摘要:
The porphyrinogen-porphyrin relationship: the discovery of artificial porphyrins Carlo Floriani Institut de Chimie Mintrale et Analytique, BCH #3307, Universite‘ de Lausanne, CH-I 015 Lausanne, Switzerland The oxidation of meso-tetrahydrotetraalkylporphyrinogen,which is the chemical and biochemical precursor of porphyrins, has been reexamined using meso-octaalkyl- porphyrinogen as a model compound. This investigation has led to the discovery of oxidized forms of porphyrinogen other than porphyrins, which we call artificial porphyrins. They contain cyclopropane moieties which function as two-electron shuttles via the formation and cleavage of a C-C bond. The metal-assisted modifications of the porphyrinogen skeleton, namely the homologation of a pyrrole to a pyridine ring using carbon monoxide and the functionalization of the aliphatic periphery, are the consequences of the bifunctional acid-base carrier properties of these metal-porphyrinogen complexes.This observation allowed us to establish general synthetic methodologies in the fields of metal-assisted C-H bond activation and C-C bond formation. Introduction The popularity of the porphyrinogen molecular skeleton (Scheme 1) is based upon its use as a chemical and biochemical precursor of porphyrins.’ This notwithstanding, its chemistry is almost unexplored, except for the spontaneous, six-electron oxidation leading to the corresponding porphyrin (Scheme 1). This is due to the fact that the meso-tetrahydrotetraalkyl- porphyrinogen is not available in a stable form, i.e.one which does not convert into a porphyrin. However, the prototype of a stable form of porphyrinogen, the rneso-octamethylporphy- rinogen, was discovered back in 1886 by Baeyer,2 and it is abbreviated as [R&&] (Scheme 1, R = Me). Until now, the molecule and its homologues have been almost totally ignored by chemists.3 The present report deals with: (i) an overview of the very recent metal-assisted redox chemistry of meso-octaalkyl-p~rphyrinogen.~This investigation has also led to the discovery of oxidized forms of porphyrinogen other than porphyrins, the so-called ‘artificial porphyrins’;4c-e (ii) the potential applica- tions of such a skeleton to coordination and organometallic chemistry with an emphasis on its very recent metal-assisted transformations.5-7 meso-Octaalkylporphyrinogen and its Deprotonated Form The syntheses of meso-octaalkylporphyrinogens are quite straightforward, and follow a slight modification of the original method of Baeyer, which involves the acid catalysed condensa- tion of pyrrole and the relevant ketone.3 Considerable amounts of linear by-products and/or polymers form, depending on the reaction conditions and the substituent at the ketone function- ality.The presence of meso sp3 carbons in 1(Scheme 2) allows the coexistence of a number of conformations which have been observed both in solution8 and in the solid state,4e as in the case of calix[4]arenes.g In the case of porphyrinogen, however, the R substituents play a significant role in the choice of the preferred conformation. Unlike its deprotonated forms, which have been obtained as alkali-metal derivatives, rneso-octaalkylporphy- rinogen is a very poor ligand for metals. The former compounds have different synthetic uses depending on the a1 kali-metal cation.The full deprotonation of 1 in the case of the rneso-octaethyl derivative (Scheme 2) occurs as a result of the formation of a very tight ion-pair complex between lithium and R R porphyrin I1 -2H/ porphyrinogen \-IH J I -2 H c I11 artificial porphyrins IV Scheme 1 ‘(‘2) C(13\ LiBu -thf G*R = Et as) C(21 EtgN,H4,1 [Et8N4Li(thf131,2 Ethyl groups and thf molecules omitted Scheme 2 Chem.Commun., 1996 1257 the tetraanion, in which the lithium binding modes emphasize the polyfunctionality of the ligand.1° A further solvated form [Et8N4Li4(thQ4]3 is currently used in our laboratories in metal complexation.10 We should comment briefly on the very peculiar bonding mode of the porphyrinogen tetraanion. 