Chiroptical and catalytic properties of doubly binaphthyl-strapped chiral porphyrins

Article abstract of DOI:10.1039/c8cc09114e

Doubly (R)-binaphthyl-strapped porphyrins with methylene chains were synthesized. The CD spectra showed the positive Cotton effect around the Soret bands, and several porphyrins showed CPL. In addition, we found that the chiral porphyrins were applicable to kinetic resolution of epoxide with CO₂.

Full text of DOI:10.1039/c8cc09114e

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Chiroptical and catalytic properties of doubly
binaphthyl-strapped chiral porphyrins†
Cite this:DOI: 10.1039/c8cc09114e
Chihiro Maeda, * Kanae Ogawa, Kosuke Sadanaga, Kazuto Takaishi
and
Received 16th November 2018,
Accepted 12th December 2018
Tadashi Ema
*
DOI: 10.1039/c8cc09114e
rsc.li/chemcomm
Doubly (R)-binaphthyl-strapped porphyrins with methylene chains were used as catalysts for the kinetic resolution of styrene oxide
were synthesized. The CD spectra showed the positive Cotton with CO2. Herein, we report the chiroptical properties and
effect around the Soret bands, and several porphyrins showed catalytic behaviors of the strapped porphyrins.
CPL. In addition, we found that the chiral porphyrins were applicable
to kinetic resolution of epoxide with CO2.
Synthetic routes to strapped porphyrins 1 and 2 are shown in
Scheme 1. First, we attempted to synthesize dialdehyde 6d by the
reaction of 3-(6-bromohexyloxy)benzaldehyde (3d) with (R)-BINOL.
Distorted aromatic p-systems have emerged as an attractive topic However, 6d was not obtained reproducibly. Therefore, the formyl
because of their unique electronic states and novel functions that group of 3d was once protected, and the resulting 4d was reacted
are not seen in planar p-molecules.1 Strapped porphyrins were with (R)-BINOL to provide 5d, which was converted into 6d and
synthesized to investigate and control the porphyrin p-system, obtained in an acceptable yield by the deprotection. Next, we
molecular recognition, and catalysis.2,3 Particularly, the introduction performed the acid-catalyzed condensation of 6d with 2 equiv. of
of chirality to porphyrins gave rise to fascinating properties,4 and pyrrole, which provided structural isomer 8d as a major product
several chiral strapped porphyrins were used to study their chir- instead of 1d. To obtain 1d selectively, the production of 8d needed
optical properties and asymmetric catalytic properties. Among to be suppressed. Therefore, we prepared bis(dipyrromethane) 7d
them, some strap bridges deform the porphyrin plane.2g,i,j,3g
by treatment of 6d with excess pyrrole. The acid-catalyzed
Recently, circularly polarized luminescence (CPL) materials condensation of 6d and 7d selectively produced 1d with 3.7%
have been extensively studied.5 Helicenes, binaphthyl derivatives, yield. Zinc complex 2d was obtained by the reaction of 1d with
and twisted molecules have been reported to show CPL.6–8 Zn(OAc)2. 1a–c and 2a–c with different methylene chain lengths
Although chiral strapped porphyrins would serve as potential were also obtained in a similar manner. Single crystals of 1d
CPL materials, CPL of chiral porphyrins have been scarcely suitable for X-ray diffraction analysis were obtained via vapor
reported.9 As another recent interesting research topic, carbon diffusion of acetonitrile into a CH2Cl2 solution of 1d. X-ray
dioxide (CO2) fixation has been actively explored.10 Synthesis of diffraction analysis unambiguously revealed the structure of 1d
cyclic carbonates from epoxides and CO2 is one of the most (Fig. 1).‡ Side view indicated the distorted structure, and the
efficient CO2 fixation method, and several metalloporphyrins mean plane deviation (MPD) of the porphyrin plane was
have been used as catalysts for the reaction.11–13 This reaction calculated to be 0.119 Å. Structural isomers 8a–d and 9a–d
is applicable for the kinetic resolution of chiral epoxides using were also prepared for comparison.
metal catalysts and organocatalysts.14–16
The 1H NMR spectra of these porphyrins exhibited down
In this work, we synthesized doubly-binaphthyl strapped field shifts for the inner NH protons (0.01 ppm, À1.41 ppm,
chiral porphyrins 1 with methylene chains and their zinc complexes À1.54 ppm, and À1.92 ppm for 1a, 1b, 1c, and 1d, respectively)
2 (Scheme 1). The effect of the methylene chain length on the as compared to those of tetraphenylporphyrin (TPP) (À2.8 ppm).
distortion of the porphyrin plane and on the chiroptical properties The ring current effect of 1a–d was probably weakened by the
including CPL was investigated. In addition, the Zn(II) complexes distortion of the porphyrin planes. Indeed, the MPDs of the
porphyrin plane of the DFT-optimized structures were calculated
Division of Applied Chemistry, Graduate School of Natural Science and Technology,
to be 0.301 Å, 0.241 Å, 0.170 Å, and 0.119 Å, and the calculated
strain energy based on TPP were 12.5 kcal molÀ1, 8.1 kcal molÀ1
,
Okayama University, Tsushima, Okayama 700-8530, Japan.
