Organolithium Reagents in Pharmaceutical Asymmetric Processes

Anatomy of long-lasting love affairs with lithium carbenoids: past and present status and future prospects

After a long adolescence, the chemistry of lithium carbenoids has currently been entering its maturity bringing a dowry of a more in-depth and less empirical knowledge of the structure and configurational stability of such double-faced intermediates; this, thanks in particular to the synergistic and harmonic cooperation between calculations and the most modern NMR techniques now at our disposal. Such knowledge has stimulated the development of fruitful stereoselective applications in the field of organic synthesis, providing in addition a rationale to observed selectivities. Such aspects together with the role played by aggregation and solvation on the structure-reactivity relationship are highlighted throughout this Minireview with selected examples extracted from recent literature.

Lithium phenolates solvated by tetrahydrofuran and 1,2-dimethoxyethane: structure determination using the method of continuous variation

The method of continuous variation in conjunction with (6)Li NMR spectroscopy was used to characterize lithium phenolates solvated by tetrahydrofuran and 1,2-dimethoxyethane. The strategy relies on the formation of ensembles of homo- and heteroaggregated phenolates. The symmetries and concentration dependencies of the heteroaggregates attest to the aggregation numbers of the homoaggregates. The structurally diverse phenols afford substrate- and solvent-dependent combinations of lithium phenolate monomers, dimers, trimers, tetramers, and pentamers. We discuss the refinement of protocols for characterizing O-lithiated species. Computational studies examine further the substituent and solvent dependencies of aggregation.

Solution structures of lithium enolates, phenolates, carboxylates, and alkoxides in the presence of N,N,N',N'-tetramethylethylenediamine: a prevalence of cyclic dimers

The method of continuous variation was used to characterize lithium enolates, phenolates, carboxylates, and alkoxides solvated by N,N,N',N'-tetramethylethylenediamine (TMEDA). The method relies on characterizing an ensemble of homo- and heteroaggregates using (6)Li NMR spectroscopy. A combination of aggregate counts and symmetries, nearly statistical distributions, and quantitative parametric fits revealed that cyclic dimers are the dominant forms. Nonstatistical distributions favoring heteroaggregated dimers were observed when hindered enolates and carboxylates were mixed with unhindered enolates. Hindered (tertiary) alkoxides form higher aggregates (possibly hexamers), whereas hindered lithium phenolates appear to form TMEDA-solvated monomers.

Lithium hexamethyldisilazide-mediated enolizations: influence of triethylamine on E/Z selectivities and enolate reactivities

Lithium hexamethyldisilazide (LiHMDS) in triethylamine (Et 3N)/toluene is shown to enolize acyclic ketones and esters rapidly and with high E/ Z selectivity. Mechanistic studies reveal a dimer-based mechanism consistent with previous studies of LiHMDS/Et 3N. E/ Z equilibration occurs when <2.0 equiv of LiHMDS are used. Studies of the aldol condensation and Ireland-Claisen rearrangement of the resulting Et 3N-solvated enolates show higher and often complementary diastereoselectivities when compared with analogous reactions in THF. The Et 3N-solvated enolates also display a marked (20-fold) acceleration of the Ireland-Claisen rearrangement with evidence of autocatalysis. A possible importance of amine-solvated enolates is discussed.

Lithium enolates of simple ketones: structure determination using the method of continuous variation

The method of continuous variation in conjunction with 6Li NMR spectroscopy was used to characterize lithium enolates derived from 1-indanone, cyclohexanone, and cyclopentanone in solution. The strategy relies on forming ensembles of homo- and heteroaggregated enolates. The enolates form exclusively chelated dimers in N,N,N',N'-tetramethylethylenediamine and cubic tetramers in tetrahydrofuran and 1,2-dimethoxyethane.

The renaissance of H/D exchange

The increasing demand for stable isotopically labeled compounds has led to an increased interest in H/D-exchange reactions at carbon centers. Today deuterium-labeled compounds are used as internal standards in mass spectrometry or to help elucidate mechanistic theories. Access to these deuterated compounds takes place significantly more efficiently and more cost effectively by exchange of hydrogen by deuterium in the target molecule than by classical synthesis. This Review will concentrate on the preparative application of the H/D-exchange reaction in the preparation of deuterium-labeled compounds. Advances over the last ten years are brought together and critically evaluated.

Theoretical investigation on carbon-centered tri-s-tetrazine and its 10 derivatives

A novel species, carbon-centered tri-s-tetrazine (C(4)N(9)H(3)), and its 10 derivatives (C(4)N(9)R(3), where R=OH, F, CN, N(3), NH(2), NO(2), N=NH, N(2)H(3), C triple bond CH, and CH=CH(2)) have been studied computationally. Density functional theory (DFT) has been used to study the geometries, electronic structure, harmonic vibrational frequencies, ionization energies of the 11 compounds at the restricted (for neutrals) and the unrestricted (for cations) B3LYP/cc-pVDZ level of theory. Atoms in molecule (AIM) and natural bond orbital (NBO) analyses have been used to obtain the bonding properties. Valence bond (VB) theory is applied to explain the unusual pyramidal structure around the carbon-center and electron arrangements of orbitals. We found: (1) All the species possess novel bonding features and geometrical structures. The atoms on the periphery of each species are sp(2) hybridized. Each of these atoms offers an orbital to form an extensive conjugation system (12)pi(15) (a pi system consisting of 12 centers and 15 electrons). The central carbon atom C13 is sp(3) hybridized, which makes the non-planar molecule shape like a straw-hat. Atom C13 also participates in the conjugated pi system with its sp(3) hybridized orbital, thus forming an extensive (13)pi(16) conjugate pi system covering the whole C(4)N(9) framework. (2) The change of charge on C13 is the largest among all the atoms when the species is ionized and the atomic charges are redistributed. In other words, C13 is the attack center for electrophilic agents. Thus, the species is carbanion-like. (3) All the species have low ionization energies (IEs). The electron ionized mainly comes from C13. They may have wide applications in organic chemistry, in organometallic chemistry and in alkyl lithium chemistry once they are synthesized.

Lithium diisopropylamide: solution kinetics and implications for organic synthesis

Lithium diisopropylamide (LDA) is a prominent reagent used in organic synthesis. In this Review, rate studies of LDA-mediated reactions are placed in the broader context of organic synthesis in three distinct segments. The first section provides a tutorial on solution kinetics, emphasizing the characteristic rate behavior caused by dominant solvation and aggregation effects. The second section summarizes substrate- and solvent-dependent mechanisms that reveal basic principles of solvation and aggregation. The final section suggests how an understanding of mechanism might be combined with empirical methods to optimize yields, rates, and selectivities of organolithium reactions and applied to organic synthesis.