Carbonyls metal, reaction with alkenes

The reactivity of metal-carbonyl clusters with unsaturated hydrocarbons gives an idea of what may happen on surfaces in heterogeneous catalysis, although the conditions and pathways are different, because surfaces of heterogeneous catalysts are not saturated with ligands whereas clusters are (see Chap. 20). Many bond activations have been shown (CO, PPhs, alkenes, alkynes, etc.), a simple one being the reaction of ethylene leading to double C-H activation. Such bond activations are common and rather facile because of the proximity of the metals in the clusters ... 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Most researchers currently agree that the hydrido mechanism is more common than the alkoxycarbonyl path in the alkoxycarbonylation of alkenes with palladium systems. However, carbalkoxy complexes are putative intermediates in carbonylation reactions giving succinates and polyketone diesters, with metals like Co, Rh, or Pd.137... 

It is important to realize that there is a great deal of overlap in the topics covered in this chapter. For example, the chemistry of metal carbonyls is intimately related to metal alkene complexes, because both types of ligands are soft bases and many complexes contain both carbonyl and alkene ligands. Also, both areas are closely associated with catalysis by complexes discussed in Chapter 22, because some of the best-known catalysts are metal carbonyls and they involve reactions of alkenes. Therefore, the separation of topics applied is certainly not a clear one. Catalysis by metal complexes embodies much of the chemistry of both metal carbonyls and metal alkene complexes. 

Since the early work dealing with Zeise s salt, many complexes have been prepared with the formula [PtL(C2H4)X2], where L = quinoline, pyridine, or ammonia and X=C1 , Br , I, or N()2. Similar compounds have been prepared that contain other alkenes than C2H4. Many of the complexes containing dienes, trienes, and tetraenes as ligands also contain carbonyl ligands. In fact, metal carbonyls are frequently starting complexes from which alkene complexes are obtained by substitution reactions. 

Pettit and coworkers—metal hydride intermediates by weak base attack over Fe carbonyl catalysts. Pettit et al.ls approached the use of metal carbonyl catalysts for the homogeneous water-gas shift reaction from the standpoint of hydroformyla-tion by the Reppe modification.7 In the typical hydroformylation reaction, an alkene is converted to the next higher aldehyde or alcohol through reaction of CO and H2 with the use of a cobalt or rhodium carbonyl catalyst. However, in the Reppe modification, the reduction is carried out with CO and H20 in lieu of H2 (Scheme 6) ... 

Abstract The transition metal mediated conversion of alkynes, alkenes, and carbon monoxide in a formal [2 + 2+1] cycloaddition process, commonly known as the Pauson-Khand reaction (PKR), is an elegant method for the construction of cyclopentenone scaffolds. During the last decade, significant improvements have been achieved in this area. For instance, catalytic PKR variants are nowadays possible with different metal sources. In addition, new asymmetric approaches were established and the reaction has been applied as a key step in various total syntheses. Recent work has also focused on the development of CO-free conditions, incorporating transfer carbonylation reactions. This review attempts to cover the most important developments in this area. 

Suga et al. (197) reported the first stereocontrolled 1,3-dipolar cycloaddition reactions of carbonyl ylides with electron-deficient alkenes using a Lewis acid catalyst. Carbonyl ylides are highly reactive 1,3-dipoles and cannot be isolated. They are mainly generated through transition metal carbenoid intermediates derived in situ from diazo precursors by treatment with a transition metal catalyst. When methyl o-(diazoacetyl)benzoate is treated with A-methylmaleimide at reflux... 

The examples chosen in connection with the reactions of alkenes and alkynes in the presence of metal carbonyls illustrate a number of the factors pertinent to the general realm of ligand reactions. 

It was discovered by Roelen in 1938 and is the oldest and largest volume catalytic reaction of alkenes, with the conversion of propylene to butyraldehyde being the mosi important. About 5 million tons of aldehydes and aldehyde derivatives (mostly alcohols) are produced annually making the process the most important industrial synthesis using a metal carbonyl complex as a catalyst. The name hydroformylation arises from the fact that in a formal sense a hydrogen atom and. formyl group are added across a double bond. The net result of the process is extension of (he carbon chain by one and introduction of oxygen into the molecule. 

Cyclopropane formation occurs from reactions between diazo compounds and alkenes, catalyzed by a wide variety of transition-metal compounds [7-9], that involve the addition of a carbene entity to a C-C double bond. This transformation is stereospecific and generally occurs with electron-rich alkenes, including substituted olefins, dienes, and vinyl ethers, but not a,(J-unsaturated carbonyl compounds or nitriles [23,24], Relative reactivities portray a highly electrophilic intermediate and an early transition state for cyclopropanation reactions [15,25], accounting in part for the relative difficulty in controlling selectivity. For intermolecular reactions, the formation of geometrical isomers, regioisomers from reactions with dienes, and enantiomers must all be taken into account. 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

The amide ions are powerful bases and may be used (i) to dehydrohalogenate halo-compounds to alkenes and alkynes, and (ii) to generate reactive anions from terminal acetylenes, and compounds having reactive a-hydrogens (e.g. carbonyl compounds, nitriles, 2-alkylpyridines, etc.) these anions may then be used in a variety of synthetic procedures, e.g. alkylations, reactions with carbonyl components, etc. A further use of the metal amides in liquid ammonia is the formation of other important bases such as sodium triphenylmethide (from sodamide and triphenylmethane). 

Hydrosilylation reactions catalyzed by iron carbonyl compounds often occur under drastic thermal conditions. Schroeder and Wrighton reported a photocatalyzed reaction oftrialkylsilanes with alkenes in the presence of Fe(CO)5 at low temperatures (0-50 °C) [48]. It is well known that irradiation of mononuclear metal carbonyls leads... 

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