Vinyl rhodium species

Freshly prepared 11 reacts readily with phenylacetylene 13 to give divinyl ketone 108 and Rh4(GO)i2 at room temperature under a GO pressure or GO atmosphere. The result strongly suggests that insertion of 13 into an Rh-Si bond in 11 must be involved at the first stage of the conversion of 11 to 108. The resulting vinyl-rhodium species 109 reacts with GO to form an acyl-rhodium intermediate 108, coupling of which with another molecule of 109 would afford 108 (Scheme 4). ... 

Rh(I) catalysts under H2 effect the reductive cychzation of diynes and enynes [119]. In the example below (1.39), Rh(C0D)20Tf (COD = 1,5-cyclooctadiene) is converted by H2 gas to a Rh hydride, which is thought to undergo oxidative cycliza-tion to produce a rhodacyclopentene intermediate. Cleavage of the Rh-C bonds by reductive elimination to form a vinyl rhodium species, and hydrogenolysis of that intermediate, forms the Rh hydride and the cychzed product. 

On the basis of the above results, Krische and co-workers proposed the mechanism depicted in (Scheme 12) (32). The catalytic cycle begins with oxidative addition of the Rh complex to enyne 25, forming the rhodacyclopentene intermediate B-I. Homoljdic hydrogen activation via either (a) hydrogen oxidative addition or (b) a-bond metathesis to form vinyl rhodium species B-II is expected. 

This type of transition metal catalyzed the Michael addition of nitriles to methyl acrylate, and methyl vinyl ketone proceeds with good to high yields with the aid of RhH(CO)(PPh3)3 as the catalyst (Eq. 63) [133]. Interestingly, benzyl cyanide also shows a high reactivity with methyl vinyl ketone. In this study, the insertion of the low-valent rhodium species into the C-H bond adjacent to the cyano group has been proposed. 

Murakami and coworkers reported a further use for this rhodium migration [80, 81]. Instead of protonolysis, they noticed that the aryl rhodium species after the vinylic to aryl migration is nucleophilic enough to attack an ester moiety in an intramolecular fashion to afford a cyclic ketone. Thus, an internal alkyne equipped with ester groups at a specific place was subjected to the rhodium-catalyzed hydroarylation conditions (Scheme 21). Indeed, the desired ketone was obtained in an 89% yield. Not only methyl esters can serve as acylation agents ethyl esters and isopropyl esters are also suitable substrates. 

The rhodium hydride tricarbonyl species easily coordinates the vinyl substrate generating the n complex (1), which is converted into the alkyl-rhodium intermediates (2) through insertion of the alkene into the Rh-H bond. Migratory insertion of the alkyl moiety on to a CO molecule coordinated to the metal center provides the acyl-rhodium species 3, which, at the end of the catalytic cycle, interacts with hydrogen via an oxidative addition, giving rise to aldehydic products and regenerating the rhodium-hydride species. 

Based on these observations, a possible catalytic cycle for the rhodium-catalyzed anfi-Markovnikov addition of thiols to alkynes is shown in Scheme 18. Oxidative addition of PhSH to WOkinson s catalyst generates hydrorhodium sulfide species (frans-HRhCl(SPh)(PPh3)2). After coordination of alkyne to the hydrorhodium sulfide species, stereoselective insertion of alkyne into Rh-H bond provides -isomer of vinylic rhodium intermediate. The subsequent reductive eliminatiOTi of anri-Markovnikov adduct in the presence of PhSH regenerates the hydrorhodium sulfide species. 

Olefin isomerization can be catalyzed by a number of catalysts such as molybdenum hexacarbonyl [13939-06-5] Mo(CO)g. This compound has also been found to catalyze the photopolymerization of vinyl monomers, the cyclization of olefins, the epoxidation of alkenes and peroxo species, the conversion of isocyanates to carbodiimides, etc. Rhodium carbonylhydrotris(triphenylphosphine) [17185-29-4] RhH(CO)(P(CgH )2)3, is a multifunctional catalyst which accelerates the isomerization and hydroformylation of alkenes. 

Reactions of rhodium(III) porphyrins with olefins and acetylenes - Ogoshi et al. [326] have described the reactions of vinyl ether with rhodium (III) porphyrins which are depicted in reaction sequence (33). Step (a) appears to be an insertion of the olefin into the Rh-Cl bond followed by alcoholysis of a chlorosemiacetal to the acetal, step (b) is the hydrolysis of the acetal to the aldehyde. The insertion is thought to start by heterolysis of the Rh-Cl bond producing a cationic species which forms a 7i-complex with the electron-rich olefin. 

Phenyl, vinyl or carbonyl substituted cyclopropanes are more easily hydrogenolyzed than are the alkyl substituted species. Such compounds are commonly cleaved over palladium catalysts at room temperature and atmospheric pressure. Hydrogenolyses run over platinum, rhodium or nickel catalysts frequently result in the saturation of the double bond, the benzene ring or the carbonyl group with the cyclopropane ring remaining intact or cleaved to only a slight extent. 20 As illustrated in Fig. 20.22 the bond broken in the... 

The catalytic activity of low-valent ruthenium species in carbene-transfer reactions is only beginning to emerge. The ruthenium(O) cluster RujCCO), catalyzed formation of ethyl 2-butyloxycyclopropane-l-carboxylate from ethyl diazoacetate and butyl vinyl ether (65 °C, excess of alkene, 0.5 mol% of catalyst yield 65%), but seems not to have been further utilized. The ruthenacarborane clusters 6 and 7 as well as the polymeric diacetatotetracarbonyl-diruthenium (8) have catalytic activity comparable to that of rhodium(II) carboxylates for the cyclopropanation of simple alkenes, cycloalkenes, 1,3-dienes, enol ethers, and styrene with diazoacetic esters. Catalyst 8 also proved exceptionally suitable for the cyclopropanation using a-diazo-a-trialkylsilylacetic esters. ... 

Unmodified rhodium catalysts are readily formed in seCOa from simple precursor complexes such as [(CO)2Rh(acac)j, [(cod)Rh(hfacac)], or [Rh( (CO)i6] [33. The resulting rhodium carbonyl species are highly active in this medium for a range of substrates including simple olefins, vinyl arenes and polar substrates such as aUyl acetate. Especially the reaction rates for internal C=C bonds are remarkably higher than those observed in liquid organic solvents under typical hydroformylation conditions (Scheme 12.13). 

Pioneered by the seminal work of Lim and Kang on the alkylation of C—H bond using a rhodium catalyst, the chelation-assisted Rh-catalyzed C—H bond functionalization reactions for new C—C bond construction have witnessed significant improvements. Various aromatic, vinylic, and even allylic C—H bonds were found possible to be cleaved by a chiral Rh catalyst. The formed C—Rh species can readily react with common unsaturated functionalities such as alkenes, alkynes, allenes, and even ketones and imines. 

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