Introduction of chirality to the remote, open face of a metalloporphyrin through coordination to the metal of a specially designed pendant arm

Article abstract of DOI:10.1246/cl.2003.20

Chirality has been introduced to a metalloporphyrin by a new approach that involves coordination to the metal of a pendant arm, with the preferred conformation adopted by the resulting chelate ring causing the meso-phenyl group to which the arm is attached to twist and thereby introduce chirality to the remote, open face of the metalloporphyrin.

Full text of DOI:10.1246/cl.2003.20

20
Chemistry Letters Vol.32, No.1 (2003)
Introduction of Chirality to the Remote, Open Face of a Metalloporphyrin through
Coordination to the Metal of a Specially Designed Pendant Arm
Akihisa Hamazawa, Toshihiro Yano, Takanori Nishioka, Isamu Kinoshita,ꢀ Kiyoshi Isobe, Shigenobu Yano,y
L. James Wright,yy and Terrence J. Collinsyyy
Department of Material Science, Graduate School of Science, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585
yDivision of Material Science, Graduate School of Human Culture, Nara Women’s University, Kitauoyanishimachi, Nara 630-8506
yyDepartment of Chemistry, The University of Auckland, Private Bag92019, Auckland 1020, N. Z.
yyyDepartment of Chemistry, Carnegie Mellon University, 4400, Fifth Av., Pittsburgh, PA 15213-2683, U. S. A.
(Received September 9, 2002; CL-020771)
Chirality has been introduced to a metalloporphyrin by a new
approach that involves coordination to the metal of a pendant arm,
with the preferred conformation adopted by the resulting chelate
ring causing the meso-phenyl group to which the arm is attached
to twist and thereby introduce chirality to the remote, open face of
the metalloporphyrin.
The study of chiral porphyrins and metalloporphyrins has
developed into an important area of scientific endeavour. Interest
in these compounds is stimulated by the diverse relevance they
Scheme 1. Syntheses of 1–4. i) 2,6-pyridinedicarbonyl dichloride, NEt3,
THF. ii) Zn(OAc)2Á2H2O, CHCl3, MeOH.
have in medicine,1 enantioselective catalysis,2 chiral rec-
ognition,3 chirogenesis4 and even chiral sensing and memory.5
Chirality has been introduced into porphyrins using several
different strategies. The most common approach involves the
attachment of chiral units either to preformed porphyrins or to the
aldehyde and pyrroles employed in the porphyrin-forming
stage.2g An alternative approach involves the introduction of
achiral substituents to the porphyrin core in such a way that
chirality is generated. For example, intrinsically chiral porphyrins
have been formed by the introduction of selectively substituted,
achiral meso-aryl groups,6 or by the addition of achiral straps,
either between the ortho-positions of adjacent meso-phenyl
substituents7 or opposite b-pyrrolicpositions. 8
chelate ring adopts a conformation that is chiral and the unit cell
contains a pair of enantiomers related by an inversion centre. In
the enantiomer depicted in Figure 1, the coordinated arm has M
helicity and the diastereotopic nature of the methylene hydrogen
atoms can be clearly seen, with one C–H bond lying parallel to the
porphyrin ring and the other directed towards the porphryin ring.
One significant feature associated with the chelate ring is that the
ortho-substituted meso-phenyl group is rotated by 22.5ꢁ from the
perpendicular to the average plane through the porphyrin ring.
The rotation of this one phenyl group introduces chirality to the
open porphyrin face remote from the chelate ring.
In this paper we describe the induction of chirality into a
metalloporphyrin by a new approach that involves coordination to
the metal of a specially designed pendant arm that is covalently
attached to the ortho-position of one of the meso-phenyl groups.
The conformation adopted by the chelated pendant arm causes the
attached meso-phenyl ring to twist with respect to the porphyrin
plane, and this twist imparts chirality to the remote, open face of
the metalloporphyrin.
The achiral porphyrin 1 was prepared by treatment of 5-(2-
aminopheny)-10,15,20-triphenylporphyrin with 2,6-pyridinedi-
carbonyl dichloride and 2-(aminomethyl)pyridine as shown in
Scheme 1. Metallation of 1 with Zn(OAc)2Á2H2O affords the zinc
porphyrin 2 and the structure of 2 was determined by X-ray
crystallography.9 The terminal pyridine of the pendant arm is
Figure 1. a) ORTEP drawing of the M helical isomer of 2. b) Schematic
presentation of pendant arm of 2. One of the methylene protons lies parrallel
to the porphyrin ring, and the other points directly towards the porphyrin
ring.
Studies of 2 by variable temperature, solution 1H NMR
spectroscopy show that the CH2 protons undergo an equilibration
process at higher temperatures that is rapid on the 1H NMR time-
scale. Thus, at 293 K the two CH2 protons of 2 are observed as a
broad singlet at 0.5 ppm. As the temperature is lowered to 243 K
this signal practically disappears, and at 193 K, the signal for the
CH2 group reappears as two sharp signals at 2.84 ppm and
À2:26 ppm (see Figure 2). The large difference in the chemical
shift observed between the signals at low temperature is
