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Synthesis of porphyrins: models of natural hemoproteins and impressive catalysts for asymmetric epoxidation of olefins
www.elsevier.nl/locate/poly Polyhedron 19 (2000) 581–586
Synthesis of porphyrins: models of natural hemoproteins and impressive catalysts for asymmetric epoxidation of olefins E. Rose a,*, M. Quelquejeu a, R.P. Pandian a, A. Lecas-Nawrocka a, A. Vilar a, G. Ricart b, J.P. Collman c, Z. Wang c, A. Straumanis c a
` Organique et Organometallique, ´ Laboratoire de Synthese UMR 7611, Universite´ P. et M. Curie, BP 181, 4 place Jussieu, 75252 Paris Cedex 5, France b ˆ Laboratoire de Spectrochimie de Masse, Batiment C4, USTL, Cite´ Scientifique, 59655 Villeneuve d’Asq Cedex, France c Department of Chemistry, Stanford University, Stanford, CA 94305, USA Received 4 October 1999; accepted 6 December 1999
Abstract Porphyrins, with two chiral binaphthyl handles, efficiently catalyze the epoxidation of terminal olefins to give epoxides with high enantiomeric excess and turnover number. q2000 Elsevier Science Ltd All rights reserved. Keywords: Porphyrins; Hemoprotein models; Chiral catalysts; Epoxidation; Olefins
Porphyrins play an important role in a bewildering array of proteins [1]. Their functions include O2 storage (myoglobin Mb), O2 transport (hemoglobin Hb), oxidation of unactivated carbon–hydrogen bonds (cytochrome P450), and oxygen reduction (cytochrome c oxidase.) In the case of Mb and Hb, the axial ligand is the nitrogen atom of a histidine residue, whilst in the case of cytochrome P450, the axial ligand is the sulfur atom of a cysteinate. The diversity of functions of these natural hemoproteins is mainly dictated by the number and the nature of the axial ligands, the spin and the oxidation state of the metal center, the nature of the polypeptide chain, and the geometry of the porphyrin ring. In the presence of O2, synthetic iron(II) porphyrins, which are not protected on both faces, tend to be oxidized irreversibly and give m-oxo Fe(III) dimers [2]. To avoid this, several models with protected faces have been developed [3–5]. For instance, we have prepared ‘gyroscope’ porphyrins 1 [4,6], which have two different handles: one pyridinic handle which is able to coordinate the iron atom of the porphyrin and one terephthalic handle which protects the active site. We have also prepared ‘basket handle’ porphyrins 2, with the same handles or with a homologated terephthalic handle. It is worth noting that 2b discriminates carbon monoxide and oxygen in a similar way to Mb and Hb [7,8]. Indeed, we observed a
partition coefficient Ms105 (M is the ratio between O2 and CO affinity), which is similar to the values described in the case of natural hemoproteins. Similarly, we prepared cytochrome P450 models 3 [4] and 4 [9] bearing handles or pickets which contain sulfur derivatives.
* Corresponding author. Tel.: q33-1-44-27-62-35; fax: q33-1-44-27-5504; e-mail: [email protected] 0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved. PII S 0 2 7 7 - 5 3 8 7 ( 9 9 ) 0 0 4 1 3 - 1
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More recently, we prepared an octa nitrophenyl porphyrin 5a by condensing pyrrole with 2,6-dinitro-4-tert-butylbenzaldehyde and correspondingly the octa aminophenyl porphyrin 5b. Condensation of acryloyl chloride with 5b gave the octa-Michael acceptor 6 which reacts with cyclen to give a porphyrin that we called a ‘barrel’ porphyrin 7 [9].