11 The four independent and conformationally flexible pyrrolyl anions can bind the metal in an ql, q3 or q5 fashion providing 4(n +2) electrons (0 d n d 4) to the metal (considering the following contribution for each pyrrolyl anion: q1, n = 0; 73, n = 0.5; q5, n = 1) (Scheme 3) depending on its needs along the reaction pathway.677J In addition, the pyrrolyl anions a-bonded to the central metal atom maintain the ability to bind 73 or q5 to another metal ion on the periphery of ligand V.4,6,7,11a,12 This peculiarity allows the meso-octaalkylporphyrinogen complexes to display an acid-base, bifunctional behaviour.lla913 We should also mention that the orientation of metal n-bonded pyrrolyl anions along with the meso-substituents provides a protected cavity for the metal. The properties of the porphyrinogen ligand mentioned above are illustrated (Scheme 4) in the complexation of an electron- rich metal, e.g. NilI (see complex 4),12 and an electron-poor metal, e.g. ZrIV (see complex 5).11a The structure of 4, which is quite similar to those of several other metal@) ion-porphy- rinogen complexes,4J2 emphasizes the binding ability of the electron-rich porphyrinogen towards alkali cations along with the a-bonding mode of the pyrrolyl anions.In complex 5, the electron poor, do zirconium(1v) forces some of the pyrrolyl anions to bind q5 to the metal. The fluxionality of this molecule over a very small range of temperature (290-330 K) is in agreement with the electronic and steric flexibility of the V 4(2+n) electron donor ligand n =u to1 Scheme 3 NiC12 Et 4r 5 Scheme 4 1258 Chem. Commun., 1996 tetraanion.’la The addition of electron-rich substrates to zirconium forces one of the n-bonded pyrrolyl anions to change to a a-bonding mode.637J1~ Complexes 4 and 5 represent the two prototypes for metal- porphyrinogen chemistry. (a)Artificial Porphyrins: Oxidized Forms of Porphyrinogen other than Porphyrins It is very difficult to understand the so-called oxidative aromatization of the porphyrinogen to the porphyrin skeleton (see Scheme l),because such a process can be formally viewed as the result of two molecular actions, namely, the removal of six electrons followed by the removal of six protons.The key part of this process is the removal of four hydrogens from the meso positions. We were interested in answering the following questions in order to understand the porphyrinogen-porphyrin transformations: ‘what happens when there are only alkyl groups in the meso-positions?’, ‘can we identify partially or fully oxidized forms other than porphyrins?’, and ‘is it possible to follow the pathway leading to these species?’ Our results are shown in Scheme 1 and displayed using the standard convention for illustrating the oxidation of the porphyrinogen skeleton.They show the formation of a conventional porphyrin, 11, from the six-electron oxidation of the meso-tetraalkyl-tetrahydroporphyrinogen and the formation of artificial por- phyrins, I11 and IV, from the two- and four-electron oxidation of the meso-octaalkylporphyrinogen. Two- and four-electron oxidized forms of porphyrinogen, like those shown in Scheme 1, would have no chance of being trapped in the case of meso-tetrahydrotetraalkylporphyrinogen, although they may be eventually identified in the meso-octaalkyl form. Scheme 5 gives a more clear picture of the oxidation of meso-octaalkylporphyrinogen tetraanion by two and four electrons in which no removal of atoms is required.Such a redox scheme is correct if one assumes that the tetraanion is bound to the transition metal, which should assist the oxidation (vide infra) of the porphyrinogen skeleton. We anticipated that oxidation by two electrons leads to the formation of a cyclopropane unit, which can undergo the reverse reduction with the cleavage of the same C-C bond. Therefore each cyclopropane unit within the artificial porphyrin acts as a two-electron shuttle. Let us first discuss the overall metal-assisted pathway and the reaction conditions leading to the generation of ‘artificial porphyrins’.4c-e Scheme 6 shows the stepwise oxidation of a parent porphyrinogen-metal complex to an artificial porphyrin.Such an oxidation allows, depending on the nature of the