Lithium bonding

You can see how the alkyl-lithium acts as tlie synthon CH3CH2 since the carbon-lithium bond breaks so that the electrons go with the carbon atom. Suggest a disconnection for TM 16. 

Methyllithium has a polar covalent carbon-lithium bond... 

ButyUithium is available as a 15—20 wt % solution in //-pentane or heptane. Noticeable decomposition occurs after alb reflux in heptane (bp 98.4°C) but not after a 15 min reflux in ben2ene (bp 80.1°C) or hexane (bp 68°C). /-ButyUithium in pentane or heptane is more stable than //-butyUithium in hexane (125). Solutions of /-butyUithium in pentane and heptane are flammable Hquids and are considered pyrophoric. The /-butyl compound is more reactive than either the n- and sec-huty. Di-//-butylether is cleaved by /-butyUithium in 4—5 h at 25°C, compared to the 2 d for j iZ-butyUithium and 32 d for //-butyUithium (126). /-ButyUithium can be assayed by aU of the techniques used for //-butyUithium. /-ButyUithium is a useful reagent in syntheses where the high reactivity of the carbon—lithium bond and smaU si2e of the lithium atom promote the synthesis of stericaUy hindered compounds, eg,... 

Compounds Containing Lithium Bonded to a Transition Metal. 

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... 

One of the most important features of dipole-stabilized a-amino-organolithium compounds is the fact that the carbon-lithium bond is in the nodal plane of the Jt orbitals... 

The short Li—Li distances (often much shorter than those in the metal and in the Li2 molecule) found in the structures of many organolithium compounds do not imply that lithium-lithium bonding is of thermodynamic importance in these species. Close approach of Li centers is usually dictated by the need for acute angles at bridging atoms (E), e.g., of (ELi)n rings (n = 2 and 3 especially). Thus, the distances represent the optimization of maximized E—Li bridge bonding and minimized Li8+—Li8+ repulsions. 

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. 

Flowever, the electrons of a covalent bond are not necessarily shared equally by the bonded atoms, especially when the affinities of the atoms for electrons are very different. Thus, carbon-fluorine and carbon-lithium bonds, although they are not ionic, are polarized such that the electrons are associated more with the atom of higher electron affinity. This is usually the atom with the higher effective nuclear charge. 

Due to its high ionic character, the carbon-lithium bond is very reactive and adds under mild conditions to ethylene or dienes and under more severe conditions to other alkenes. Some functionalized alkenes can be used, and high regio- and stereo-selectivity is usually observed in these carbolithiation reactions, especially if a precoordination of the lithium organometallic with the alkene is possible. Intramolecular carbolithiations of alkenes proceed under mild conditions and allow the preparation of several stereochemically well defined mono- and bi-cyclic compounds. Alkynes are too reactive, and can lead, with organolithium derivatives, to several side reactions, and seldom afford the desired carbolithiated product in good yield. 

The allylic- and benzylic-lithium active centers, which can be characterized as having polarized covalent carbon-lithium bonds in hydrocarbon solvents, have been extensively studied with regard to their structure, their kinetic behavior in the propagation event, and their association states. These latter two topics are the subject of this... 

Furthermore, several of Worsfold s assessments seem to be open to question. The assertion that the association (between the allylic-lithium active centers) is between ionic species can be contrasted with the evidence provided by NMR spectroscopy 36,134 143) which has shown that the carbon-lithium bond of allylic-lithium species can possess considerable covalent character. Worsfold has also previously published 43 > concentrated solution viscosity results where the ratio of flow times, before and after termination, of a poly(isoprenyl)lithium solution was about 15. This finding is clearly incompatible with the conclusion that viscometry cannot detect the presence of aggregates greater than dimeric. 

---