In parallel, we developed a new class of catalysts for asymmetric epoxidation of olefins based on cytochrome P450 models. Indeed, it is well known that by using a strong oxidant, it is possible to oxidize the iron atom to a high oxidation state able to epoxidize olefins [10]. We undertook the mixed condensation of pyrrole with two aldehydes, namely pentafluorobenzaldehyde and 2,6-dinitro-4-tert-butylbenzaldehyde [11]. The first aldehyde was used to obtain catalysts with robust pentafluoro phenyl groups while the tert-butylbenzaldehyde was selected in order to increase the solubility of the catalyst. Thus, we obtained porphyrins 14, 15 and 17 with two, four and six nitro or amino groups respectively (RsNO2, NH2), Table 2. Condensation of different chiral acylchlorides with the amino-porphyrins allowed us to prepare porphyrins 15 and 17 (Rs–NH–C(O)–C*(CF3)(C6H5)(OMe)) [11]. Unfortunately, epoxidations conducted with porphyrins bearing Mosher pickets only gave low enantiomeric excess, Table 2. Other chiral porphyrins have been obtained by condensing a chiral binap diacid chloride with aminophenyl porphyrin. The iron-porphyrin 8 called a ‘seat’ porphyrin, was used to epoxidize different terminal olefins in the presence of iodosylbenzene. We respectively obtained 59% and 85% enantiomeric excesses for styrene and pentafluorostyrene [18]. Table 1 Type I system: single-faced protected ansa porphyrins. Summary of percentages of enantiomeric excess obtained with catalysts 1–13 and styrene and ortho-nitrostyrene
a
Refs. [21,22]. Refs. [23]. c Refs. [24,25]. d Ref. [26]. b
Table 2 Type II system: double-faced protected picket porphyrins. Summary of percentages of enantiomeric excess obtained with catalysts 14–19 and styrene
a
Ref. [11]. Refs. [11–14] (see also Refs. [15–17]). c Refs. [27–33]. d In preparation. e Too bulky with R_–NH–C(O)–CU(OMe)(CF3)(C6H5). b
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Table 3 Type III system: double-faced protected ansa porphyrins. Summary of percentages of enantiomeric excess obtained with catalysts 20–23, 8 and 9 and styrene and ortho-nitrostyrene
a
Refs. [7,8,34–36]. Refs. [6,8]. c Refs. [37–40]. d Refs. [37–41]. e This work. f Refs. [18,20]. b
These results can be compared with those obtained using a similar catalyst 9 which is prepared by condensing the same binap diacylchloride with the a2b2 atropoisomer of tetrakis2-aminophenylporphyrin. This atropoisomer is obtained in 92% yield by heating the statistical mixture (abab/aaaa/ a2b2/a3bs1/1/2/4) of tetrakis-2-nitrophenyl Zn porphyrin atropoisomers in naphthalene [19]. Interestingly, with the bis-binaphthylporphyrin 9, we obtained 83% and 88% enantiomeric excess for the epoxidation of styrene and pentafluorostyrene [20]. The best enantiomeric excess yet ever observed with our porphyrin catalyst was obtained in the case of tert-butylethylene with a 92% enantiomeric excess. Surprisingly, we observed an increase of the enantiomeric excess during the initial period of the styrene epoxidation using 9. Indeed, enantiomeric excess values raised from 65% at 10 turnovers to 83% at 200 turnovers. From 100 to 2000 turnovers, the reaction maintains 83% enantiomeric excess. The complex catalyzes the epoxidation at a rate of 2400 turnovers per hour if iodosylbenzene is added portionwise to the reac-
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tion mixture. Indeed, after 5000 turnovers the enantiomeric excess is still about 75%. We believe that oxidative demethylation of the binap strap occurs during the initial reaction corresponding to the transformation to a quinone-like structure. Probably, the quinone-type catalyst is less hindered than the original methoxy catalyst, allowing easier access of the olefin to the active site with a good facial discrimination. At this stage, we can compare the properties of the different catalysts described in the literature (Tables 1–3). A classification of the catalysts into three structure-types can be proposed: type I systems correspond to single-faced protected ansa porphyrins, type II systems are bis-faced picket porphyrins, type III systems are bis-faced protected ansa porphyrins. In the case of type I porphyrins, enantiomeric excess can reach 58% with catalyst 10 [21,22] for the epoxidation of indene with PhIO in the presence of different imidazoles as blocking ligands (Fig. 1). When the handle of the catalyst 12 is too far from the metal center, a low 13% enantiomeric excess is obtained for the epoxidation of styrene [24,25]. Using Mn-‘threitol’ porphyrins with a small strap 13c, the enantiomeric excess can reach 74% with ortho-nitrostyrene. However, the numbers of turnovers remain low when the unhindered face of the porphyrin is blocked with a bulky imidazole ligand, (Table 1, Fig. 1) [23,26]. In the case of type II catalysts, the Mn catalyst 18b shows 76% enantiomeric excess for the epoxidation of cis-b-methylstyrene [13]. Catalysts 18c, 18d give 70% enantiomeric excess of the R epoxide of styrene [14]. In the particular cases of catalysts 19a–19c, low turnovers and enantiomeric excess are obtained. With the catalyst 19d, the turnover is good but not the enantiomeric excess [27]. An improvement has been described by Marchon and co-workers using Mn
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Fig. 1. Type I system: single-faced protected ansa porphyrins.
