Lithium, alkyls bonding

For application in organic synthesis, the regiochemistry of insertion of carbenoids into un-symmetrical zirconacydes needs to be predictable. In the case of insertion into mono- and bicydic zirconacydopentenes where there is an wide variety of metal carbenoids insert selectively into the zirconium—alkyl bond [48,59,86], For more complex systems, the regiocon-trol has only been studied for the insertion of lithium chloroallylides (as in Section 3.3.2) [60]. Representative examples of regiocontrol relating to the insertion of lithium chloroal-lylide are shown in Fig. 3.2. 

In spite of the general ambiphilicity of phosphonio-substituted phosphoHde derivatives, the aromaticity of the phosphoHde ring [10, 11] tends to reduce their electrophilicity while the intramolecular compensation of the negative charge by the phosphonio-substituents lowers at the same time their nucle-ophilicity [15, 16]. Bis-phosphonio-benzophospholides and -1,2,4-diaza-phospholides are therefore less reactive towards electrophiles and nucleophiles than other types of phosphorus containing multiple-bond systems and lack the notorious hydrolytic instabihty of many of these species [15, 16, 24]. Reactions are observed, however, with sufficiently strong electrophiles such as triflic acid or methyl triflate, or nucleophiles such as OH" or lithium alkyls, respectively. 

Whereas SiH-containing silylphosphanes such as H3Si—PEt2 react with LiPEtj by substituting the SiH group, lithium alkyls on the other hand cleave the Si—P bond (ii) as shown in the following case. 

Phosphorus-silicon bonds in trimethylsilyl phosphanes can be cleaved by lithium alkyls 11). Such reactions occur in most cases even below 20°C in the presence of a solvating ether like THF or DME. 

In pure EtjO (without added THF) the reaction times are considerably lengthened. Accordingly, in the first reaction step, a P—P bond of 85 is cleaved by the nucleophilic attacking lithium alkyL Because of the symmetrical structure of 85 only one primary product can be yielded by the opening of its four-membered ring. When cooled THF is added to this solution of compound 86, or of 87, rapid isomerization to the P2 n-tetraphosphide follows, as illustrated by Eq. (14). 

In a first experiment a pressure of 2 bar of CO at — I00°C was applied to a saturated solution of n-BuLi in liquid xenon. Surprisingly, no free CO was detected, but a stretching vibrational mode of the carbonyl adduct of the lithium alkyl was observed at 2047 cm (triple-bonded CO group). Warming up to —30°C led to the appearance of a new v(CO) peak at 1635 cm (double-bonded CO group), while the IR band of the carbonyl adduct vanished. The new absorption was therefore attributed to the acyllithium compound, which also decomposed at slightly higher temperature (—20°C) (equation 1) . ... 

Let us now focus our attention on the interaction between lithium alkyls and Group III derivatives. These species are often considered to be metalates with discrete MR4 ions present, but a variety of studies show that substantial metal-anion interactions occur both in solution and in the solid state (45, 96, 131). More thorough examination of both of the structures and spectroscopic properties of these derivatives shows that they must be included in any treatment involving electron-deficient bonding. 

Organolithum compounds (lithium alkyls) are the most valuable initiators in anionic polymerization.120168 169172-175 Since living anionic polymerization requires the fastest possible initiation, sec- or ferf-butyllithium is usually used. Lithium alkyls add readily to the double bond of styrene [Eq. (13.32)] or conjugated dienes and form free ions or an ion pair depending on the solvent ... 

SAMPLE SOLUTION (a) The metal lithium provides the base name for (CH3)3CLi. The alkyl group to which lithium is bonded is ferf-butyl, and so the name of this organometallic compound is fe/t-butyllithium. An alternative, equally correct name is 1,1-dimethylethyllithium. ... 

Mechanisms of the above type are very plausible but two points should be considered. Firstly, all these transition states are equally plausible for butadiene and isoprene whereas butadiene gives a mixed cis-trans product with lithium alkyls in hydrocarbons. Secondly, it is not certain that these carbon-lithium bonds are essentially covalent in hydrocarbons. There is evidence that the lithium compounds of conjugated monomers still exist as charge delocalized ion-pairs in the associated state in hydrocarbons (48). The characteristic ultra-violet absorption band attributable to this kind of anion pair persists almost unchanged in different solvents and alkali metals. The monomeric form active in the propagation step could possibly contain a more covalent carbon-lithium bond but we cannot be sure of this. 

Fig. 10. Proposed intermediate for C02 insertion into metal alkyl bond in the presence of lithium counterion.

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. 

The unique feature of the alkyllithium compounds that makes them useful as diene initiators is their character as exceedingly powerful bases yet they are soluble in organic solvents and quite thermally stable. Alkyllithium compounds are sufficiently basic to add to hydrocarbon monomers. However, lithium salts of stabilized anions, such as acetylide and fluorenyl anions, are too weakly basic to add to such double bonds. Similarly, alkoxides and mercaptides fail to react with hydrocarbon monomers, but lithium alkyl amides react analogously to alkyllithium compounds. 

