Complex, Rhodium porphyrin

The syntheses and spectroscopic and electrochemical characterization of the rhodium and iridium porphyrin complexes (Por)IVI(R) and (Por)M(R)(L) have been summarized in three review articles.The classical syntheses involve Rh(Por)X with RLi or RMgBr, and [Rh(Por) with RX. In addition, reactions of the rhodium and iridium dimers have led to a wide variety of rhodium a-bonded complexes. For example, Rh(OEP)]2 reacts with benzyl bromide to give benzyl rhodium complexes, and with monosubstituted alkenes and alkynes to give a-alkyl and fT-vinyl products, respectively. More recent synthetic methods are summarized below. Although the development of iridium porphyrin chemistry has lagged behind that of rhodium, there have been few surprises and reactions of [IrfPorih and lr(Por)H parallel those of the rhodium congeners quite closely.Selected structural data for rr-bonded rhodium and iridium porphyrin complexes are collected in Table VI, and several examples are shown in Fig. 7. ... 

Metal complexes of tetra-4-ferf-butylphthalocyanine [PcM, M = Mn(III)OAc, Cu(II), Co(II), Ni(II), Fe(II) (C5H5N)2, Rh(III)Cl] have also been tested for their stereoselective potential in the cyclopropanation of styrene with ethyl diazoacetate 101K The Co(II) and Rh(I) complexes, already highly active at room temperature, produced the 2-phenylcyclopropanecarboxylic esters in a E Z isomer ratio of 1.0-1.2 which compares well with the value obtained with the rhodium(III) porphyrin 47 a (1.2). In the other cases, E.Z ratios of 2.0-2.2 were observed, except for M = Fe(II) (C5HsN)2 where it was (3.0) the E.Z ratio of the purely thermal reaction was 2.0. 

The q1-coordinated carbene complexes 421 (R = Ph)411 and 422412) are rather stable thermally. As metal-free product of thermal decomposition [421 (R = Ph) 110 °C, 422 PPh3, 105 °C], one finds the formal carbene dimer, tetraphenylethylene, in both cases. Carbene transfer from 422 onto 1,1-diphenylethylene does not occur, however. Among all isolated carbene complexes, 422 may be considered the only connecting link between stoichiometric diazoalkane reactions and catalytic decomposition [except for the somewhat different results with rhodium(III) porphyrins, see above] 422 is obtained from diazodiphenylmethane and [Rh(CO)2Cl]2, which is also known to be an efficient catalyst for cyclopropanation and S-ylide formation with diazoesters 66). 

The preparation of cyclopropanes by intermolecular cyclopropanation with acceptor-substituted carbene complexes is one of the most important C-C-bond-forming reactions. Several reviews [995,1072-1074,1076,1077,1081] and monographs have appeared. In recent decades chemists have focused on stereoselective intermolecular cyclopropanations, and several useful catalyst have been developed for this purpose. Complexes which catalyze intermolecular cyclopropanations with high enantiose-lectivity include copper complexes [1025,1026,1028,1029,1031,1373,1398-1400], cobalt complexes [1033-1035], ruthenium porphyrin complexes [1041,1042,1230], C2-symmetric ruthenium complexes [948,1044,1045], and different types of rhodium complexes [955,998,999,1002-1004,1010,1062,1353,1401-1405], Particularly efficient catalysts for intermolecular cyclopropanation are C2-symmetric cop-per(I) complexes, as those shown in Figure 4.20. These complexes enable the formation of enantiomerically enriched cyclopropanes with enantiomeric excesses greater than 99%. Illustrative examples of intermolecular cyclopropanations are listed in Table 4.24. 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

The rhodium complex [CpRh(bipy)Cl2] is reported (162) to act as one-half of a redox couple that, in concert with a manganese porphyrin system, catalyzes the epoxidation of olefins by dioxygen. In this two-phase system, the aqueous phase contains sodium formate, and the organic phase is a trichloroethane solution of [Mnm(tpp)]1+ and the rhodium complex (tpp = meso-tetraphenylporphyrin). Apparently, the rhodium complex catalyzes the reduction of [Mnin(tpp)]1+ by formate, and the manganese(II) species thus formed binds dioxygen and reacts with the substrate olefin to form the epoxide. However, the intermedi-... 

The inefficiency of the platinum/hydrogen reduction system and the dangers involved with the combination of molecular oxygen and molecular hydrogen led to a search for alternatives for the reduction of the manganese porphyrin. It was, for example, found that a rhodium complex in combination with formate ions could be used as a reductant and, at the same time, as a phase-transfer catalyst in a biphasic system, with the formate ions dissolved in the aqueous layer and the manganese porphyrin and the alkene substrate in the organic layer [28]. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

Using the metalloradical reactivity of the Rh(II)OEP (OEP = 2,3,7,8,12,13,17,18-octaethylphorphynato) dimer, the preparation of silyl rhodium complexes was achieved by the hydrogen elimination reaction with silanes I R SiH (R = R = Et, Ph R = Me, R = Ph, OEt). The Rh—Si bond length of 2.32(1) A, found when R = Et, is comparable to those in other Rh(III) complexes (Table 11). The crystal packing indicates that all the ethyl groups on the porphyrin periphery are directed toward the silyl group. Consequently, the aromatic part of one complex molecule is in contact with the aromatic part of the next molecule and the aliphatic part is in contact with the aliphatic part of the next molecule204. 

