Rhodium carboxylates studies

As corroborated by deuterium labeling studies, the catalytic mechanism likely involves oxidative dimerization of acetylene to form a rhodacyclopen-tadiene [113] followed by carbonyl insertion [114,115]. Protonolytic cleavage of the resulting oxarhodacycloheptadiene by the Bronsted acid co-catalyst gives rise to a vinyl rhodium carboxylate, which upon hydrogenolysis through a six-centered transition structure and subsequent C - H reductive elimina-... 

With the long chain a-diazo ketone. 6-diazo-7-tridecanone, 1,5-insertion could proceed with placement of the carbonyl outside the ring, or included in the ring. In fact, only the product 7, from the first of these two cyclization modes, is observed67. The alternative cyclopentane 9 is not formed. As with the a-diazo ester, the relative proportion of 1,2- and 1,5-products depends on the rhodium carboxylate employed. Throughout these studies, it has been observed that the olefin 8, obtained from 1,2-elimination, is cleanly Z-configured67 68. 

Hubert in 1976 reported that rhodium acetate efficiently catalyzes diazo insertion into an alkene, to give the cyclopropane. In 1979, Southgate and Ponsford reported that rhodium acetate also catalyzes diazo insertion into a C—H bond. Prompted by these studies, Wenkert then demonstrated that cyclization of (58) to (59) proceeded much more efficiently with the rhodium carboxylates than it had with copper salt catalysis (equation 23). ... 

When a large excess of carboxylic acid is present, rhodium carboxylate is not converted into hydride [1]. A preferred dionate is 2,2,6,6-tetramethyl-3,5-heptanedionate (dipivaloylmethane) especially because its solutions are stable upon storage [2]. The use of stock solutions is recommended because very small amounts of rhodium need to be applied when active catalysts are being studied. In batch reactions the formation of rhodium hydride species may be slow compared to catalysis. If so, one should pre-heat the catalyst system to allow the formation ofthe catalyst before adding the alkene. 

The biological studies on the rhodium carboxylates have been reviewed and discussed [3, 9, 10]. A number of interesting points have emerged, emphasizing the diversity of metal-based complexes with antitumour activity, although for rhodium carboxylates the acute toxicity probably eliminates these complexes from further development. The results are summarized below ... 

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. 

The structure of rhodium(II) carboxylate-derived carbene complexes has been assessed both by quantum mechanical calculations [19,20] and by the study of rhodium(II) carboxylate isonitrile complexes [20,21]. Recent investigations [20] suggest, that also in these highly electrophilic carbene complexes there is a significant n backbonding from rhodium to carbon. 

Hydrogenation of unsaturated carboxylic acids, such as acrylic, methacryUc, maleic, fumaric, cinnamic etc. acids was studied in aqueous solutions with a RhCU/TPPTS catalyst in the presence of p-CD and permethylated P-cyclodextrin [7]. In general, cyclodextrins caused an acceleration of these reactions. It is hard to make firm conclusions with regard the nature of this effect, since the catalyst itself is rather undefined (probably a phosphine-stabilized colloidal rhodium suspension, see 3.1.2) moreover the interaction of the substrates with the cyclodextrins was not studied separately. 

By 1984, the palladium-catalyzed aUyhc alkylation reaction had been extensively studied as a method for carbon-carbon bond formation, whereas the synthetic utility of other metal catalysts was largely unexplored [1, 2]. Hence, prior to this period rhodium s abihty to catalyze this transformation was cited in only a single reference, which described it as being poor by comparison with the analogous palladium-catalyzed version [6]. Nonetheless, Yamamoto and Tsuji independently described the first rhodium-catalyzed decarboxylation of allylic phenyl carbonates and the intramolecular decarboxylative aUylation of aUyl y9-keto carboxylates respectively [7, 8]. These findings undoubtedly laid the groundwork for Tsuji s seminal work on the regiospecific rho-... 

Alcohols. Similarly to acids metals can also catalyze the transformation of alcohols to produce carboxylic acids. Cobalt, rhodium, and iridium were studied... 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

The aziridination of olefins, which forms a three-membered nitrogen heterocycle, is one important nitrene transfer reaction. Aziridination shows an advantage over the more classic olefin hydroamination reaction in some syntheses because the three-membered ring that is formed can be further modified. More recently, intramolecular amidation and intermolecular amination of C-H bonds into new C-N bonds has been developed with various metal catalysts. When compared with conventional substitution or nucleophilic addition routes, the direct formation of C-N bonds from C-H bonds reduces the number of synthetic steps and improves overall efficiency.2 After early work on iron, manganese, and copper,6 Muller, Dauban, Dodd, Du Bois, and others developed different dirhodium carboxylate catalyst systems that catalyze C-N bond formation starting from nitrene precursors,7 while Che studied a ruthenium porphyrin catalyst system extensively.8 The rhodium and ruthenium systems are... 

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