Axially chiral β, β-bisporphyrins: Synthesis and configurational stability tuned by the central metals

Article abstract of DOI:10.1021/ja8055886

In this paper, the synthesis of a new type of intrinsically chiral, directly β,β'-linked, octa-mesoaryl-substituted bisporphyrins is described, by using an optimized Suzuki-Miyaura coupling procedure. This strategy leads to a broad variety of such axially

Full text of DOI:10.1021/ja8055886

Published on Web 12/04/2008
Axially Chiral ꢀ,ꢀ′-Bisporphyrins: Synthesis and
Configurational Stability Tuned by the Central Metals
Gerhard Bringmann,*,† Daniel C. G. Go¨tz,† Tobias A. M. Gulder,† Tim H. Gehrke,†
Torsten Bruhn,† Thomas Kupfer,‡ Krzysztof Radacki,‡ Holger Braunschweig,‡
Alexander Heckmann,† and Christoph Lambert†
Institute of Organic Chemistry and Ro¨ntgen Research Center for Complex Material Systems,
UniVersity of Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, and Institute of Inorganic
Chemistry, UniVersity of Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received July 17, 2008; E-mail: bringman@chemie.uni-wuerzburg.de
Abstract: In this paper, the synthesis of a new type of intrinsically chiral, directly ꢀ,ꢀ′-linked, octa-meso-
aryl-substituted bisporphyrins is described, by using an optimized Suzuki-Miyaura coupling procedure.
This strategy leads to a broad variety of such axially chiral ‘superbiaryls’, differing in their metalation states
and substitution patterns. On the basis of an efficient route to as-yet-unknown ꢀ-boronic acid esters of
various meso-tetraarylporphyrins (TAPs) by a two-step bromination-borylation protocol, 18 axially chiral
bisporphyrin derivatives were prepared in good to excellent yields. As compared to all other directly linked
dimeric porphyrin systems, the joint presence of eight bulky meso substituents and the peripheral position
of the porphyrin-porphyrin linkage is unprecedented. The axial configurations and rotational barriers of
the pure atropo-enantiomers were investigated by HPLC-CD experiments on a chiral phase in combination
with quantum chemical CD calculations. According to the HPLC experiments and in agreement with quantum
chemical calculations by applying the dispersion-corrected BLYP method, the configurational stability of
the central porphyrin-porphyrin axis strongly depends on the nature of the central metals. Cyclovoltammetric
studies proved the systematic influence of the meso substituents and of the metal ions on the oxidation
potentials of the bisporphyrins. The novel axially chiral bis(tetrapyrrole) compounds described here are
potentially useful as substrates for asymmetric catalysis, biomimetic studies on directed electron-transfer
processes, for photodynamic therapy (PDT), and for chiral recognition.
Introduction
established in the late 1840s: the principle of chirality. Whereas
the substantial influence of stereogenic centers was thus
Since the structural elucidation of chlorophyll and heme by
Willsta¨tter and Fischer in the early 20th century, the interest in
chlorins, porphyrins, and related tetrapyrrolic natural products
has substantially grown.1 Besides structural work, the investiga-
tion of the biosynthetic origin and the total synthesis of naturally
occurring tetrapyrrolic macrocycles were ‘early’ major tasks in
tetrapyrrole research,2 including the brilliant synthesis of vitamin
B12 by Woodward and Eschenmoser3 in 1973. After the
development of various methods for the functionalization of
natural and fully synthetic porphyrinoids,4 the interest has
concentrated on possible applications of tetrapyrrole derivatives
during the past decades.5 Today, porphyrins play an important
role in modern applied organic chemistry, e.g., as metal-organic
catalysts,6 as potent substrates in boron neutron capture therapy
(BNCT),7 in photodynamic therapy (PDT),8 in material sci-
ences,9 and in biomimetic solar-energy conversion.10
recognized by the research community long ago, the significance
of chiral axes was correctly described for the first time by
Christie and Kenner no earlier than 1922.11 Meanwhile, axial
chiralityslike chirality originating from stereogenic centerssis
known to be important for the pharmacological activity of
biarylic natural products,12 which, in turn, has triggered the
development of synthetic pathways to axially chiral natural and
unnatural target molecules, among them reagents, ligands, and
catalysts for asymmetric synthesis.12,13
(3) (a) Woodward, R. B. Pure Appl. Chem. 1971, 25, 283–304. (b)
Woodward, R. B. Pure Appl. Chem. 1973, 33, 145–177.
(4) (a) The Porphyrin Handbook; Kadish, K. M.; Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, 2000, Vols. I-II. and references
cited therein. (b) The Porphyrins; Dolphin, D., Ed.; Academic Press:
New York, 1978, Vols. I-II and references cited therein. (c) Shan-
mugathasan, S.; Edwards, C.; Boyle, R. W. Tetrahedron 2000, 56,
1025–1046 and references cited therein.
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Eds.; Academic Press: San Diego, 2000, Vol. VI and references cited
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Int. Ed. 1998, 37, 1131–1133.
Independent from the fruitful development of tetrapyrrole
chemistry, a chemical concept of fundamental importance was
† Institute of Organic Chemistry.
‡ Institute of Inorganic Chemistry.
(1) Battersby, A. R. Nat. Prod. Rep. 2000, 17, 507–526 and references
cited therein.
(2) (a) Battersby, A. R. Nat. Prod. Rep 1987, 4, 77–87 and references
cited therein. (b) Battersby, A. R. Pure Appl. Chem. 1993, 65, 1113–
1122 and references cited therein.
9
17812 J. AM. CHEM. SOC. 2008, 130, 17812–17825
10.1021/ja8055886 CCC: $40.75
2008 American Chemical Society

Axially Chiral ꢀ,ꢀ′-Bisporphyrins
A R T I C L E S
Figure 1. Axial chirality in directly linked bisporphyrins: through lateral meso substituents (left) or per se, by the rather ‘tangential’ coupling site (right).
Still, it is only quite recently that the useful properties of
porphyrins have been combined with the phenomenon of
chirality.14 The chirality of literature-known porphyrin deriva-
tives often originates either from chiral centers or from
rotationally hindered, axially chiral substituents attached to the
ꢀ or meso positions.15 Even planar-chiral porphyrin structures
have been generated by unsymmetric ansa-like capping of the
porphyrin moiety,16 by N-methylation of a pyrrolic nitrogen
starting from prochiral porphyrin derivatives,17 or by axial
coordination of the central metal, e.g., in a cobalt(III) etiopor-
phyrin.18
In contrast to the structural variety represented by such
common, monomeric chiral porphyrins, there are only few
examples of chiral porphyrin dimers. For most of such optically
active bisporphyrins, a spacer that links twosas such achirals
porphyrin subunits carries the chiral information.19 Even though
directly linked porphyrin complexes display a structural motif
frequently used for the investigation of electron and energy
transfer,20,21 only few bisporphyrins that are based on direct
meso-meso,22,23 meso-ꢀ,24,25 or ꢀ-ꢀ26-29 linkages have so
far been described. Only Osuka’s group proved the configura-
tional stability of three directly linked bisporphyrins, two of
them being axially chiral,30,31 one of them being doubly linked
and therefore helically chiral.21 However, all of the singly linked
representatives have onesrestrictingsproperty in common: their
chirality results from an unsymmetric substitution pattern,
exclusively, since the basic bisporphyrin framework itself has
a plane of symmetry and is therefore achiral (Figure 1).
Consequently, such axially chiral dimeric porphyrins are limited
to special peripheral substitution patterns that make them
unsymmetric, which moreover requires a cumbersome synthesis
of their monomeric building blocks.
Dimeric porphyrins, catenated porphyrin assemblies, and
multiporphyrin arrays are of growing importance,32 i.e., because
such systems possess remarkable spectral properties that are
useful for electronic and optical applications.20,21,33 Moreover,
porphyrin-porphyrin (or generally, tetrapyrrole-tetrapyrrole)
interactions are the basis of fundamental biological processes,34
and thus especially of studies on ‘bioinspired’ electron transfer.35
For these reasons, the synthesis of directly ꢀ,ꢀ′-linked bispor-
phyrins, which are intrinsically axially chiral, independent from
their individual substitution pattern, and their stereochemical,
spectral, and electronical characterization constitute rewarding
tasks.
We herein describe a convenient, highly effective, and
variable route for the synthesis of such systems and their
stereoanalysis by chromatography on a chiral phase with HPLC-
CD coupling, combined with quantum chemical CD calcula-
tions.36 Key step of the synthesis is a Pd-catalyzed C-C
coupling following an optimized Suzuki-Miyaura protocol.37
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9
J. AM. CHEM. SOC. VOL. 130, NO. 52, 2008 17813

A R T I C L E S
Bringmann et al.
