornene by ethyldiazoacetate (EDA), have been investigated. The electronic effects of the bromine groups in the β-positions of the porphyrins can be correlated to the syn/anti ratio obtained for the reaction products in the case of styrene, showing a good linear correlation between the sum of the Hammett's σ_p and the logarithm of the obtained selectivity. A decrease in the stereochemical ratio on increasing the number of halogen atoms was observed. This is not the case for cyclohexene and norbornene which show a different behavior and do not give good linear correlation. Furthermore, when the rhodium porphyrins have bulky methoxy substituents in the ortho-positions of the phenyl rings, comparing with the simple rhodium meso-tetraphenylporphyrin, the isomeric ratios decrease dramatically for styrene whilst for the other substrates remain almost in the same range. On the contrary, using a rhodium porphyrin bearing chlorine groups in the ortho-positions of the phenyl rings, the ratios increase, reaching higher values. These observations suggest the presence of different factors involved in determining the isomeric ratios. The electron-withdrawing effects of the substituents in the β-positions seem to influence the stereoselectivity giving the less hindered isomer while the chlorine bulky substituents in the ortho-phenyl positions can compensate this trend.

Full text of DOI:10.1016/S1381-1169(02)00080-8

Journal of Molecular Catalysis A: Chemical 185 (2002) 127–133
Electronic and steric effects on the stereoselectivity of
cyclopropanation reactions catalysed by rhodium
meso-tetraphenylporphyrins
Pietro Tagliatesta*, Alessandra Pastorini
Dipartimento di Scienze e Tecnologie Chimiche, Universita’ degli Studi di Roma, Tor Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy
Received 28 November 2001; received in revised form 18 February 2002; accepted 18 February 2002
Abstract
The rhodium meso-tetraphenylporphyrins catalysed cyclopropanation reactions of styrene, cyclohexene and norbornene
by ethyldiazoacetate (EDA), have been investigated. The electronic effects of the bromine groups in the ␤-positions of
the porphyrins can be correlated to the syn/anti ratio obtained for the reaction products in the case of styrene, showing a
good linear correlation between the sum of the Hammett’s σp and the logarithm of the obtained selectivity. A decrease in
the stereochemical ratio on increasing the number of halogen atoms was observed. This is not the case for cyclohexene
and norbornene which show a different behavior and do not give good linear correlation. Furthermore, when the rhodium
porphyrins have bulky methoxy substituents in the ortho-positions of the phenyl rings, comparing with the simple rhodium
meso-tetraphenylporphyrin, the isomeric ratios decrease dramatically for styrene whilst for the other substrates remain almost
in the same range. On the contrary, using a rhodium porphyrin bearing chlorine groups in the ortho-positions of the phenyl
rings, the ratios increase, reaching higher values. These observations suggest the presence of different factors involved in
determining the isomeric ratios. The electron-withdrawing effects of the substituents in the ␤-positions seem to influence
the stereoselectivity giving the less hindered isomer while the chlorine bulky substituents in the ortho-phenyl positions can
compensate this trend. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Metalloporphyrins; Cyclopropanation; Catalysts; Rhodium; Olefins
1. Introduction
their exquisite prerogative in reverting the syn/anti
ratio of the reaction products obtained with normal
catalysts [6–9]. Iron, rhodium and osmium porphyrins
have been successfully used in the cyclopropana-
tion reactions of simple olefins by ethyldiazoacetate
(EDA). Reaction yields, turnovers number and stere-
oselectivities obtained with the use of such catalysts
have been strictly correlated to the nature of the metal
and the macrocycle [6–9].
The reaction mechanism of the metalloporphyrins
catalysed cyclopropanation reactions is not com-
pletely elucidated because of the liability of the bond
between the central metal and the acetate residue. The
Transition metal complexes are active catalysts for
the cyclopropanation of olefins by diazocompounds
[1,2]. This reaction is widely employed in organic
synthesis and several copper, rhodium and osmium
complexes have been reported to be efficient in the
formation of cyclopropanes [3–5].
Among the catalysts studied, much interest has
been devoted to the metalloporphyrins because of
∗
Corresponding author.: Fax: +39-6-725-94754.
E-mail address: pietro.tagliatesta@uniroma2.it (P. Tagliatesta).
1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(02)00080-8

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P. Tagliatesta, A. Pastorini / Journal of Molecular Catalysis A: Chemical 185 (2002) 127–133
intermediate of the reaction, showed in Eq. (1), has
been tentatively proposed, in the case of rhodium,
by Callot and co-workers [6,7] and later studied
by Maxwell and Kodadek, who used NMR spec-
troscopy for detecting the possible carbene species
[10].