ꢀ
axially coordinated to the zinc atom (Zn–Npyridine = 2.234(4) A)
ꢀ
and the Zn(II) ion deviates 0.366(2) A from the 4Npyrrole plane
toward the axial pyridine. The 2,6-diamidepyridine group, with
hydrogen bonds between the amide nitrogen atom and the central
pyridine nitrogen atom, is rigidly planar. The chelate ring gains
flexibility through rotations about the bonds to the phenyl group
and the methylene carbon atom. In order to accommodate
coordination of the terminal pyridine nitrogen atom to zinc, the
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.1 (2003)
21
and that the origin of the chirality must therefore be mainly caused
by the coordination of the side arm.
In conclusion, we report here a new approach to the synthesis
of chiral metalloporphyrins. In 2, a specially designed pendant
arm, which is covalently attached to the ortho-position of a meso-
phenyl group, coordinates to the metal. The arm has limited
flexibility and the chirality arises from the conformation adopted
by the chelate ring and the associated twisting of the attached
meso-phenyl ring with respect to the porphyrin plane. The two
enantiomers of 2rapidly interconvert at 293 K. As they do this, the
helicity of the coordinated arm and the direction of tilt of the
meso-phenyl ring changes. When one of the hydrogens of the
methylene group is replaced by methyl, as in metalloporphyrin 4,
the chelate ring is locked into one conformation and only one
enantiomer is observed. Importantly, the imposed, controlled
rotation of the meso-phenyl group with respect to the porphyrin
plane introduces chirality to the remote, open face of the
porphyrin. In principle this feature could be utilized in the design
of chiral, synthetic enzymes, such as those modelled on the heme-
Figure 2. Variable-temperature 1H NMR spectra (400 MHz) of a solution
of 2 in CD2Cl2 between 193 and 293 K.
consistent with the two CH2 protons being located in very
different environments, as was found in the solid-state structure.
The coalescence of these signals as the temperature rises is
interpreted as resulting from a rapid interchange between the two
enantiomers of 2 through a simple conformational change of the
chelate ring. From the difference in chemical shifts of the
diastereotopicmethylene protons at low temperature and the
coalescence temperature of 243 K, ÁGz for this dynamicprocess
containing cytochromes P450
.
This work is financially supported by Ministry of Culture,
Sports and Science.
was estimated as 10.2 kJ molÀ1 10
.
If (R)-1-(2-pyridyl)ethylamine11 is used instead of 2-(amino-
methyl)pyridine in the synthesis of the pendant arm porphyrin
(see Scheme 1) compound 3 is produced. Compound 3 is an
analogue of 1 in which one of the methylene hydrogen atoms has
been replaced with a methyl group. Metallation of 3 with
Zn(OAc)2Á2H2O produces 4, which is the corresponding ana-
logue of 2. 1H NMR data for 4 indicate that the terminal pyridine
of the pendant arm is coordinated to zinc in the same manner as it
is in 2. The resonances of the two amide protons, which are
strongly influenced by the porphyrin ring current, are observed at
4.55 and 10.72 ppm. The corresponding resonances in 2 appear at
4.16 and 11.2 ppm. Only one sharp signal is observed in each case
for the CH (À1:67 ppm) and CH3 (0.36 ppm) protons of the
CHCH3 group over the entire temperature range investigated
(193–293 K). This indicates that the chelate ring in 4 is
conformationally locked and only one enantiomer is formed.
The data is most consistent with this enantiomer being the one that
has the methylene proton pointing in towards the porphyrin ring.
Figure 3 shows the circular dichroism spectra of 3 and 4. A
significant Cotton effect appears around the Soret band of 4, but
this is not observed for 3. The difference between the CD spectra
for 3 and 4 indicates the presence of chirality for 4 but not for 3,
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Crystal data for 2: C57H38N8O2Zn, Mr ¼ 932:36, purple blocks, monoclinic,
ꢀ
ꢀ
ꢀ
space group P21=c (no. 14), a ¼ 11:742ð6Þ A, b ¼ 17:379ð7Þ A, c ¼ 22:29ð1Þ A,
ꢀ ¼ 99:267ð9Þꢁ, V ¼ 4489ð3Þ A , Z ¼ 4, Fð000Þ ¼ 1928:00; Dcalcd
¼
ꢀ 3
1:379 g c mÀ3, ꢁ(Mo Ka) = 6.03 cmÀ1. Crystal dimensions: 0:30 ꢂ 0:10 ꢂ
0:40 mm3. A total of 33819 reflections was collected, 9885 unique (Rint ¼ 0:117).
The structure was solved by direct method (SIR92), and developed through
subsequet cycles of least squares refinement and difference Fourier synthesis, final
R1 ¼ 0:068 and Rw ¼ 0:190 for 5330 refluctions (I > 2ꢂðIÞ) with a GOF of 0.98.
Data were measured on a Rigaku/MSC mercury CCD diffractometer with graphite
ꢀ
monochromaized Mo Ka (l ¼ 0:71070 A) radiation at 153 K.
10 The 1H NMR behavior of the CH2 protons does not change with addition of up to
10 equivalent of CD3CN. The chemical shift of the 6th position of the coordinated
pyridine stays in the same field from 193–293 K indicating the same coordination
for these temperature regions.
11 T. Yano, R. Tanaka, T. Nishioka, I. Kinoshita, K. Isobe, L. J. Wright, and T. J.
Collins, Chem. Comm., 2002, 1396.
Figure 3. Circular dichroism spectra of solutions of 3 and 4 in CH2Cl2.