Organometallic compounds of lithium

Pentamethylantimony was the first It6Sb species to be obtained by Wittig and Torssell in 1955 (104) Various other derivatives of this type have since been synthesized, and a range of preparative methods has been described. Among these are the alkylation of RiSbX, R3SbX2, and SbX species with organometallic compounds of lithium, magnesium, aluminum,... 

Example 7.9 Among the elements of the first group of the periodic table, only organometallic compounds of lithium show a considerable tendency to give stereospecific catalysts. Also, this tendency decreases with increasing atomic radius of the elements of this group. Explain this observation. 

Organometallic compounds of lithium and magnesium are of great importance in organic synthesis. They are relatively stable in ether solutions, but their carbon—metal bonds have considerable ionic character. Because of this ionic nature, the carbon atom that is bonded to the metal atom of an organolithium or organomagnesium compound is a strong base and powerful nucleophile. We shall soon see reactions that illustrate both of these properties. 

This is the most commonly utilized method for synthesis of transition metal complexes with M —C a bonds. The following substrates are utilized halides, acetylacetonates, acetates, /-electron element alcoholates, as well as organometallic compounds of lithium, magnesium, aluminum, sodium, zinc, mercury, elements of group 14, etc. Ethers and hydrocarbons are used as solvents. In general, the synthesis should be carried out at lower temperatures and under inert atmosphere. 

Fraenkel G, Dayagi S, Kobayashi S (1968) Nuclear Magnetic Resonance and Ultraviolet Spectroscopy of Substituted Aromatic Organometallic Compounds of Lithium, Magnesium, and Calcium. J Phys Chem 72 953-961... 

Organometallic compounds of lithium and magnesium are most conveniently prepared by direct reaction of a haloalkane with the metal suspended in ethoxyethane (diethyl ether) or oxacyclopentane (tetrahydrofuran, THE). The reactivity of the haloalkanes increases in the... 

Organometallic compounds of copper were known for a long time before their versatil ity in synthetic organic chemistry was fully appreciated The most useful ones are the lithium dialkylcuprates which result when a copper(I) halide reacts with two equivalents of an alkyllithium in diethyl ether or tetrahydrofuran... 

The compounds of lithium and magnesium are the most important of the group IA and IIA organometallics from a synthetic perspective. The metals in these two groups are the most electropositive of the elements. The polarity of the metal-carbon bond is such as to place high electron density on carbon. This electronic distribution is responsible for the strong nucleophilicity and basicity of these compounds. 

The most widely used method for the preparation of organometallic compounds of the heavier alkali metals is from the corresponding organo-lithium reagent and an alkoxide. 

In this chapter we will restrict our discussion of organometallic compounds to the alkyl and aryl compounds of magnesium and lithium, and the sodium and potassium salts of 1-alkynes. These substances normally are derived directly or indirectly from organohalogen compounds and are used very widely in organic synthesis. Organometallic compounds of transition metals and of boron are discussed in Chapters 11 and 31. 

Table 12.1 shows some organometallic lithium compounds. It is seen from their formulas that these compounds are ionic. As discussed in Section 12.2, 1A metals have low electronegativities and form ionic compounds with hydrocarbon anions. Of these elements, lithium tends to form metal-carbon bonds with the most covalent character therefore, lithium compounds are more stable (though generally quite reactive) than other organometallic compounds of group 1A metals, most... 

As discussed in Section 12.2, group 1A metals form ionic metal-carbon bonds. Organometallic compounds of group 1A metals other than lithium have metal-carbon bonds with less of a covalent character than the corresponding bonds in lithium compounds and tend to be especially reactive. Compounds of rubidium and cesium are rarely encountered outside the laboratory, so their toxicological significance is relatively minor. Therefore, aside from lithium compounds, the toxicology of sodium and potassium compounds is of most concern. 

It is, therefore, to be expected that a compound such as trimethyl-methylene stiborane, (CHs SbCtE, should be relatively unstable and attempts to synthesize this species would have to be carried out at low temperature. Another difficulty arises from the experimental fact (104) that all conventional methods of synthesis for stibonium ylides lead to pentaalkylstiboranes instead of ylides (see Introduction). Thus mcthyl-ation of (CH3)4SbX or (CHY) sHbX2 halides by organometallic reagents of lithium, magnesium, aluminum, or zinc invariably yield (CHY) 5Sb as the sole product. 

Ziegler-Natta Catalysts (Heterogeneous). These systems consist of a combination of a transition metal compound from groups IV to VIII and an organometallic compound of a group I—III metal.23 The transition metal compound is called the catalyst and the organometallic compound the cocatalyst. Typically the catalyst is a halide or oxyhalide of titanium, chromium, vanadium, zirconium, or molybdenum. The cocatalyst is often an alkyl, aryl, or halide of aluminum, lithium, zinc, tin, cadmium, magnesium, or beryllium.24 One of the most important catalyst systems is the titanium trihalides or tetra-halides combined with a trialkylaluminum compound. 

Other useful organometallic reagents for the preparation of tertiary stibines or related heterocyclic compounds are compounds of lithium, aluminum, tin, copper, zirconium, and other metals. " In most cases, these metal compounds are reacted with antimony chlorides or bromides. Good yields of tertiary stibines were, however, also obtained by the interaction (equation 3) of triaUcylaluminium reagents and tris(dimethylamino)stibine. ... 

Organometallic compounds of magnesium, mercury, or lithium have been treated with iodine or bromine to form organic halides. The method has been successful for obtaining neopentyl iodide where other methods have failed (92%). It has been found convenient in the synthesis of 9-iodoanthracene (53%) and certain heterocyclic halides. ... 

Ligand exchanges ( redistribution ) between two different organometallic compounds are widely used in preparative organometallic chemistry °. Lithium organic compounds are frequently applied in such syntheses, while organometallic compounds of Sb and Bi are employed as the second reagent. 

Very often, cyclic compounds are made by the adapting of a standard method of preparation to the job of closing a ring. For example, we have seen (Sec. 3.17) that the alkyl groups of two alkyl halides can be coupled together through conversion of one halide into an organometallic compound (a lithium dialkylcopper) ... 

---