Rhodium addition initiated

Applying more drastic conditions (100 bar of CO 175 °C), [Rh6(CO)i6] also catalyzes cyclocarbonylation of 2-alkynylphenols. The mechanism involves here also an 0 - H oxidative addition initial step. The a-methylene lactone is further reduced since rhodium catalyzes the water-gas-shift reaction and water is present in the medium [145]. 

Rhodium Transfer Experiments. The supported catalyst was prepared in a manner similar to that described (20) using 600 mg of the phosphinated resin (20-40 mesh) and equimolar amounts of [RhCODCl]2 or [Rh(C2H4)2Cl]2 in benzene under nitrogen. To the flask, the second resin sample (600 mg) in a nylon sack with a porosity of 250 fi was added swiftly, and the flask was again purged with nitrogen. The resins suspended in benzene were stirred for various times in the presence or absence of additives (acetophenone and/or a-naphthylphenylsilane). The two resins were then separated and analyzed for their rhodium content. Up to 10% of the rhodium present initially could be transferred from one resin to the other. Control experiments demonstrated that the nylon sack did not inhibit the migration of the soluble complexes. 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

The reaction with Mel proceeds in two stages. Initial reaction is oxidative addition to give a rhodium(III) species, isolated as a Mel adduct... 

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

In conclnsion, it was shown that the hydrogenolysis of glycerol in the presence of heterogeneous rhodium-based catalysts yielded mainly either 1,2-, or 1,3-propane diol. Many parameters influenced the activity and the selectivity of the catalysts, particnlarly the presence of metal additives and the initial pH value. 1,2-propanediol can be obtained nearly quantitatively at high pH. Further woik is currently under progress in order to optimize this reaction. 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

In the reductive aldol condensation of an ,/J-unsaturated ester and an aldehyde shown in Eq. 291, the initial step is believed to be the addition of an in situ formed rhodium hydride to the a,/Tunsaturated ester, followed by reaction of the resulting rhodium enolate with the aldehyde.470 The reaction has been carried out both inter-470 and intramolecularly471,472 as well as in an asymmetric fashion (Eq. 291). 

The initial step is an oxidative addition of RhCI(PPh3)3 to a C-0 bond of the ester moiety and produces rhodium-carbon and rhodium-oxygen bonds. Adjacent rhodium species can undergo further reaction with the formation of anhydride linkages. This anhydride formation may occur between adjacent pairs of reactants, between pairs in the same chain, or between pairs that are present in different chains. All of these reactions are observed, and in however the last reaction is the one of interest here since this leads to cross-linking and char formation. Rhodium is present in both the chary material and in the soluble fractions. From the reaction pathway in order for rhodium elimination to occur, two rhodium-inserted... 

The current mechanistic understanding of these reductive cyclization processes is largely conjecture. Stepwise oxidative addition, migratory insertion, and reductive elimination (see Scheme 26) is a widely proposed mechanism. However, other mechanisms - such as initial cyclometallation - are to afford a rhodacyclopentadiene followed by either oxidative addition to a rhodium(v) intermediate or (perhaps more likely) bond metathesis with an additional molecule of silane (Scheme 28). 

These reports announced the rapid development of a large variety of monodentate ligands for rhodium-catalyzed enantioselective hydrogenation. It was shown that the substrate scope for catalysts based on monodentate ligands is most probably at least as big as for their bidentate counterparts. Also, initial doubts about the activity and stability of the monodentate ligand-catalysts have been taken away. Several reports show that substrate catalyst ratios (SCRs) of 103 or higher, essential for industrial application, are possible. In addition, reaction rates are in the studied cases comparable to those reached by catalysts based on state-of-the-art bidentate ligands [16]. 

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