Rhodium benzoate

In a direct competition between 1,2- and 1,5-insertion into methylene C—H, the relative proportion of products depends on the rhodium carboxylate used. Rhodium benzoate is the most efficient catalyst so far found for the cyclization of a-diazo ester (97) to (98) (equation 34). ... 

The absence of mesomorphism in these compounds was explained on the basis of space-filling requirements. Thus, the intercalation of pyrazine between the binuclear units creates free volume which needs to be filled to obtain a stable, condensed phase when the carboxylates bear only one chain, the interdimeric space is likely filled by the aliphatic chains belonging to a different polymeric chain, giving rise to a crossed structure which prevents the formation of a columnar mesophase. However, as will be seen later, liquid-crystalline behavior was induced in the case of mixed-valence diruthenium(II,III) carboxylate complexes with bulky equatorial Kgands bearing two and three aliphatic chains as with such ligands, it was possible to fill the interdimeric space and thus to induce a thermotropic columnar mesophase. Very recently, the synthesis, characterization, and mesomorphic properties of pyrazine-polymerized divalent rhodium benzoates have also been reported (99). " Most of these compounds exhibit columnar (Colh, Coir, CoIn) and cubic mesophases with melting transition temperatures close to, or even below, room temperature. 

Reactions with alkynes may lead to the formation of cyclized products. The reaction of iodobenzenes with two equivalents of an alkyne has been shown to give naphthalene derivatives in the presence of cobalt catalyst with a manganese reduc-tant. The process, shown in Scheme 15, is thought to involve oxidative addition of the aryliodide to cobalt followed by double alkyne insertion. The cobalt-catalysed annulation step probably involves an pathway. The cyclopentadienyl-rhodium-catalysed annulation of benzoic acids with alkynes has been used to form isocoumarin derivatives, such as (126). The process is thought to involve cyclorhodation at the ortho-position of a rhodium benzoate intermediate, followed by alkyne insertion to form a seven-membered rhodacycle and reductive elimination The silver-catalysed annulations of diphenylphosphine oxides with alkynes proceed in the absence of rhodium. Benzophosphole oxides such as (127), formed with diphenylethyne, are produced. Here, the proposed mechanism involves homolytic cleavage of the phosphorus-hydrogen bond to give a radical which can add to the alkyne and subsequently cyclize. ... 

From a study of the decompositions of several rhodium(II) carboxylates, Kitchen and Bear [1111] conclude that in alkanoates (e.g. acetates) the a-carbon—H bond is weakest and that, on reaction, this proton is transferred to an oxygen atom of another carboxylate group. Reduction of the metal ion is followed by decomposition of the a-lactone to CO and an aldehyde which, in turn, can further reduce metal ions and also protonate two carboxyl groups. Thus reaction yields the metal and an acid as products. In aromatic carboxylates (e.g. benzoates), the bond between the carboxyl group and the aromatic ring is the weakest. The phenyl radical formed on rupture of this linkage is capable of proton abstraction from water so that no acid product is given and the solid product is an oxide. 

Figure 3. Hydrogenation of methyl benzoate to methyl cyclohexanoate with rhodium on y-alumina catalysts. (Reprinted from Ref. [24], 2003, with permission from Elsevier.)...

The search for catalysts which are able to reverse the ratio of cyclopropane diastereomers in favor of the thermodynamically less stable isomer has met with only moderate success to date. Rh(II) pivalate and some ring-substituted Rh(II) benzoates induce cw-selectivity in the production of permethric acid esters 77,98 99 contrary to rhodium(II) acetate, which gives a 1 1 mixture 74,77,98), and some copper catalysts 98) (Scheme 10). 

Arya et al. used solid phase synthesis to prepare immobilised dendritic catalysts with the rhodium centre in a shielded environment to mimic nature s approach of protecting active sites in a macromolecular environment (e.g. catalytic sites inside enzymes) [51], Two generations PS immobilised rhodium-complexed dendrimers, 6 and the more shielded 7, were synthesised.The PS resin immobilised rhodium-complexed dendrimers were used in the hydroformylation of styrene, p-methoxystyrene, vinyl acetate and vinyl benzoate using a total pressure of 70 bar 1 1 CO/H2 at 45 °C in CH2C12. 

Fig. II. Effects on rate and catalyst stability of using sulfolane-tetraglyme mixtures as solvent ( ) methanol ( ) ethylene glycol ( ) rhodium recovery. Reaction conditions 75 ml solvent, 3 mmol Rh, 0.65 mmol cesium benzoate, 544 atm, H2/CO = 1, 240 C, 4 hr (94).

Support for the role of the o-DPPB substituent as a catalyst-directing group was provided in a control experiment with the benzoate 7. Thus, exchanging the phosphorus of the o-DPPB group with a CH moiety, itself not able to coordinate to the catalytically active rhodium center, caused a complete loss of stereoselectivity in the hydroformylation reaction [10]. 

The reaction of diazoketone 621 with either silver benzoate in methanol or with rhodium acetate in dichloromethane led to the formation of the stable azetinium salt 622, which was isolated as yellow crystals (Equation 236) <1998AGE2229>. 

Kambara and Nishimura studied the hydrogenation of the methyl esters of o-, m-, and p-methoxybenzoic acid over platinum group metals in f-butyl alcohol at 60°C and atmospheric hydrogen pressure.197 As seen from the results shown in Table 11.15, hydrogenolysis occurred most extensively over platinum and iridium and much less extensively over rhodium and ruthenium, especially with the meta and para isomers. It is noted that the ortho isomer was hydrogenolyzed as extensively as the meta isomer over platinum and iridium, and much more extensively than the meta and para isomers over rhodium and ruthenium. Over Pd-C the most extensively hydrogenolyzed was the meta isomer. One of the reasons for the extensive hydrogenolysis in the case of the ortho isomer can be attributed to the direct formation of methyl benzoate, which amounted to 17.1% over rhodium, 20.7% over ruthenium, and 16.6% over platinum,... 

The alkaloid ( + )-retronecine (883, Scheme 129) is structurally similar to ( + )-heliotridine (850), with the exception that the stereocenter at C-7 is of opposite configuration. The basic approach to its synthesis involves a carbenoid displacement similar to that in the previous scheme. The acetyl protecting group of the common intermediate 875 (R=SPh) is changed to a TBS group, and the benzoate is converted to pivalate. Carbenoid displacement with dibenzyl a-diazomalonate in the presence of rhodium acetate gives 879. Reductive desulfurization... 

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