Metal(II) species, homobimetallic complexes - During a rhodium insertion into H2(TPP) using [Rh(CO)2Cl]2 in HOAc/NaOAc, a paramagnetic rhodium(II) porphyrin was observed [57] which could be transformed into a hydridorho-dium(III) porphyrin with dihydrogen (path — p) and subsequently into its corresponding base, the anion [Rh(TPP)], which may be likewise regarded as... 

Molecular recognition with rhodium complexes of functionalized porphyrins - The coordination abilities of rhodium(III) porphyrins outlined in Scheme 3 (paths b and g) were used to design porphyrins with special receptor properties, i.e. cis-or trans-5,15-bis(2-hydroxy-l-naphthyl)octaethylporphyrin [H2(npOEP)], trans-5,15-bis(8-quinolyl) porphyrin, or tetrakis(2-hydroxyphenyl) porphyrin. After Rh insertion, these porphyrins provide lateral OH or N donor groups at a fixed distance from the coordination site at the Rh atom. Unlike RhCl(TPP), RhCl(npOEP) activates acetone in a manner that an a-metallation of acetone takes place, yielding Rh(CH2COMeXnpOEP) [283]. 

Replacement of the 2-naphthyl groups by 2-dimethylaminomethylphenyl groups in H2(npOEP) also led to a rhodium porphyrin being able to extract leucine from water, however, the situation is complicated by dimerization of the rhodium porphyrin due to intermolecular amine-rhodium bonding [286]. A rhodium complex of a trifunctional chiral bis(2-hydroxynaphthyl)porphyrin related to the above-mentioned RhCl(npOEP) system was used to separate diastereomers formed via two-point fixation of amino acids [287],... 

Reactions of rhodium(III) porphyrins with olefins and acetylenes - Ogoshi et al. [326] have described the reactions of vinyl ether with rhodium (III) porphyrins which are depicted in reaction sequence (33). Step (a) appears to be an insertion of the olefin into the Rh-Cl bond followed by alcoholysis of a chlorosemiacetal to the acetal, step (b) is the hydrolysis of the acetal to the aldehyde. The insertion is thought to start by heterolysis of the Rh-Cl bond producing a cationic species which forms a 7i-complex with the electron-rich olefin. 

Reactions of monomeric and dimeric rhodium(II) porphyrins with carbon monoxide - As already reported in Sect. 3.3, a carbonylrhodium(II) porphyrin behaves as an acyl radical. Hence, if possible, dimerization or coupling reactions occur. Evidence for the formation of isomeric 2 1 Rh(P) CO adducts, namely a monoadduct of the dimer and a metallo ketone complex, and a dimeric 1 1 adduct in the reaction of [Rh(OEP)]2 with carbon monoxide according to sequences (38) and (39) has been presented [340,341] solution equilibria and structures have been studied essentially by lHNMR, 13CNMR, and IR spectroscopy. The first half of sequence (38) and reaction (39) occurred in parallel at CO pressures up to 12 atm at 297 K. At higher pressures, or at lower temperatures, the double-insertion of CO shown in the last step of (38) was observed. 

The formation of 7 j/ complexes has also been reported for rhodium(II) porphyrin complexes on oxygenation and the iron dioxygen complexes will also undergo inner sphere reduction to give Fe(III)OOFe(ni) ... 

The iridium complex contained a tightly bound carbonyl group, unlike the rhodium complex, in accord with the stronger bonding of CO groups to 5d than to transition metals (216). Related iron and ruthenium carbonyl porphyrins have also been described 14, 217, 461). 

So far, while there is a relative abundance of synthetically useful cyclopropana-tion catalysts, all of them provide a mixture of diastereomers with the anti product predominating. Thus, a catalyst able to provide optically active syn cyclopropyl esters would constitute a useful complement to existing methodology. Rhodium complexes of bulky porphyrins ( chiral fortress porphyrins) have been developed for this purpose [27]. The porphyrin ligands bear chiralbinaphthyl groups appended directly to the meso positions. Their rhodium(III) complexes provide predominantly the syn cyclopropane with diazoesters, with very good stereoselectivity in some cases. However, the enantioselectivities observed are modest. 

Mechanistic studies of rhodium porphyrins as cyclopropanation catalysts have resulted in the spectroscopic identification of several potential intermediates in the reaction of ethyl diazoacetate with olefins, including a diazoniumfethoxy-carbonyl)methyl-rhodium complex formed by electrophilic addition of the rhodium center to the a-C atom of ethyl diazoacetate [29]. It is not known if analogous intermediates are also formed in analogous reactions of copper catalysts. However, the initial part of the catalytic cycle leading to the metal carbene intermediate is not of primary concern for the enantioselective reactions described herein. It is the second part, the reaction of the metal-carbene complex with the substrate, that is the enantioselective step. 

The (octaethylporphyrin) rhodium dimer [Rh(OEP)]2 reacts in the presence of H2 and CO to produce the formyl species, Rh(OEP)CHO . The rhodium(II) porphyrins are the only systems to date to react with H2 and CO to produce formyls directly. Alternatively, the metalloformyl complex Rh(OEP)CHO can be prepared as ... 

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