Table 1. Optimization of the Reaction Conditions for the Formation of the Boronic Ester 3aa
entry
solvent
additive
borane
catalyst
catalyst loading [mol %]
t [h]
hydro-debrominationb
2 [%]
3a [%]c
b
b
1e
2f
3
4
5
6
7
8
9
1,2-DCE
1,4-dioxane
DMF
toluene
toluene
NEt3
KOAc
K2CO3
K2CO3
K2CO3
KOAc
PinBH
(BPin)2
(BPin)2
(BPin)2
(BPin)2
(BPin)2
(BPin)2
(BPin)2
(BPin)2
(BPin)2
Pd(PPh3)2Cl2
Pd(PtBu3)2
20
10
20
20
20
20
20
20
20
5
1
14
3
+
+
-
20-30
0
0
45-55
45-55
60-65
70
70
67
69
traces
-
Pd(dppf)Cl2
Pd(dppf)Cl2
Pd(dppf)Cl2
Pd(dppf)Cl2
Pd(dppf)Cl2
Pd(dppf)Cl2
Pd(dppf)Cl2
Pd(dppf)Cl2
97c
b
24
1d
12
3
+
+
b
+
+
b
toluene
-
-
b
toluene/H2O
toluene/H2O
toluene/H2O
toluene/H2O
KOAc
traces
traces
traces
traces
-
KOAc
1d
1
-
-
-
KOAc PTCg
KOAc
10
46
a Reactions were carried out under Ar with bromoporphyrin 1a (30 mg, 1.0 equiv), bis(pinacolato)diboron (2.5 equiv), base (10 equiv), and Pd
catalyst. b Observed by TLC. c Isolated yields. d Assisted by microwave irradiation. e Reaction was carried out with bromoporphyrin 1a (30 mg, 1.0
equiv), pinacolborane (8.5 equiv), base (13 equiv), and Pd catalyst (3.0 mol %) at 90 °C as in ref 38. f Reaction was carried out with bromoporphyrin 1a
(30 mg, 1.0 equiv), bis(pinacolato)diboron (5.0 equiv), base (5.0 equiv), and Pd catalyst (10 mol %) as in ref 40. g 18-Crown-6 (10 mol %) was used as
a phase-transfer catalyst.
Generating a vast chiral cavity the synthesized ‘superbiaryls’
should open new perspectives, e.g., as reagents, ligands, or
catalysts in asymmetric synthesis. The required free-base
ꢀ-boronic esters of meso-tetraaryl-substituted porphyrins (TAPs)
have not yet been synthesized either.36 Cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) measurements were
performed to investigate the trends of oxidation potentials of
all bisporphyrins depending on the type of meso substituents
and metal ions. A significant result of the work is the discovery
that the rotational barriers of the bisporphyrins can be tuned by
the choice of the central metals.
Results and Discussion
(12) Bringmann, G.; Gu¨nther, C.; Ochse, M.; Schupp, O.; Tasler, S. In
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Mortimer, A. J.; Keller, P. A.; Gresser, M. J.; Garner, J.; Breuning,
M. Angew. Chem., Int. Ed. 2005, 44, 5384–5427.
Synthesis of ꢀ-Borylated TAPs. The scheduled construction
of such bisporphyrins required the availability of ꢀ-boronic acid
esters of TAP derivatives. While a versatile access to meso
‘Suzuki porphyrins’38,39 had already been described, there had
as yet not been any report on the synthesis of metal-free
ꢀ-borylated tetraarylporphyrins. The only known protocol for
the synthesis of a metalated ꢀ-boronic acid ester of TPP (with
zinc as the central metal) has recently been published by Zhang
et al.,40 starting from the respective brominated precursor. In
our hands, this procedure failed to give the desired ꢀ-borylated
tetraarylporphyrins both when starting from metal-free (Table
1, entry 2) or zincated bromoporphyrins.
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9
17814 J. AM. CHEM. SOC. VOL. 130, NO. 52, 2008

Axially Chiral ꢀ,ꢀ′-Bisporphyrins
A R T I C L E S
Table 2. Synthesis of the New ꢀ-Boronic Acid Esters 3a-k of
Metalated and Nonmetalated TAPsa
The Ir-catalyzed borylation procedure starting from a bromine-
free porphyrin, i.e., by direct C-H activation as elaborated by
Hata et al.,41 was not applicable to the synthesis of ꢀ-borylated
TAPs either, as it is known to be limited to the preparation of
porphyrin boronic acid esters having a free meso position
adjacent to the center of the transmetalation.41 Reaction of
brominated TPP (1a) under the conditions described by Deng
et al., with DMF as the solvent,27 resulted in an almost exclusive
formation of the undesired Heck product42 2 (Table 1, entry
3), showing the necessity to establish a basically new access to
the desired porphyrin derivatives bearing four meso-aryl sub-
stituents.36 The use of toluene as a less polar solvent finally led
to the formation of the desired product 3a, which was isolated
in up to 55% yield (Table 1, entries 4 and 5). Nevertheless, the
purification of the boronic acid ester was still cumbersome due
to the presence of undesired byproductssthe hydrodebrominated
b
product (yield [%] )
entry
reactant
Ar
M
t [h]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
1g
1h
1i
phenyl
4-tolyl
4-chlorophenyl
4-methoxyphenyl
phenyl
4-tolyl
phenyl
4-tolyl
phenyl
2H
2H
2H
2H
Zn
Zn
Ni
3
3
2
3
3
3
3
3
4
4
4
3a (70)
3b (65)
3c (56)
3d (64)
3e (62)
3f (67)
3g (58)
3h (60)
3i (53)
3j (50)
3k (61)
Ni
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10321.
9
10
11
Cu
Cu
Pd
1j
1k
4-tolyl
phenyl
a Reactions were carried out under Ar with the respective
bromoporphyrin 1 (1.0 equiv), bis(pinacolato)diboron (2.5 equiv), KOAc
(10 equiv), and Pd catalyst (20 mol %). b Isolated yields.
(22) (a) Susumu, K.; Shimidzu, T.; Tanaka, K.; Segawa, H. Tetrahedron
Lett. 1996, 37, 8399–8402. (b) Osuka, A.; Shimidzu, H. Angew. Chem.,
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parent compound (TPP) and the likewise formed ring-annulated
product 2swhich substantially limited the isolated yields of 3a.
Replacement of K2CO3 by KOAc as a more nucleophilic base
increased the isolated yield up to 65% (Table 1, entry 6),
presumably due to an additional activation of the boron-
transferring agent by intermediate formation of a quaternary
boronate anion.43 Using toluene/H2O as a two-phase solvent
system finally led to a further improvement of the yield and to
a significant reduction of the reaction time, which was again
shortened by microwave irradiation or by the additional use of
18-crown-6 as a phase-transfer catalyst (Table 1, entries 5, 7,
8, and 9).44 The borylation also succeeded by using a far smaller
catalyst loading of 5 mol % (Table 1, entry 10), giving identical
yields but requiring significantly extended reaction times.
Using these optimized Pd-catalyzed transmetalation condi-
tions, several as-yet-unknown air- and water-stable porphyrin
boronic acid esters 3a-k were synthesized from the respective
ꢀ-brominated tetraarylporphyrins 1a-k in good yields and short
reaction times (Table 2). Electron-withdrawing chlorine sub-
stituents (Table 2, entry 3) facilitated the competing hydrode-
bromination at the catalytic Pd center, presumably by an
oxidative addition of hydridic boron species, followed by
transmetalation and reductive elimination.45 This would explain
the slightly decreased yields as compared to the other substrates
(24) Senge, M. O.; Ro¨ꢀler, B.; von Gersdorff, J.; Scha¨fer, A.; Kurreck, H.
Tetrahedron Lett. 2004, 45, 3363–3367.
(25) Ogawa, T.; Nishimoto, Y.; Yoshida, N.; Ono, N.; Osuka, A. Angew.
Chem., Int. Ed. 1999, 38, 176–179.
(26) For ꢀ,ꢀ′-bisporphyrins without meso substituents that might be axially
chiral but have not been stereochemically characterized, see refs 27
and 28.
(27) Deng, Y.; Chang, C. K.; Nocera, D. G. Angew. Chem., Int. Ed. 2000,
39, 1066–1068.
(28) (a) Paine III, J. B.; Dolphin, D. Can. J. Chem. 1978, 56, 1710–1712.
(b) Uno, H.; Kitawaki, Y.; Ono, N. Chem. Commun. 2002, 116–117.
(29) For a directly ꢀ,ꢀ′-linked, yet N-confused bisporphyrin, which has
been stereochemically characterized more recently, see: (a) Siczek,
M.; Chmielewski, P. J. Angew. Chem., Int. Ed. 2007, 46, 7432–7436.
For its synthesis, see: (b) Chmielewski, P. J. Angew. Chem., Int. Ed.
2004, 43, 5655–5658.
(30) Nakamura, Y.; Hwang, I.; Aratani, N.; Ahn, T. K.; Ko, D. M.; Takagi,
A.; Kawai, T.; Matsumoto, T.; Kim, D.; Osuka, A. J. Am. Chem. Soc.
2005, 127, 236–246.
(31) Yoshida, N.; Osuka, A. Tetrahedron Lett. 2000, 41, 9287–9291.
(32) (a) Vicente, M. G. H.; Jaquinod, L.; Smith, K. M. Chem. Commun.
1999, 1771–1782. (b) Burrell, A. K.; Officer, D. L.; Plieger, P. G.;
Reid, D. C. W. Chem. ReV. 2001, 101, 2751–2796.
(33) Pescitelli, G.; Gabriel, S.; Wang, Y.; Fleischhauer, J.; Woody, R. W.;
Berova, N. J. Am. Chem. Soc. 2003, 125, 7613–7628.
(34) (a) van Amerongen, H.; Valkunas, L.; van Grondelle, R. Photosynthetic
Excitons; World Scientific: Singapore, New Jersey, London, Hong
Kong, 2000. (b) Blankenship, R. E. Molecular Mechanisms of
Photosynthesis; Blackwell Science: Oxford, 2002.
