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1.10 Acidic Clay-Directed Polymerization-Cyclization of Aldehydes and Pyrrole to Porphyrins
1.10 Acidic Clay-Directed Polymerization-Cychtion of Aldehydes and m o l e to Porphyrins Makoto ONAKA,' Tomotaka SHINODA,' Yusuke IZUMI,' and Ernest N O L E d a Department of Applied Chemistry, Faculty of Engineering, Nagoya University, Chikusa, Nagoya 464, Japan; b Department of Chemistry, Colgate University, Hamilton, New York 13346-1398, U. S. A.
Abstract
Intrinsic nanospaces of acidic clay K10 were found to serve as micro reactors directing polymerization-cyclization of aldehydes and pyrrole to porphyrins efficiently.
1. INTRODUCTION It has been focused on porphyrin chemistry from a variety of interests concerning organic chemistry, inorganic chemistry, physical chemistry, analytical chemistry, biochemistry, and so on. From a standpoint of synthetic organic chemistry, it is very significant to develop efficient methodologies for creating porphyrin compounds which can provide us useful functions. It has been suggested that minerals serve as possible catalysts for porphyrin abiogenesis from pyrroles and aldehydes in the prebiotic era. By analogy, mineral clays are expected to be promising candidates for promoting artificial porphyrin formation in vitro. Symmetrical meso-tetrasubstituted porphyrins are synthesized directly from their components, pyrroles and aldehydes, as shown in Scheme 1: pyrroles and aldehydes alternately oligomerize to form 0, as an intermediate. An intramolecular cyclization of 0, leads to porphyrinogen, a precursor to porphyrin, otherwise linear copolymers are produced from 0,. The intramolecular cyclization to porphyrinogen is normally not a predominant process in the copolymerization of pyrroles and aldehydes. We supposed that if the porphyrinogen formation was carried out in a restricted nano-size space, the intramolecular cyclization would be more dominant than the cyclization conducted in a homogeneous solution, and the longer copolymer formation from 0, would be suppressed. Based on this idea, we applied different acidic clays as reaction media (micro reactors) to mesotetrasubstituted porphyrin synthesis from pyrrole and aldehydes, and we discovered [ 11 that montmorillonite K10 [2] is the best clay for producing the porphyrins in much higher yields, compared with the conventional methodologies which have been developed so far [3].
2. EXPERIMENTAL METHODS
2.1. Materials. Montmorillonite K10 was purchased from Aldrich and used. Fe-Mont (iron ion-exchanged montmorillonite) was prepared by cation exchange from sodium ion-exchanged montmorillonite "Kunipia F' supplied by Kunimine Industries Co. Japan. Si02-A1203 was JRC-SAL-2 from the 85
86
M. ONAKA, T. SHINODA, Y. IZUMI and E. NOLEN
Catalysis Society of Japan. Aldehydes, pyrrole, BF,*OEb, and CF3C02H were distilled before use. CH2C12 was dried on molecular sieves 4A.
R-CHO
H H
-+
- H20
H
R
-
+
R
-R
(
R
Porphyrinogen
08)
Oxidation
Linea copolymer
R
1-(
R
Scheme 1. Porphyrin formation from aldehyde and pyrrole. 2.2. Reaction procedure. We conducted two synthetic steps in one pot: polymerization-cyclization to porphyrinogen from aldehyde and pyrrole, followed by oxidation with p-chloranil to porphyrin. A clay (1 g) was activated at 120 OC and below 0.5 Torr for 3 h in a 200-ml flask, and then N, was introduced. The flask was shielded from light with foil. To the flask were added dry CH2C12 (95 ml) and then a CH2CI2 (5 ml) solution of aldehyde (1 mmol). To the well-stirred mixture was introduced dropwise neat pyrrole (1 mmol) at room temperature, and the stirring was continued for 1 h. Solid p-chloranil (0.75 mmol) was added and the mixture was gently refluxed at 45 OC for 1 h. Solid materials were removed through a Celite pad and washed with AcOEt or CH,CI, (60 ml). The combined filtrate contained free base porphyrin, and was condensed and adsorbed on Florisil ( 2 g).