metal and the oxidation agent, the isolation of all the reported species. X 4-x -2e--+2e-Scheme 5 Although the formation of a cyclopropane unit is an overall two-electron oxidation (see Scheme 5), it proceeds via two monoelectronic steps, the first being the metal@) to metal(II1) oxidation followed by the formation of a cyclopropane unit and the concomitant reduction of MI11 to M11.4cThe monoelectronic pathway has been elucidated by using CuC12 as oxidizing agent in the case of CU" and Co", which have an accessible +3 oxidation state. The use of para-benzoquinone as oxidant led to the two-electron oxidation product, that is the cyclopropane, regardless of the transition metal, and it works only in case of the formation of the first cyclopropane.The monoelectronic pathway emphasizes the effectiveness of the metal-to-ligand intramolecular electron transfer. In addition, we can reasonably assume that the cyclopropane unit is masking a +2 oxidation state for the bound metaL4c.d The disproportionation of 7 to 8 and 6, or the reaction of 8 with 6 leading to 7,4c both being solvent-dependent reactions, emphasize how the formation and cleavage of a cyclopropane unit can occur as a result of an intermolecular electron transfer process. The fully oxidized form of meso-octaalkylporphy-rinogen containing two cyclopropane units and obtained using CuC12 as oxidant, has only been obtained for Co,4d Fe4d and Mn.14 The use of CuC12 gives the inconvenient cluster [Cu4C15]-as a counter anion for 9 bonded to the four pyrrolic C=C double bonds.However, [Cu4C1,]-does not impart any extra stabilization to the biscyclopropane form, as evidenced by the isolation of 9.[FeC14]2-.4d Reduction of the cyclopropane forms 8 and 9 with lithium metal reverses this process to give the parent porphyrinogen-metal complexes via the same monoelectronic pathway.4~-eTherefore, when complexes 8 are reacted with 1 equiv. of a reducing agent, 7 is formed. A variety battery; (iii) the metal-to-ligand synergism allows one to use the of reducing agents can cleave the cyclopropane unit and restore cyclopropane to mask high oxidation states of the metal, each the porphyrinogen skeleton, the mildest ones being S2-and cyclopropane unit having a contribution of +2 per metal; (iv) the RS-.14 Scheme 6 may be extremely suggestive of a possible redox processes can be tailored to occur exclusively at the R R Li(thf)2RR :BR i,LiBu, thf M = Fe WOC4 R NHHN kii9 MC12 d'N / \ ' \ R R (thfhLi 10 k3 Li(thf), R 12 11 8 M=Co,Ni,Cu 7 M=Co,Cu .I.10 CUC1,1 1-'rM= IM=Fe Et Et Et Et X-P Et atE\N Et tat-d N-= [CoCI]+,x-= [CqC15]-/\ \ /Et Et Et Et = [FeCl]+, X-= [Cu&15]-A B 9 M = [FeC1]2+,X-= [FeC14]2-i3 Scheme 6 Reductions carried out with Li: R = Et Scheme 7 Chem.Commun., 1996 1259 stepwise aromatization pathway of the porphyrinogen leading to porphyrins.In this case, the formation of the cyclopropane derivative would not be restricted to the oxidation of meso-octaalkylporphyrinogen, but may be a fundamental inter-mediate in the formation of porphyrins preceding the removal of the four meso-hydrogens. One question which arises at this point is: can we provide these artificial porphyrins as metal-free ligands? As for many of the metal-porphyrin complexes, the synthesis is better achieved via the oxidation of the corresponding metal-porphyrinogen precursor. However, quite recently we were able to study the redox chemistry of high valent, early transition metal-porphy-rinogen complexes, thus making available I11 and IV (Scheme 1) as free ligands. The reactions in Scheme 7 have been converted into synthetic methods for 12 and 13B.l5 Although their exploitation is at an early stage, these preliminary accounts on the discovery of 'artificial porphyrins' allow one to catch a glimpse of the potential of such molecules in electron-transfer processes, A few facts should be emphas-ized: (i) unlike the porphyrinogen-porphyrin couple, in the meso-octaalkylporphyrinogen-artificial porphyrin couple the redox interconversion between the two forms is energetically quite easy.It is particularly attractive to follow the small conformational