E-mail: cmaeda@okayama-u.ac.jp, ema@cc.okayama-u.ac.jp
5.1 kcal molÀ1, and 3.7 kcal molÀ1 for 1a, 1b, 1c, and 1d, respectively
(ESI†).17 The strain energies for the corresponding Zn(II) porphyrins
† Electronic supplementary information (ESI) available: Experimental procedures
and compound data. CCDC 1873097. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c8cc09114e
2a, 2b, 2c, and 2d were estimated to be 12.4 kcal molÀ1
,
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Fig. 2 (a) UV/vis absorption (left) and fluorescence (right) spectra of 1a–d
in 1,4-dioxane. (b) CD spectra (left) of 1a–d and CPL spectra (right) of 1b–d
in 1,4-dioxane.
The UV/vis absorption and fluorescence spectra of 1 and 2
are shown in Fig. 2a, and the data are summarized with TPP
and ZnTPP in Table 1. The absorption spectra of 1 showed
bands at ca. 230 nm derived from binaphthyl moieties, and
420 nm and 500–650 nm as Soret and Q-bands, respectively.
The absorption maxima of the free-base porphyrins increased
in the order: TPP o 1d o 1c o 1b o 1a. The fluorescence
maxima also increased in the same order, while the fluorescent
quantum yields decreased with a decrease in the methylene
chain length. In particular, the molar absorption coefficient of
1a is low probably because of the weak aromaticity and/or
the strong intramolecular interaction between the binaphthyl
and porphyrin moieties. Zinc complexes 2a–d also showed
red-shifted bands as the methylene chain length decreased.
On the other hand, 8a–d or 9a–d gave almost unchanged
spectra as compared with TPP or ZnTPP, regardless of the
length of the methylene chain (ESI†).
Scheme 1 Synthesis of doubly binaphthyl-strapped porphyrins 1, 2, 8,
and 9.
The CD spectra of 1a–d are shown in Fig. 2b. Notably, positive
Cotton effects were observed in the porphyrin Soret region
(420–450 nm), suggesting induced CD effect.18 Interestingly, the
Table 1 Photophysical data in 1,4-dioxane
Fig. 1 X-ray crystal structure of 1d. Hydrogen atoms except for NH
protons are omitted for clarity. Side view of the tetraphenylporphyrin
moiety is shown. Thermal ellipsoids are at the 30% probability level.
a
b
Compd
lSoret (nm)
lQ (nm)
lF (nm)
FF
1a
1b
1c
1d
TPP
2a
2b
2c
2d
430
423
422
420
416
439
430
428
426
423
529, 568, 601, 654
520, 555, 595, 648
518, 553, 593, 647
515, 550, 590, 645
513, 547, 591, 648
573, 618
562, 603
559, 596
555, 594
555, 594
663, 724
655, 717
650, 715
649, 714
647, 713
623
612, 660
608, 655
602, 651
601, 605
0.030
0.053
0.066
0.066
0.088
0.011
0.026
0.026
0.023
0.028
8.1 kcal molÀ1, 4.9 kcal molÀ1, and 3.7 kcal molÀ1, respectively.
The nucleus independent chemical shift (NICS) values were
calculated to be À12.8, À13.9, À13,7, À13.3, and À14.4 at the
central positions of 1a, 1b, 1c, 1d, and TPP, respectively,
suggesting weakened aromaticity of 1a–d, which is consistent
with the results of NMR. The effect of distortion was also
observed in the photophysical properties as shown below.
ZnTPP
a
b
Excited at lSoret
.
Absolute fluorescence quantum yields.
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a
Table 2 Kinetic resolution of styrene oxide with CO2
In summary, we have synthesized doubly-binaphthyl strapped
chiral porphyrins. The structures were characterized by NMR
spectroscopy and X-ray diffraction analysis. The 1H NMR spectra,
X-ray structure, and DFT-optimized structures indicated the
distorted porphyrin planes. The distortion affected the photo-
physical properties. Notably, the free-base porphyrins 1a–d
showed strong CD signals in the Soret region, and 1b–d showed
moderate CPL. The dependence on the methylene chain length
was also observed for the kinetic resolution of styrene oxide with
CO2 using the chiral Zn(II) porphyrins as catalysts. Among the
Zn(II) porphyrins, 2c showed the highest enantioselectivity of
s = 4.1. Further investigations on the development of chiral
metalloporphyrins as CPL materials and chiral catalysts are
currently underway in our laboratory.