2. Experimental
2.1. General
Dry chloroform (CHCl3) was distilled over P2O5
under nitrogen before the use. All other reagents and
solvents were of the highest analytical grade and
used without further purification. Chromatographic
purifications were performed on silica gel (35–70
mesh, Merck) columns. Thin-layer chromatogra-
phy was carried out using Merck Kiesegel 60 F254
plates.
(1)
1H NMR spectra were recorded as CDCl3 solutions
on a Bruker AM 400 instrument using tetramethyl-
silane (TMS) as an internal standard. Electronic ab-
sorption spectra of the compounds were recorded on a
Varian Cary 50 scan UV–VIS spectrophotometer. FAB
mass spectra were measured on a VG-Quattro spec-
trometer using m-nitrobenzylic alcohol as a matrix.
GC analyses were performed on a Carlo Erba
HRGC 5160 instrument equipped with a 30 m Su-
pelco SPB-35 GC column and FID detector.
However, the key intermediate carbene species has
never been isolated and fully characterised.
Recently, we have studied the synthesis and
electrochemistry of several ␤-halogen substituted
metalloporphyrins and explored their stability and
reactivity during the electrochemical oxidation and
reduction [11–15]. Furthermore, we have also used
some manganese and iron derivatives of such por-
phyrins as catalysts in the oxidation reactions of
organic substrates [16–18]. The possibility to vary
the electronic properties and steric hindrance of the
meso-tetraphenylporphyrins, introducing different
groups on the ␤-positions and/or on the phenyl rings,
prompted us to investigate which parameters govern
the stereochemical results obtained in the cyclo-
propanation reactions using their rhodium derivatives
as catalysts and in this paper we report the results
obtained for standard olefins.
Elemental analyses (C, H and N) were carried out
by the Analytical Laboratory of the University of
Padova (Italy) and are all in excellent agreement with
the expected values.
2.2. Synthesis of the porphyrins
The free base porphyrins were synthesised by liter-
ature methods [12,14,19–21].
2.3. Rh(III) derivatives
The catalysts were RhTPPCl (1), RhTDMPPCl
(2), Rh(Cl16)TDMPPCl (3), Rh(Br2)TPPCl (4),
Rh(Br3)TPPCl (5), Rh(Br4)TPPCl (6), Rh(Br8)TPPCl
(7) and RhTDCPPCl (8), where TPP is the dianion of
5,10,15,20-tetraphenylporphyrin, TDMPP the dianion
of 5,10,15,20-tetra(2ꢀ,6ꢀ-dimethoxyphenyl)porphyrin,
(Cl16)TDMPP the dianion of 2,3,7,8,12,13,17,18-oct-
achloro-5,10,15,20-tetra(2ꢀ,6ꢀ-dimethoxy-3ꢀ,5ꢀ-dichl-
orophenyl)porphyrin, (Br2)TPP the dianion of 2,3-
dibromo-5,10,15,20-tetraphenylporphyrin, (Br3)TPP
the dianion of 2,3,12-tribromo-5,10,15,20-tetrapheny-
lporphyrin, (Br4)TPP the dianion of 2,3,12,13-tetrabr-
omo-5,10,15,20-tetraphenylporphyrin, (Br8)TPP the
dianion of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-
tetraphenylporphyrin and TDCPP the dianion of
5,10,15,20-tetra(2ꢀ,6ꢀ-dichlorophenyl)porphyrin.
The insertion of rhodium in the free bases has been
carried out according to literature procedures [22]. In
a typical preparation, the free base (100 mg) was dis-
solved in benzene and [Rh(CO)2Cl]2 (100 mg) was
added. The reaction mixture was stirred at 60 ◦C for
3 days, while protecting by a CaCl2 valve and in the
presence of air oxygen. After the evaporation of the
solvent under vacuum, the residue was dissolved in
chloroform and purified by column chromatography
on silica gel, eluting with chloroform. The fraction
containing the desired product was evaporated to small
volume and methanol was added to precipitate the pure
compound. The yields of the metal insertions were al-
ways in the range 60–70%. The UV–VIS, mass and 1H

P. Tagliatesta, A. Pastorini / Journal of Molecular Catalysis A: Chemical 185 (2002) 127–133
129
Table 1
products were performed under the following con-
ditions: initial (5 min) and final temperatures of 70
UV–VIS characterisation of the new rhodium porphyrin com-
pounds in CHCl3
and 200 ◦C, respectively, heating rate 10 ◦C min−1
.