Acidic Clay-Directed Polymerization-Cyclizationof Aldehydes and Pyrrole
87
The adsorbate was placed on the top of an A1203 (Merck Aluminum Oxide 90, Activity 11-111, 100 g) column and developed with hexane-AcOEt or hexane-CH2CI2. The porphyrin fraction was collected, condensed, charged on an alumina (100 g) column, and purified again. The purified porphyrin was dried at 80 OC and below 0.5 Torr for 6 h. 3. RESULTS AND DISCUSSION
It was found that the montmorillonite K10-mediated porphyrin formation is especially outstanding in the meso-tetraalkylporphyrinsynthesis from aliphatic aldehydes and pyrrole, because conventional methods under the influence of homogeneous acids only afforded the porphyrins in poor yields. Table 1 shows the comparison between the present K10 method and the conventional methods using homogeneous acids (BF3*0E%and CF3C02H) for the syntheses of typical meso-tetraalkylporphyrins. Table 1. Syntheses of meso-tetraalkylporphyrins.a)
R -(CH2)4CH3 -(CH2)9CH3 -(CH,),Cl -(CH,),CH=CH, Cyclohexyl
K10
Yields / % BF,*OEt,
CF3CO2H
46 39 40 24 35
20 20 21 21 14
15 16 13 11 0
a) Aldehyde, Pyrrole (1 mmol:JO-2 M), K10 (1 g), BF3*OEt2(0.1 mmol, CF3C02H (0.5 mmol, 5x10 M), in CH2C12
M),
As shown in Table 2, K10 was found to give a remarkably high yield of meso-tetrapentylporphyrin among solid strong acids of montmorillonites (K10 and Fe-Mont [4]) as well as amorphous silicaalumina. In order to clarify the origin of the difference in porphyrin synthesis between K10 and another montmorillonite Fe-Mont, the morphology of K10 and Fe-Mont was examined in berms of specific surface area and powder X-ray diffractometry. Fe-Mont has a surface area of 26 m /g and a peak at 2 28 = 5.6 in X-ray diffraction, while K10 has 223 m /g and no diffraction peaks. Consequently it can be deduced that Fe-Mont has a typical laminated structure (Fig. la) with a basal spacing of 1.58 nm, whereas K10 is delaminated or a cardhouse structure, as shown in Fig. lb. Recently Table 2. Formation of meso-tetrapentYlporphyrin. Pinnavaia reported that the pore structure of K10 is mainly composed of mesopores ranging between 3 and 5 nm [5]. The molecular size Solid acid Yield / % of meso-tetrapentylporphyrin is estimated to be 1.8 nm from a molecular model. Therefore, K10 46 the three-dimensional mesopores in K10 serve as suitable reaction media (templates) for Fe-Mont 4 formation of bulky porphyrinogens and Si02-A1203 trace porphyrins. O
88
M. ONAKA, T. SHINODA, Y. IZUMI and E. NOLEN
a) Laminated structure (Fe-Mont)
b) Card-h&se structure (Montmorillonite K10)
Fig. 1. Morphology of montmorillonites.
In addition, meso-tetraarylporphyrins were successfully synthesized from aromatic aldehydes and pyrrole by using K10 clay. Interestingly, the yields of meso-tetraarylporphyrinsvaried depending on the substituents and their substitution patterns under both heterogeneous and homogeneous conditions. Finally, it should be noted that Table 3. meso-Tetraarylporphyrin Syntheses.a) the present K10 method has another synihetic advantage: porphyrin products can easily be separated and Yields (%) isolated from by-product polymers, R K10 BF,*OEt, CF3CO2H which tend to be selectively adsorbed on K10 and separated from a solution 30 42 36 phase in the filtration procedure. Ph O-MeC6H4 P-MeCbH, o-MeOC6H4 p-MeOC6H4 O-CIC6H4
21 20 30 trace
45b) 28 2Ob) trace
31 29 trace trace
3
2ab)
9
p-CIC6H4
9
trace
17
a) Aldehyde ( 1 mmol, loT2M), Pyrrol (1 mmol, M), K10 (1 g), BF,*OEt 0.1 mmol, 10'3M),TFA (0.5 mmol, 5x10 M), in CH2CIZ b) Data were quoted from Ref. 3.
meso-Tetraalkylporphyrins have not been studied intensively concerning their physical and chemical properties and applications to catalysis from lack of their efficient syntheses. Uncovering the new chemical characteristics of meso-tetraalkylporphyrins is in progress.
4
4. REFERENCES AND NOTES.
[l] M. Onaka, T. Shinoda, Y. Izumi, E. Nolen, Chem. Lett. 1993, 117; Tetrahedron Lett, 1993, 34, 2625. [2] K10 is a sulfuric acid-leached montmorillonite. [3] a) J. S . Lindsey, I. C. Schreiman, H. C. Hsu, P. C. Kearney, A. M. Marguerettaz, J . Org. Chem., 1987,52,827; J. S. Lindsey, R. W. Wagner, ibid., 1989,54, 828. [4] K10 and Fe-Mont were found to have almost the same acid strength: M. Onaka, Y. Hosokawa, K. Higuchi, Y. Izumi, Tetrahedron Lett., 1993,34, 1171. [5] J. -R. Butruille, T. J. Pinnavaia, Catal. Today, 1992, 14, 141.