chiroporphyrins 19f [31–33]. Indeed, they obtained 86% enantiomeric excess with the bulky porphyrin 19f for the epoxidation of 1,2-dihydronaphthalene (Fig. 2, Table 2) [31]. With type III catalysts, the enantiomeric excess can reach 72% for the epoxidation of cis-b-methylstyrene with the bisansa derivative 20c, but the turnover barely reaches 300 [36] and is only 2 in the case of catalyst 20b [34]. The enantiomeric excess can be improved using staggered and eclipsed catalysts 22 and 23 (twin-coronet porphyrins) [37–40]; an enantiomeric excess of up to 54% in the case of styrene (Fig.
3, Table 3) [37–41] can be obtained but yields for the preparation of the catalysts are low. However, the enantiomeric excess can reach 96% for the epoxidation of the electrondeficient 3,5-dinitrostyrene [37–40]. In conclusion, we have prepared chiral bis-handle porphyrins that are good models of Mb in terms of discriminating between CO and O2. We have also prepared C2-symmetric chiral porphyrins bearing binaphthyl handles. These are efficient catalysts for olefin epoxidation giving high enantiomeric excess and turnover numbers. In particular, catalyst 9 gives the highest enantiomeric excess ever obtained for the
Fig. 2. Type II system: double-faced protected picket porphyrins.
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Fig. 3. Type III system: double-faced protected ansa porphyrins.
epoxidation of aliphatic olefins, 92% enantiomeric excess for the epoxidation of tert-butylethylene, and the turnovers are unexpectedly high!
Acknowledgements We thank NATO (Grant No. 960485), the NSF (Grant No. CHE-9612725) and CNRS for financial support (RPP).
References [1] J.A. Shelnutt, X.-Z. Song, J.-G. Ma, S.-L. Jia, W. Jentzen, C. Medforth, J. Chem. Soc. Rev. 27 (1998) 31. [2] J.P. Collman, R.R. Gagne, C.A. Reed, T.R. Halbert, G. Lang, W.T. Robinson, J. Am. Chem. Soc. 97 (1975) 1427. [3] M. Momenteau, Models of hemoprotein active sites. In: A.D. Hamilton (Ed.), Supramolecular Control of Structure and Reactivity, Wiley, New York, 1996, pp. 155–223. [4] E. Rose, A. Lecas, M. Quelquejeu, A. Kossanyi, B. Boitrel, Coord. Chem. Rev. 178–180 (1998) 1407. [5] C. Kalodimos, I. Gerothanassis, E. Rose, G.E. Hawkes, R. Pierattelli, J. Am. Chem. Soc. 121 (1999) 2903. [6] B. Boitrel, A. Lecas, Z. Renko, E. Rose, J. Chem. Soc., Chem. Commun. (1985) 1820. [7] E. Rose, B. Boitrel, M. Quelquejeu, A. Kossanyi, Tetrahedron Lett. 34 (1993) 7267. [8] B. Boitrel, A. Lecas, Z. Renko, E. Rose, New J. Chem. 13 (1989) 73. [9] E. Rose, A. Kossanyi, M. Quelquejeu, M. Soleilhavoup, F. Duwavran, N. Bernard, A. Lecas, J. Am. Chem. Soc. 118 (1996) 1567. [10] J.T. Groves, Y. Watanabe, J. Am. Chem. Soc. 108 (1986) 507. [11] E. Rose, M. Soleilhavoup, L. Christ-Tommasino, G. Moreau, J.P. Collman, M. Quelquejeu, A. Straumanis, J. Org. Chem. 63 (1998) 2042.
