Rhodium labeling experiments

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

Previous studies on various aliphatic and aromatic alkynes have evidenced that this type of furan-2-ones could also be produced in the presence of the rhodium carbonyl cluster [Rh4(CO)i2] [39-41]. Labeling experiments using D2O allowed the authors to assign the origin of the hydrogen atoms (Scheme 6). Noteworthy, dicobalt octacarbonyl follows a different catalytic... 

According to a deuterium-labeling experiment, Miyuara s hydroboration is actually a 1,1-addition process with concomitant 1,2-H-shift, rather than a true transaddition. The olefin geometry of the product boronate is presumably determined during an insertion of boron or hydrogen into the a-position of rhodium vinylidene intermediate 43 (Table 9.8). 

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

The mechanism of the catalysis of the CO reduction of NO via (113) using this rhodium system has been investigated by kinetic studies (187) and by an isotope labeling experiment (234). Scheme I presents the mechanism as currently viewed. From initial rate studies it was established that the reaction is first order in total rhodium concentration, indicating that N—N bond formation occurs intramolecularly. The rate also appears to be first order in the partial pressure of CO and zeroth order in the partial pressure of NO over the range 200-400 Torr. We conclude from these results that the rate determining step is attack of CO on the catalytically active species (42) (187). 

The bis( -methylene)rhodium complex (Structure 8) undergoes thermal decomposition at 300-350 °C to yield mainly propene and methane H-labeling experiments showed that the propene originates rather selectively (>75 %) from two CH2 groups and one CH3 unit a //-methylidene (CH) intermediate 9 (from H abstraction) with consecutive isomerizations is assumed to yield the propene (Scheme 3). 

Other rhodium complex also catalyzed the addition of the C-H bond in aldehyde to olefins [115-117]. The use of paraformaldehyde results in the formation of aldehydes [115]. Marder et al. proposed the reaction mechanism of CpRh(eth-ylene)2-catalyzed addition of C-H bond in aldehyde to ethylene by the use of isotope-labeling experiments [117]. They suggested that insertion of ethylene to the Rh-H bond must take place rapidly and reversibly, and this equilibrium must be established significantly faster than either aldehyde reductive elimination or product formation (Scheme 4). 

The asymmetric reduction of raeso-epoxides with hydrogen or hydrides has been scarcely explored, despite the synthetic utility of the chiral secondary alcohol products. The lone example has been provided by Chan, who treated the disodium salt of epoxysuccinic acid with H2 or MeOH as the reducing agent in the presence of a chiral rhodium catalyst (Scheme 13) [27]. Deuterium labeling experiments established that the reduction proceeded through direct cleavage of the epoxide C-O bond, rather than isomerization to the ketone followed by carbonyl reduction. 

Scheme 18 Isotope labeling experiment and the proposed mechanism for the rhodium migration...

Two homogeneous metal complex water-gas shift catalyst systems have recently appeared 98, 99). The more active of these comes from our Rochester laboratory (99, 99a). It is composed of rhodium carbonyl iodide under CO in an acetic acid solution of hydriodic acid and water. The catalyst system is active at less than 95°C and less than 1 atm CO pressure. Catalysis of the water-gas shift reaction has been unequivocally established by monitoring the CO reactant and the H2 and C02 products by gas chromatography The amount of CO consumed matches closely with the amounts of H2 and C02 product evolved throughout the reaction (99). Mass spectrometry confirms the identity of the C02 and H2 products. The reaction conditions have not yet been optimized, but efficiencies of 9 cycles/day have been recorded at 90°C under 0.5 atm of CO. Appropriate control experiments have been carried out, and have established the necessity of both strong acid and iodide. In addition, a reaction carried out with labeled 13CO yielded the same amount of label in the C02 product, ruling out any possible contribution of acetic acid decomposition to C02 production (99). 

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