Rhodium carbon

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 reaction of (TPP)Rh with terminal alkenes or alkynes is of special interest due to the cleavage of the carbon-carbon bond adjacent to either the alkene or the alkyne functionality and results in the ultimate formation of (TPP)Rh(R). This overall reaction implies activation of a relatively inert carbon-carbon bond, especially for the case of the terminal alkene. However, the ultimate formation of (P)Rh(R) is not surprising if one considers the relative stability of the rhodium carbon bond in this species(17). 

A concerted mechanism has also been discussed [29,30], involving either a 2+2+1 or 3+2 mechanism. To avoid trimolecular reactions this requires an interaction between Rh(I) and silanes prior to the reaction with a ketone. Interaction of silanes not leading to oxidative addition usually requires high-valent metals as we have seen in Chapter 2. The model is shown in Figure 18.16 it proved useful for the explanation of the enantiomers formed in different instances. The formation of a rhodium-carbon bond is included and thus formation of silyl enol ethers remains a viable side-path. 

The mechanism is well understood, involving complexation of the rhodium with iodine and carbon monoxide, reaction with methyl iodide (formed from the methanol with hydrogen iodide), insertion of CO in the rhodium-carbon bond, and hydrolysis to give product with regeneration of the complex and more hydrogen iodide. 

The ester is prepared by catalytic hydrogenation of 4-tert-butylphenol followed by acetylation of the resulting 4-tert-butylcyclohexanol [132]. If Raney nickel is used as the catalyst, a high percentage of the trans isomer is obtained. A rhodium-carbon catalyst yields a high percentage of the cis isomer. The trans alcohol can be isomerized by alkaline catalysts the lower-boiling cis alcohol is then removed continuously from the mixture by distillation [133]. 

Carbon dioxide insertion into a rhodium-carbon bond has been found in the reaction of Rh(Ph)(PPh3)3 with C02 under 20 atm of pressure at room... 

The latter undergoes insertion of ethylene into the rhodium-carbon cr bond to give 7.36. The formal mechanism for the formation of 7.36 is shown by reaction 7.10. Complex 7.36 undergoes /3-elimination to generate 7.37, which liberates 1,4-hexadiene and completes the catalytic cycle. 

Various rhodium complexes such as Rh(acac)(CH2=CH2)2, Rh(acac)(CO)2 and [Rh(cod)(CH3CN)2]BF4 plus a phosphine ligand efficiently catalyzed the addition to a,P unsaturated esters. The reaction rates were highly dependent on the substituent (R1) which may affect the rate of insertion of 7 to the rhodium-carbon bond. Thus, the addition to 7f... 

Keywords Rhodium, Carbon-Hydrogen Insertion, Cyclopropanation, Chiral, Asymmetric, Enantioselective, Intermolecular, Intramolecular, Diazocarbonyl Compounds... 

Compounds with Rhodium-Carbon cr-Bonds Compounds of Ligands Containing Locaiized 7T-Bonds... 

Various 2-furanone chiral building blocks are readily accessible from 0-acetyllactate derivative 222 according to the series of reactions outlined in Scheme 37. Deprotonation of 222 with 2-4 equivalents of LiHMDS gives (/S)-7-methyltetronic acid (257) in nearly quantitative yield [88]. Reduction of 257 with ammonia—borane affords a 25 75 mixture of 258 and 259, whereas catalytic hydrogenation over rhodium/carbon produces an 85 15 mixture of 258 and 259 [89]. Dehydration of the mixture with phosphorus oxychloride furnishes the 55 -butenolide 260 [(+ )-angelica lactone]. Dihydrofiiranone 262 is made by benzoylation of the tetrabutylammonium salt of 257 followed by catalytic hydrogenation. 

Starting with initially clean rhodium and iron surfaces, the surface rapidly becomes covered with a monolayer of active carbon. This active carbon layer appears to hydrogenate directly to produce methane by a mechanism which is very similar for both rhodium and iron. While rhodium-carbon monolayer systems remain stable indefinitely under our... 

Thermodynainic Determination of Rhodium-Carbon Bond Strengths in Tp Rh(CNR)(R)H 72... 

Thermodynamic Determination of Rhodium-Carbon Bcmd Strengths in Tp Rh(PMe3)(R)H 80... 

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