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[12] R.L. Halterman, S. Jan, J. Org. Chem. 56 (1991) 5253. [13] R.L. Halterman, S.T. Jan, H.L. Nimmons, D.J. Standlee, M.A. Khan, Tetrahedron 53 (1997) 11257. [14] J.F. Barry, L. Campbell, D.W. Smith, T. Kodadek, Tetrahedron 53 (1997) 7753. [15] R. Zhang, W-Y. Yu, T-S. Lai, C-M. Che, Chem. Commun. (1999) 1791. [16] L.A. Campbell, T. Kodadek, J. Mol. Catal. A. 113 (1996) 293. [17] K.S. Suslick, S. Van Deusen-Jeffries, in: J.L. Atwood, J.E.D. Davies, ¨ D.D. MacNicol, F. Vogtle, J.M. Lehn (Eds.), Comprehensive Supramolecular Chemistry, vol. 5, Elsevier, Oxford, 1996, p. 141. [18] E. Rose, manuscript in preparation. [19] E. Rose, A. Cardon-Pilotaz, M. Quelquejeu, N. Bernard, A. Kossanyi, ` B. Desmazieres, J. Org. Chem. 60 (1995) 3919. [20] J.P. Collman, Z. Wang, A. Straumanis, M. Quelquejeu, E. Rose, J. Am. Chem. Soc. 121 (1999) 460. [21] K. Konoshi, K. Oda, K. Nishida, T. Aida, S. Inoue, J. Am. Chem. Soc. 114 (1992) 1313. [22] S. Inoue, T. Aida, K. Konoshi, J. Mol. Catal. 74 (1992) 121. [23] J.P. Collman, V.J. Lee, C.J. Kellen-Yuen, X. Zhang, J.A. Ibers, J.I. Brauman, J. Am. Chem. Soc. 117 (1995) 692. [24] J.P. Collman, J.I. Brauman, J.P. Fitzgerald, P.D. Hampton, Y. Naruta, J.W. Sparapany, J.A. Ibers, J. Am. Chem. Soc. 110 (1988) 3477. [25] J.P. Collman, X. Zhang, R.H. Hembre, J.I. Brauman, J. Am. Chem. Soc. 112 (1990) 5356. [26] J.P. Collman, V.J. Lee, X. Zhang, J.A. Ibers, J.I. Brauman, J. Am. Chem. Soc. 115 (1993) 3834. [27] J.T. Groves, R.S. Myers, J. Am. Chem. Soc. 105 (1983) 5791. [28] S. O’Malley, T. Kodadek, J. Am. Chem. Soc. 111 (1989) 9116. [29] S. Licoccia, M. Paci, P. Tagliatesta, R. Paolesse, S. Antonaroli, T. Boschi, Magn. Res. Chem. 29 (1991) 1084. [30] P. Maillard, J.L. Guerquin-Kern, M. Momenteau, Tetrahedron Lett. 32 (1991) 4901. ´ ´ [31] C. Perrollier, J. Pecaut, R. Ramasseul, J.C. Marchon, Inorg. Chem. 38 (1999) 3758. ´ ´ [32] C. Perrollier, J. Pecaut, R. Ramasseul, R. Bau, J.C. Marchon, Chem. Commun. (1999) 1597.
