Compounds of Rhodium O

It is believed to be metal-metal bonded (PF3)4Rh-Rh(PF3)4 and readily reacts with hydrogen 

Electrochemical reduction in MeCN of various RhCl(R3P)3 complexes give the diamagnetic Rh(R3P)4 (R3P = Ph3P, Mc2PhP), which are probably analogous to the PF3 complex. 

The best defined rhodium(O) compound [41] is diamagnetic Rh2(PF3)8 RhCl3-------------------------— Rh2(PF3)8 

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Many compounds of rhodium (III) may readily be obtained by this general catalytic procedure. Rhodium(III) bromide may be substituted for the rhodium trichloride, and weakly basic nitrogenous ligands (2,2 -bipyridine, o- (or l,10-)phen-anthroline, or dioximes) for the pyridine. The products all have the halogen atom in trans positions. 

A selective hydrogenabon of di-subsbtuted benzenes such as xylene, methylan-isole, and cresol was also observed with these aqueous suspensions of rhodium(O) nanoparbcles. In all cases, the ds-compound is the major product (near 95%). The cis/trans ratio decreases with the position of the substituents o>m>p. A comparison with a similar iridium(O) system shows that the nature of the metal does not seem to be important. 

To summarize, the investigation of rhodium(O) colloids as nanocatalysts for the hydrogenation of a large series of aromatic compounds has shown that (i) the... 

Rhodium-catalyzed hydroformylation of -(substituted amino)benzyl-amines (387, X = H2) and -(substituted amino)benzamides (387, R = H, X = O) in the presence of rhodium(II) acetate dimer and triphenylphos-phine in deoxygenated ethyl acetate gave a 7 3 mixture of 1,2,3,4,4 ,5-hexahydro-6//-pyrido[l,2-a]quinazolines (388, X = H2,0) and isomeric 3-methyl-l,2,3,3fl,4,5-hexahydropyrrolo[l,2-a]quinazolines (389, X = H2, O) (94AJC1061). The methyl derivative of benzylamine 387 (R = Me, X = H2) afforded a mixture of diastereoisomers 390 and 391 (X = H2). Their ratio depended on the reaction time. Longer reaction times gave more 391 (X = H2), containing the methyl group in an equatorial position. Compound 390 isomerized into 391 (X = H2), under the aforementioned conditions. The benzamide derivative (387, R = Me, X = O) yielded only one isomer (391, X = O), independent of the reaction period. 

A detailed study of over 45 catalysts, primarily from Group VIII metal salts and complexes, showed palladium(II) compounds to be the most effective in the dehydrogenation of a variety of aldehydes and ketones. Soluble palladium(II) salts and complexes such as dichloro(triphenylphosphine)palladium(II) and palladium(II) acetylacetonate have been shown to be optimal, with the salts of rhodium, osmium, iridium and platinum having reduced efficacy. Since the dehydrogenation reaction is accompanied by reduction of the palladium(II) catalyst to palladium(O), oxygen and a cooxidant are required to effect reoxidation. Copper(II) salts are favored cooxidants, but quinones, and especially p-benzoquinone, are also effective (Scheme... 

Preparative Methods substituted 2,3-methanoamino acids are difficult to prepare. Unfortunately, most of the reported syntheses give racemic materials whereas stereochemically pure compounds are required for studies of cyclopropane-based peptidomimetics. The only 2,3-methanologs of protein amino acids prepared in optically active form are ( )- and (Z)-cyclo-Phe and -Tyr, all four stereoisomers of cyc/o-Met, (Z)-cyclo-Arg and (25,35)-(Z)-cyc/o-Trp, although several routes to enantio-enriched 2,3-methanologs of simple nonproteogenic amino acids have been reported. " The most practical synthesis of the title compound is that based on a diastereoselective, rhodium-catalyzed cyclopropanation reaction. ... 

The hydrogenolysis of acetals and ketals does not take place over most catalysts under normal hydrogenation conditions, even at relatively high temperatures and pressures. About the only reasonable hydrogenolytic cleavage of these compounds takes place over rhodium at 50°-80°C under 3-4 atmospheres of hydrogen in the presence of a trace of acid. O jhe formation of ethers in this... 

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