A widely exploited procedure for bringing about carbenoid reactions of organic mono- and fifem-dihalides is by use of lithium alkyls. Examples are given in equations (11) and (12). Dimeric olefin formation, stereospecific cyclopropane formation from olefins, and insertion into carbon-hydrogen bonds have all been observed in suitable cases, together with further reactions of these products with excess of the lithium alkyl. 

Butyl lithium, sec-butyl lithium, tert-butyl lithium, and phenyl lithium cleave Te-phenyl and Te-alkyl bonds in tetrahydrofuran at — 78°. These reactions, in which on organic group bonded to tellurium is exchanged for the organic group associated with the organic lithium compound, are useful for the preparation of organic lithium compounds that are otherwise available only with difficulty or not at all. 

When lithium alkyl catalysts are used in non-solvating media such as aliphatic hydrocarbons, the polymer-lithium bond is not sufficiently ionic to initiate anionic polymerization so that the monomer must first complex with vacant orbitals in the lithium. A partial positive charge is induced on the monomer in the complex, and this facilitates migration of the polymer anion to the most electrophilic carbon of the complexed monomer. This type of polymerization is more appropriately termed coordinated anionic and will be discussed in the next section. There does not appear to be any evidence that alkyl derivatives of metals which are less electropositive than lithium and magnesium can initiate simple anionic polymerization. 

However, the well-known ability of organolithium compounds to form associated species or to form complexes with electron donor compounds (240—242) provides strong support for mechanisms involving cationic attack by the lithium cation on the monomer prior to an anionic addition. With three orbitals available for coordination, a monomeric lithium alkyl should be able to complex both double bonds of a diolefin to provide the orientation for making cis-1,4 polymer and still have an orbital available for forming associated species in hydrocarbon solvents. The lithium orbitals are presumed to be directed tetrahedrally. Looking at the top of a tetrahedron with the fourth lithium oibital above and normal to the plane of the paper, the complex could have structure A below. In the transition state B for the addition step, the structure... 

The structures of the organic derivatives of the Group IA and IIA metals are not simple because many of them involve molecular association. For example, the lithium alkyls are tetramers in which the lithium atoms reside at the corners of a tetrahedron and the carbon atoms bonded to them are located above the triangular faces of the tetrahedron as shown in Figure 7.2. 

A study35,36 concerning the ambidentity of the lithium salts of imines resulted in the discovery that the lithium is bonded to the nitrogen, e.g. compare 8 (equation 3). However, the evolution of the products on the carbon or nitrogen is influenced by the nature of the electrophile reactants like methyl iodide that ionize with difficulty will form a six-centered transition state 21 where the C-alkylation product will be evolved after elimination of lithium iodide. In the same way, the intermediate in aldol condensation is 22. 

Addition of carbon nucleophiles to the C=C bond of a compound la,b includes reactions of enolizable carbonyl compounds, enol ethers, and ena-mines, as well as lithium alkyls and zinc alkyls. Condensation of the enolizable ketone 68 with la,b (M = Cr, W)26 is induced, for example, by catalytic amounts of triethylamine in pentane and under these conditions affords a 90% yield of crystalline pyranylidene complex 57 directly from the reaction mixture.102 This reaction proceeds via the 2-ethoxy-l-metallatriene L, which, because of the presence of triethylamine, rapidly undergoes ring closure to the pyranylidene (pyrylium ylide) complex 69 by 1,6-elimination of ethanol (Scheme 22). Chromanylidene complexes 71 are obtained from condensation of a /3-tetraIone 70 (R = H, OMe) with compound 1a,b. 

Lithium Alkyl Cuprates. These important species are commonly used in ether or a similar solvent for a wide variety of organic syntheses. They are especially useful for C—C bond formation by interaction with organic halides ... 

Other metal alkyls, like triphenylmelhyl sodium and potassium benzyl, are sometimes used. They have lower solubilities in organic solvents than lithium alkyls because of the greater ionic character of the Na—C and K—C bonds. 

The character of the counterion and the solvent both affect the microstruclure of polymers made anionically from dienes. In general, the proportion of 1,4 chains is highest for Li and decreases with decreasing clecironegativity and increasing size of the alkali metals in the order Li > Na > K > Rb > Cs. A very high (>90%) 1,4 content is achieved only with lithium alkyl or lithium metal initiation in hydrocarbon solvents. The properties of polymers of conjugated diolefins tend to be like those of thermoplastics if the monomer enchainment is 1,2 or 3,4 [reactions (4-3) and (4-4)]. Elastomeric behavior is realized from 1,4 polymerization and particularly if the polymer structure is cis about ihe residual double bond. 

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