Lithium-oxygen interaction

The other interactions which must be considered are the Lithium-proton interaction, and the Lithium-Oxygen interaction. The former is purely repulsive. 

Recently, Boche et al. (5) have reported the synthesis of a highly interesting crystalline adduct of 1,3-diphenylallyllithium with diethyl ether. The crystal structure of this complex (I in Fig. 1) shows symmetrical it bonding between lithium atoms and allylic fragments. Each lithium atom interacts with two allylic 7t systems and further with the oxygen atom of a diethyl ether molecule. An exo, exo orientation of the phenyl ligands has been observed in this coordination polymer. 

The preferred formation of the kinetically favored (Z)-silylketene acetal with amide bases in THF can be rationalized by a cyclic transition state model (128) that enables a close interaction between Li cation, carbonyl oxygen and base (Scheme 23). The presence of additives such as HMPA or DMPU results in a greater degree of solvation of the lithium cation and a weakened Li -caibonyl oxygen interaction. Accordingly, the association between base and ester is diminished and the 1,3-diaxial strain in transition state (129) is reduced, whereas transition state (128) is still destabilized by A -strain." In the presence of a slight excess of ester in the enolization mixture, a kinetic resolution process accounts for an additional increase in the ratio of the ( )- vj. the (Z)-lithium enolate (Table 3). ° ... 

The structural information at an atomic level is essential for understanding the various properties of supercooled and glassy solutions. X-ray and neutron diffraction enables us to obtain direct structure information (bond distance and coordination number) of ionic solutions in terms of the radial distribution function. In the case of aqueous lithium halide solutions. X-ray diffraction data are dominated by halide-oxygen, halide-oxygen, and oxygen-oxygen interactions. On the contrary, neutron isotopic substitution... 

In the lithiation of fluoroanisoles (15) and (16) and their derivatives, butyllithium exclusively deprotonates the less acidic protons from the position ortho to the alkoxyl group. On the other hand, deprotonation takes place at the more acidic site, i.e. the ortho-position next to the fluorine substituent on reaction of the substrates with super bases, such as BuLi— t-BuOK or BuLi—N,N,N/,N//,N//-pentamethyldiethylenetriamine, in which lithium cation is stabilized by chelation in the combined base-system (Scheme 3.5) [ 14]. The lithium cation interacts preferentially with the more Lewis basic alkoxyl oxygen to locate butyllithium close to the position ortho to the alkoxyl group, enhancing kinetic deprotonation (see 17 in Scheme 3.5). 

They further report a bond length for the litbitin-Oxygen "bond" of 1.91 A. From the point o viev the Polarization Model, there exists no such thinn as lone-pair electrons. The non-planar structtire nevertheless occurs unless one asstimes that the Lithium proton interaction should be made more repulsive than Coulombic. 

Most other studies have indicated considerably more complex behavior. The rate data for reaction of 3-methyl-l-phenylbutanone with 5-butyllithium or n-butyllithium in cyclohexane can be fit to a mechanism involving product formation both through a complex of the ketone with alkyllithium aggregate and by reaction with dissociated alkyllithium. Evidence for the initial formation of a complex can be observed in the form of a shift in the carbonyl absorption band in the IR spectrum. Complex formation presumably involves a Lewis acid-Lewis base interaction between the carbonyl oxygen and lithium ions in the alkyllithium cluster. 

Electrostatic and orbital interactions may steer reaction toward either carbon or oxygen. First, examine the electrostatic potential map for cyclohexanone lithium enolate. Which atom is more negatively charged, carbon or oxygen Is the difference significant If it is, what would be the favored mode of addition Does either methylation or silylation appear to be guided by electrostatics Explain. 

The anion is assumed to exist preferentially in the anti conformation A, which offers a stabilizing 7rc -> minimizes steric interactions69,71 11. The nonplanar nature of the carhanionic center and coordination of the lithium to the oxygen atom could be demonstrated for a related compound by an X-ray analysis72. Usually, the electrophile attacks the carhanionic center with stereoinversion. 

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