Lithium-alkene interactions

Lithium can interact with 7r-systems in alkenes, alkynes and arenes. This may account for the ability of lithium alkyls to initiate the polymerization of dienes (p. 48). 

Tetrasubstituted alkenes (214) were obtained with high Z selectivity (>99 1) by reaction of ynolates (211) with a-oxy- and a-amino-ketones (212 X = OR, NR2) (g) at room temperature. According to experimental and theoretical studies, the high Z selectivity is induced by orbital interactions in the ring opening of the /3-lactone enolate intermediate (213), rather than by the initially presumed chelation of the lithium atom.260... 

To the best of our knowledge, X-ray structural data of complexes with simple dihapto interactions between a lithium atom and the n system of an alkene or alkyne ligand are unknown, but there is some spectroscopic evidence for weak it interactions in solutions of 3-alkenyllithium compounds from 7Li-and H-NMR data (4). Interactions of this sort are presumably important in addition (polymerization) reactions between organolithium compounds and alkenes or alkynes. 

The key to the high stereoselectivity of these reactions, relative to their radical counterparts, is their late, product-like transition state.152 As early as 1974, it was suggested by Oliver that cyclisations of organolithiums onto unactivated alkenes was promoted by an interaction between the lithium atom and the C=C double bond. There is now plenty of evidence that this is the case in 1991, ab initio calculations by Bailey and coworkers125 showed that not only is there a sound theoretical basis for treating structures such as 338-340 as reasonable... 

Solvent plays an important role in these reactions. A highly basic solvent, usually THF, is used to provide stabilization of the lithium halocarbenoid, due to coordination which disrupts the internal Li-Br interaction which would otherwise lead to a-elimination (equation 20). The extreme low temperature of many of the reactions requires special solvent systems, commonly a mixture of THF, diethyl ether, and petroleum ether or pentane in a ratio of 4 1 1 (Trapp mixture). Solvent has also been noted to alter the mechanistic course of the reaction for example, reaction of ketones with lithiobromomethane in hexane was observed to give rise to alkenes rather than epoxides. ... 

Ti, Zr and Hf enolates do not react via a transition state (29) akin to the (70) proposed later for enolates with lithium as a small counterion. With its large counterion, (29) would exhibit a destabilizing interaction between the voluminous cyclopentadienyl ligands and the substituent R . No such interaction occurs in the preferred transition state (30). (30) precedes m-alkenes (31) with syn configuration (Scheme 3). 

As indicated above, (j5-hydroxyalkyl)phosphonic diesters can be obtained by protonation of the ions 154 under carefully controlled conditions, and the formation of such products (unaccompanied by an alkene) has been observed directly in the interaction of benzophenone and the anion from diethyl methylphosphonate, and also from dialkyl (prop-2-enyl)phosphonate anions and aldehydes under kinetic control The ionic intermediates, such as 154 (Z = Ph, CN or COOEt) from PhCHO, are stabilized in the presence of lithium or magnesium ions, so aiding in the isolation of the corresponding hydroxyalkyl)phosphonic diesters The addition of KOBu to 154b, prepared by an independent route, produces the orange colour characteristic of the ions from 153 more-... 

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