Organolithium compounds, hydrogenation

With a difluoroethylene containing hydrogen and chlonne, where both groups can be replaced by lithium, a mixture of two organolithium compounds is formed in a 2 1 ratio, indicating a more facile replacement of chlorine [63] (equation 30)... 

The reaction of tnfluoromethyl-substituted A -acyl umnes toward nucleophiles in many aspects parallels that of the parent polyfluoro ketones Heteronucleophiles and carbon nucleophiles, such as enarmnes [37, 38], enol ethers [38, 39, 40], hydrogen cyanide [34], tnmethylsilylcarbomlnle [2,47], alkynes [42], electron-nch heterocycles [43], 1,3-dicarbonyl compounds [44], organolithium compounds [45, 46, 47, 48], and Gngnard compounds [49,50], readily undergo hydroxyalkylation with hexafluoroace-tone and amidoalkylation with acyl imines denved from hexafluoroacetone... 

Metalation of Thiophenes with Organolithium Compounds (Nucleophilic Substitution on Hydrogen)... 

The crystal structures of many organolithium compounds have been determined.44 Phenyllithium has been crystallized as an ether solvate. The structure is tetrameric with lithium and carbon atoms at alternating corners of a highly distorted cube. The lithium atoms form a tetrahedron and the carbons are associated with the faces of the tetrahedron. Each carbon is 2.33 A from the three neighboring lithium atoms and an ether molecule is coordinated to each lithium atom. Figures 7.2a and b show, respectively, the Li-C cluster and the complete array of atoms, except for hydrogen 45 Section 6.2 of Part A provides additional information on the structure of organolithium compounds. 

In their papers Rodionov and coworkers described the polymerization of organolithium compounds in terms of the formation of lithium bonds (Scheme 1), analogous to hydrogen bonds, which brought about cyclic or linear association of these compounds in solution . However, the strong association of alkyllithium compounds, persisting even in the vapour phase, indicates that their association takes place through the formation of... 

A from the three neighboring lithium atoms. An ether molecule is coordinated to each lithium atom. Figure 7.2a shows the Li-C cluster, and Fig. 7.2b shows the complete array of atoms, except for hydrogen.36 Section 7.1 of Part A provides additional information on the structure of organolithium compounds. 

Although of limited utility for other types of Grignard reagents, metalation is an important means of preparing a variety of organolithium compounds. The position of lithiation is determined by the relative acidity of the available hydrogens and the directing effect of... 

The competition between insertion and hydrogen transfer is also crucial to the selectivity of the reaction of aluminium alkyls with carbonyl compounds. Aluminium alkyls, like organolithium compounds and Grignard reagents, can add to aldehydes and ketones to form secondary or tertiary alcohols, respectively. If the aluminium alkyl has a j -hydrogen, however, reduction of the carbonyl compound is a common side reaction, and can even become the main reaction [16]. Most authors seem to accept that reduction involves direct j5-hydrogen transfer to ketone. 

Benzyl methyl ether or allyl methyl ethers can be selectively metalated at the benzylic/allylic position by treatment with BuLi or sBuLi in THF at -40 °C to -80 C, and the resulting organolithium compounds react with primary and secondary alkyl halides, epoxides, aldehydes, or other electrophiles to yield the expected products [187, 252, 253]. With allyl ethers mixtures of a- and y-alkylated products can result [254], but transmetalation of the lithiated allyl ethers with indium yields y-metalated enol ethers, which are attacked by electrophiles at the a position (Scheme 5.29). Ethers with ft hydrogen usually undergo rapid elimination when treated with strong bases, and cannot be readily C-alkylated (last reaction, Scheme 5.29). Metalation of benzyl ethers at room temperature can also lead to metalation of the arene [255] (Section 5.3.11) or to Wittig rearrangement [256]. Epoxides have been lithiated and silylated by treatment with sBuLi at -90 °C in the presence of a diamine and a silyl chloride [257]. 

Organomagnesium compounds are kinetically less-reactive bases toward weakly acidic hydrogens than organolithium compounds. Usually, Metalation using an organomagnesium compound is practically limited to acid hydrogens with pXa of about 25 or lower. This includes aUcynes (equation 14) and derivatives of cyclopentadiene, indene, and fluorene. Even... 

Organolithium compounds RLi (R = Me, Et, //-Bu, Ph) are excellent dehydrohalo-genating agents of vinyl halides in ether or THE solution under very mild conditions at temperatures below 0 °C. In a first step the acidic geminal hydrogen is replaced by lithium in a slow step to give an alkenyllithium which can be isolated at low temperature. In a second fast step lithium halide is eliminated to furnish an acetylene (equation 61). Two equivalents of alkyllithium are needed according to this equation. 

Several Lewis acid-base interactions between alkali metal cations and heteroatom-containing molecules are indispensable in the promotion of reactions involved in critically important and fundamental transformations—deprotonation with lithium amides at the a-hydrogens of carbonyl or imino compounds and the addition of organolithium compounds to such electrophilic substrates. Because it is impossible to cover the multitude of these and other closely related subjects, this chapter describes only briefly general aspects of current interest. 

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