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[33] M. Veyrat, O. Maury, F. Faverjon, D.E. Over, R. Ramasseul, J.C. Marchon, I. Turowska-Tyrk, W.R. Scheidt, Angew. Chem., Int. Ed. Engl. 33 (1994) 220. [34] D. Mansuy, P. Battioni, J.P. Renaud, P. Guerin, J. Chem. Soc., Chem. Commun. (1985) 155. [35] J.P. Collman, X. Zhang, V.J. Lee, J.I. Brauman, J. Chem. Soc., Chem. Commun. (1992) 1647. [36] J.T. Groves, P. Viski, J. Org. Chem. 55 (1990) 3628.
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[37] Y. Naruta, F. Tani, N. Ishihara, K. Maruyama, J. Am. Chem. Soc. 113 (1991) 6865. [38] Y. Naruta, F. Tani, K. Maruyama, Chem. Lett. (1989) 1269. [39] Y. Naruta, N. Ishihara, F. Tani, K. Maruyama, Bull. Chem. Soc. Jpn. 66 (1993) 158. [40] Y. Naruta, N. Ishihara, F. Tani, K. Maruyama, Chem. Lett. (1991) 1933. [41] Z. Gross, S. Ini, J. Org. Chem. 62 (1997) 5514.
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Synthesis of porphyrins: models of natural hemoproteins and impressive catalysts for asymmetric epoxidation of olefins E. Rose a,*, M. Quelquejeu a, R.P. Pandian a, A. Lecas-Nawrocka a, A. Vilar a, G. Ricart b, J.P. Collman c, Z. Wang c, A. Straumanis c a
` Organique et Organometallique, ´ Laboratoire de Synthese UMR 7611, Universite´ P. et M. Curie, BP 181, 4 place Jussieu, 75252 Paris Cedex 5, France b ˆ Laboratoire de Spectrochimie de Masse, Batiment C4, USTL, Cite´ Scientifique, 59655 Villeneuve d’Asq Cedex, France c Department of Chemistry, Stanford University, Stanford, CA 94305, USA Received 4 October 1999; accepted 6 December 1999
Abstract Porphyrins, with two chiral binaphthyl handles, efficiently catalyze the epoxidation of terminal olefins to give epoxides with high enantiomeric excess and turnover number. q2000 Elsevier Science Ltd All rights reserved. Keywords: Porphyrins; Hemoprotein models; Chiral catalysts; Epoxidation; Olefins
Porphyrins play an important role in a bewildering array of proteins [1]. Their functions include O2 storage (myoglobin Mb), O2 transport (hemoglobin Hb), oxidation of unactivated carbon–hydrogen bonds (cytochrome P450), and oxygen reduction (cytochrome c oxidase.) In the case of Mb and Hb, the axial ligand is the nitrogen atom of a histidine residue, whilst in the case of cytochrome P450, the axial ligand is the sulfur atom of a cysteinate. The diversity of functions of these natural hemoproteins is mainly dictated by the number and the nature of the axial ligands, the spin and the oxidation state of the metal center, the nature of the polypeptide chain, and the geometry of the porphyrin ring. In the presence of O2, synthetic iron(II) porphyrins, which are not protected on both faces, tend to be oxidized irreversibly and give m-oxo Fe(III) dimers [2]. To avoid this, several models with protected faces have been developed [3–5]. For instance, we have prepared ‘gyroscope’ porphyrins 1 [4,6], which have two different handles: one pyridinic handle which is able to coordinate the iron atom of the porphyrin and one terephthalic handle which protects the active site. We have also prepared ‘basket handle’ porphyrins 2, with the same handles or with a homologated terephthalic handle. It is worth noting that 2b discriminates carbon monoxide and oxygen in a similar way to Mb and Hb [7,8]. Indeed, we observed a
partition coefficient Ms105 (M is the ratio between O2 and CO affinity), which is similar to the values described in the case of natural hemoproteins. Similarly, we prepared cytochrome P450 models 3 [4] and 4 [9] bearing handles or pickets which contain sulfur derivatives.
