Catalysis metal-alkene reactions

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. 

In heterogeneous metal catalysis alkanes, alkenes, and aromatics adsorbed on the metal surface rapidly exchange hydrogen and deuterium. The multiple adsorption of reactants and intermediates lowers the barriers for such exchange processes. Hydrogenation of unsaturated aliphatics and isomerisation can be accomplished under mild conditions. Catalytic dehydrogenation of alkanes to alkenes requires temperatures >200 °C, but this is because of the thermodynamics of this reaction. 

The development and application of catalytic enantioselective 1,3-dipolar cycloadditions is a relatively new area. Compared to the broad application of asymmetric catalysis in carbo- and hetero-Diels-Alder reactions (337,338), which has evolved since the mid-1980s, the use of enantioselective metal catalysts in asymmetric 1,3-dipolar cycloadditions remained almost unexplored until 1993 (5). In particular, the asymmetric metal-catalyzed reactions of nitrones with alkenes has received considerable attention during the past 5 years. 

In a number of classes of systems, the catalytic and other chemical effects of metal ions on reactions of organic and inorganic molecules are generally recognized the catalysis of nucleophilic reactions such as ester hydrolysis the reactions of alkenes and alkynes in the presence of metal carbonyls (8, 9, 69) stereospecific polymerization in the presence of Ziegler catalysts (20, 55, 56) the activation of such small molecules as H2 (37), 02 (13), H202 (13), and possibly N2 (58) and aromatic substitution reactions of metal-cyclopentadienyl compounds (59, 63). 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

The importance and relevance of homogeneous catalysis in polymerization reactions have increased tremendously in the past few years for two reasons. First, from about the beginning of the early 1990s a special class of sandwich complexes has been used as homogeneous catalysts. These catalysts, often referred to as metallocene catalysts, can effect the polymerization of a wide variety of alkenes to give polymers of unique properties. Second, the molecular mechanism of polymerization is best understood on the basis of what is known about the chemistry of metal-alkyl, metal-alkene, and other related complexes. 

Transition metals have already established a prominent role in synthetic silicon chemistry [1 - 5]. This is well illustrated by the Direct Process, which is a copper-mediated combination of elemental silicon and methyl chloride to produce methylchlorosilanes, and primarily dimethyldichlorosilane. This process is practiced on a large, worldwide scale, and is the basis for the silicones industry [6]. Other transition metal-catalyzed reactions that have proven to be synthetically usefiil include hydrosilation [7], silane alcdiolysis [8], and additions of Si-Si bonds to alkenes [9]. However, transition metal catalysis still holds considerable promise for enabling the production of new silicon-based compounds and materials. For example, transition metal-based catalysts may promote the direct conversion of elemental silicon to organosilanes via reactions with organic compounds such as ethers. In addition, they may play a strong role in the future... 

Zhang, X. Hill, C. L. Alkene Epoxidation by p-Cyano-lV-lV-dimethylaniline IV-oxide Catalyzed by d-Electron-transition-metal-substituted Polyoxometalates. In Catalysis of Organic Reactions. Malz, R. E. J., Ed. Marcel Dekker New York, 1996 pp 445-450. 

Transition metal catalysis of the Michael reaction of 1,3-dicarbonyl compounds with acceptor activated alkenes has been known since the early 1980 s 2>3 It is a valuable alternative to the classic base catalysis of the reaction. Because of the mild and neutral conditions, the chemoselectivity of these reactions is superior to that provided by base catalysis, since the latter suffers from various unwanted side or subsequent reactions, such as aldol cyclizations, ester solvolyses or retro-Claisen type decompositions. A number of transition metal and lanthanide compounds have been reported to catalyze the Michael reaction, but FeCb 6 H20 is one of the most efficient systems to date. A number of 3-diketones or p-oxo esters and MVK are cleanly converted to the corresponding Michael reaction products within a few hours at room... 

Cyclopropenes may undergo [2 + 2] cycloaddition to themselves, to form dimers, or to other alkenes the reactions may be brought about by photolysis, metal catalysis, Lewis acid catalysis, or simply by heat. Due to the difficulties that can be encountered in completely purifying some cyclopropenes, it is possible that some thermal reactions are actually initiated by small quantities of impurity. The following sections include those reactions occurring under thermal and Lewis acid catalysis, Those brought about by photolysis are covered in Section Ll.6.2.3.1,1. metal-induced reactions are dealt with in Section 1.1.6.3.3. 

In connection with the activation of saturated hydrocarbons via homogeneous catalysis, we have examined transition metal catalyzed reactions of various strained hydrocarbon systems that have unique steric and electronic properties. Strained carbon-to-carbon single bonds have considerable TT-bonding character. The chemistry of these substrates should be intermediate between well-documented transition metal chemistry of alkenes and rather unclarified alkane chemistry (1, 2, 3). Our attention has been focused particularly on the stereoselectivity, regioselectivity, and periselec-tivity of the Ni(0)-catalyzed reactions (4-14). 

Now for another case where a transition metal catalysis facilitates a reaction that would not occur under normal conditions nucleophilic attack on an isolated double bond. Usually alkenes react with nucieophiies only when conjugated with an eiectron-withdrawing group. But coordination of an electron-rich aikene to a transition metai ion such as palladium(ll) changes its reactivity dramatically electron density is drawn towards the metal and away from the n orbitals of the aikene. This leads to activation towards attack by nucleophiles, just as in conjugate addition, and unusual chemistry follows. Unusual, that is, for the aikene the palladium centre behaves exactly as expected. 

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