(38) Hyslop, A. G.; Kellet, M. A.; Iovine, P. M.; Therien, M. J. J. Am.
Chem. Soc. 1998, 120, 12676–12677. (a) In our hands these conditions
published for the synthesis of meso-boronic acid esters of TAPs by
Hyslop et al. did give ꢀ-borylated TAPs starting from the respective
ꢀ-brominated porphyrins, too, but in very low yields (e.g., 25-30%
for 3a as compared to 70% with our optimized procedure, Table 1,
entries 1 and 8).
(35) For recent reviews on artificial photosynthetic model systems derived
from porphyrins, see: (a) Guldi, D. M. Chem. Soc. ReV. 2002, 31,
22–36. (b) Kobuke, Y.; Ogawa, K. Bull. Chem. Soc. Jpn. 2003, 76,
689–708. (c) Imahori, H. J. Phys. Chem. B 2004, 108, 6130–6143.
(d) Fukuzumi, S. Bull. Chem. Soc. Jpn. 2006, 79, 177–195. (e) Satake,
A.; Kobuke, Y. Org. Biomol. Chem. 2007, 5, 1679–1691. (f) Hori,
T.; Nakamura, Y.; Aratani, N.; Osuka, A. J. Organomet. Chem. 2007,
692, 148–155.
(39) For some recent synthetic applications of this procedure,38 see: (a)
Tsuda, A.; Nakamura, T.; Sakamoto, S.; Yamaguchi, K.; Osuka, A.
Angew. Chem., Int. Ed. 2002, 41, 2817–2820. (b) Kang, Y. K.;
Rubtsov, I. V.; Iovine, P. M.; Chen, J.; Therien, M. J. J. Am. Chem.
Soc. 2002, 124, 8275–8279. (c) Chng, L. L.; Chang, C. J.; Nocera,
D. G. J. Org. Chem. 2003, 68, 4075–4078. (d) Cheng, F.; Zhang, S.;
Adronov, A.; Echegoyen, L.; Diederich, F. Chem. Eur. J. 2006, 12,
6062–6070.
(36) For preliminary results on two first representatives, see: Bringmann,
G.; Ru¨denauer, S.; Go¨tz, D. C. G.; Gulder, T. A. M.; Reichert, M.
Org. Lett. 2006, 8, 4743–4746.
(37) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457–2483. (b)
Suzuki, A. Proc. Jpn. Acad. 2004, 80, 359–371.
(40) Zhang, C.; Suslick, K. S. J. Porphyrins Phthalocyanines 2005, 9, 659–
666.
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A R T I C L E S
Bringmann et al.
significantly tilted relative to the planar backbone by 75.4°.
Similar to the structure of a meso-boryl porphyrin (torsion angle
52.0°) published by Hyslop et al.,38 steric interactions with the
adjacent meso-phenyl groups prevent the electronically favored
coplanar orientation. The observed B-C bond length of 1.572
Å is in accordance with data previously published for meso-
(1.57 Å)38 and ꢀ-borylated (1.544-1.555 Å)41 porphyrins.
In conclusion, an efficient and variable route to new ꢀ-boronic
acid esters of metalated and nonmetalated tetraarylporphyrins
was established, simultaneously preventing the competitive
cyclization by intramolecular Heck reaction. The porphyrin
derivatives thus obtained are not only of importance for the
synthesis of directly ꢀ,ꢀ′-linked bisporphyrins (see below), but
are also promising precursors to a broad variety of ꢀ-derivatized
tetraarylporphyrins46 and building blocks for the design of
supramolecular porphyrin assemblies.32
Synthesis of ꢀ,ꢀ′-Linked Bisporphyrins. As reported earlier,36
we have recently achieved the first synthetic access to intrinsi-
cally axially chiral, directly ꢀ,ꢀ′-linked bisporphyrinssa novel
class of ‘superbiaryls’sby a Suzuki-type cross-coupling reac-
tion. Encouraged by these results, our efforts focused on the
development of a convenient method for the synthesis of a broad
range of such porphyrin-derived extended heterobiaryls, differ-
ing in their substitution patterns and metalation states (Table
3). To minimize the occurrence of reduced yields caused by
the above-mentioned intramolecular Heck coupling and hydro-
debromination reactions, polar solvents were initially avoided.
While first attempts to achieve the coupling to generate bis-
TAPs using K3PO4 as the base succeeded for the meso-phenyl-
substituted derivative rac-4a,36 these conditions failed to deliver
the tolyl substituted product rac-4b (Table 3, entry 1). Neither
microwave irradiation nor the use of KF as an additive effected
a sufficient activation of the coupling partners (Table 3, entries
2 and 3). Optimum conditions in favor of the intermolecular
coupling as compared to the undesired side reactions were finally
found by activating the boron organyl as its more nucleophilic
at-complex using Ba(OH)2 ·8H2O as a stronger additive (Table
3, entry 5).37,47
Figure 2. Molecular structure (X-ray) of {2-(4′,4′,5′,5′-tetramethyl-
[1′,3′,2′]dioxaborolan-2′-yl)-5,10,15,20-tetraphenylporphyrinato}-cop-
per(II) 3i. Hydrogen atoms and solvent molecules (benzene) have been
omitted for clarity. The thermal ellipsoids are shown at 50% probability.
As in the case of the formation of the boronic acid esters
(see above), the use of toluene/H2O as the solvent system led
to a substantially reduced reaction time and, in addition,
permitted to work with a significantly smaller excess of
brominated starting material 1b (Table 3, entry 5), apparently
by further increasing the concentration of Ba(OH)2 ·8H2O in
the reaction system. These improvements thus provided an
efficient method for the synthesis of various ꢀ,ꢀ′-linked bispor-
phyrins on a 300-mg scale in good, perfectly reproducible yields
(Table 4).
The influence of the metalation states of the coupling
partners on the reaction course was most different, depending
on the respective metal: While in the case of the zinc(II)
derivative, rac-4f, the yield was comparable to that of the
metal-free analogue rac-4a (Table 4, entry 6), the use of the
nickel(II) porphyrin boronic acid esters 3g and 3h led to
decreased yields in the coupling step (Table 4, entries 7 and
8). The reaction of the borylated copper(II) porphyrin 3i with
the palladium(II) derivative 1k finally resulted in significantly
lower yields due to extensive hydrodebromination and
hydrodeborylation (Table 4, entry 9).
bearing less electron-poor substituents on the meso-aryl residues.
The influence of the central metal ion was found to be small in
the case of the zinc(II), nickel(II), and palladium(II) derivatives
1e-1h and 1k, (Table 2, entries 5-8), while borylation of the
brominated copper(II) porphyrins 1i and 1j led to significantly
reduced yields (Table 2, entries 9 and 10).
The structure of the ꢀ-borylated porphyrins thus synthesized
was verified by an X-ray diffraction analysis of 3i, using crystals
obtained by slow evaporation of a saturated solution in benzene
(Figure 2): introduction of the boryl group in the ꢀ-position
did not result in any distortion of the entirely flat porphyrin
macrocycle. In contrast to previously described ꢀ-borylated
porphyrins with a coplanar arrangement of the porphyrin core
and the boronic acid ester unit,41 the dioxaborolane ring is
(41) (a) Hata, H.; Shinokubo, H.; Osuka, A. J. Am. Chem. Soc. 2005, 127,
8264–8265. For a recent review on the regioselective borylation of
porphyrins by C-H activation, see: (b) Hata, H.; Yamaguchi, S.; Mori,
G.; Nakazono, S.; Katoh, T.; Takatsu, K.; Hiroto, S.; Shinokubo, H.;
Osuka, A. Chem. Asian J. 2007, 2, 849–859.
(42) In the bisporphyrin synthesis by Deng et al.,27 the monomeric coupling
partners had no meso substituents, so that no competing intramolecular
Heck-like cyclization could occur.
(43) Using KOAc not even traces of the Heck-annulated product 2 were
detected by TLC.
(44) Kappe, C. O. Angew. Chem., Int. Ed. 2004, 43, 6250–6284.
(45) For similar results, see: Murata, M.; Watanabe, S.; Masuda, Y. J. Org.
Chem. 1997, 62, 6458–6459.
(46) For similar derivatization of porphyrins without meso-substituents
starting from the respective ꢀ-boronic acid esters, see, e.g., Deng et
al.27
(47) Suzuki, A. Pure Appl. Chem. 1994, 66, 213–222.
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Axially Chiral ꢀ,ꢀ′-Bisporphyrins
A R T I C L E S
Table 3. Optimization of the Coupling Conditions of 1b and 3b To Form the ꢀ,ꢀ′-Bisporphyrin rac-4ba
entry
solvent
base
t [h]
equiv of 1b
hydro-debrominationb
Heck reactionc
rac-4b [%]c
1
2
3
4
5
toluene
toluene
toluene
toluene
K3PO4
K3PO4
K3PO4/KF
Ba(OH)2
Ba(OH)2
15
1d
15
24
14
4
4
4
4
+
+
0
0
+
+
+
traces
traces
+
traces
traces
traces
51
71
toluene/H2O
1.2
a Reactions were carried out under Ar with porphyrin boronic acid ester 3b (30 mg, 1.0 equiv), base (10 equiv), and Pd catalyst (20 mol %).
b Observed by TLC. c Isolated yields. d Assisted by microwave irradiation.
