Rhodium porphyrin dimer

Using the very bulky rhodium porphyrins Rh(TTEPP)- and Rh(TTiPP)- (which contain triethylphenyl and triisopropylphenyl groups), neither of which can dimerize. direct evidence for an alkene adduct and its subsequent dimerization to the four-carbon bridged product has been obtained. Reaction of Rh(TTEPP)- with ethene... 

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

TPP)Rh(L)J+C1 in the presence of an alkyl halide leads to a given (P)Rh(R) or (P)Rh(RX) complex. The yield was nearly quantitative (>80X) in most cases based on the rhodium porphyrin starting species. However, it should be noted that excess alkyl halide was used in Equation 3 in order to suppress the competing dimerization reaction shown in Equation 1. The ultimate (P)Rh(R) products generated by electrosynthesis were also characterized by H l MR, which demonstrated the formation of only one porphyrin product(lA). No reaction is observed between (P)Rh and aryl halides but this is expected from chemical reactivity studles(10,15). Table I also presents electronic absorption spectra and the reduction and oxidation potentials of the electrogenerated (P)Rh(R) complexes. 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

The main reactions of rhodium or iridium porphyrins are depicted in Scheme 3 and compiled in Table 6. This comparative table shows that not in all cases have the analogous situations been studied for rhodium and iridium porphyrins as a whole, a systematic study of iridium porphyrins has commenced only recently. As already mentioned in Table 4, the main starting materials are the aquachlo-rorhodium(III) or carbonylchloroiridium(III) species, i.e. the inspection of Scheme 3 will start from the compounds MC1(P)L (M = Rh L = H20) or MCl(P)CO (M = Ir). Alternative access to the chemistry of rhodium porphyrin chemistry originates at a bare Rh(II) species Rh(P) which is in equilibrium with its metal-metal bonded dimer, [Rh(P)]2 (paths q, — q see below). 

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],... 

The steric properties of the porphyrin ring help control the nuclearity of the species. For example, the rhodium-tetraphenylporphyrin (TPP) complex in the equilibrium of Equation 4.36 is predominantly dimeric, while the tetramesitylporphyrin (TMP) complex in Equation 4.37 is predominantly monomeric. - Generally, the bond dissociation energies of the Rh-Rh bond in dimeric rhodium-porphyrin complexes He in the range of 8-25 kcal/mol. " ... 

The electrooxidation of Rh(III) porphyrins is relatively straightforward and consists for the most part of two successive one-electron transfers, both of which involve the porphyrin 7r-ring system. An assignment of the electron-transfer site has often been made on the basis of ESR and/or UV-visible spectra of the singly oxidized products. Other types of rhodium porphyrins, which have been electrochemically examined include [(TPP)Rh(L)2] Cl and (TPP)Rh(L)Cl, where L = NMe2, both of which were proposed to form a dimeric Rh(II) species after an initial one-electron... 

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

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. ... 

Radiolytic reduction has been investigated as a means of producing transient Rh(II) porphyrin products, and as in the above study, the observed products were strongly dependent on pH and solvent. Radiolytic reduction of Rh(TMP)Cl in alcohol formed transient Rh(TMP)- which was prevented from dimerization by the bulky TMP ligand. In alkaline 2-propanol the product is [Rh(TMP)r. in weakly acidic 2-propanol the hydride Rh(TMP)H is formed, and in strongly acidic 2-propanol the alkylated rhodium(III) porphyrins Rh(TMP)CH3 and Rh(TMP) (C(CH 3)20H) are observed. The alkyl products result from reaction of Rh(TMP)-with CH3- and C(CH3)20H formed by radiolysis of the 2-propanol solvent. 

Similar to the intramolecular insertion into an unactivated C—H bond, the intermolecular version of this reaction meets with greatly improved yields when rhodium carbenes are involved. For the insertion of an alkoxycarbonylcarbene fragment into C—H bonds of acyclic alkanes and cycloalkanes, rhodium(II) perfluorocarb-oxylates 286), rhodium(II) pivalate or some other carboxylates 287,288 and rhodium-(III) porphyrins 287 > proved to be well suited (Tables 19 and 20). In the era of copper catalysts, this reaction type ranked as a quite uncommon process 14), mainly because the yields were low, even in the absence of other functional groups in the substrate which would be more susceptible to carbenoid attack. For example, CuS04(CuCl)-catalyzed decomposition of ethyl diazoacetate in a large excess of cyclohexane was reported to give 24% (15%) of C/H insertion, but 40% (61 %) of the two carbene dimers 289). 

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). 

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. 

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. 

A clean formation of [Rh(OEP)]2 proceeds via thermolysis [269] or photolysis [273] with loss of dihydrogen from or autoxidation of the hydride RhH(OEP) (path p). The tetramesitylporphyrin complex, Rh(TMP) [61], does not dimerize at all due to the sterically hindrance created by the two ortho-methyl groups of each phenyl ring (see Ru(TMP) ), however, the meta-methyl groups of the rhodium(II) derivative prepared from tetra-kis(3,5-xylyl) porphyrin [H2(TXP)] do not prevent dimerization, and the complex is isolated as a dimer [Rh(TXP)] 2 which dissociates (path — q) prior to chemical reactions. Photolysis of RhMe(TMP) [274] (path r) is another suitable access to Rh(TMP) [271]. 

The dimeric iridium(II) porphyrinates, [Ir(P)]2, are far less well studied [222,270] than their rhodium analogs. The formation of [Ir(OEP)]2 is cleanly achieved by photolysis of IrMe(OEP) (path r, q) [272]. Hydrogenolysis of the dimer (paths — q, — p) yields the hydride, neat toluene a mixture of the hydride and the benzyliridium(III) compound [paths — q, — p, — r, similarly to Rh(II) porphyrins]. 

Organometallic reactions of dimeric rhodium (II) or iridium(II) porphyrins - The review of Guilard, Radish, and coworkers [306] has already been cited. This gives a clear evaluation of the reactions of [Rh(OEP)]2 and [Ir(OEP)]2 with a variety of organic substrates as far as described up to 1987. The important reactions with aliphatic and benzylic CH bonds have already been mentioned in Sect. 3.3, see Eq. (20). Here, some more recent developments, especially concerning the reactions with CO or olefins, will be elaborated. 

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. 

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

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