Alkenes rhodium complex catalyzed

When substituted silanes are used instead of hydrogen, the process is referred to as silylformylation or silylcarbonylation. Only rhodium complexes catalyze the transformation of unsaturated compounds to silylaldehydes via the silylformylation reaction. Iridium complexes also are able to catalyze the simultaneous incorporation of substituted silanes and CO into unsaturated compounds, although during the reaction other types of product are formed. In the presence of [ IrCl(C03) ] and [Ir4(CO)i2]) the alkenes react with trisubstituted silanes and CO to give enol silyl ethers of acyl silanes [58] according to Scheme 14.10. 

Copper and rhodium complexes catalyze the reaction of alkenes with diazoacetate to give alkyl cyclopropanecarboxylates [13]. In the presence of Cu(acac)2, the reaction of carbohydrate enol ether 20 with methyl diazoacetate afforded a 1 4 mixture of cis- and frani-cyclopropanes 21 and 22 (c -product 21 was obtained with 95% de). When the reaction was catalyzed by CuOTf in the presence of hgand 23, the tranj -product 22 was obtained with 60% de (Scheme 10.4). The absolute configuration of the major diastereomer was not given [19]. 

Whereas water-soluble rhodium complexes catalyze the biphasic hydrogenation of various unsaturated substrates but rapidly form colloids, the addition of yd-CD has been shown to lead to efficient homogeneous catalytic systems [40]. For instance, in the absence of transfer agent, as reported by Larpent et al. [41], maleic acid is completely transformed in 83 h instead of a reaction time of 17 h with yd-CD. Similarly, the hydrosilylation reaction of an alkene catalyzed by a platinum precursor can be notably accelerated by the presence of yd-CD the case of a Lamoreaux Pt Pl catalyst is more spectacular since the yield can reach 100% in 12 min, whereas no transformation of the substrate occurs in 24 h in the absence of yd-CD [42]. 

Several independent protocols using a combination of transition metal-catalyzed stereoselective hydrosilylation, such as palladium-catalyzed crosscoupling sequence leading to stereodefined r-conjugated alkene derivatives, have been successfully developed in the last decade (4). Alkenylsilanes or siloxanes, prepared via platinum or rhodium complex-catalyzed intermolecular hydrosilylation of terminal alkynes have been highly stereospecifically cross-coupled with aryl and alkenyl halides to give unsymmetrical stilbenes, alkenylbenzenes, and conjugated dienes (Scheme 24) (4). 

Calixarene phosphite 30 with unsubstituted hydroxyl group was used as a ligand in the rhodium complex-catalyzed hydroformylation of 1-hexene (Scheme 4.19) [78]. The almost quantitative conversion of initial alkene was observed in 3 hours with aldehyde yields of 80-85%. Unfortunately, the selectivity was low because the aldehydes with normal and isomerized chains were formed in similar amounts. A number of structurally similar phosphites 31 as components of catalytic systems for the hydroformylation of 1-octene are described (Scheme 4.19) [79], It was shown that the nature and steric volume of R did not significantly affect main processes. Thus, the depth of conversion was 80-90% with about 60% selectivity in all cases. 

For further study of this reaction, a-methylene lactones 2.181 were chosen as alkenes because they form spirocyclic compounds in the reaction with diyne 2.180. The rhodium complex catalyzes this reaction to... 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

Another reaction involving combining alkenes is the dimerization of ethene. That reaction is catalyzed by a rhodium complex, RhClL3 as is illustrated in Figure 22.12. 

Directed intramolecular transfer hydrogenations are catalyzed by rhodium complexes with the pendant alkene acting as an internal sacrificial olefin (Equation (39)). 

Alkynes react with the bulky germanium hydride (MejSdjGeH to selectively yield (Z)-alkenes (Equation (105)).67 The hydrogermylation of alkynols or alkynes can be catalyzed by a rhodium complex (Equation (106), Table 18) and some of the intermediates were identified (Scheme 16).132 Similar rhodium species react with alkynes to yield alkenyl complexes,133 and other transition metal complexes have been employed as hydrogermylation catalysts including those containing palladium.134,135... 

These examples of rhodium-catalyzed [4 + 2] reactions provide the first chemo- and stereoselective carbocycHzations of tethered diene-aUenes, through the facial differentiation of the rhodium-complex. This presumably results in the kinetic formation of a preferred metaUacycle intermediate, which subsequently undergoes alkene insertion... 

Rhodium complexes were generally found to be more effective than iridium, but on the whole they show moderate activity in alkene oxygenation reactions. Significantly, epoxides, a typical product of the oxidation of olefins catalyzed by the middle transition metals, have rarely been evoked as products [18-22]. Although allylic alcohols [23] or ethers [24] have sometimes been described as products, the above cited rhodium and iridiiun complexes are characterized by an excellent selectivity in the oxygenation of terminal alkenes to methyl ketones. 

As an alternative to addition of anionic nucleophiles followed by reoxidation, rhodium(l)-catalyzed C-H activation allowed the nucleophilic addition of alkenes to the intermediate Rh(i) carbene complex <2002JA13964, 2004JOC7329>. Purine behaved anomalously compared to other heterocycles, for which selective monoalkylation was observed, and underwent sequential substitution first at C-8 and then at C-6 (Equation 8). Caffeine was monoalkylated at C-8 in low yield (15%). Selectivity for C-8-arylation was also observed in the palladium-catalyzed C-H activation of 6-phenyl-9-benzylpurine (aryl iodides, 0.05 equiv Pd(OAc)2, 3 equiv Cul, 2.5 equiv CS2CO3, DMF, 160 °C, 60 h, 48-95% yields) <2006OL5389>. 

Chauvin, Y., Mussmann, L., and Olivier, H., A novel class of versatile solvents for two-phase catalysis hydrogenation, isomerization, and hydroformylation of alkenes catalyzed by rhodium complexes in liquid 1,3-dialkylimidazolium salts, Angew. Chem. Int. Ed., 34, 2698-2700,1996. 

Homogeneous catalysis by transition metal complexes almost always involves processes in which product-catalyst separation and catalyst recycling are important issues. For years, researchers have worked to find effective ways to isolate metal-complex catalysts in phases separate from those containing the catalyst, usually by anchoring the metal complex to a solid surface. As summarized by Driessen-Holscher, it is now evident that the method that has met with most practical success in this direction involves the use of multiple liquid phases. For example, rhodium complexes with water-soluble sulfonated ligands are used to catalyze alkene hydroformyla-tion, and the aqueous-phase catalyst and the organic products are easily separated as insoluble liquid phases. 

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... 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Although no direct oxygen transfer reactions from well-characterized rhodium-hydroperoxo or -alkylperoxo complexes to alkenes have yet been reported, these species are probably involved in the rhodium-copper catalyzed ketonization of terminal alkenes by 02, as previously shown in Section 61.3.2.1.3. Rhodium trichloride has been used to catalyze the ketonization of terminal alkenes by H202 or TBHP in alcoholic solvents, but these reactions occur less efficiently than with the Rh/Cu/02 system.207... 

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