This work was supported by JSPS KAKENHI Grant Number
JP16H01030 in Precisely Designed Catalysts with Customized
Scaffolding. We thank Prof. A. Osuka and Dr T. Tanaka
(Kyoto University) for X-ray diffraction analysis and fluorescence
measurements, and Prof. H. Yorimitsu (Kyoto University) for
mass measurements.
Yield%c (ee%)d
Entry
Cat.
cb (%)
(S)-11
(R)-10
s valuee
1
2
3
4
2a
2b
2c
2d
13
2c
25
10
10
41
48
27
20 (9.4)
10 (25)
7 (44)
40 (22)
45 (2.9)
18 (54)
62 (3.1)
71 (2.7)
72 (5.1)
31 (15)
45 (2.7)
55 (20)
1.2
1.7
2.7
1.8
1.1
4.1
5
6 f
a
Reaction conditions: styrene oxide (0.3 mmol), Zn(II) porphyrin
(0.05 mol%), TBAB (0.1 mol%), CO2 (1.7 MPa) in a 30 mL autoclave.
b
c
Conversion calculated from c = ee(10)/(ee(10) + ee(11)). Isolated
d
e
yield. Determined by HPLC or GC analyses. Calculated from
f
s = ln[1 À c(1 + ee(11))]/ln[1 À c(1 À ee(11))]. Reaction conditions: 2c
(0.5 mol%), TBAB (1 mol%), 6 h.
De values at lSoret increased in the order: 1d o 1c E 1b o 1a.
TD-DFT calculations indicated that the CD intensity increased
with increasing distortion. 2a–d also showed CD signals in the
Soret region (ESI†), which implies that there was a chiral space
around the porphyrin planes and that these Zn(II) porphyrins
may be applicable to asymmetric reactions. On the other hand,
8a–d and 9a–d showed no appreciable CD signals in the Soret
and Q regions.
Conflicts of interest
There are no conflicts to declare.
Notes and references
‡ Crystal data for 1d: formula: 2(C108H98N4O8)Á3(CH2Cl2)Á4(C2H3N)Á
Since these chiral porphyrins showed fluorescence, we measured
CPL. Fortunately, 1b–d showed CPL in the region where fluores-
cence was observed (Fig. 2b). The Kuhn’s anisotropy factors
(glum) were À6.8 Â 10À4, À8.8 Â 10À4, and À5.4 Â 10À4, for 1b,
1c, and 1d, respectively.19 In contrast, CPL of 1a was not
observed. Other porphyrins showed no detectable CPL signals.
Finally, we used Zn(II) porphyrins 2a–d and 9a–d as chiral
catalysts for the kinetic resolution of styrene oxide (10) with
CO2. 10 was heated at 50 1C in the presence of 0.05 mol% of
each Zn porphyrin with TBAB (0.1 mol%) under CO2 (1.7 MPa)
for 24 h (Table 2). Products 10 and 11 were isolated, and their
enantiomeric excesses were analysed. 2a–d showed the s values
of 1.2–2.7, while 9a–d showed no enantioselectivity at all (ESI†).
In addition, singly-strapped porphyrins 12 and 13 were synthesized,
and Zn(II) complex 13 was used as a reference catalyst (Scheme 2). 13
recorded an s value of only 1.1, which indicates the importance of
the double straps (entries 3 and 5). Since 2c showed the highest s
value, the reaction conditions were optimized, and a higher s value
of 4.1 was recorded by using 0.5 mol% of 2c with the reaction time
of 6 h (entry 6).
2(C2N), Mw
= 1830.45, orthorhombic, space group P212121 (19),
a = 17.656(7), b = 24.127(9), c = 24.239(9) Å, V = 10325(7) Å3, Z = 4,
rcalcd = 1.177, T = À180 1C, 17 869 measured reflection, 9895 unique
reflections (Rint = 0.0633), flack 0.133(14), R1 = 0.0970, wR2 = 0.3130 (all
data), GOF = 1.077.
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Scheme 2 Synthesis of singly-binaphthyl-strapped porphyrins 12 and 13.
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10 For reviews of CO2 fixations, see: (a) M. Cokoja, C. Bruckmeier, 19 glum defined as 2(IL À IR)/(IL + IR) was calculated as an average value
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Chem. Commun.
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