Rhodium complexes stereochemistry

The strategy of the catalyst development was to use a rhodium complex similar to those of the Wilkinson hydrogenation but containing bulky chiral ligands in an attempt to direct the stereochemistry of the catalytic reaction to favor the desired L isomer of the product (17). Active and stereoselective catalysts have been found and used in commercial practice, although there is now a more economical route to L-dopa than through hydrogenation of the prochiral precursor. 

A variety of rhodium complexes, including [Rh(CO)2Cl]2 and [Rh(COD)Cl]2 when used in combination with a variety of bisphosphine ligands, will catalyze the ring opening of vinyl epoxides in the presence of aniline nucleophiles [19, 20]. These reactions occur under very mild and neutral conditions (at room temperature or with mild heating) and are highly regio- and stereoselective. In all cases, nucleophilic attack occurs at the allylic epoxide carbon atom and proceeds with inversion of stereochemistry (Scheme 9.11). 

A combination of rhodium complexes and phosphates promotes a highly regioselective allylic alkylation of unsym-metric allylic esters, where alkylation occurs at the more substituted allylic terminus of the esters (Equation (46)). As Evans and his co-workers reported, both the regio- and stereochemistry of the starting allylic esters are maintained in the allylic alkylated products (Equation (47)). Thus, the rhodium-catalyzed allylic alkylation takes place at the carbon substituted by a leaving group with net retention of configuration. A variety of carbon-centered... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

Asymmetric hydrogenation. Procbiral u,/ -unsaturated acids and their derivatives can be hydrogenated with high stereoselectivity by rhodium complexes with 1, such as (BPPM)Rh(COD)Cl and (BPPM)Rh(COD)+ClCV, in which COD = 1,5-eyclooctadiene. The stereoselectivity is dependent in part on the hydrogen pressure, ami the effect can be attenuated by addition of triethylamine, which also increases Ihc optical yield. The stereoselectivity is markedly controlled by the stereochemistry of the double bond.1... 

During our investigation, PPhs and CH3SO3H were found to add to unactivated alkynes in the presence of a palladium or rhodium complex. The regiochemistry and stereochemistry could be controlled by a judicious selection of the metal catalyst. Alkenylphosphonium salts have various applications in synthetic chemistry,and the present method enables an easy access to the organophosphorous compounds using readily available starting materials. 

The rhodium complex [RhCl(PPh3)3] readily brings about stoichiometric decarbonylation of aldehydes, acyl halides and diketones. A typical aldehyde decarbonylation is illustrated by equation (69). a,3-Unsaturated aldehydes are decarbonylated stereospecifically (equation 70), while with chiral aldehydes the stereochemistry is largely retained (equation 71). ° ... 

The influence which the other ligands have on the alkylation of d complexes is illustrated by the addition of methyl iodide to the tris(phos-phine)-rhodium complex (XL) (82a) but not to the similar complex RhCl(CO)(PPh3)2 in which a CO group has replaced a phosphine. However, the analogous iridium complex IrCl(CO)(PPh3)2 reacts with methyl iodide (see Section II,B) (2J, 41, 67), The rhodium adduct (XLI) is novel inasmuch as it contains two molecules of methyl iodide, the second apparently being bound through iodine (82a). The detailed stereochemistry... 

An -ray structure determination of a cationic intermediate (8) isolated during the acylation of [Fe(CO)8(rraAw, ra j-hexa-2,4-diene)] establishes that Friedel-Crafts acylation involves stereospecific endo attack. The stereochemistry of reaction parallels protonation of tricarbonyl(diene)iron, cyclopentadienyl(cyclohexa-l,3-diene)rhodium complexes, and ( j -cyclopentadienyl)rhodium complexes of limonene (9), a-phellandrene (10), and carvone (11). ... 

It has been shown that the stereochemistry of the hydrosilylation of 1-aUcynes giving 1-silyl-l-alkenes depends on the catalysts or promoters used. For example, the reactions under radical conditions give the cis-product predominantly via trans-addition , while the platinum-catalyzed reactions afford the trans-product via exclusive cts-addition. In the reactions catalyzed by rhodium complexes, thermodynamically unfavorable c/s-1-silyl-l-alkenes are formed via apparent trans-addition as the major or almost exclusive product. Since the trans-addition of HSiEts to 1-alkynes catalyz by RhCl(PPh3)3 was first reported in 1974 , there have been controversy and dispute on the mechanism of this mysterious trans-addition that is vray rare in transition-metal-catalyzed addition reactions to aUtynes. Recently, iridium 4i6 mthenium complexes were also found to give the ds-product with extremely high selectivity (vide supra). 

We have studied the reactions of SF4 with analogous complexes of rhodium. In general the results are similar, though the rhodium complexes are less stable and the initial reactions less simple, leading to the formation of more isomeric products. Fluxional processes are also similar, and are affected by ligand and by stereochemistry in the same sort of way. 

The control of the absolute stereochemistry of the newly generated stereogenic centre has been targeted at first by Wender who investigated the use of cationic rhodium complexes generated from [Rh(nbd)Cl]2, AgSbFg and (/ )-BINAP under... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

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