Porphyrins, rhodium , preparation

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

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. 

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. 

Many papers formulate the starting chlororhodium(III) porphyrin just as RhCl(P), as if a trans ligand L in MC1(P)L were easily lost (path c, X = Cl). However, the conditions of preparations point to a predominance of hexacoor-dinate aqua species, RhCI(P)H20. Only in one case the formation of a pentacoordinated rhodium(III) halide, the iodide RhI(TMP), seems well-documented [61], see Sect. 2.1.3. The formation of interesting heterobimetallic porphyrins, e.g. (TPP)RhMn(CO)s, [path c, X = Mn(CO)s] was formulated as starting from RhCl(TPP) [63], but the work referred to [264] clearly stated that hexacoordinate species, namely RhCl(TPP)H20, RhCl(TPP)(EtOH), or RhCl(TPP)CO were involved. On the other hand, the heterobimetallic species appear to be pentacoordinate about the rhodium, in accord with many metal-metal-bonded porphyrin complexes [222] (see also below). 

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]. 

A complementary, but less pronounced, increase in cis syri) selectivity is provided when diazoacetates with rather small ester groups and rhodium catalysts with bulky ligands, such as iodorhodium(IIl) porphyrins and mcso-tetrakis(2,4,6-triarylbenzoato)di-rhodium(ll) complexes, are employed. For ethyl 2-alkylcyclopropane-l-carboxylates so obtained, a cisfran.s ratio of 2-4 was typical. Notable cis selectivities have also been achieved in the synthesis of ethyl 2-phenylcyclopropane-l-carboxylate with the catalyst [(r/ -C5H5)Fe(CO)2(THF)][BFJ (cis/trans 5.25, see Section 1.2.1.2.4.2.6.3.1.) and copper catalysts prepared in situ from a copper salt [copper(I) iodide, copper(II) acetate or triflate] and sodium tetrakis(7,8,8-trimethyl-4,5,6,7-tetrahydro-2Ff-4,7-methanoindazolyl)borate (3) cis/ trans 2.1-3.2). °... 

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 ... 

Rhodium complexes of the bulky tetra-(3,5-dimethyl-phenyl) porphyrin (TXP) ligand and the more sterically demanding tetra-(2,4,6-trimethylphenyl) porphyrin (TMP) were prepared as part of an effort to examine the role of... 

A similar type of chiral rhodium porphyrin was found to be effective for the carbene-insertion reaction to olefins, where formation of the carbene complex takes place. Chiral rhodium complexes for catalytic stereoselective-carbene addition to olefins were prepared by condensation of a chiral aldehyde and pyrrole. Formation of the metal-carbene complex and substrate access to the catalytic center are crucial to the production of optically active cyclopropane derivatives. Optically active a-methoxy-a-(trifluoro-methyOphenylacetyl groups are linked witfi the amino groups of a,p,0L,p isomers of tetrakis-(2-aminophenyI)por-phyrin through amide bonds. Oxidation reactions of the... 

Zhang X-X, Wayland BB (1994) Rhodium(ll) porphyrin bimetalloradical complexes preparation and enhanced reactivity with CH4 and H2. J Am Chem Soc 116(17) 7897-7898... 

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