Metallation, of alkenes

Metallation of alkenes with rc-BuLi-KOC(CH3)3 provides a route that is stereoselective for Z-allylic silanes.68 (See p. 632 for discussion of this metallation method.)... 

This route has, for example, been used to make the butyl compounds.15 The trimethylsilylmethyl derivatives have also been made1617 they can be precipitated from cyclohexane, washed free from organomercury starting materials, and obtained without organometal-alkoxide contaminants. Alternatively, KCH2SiMe3 may be used in situ, e.g., for metallation of alkenes.18... 

Morton and Brown (1947) and Morton et al. (1950) have studied the metallation of alkenes and have found that propylene and isobutylene metallate much more easily than ethylene. If the CH3 groups are attached to opposite sides of the double bond, then the hydrocarbon becomes less acidic, but even then only the hydrogen atom in the allyl... 

Alkenes lacking hydrogen atoms at allylic positions are much less acidic than ordinary unsaturated hydrocarbons. Superbases regioselectively exchange allylic protons in alkenes whenever there is a choice. However, a few examples of metallation of alkenic C-H bonds with superbases are known and a compilation of them is reported in Table 1. Ethylene itself has been deprotonated by the superbasic mixture constituted by butyllithium, potassium tert-butoxide, and TMEDA.41... 

Table 1 Metallation of alkenes in the vinylic position Metallated alkene Superbase...

Metallation of Alkenes, Dienes, and Polyenes at the Allylic Position... 

The selective and efficient metallation of alkenes in the allylic position by the Schlosser s base is certainly one of the major achievements in the field of superbase chemistry. A complete list of successful alkene metallations by superbases would be too long for the purpose of this chapter but some representative examples are shown in Tables 2 and 3 as a general trend for the unsubstituted and heterosubstituted alkenes, respectively. 

The superbase metallation of alkenes in the allylic position followed by reaction with electrophiles is one of the best methods to access Z-alkenes with new functional groups. 

Dialkylallyl boranes have also been prepared119 by metallation of alkenes with trimethylsilylmethylpotassium followed by trapping of the allylpotassium species with dialkylchloroboranes. Such allylboranes have been then used... 

Alkenyllithium compounds can also be prepared by metallation of alkenes, particularly when alkenyl hydrogens are rendered acidic by an a-substituent (equation 22). Transmetallation of alkenyl stannanes with organolithium reagents gives alkenyllithium compounds with retention of alkene stereochemistry (equation 23). Tin-lithium transmetallation has been used to prepare 1,4-dilithio-l,3-butadiene. Monosubstituted alkenyllithium compounds RHC=CHLi, can also be prepared from the corresponding diorganotel-luride, RHC=CHTeBu, by reaction with butyllithium in... 

Table 1 Metalation of alkenes with n-butyllithium/potassium tert-butoxide...

Khan, M. M. T. 1974, Homogeneous Catalysis by Metal Complexes, Vol. II, Activation of Alkenes and Alkynes, Academic Press New York - London... 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6 1-6 3) A mechanism for metal-ammonia reduction of alkynes is outlined m Figure 9 4... 

The preparation and structure determination of ferrocene marked the beginning of metallocene chemistry Metallocenes are organometallic compounds that bear cyclo pentadiemde ligands A large number are known even some m which uranium is the metal Metallocenes are not only stucturally interesting but many of them have useful applications as catalysts for industrial processes Zirconium based metallocenes for example are the most widely used catalysts for Ziegler-Natta polymerization of alkenes We 11 have more to say about them m Section 14 15... 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Heterogeneous reaction (Section 6 1) A reaction involving two or more substances present in different phases Hydro genation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst Heterolytic cleavage (Section 4 16) Dissociation of a two electron covalent bond in such a way that both electrons are retained by one of the initially bonded atoms Hexose (Section 25 4) A carbohydrate with six carbon atoms High density lipoprotein (HDL) (Section 26 11) A protein that carries cholesterol from the tissues to the liver where it is metabolized HDL is often called good cholesterol Histones (Section 28 9) Proteins that are associated with DNA in nucleosomes... 

Uses. Magnesium alkyls are used as polymerization catalysts for alpha-alkenes and dienes, such as the polymerization of ethylene (qv), and in combination with aluminum alkyls and the transition-metal haUdes (16—18). Magnesium alkyls have been used in conjunction with other compounds in the polymerization of alkene oxides, alkene sulfides, acrylonitrile (qv), and polar vinyl monomers (19—22). Magnesium alkyls can be used as a Hquid detergents (23). Also, magnesium alkyls have been used as fuel additives and for the suppression of soot in combustion of residual furnace oil (24). 

Heterogeneous reaction (Section 6.1) A reaction involving two or more substances present in different phases. Hydrogenation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst. 

An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

Other convenient routes to carboranes, selected from the growing number of recently reported syntheses, are as follows. Monocarbon carboranes can be prepared in good yield by the transition-metal catalysed hydroboration of alkenes followed by thermal rearrangement of the intermediate product, c.gP" ... 

Arsine complexes are especially stable for b-class metals such as Rh, Pd and Pt, and such complexes have found considerable industrial use in hydrogenation or hydroformylation of alkenes. 

The reverse reaction (formation of metal alkyls by addition of alkenes to M-H) is the basis of several important catalytic reactions such as alkene hydrogenation, hydroformylation, hydroboration, and isomerization. A good example of decomposition by y3-elimination is the first-order intramolecular reaction ... 

When a mixture of alkenes 1 and 2 or an unsymmetrically substituted alkene 3 is treated with an appropriate transition-metal catalyst, a mixture of products (including fi/Z-isomers) from apparent interchange of alkylidene moieties is obtained by a process called alkene metathesis. With the development of new catalysts in recent years, alkene metathesis has become a useful synthetic method. Special synthetic applications are, for example, ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROM) (see below). 

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