Compound
λ (nm) (× 10−4 ε dm−3 mol−1 cm−1
)
The chemical yields were not optimised. The obtained
data were reproducible within 2% for several exper-
iments.
RhTDMPPCl
420 (2.25); 532 (0.37); 607 (0.16)
449 (6.40); 561 (0.87); 596 (0.28)
426 (22.5); 538 (2.12); 571 (0.38)
426 (23.1); 538 (2.30); 568 (0.56)
433 (20.6); 546 (1.96); 581 (0.92)
456 (20.0); 567 (1.97); 605 (0.46)
420 (6.07); 534 (10.96); 567 (0.25)
Rh(Cl16)TDMPPCl
Rh(Br2)TPPCl
Rh(Br3)TPPCl
Rh(Br4)TPPCl
Rh(Br8)TPPCl
RhTDCPPCl
3. Results and discussion
The molecular structures of the catalysts employed
in this study are reported in Fig. 1.
NMR data for the new compounds have been reported
in Tables 1 and 2, respectively.
It is well known from previous studies [11–13,15,24]
that (PorBrx)M, where PorBrx is the dianion of dif-
ferent ␤-halogenated TPP, M = Fe, Co, Zn or H2
and x ≥ 3 or 4, show saddle-shaped distortion of the
macrocycles and formation of a pocket on both sides
of the ring. This fact suggested the possibility to di-
rect the cyclopropanation reaction to give the most
hindered isomer in excess without building compli-
cated porphyrinic structures [8]. For this purpose, we
decided to use RhTDMPPCl (2) and RhTDCPPCl (8),
which show good steric hindrance on both sides of the
macrocycle due to presence of bulky groups. The start-
ing free bases of such metal derivatives are now avail-
able in grams quantity by new synthetic methods much
cheaper than those previously reported in the literature
and comparable with those for obtaining the simple
TPP [21].
2.4. General procedure for the
cyclopropanation reactions
The reactions and work-up were performed as re-
ported in the literature [23]. The GC analysis of the
Table 2
NMR and mass spectra characterisation of the new porphyrin
compounds
RhTDMPPCl (2)
δH: 8.69 (s, 8H, pyrr.); 7.65–7.76 (m, 8H, m-phenyl);
6.96–7.05 (m, 4H, p-phenyl); 3.51 (s, 24H, –OCH3)
FAB-mass: 955 (M − H–Cl)+
Rh(Cl16)TDMPPCl (3)
δH: 7.93 (s, 4H, p-phenyl); 3.00 (s, 24H, –OCH3)
FAB-mass: 1508 (M + H–Cl)+
We also used the rhodium derivative (3) of a
perchlorinated porphyrin similar to those showing
distorted and hindered structures [24]. Furthermore,
we examined the possibility to quantify the influence
of the ␤-bromine groups on the stereoselectivity of
the reaction. Looking at the syn/anti ratios given in
Tables 3, 4 and 5 and comparing them with the results
obtained for the reactions catalysed by CuCl, it is ev-
ident that the situation is complicated and not easy to
rationalise.
From the data obtained for styrene (Table 3; en-
tries 2–6), it is clear that there is a small but evident
influence of the ␤-bromination on the stereoselec-
tivity of the reaction and plotting the logarithm of
the syn/anti ratio against the sum of the Hammett’s
σp of the bromine groups, a good linear correla-
tion (r2 = 0.98; ρ = −0.132) is evident (Fig. 2).
The selectivity changes on increasing the number
of the halogens and this fact, in our opinion, is due
Rh(Br2)TPPCl (4)
δH: 8.68–9.06 (m, 6H, pyrr.); 8.01–8.19 (m, 8H,
o-phenyl); 7.71–7.79 (m, 12H, m,p-phenyl)
FAB-mass: 871 (M − 2H–Cl)+
Rh(Br3)TPPCl (5)
δH: 8.71–9.08 (m, 5H, pyrr.); 8.06–8.25 (m, 8H,
o-phenyl); 7.71–7.79 (m, 12H, m,p-phenyl)
FAB-mass: 952 (M − Cl)+
Rh(Br4)TPPCl (6)
δH: 8.68 (s, 4H, pyrr.); 8.0–8.17 (dd, 8H, o-phenyl);
7.6–7.8 (m, 12H, m,p-phenyl)
FAB-mass: 1031 (M − Cl)+
Rh(Br8)TPPCl (7)
δH: 8.0–8.25 (dd, 8H, o- phenyl); 7.6–7.9
(m, 12H, m,p-phenyl)
FAB-mass: 1346 (M − H–Cl)+
RhTDCPPCl (8)
δH: 8.70 (s, 8H, pyrr.); 7.7–7.85 (m, 8H, m-phenyl);
7.65–7.7 (m, 4H, p-phenyl)
FAB-mass: 1026 (M − H–Cl)+

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P. Tagliatesta, A. Pastorini / Journal of Molecular Catalysis A: Chemical 185 (2002) 127–133
Fig. 1. Rhodium(III) porphyrins employed in this work.