Table 4. Synthesis of Directly ꢀ,ꢀ′-Linked Bisporphyrinsa
entry
reactant
Ar1
M1
reactant
Ar2
M2
t [h]
product (yield [%]b)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1b
1b
1e
1f
phenyl
4-tolyl
4-chlorophenyl
4-methoxyphenyl
4-tolyl
4-tolyl
phenyl
4-tolyl
phenyl
2H
2H
2H
2H
2H
2H
Zn
Zn
Pd
3a
3b
3c
3d
3a
3f
3g
3h
3i
phenyl
4-tolyl
4-chlorophenyl
4-methoxyphenyl
phenyl
4-tolyl
phenyl
4-tolyl
phenyl
2H
2H
2H
2H
2H
Zn
Ni
14
14
40
18
17
18
15
16
2
rac-4a (73)
rac-4b (71)
rac-4c (70)
rac-4d (65)
rac-4e (80)
rac-4f (73)
rac-4g (58)
rac-4h (50)
rac-4i (33)
Ni
Cu
1k
a Reactions were carried out under Ar with porphyrin boronic acid ester 3 (1.0 equiv), bromoporphyrin 1 (1.2 equiv), Ba(OH)2 ·8H2O (10 equiv), and
Pd catalyst (20 mol %). b Isolated yields.
An advantage of the chosen synthetic strategy is that it permits
access not only to the C2-symmetric bisporphyrins rac-4a-d,
but, for the first time, also to the constitutionally unsymmetric
representatives rac-4e-i. The decreased symmetry of rac-4e
results from the different substitution patterns in the two
monomeric porphyrin subunits, whereas in the cases of rac-
4f-i it is the consequence of the presence of only one metal
center or the combination of two different metals in the two
porphyrin portions. Subsequent metalation of the free-base
bisporphyrins rac-4a-e by literature-known procedures for the
metalation of monomeric porphyrins48 was achieved in good
to excellent yields (Table 5). The corresponding nickel insertion
into the monozincated derivative rac-4f, however, resulted in a
scrambling of the metal centers even under mild conditions,
leading to a mixture of homo- and heterogeneously bismetalated
products.49 Still, the elaborated synthetic procedure permits
generation of a broad structural diversity of these and related
bisporphyrin systems.
Structure of the Bisporphyrins. Earlier optimizations of the
bisporphyrin rac-4a with semiempirical (AM1) methods and
of rac-4a and rac-5a with density functional calculations
(B3LYP/3-21G) had yielded structures with an angle of nearly
90° between the two porphyrin planes and with an unexpectedly
large distance between the phenyl substituents next to the central
axis and the porphyrin subunits.36 This was surprising at first
sight because one would have supposed a larger Coulomb
interaction, but in view of the fact that normal DFT methods
cannot take into account such interactions, this seemed under-
standable. Thus, the dispersion correction introduced by
Grimme,50 which considers Coulomb interactions at a semiem-
pirical level, was now used together with the BLYP functional
and SVP51 as the basis set for C, N, and H, and TZVP52 for the
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A R T I C L E S
Bringmann et al.
Table 5. Full Metalation of Directly ꢀ,ꢀ′-Linked Bisporphyrinsa
entry
reactant
metal source
solvent
T [°C]
t [h]
M
product (yield [%]b)
1
2
3
4
5
7
8
9
rac-4a
rac-4b
rac-4c
rac-4d
rac-4e
rac-4a
rac-4a
rac-4b
rac-4a
ZnCl2
ZnCl2
ZnCl2
ZnCl2
DMF
DMF
DMF
DMF
130
130
130
130
130
62
84
84
191
2
2
2
2
2
0.5
16
18
2
Zn
Zn
Zn
Zn
Zn
Cu
Ni
Ni
Pd
rac-5a (96)
rac-5b (98)
rac-5c (90)
rac-5d (89)
rac-5e (80)
rac-5f (78)
rac-5g (78)
rac-5h (93)
rac-5i (90)
ZnCl2
DMF
Cu(OAc)2
Ni(acac)2
Ni(acac)2
PdCl2
CHCl3
1,2-DCE
1,2-DCE
benzonitrile
10
a Reactions were carried out with ꢀ,ꢀ′-bisporphyrin rac-4 (1.0 equiv) and the respective metal salt (20 equiv). b Isolated yields.
Figure 3. Molecular structure (X-ray) of ꢀ,ꢀ′-bis(5,10,15,20-tetraphenylporphyrin) rac-4a (left; hydrogen atoms and solvent molecules have been omitted
for clarity) and detailed top view of possible π-stacking interactions (right; hydrogen atoms, meso phenyl groups and the second porphyrin moiety omitted).
The ellipsoids are shown at 50% probability. Of the two atropo-enantiomers present in the crystal, arbitrarily only the P isomer is shown.
metal centers to further optimize the structures of the bispor-
phyrins rac-4a, rac-4g, rac-5a, rac-5f, rac-5g, and rac-5i, giving
structures with a clear π-stacking between the phenyl rings at
C54 and C42 and the porphyrin planes. These interactions were
approved by subsequent X-ray diffraction studies of one of the
racemic bisporphyrins, rac-4a (see below), yielding nearly the
same structure as the calculated one.
Crystals of rac-4a suitable for this X-ray diffraction analysis
were obtained by slow diffusion of n-hexane into a saturated
solution of the compound in dichloromethane. The bisporphyrin
crystallized as violet plates in a racemic form, in the triclinic
space group P1. The monomeric porphyrin subunits were found
to be no longer planar as observed for the parent compound
TPP,53 but showed a substantially distorted saddle-shape
geometry, which might be explained by disfavored steric
interactions of the bulky meso substituents in the two porphyrin
moieties (Figure 3). Furthermore, the bent structure facilitates
intramolecular π-stacking of the meso phenyl rings next to the
chiral axis and the central pyrrolic subunits linked by the
porphyrin-porphyrin bond: the intercentroid distance of 3.446 Å
(48) For the insertion of zinc(II), see: (a) Adler, A. D.; Longo, F. R. J. Inorg.
Nucl. Chem. 1970, 32, 2443–2445. For the insertion of nickel(II), see:
(b) Annoni, E.; Pizzotti, M.; Ugo, R.; Quici, S.; Morotti, T.; Bruschi,
M.; Mussini, P. Eur. J. Inorg. Chem. 2005, 3857–3874. For the
insertion of copper(II), see: (c) Hosseini, A.; Taylor, S.; Accorsi, G.;
Armaroli, N.; Reed, C. A.; Boyd, P. D. W. J. Am. Chem. Soc. 2006,
128, 15903–15913. For the insertion of palladium(II), see: (d) Finikova,
O. S.; Cheprakov, A. V.; Vinogradov, S. A. J. Org. Chem. 2005, 70,
9562–9572.
(51) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571–7.
(52) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829–
35.
(49) As an example, reaction of rac-4f with Ni(OAc)2 ·4H2O (10 equiv) in
CHCl3/MeOH at room temperature resulted in a mixture of bispor-
phyrins bearing either two nickel(II), two zinc(II), or two different
metal centers.
(53) (a) Fleischer, E. B. Acc. Chem. Res. 1970, 3, 105–112. (b) Kano, K.;
Fukuda, K.; Wakami, H.; Nishiyabu, R.; Pasternack, R. F. J. Am.
Chem. Soc. 2000, 122, 7494–7502.
(50) Grimme, S. J. Comput. Chem. 2006, 27, 1787–1799.
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Axially Chiral ꢀ,ꢀ′-Bisporphyrins
A R T I C L E S
Figure 4. View along the porphyrin-porphyrin axis of 4a: Molecular
structure (X-ray, left; hydrogen atoms, phenyl moieties on C18, C42, C54,
and C78 omitted for clarity) and schematic side view (right). The ellipsoids
are shown at 50% probability.
Figure 5. Side view of the optimized (BLYP-D/SVP, TZVP) structures
a) of rac-4a (M1/M2 ) 2H/2H) and b) rac-5g (M1/M2 ) Ni/Ni); some of
the phenyl substituents were omitted for reasons of clarity. While the free-
base structure shows nearly planar porphyrin subunits, the molecular portions
of the bis-metalated compound adopt a ruffled conformation.
between the two slipped ring systems is typical of such π-π
interactions;54 the tilt angle of the phenyl ring relative to the
porphyrin unit is 15.0°. The phenyl substituents next to the central
axis (connected to C54 and C42) adopt two different orientations
in relation to the respective adjacent porphyrin moiety: the phenyl
ring at C54 is arranged fully above the porphyrin plane, while the
other one has a more peripheral position (Figure 3). Consequently,
the bisporphyrin slightly deviates from perfect C2 symmetry in the
crystal structure. The close proximity of the phenyl portions at C42
and C54 and the respective coplanar porphyrin subunit is also
consistent with the significant diastereotopic differentiation of the
corresponding phenyl protons, which had already hinted at a
restricted rotation about the porphyrin-phenyl axis in previous
NMR investigations.36,55
The mean planes of the porphyrin macrocycles as defined
by the four N atoms of the respective monomeric substructures
are not orthogonal to each other but form a torsion angle of
57.2° (Figure 4), which lies between typical values previously
published for ꢀ,ꢀ′-linked (51.7°)27 and meso,meso′-linked
(65.0-84.0°)23 bisporphyrins. If, however, one defines the
dihedral angle between the two molecular portions by the mutual
array of the two pyrrole rings that form the inner part of the
heterobiaryl system (which has not been done for the other
derivatives in the literature), the angle is much higher, viz 75.9°,
thus reflecting the high degree of molecular torsion of the two
porphyrin subunits.
between the two macrocycles. Nonetheless, the central C-C
axes of all these ꢀ,ꢀ′-bisporphyrins (‘conventional’ or N-
confused) are significantly shorter than typical Caryl-Caryl
bonds,56 suggesting greater electronic interaction between the
two ꢀ,ꢀ′-bridged porphyrin subunits than in the case of
meso,meso′-linked bisporphyrins, which show a typical bond
length of 1.51 Å.23 The described crystal structure is unique as
no X-ray diffraction analysis of a ꢀ,ꢀ′-bisporphyrin with meso-
aryl substituents has so far been reported.