changes which parallel the conversion of porphyrinogen into artificial porphyrins, as shown in Scheme 8 for the case of a cobalt-porphyrinogen complex. Such small conformational changes may be one of the reasons for the easy porphyrinogen-artificial porphyrin interconversion; (ii) the cyclopropane unit functions as a two-electron shuttle, thus the C-C bond formation and breakage behaves as a molecular metal, at the ligand, or at the metal-ligand sites; (v)the cleavage and formation of cyclopropane can be used for planning long- range electron-transfer processes; (vi) the appropriate site opening of the cyclopropane may lead to a ring-contracted corrinoid-type skeleton, or to other modifications of the porphyrinogen frame.(b)Modifying the Porphyrinogen Skeleton using Organometallic Methodologies In the context of the porphyrinogen-artificial porphyrin rela- tionship, modification of the porphyrinogen skeleton may shed more light on the mechanism of this transformation and make available other types of artificial porphyrins.Traditionally, such modifications of the porphyrinogen skeleton may be achieved using conventional organic assembly methods, but we will approach this problem using organometallic methodologies applied directly to the porphyrinogen skeleton.' These have been essentially based on the discovery that meso-octa-alkylporphyrinogen-metal complexes behave as carriers for As a result, we have been able to: polar organ~metallics.~~~ (i) functionalize the meso-octaalkylporphyrinogen at the ali- phatic periphery, or, in more general terms, to establish a novel entry into intra- and inter-molecular C-H bond activation;5*6 (ii) change the redox properties of the porphyrinogen skeleton by replacing pyrrole rings with pyridine 0nes.7,~3~ This was carried out via a direct homologation of a pyrrole to pyridine within the porphyrinogen structure using carbon monoxide under mild conditions (Scheme 9).(i)Regiochemically controlled mono- and bis-homologa- tion of porphyrinogen using carbon monoxide. The homo- logationl6 of porphyrinogen involves the introduction of one carbon atom into one or two pyrrole rings, as depicted in Scheme 9. Such a homologation of pyrrole to pyridine leads to the otherwise barely accessible trispyrrole-monopyridine and bispyrrole-bispyridine porphyrinogen-based macrocycles.7~13c The homologation of pyrrole to pyridine has been success- fully achieved by exploiting some well known organometallic reactions, such as the insertion of carbon monoxide into Zr-H and Zr-C bonds via the migration of the hydride and alkyl ligands.This often studied reaction produces y2-formyl and y2-acyl carbenium ions. l8 When sterically protected towards dimerization (which would yield an enediolato complex)17 or reduction, a carbenium ion will react as such. The meso-octaethylporphyrinogen ligand exhibits some in- teresting characteristics which assist the transformations men- tioned above. Specifically, the conformations derived from the sp3 carbons in the meso-positions and the porphyrinogen to zirconium bonding modes create cavities, thus to assure the stabilization of reactive intermediates at the metal and the geometrical proximity of reactive sites (pyrroles).7 Further, the electron-rich periphery of porphyrinogen is capable of binding an alkali-metal cation4Jj.7.1 la312 and this property is of major assistance in the homologation reaction pathway.This property makes the early transition metal-porphyrinogen complexes particularly novel carriers for polar substrates, such as metal hydrides and alkali-metal organometallics, la,13 as exemplified in Scheme 10. The addition of a carbenium y2-formyl or @'-acyl unit to a pyrrole ring, followed by the complete cleavage of a C-0 multiple19 bond due to the presence of very oxophilic metal centres such as zircoilium and potassium, led to the homologa- tion of a pyrrole to a pyridine ring (Scheme 11).It is the reaction of these metal hydride bridged dimers with carbon monoxide which paves the way for the pyrrole to I c** VIII IX Scheme 9 I5 alkyi t Li(thf),I 14 15 Scheme 10 [Et,N,(A)COCI], 8 [Et,N4(A)CoCI]+, 9 C(4)...C(6) 2.49 A C(4)...C(6) 1.61 A C(14)...C(16) 1.52 A C(14)...C(16) 1.64 A Scheme 8 1260 Chem. Commun., 1996 pyridine transformation. Migratory insertion reactions of car- bon monoxide with Zr-H (and Zr-C) bonds have mainly focused on cyclopentadienyl- and alkoxo-based systems. l7 The analogous reaction performed with 15 emphasizes the unusual role that porphyrinogen can play as an auxiliary ligand. Indeed, the ability to bind alkali-metal cations has already been alluded to. Furthermore, the considerable three-dimensional bulk of the ligand provides the necessary steric protection for the organo- metallic functionality.We have investigated the large-scale synthesis of 17 as either a multiple step or a one-pot preparation. Both strategies gave comparable yields (40-60%)7 and we are now able to produce 17 in quantities of up to 50 g following the sequence shown in Scheme 11. One major stereochemical consideration which should be addressed in the pyrrole to pyridine conversion is the regiochemistry of the homologation reaction. As a result of the attack of a carbenium ion on the pyrrole ring, the formyl carbon should be in the meta position of the final pyridine fragment. This regiochemistry is observed in the ring expansion of pyrrole to pyridine when using carbenes.20 This regiochemistry has been found in the reaction of Zr-alkyl derivatives 14 with carbon monoxide leading exclusively to meta-substituted pyridines.Attempts to modify the regiochemistry of the homologation of the meso-octaethylporphyrinogen [Et8N4H4], and to proceed further with the homologation of a second pyrrole ring have been successfully negotiated using metals other than zirconium. For instance, the use of niobium7 and titaniuml3C allowed the preparation of para-substituted pyridine rings. For the second homologation, the sequence followed is shown in Scheme 12.7 The reasons for the compulsory use of Hf instead of Zr is still obscure. The organometallic pathway leading from 17 to 18 is quite similar to that clarified for the homologation of 1 to 17 in Scheme 11. Et Et 1 Scheme 11 Et Et 17 18 Scheme 12 Between the two possible positional isomers (consider the two non-equivalent meta positions), 18 is the only one we have identified.7 The results reported emphasize how we can modify the skeleton and control the related regiochemistry of rather complex structures, like those of macrocyclic polypyrroles, using organometallic methodologies.In addition, this process does not remain just a chemical curiosity, since we can use this transformation as a good preparative method for a novel class of macrocycles. The introduction of one or two pyridines into a tetrapyrrolic macrocycle greatly modifies the geometric and electronic properties of this very important class of compounds, and could lead to porphyrins other than those shown in Scheme 1.(ii) The functionalization of aliphatic chains at the porphyrinogen periphery. Although electrophilic, metal-mediated aliphatic C-H cleavage is precedented in the lit- erature,z1 its application to the functionalization of complex substrates is almost unknown. We review here a novel electrophilic C-H bond activation and its application to the functionalization of the porphyrinogen skeleton (see Scheme 13). Unlike the usual approach, in which a metal-bonded alkyl or hydride is employed to remove an aliphatic hydrogen in intra- or inter-molecular processes,21 the following strategy has been explored. The removal of an aliphatic hydrogen by an alkali- metal hydride M*H, assisted by an electrophilic metal, leads to the formation of a polar alkali metal--alkyl species complexed by L,M (Scheme 13).11a,13 Therefore, the process depicted at the top of Scheme 13 is achieved intra-molecularly using alkali-metal hydrides.This type of reaction is particularly intriguing and very dependent on factors such as the MH :Zr ratio, the nature of the solvent, and the nature of the alkali-metal cation.l3b The reaction of 5 with KH 5-KH excess thf KH excess rneseEt not shown 21 22 Scheme 13 Chem. Commun., 1996 1261 meso-Et not shown 25 Scheme 14 KH