to different factors; the electrophilic character of the
metal is clearly enhanced by the electron-withdrawing
effects of the ␤-substituents and can be related to the
mechanism proposed by Kodadek and co-workers [9]
for the rhodium porphyrins catalysis. In this mech-
anism, the olefin can approach the metallocarbene
intermediate in a perpendicular orientation relative to
the metal–carbon bond axis and giving after a rotation
the arrangement found in the final syn or anti product.
Our results are in agreement with this interpretation
but we also believe that, during the catalytic pro-
cess, styrene can approach the core of the macrocycle
through a ␲–␲ interaction with the higher halogenated
pyrrole rings stabilising the transition state which
leads to the anti product. Accordingly with this mech-
anism, it might be also expected that bulky groups
located on the porphyrin, like methoxy, could give
higher ratios. This is not the case for catalysts 2 and
3, the first having bulky electron-donating methoxy
groups on all the ortho-positions of the phenyl rings
[14] and the second also bearing chlorine atoms on
all the ␤-pyrrole and meta-phenyl positions, show an
opposite trend with a syn/anti ratio around 0.4/0.5
(Table 3; entries 7 and 8). Similar data were also re-
ported for osmium(II) and iron(II) catalysed reactions
[9,25] but only for styrene and the results obtained

P. Tagliatesta, A. Pastorini / Journal of Molecular Catalysis A: Chemical 185 (2002) 127–133
131
Fig. 2. Correlation diagram of the logarithm of the syn/anti ratio vs. the sum of Hammett’s σp for the cyclopropanation of styrene catalysed
by Rh(Brx)TPPCl.
for our catalysts are in agreement with the model pro-
posed for the iron in which the larger substituent, i.e.
phenyl, in the transition state, is projected far from the
ester group of the pre-formed carbene. In our case, this
effect could be emphasised by the methoxy groups
which insist in the same direction of the carbene ester,
pushing the phenyl of styrene in the opposite side.
A tentative schematic representation of the substrate
approaching the catalyst–carbene intermediate is
reported in Eq. (2).
(2)
Table 3
Table 4
Molar ratios (syn/anti) and yields in parentheses for the cyclo-
propanation reaction of styrene with EDA catalysed by rhodium
porphyrins
Molar ratios (syn/anti) and yields in parentheses for the cyclopropa-
nation reaction of cyclohexene with EDA catalysed by rhodium
porphyrins
Entrya
Catalyst
Molar ratio
Entrya
Catalyst
Molar ratio
1
2
3
4
5
6
7
8
9
CuCl
RhTPPCl
0.6 (79.3)
1.3 (70.4)
1.1 (95.0)
1.0 (90.1)
0.9 (91.2)
0.7 (26.5)
0.5 (27.7)b
0.4 (88.8)
1.7 (43.7)b
1
2
3
4
5
6
7
8
9
CuCl
RhTPPCl
0.2 (60.4)
0.8 (61.5)
0.6 (51.7)
0.5 (69.0)
0.6 (59.8)
0.5 (8.2)
0.7 (9.4)b
0.9 (59.1)
1.5 (80.1)b
Rh(Br2)TPPCl
Rh(Br3)TPPCl
Rh(Br4)TPPCl
Rh(Br8)TPPCl
Rh(Cl16)TDMPPCl
RhTDMPPCl
RhTDCPPCl
Rh(Br2)TPPCl
Rh(Br3)TPPCl
Rh(Br4)TPPCl
Rh(Br8)TPPCl
Rh(Cl16)TDMPPCl
RhTDMPPCl
RhTDCPPCl
a Reactions were carried out at 60 ◦C with substrate/EDA/
catalyst molarratio = 2500/1000/1.
a Reactions were carried out at 60 ◦C with substrate/EDA/
catalyst molarratio = 2500/1000/1.
b Room temperature.
b Room temperature.

