Rhodium macrocycles

Favorable thermodynamics (AG° (298K) <0) for obtaining acetylene diolates from M-M bonded complexes occurs when 2(M-0)>(M-M)+ 147 kcal while single metal units require that the M-0 bond energy exceed 78 kcal (Table I, entries u,v). This limiting type of CO coupling is best known for reactions of alkali metals with CO which form solid ionic acetylene diolate compounds.Rhodium macrocycle complexes have Rh-0 bond energies 50-60 kcal and thus are excluded as potential candidates for acetylene diolate formation. 

Rhodium macrocycles [((Rh(acac))2()U-L)2] (89 L = dppa 90 L = dppda) have been prepared from [Rh(acac)(cyclooctene)2] and L (Fig. 2.32) [95]. These compounds proved to be effective catalysts for the hydroboration of unhindered vinyl arenes, giving selectivities up to 98%. 

Pentapyrrolic macrocycles, 2,888 2,1,2-Pen tathiadiazol e-4,7-dicarbonitrile in hydrogen production from water, 6, 508 Pentatungstobis(organophosphonates), 3, 1053 4-Penten-l-al reaction with ethylene catalysts, rhodium complexes, 6, 300... 

Grieco in the total synthesis of (—)-epothilone B 134 used a rhodium-catalyzed hydroboration as a key step in the synthesis of the macrocyclic ring (Figure 15).141 Completion of the synthesis of the C(3)-C(12) fragment was carried out using a rhodium-catalyzed hydroboration as the key step. 

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

Mach-Zehnder interferometer, 22 153 Mackenzie-Shuttleworth model, 23 75 Mackie line effect, 19 209, 210 Macor glass-ceramic, 22 635 Macquartite, 6 471t Macrinite, 6 707t Macrobicyclic effect, 24 39 Macrocrystalline wax, 26 214 Macrocycles, 24 35, 45 developments of, 24 36 Macrocyclic complexes, rhodium, 29 645 Macrocyclic compounds, chelating agents, 5 710, 713t... 

Secondary and tertiary amines can be obtained if the hydroformylation of olefins is conducted in the presence of primary and secondary amines under elevated hydrogen partial pressures. Here the rhodium catalyst is involved in both steps, the hydroformylation of an olefin as well as the hydrogenation of the imine or enamine resulting from a condensation of the oxo-aldehyde with the amine (Scheme 14). This combination of hydroformylation and reductive amination is also known as hydroaminomethylation and has been applied to the synthesis of various substrates of pharmaceutical interest [55-57] as well as to the synthesis of macrocycles [60-63] and dendrimers [64,65]. 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

The complexes in Table I have been assigned an end-on geometry on the basis of spectroscopic data, chemical behavior, and, in the case of a macrocyclic rhodium complex, X-ray crystallographic data (53). The 0-0 stretching frequencies and 0-0 bond lengths are useful indicators of the electronic structure of coordinated dioxygen (54-59),... 

Coordinated secondary amines can also be alkylated, but only after deprotonation by a strong base generates a suitable nucleophile. Work on rhodium(III) complexes of ethylenediamine12 has been extended to nickel(II) complexes of various fully saturated macrocycles such as cyclam (Scheme l).13,14 The methylated cyclam complex is kinetically inert, unlike the isomer with all four methyl groups on the same side of the ring, which is obtained on reaction of the preformed tetramethyl cyclam with nickel ions. 

The reactions of dihydrobilin (1,19-dideoxybiladiene-a, c) with transition metals are strongly influenced by the nature of the metal ion. Thus with Mn(OAc)3 or FeClj the corresponding metallocorrolates have been obtained in high yield, in the presence of chromium or ruthenium salts the reaction product isolated has been the metal free macrocycle, while coordination of rhodium requires the presence of an axial ligand such as a phosphine, arsine or amine [21]. Neutral pentacoordinated rhodium complexes have thus been obtained. Although analysis of the electronic spectra of the reaction mixtures demonstrated that cyclization of the open-chain precursor and formation of metallocorrolates occur even in the absence of extra ligands, no axially unsubstituted rhodium derivative has been reported. 

The stabilizing effect of an axial ligand has been previously observed in the synthesis of cobalt corrolates. Such an effect has been used to synthesize the complex where no peripheral p substituents are present on the macrocycle, which decomposes if attempts are made to isolate it in the absence of triphenyl-phosphine [10]. The behavior of rhodium closely resembled that of cobalt and it seems to be even more sensitive to the presence of axial ligands. [Rh(CO)2Cl]2 has also used as a metal carrier with such a starting material a hexacoordinated derivative has been isolated. The reaction follows a pathway similar to that observed for rhodium porphyrinates the first product is a Rh+ complex which is then oxidized to a Rh3+ derivative [29]. 

The distortion is the result of the strain imposed on the whole molecule by the geometric requirements of the corrole structure. It is not, however, very significant the rhodium atom is displaced by only 0.26 A from the plane of the four coordinating nitrogen atoms, a value much smaller than that observed in the structure of the P-unsubstituted complex Co(Corrole)PPh3 shown in Fig. 12 where the cobalt atom is displaced by 0.38 A from the macrocycle plane [32]. In both compounds the four coordinating nitrogen atoms are strictly coplanar. 

It is interesting to note that in the structure of both rhodium and cobalt complexes the corrole framework shows a higher degree of planarity than that observed in the free ligand where a large distortion is induced by contacts between the inner hydrogen atoms [11]. The free base corrole macrocycle in its unprotonated form seems to be very flexible and the macrocycle core can expand to accommodate a large metal ion such as Rh3+ in a [Pg.89]

Collman et al. prepared halopropyl rhodium complexes by the reaction of dihalopropanes with a macrocyclic rhodium(I) complex [Eq. (29)] (180). The ratio of mononuclear to binuclear product was found to be solvent dependent, with the variations in product ratios in different solvents being related to the solubility of the Rh(I) macrocycle. 

Sargeson and co-workers have synthesized, characterized and studied the chemistry of a variety of remarkable macrocyclic amine complexes of RhIH, in which the rhodium is virtually isolated... 

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