* Corresponding author. Tel.: q33-1-44-27-62-35; fax: q33-1-44-27-5504; e-mail: [email protected] 0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved. PII S 0 2 7 7 - 5 3 8 7 ( 9 9 ) 0 0 4 1 3 - 1
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More recently, we prepared an octa nitrophenyl porphyrin 5a by condensing pyrrole with 2,6-dinitro-4-tert-butylbenzaldehyde and correspondingly the octa aminophenyl porphyrin 5b. Condensation of acryloyl chloride with 5b gave the octa-Michael acceptor 6 which reacts with cyclen to give a porphyrin that we called a ‘barrel’ porphyrin 7 [9].
In parallel, we developed a new class of catalysts for asymmetric epoxidation of olefins based on cytochrome P450 models. Indeed, it is well known that by using a strong oxidant, it is possible to oxidize the iron atom to a high oxidation state able to epoxidize olefins [10]. We undertook the mixed condensation of pyrrole with two aldehydes, namely pentafluorobenzaldehyde and 2,6-dinitro-4-tert-butylbenzaldehyde [11]. The first aldehyde was used to obtain catalysts with robust pentafluoro phenyl groups while the tert-butylbenzaldehyde was selected in order to increase the solubility of the catalyst. Thus, we obtained porphyrins 14, 15 and 17 with two, four and six nitro or amino groups respectively (RsNO2, NH2), Table 2. Condensation of different chiral acylchlorides with the amino-porphyrins allowed us to prepare porphyrins 15 and 17 (Rs–NH–C(O)–C*(CF3)(C6H5)(OMe)) [11]. Unfortunately, epoxidations conducted with porphyrins bearing Mosher pickets only gave low enantiomeric excess, Table 2. Other chiral porphyrins have been obtained by condensing a chiral binap diacid chloride with aminophenyl porphyrin. The iron-porphyrin 8 called a ‘seat’ porphyrin, was used to epoxidize different terminal olefins in the presence of iodosylbenzene. We respectively obtained 59% and 85% enantiomeric excesses for styrene and pentafluorostyrene [18]. Table 1 Type I system: single-faced protected ansa porphyrins. Summary of percentages of enantiomeric excess obtained with catalysts 1–13 and styrene and ortho-nitrostyrene
a
Refs. [21,22]. Refs. [23]. c Refs. [24,25]. d Ref. [26]. b
Table 2 Type II system: double-faced protected picket porphyrins. Summary of percentages of enantiomeric excess obtained with catalysts 14–19 and styrene
a
Ref. [11]. Refs. [11–14] (see also Refs. [15–17]). c Refs. [27–33]. d In preparation. e Too bulky with R_–NH–C(O)–CU(OMe)(CF3)(C6H5). b
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Table 3 Type III system: double-faced protected ansa porphyrins. Summary of percentages of enantiomeric excess obtained with catalysts 20–23, 8 and 9 and styrene and ortho-nitrostyrene
a
Refs. [7,8,34–36]. Refs. [6,8]. c Refs. [37–40]. d Refs. [37–41]. e This work. f Refs. [18,20]. b
These results can be compared with those obtained using a similar catalyst 9 which is prepared by condensing the same binap diacylchloride with the a2b2 atropoisomer of tetrakis2-aminophenylporphyrin. This atropoisomer is obtained in 92% yield by heating the statistical mixture (abab/aaaa/ a2b2/a3bs1/1/2/4) of tetrakis-2-nitrophenyl Zn porphyrin atropoisomers in naphthalene [19]. Interestingly, with the bis-binaphthylporphyrin 9, we obtained 83% and 88% enantiomeric excess for the epoxidation of styrene and pentafluorostyrene [20]. The best enantiomeric excess yet ever observed with our porphyrin catalyst was obtained in the case of tert-butylethylene with a 92% enantiomeric excess. Surprisingly, we observed an increase of the enantiomeric excess during the initial period of the styrene epoxidation using 9. Indeed, enantiomeric excess values raised from 65% at 10 turnovers to 83% at 200 turnovers. From 100 to 2000 turnovers, the reaction maintains 83% enantiomeric excess. The complex catalyzes the epoxidation at a rate of 2400 turnovers per hour if iodosylbenzene is added portionwise to the reac-