The calculated structures of rac-4a, rac-4g, rac-5a, rac-5f,
and rac-5i look very similar to each other because in the free
base and most of the metalated bisporphyrins the porphyrin
subunits are close to planarity. The only exception is the
metalated dimer rac-5g, where the two nickel(II) porphyrin
moieties are largely distorted and have a ruffled conformation
(Figure 5).
Spectral Properties. As expected, the UV spectra of all metal-
free ꢀ,ꢀ′-bisporphyrins, rac-4a-e, showed the B band (Soret
band) at about 425 nm and the four Q bands between 520 and
660 nm. As in the cases of achiral meso,meso-linked porphyrin
dimers22,23 and alkyl-substituted ꢀ,ꢀ′-bisporphyrins that have
no meso substituents,26 the interaction of the electric transition
dipole moments of the single porphyrin moieties connected with
each other results in an exciton coupling. As a consequence, a
Davydov splitting of the B band was observed (Figure 6a), with
an energy gap of about 800 cm-1 (energetic difference between
B1 and B2), which is typical of ꢀ,ꢀ′-linked bisporphyrins,27 while
the meso,ꢀ- and meso,meso-coupled ones display B-band
splittings of about 1300 and 2100 cm-1, respectively.25 The
comparison between the UV spectrum and the CD curves of
the respective atropo-enantiomers proved that the doublet of
the Soret band indeed belonged to an exciton splitting,
since the wavelengths of B1 and B2 in the former one matched
the extreme values in the latter ones, while the wavelength of
the relative minimum between B1 and B2 corresponded to the
inflection points of the CD couplets (Figure 6c).
The length of the central C1-C49 bond (1.470 Å) correlates
with the respective values observed for a similar ꢀ,ꢀ′-linked
bisporphyrin, yet without phenyl portions in the meso positions
(1.46 Å),27 and with those of an N-confused ꢀ,ꢀ′-bisporphyrin
(1.468 Å).29 As mentioned the directly linked pyrrole subunits
are substantially tilted against each other (75.9°), which, at first
sight, should be unfavorable for the electronic ‘communication’
(54) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525–
5534.
(55) For a report on the electronic influence of para substituents on the
rotational barriers of the aryl residues in ruthenium, indium, and
titanium complexes of meso-tetraarylporphyrins, see: Eaton, S. S.;
Eaton, G. R. J. Am. Chem. Soc. 1977, 99, 6594–6599.
(56) Handbook of Chemistry and Physics, 79th ed.; Lide, D. R., Ed.; CRC
Press: Boca Raton, Boston, London, New York, Washington D.C.,
1998.
In addition, the B band appeared broadened and slightly
decreased in intensity, whereas the Q bands exhibited an
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A R T I C L E S
Bringmann et al.
Figure 6. (a) Comparison of the UV spectra of monomeric porphyrins and ꢀ,ꢀ′-bisporphyrins, exemplarily shown for the monomer 5,10,15,20-tetra(4-
tolyl)porphyrin (left), its metal-free dimer, rac-4b (center), and the fully zincated derivative rac-5b (right). (b) Overlay of the UV spectra of 5,10,15,20-
tetra(4-tolyl)porphyrin and rac-4b. (c) Correlation of the CD curves of the respective atropisomers of rac-5b to its UV spectrum.
Stereochemical Analysis. For stereochemical analysis, the
(racemic) metalated bisporphyrins thus prepared had to be
resolved into their atropo-enantiomers by HPLC on a chiral
phase, which had initially, for rac-5a (and for rac-5b), suc-
ceeded by using a Chirex-3010 column (Phenomenex) at room
temperature. The configuration at the biaryl axis of the enan-
tiomers P-5a and M-5a (and of P- and M-5b) had been
established by HPLC-CD experiments in the stopped-flow mode
in combination with quantum chemical CD calculations, reveal-
ing the P atropo-enantiomer to be the faster one.36 Since these
earlier calculations had only been done for the free base as a
Figure 7. UV spectra of the metal-free bisporphyrin rac-4b (blue), of its
simplified model compound and because the new, higher-level
monozincated derivative rac-4f (purple), and the fully metalated (two times
methods used in this paper to optimize the structure now yielded
Zn) analogue, rac-5b (red), in dichloromethane. The spectrum of the
significantly more accurate structures, the CD spectrum of the
bisporphyrin rac-5a was recalculated, too, and these calculations
confirmed the absolute configurations attributed before (see
Supporting Information).
monometalated bisporphyrin rac-4f is composed of the spectrum of the
metal-free bisporphyrin rac-4b and that of its fully metalated derivative,
rac-5b.
increased intensity (Figure 6b). Furthermore, the entire UV
spectra of ꢀ,ꢀ′-bisporphyrins were red-shifted as compared to
the respective monomeric forms (Figure 6b), indicative of the
occurrence of further effects, like electronic couplings27 or
charge-transfer transitions.31
Individual optimization of the HPLC conditions for the
metalated dimers rac-4g, rac-4h, and rac-5b-e resulted in clear
baseline separations with short retention times and perfectly
mirror-shaped CD curves (recorded in the stopped-flow mode)
for the respective atropo-enantiomers (Figure 8). As expected,
all CD spectra were strongly related to those of 5a, permitting
configurational assignment by comparison of these curves with
the ones calculated for the enantiomers of the parent dimer 5a,
P-5a, and M-5a. This clearly evidenced the chromatographically
faster atropo-enantiomers of 4g, 4h, and 5a-e to be P-
configured, and, as a consequence, the slower ones to have the
M configuration at the porphyrin-porphyrin axis. The palla-
dium(II) and copper(II) bisporphyrins rac-4i, rac-5f, and rac-
5i, by contrast, showed an opposite chromatographical behavior,
with the M enantiomer being the faster eluting one. That this
inversion is due to a different chromatographic behavior and
not a consequence of inverted CD spectra is confirmed by
satisfying accordance of the experimental CD curves of the
copper(II)/copper(II) and palladium(II)/palladium(II) bispor-
As for monomeric porphyrins,57 full metalation of the
synthesized dimers gave rise to a higher molecular symmetry,
leading to a degeneration of the transitions Q1/Q2 and Q1′/Q2′
to give only two Q bands (Q and Q′, respectively) for the
substances rac-4g-i and rac-5a-i (Figure 6a). The UV profile
of the monometalated bisporphyrin rac-4f displays the bands
typical of both, the metal-free bisporphyrin (rac-4b) and the
fully metalated one (rac-5b, Figure 7). A more detailed
assignment of electronic transitions and dicussion of the
excitonic effects in all bisporphyrins is hampered by a strong
overlap of multiple B bands, which is caused by the low
symmetry even of fully metalated bisporphyrins.
(57) (a) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138–163. (b) Gouterman,
M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York,
1978, Vol. III, pp 1-165.
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17820 J. AM. CHEM. SOC. VOL. 130, NO. 52, 2008

Axially Chiral ꢀ,ꢀ′-Bisporphyrins
A R T I C L E S
Figure 8. Stereochemical characterization of the bisporphyrins by online HPLC-CD measurements on a chiral phase, exemplarily shown for 4g, 4h, and
5a-e.
phyrins rac-5f and rac-5i with the spectra calculated for these
compounds. Moreover, the computed structures of the different
bisporphyrins are very similar to each other, independent from
the central metal. These findings preclude that there are metal-
induced conformational changes that might lead to fully inverted
CD spectra.
Although the simple exciton coupling theory is not generally
applicable to optically active porphyrin dimers,58 all of the
stereochemical assignments thus achieved were in agreement
with the results when applying the exciton chirality59 approach,
according to which those enantiomers of the fully metalated
dimeric porphyrins that possess a positive first couplet around
450 nm should be interpreted to have a ‘positive chirality’, i.e.,
to be P-configured; and, vice versa the peaks with a negative
couplet should correspond to the M enantiomers.
Configurational Stability Depending on the Central Metal
Atoms. Unexpectedly, all attempts to resolve the free-base
bisporphyrins rac-4a-e, the monometalated dimer rac-4f, or the
fully metalated nickel(II) dimers rac-5g and rac-5h into their
atropo-enantiomers failed, even on a variety of different chiral
HPLC phases.60 One reason for the observed chromatographic
behavior could be that enantiomer-specific interactions of the
bisporphyrins with the stationary phase are more differentiated
in the presence of certain metal centers. Alternatively, the nature
of the central metal might influence the geometry of the dimer,
possibly including the configurational stability at the axis; in
fact, the metal center has been reported to rigidify the molecular
framework and, thus, to influence rotational barriers of attached
aryl (though not porphyrin) substituents.61 Hence, it is imagin-
able that the metal insertion into the porphyrin backbone might
not only effect a conformational change of the macrocycle,62
but could also increase the rotational barrier about the
porphyrin-porphyrin axis. In this case the failure to resolve
the enantiomers of the metal-free, the monometalated, and the
nickelated bisporphyrins might be due to their configurational
instability. This hypothesis was supported by the fact that
complete racemization was observed when submitting an
enantiopure sample of 5a to demetalation (room temperature,
aqueous 1 N HCl) and subsequent remetalation under mildest
possible conditions, which hinted at a significantly lowered
rotational barrier of the generated metal-free intermediate, yet
not fully excluding an acid-catalyzed racemization process.
