Reactions of metal alkyls

Most metals are extremely reactive when they are in the form of powders, and they may react explosively with oxygen, halogens, sulfur, or other oxidizing agents. In some ways, metal alkyls behave as if they contained the metals in an atomic form. As a result, some metal alkyls are spontaneously flammable in air as shown in the following cases  

Those that are not spontaneously flammable will bum readily as a result of the formation of very stable combustion products. For example, the combustion of trimethylboron produces 

Virtually all reactions of this type are extremely exothermic. However, tetraethyllead, Pb(C2H5)4, is stable in air, as are SiR4, GeR4, and HgR2 compounds, although they will all readily bum. 

Most metal alkyls react vigorously (some explosively) with water to produce a hydrocarbon and a metal hydroxide. Typical reactions are the following  

In these reactions, the polar nature of the metal-carbon bond is indicated by the fact that the electronegativity of carbon is approximately 2.5, whereas that of most metals is in the range of 1.0 to 1.5. Thus, many reactions take place as if R is present, with the alkyl group behaving as a strong base. This behavior extends even to reactions with NH3, which is normally basic, but an amide is formed as a result the removal of H+  

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B. The Controlled Reaction of Metal Alkyls with Elemental Fluorine... 

A reaction which is rather new and not mentioned in older textbooks is the so-called o-bond metathesis. It is a concerted 2+2 reaction immediately followed by its retrograde reaction giving metathesis. Both late and early transition metal alkyls are prone to this reaction, but for d° early transition metals there is no other mechanism than o-bond metathesis at hand. Many similar reactions such as the reaction of metal alkyls with other HX compounds could be described as if they would follow this pathway, but the use of the term o-bond metathesis is restricted to those reactions in which one reacting species is a metal hydrocarbyl or metal hydride and the other reactant is a hydrocarbon or dihydrogen. In Figure 2.30 the reaction has been depicted. 

Kinetic and Mechanistic Studies of Stoichiometric Reaction Steps 3.3.2.1 Migratory CO Insertion Reactions of Metal Alkyls... 

The diene polymerizations catalyzed by organometallic compounds are probably further examples of diene insertion reactions of metal alkyls (25). But there is little evidence on the detailed mechanisms of these polymerizations. 

One useful type of reaction of metal alkyls is the transfer of an alkyl group to another metal as shown in the following examples ... 

In addition to reaction of metal alkyl with the lateral edges of the titanium trichloride crystal, reaction also occurs at the main faces although this does not initiate polymerization. A study of the stoichiometry and mechanism of the TiCl3 /AlMe3 reaction indicates that a complex of the structure TiCl3—TiCl2 AlMej is formed on the 001 face, in which the titanium and chlorine atoms maintain their original positions in... 

At this point Ziegler and his coworkers carried out experiments on the effects of adding various other metal compounds to triethylaluminum. In one of these experiments with zirconium acetylacetonate, ethylene, and triethylaluminum, they found, to their surprise, an autoclave filled with a solid cake of snow-white polyethylene (1. ) Further work revealed that aluminum alkyls in conjunction with certain transition metal compounds of Groups IV-VI, as well as uranium and thorium, were active ethylene polymerization catalysts. Ultimately, Ziegler catalysts were described to be the product of reaction of metal alkyls, aryls, or hydrides of Groups I-IV and certain transition metal compounds of Groups IV-VIII (Reaction 4). The choice of a particular catalyst and experimental conditions is dictated by the structure of the monomer to be polymerized. 

The reaction of metal alkyl compounds with dihydrogen, leading to hydride species with elimination of alkanes, is commonly observed for Lewis acidic metal centers. This reaction has been used to prepare hydride complexes of the lanthanides, and of titanium(III), see equation 8 [39]. 

Dinuclear elimination reactions of metal alkyls (Equation 2.4, L = all other ligands) have been studied less intensely than, for example, similar reactions of metal amides (Equation 2.5). The following characteristics are common to known dinuclear elimination reactions of metal alkyls (i) a site of unsaturation must be present cis to the alkyl ligand and (ii) the other reagent must be a hydrido-complex. 

Transition-metal-silyl complexes are also formed by the reactions of metal-alkyl complexes with silanes to form free alkane and a metal-silyl complex. Two examples are shown in Equations 4.114 and 4.115. ° The synthesis of silyl complexes by this method has been accomplished with both early and late transition metal complexes. The formation of metal-silyl complexes from late-metal-alkyl complexes resembles the hydrogenolysis of metal-alkyl complexes to form metal hydrides and an alkane. The mechanisms of these reactions are discussed in Chapter 6. In brief, these reactions with late transition metal complexes to form silyl complexes typically occur by a sequence of oxidative addition of the silane, followed by reductive elimination of alkane. An example of this is shown in the coupling of 1,2-bis-dimethylsilyl benzene with a dimethyl platinum(II) complex (Equation 4.114). Similar reactions occur with d° early metal complexes by a a-bond metathesis process that avoids these redox events. For example, the reaction of Cp ScPh with MesSiH, has been shown to proceed through this pathway (Equation 4.115). ... 

Nearby C=C bonds will insert into the product acyls, a reaction that has been used in a synthesis of aphidicolin (Eq. 14.76 ). We will see further examples of CO insertion reactions of metal alkyls in the next section. 

VIII. Redistribution Reactions of Metal Alkyls and Metal Halides. . . 107... 

REDISTRIBUTION REACTIONS OF METAL ALKYLS AND METAL HALIDES... 

The structural consequences of bonding for carbene complexes are discussed. Photochemical reactions of metal alkyl complexes result in very reactive intermediates that can be trapped and used preparatively. Matrix isolation studies of organometallic intermediates are considered in detail. Transition metals in organic synthesis for the year 1982 are reviewed extensively and a further article concerns a-(1,n)-alkanediy1 complexes. ... 

The most broadly applied laboratory approach of this type is the reaction of metal alkyl amides, usually fcw-(trimethylsilyl)-amides with the stoichiometric amounts of alcohols. The starting reagents even in this case are not available commercially, but can be obtained more-or-less easily by reaction of the corresponding metal chlorides with commercially available LiN(SiR3)2 in anhydrous diethyl ether ... 

In the reaction of metal alkyls with lead(ll) compounds elemental lead is always precipitated according to the general equation ... 

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