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tion mixture. Indeed, after 5000 turnovers the enantiomeric excess is still about 75%. We believe that oxidative demethylation of the binap strap occurs during the initial reaction corresponding to the transformation to a quinone-like structure. Probably, the quinone-type catalyst is less hindered than the original methoxy catalyst, allowing easier access of the olefin to the active site with a good facial discrimination. At this stage, we can compare the properties of the different catalysts described in the literature (Tables 1–3). A classification of the catalysts into three structure-types can be proposed: type I systems correspond to single-faced protected ansa porphyrins, type II systems are bis-faced picket porphyrins, type III systems are bis-faced protected ansa porphyrins. In the case of type I porphyrins, enantiomeric excess can reach 58% with catalyst 10 [21,22] for the epoxidation of indene with PhIO in the presence of different imidazoles as blocking ligands (Fig. 1). When the handle of the catalyst 12 is too far from the metal center, a low 13% enantiomeric excess is obtained for the epoxidation of styrene [24,25]. Using Mn-‘threitol’ porphyrins with a small strap 13c, the enantiomeric excess can reach 74% with ortho-nitrostyrene. However, the numbers of turnovers remain low when the unhindered face of the porphyrin is blocked with a bulky imidazole ligand, (Table 1, Fig. 1) [23,26]. In the case of type II catalysts, the Mn catalyst 18b shows 76% enantiomeric excess for the epoxidation of cis-b-methylstyrene [13]. Catalysts 18c, 18d give 70% enantiomeric excess of the R epoxide of styrene [14]. In the particular cases of catalysts 19a–19c, low turnovers and enantiomeric excess are obtained. With the catalyst 19d, the turnover is good but not the enantiomeric excess [27]. An improvement has been described by Marchon and co-workers using Mn
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Fig. 1. Type I system: single-faced protected ansa porphyrins.
chiroporphyrins 19f [31–33]. Indeed, they obtained 86% enantiomeric excess with the bulky porphyrin 19f for the epoxidation of 1,2-dihydronaphthalene (Fig. 2, Table 2) [31]. With type III catalysts, the enantiomeric excess can reach 72% for the epoxidation of cis-b-methylstyrene with the bisansa derivative 20c, but the turnover barely reaches 300 [36] and is only 2 in the case of catalyst 20b [34]. The enantiomeric excess can be improved using staggered and eclipsed catalysts 22 and 23 (twin-coronet porphyrins) [37–40]; an enantiomeric excess of up to 54% in the case of styrene (Fig.
3, Table 3) [37–41] can be obtained but yields for the preparation of the catalysts are low. However, the enantiomeric excess can reach 96% for the epoxidation of the electrondeficient 3,5-dinitrostyrene [37–40]. In conclusion, we have prepared chiral bis-handle porphyrins that are good models of Mb in terms of discriminating between CO and O2. We have also prepared C2-symmetric chiral porphyrins bearing binaphthyl handles. These are efficient catalysts for olefin epoxidation giving high enantiomeric excess and turnover numbers. In particular, catalyst 9 gives the highest enantiomeric excess ever obtained for the
Fig. 2. Type II system: double-faced protected picket porphyrins.
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Fig. 3. Type III system: double-faced protected ansa porphyrins.
epoxidation of aliphatic olefins, 92% enantiomeric excess for the epoxidation of tert-butylethylene, and the turnovers are unexpectedly high!
Acknowledgements We thank NATO (Grant No. 960485), the NSF (Grant No. CHE-9612725) and CNRS for financial support (RPP).
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