exemplifies the complexity of this chemistry (Scheme 13). The mixture of 21 and 22 obtained from 20 is accounted for by the facile o-bond metathesis of Zr-C and Zr-H bonds and is also the entry into the intermolecular C-H bond activation.536 The alkali-metal ion plays a major role in the metallation reaction.Using LiH and NaH, instead of KH, even under very drastic conditions and in a large excess, a single ethyl group undergoes metallation, 23. The Zr-C bonds formed via the intramolecular metallation of the porphyrinogen periphery should readily allow its functional- ization using conventional organometallic methodologies.5,6~ Among the reactions that have been explored, the insertions of isocyanides and carbon monoxide are summarized in Scheme 14.6~ In conclusion, the metallation of aliphatic chains at the periphery of meso-octaethylporphyrinogen has been achieved via the electrophilic activation of the C-H bond followed by the removal of the P-proton using alkali-metal hydrides.The success of this class of reactions depends on two important features. First, the bifunctional nature of metal-porphyrinogen complexes allows the binding of cations at the electron-rich periphery and, as a consequence, their ability to function as polar organometallic carriers. Secondly, their reactivity is very dependent on the conformation of the porphyrinogen, which allows the C-H bond of the periphery to come in close proximity to the central electrophilic metal. Finally, an intramolecular o-bond metathesis between Zr-C and C-H bonds was observed in the conversion of the ethyl groups of porphyrinogen from the P-to the a-metallated forms.The results we have reported emphasize how appropriate organometallic methodologies (bifunctional complexes, con- formational effects, polar organometallic carriers) allow the activation and functionalization of aliphatic substituents in large organic molecules. Future Prospects In this brief survey, I have been able to give an exhaustive account of all of our recent results in the field of porphyrinogen- transition-metal chemistry. Furthermore, the work that is 1262 Chem. Commun., 1996 described has only been briefly outlined. The future prospects for this field are quite exciting, and some of major directions may be: (i) the exploitation of the artificial porphyrins; (ii)the chemistry of the porphyrinogen-metal complexes allowing the development of systems whose redox chemistry is associated with the formation and cleavage of C-C bonds, as in the artificial porphyrins; (iii)the use of transition metal-porphy- rinogen complexes as carriers for polar organometallics, ion- pairs and salts leading to the development of novel synthetic methodologies in organic chemistry.Acknowledgments This is the result of experimental and intellectual contributions by many co-workers, whose names are listed in the references. It has been supported over the years by the Fonds National Suisse de la Recherche Scientifique, Ciba-Geigy SA, and Action COST (European program for Scientific Research). I gratefully thank them all for their help and support.Carlo Floriani’s research interest is in molecular design to target specific functions. After obtaining his doctoral degree in organic chemistry (Milano, Italy), he went to CERI (Geneva) for a post-doctoral training in organometallic chemistry. He returned to Italy and became a Professor of Inorganic Chemistry at the University of Pisa. He then moved to Columbia University (New York, USA) as a Professor of Chemistry, followed by his return to Europe at the University of Lausanne (Switzerland). He has been honoured, among other things, by the award of the Centenary Lecture and the silver medal by the Royal Society of Chemistry. References 1 Porphyrins and Metalloporphyrins, ed. K. M. Smith, Elsevier, Amsterdam, The Netherlands, 1975; The Porphyrins, ed.D. Dolphin, Academic, New York, 1978; T. Mashiko and D. Dolphin, in Comprehensive Coordination Chemistry, ed. G. Wilkinson, R. D. Gillard and J. A. 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ISSN:1359-7345
DOI:10.1039/CC9960001257
出版商:RSC
年代:1996
数据来源: RSC