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P. Tagliatesta, A. Pastorini / Journal of Molecular Catalysis A: Chemical 185 (2002) 127–133
Table 5
For styrene we obtained a ratio of 1.7/0.98, for cy-
clohexene 1.5/1.17 and for norbornene 3.5/2.14, at
room temperature. These last results, in our opinion,
are quite remarkable for several reasons, first of all
because of the low cost of the starting free base. Fur-
thermore, the result for norbornene, to the best of our
knowledge, is the higher so far reported in the litera-
ture and also the other ratios obtained for styrene and
cyclohexene are interesting even compared with the
values obtained by O’Malley and Kodadek who used
more complicated and expensive porphyrin catalysts
[8].
Molar ratios (syn/anti) and yields in parentheses for the cyclopropa-
nation reaction of norbornene with EDA catalysed by rhodium
porphyrins
Entrya
Catalyst
Molar ratio
1
2
3
4
5
6
7
8
9
CuCl
RhTPPCl
0.4 (90.7)
1.4 (71.0)
1.5 (31.8)
1.5 (53.6)
1.0 (66.4)
0.6 (10.1)
1.2 (80.6)b
1.5 (55.6)
3.5 (50.2)b
Rh(Br2)TPPCl
Rh(Br3)TPPCl
Rh(Br4)TPPCl
Rh(Br8)TPPCl
Rh(Cl16)TDMPPCl
RhTDMPPCl
RhTDCPPCl
a Reactions were carried out at 60 ◦C with substrate/EDA/
catalyst molarratio = 2500/1000/1.
4. Conclusions
b Room temperature.
The stereoselectivity of the rhodium tetraphenyl-
porphyrins catalysed cyclopropanation reaction is the
result of different factors which can be tentatively
rationalised in terms of steric and/or electronic de-
mands. The electron-withdrawing properties of the
substituents in the ␤-positions decrease the syn/anti
ratios for all the examined substrates. This effect is
more pronounced in the case of styrene, which also
show a good linear correlation between the number
of halogen atoms and the stereochemical results. This
change of stereoselectivity can be attributed to the ge-
ometry of the approach of the substrate to the core of
the macrocycle which is more similar to that reported
for the iron porphyrins catalysis. For the catalysts hav-
ing methoxy groups in the 2ꢀ,6ꢀ-phenyl positions, the
effect of the bulky groups goes in the same direction,
while the catalyst bearing chlorine in the same posi-
tions gives better results.
The situation is different for cyclohexene or nor-
bornene, which show different behaviours.
For cyclohexene, the most conformationally flex-
ible substrate among those examined, we obtained
almost the same syn/anti ratio for all the above cited
catalysts (Table 4; entries 2–6). This is not surpris-
ingly and such effect can be attributed to the possi-
bility for cyclohexene to have the access to different
conformations. This fact can help the approach to the
metal centre, minimising the steric effects induced by
the bulky substituents of the catalysts 2 and 3 or by
the saddle-shaped deformations of the macrocycle 7.
On the contrary, norbornene, a more rigid substrate,
shows a non-linear decrease in the stereochemical ra-
tio, as observed for styrene, on increasing the number
of the halogen atoms on the ␤-positions. For the cata-
lysts 2 and 3 having bulky groups in the ortho-phenyl
positions, the stereochemical ratios for norbornene
increase again (Table 5; entries 7 and 8) and this fact
might be due to a decreasing in the conformational
freedom of the norbornene in the porphyrins cavity.
Finally, porphyrin 8 shows the most interesting results
among all reported in this paper.
Other studies on porphyrin catalysts bearing halo-
gens in 2ꢀ,6ꢀ-positions and electron-donating groups
in ␤-position are currently underway.
5. Synopsis
When compared with the data obtained by Callot
and co-workers [6,7], who used the rhodium deriva-
tives of porphyrins bearing other bulky substituents
as catalysts for the reaction of EDA with the same
substrates, we were able to obtain a good improve-
ment of the stereochemical results (Tables 3, 4 and 5;
entries 9).
The rhodium porphyrins cyclopropanation reac-
tions of three standard olefins have been widely
studied. The isomeric ratios of reaction products has
been correlated to the structures of the catalysts. The
perchlorinated macrocycle RhTDCPPCl affords good
stereochemical results.

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133
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The financial support of the Italian Consiglio
Nazionale delle Ricerche and Ministero della
Universita’ e della Ricerca Scientifica e Tecnologica
is gratefully acknowledged. We wish to thank Mr.
Alessandro Leoni for his valuable technical support.
We also thank Mrs. Giuseppe D’Arcangelo and Fabio
Bertocchi for recording mass and NMR spectra,
respectively.
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