These findings made it rewarding to further investigate the metal
influence on the configurational stability.
In a first set of experiments the rotational stability of the
stereogenic axes of the homometalated dimers rac-5a (M1/M2
) Zn/Zn), rac-5f (M1/M2 ) Cu/Cu), and rac-5i (M1/M2 ) Pd/
Pd) at room temperature was investigated using the respective
P-configured stereoisomers: Enantiopure samples were isolated
by repeated HPLC runs on a semipreparative Chirex-3010
column, stored at room temperature, and analyzed by HPLC at
regular intervals (Figure 9a).
After 24 h no interconversion of P-5a into the corresponding
enantiomer M-5a was observed, suggesting that the porphyrin–
porphyrin axis of P-5a is rotationally fully stable at room
temperature. Within the same period, the enantiomeric purity
of the palladium(II) bisporphyrin rac-5i decreased to 82% ee,
while the copper(II) derivative rac-5f was already almost
racemic (4% ee). Thus, the barrier of the racemization process
strongly depends on the central metals, decreasing in the order
zinc(II)/zinc(II) > palladium(II)/palladium(II) > copper(II)/
copper(II) > nickel(II)/nickel(II) ≈ zinc(II)/free-base ≈ free-
base/free-base (Figure 9a). This remarkable phenomenonsthe
metal influence on the rotational barrier of a porphyrin-porphyrin
axisshad not previously been described in the literature. By
(58) Muranaka, A.; Asano, Y.; Tsuda, A.; Osuka, A.; Kobayashi, N.
ChemPhysChem 2006, 7, 1235–1240.
(59) Harada, N.; Nkanishi, K. Circular Dichroic Spectroscopy: Exciton
Coupling in Organic Stereochemistry; University Science Books: Mill
Valley, CA, 1983.
(61) Medforth, C. J.; Haddad, R. E.; Muzzi, C. M.; Dooley, N. R.; Jaquinod,
L.; Shyr, D. C.; Nurco, D. J.; Olmstead, M. M.; Smith, K. M.; Ma,
J.-G.; Shelnutt, J. A. Inorg. Chem. 2003, 42, 2227-2241. For an
earlier, yet more special report on phenyl ring rotation in monomeric
ruthenium, indium, and titanium tetraarylporphyrins, see ref 55.
(62) (a) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin) 1987, 64, 1–
70. (b) Senge, M. O. J. Photochem. Photobiol., B 1992, 16, 3–36.
(60) In agreement with these results, the enantiomeric resolution of
previously reported axially chiral meso,meso′-linked bisporphyrins by
HPLC on a chiral phase had likewise succeeded only for the fully
metalated derivatives.30
9
J. AM. CHEM. SOC. VOL. 130, NO. 52, 2008 17821

A R T I C L E S
Bringmann et al.
Figure 9. (a) Metal-dependence of the racemization of symmetrically (5a, 5i, and 5f) and nonsymmetrically (4d and 4i) metalated dimeric porphyrins over
time at room temperature. (b) Determination of rate constants at room temperature assuming a first-order kinetic. The metal-free, the monometalated, and
the nickelated bisporphyrins could not be resolved due to their configurational instability.
) Pd/Pd), and the heterogeneously metalated 4i (M1/M2 ) Pd/
Cu) and 4d (M1/M2 ) Ni/Zn) at room temperature (Figure 8b),
permitted to rank the synthesized ‘superbiaryls’ by their
stabilities: zinc(II)/zinc(II) (114.9 kJ mol-1) > palladium(II)/
palladium(II) > palladium(II)/copper(II) > copper(II)/copper(II)
(101.1 kJ mol-1) > nickel(II)/zinc(II) > nickel(II)/nickel(II) ≈
zinc(II)/free-base ≈ free-base/free-base (not resolvable, even at
0 °C). Therefore, the decisive factor for the rotational stability
might indeed be a conformational change of the porphyrin
subunits due to the ion radius and/or the electronic character of
the central metals. The mixed metalated bis-TAP 4i (M1/M2 )
Figure 10. Experimental determination of the atropisomerization rate,
Pd/Cu) nicely confirms the expected tendencies, as it exhibits
exemplarily shown for rac-5a, by resolution of its two atropo-enantiomers,
an energy of activation right in between the respective homo-
P-5a (peak A, blue) and M-5a (peak B, red), followed by thermal
metalated systems. The nickel(II)/zinc(II) derivative 4d, how-
ever, suggests that the conformationally labile part of the
bisporphyrins is decisive for the rotational barrier as its energy
of activation is even lower than the one of the semistable
copper(II)/copper(II) dimer 5f, although it has at least one
zincated porphyrin half.
equilibration (here exemplarily shown for 90 °C) of M-5a over time in
toluene, monitored by analytical HPLC on a chiral phase (Chirex 3010).
in-depth investigation of the isomerization barriers of the
configurationally most stable bisporphyrin 5a and the semistable,
but still separable copper(II) derivative 5f, the range of the
energies of activation for the atropo-enantiomerization of this
new class of optically active bisporphyrins was determined.
The rotational stability of the biaryl axis of the bisporphyrin
5a was investigated more closely by HPLC-UV measurements
of the temperature-dependent atropisomerization of the above
prepared enantiopure material, this time exemplarily for M-5a
(Figure 10). The atropo-enantiomerization was monitored at
three different temperatures (90, 100, 110 °C),63 permitting
evaluation of the respective rate constants of the racemization
process, and thus to calculate the Gibbs free energy of activation
at room temperature (25 °C) by the Eyring equation. The
experimental value thus obtained, ∆Grot(5a)exp ) 114.9 kJ mol-1,
clearly proved the rotational stability of the 2-fold zincated
bisporphyrin 5a.
The isomerization process of the copper(II) bisporphyrin 5f
was investigated analogously at 25, 40, and 62 °C, resulting in
a rotational barrier of ∆Grot(5f)exp ) 101.1 kJ mol-1. This value
is thus in good agreement with the observed slow interconver-
sion of the atropo-enantiomers at room temperature. The Gibbs
free energies of activation of the chromatographically separable
dimers thus range from 101.1 kJ mol-1 for the configurationally
semistable bisporphyrin 5f (M1/M2 ) Cu/Cu) to 114.9 kJ mol-1
for the fully stable derivative 5a (M1/M2 ) Zn/Zn). The
determination of these boundary values, together with the
likewise experimentally measured rate constants of 5i (M1/M2
Computational Studies on the Atropisomerization Process.
To get deeper insight into this observed metal-dependent
configurational behavior, the rotational barriers of the different
bisporphyrins were calculated with the BLYP-D method and
the above-mentioned combination of basis sets. Two possible
transition states were found for each of the calculated bispor-
phyrins, of which only the energetically clearly favored ones
will be discussed here. These transition states look quite similar
for all bisporphyrins (except for M1/M2 ) Ni/Ni, see below),
with the porphyrin planes themselves being slightly saddle-
shaped distorted and nonplanar (Figure 11). The angle between
the porphyrin planes varies depending on the kind of central
atoms (metal or, in the case of the free base, the two hydrogens).
While in previous investigations based on simpler theoretical
methods, nearly no difference in the rotational barriers between
zinc(II)/zinc(II) and the free base had been found,36 the
dispersion-corrected calculations now provided clear differences
between the investigated bisporphyrins. Unfortunately, the
calculated barriers are systematically, by about 50 kJ mol-1
,
too high in comparison to the experimental values (Table 6)
(63) The new dimers described in this paper are configurationally not as
stable as the meso,meso′-linked bisporphyrins previously reported.31
The synthesis of ꢀ,ꢀ′-bisporphyrins with additional ꢀ-substituents next
to the central axis is currently in progress in our group.
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17822 J. AM. CHEM. SOC. VOL. 130, NO. 52, 2008

Axially Chiral ꢀ,ꢀ′-Bisporphyrins
A R T I C L E S
Table 7. Redox Potentials of rac-4a-i and rac-5a-i As
Determined by DPV (Scan Rate: 20 mVs-1), Referenced against
Fc/Fc+ As the Internal Standard (0.1 M TBAH in CH2Cl2)
E1/2 (Ox1)
[mV]
E1/2 (Ox2)
[mV]
E1/2 (Ox3)
[mV]
E1/2 (Ox4)
[mV]
∆E (Ox1 - Ox2)
[mV]
rac-4a
rac-4b
rac-4c
rac-4d
rac-4e
rac-4f
rac-4g
rac-4h
rac-4i
rac-5a
rac-5b
rac-5c
rac-5d
rac-5e
rac-5f
rac-5g
rac-5h
rac-5i
480
470
565
550
825a
85
80
-
-
820a
895a
675a
830a
640a
445a
450
340
390
370
485
570
490
570
550
720
(120)b
(150)b
(180)b
(180)b
(235)b
-
650
690
660
965
890
950
925
1140
410a
380a
490a
335a
655a
650a
735a
585a
650a
940a
960a
925a
1100a
-
-
-
350
430
510
455
595
415
535
650
590
710
(65)b
105
140
135
115
a Two-electron process not resolvable even by DPV. b ∆E resulting
from different oxidation potentials of two different porphyrin subunits.
