Organolithiums polarized bond

Germyl, Stannyl, and Plumbyl Anions The preparative methods for the synthesis of the germyl, stannyl, and plumbyl anions are essentially the same as those mentioned above for the silyl anions. The most widely used methods are (1) reduction of halides R3EX (R = alkyl, aryl E = Ge, Sn, Pb X = Cl, Br) with alkali metals and (2) reductive cleavage of the E-E bond of R3E-ER3 (R = alkyl, aryl E = Ge, Sn, Pb) with alkali metals or organolithium reagents. Due to the favorable polarization of the (E = Ge, Sn, Pb) bond, the direct metalation... 

Alkyl derivatives of the alkaline-earth metals have also been used to initiate anionic polymerization. Organomagnesium compounds are considerably less active than organolithiums, as a result of the much less polarized metal-carbon bond. They can only initiate polymerization of monomers more reactive than styrene and 1,3-dienes, such as 2- and 4-vinylpyridines, and acrylic and methacrylic esters. Organostrontium and organobarium compounds, possessing more polar metal-carbon bonds, are able to polymerize styrene and 1,3-dienes as well as the more reactive monomers. 

The crystal structures of numerous organolithium compounds and complexes with 0—Li bonds are now available (2-5). Table IV lists a number of these species, as well as two derivatives of heavier alkali metals. As with the C—Li derivatives just discussed (Tables II and III), clustered (ROLi) tetramers and hexamers, as well as ring dimers, are prevalent. Note that (OLi)2,3 ring systems also are pseudoplanar (Fig. 21a). However, extensive stacking leading to polymers will only occur if the substituents on 0 are small and if polar ligands are absent. Otherwise, limited (double) stacks or unstacked rings form. 

The most important synthetic use of Grignard reagents and organolithium reagents is to form new carbon-carbon bonds by addition to polar multiple bonds, particularly carbonyl bonds. An example is the addition of methyl-magnesium iodide to methanal ... 

From the viewpoint of polar, yet covalent Li—O and Li—N bonds, lithium would be unable to reach a valence electron octet in the absence of bonding partners besides the heteroatom. The lithium thus has to surround itself by other donors in much the same way as has been seen in the case of the organolithium compounds (cf. Section 10.1). 

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. 

We possess more information on systems containing polar monomers. Organolithium and organomagnesium compounds initiate the polymerization of a number of monomers with an electron-withdrawing substituent. These polymerizations are rarely of the living type. The initiator usually reacts not only with the double bond of the monomer, but also with the polar substituent (both on the monomer and the polymer) yielding inactive products. 

Deprotonation of the hydroxyalkylpolysilanes la-lc with two or more equivalents methyllithium, rbutyllithium, or phenyllithium, respectively, leads to the trisilanes 6a-6e, which are formed by addition of the excess organolithium reagent to the polar Si=C-bond (Eq. 4). 

In view of the trend to more controlled and stereoselective reactions with readily available, less expensive and environmentally non-problematic reagents, the light-induced inner-sphere electron transfer between M-C bonds of less polar co-ordinating organometallics (Zn, Al) and the organic substrate seems to be a particularly attractive alternative to thermal reactions from organolithium or -magnesium compounds. 

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