Enantioselective Carbolithiations. In: Hodgson DM, editor. Organolithiums in Enantioselective Synthesis. Berlin: Springer Berlin Heidelberg; 2003. pp. 287-310. [DOI: 10.1007/3-540-36117-0_9]

Inter- and intramolecular enantioselective carbolithiation reactions

In this review we summarize recent developments in inter- and intramolecular enantioselective carbolithiation reactions carried out in the presence of a chiral ligand for lithium, such as (-)-sparteine, to promote facial selection on a C=C bond. This is an attractive approach for the construction of new carbon-carbon bonds in an asymmetric fashion, with the possibility of introducing further functionalization on the molecule by trapping the reactive organolithium intermediates with electrophiles.

Intramolecular Carbolithiation of 2,6-Dilithio-1,6-heptadienes: An Experimental and Theoretical Study

2,6-Dilithio-1,6-heptadienes 3 undergo intramolecular carbolithiation in Et(2)O/N,N,N',N'-tetramethylethylenediamine (TMEDA) at the lithiated double bonds to afford 1,2-bis(lithiomethyl)cyclopentenes 5. Reaction of these dianions with electrophiles affords a number of 1,2-difunctionalized cyclopentene derivatives 7-10. The ease of carbolithiation of 2,6-dilithio-1,6-heptadiene (3a) compared to that of 2-lithio-1,6-heptadiene (14) has been studied experimentally. A series of ab initio molecular-orbital calculations on the course of the reaction were carried out and the results were compared to those for the corresponding intramolecular carbolithiation of an isolated double bond. The Li-C interactions found in the transition state by this theoretical study support a carbolithiation pathway for the cyclization of 2,6-dilithio-1,6-heptadienes.

Lithium Diisopropylamide-Mediated Enolization: Catalysis by Hemilabile Ligands

Structural, kinetic, and computational studies reveal the mechanistic complexities of a lithium diisopropylamide (LDA)-mediated ester enolization. Hemilabile amino ether MeOCH2CH2NMe2, binding as an eta1 (ether-bound) ligand in the reactant and as an eta2 (chelating) ligand in the transition structure, accelerates the enolization 10,000-fold compared with n-BuOMe. At the onset of the reaction, a dimer-based enolization prevails. As the reaction proceeds, significantly less reactive LDA-enolate mixed dimers appear and divert the reaction through monomer- and mixed dimer-based pathways. The mechanistic and computational investigations lead to a proof-of-principle ligand-catalyzed enolization in which an ancillary ligand allows the catalytic ligand to re-enter the catalytic cycle.

Intramolecular Carbolithiation of Allylo-Lithioaryl Ethers: A New Enantioselective Synthesis of Functionalized 2,3-Dihydrobenzofurans

A new and easy method for the diastereoselective synthesis of 3-functionalized 2,3-dihydrobenzofuran derivatives from allyl 2-bromoaryl ethers is described. The key step of this transformation involves an intramolecular carbolithiation reaction of allyl 2-lithioaryl ethers. The substituents in both the allyl and the aryl moieties play an important and decisive role in stopping the reaction at the benzofuran thus avoiding a gamma-elimination reaction. Finally, this process is amenable to the synthesis of enantiomerically enriched compounds by using (-)-sparteine as a chiral inductor.