Figure 11. BLYP-D/SVP and TZVP-optimized transition states for the
atropisomerization of 4i (stereochemically stable) and 5g (configurationally
unstable). The ruffled conformation of the porphyrin subunits in 5g yields
a substantially different transition state as compared to that of 4i, which
has slightly saddle-shaped porphyrin moieties.
of configurational stability as experimentally established above.
The absence of a central metal in the free base makes it easier
for the porphyrin rings to attain the nonplanar arrangement as
required for the transition state. Whereas in the case of zinc(II),
palladium(II), and copper(II) the ground-state has planar por-
phyrin subunits, so that the system has to deviate from this
orientation in the transition state, the porphyrin plane with nickel
already shows a ground-state orientation closely related to the
transition state, thus leading to a much lower rotational barrier.
The results show that the rigidity of the porphyrin subunits
causes the largely different barriers observed. Consequently, the
better the central metal fits into the porphyrin ring and, hence,
the more planar (and ‘relaxed’) this ring is in the ground state,
the higher will be the energy required to overcome the ring
strain in the transition state and, thus, the more stable is the
configuration at the central axis.
Table 6. Calculated (BLYP-D; SVP as a Basis Set for C, H, and N
and TZVP for Metal Atoms) and Experimental Rotational Barriers
of Selected Bisporphyrinsa
central atoms
∆E (gas phase) [kJ mol-1
]
∆G (exp.) [kJ mol-1
]
rac-4a
rac-5g
rac-4g
rac-5f
rac-5i
rac-5a
2H/2H
Ni/Ni
Zn/Ni
Cu/Cu
Pd/Pd
Zn/Zn
99 (94)
-
-
128 (129)
153 (149)
156 (153)
156 (153)
163 (160)
b
-
101
b
-
115
a The results of the COSMO calculations are given in parentheses.
b Not measured.
and even calculations in a solvent field with COSMO64 or
calculations with a higher-level method, viz. the dispersion-
corrected double hybrid functional B2PLYP-D,65 exemplarily
performed for the bis-zinc(II) and the bis-palladium(II) case (viz.
bisporphyrins rac-5a and rac-5i), did not give significantly lower
values. Nonetheless, the results confirm the experimentally
determined order of stability with zinc(II)/zinc(II) (exptl 115
kJ mol-1, calcd 163 kJ mol-1) possessing the most stable axial
configuration and with copper(II)/copper(II) (exptl 101 kJ mol-1,
calcd 156 kJ mol-1) and zinc(II)/nickel(II) (calcd 152 kJ mol-1)
having the configurationally least stable arrays.66 Keeping in
mind that the energy is calculated too high (by about 50 kJ
mol-1), the nickel(II)/nickel(II) (calcd 128 kJ mol-1) and the
free-base bisporphyrins (calcd 99 kJ/mol) are configurationally
unstable as also seen in the experiment.
In the bis-nickel system rac-4i, the porphyrin moieties are
ruffled (and, possibly, more flexible due to the smaller central
metal) and the transition state is clearly different from all other
ones investigated so far. This divergent orientation and thus the
resulting smaller steric hindrance is the reason for the much
lower rotational barrier.
The versatile synthetic route to the novel class of ‘superbi-
aryls’ as presented in this paper permits free variation of the
central metals, thus paving the way to selectively tune the
rotational barrier of these porphyrin-derived ‘superbiaryls’ and
permitting the design of bimetal systems with defined rotational
stability at the biaryl axis.
Cyclovoltammetric Studies. For an investigation of the
oxidation properties of all metalated and nonmetalated bispor-
phyrins, rac-4a-4i and rac-5a-5i, cyclic voltammograms
(CVs) in CH2Cl2/tetrabutylammonium hexafluorophosphate
(TBAH) solution (Table 7) were measured. All bisporphyrins
showed at least two oxidation waves which were assigned to
four oxidation processes: (a) P-P f P+-P, (b) P+-P f
P+-P+, (c) P+-P+ f P2+-P+, (d) P2+-P+ f P2+-P2+. In most
cases, processes (a) and (b), as well as processes (c) and (d),
were found to be covered by one unresolved wave. For a more
accurate determination of the separation between the redox
potentials of the first and second oxidation steps (processes (a)
As mentioned above, the differences in the optimized
structures are small, but still large enough to explain the order
(64) Klamt, A. COSMO-RS: From Quantum Chemistry to Fluid Phase
Thermodynamics and Drug Design; Elsevier: Amsterdam, 2005.
(65) Grimme, S. J. Chem. Phys. 2006, 124, 034108/1–034108/16.
(66) Regarding the rotational barriers the calculations fail to differentiate
between the Pd/Pd and the Cu/Cu dimer, certainly because the Pd/Pd
complex should be influenced by relativistic effects that can not be
taken into account with the used methods.
9
J. AM. CHEM. SOC. VOL. 130, NO. 52, 2008 17823

A R T I C L E S
Bringmann et al.
Figure 13. CV (top) and DPV (bottom) of rac-5g in 0.1 M TBAH in
CH2Cl2 referenced vs Fc/Fc+ as the internal standard. The first two oxidation
processes are clearly split.
Figure 12. CVs (scan rate: 250 mV s-1) and DPVs of 5d, 5b, 5a, 5c with
decreasing donor strength of the meso substituents in this order.
reaction conditions now open up an efficient access to ꢀ-bory-
lated porphyrins with four meso-aryl substituents. These as yet
unknown porphyrin derivatives should be of high synthetic
value, not only in terms of the construction of bisporphyrins
described in this paper, but also as versatile building blocks for
the design of diverse ꢀ-functionalized porphyrins46 and large
porphyrin assemblies.32
The key step of the synthesis was a Suzuki coupling of the
boronic acid esters 3a-k with the brominated porphyrins 1a-k.
By application of the optimized procedure, various novel ꢀ,ꢀ′-
bisporphyrins have become available in good yields differing
in their substitution pattern and in their metalation state. The
coupling step is highly efficient and variable, which was
demonstrated by the directed synthesis of unsymmetric bispor-
phyrins either bearing different meso-aryl residues in the
porphyrin moieties (like rac-4e and rac-5e) or two different
metal centers (rac-4g-i), and by the preparation of monometa-
lated representatives, as exemplarily shown by the synthesis of
rac-4f.
and (b), see above), differential pulse voltammetry (DPV) was
performed. Table 7 shows the determined redox potentials and
∆E values.
The oxidation potentials of the porphyrins are known to be
influenced by two effects: the donor strength of the meso
substituents and the electronic character of metal ion. As
expected, the oxidation potentials of both, the metalated and
the free-base porphyrins increased with decreasing donor
strength of the meso substituents, viz. 4d < 4b < 4a < 4c and
5d < 5b < 5a < 5c (Table 7). As an example, the CVs and
DPVs of zincated compounds rac-5a-5d are shown in Figure
12.
Furthermore, even the type of metal was found to influence
the oxidation potentials of the bisporphyrins significantly. As
indicated by the data in Table 7, the oxidation potentials
increased in the order zinc(II) < copper(II) < free-base <
nickel(II) < palladium(II) for porphyrins rac-5a, rac-5f, rac-
4a, rac-5g, and rac-5i with identical meso substituents. Another
effect that occurred upon exchange of the metal ion, was a
variation of the potential splitting (∆E) between the first
oxidation step and the second one. All zinc(II) bisporphyrins
showed a single wave for the first two oxidation processes,
which was not resolvable even by DPV like, e.g., rac-5a in
Figure 12. For the copper(II), nickel(II), and palladium(II)
derivatives and for the free-base bisporphyrin rac-4a, by
contrast, well separated waves were observed at least in the DPV
measurements (see, e.g., CV and DPV of rac-5g in Figure 13).67
In their metalated form, most of the novel-type bisporphyrins
thus prepared were shown to be rotationally stable and thus
axially chiral and were fully characterized stereochemically by
stopped-flow HPLC-CD experiments assisted by quantum
chemical CD calculations. The barrier of rotation about the chiral
porphyrin-porphyrin axis was found to be strongly dependent
on the nature of the metal centers coordinated by the bispor-
(67) It is tempting to interpret this potential splitting as being due to different
electronic ‘communication’ between the two porphyrin subunits, also
because many other factors like, e.g., ion pairing effects, control the
redox splitting significantly.68 On the other hand, Arnold et al.69
observed an analogous behavior for butadiyne-bridged metalated
octaethyl-substituted bis(octaethylporphyrins) and attributed this wave
splitting to aggregate formation of cationic porphyrin units.70 Despite
the apparent steric hindrance by the meso substituents it might indeed
be imaginable that the meso-aryl substituted bisporphyrins in this paper
may form aggregates.
Conclusion
A series of novel, directly ꢀ,ꢀ′-linkedsand thus intrinsically
axially chiralsbisporphyrins have been synthesized. The prepa-
ration involved a Pd-catalyzed Miyaura-like transmetalation of
monobrominated porphyrins. This reaction, which is unpre-
cedented in porphyrin chemistry with respect to the sterical
requirements at the coupling position, necessitated the develop-
ment of a method for the formation of a variety of novel
ꢀ-boronic acid esters of tetraarylporphyrins. Our optimized
(68) Barrie`re, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980–3989.
