Carbon-metal bond, polarity

The LIC-KOR reagent consisting of stoichiometrically equal amounts of butyllithium ( LIC ) and potassium feri-butoxide ( KOR ) was conceived in Heidelberg and optimized in a trial-and-error effort . The fundamental idea was simple. To activate butyllithium optimally by deaggregation and carbon-metal bond polarization, a ligand was required that would surpass as an electron donor any crown ether but not suffer from the drawback of the latter, i.e. its proneness to /3-elimination. Whereas pinacolates and other v/c-diolates proved too labile to be generally useful, potassium terf-butoxide or any other bulky, hence relatively soluble, potassium or cesium alkoxide was found to serve the purpose. ... 

The existence of the assumed local maximum on curve 2 is due to the fact that the mutual donor-acceptor interaction between the monomer and the active center is probably most effective when the asymmetry of the former and the carbon-metal bond polarity (pseudosymmetry) in the latter supplement each other to a certain asymmetry which is attained, e.g. in the interaction between the symmetric vinyl monomer and the contact ion pair. It should be noted that the requirement of a certain asymmetry of the vinyl reagent is an indispensable general condition of cycloaddition [24, 25]. 

It should be emphasized that here and below that it is not the exact shape of the curves which is significant but only the tendency represented by these curves towards an increase or decrease in mutual reactivity of monomers and active centers depending on the carbon-metal bond polarity and hence depending on the process conditions. 

Carbon is more electronegative than metals and carbon-metal bonds are polarized so that carbon bears a partial to complete negative charge and the metal bears a partial to complete positive charge... 

Which compound m each of the following pairs would you expect to have the more polar carbon-metal bond" Compare the models on Learning By Modeling with respect to the charge on the carbon bonded to the metal... 

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. 

Whether a carbon-metal bond is ionic or polar-covalent is determined chiefly by the electronegativity of the metal and the structure of the organic part of the molecule. Ionic bonds become more likely as the negative charge on the metalbearing carbon is decreased by resonance or field effects. Thus the sodium salt of acetoacetic ester has a more ionic carbon-sodium bond than methylsodium. 

With less polar solvents and more basic allyl anions the compounds are present as ion pairs. The carbon-metal bond with the alkali and alkaline earth metals are known to have high ionic character. The allyl compounds behave accordingly as salts. The structures of allyl compounds of the alkali and alkaline earth metals are of two fundamental types, a 41 (or metal cation is associated closely with a single terminal allylic carbon, and the rf 1 (or ji) type, 15, in which the cation bridges the two terminal allylic positions. 

Such metallorganic compound or coordination complex contains an electropositive metal for which the carbon-metal bond may be considered at least partly polarized, so that the carbon atom has a partially ionic character and behaves as a carbanion. 

B-2. Rank the following species in order of increasing polarity of the carbon-metal bond... 

Compared to the lithium alkyls, the carbon-metal bond in the corresponding sodium and potassium compounds is more polar and thus the lower alkyl derivatives are no longer soluble in hydrocarbons nor are they volatile (262). Therefore, little has been done to elucidate any exchange reactions in which they might participate. 

In both systems the hydrogen molecule is apparently split into ions, rather than atoms. Weight is lent to this conclusion by the fact that the order of increasing ease of hydrogenolysis of phenyl derivatives is Ca, Li, Na, K, Rb, and Cs this is also the order of increasing polarity of the carbon-metal bond. 

Organometallic compound (Section 18.5) A compound with a covalent carbon—metal bond. Because the metal is less electronegative than carbon, the bond is polarized in the opposite direction to that found in most organic compounds that is, the negative end of the dipole is on the carbon and the positive end is on the metal. These often make useful carbon nucleophiles. 

In the majority of cases, organolithium compounds and Grignard reagents contain polarized but covalent carbon—metal bonds. Lithioalkanes, -alkenes, and -aromatics, on the one hand, and alkyl, alkenyl, and aryl magnesium halides, on the other hand, are therefore formulated with a hyphen between the metal and the neighboring C atom. Only lithiated alkynes and alkynyl Grignard reagents are considered to be ionic—that is, species with carbon, metal bonds similar to those in LiCN or Mg(CN)2. 

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). 

Sequence 16 clearly demonstrates electrophilic attack by ozone in these reactions. As noted by Jensen and Rickborn (2), the rate of electrophilic cleavage of a carbon-metal bond increases as the polarization of that bond increases. This, in turn, is a direct consequence of the electronegativity of the second atom attached to mercury. The following representations, based on Pauling electronegativity values, illustrate this relationship. 

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