Abstract
Intrinsic nanospaces of acidic clay K10 were found to serve as micro reactors directing polymerization-cyclization of aldehydes and pyrrole to porphyrins efficiently.
1. INTRODUCTION It has been focused on porphyrin chemistry from a variety of interests concerning organic chemistry, inorganic chemistry, physical chemistry, analytical chemistry, biochemistry, and so on. From a standpoint of synthetic organic chemistry, it is very significant to develop efficient methodologies for creating porphyrin compounds which can provide us useful functions. It has been suggested that minerals serve as possible catalysts for porphyrin abiogenesis from pyrroles and aldehydes in the prebiotic era. By analogy, mineral clays are expected to be promising candidates for promoting artificial porphyrin formation in vitro. Symmetrical meso-tetrasubstituted porphyrins are synthesized directly from their components, pyrroles and aldehydes, as shown in Scheme 1: pyrroles and aldehydes alternately oligomerize to form 0, as an intermediate. An intramolecular cyclization of 0, leads to porphyrinogen, a precursor to porphyrin, otherwise linear copolymers are produced from 0,. The intramolecular cyclization to porphyrinogen is normally not a predominant process in the copolymerization of pyrroles and aldehydes. We supposed that if the porphyrinogen formation was carried out in a restricted nano-size space, the intramolecular cyclization would be more dominant than the cyclization conducted in a homogeneous solution, and the longer copolymer formation from 0, would be suppressed. Based on this idea, we applied different acidic clays as reaction media (micro reactors) to mesotetrasubstituted porphyrin synthesis from pyrrole and aldehydes, and we discovered [ 11 that montmorillonite K10 [2] is the best clay for producing the porphyrins in much higher yields, compared with the conventional methodologies which have been developed so far [3].
2. EXPERIMENTAL METHODS
2.1. Materials. Montmorillonite K10 was purchased from Aldrich and used. Fe-Mont (iron ion-exchanged montmorillonite) was prepared by cation exchange from sodium ion-exchanged montmorillonite "Kunipia F' supplied by Kunimine Industries Co. Japan. Si02-A1203 was JRC-SAL-2 from the 85
86
M. ONAKA, T. SHINODA, Y. IZUMI and E. NOLEN
Catalysis Society of Japan. Aldehydes, pyrrole, BF,*OEb, and CF3C02H were distilled before use. CH2C12 was dried on molecular sieves 4A.
R-CHO
H H
-+
- H20
H
R
-
+
R
-R
(
R
Porphyrinogen
08)
Oxidation
Linea copolymer
R
1-(
R
Scheme 1. Porphyrin formation from aldehyde and pyrrole. 2.2. Reaction procedure. We conducted two synthetic steps in one pot: polymerization-cyclization to porphyrinogen from aldehyde and pyrrole, followed by oxidation with p-chloranil to porphyrin. A clay (1 g) was activated at 120 OC and below 0.5 Torr for 3 h in a 200-ml flask, and then N, was introduced. The flask was shielded from light with foil. To the flask were added dry CH2C12 (95 ml) and then a CH2CI2 (5 ml) solution of aldehyde (1 mmol). To the well-stirred mixture was introduced dropwise neat pyrrole (1 mmol) at room temperature, and the stirring was continued for 1 h. Solid p-chloranil (0.75 mmol) was added and the mixture was gently refluxed at 45 OC for 1 h. Solid materials were removed through a Celite pad and washed with AcOEt or CH,CI, (60 ml). The combined filtrate contained free base porphyrin, and was condensed and adsorbed on Florisil ( 2 g).
