Alkenes rhodium-catalyzed reactions

Introduction of two different chalcogen elements to the C-C unsaturated bond is of particular interest from both synthetic and mechanistic viewpoints. Therefore, extensive studies have been carried out on the development of the (RE)2/(R E )2 binary system without using RE-E R compounds, which are difficult to prepare. (Z)-l-Seleno-2-thio-1-alkenes are produced regio- and stereoselectively when a mixture of diaryl disulfides and diaryl diselenides is subjected to a rhodium-catalyzed reaction with alkynes (Equation (68)).193... 

In sharp contrast to the unique pattern for the incorporation of carbon monoxide into the 1,6-diyne 63, aldehyde 77 was obtained as the sole product in the rhodium-catalyzed reaction of 1,6-enyne 76 with a molar equivalent of Me2PhSiH under CO (Scheme 6.15, mode 1) [22]. This result can be explained by the stepwise insertion of the acetylenic and vinylic moieties into the Rh-Si bond, the formyl group being generated by the reductive elimination to afford 77. The fact that a formyl group can be introduced to the ole-finic moiety of 76 under mild conditions should be stressed, since enoxysilanes are isolated in the rhodium-catalyzed silylformylation of simple alkenes under forcing conditions. The 1,6-enyne 76 is used as a typical model for Pauson-Khand reactions (Scheme 6.15, mode 2) [23], whereas formation of the corresponding product was completely suppressed in the presence of a hydrosilane. The selective formation of 79 in the absence of CO (Scheme 6.15, mode 3) supports the stepwise insertion of the acetylenic and olefmic moieties in the same molecules into the Rh-Si bond. 

Tab. 8.1 summarizes the various substrates that were subjected to the rhodium-catalyzed reaction using a Rh-dppb catalyst system. Only ds-alkenes were cycloisomerized under these conditions, because the trans-alkenes simply did not react. Moreover, the formation of the y-butyrolactone (Tab. 8.1, entry 8) is significant, because the corresponding palladium-, ruthenium-, and titanium-catalyzed Alder-ene versions of this reaction have not been reported. In each of the precursors shown in Tab. 8.1 (excluding entry 7), a methyl group is attached to the alkene. This leads to cycloisomerization products possessing a terminal alkene, thus avoiding any stereochemical issues. Also,... 

It was recently shown by Zhang and coworkers that Ru(PPh3)3Cl2 is a suitable catalyst for the alkylative coupling of tertiary alcohols 186 to primary alcohols 185 leading to branched alcohols 187 in 32-98% yield (Fig. 46) [258]. The reaction required the presence of a Lewis acid, such as BF3 OEt2. It mediates the dehydration of the tertiary alcohol to a 1,1-disubstituted alkene, which coordinates the ruthenium catalyst. The further course is likely to be similar to the corresponding iron- or rhodium-catalyzed reactions (see Sects. 2.8 and 6). 

Mn(0Ac)3 2H20 <03EJ01410> <03S1977>. Muller, on the other hand, investigated the enantioselective rhodium-catalyzed reactions between 2-diazocyclohexane-l,3-diones and alkenes, which led to products with similar structures <03HCA3164>. Palladium-mediated coupling of alkenes and a 1,3-dicarbonyl derivative was utilized in the total synthesis of ( )-brevione B <03TL5235>. 

There have been reports of a novel synthesis of 2,4,5-trialkyl-imidazoles by the rhodium-catalyzed reactions of alkenes with carbon monoxide and ammonia. Yields are 50-60%. Benzylamine and derivatives react with carbon tetrachloride in the presence of a catalytic amount of metal carbonyls to yield 2,4,5-triarylimidazoles and -imidazolines. The suggested reaction mechanism implicates an initially formed radical species which coordinates with the metal carbonyl. 

Table 9. Ethoxycarbonylcyclopropanes by Rhodium-Catalyzed Reaction Between Equimolar Amounts of an Alkene and Ethyl Diazoacetate ...

Similar to the thermal reactions (Section 1.2.1.2.4.2.6.1.), photochemically induced intramolecular cyclopropanation of alkenes by an acylcarbene is very rare.82 An example has already been given, another is the cyclopropanation of 20 to give 21.127 Structurally related 2-[al-lyl(methylsulfonyl)amino]-4-diazocyclohexa-2,5-dien-4-ones react analogously, but copper- or rhodium-catalyzed reactions give better yields.1273... 

