Alkylrhodium intermediates

An explanation for the dehydrogenation process has been offered (Scheme 13). An alkylrhodium intermediate can undergo a /3-elimination process to afford the unsaturated analog A, which is carbonylated by the usual process. 

Using sodium tetraphenylborate under anhydrous conditions and [RhCl(COD)]2 as a catalyst, a cascade reaction of dienyne 292 has been carried out, which ends by acylation of the alkylrhodium intermediate and the subsequent Rh -OMe formation, to afford bicyclo[2.2.1]-heptan-2-one 293 in good yield (Scheme 75).282... 

The formation of alkylrhodium intermediates have been reported. " Dirhodium- (g) catalysed reactions of aryl-substituted tetrahydropyranone diazoacetoacetates produce ylide intermediates and give oxabicyclo[4.2.1]-nonane diastereoisomers (Scheme 154) ... 

The proposed catalytic cycle is described in Scheme 4.23. Transmetallation of aryl-boronate esters to hydroxorhodium species, giving an arylrhodium intermediate, is followed by insertion of the norbomene derivative. Then, the alkylrhodium intermediate, which has no hydrogens for p-hydrogen elimination, undergoes insertion of... 

Cramer N, Seiser T (2011) Beta-carbon elimination from cyclobutanols a clean access to alkylrhodium intermediates bearing a quaternary stereogenic center. Synlett 4 449-460. doi 10.1055/s-0030-1259536... 

In a series of electronically distinct but sterically equal ligands 4, it was found that the overall selectivity for linear aldehyde was constant, whereas the linear branched ratio and the rate increased concomitantly with the ee/ea ratio in the hydrido isomers (Table 1.2) [20]. The higher 1/b ratio was because of an increase in the 2-octene formation - the escape route for the formed branched alkylrhodium intermediate. 

In a continuation of the work developed in the 1990s, several theoretical calculations on catalytic carbonylation reactions were performed, some of them proposing rhodium-acyl intermediates. The olefin insertion in the Rh-H bond to generate alkylrhodium intermediates was theoretically studied in order to determine the factors affecting the... 

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

Some examples of this method are given in Table 11. Most of the entries proceed in about 60-80% yield, with the exception of secondary and tertiary alkyllithiums. In these cases, the facility of -hydride elimination in the intermediate alkylrhodium complex predominates over oxidative addition to the acid halide. One example of formation of an optically active ketone proceeded without racemization at the a-center. 

Regioselectivity in hydroformylation is influenced by electronic and steric effects [4, 5]. Thus the formation of the C a-Rh bond is favored over that of the C P-Rh bond by the well known P-silicon effect (Fig. 3), which stabilizes a positive charge on the p-C atom. From the resulting intermediate la the /50-product should form predominantly. On the other hand, steric effects induced by bulky substituents on silicon or rhodium would favor the sterically less hindered normal alkyl rhodium complex with the C P Rh intermediate Ila as the precusor to the -aldehyde. The observed //so-ratios very close to 1 1 for the Rh-catalyzed hydroformylation of vinyltrimethylsilane indicate that the electronic P-effect obviously is canceled out by the steric demand of the MesSi-groups. Since addition of PPha will favor an active complex with a larger number of bulky phosphine ligands (L = PPhs in Fig. 2), the formation of the linear alkylrhodium complex intermediate Ila to lid is prefered [6]. 

To account for these results, the hydrosilylation reaction in the presence of Wilkinson s catalyst was proposed to occur via a cis addition process (according to the Chalk and Harrod mechanism, cf. Sect. IV-A-l-a) with concomitant isomerization taking place, the catalyst retaining the olefinic product in its coordination sphere. Conversion of the more stable trans to cis product is proposed to be directed by steric interactions in an intermediate o-alkylrhodium complex. 

Alkylrhodiums form from the reaction of Rh hydrides and alkenes. This reaction is important in hydrogenation, asymmetric hydrogenation, alkene isomerization and hydroformylation and other catalytic processes. The regiochemistry seen in this reaction is the subject of theoretical study that rationalizes the formation of the less substituted (T-alkyrhodium intermediate on electronic grounds Several Rh complexes form stable ff-alkylrhodiums on reaction of Rh hydrides with fluorinated alkenes (see Table 8) . 

Formation of a a-alkylrhodium by insertion of an alkene into a Rh—H bond is a key step in homogeneous asymmetric hydrogenation of alkenes by Rh(I) catalysts. Such intermediates are characterized using multinuclear NMR both in hydrogenation of (Z)-a-acetamidocinnamate by [Rh(Ph2 PCH2 CHj PPhj)] and in the asymmetric hydrogenation of a-benzamidocinnamic acid or its methyl ester by (R,R)-l,2-bis(o-methoxy-phenylphenylphosphino)ethane complexes of Rh(I) . 

In the course of characterization of this new alkylrhodium hydride it was demonstrated that the amide remained bound and that the double bond had been reduced because of the loss of Ci C3 coupling in a doubly labeled sample of enamide on going from (57) to (66). It is possible that the carboxylate group occupies the coordination site trans to hydride, because it experiences a downfield chemical shift of 12 ppm relative to the free acid or ester. This may be due to the change in its environment combining the effects of saturation and jS-rhodium deshielding, and an alternative structure, (67), cannot be ruled out on current evidence. Certainly the intermediate is different in detail from the l,2-W5(diphenylphos-... 

Several observations led to the proposal that some of the catalysts containing metals other than platinum do not react by the Chalk-Harrod mechanism. First, carbon-silicon bond-forming reductive elimination occurs with a sufficiently small number of complexes to suggest that formation of the C-Si bond by insertion of olefin into the metal-silicon bond could be faster than formation of the C-Si by reductive elimination. Second, the formation of vinylsilane as side products - or as the major products in some reactions of silanes with alkenes cannot be explained by the Chalk-Harrod mechanism. Instead, insertion of olefin into the M-Si bond, followed by p-hydrogen elimination from the resulting p-silylalkyl complex, would lead to vinylsilane products. This sequence is shown in Equation 16.39. Third, computational studies have indicated that the barrier for insertion of ethylene into the Rh-Si bond of the intermediate generated from a model of Wilkinson s catalyst is much lower than the barrier for reductive elimination to form a C-Si bond from the alkylrhodium-silyl complex. ... 

The overall transformation presumably proceeds through the initial formation of a catalytically active alkylrhodium species 389 capable of carbometaUative addition across the unsaturated carbon-carbon multiple bond of the substrate. TTie corresponding alkenylrhodium intermediates 391 undergo a transmetallation, furnishing the corresponding alkenylzinc entities 390 (Scheme 10.132). 

The reaction of the rhodium(l) complex (CXLV) and organolithium or organomagnesium compounds produces the alkylrhodiums (CXLVI). Oxidative addition of an acid chloride to this intermediate, followed by reductive elimination, leads to unsymmetrical ketones in good yields. The alkyirhodium (CXLVI) does not react with aldehydes, esters, or nitriles. An additional benefit is that the starting rhodium complex (CXLV) is regenerated in reusable form in the last step (Hegedus et al., 1973, 1975a). Reaction of... 

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