Acidic Clay-Directed Polymerization-Cyclizationof Aldehydes and Pyrrole
87
The adsorbate was placed on the top of an A1203 (Merck Aluminum Oxide 90, Activity 11-111, 100 g) column and developed with hexane-AcOEt or hexane-CH2CI2. The porphyrin fraction was collected, condensed, charged on an alumina (100 g) column, and purified again. The purified porphyrin was dried at 80 OC and below 0.5 Torr for 6 h. 3. RESULTS AND DISCUSSION
It was found that the montmorillonite K10-mediated porphyrin formation is especially outstanding in the meso-tetraalkylporphyrinsynthesis from aliphatic aldehydes and pyrrole, because conventional methods under the influence of homogeneous acids only afforded the porphyrins in poor yields. Table 1 shows the comparison between the present K10 method and the conventional methods using homogeneous acids (BF3*0E%and CF3C02H) for the syntheses of typical meso-tetraalkylporphyrins. Table 1. Syntheses of meso-tetraalkylporphyrins.a)
R -(CH2)4CH3 -(CH2)9CH3 -(CH,),Cl -(CH,),CH=CH, Cyclohexyl
K10
Yields / % BF,*OEt,
CF3CO2H
46 39 40 24 35
20 20 21 21 14
15 16 13 11 0
a) Aldehyde, Pyrrole (1 mmol:JO-2 M), K10 (1 g), BF3*OEt2(0.1 mmol, CF3C02H (0.5 mmol, 5x10 M), in CH2C12
M),
As shown in Table 2, K10 was found to give a remarkably high yield of meso-tetrapentylporphyrin among solid strong acids of montmorillonites (K10 and Fe-Mont [4]) as well as amorphous silicaalumina. In order to clarify the origin of the difference in porphyrin synthesis between K10 and another montmorillonite Fe-Mont, the morphology of K10 and Fe-Mont was examined in berms of specific surface area and powder X-ray diffractometry. Fe-Mont has a surface area of 26 m /g and a peak at 2 28 = 5.6 in X-ray diffraction, while K10 has 223 m /g and no diffraction peaks. Consequently it can be deduced that Fe-Mont has a typical laminated structure (Fig. la) with a basal spacing of 1.58 nm, whereas K10 is delaminated or a cardhouse structure, as shown in Fig. lb. Recently Table 2. Formation of meso-tetrapentYlporphyrin. Pinnavaia reported that the pore structure of K10 is mainly composed of mesopores ranging between 3 and 5 nm [5]. The molecular size Solid acid Yield / % of meso-tetrapentylporphyrin is estimated to be 1.8 nm from a molecular model. Therefore, K10 46 the three-dimensional mesopores in K10 serve as suitable reaction media (templates) for Fe-Mont 4 formation of bulky porphyrinogens and Si02-A1203 trace porphyrins. O
88
M. ONAKA, T. SHINODA, Y. IZUMI and E. NOLEN
a) Laminated structure (Fe-Mont)
b) Card-h&se structure (Montmorillonite K10)
Fig. 1. Morphology of montmorillonites.
In addition, meso-tetraarylporphyrins were successfully synthesized from aromatic aldehydes and pyrrole by using K10 clay. Interestingly, the yields of meso-tetraarylporphyrinsvaried depending on the substituents and their substitution patterns under both heterogeneous and homogeneous conditions. Finally, it should be noted that Table 3. meso-Tetraarylporphyrin Syntheses.a) the present K10 method has another synihetic advantage: porphyrin products can easily be separated and Yields (%) isolated from by-product polymers, R K10 BF,*OEt, CF3CO2H which tend to be selectively adsorbed on K10 and separated from a solution 30 42 36 phase in the filtration procedure. Ph O-MeC6H4 P-MeCbH, o-MeOC6H4 p-MeOC6H4 O-CIC6H4
21 20 30 trace
45b) 28 2Ob) trace
31 29 trace trace
3
2ab)
9
p-CIC6H4
9
trace
17
a) Aldehyde ( 1 mmol, loT2M), Pyrrol (1 mmol, M), K10 (1 g), BF,*OEt 0.1 mmol, 10'3M),TFA (0.5 mmol, 5x10 M), in CH2CIZ b) Data were quoted from Ref. 3.
meso-Tetraalkylporphyrins have not been studied intensively concerning their physical and chemical properties and applications to catalysis from lack of their efficient syntheses. Uncovering the new chemical characteristics of meso-tetraalkylporphyrins is in progress.
4
4. REFERENCES AND NOTES.
[l] M. Onaka, T. Shinoda, Y. Izumi, E. Nolen, Chem. Lett. 1993, 117; Tetrahedron Lett, 1993, 34, 2625. [2] K10 is a sulfuric acid-leached montmorillonite. [3] a) J. S . Lindsey, I. C. Schreiman, H. C. Hsu, P. C. Kearney, A. M. Marguerettaz, J . Org. Chem., 1987,52,827; J. S. Lindsey, R. W. Wagner, ibid., 1989,54, 828. [4] K10 and Fe-Mont were found to have almost the same acid strength: M. Onaka, Y. Hosokawa, K. Higuchi, Y. Izumi, Tetrahedron Lett., 1993,34, 1171. [5] J. -R. Butruille, T. J. Pinnavaia, Catal. Today, 1992, 14, 141.