Organolithium compounds electronic structure

Compound 388 is an acylating agent for electron-deficient alkenes, in a Michael addition process. It is formed by treating molybdenum hexacarbonyl with an organolithium compound, followed by quenching the intermediate 387 with boron trifluoride (equation 104). The structure of 388 (R = Ph) can be elucidated by NMR spectroscopy. Other examples of enantioselective and diastereoselective Michael-type additions involving lithium-containing intermediates in the presence of chiral additives can be found elsewhere in the literature . 

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 highly polar character of organolithium compounds causes strong association. The geometry of the coordination sphere is determined essentially by steric effects, as in ionic structures, rather than by interaction of electron pairs. Even where lithium may appear to possess an octet configuration, it is not envisaged that the electrons are strongly held by the valence orbitals of the metal in covalent bonds. 

The dissociation energy of alkyllithium is very large. In the case of MeLi (dimer, trimer and tetramer) they are —42, —82 and — 128kcaI/mole, respectively [42]. Organolithium compounds are non-transition metal compounds but they can form t-bond structures. The elements of non-transition metal compounds which can form the 7t-bond, are Na, Be, Mg, Ca, B, Al, Ga, In, Tl, Ge, Sn, Pb, P, As, Sb, S, Se, Te, etc., besides Li [44]. The olefinic rr-bond with transition metals is well-known the coordination of the Tt-bond is such that the electrons of the olefinic n bond are donated to the vacant d orbitals, and the backdonation of the rr-bond is such that the electrons of the metal d-orbitals are donated to the antibonding n orbital of the olefin. However, as non-transition metals have no vacant d orbitals, the r-electrons of olefins only partially move to the s- or p-orbital of the metal. Then, the electrons largely remain in a non-bonding orbital, and the backdonation is therefore almost none [44]. 

Trimethylbenzene has three electron-donating methyl groups and it is liable to form / -complexes compared with benzene. However, in the case of iron complexes, it is liable to form Fe complexes as shown in Table 15.2 and eq. (15.21), and the reaction with organolithium compound yields a ferrocene-like sandwich structure by the alkylation of the benzene ring [66]. 

Currently, the analysis of the molecular structure based on topological examination of the electron density has gained wide acceptance [19,21-26]. This approach has led to some unexpected conclusions as to the character of bonds in certain compounds. For example, a study of the C—Li bonds in organolithium compounds [27] has shown that in compound V, notwithstanding the large distance between the carbon atoms and C4 equal to 3.121 A, there... 

Insight into the structure of organolithium compounds can be deduced from their X-ray crystal structures, recognizing that the energetics in the solid state may perturb the structure compared to the structure in solution [3]. A model for 1,1-diphenylalkyllithium compounds has been provided by the 2-electron reduction of 1,1-diphenylethylene with lithium metal in diethyl ether as shown in Eq. (5) [52] ... 

The products obtained from the reaction of (chloromethyl)trimethylsilane with organolithium reagents depend very much on the structure of the lithium compound. While lithium 2,2,6,6-tetramethylpiperidide initiates an a-elimination as described above, the treatment with sec-butyllithium leads to the formation of chloro(trimethylsilyl)methyllithium (11). This reagent cyclopropanates an electron-deficient alkene through sequential Michael addition and intramolecular ring closure (MIRC reaction), for example, the formation of cyclopropane 12. 

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