Carbon-metal bond, reactivity

In anionic polymerization, as in carbonium ion polymerization, termination does not involve bimolecular reaction between two growing chains. Neither can recombination of ions lead to termination, since a carbon-metal bond is highly polar, in the case of alkali metals frequently completely ionized, and in every case very reactive. The termination step leading to the formation of a terminal C=C double bond is not too probable. This reaction involves the formation of a metal hydride, and this does not contribute greatly to the driving force. Consequently, such a termination is observed at higher temperatures only and it is probably more common in coordination polymerization where the metals involved are less electropositive. 

Large R2 substituents induce effective ion-separation between the cationic active species and an anionic cocatalyst, which allows more space for ethylene coordination to the metal and for its insertion into the carbon-metal bond. In addition, electronically, the ion separation increases the electrophilicity of the catalytically active species and hence enhances the reactivity toward ethylene. 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

The a-elimination process is a very fast and effective reaction of trifluoromethyl carbanions (Figure 1.9)." Consequently, the corresponding organometallic species (Li, Mg) cannot be used in organic synthesis. When the carbon-metal bond is close to a covalent bond, the anionic species is more stable, but has almost no reactivity toward electrophiles. Zinc, and especially silicon, derivatives constitute the best compromises." When the fluoroalkyl chain is longer, organometallics are more stable and can be used in synthesis (Figure 1.10)." ... 

Due to the poor efficiency of trifluoromethylated organometaUic derivatives as trifluoromethylating reagents, nucleophilic trifluoromethylation has remained unattractive for a long time. Indeed, in the absence of stabilization, the trifluoromethyl anion is very unstable and is quickly transformed into difluorocarbene (cf. Chapter 1). When the carbon-metal bond is relatively covalent, the organometaUic species becomes more stable but it is then less reactive toward an electrophile. On the synthetic level, only zinc and copper derivatives have found real applications in... 

When 303 was directly treated with Me2Cu(CN)Li2, the transmetallation failed to discriminate between the two carbon-metal bonds. By contrast, the allylzincation of the alkynyllithium derived from the propargylic alcohol 309 produced the alkenyl 1,1-dimetallic species 310, in which the two carbon-metal bonds exhibit different reactivities due to the presence of a metal-alkoxide. Indeed, transmetallation with Me2Cu(CN)Li2 led to the alkenyl copper-zinc species 311, which was relatively poorly reactive towards electrophiles but underwent successful 1,4-addition to ethyl propiolate leading to 312 in satisfactory overall yield (equation 145)180. 

Wilkinson s catalyst after its discoverer, G. Wilkinson. In 1973, the Nobel Prize in chemistry was awarded jointly to Wilkinson and E. O. H. Fischer for their respective contributions to the field of organometallic chemistry. As you will see in this and later chapters, compounds with carbon-metal bonds (organometallic compounds) are extremely useful reagents, reactive intermediates, or catalysts in organic reactions. To a very large extent, the work of Fischer and Wilkinson created the current interest and developments in the field of transition-metal organic chemistry, which will be discussed in Chapter 31. 

A number of transition metal complexes react with alkenes, alkynes and dienes to afford insertion products (see Volume 4, Part 3). A general problem is that the newly formed carbon-metal bond is usually quite reactive and can undergo a variety of transformations, such as -hydride elimination or another insertion reaction, before being trapped by an electrophile.200 Usually, a better stability and lower reactivity is observed if the first carbometallation step leads to a metallacycle. It is worthy to note that the carbometallation of perfluorinated alkenes and alkynes constitutes a large fraction of the substrates investigated with transition metal complexes.20015... 

Organometallic compounds which contain a carbon-metal bond are the most reactive carbon nucleophiles. In most cases they are also powerful bases and must be prepared and used under strictly anhydrous and aprotic conditions. A very common way to produce organometallic compounds is to reduce alkyl halides with active metals. Grignard reagents and organolithium compounds are routinely produced in this manner. The transformation is a two-electron reduction of the alkyl halide to a carbanion equivalent the metal is oxidized. 

Why are organotitanium reagents more chemoselective than the lithium and magnesium counterparts It is clear from the above presentation that the rate of reaction of classical carbanions is considerably higher than those of the titanated species. Generally, more reactive species are less selective. However, this does not really answer the above question, since the phenomenon of different rates remains unclear. Apart from steric factors, we believe it has to do with the different polarity of the carbon-metal bond. Whereas C—Li bonds are highly polar (and ionic in certain cases) 57,86,87) anaj0gS appear to be considerably less so (Sect. B). 

Many organometallic compounds that have main group metal-hydrogen or metal-metal bonds undergo 1,2-hydrometallation or 1,2-dimetallation of alkynes. Pd complexes are good catalysts for these processes [118]. Since the resulting products contain one or two reactive carbon-metal bonds they are well suited for further transformations, particularly in a sequential fashion. 

The difference in reactivity between the a-stannyl sulfide 16A and the a-silyl sulfide 16B can be explained by comparing the two-center energies of their carbon-metal bonds. Semiempirical molecular orbital calculation revealed that the bond energies decrease in the order of 2-silyl, 2-germyl, and 2-stannyl-1,3-dithiane cation radicals. As the silyl dithiane was completely consumed by the oxidation under the... 

In fact, cleavage of the carbon-zirconium bond occurs classically with various electrophiles (see Electrophile), generally under mild conditions. In addition, the other carbon-metal bond in gem metaUozirconocenes undergoes a broad range of transformations. The different reactivities of the carbon-metal and carbon-zirconium bonds are the results of the difference of electronegativity of the two metal centers and the two different bond polarities. 

Boronates are more stable since they are stabilized owing to the donor effect of oxygen lone-pairs to the empty orbital of the boron. The two different carbon-metal bonds afford particnlar reactivity. For example, addition of propargylbromide on (94) in presence of a catalytic amount of copper cyanide nndergoes a carbon-carbon bond formation with exclusive cleavage of the C-Zr bond. The subsequent borylallene, by treatment with a ,/3-unsaturated aldehydes, affords two trienes, depending on the reaction conditions (Scheme 20). 

Here M denotes a monomer and the asterisk means an active site which could be a radical, ion (with an appropriate counterion), or a carbon-metal bond. The reactivity of the active site is assumed to be determined solely by the nature of the terminal monomer residue which carries this site. Thus, for copolymerizations of monomers A and B, the two species AABAABA and BBABAA would be indistinguishable. 

The more polar the carbon-metal bond, the more reactive the organometallic reagent. 

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