(69) Arnold, D. P.; Heath, G. A.; James, D. A. J. Porphyrins Phthalocya-
nines 1999, 3, 5–31.
(70) Brancato-Buentello, K. E.; Kang, S.-J.; Scheidt, W. R. J. Am. Chem.
Soc. 1997, 119, 2839–2846.
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17824 J. AM. CHEM. SOC. VOL. 130, NO. 52, 2008

Axially Chiral ꢀ,ꢀ′-Bisporphyrins
A R T I C L E S
1474 (m), 1349 (m), 1141 (s), 1029 (s), 964 (m), 800 (m) cm-1
;
phyrin backbone, thus offering the opportunity to selectively
manipulate the chiroptical properties of these novel ‘superbi-
aryls’. Such a phenomenon or a similar metal influence on the
atropisomerization process of bisporphyrins has not been
described so far.
1H (400 MHz, CDCl3): δ -2.64 (s, 2H, NH), 1.20 (s, 12H, pinacol),
7.67-7.85 (m, 12H, m/p-Ar-H), 8.15-8.31 (m, 8H, o-Ar-H), 8.65
(d, J ) 4.8 Hz, 1H, ꢀ-pyrrole-H), 8.74 (d, J ) 4.8 Hz, 1H,
ꢀ-pyrrole-H), 8.78-8.86 (m, 4H, ꢀ-pyrrole-H), 9.14 (s, 1H,
ꢀ-pyrrole-H); 13C NMR (100 MHz, CDCl3): δ 25.6, 84.1, 127.0,
127.1, 128.0, 128.1, 134.9, 135.0, 135.2, 136.6, 161.0; MS (EI, 70
eV): m/z (%) ) 740 (100) [M+], 613 (15) [M+ - BPin]; HRMS
(ESI) calcd for C50H42BN4O2 [M + H+] 741.3401, found 741.3401;
UV-vis (CH2Cl2): λmax (log ε) ) 421 (5.56), 519 (4.20), 555 (3.86),
596 (3.71), 656 (3.74) nm; Anal. Calcd for C50H41BN4O2: C, 81.08;
H, 5.58; N, 7.56; found: C, 81.62; H, 5.65; N, 7.60.
This previously not observed tuning of a chiral porphyrin-
porphyrin axis by variation of the central metals in the porphyrin
subunits was additionally confirmed and further rationalized by
quantum chemical calculations with the BLYP-D method. Using
the dispersion correction, which turned out to be a substantial
improvement for the calculations on the porphyrin dimers, the
computational investigations showed that the ring strain of the
monomeric porphyrin subunits as a consequence of the presence
and type of the central metals is decisive for the rotational
barrier. Whereas zinc(II), palladium(II), and copper(II) fit quite
well into the porphyrin ring and therefore result in planar
subunits, nickel(II) tends to be tetrahedrally coordinated in the
ground state, leading to a ruffled conformation of the nickelated
porphyrin moieties and to a significantly different structure of
the transition state in comparison to those of the other metalated
bisporphyrins and, thus, to a much lower rotational barrier.
In contrast to directly linked chiral bis-TAPs reported
previously,30 the axial chirality of these extended biaryls is
independent from the substitution pattern of the porphyrin
backbone,29 which permits a more convenient preparation,
without any restrictions concerning the symmetry of the
monomeric substructures. The described bisporphyrins open a
vast, stereochemically largely differentiated cavity including two
freely combinable metal centers, giving rise to promising
applications of such compounds as mono- or bimetallic catalysts
in asymmetric synthesis or as chiral reporter groups. Moreover,
ꢀ,ꢀ′-bisporphyrins might open new perspectives for the design
of optical and electronic devices, and of novel materials with
unique chiral and chiroptical properties. These expected proper-
ties and possible applications make the development of a first
directed access to enantiopure material by the atropo-enanti-
oselective synthesis of axially chiral porphyrin-derived com-
pounds a rewarding task. This work is in progress.
Synthesis of ꢀ,ꢀ′-Bisporphyrins 4 (Typical Procedure): ꢀ,ꢀ′-
Bis(5,10,15,20-tetraphenyl-porphyrin) (rac-4a). A Schlenck flask
filled with boronic ester 3a (300 mg, 406 µmol, 1 equiv), 2-bromo-
5,10,15,20-tetraphenylporphyrin (1a, 338 mg, 488 µmol, 1.2 equiv),
and Ba(OH)2 ·8H2O (1.28 g, 4.06 mmol, 10 equiv) was evacuated
and flushed with nitrogen three times. Toluene (160 mL) and
deionized H2O (32 mL) were added and the solution was degassed
by a nitrogen stream in an ultrasonic bath for 15 min. Pd(PPh3)4
(93.8 mg, 81.2 µmol, 20 mol %) was added and the resulting
mixture was refluxed for 14 h with vigorous stirring. After addition
of H2O and CH2Cl2, the organic phase was separated and the
aqueous layer was extracted three times with CH2Cl2. The combined
organic layers were washed with H2O and saturated aqueous NH4Cl
and dried over MgSO4. Concentration of the filtrate and purification
of the resulting residue by column chromatography on silica gel
using petroleum ether/CH2Cl2 (4:1 to 0:1, v/v) as the eluent afforded
rac-4a as a purple solid (364 mg, 296 µmol, 73%). mp (CH2Cl2/
MeOH) > 300 °C; IR (KBr): ν˜ ) 3439 (w), 2921 (m), 1616 (w),
1
1511 (m), 1461 (m), 1020 (m), 964 (m), 795 (s) cm-1; H (400
MHz, CDCl3): δ -2.71 (bs, 2H, NH), 3.98 (bs, 1H, Ar-H), 4.92
(bs, 1H, Ar-H), 6.34 (bs, 1H, Ar-H), 6.57 (d, J ) 6.7 Hz, 1H,
Ar-H), 7.30 (d, J ) 7.4 Hz, 1H, Ar-H), 7.62-7.88 (m, 9H,
Ar-H), 8.02 (d, J ) 4.8 Hz, 1H, ꢀ-pyrrole-H), 8.10 (bs, 1H Ar-H),
8.17-8.29 (m, 3H, Ar-H), 8.36-8.46 (m, 2H, Ar-H), 8.54 (d, J
) 4.8 Hz, 1H, ꢀ-pyrrole-H), 8.73 (s, 1H, ꢀ-pyrrole-H), 8.82 (d, J
) 4.8 Hz, 1H, ꢀ-pyrrole-H), 8.85-8.88 (m, 2H, ꢀ-pyrrole-H), 8.90
(d, J ) 4.8 Hz, 1H, ꢀ-pyrrole-H); 13C NMR (100 MHz, CDCl3): δ
45.9, 119.8, 119.9, 120.0, 120.3, 123.2, 124.4, 124.9, 126.6, 126.8,
127.6, 127.7, 127.8, 130.7, 134.5, 134.6, 135.0, 139.8, 142.2, 142.4,
142.7, 145.2; HRMS (ESI) calcd for C88H59N8 [M + H+]
1227.4857, found 1227.4869; UV-vis (CH2Cl2): λmax (log ε) )
423 (5.55), 438 (5.54), 523 (4.64), 555 (4.15), 596 (4.10), 652 (3.74)
nm.
Experimental Section
Synthesis of ꢀ-Borylated Tetraarylporphyrins 3 (Typical
Procedure): 2-(4′,4′,5′,5′-Tetramethyl-[1′,3′,2′]dioxaborolan-2′-
yl)-5,10,15,20-tetraphenylporphyrin (3a). Under a nitrogen at-
mosphere, a suspension of 2-bromo-5,10,15,20-tetraphenylporphyrin
(1a, 558 mg, 804 µmol, 1 equiv), KOAc (790 mg, 8.04 mmol, 10
equiv), and 4,4,5,5-tetramethyl-2-(4′,4′,5′,5′-tetramethyl-[1′,3′,2′]-
dioxaborolan-2-yl)-[1,3,2]dioxaborolane (510 mg, 2.02 mmol, 2.5
equiv) in toluene (200 mL) and deionized H2O (40 mL) was
degassed with nitrogen in an ultrasonic bath for 15 min. Pd(dppf)Cl2
(131 mg, 161 µmol, 20 mol %) was added and the reaction mixture
was heated to 110 °C for 3 h with vigorous stirring. After addition
of CH2Cl2 to the suspension, the organic phase was washed with
H2O and saturated aqueous NH4Cl. The combined organic layers
were dried over MgSO4, the solvent was removed in vacuo, and
the resulting solid was purified by column chromatography on silica
gel using petroleum ether/CH2Cl2 (5:1 to 0:1, v/v) as the eluent to
yield 3a as a purple solid (414 mg, 558 µmol, 70%). mp (CH2Cl2/
MeOH) > 300 °C; IR (KBr): ν˜ ) 3450 (w), 2976 (m), 2853 (w),
Acknowledgment. This work was supported by the Fonds der
Chemischen Industrie, the Degussa Stiftung, and the Studienstiftung
des deutschen Volkes e.V. (fellowships to D. C. G. Go¨tz).
Supporting Information Available: General experimental
1
details, complete experimental procedures, copies of the H
spectra, CD spectra, chromatographic conditions for the resolu-
tion of the atropo-enantiomers, data of the kinetic study of the
atropo-enantiomerization, UV data, information concerning
the computational methods, and details of the crystal structure
determination. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA8055886
9
J. AM. CHEM. SOC. VOL. 130, NO. 52, 2008 17825