There are few examples of the reactions of alkenes with diazoesters of chiral alcohols which give high face discrimination in rhodium-catalyzed reactions [936, 1497). However, Davies and coworkers [192, 193, 1503] have performed the reactions of alkenes with vinyldiazoesters of chiral alcohols under Rh2(OAc)4 or better yet Rh2(OCOC7H j 5)4 catalysis. The ester of (K)-pantolactone 1.16 is the most efficient substrate, and ftum-cyclopropanecarboxylates are obtained highly selectively. Starting from 7.141 (R = Ph), the enantioenriched plant hormone l-amino-2-phenylcyclopropanecarboxylic acid has been prepared (Figure 7.88). 

In rhodium-catalyzed reactions of vinyldiazoesters with alkenes various chiral alcohols have been examined, whereby (.R)-pantolactone was found to be the best choice for the synthesis of trans cyclopropanecarboxylates. Cleavage of the auxiliary was achieved with 6N HCl. 

The rhodium-catalyzed reaction of 2,3-disubstituted cyclobutenone 89 with electron-deficient alkenes dehvered 2,3,6-substituted phenols 90 (Scheme 3.51) [60]. 

Lately, Kuwano and coworkers reported on the 1,2-migration in the rhodium catalyzed reaction of alkenyl acetates and arylboron reagents, producing the isomerized frani-alkenes effectively. The catalytic system proved less sensitive to the steric interactions of the Cl substituent resulting in a broad scope and good regioselectivities (Scheme 2.5) [16]. 

In contrast to nickel- and rhodium-catalyzed reactions, palladium cycloisomerizations of 1,6-dienes produce different isomers as the major products [62,63]. Palladium catalysts show a preference on electron-deficient alkenes for the... 

Metalla-Diels-Alder Reaction via Metal Exchange A significant metal effect on the reactivity of a,(3-unsaturated Fischer carbene complexes is found in the reaction with alkenes and alkynes. Thus, the reaction of electron-poor alkynes with chromium alkenyl(alkoxy)car-benes is usually slow [26], whereas rhodium-catalyzed reactions of chromium alkenyl(methoxy)carbenes 75 with electron-poor alkynes 76 readily afford polysubstituted cyclopentenones 78, 79, and 80 (Scheme 5.17) [27]. Although the reactions proceed with high regioselectivity, the regioselectivity depends on the substituent (R ) of... 

Abstract Recent advances in synthetic aspects of the rhodium-catalyzed hydroformylation of alkenes are reviewed. Emphasis is given to practical improvements, efficient new catalysts for regioselective and enantioselective hydroformylation, and to applications of the reaction in organic synthesis. Furthermore, new developments in directed hydroformylation are covered as well as new approaches toward efficient hydroformylation catalysts employing the concept of self-assembly. 

Wilkinson s catalyst has also been utilized for the hydroboration of other alkenes. Sulfone derivatives of allyl alcohol can be hydroborated with HBcat and subsequently oxidized to give the secondary rather than primary alcohol. This reactivity proves to be independent of substituents on the sulfur atom.36 Similarly, thioalkenes undergo anti-Markovnikoff addition to afford a-thioboronate esters.37 The benefits of metal-catalyzed reactions come to the fore in the hydroboration of bromoalkenes (higher yields, shorter reaction times), although the benefits were less clear for the corresponding chloroalkenes (Table 3).38,39 Dienes can be hydroborated using both rhodium and palladium catalysts [Pd(PPh3)4] reacts readily with 1,3-dienes, but cyclic dienes are more active towards [Rh4(CO)i2].40... 

Another important reaction principle in modem organic synthesis is carbon-hydrogen bond activation [159]. Bergman, Ellman, and coworkers have introduced a protocol that allows otherwise extremely sluggish inter- and intramolecular rhodium-catalyzed C-H bond activation to occur efficiently under microwave heating conditions. In their investigations, these authors found that heating of alkene-tethered benzimidazoles in a mixture of 1,2-dichlorobenzene and acetone in the presence of di-//-... 

Arylphosphines in rhodium catalyzed hydroformylation reactions exchange an aryl group for an alkyl, principally linear alkyl, corresponding to the alkene being hydro-formylated to give an alkyldiarylphosphine [22](see Equation 2.5). 

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

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