η1-(1S,2E)-1-(N, N-Diisopropylcarbamoyloxy)-3-trimethylsilyl-allyllithium-(-)-Spartein: Struktur einer chiralen, Carbamoyloxy-substituierten Allyllithium-Verbindung

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.

Enantioselective Lithiation of O-Alkyl and O-Alk-2-enyl Carbamates in the Presence of (−)-Sparteine and (−)-α-Isosparteine. A Theoretical Study

Quantum chemical DFT calculations at the B3LYP/6-31G(d) level have been used to study the enantioselective lithiation/deprotonation of O-alkyl and O-alk-2-enyl carbamates in the presence of (-)-sparteine and (-)-alpha-isosparteine. Complete geometry optimization of the precomplexes consisting of the carbamate, the chiral ligand, and the base (iPrLi), for the transition states of the proton-transfer reaction, and for the resulting lithio carbamates have been performed in order to quantify activation barriers and reaction energies. For the lithiodeprotonation of ethyl carbamate 12 in the presence of (-)-sparteine (5) the preferred abstraction of the pro-S proton (by 2.75 kcal/mol) gives the (S)-lithio derivate (S)-14, which is in good agreement with experimental observations. (-)-alpha-Isosparteine (6) is predicted to be significantly less selective favoring the abstraction of the pro-R proton by 1.2 kcal/mol. The O-alkenyl carbamate 17 as an example of an allylic carbamate is more easily lithiated than 12. As for 12 (-)-sparteine (5) favors the abstraction of the pro-S proton, although with smaller preference (0.7 kcal/mol). Structural parameters are discussed to rationalize the theoretical results.

Computational, ReactIR-, and NMR-spectroscopic investigations on the chiral formyl anion equivalent N-(alpha-lithiomethylthiomethyl)-4-isopropyl-5,5-diphenyloxazolidin-2-one and related compounds

The 4-isopropyl-3-methylthiomethyl-5,5-diphenyloxazolidin-2-one is readily lithiated in THF on the exocyclic CH2 group (1 --> 2) to give a synthetically useful chiral nucleophilic formylating reagent. We have now studied the lithiation reaction by ReactIR spectroscopy and the structure of the organolithium reagent by computational methods and by NMR-spectroscopic measurements. The lithiation is complete at -78 degrees C within 90 seconds, and it is accompanied by a decrease of the C=O wavenumber by 50 cm(-1). The NMR data (collected in [D8]THF) give no evidence for 13C,6Li coupling or for aggregation; from DPFGSE-ROE spectra the single diastereoisomer of the lithium compound 2 seen in the NMR spectra (NMR-spectroscopic measurements from -105 to -20 degrees C) is assigned like configuration; the 13C=O signal of the oxazolidinone undergoes a 7 ppm downfield shift upon lithiation (1 --> 2); line-shape analyses of the signals from the diastereotopic CH2 and CMe2 protons in lithiated 4,4-dimethyl-3-methylthiomethyloxazolidin-2-one (model compound 7) reveal a deltaH(double dagger) of 8.9 +/- 0.2 kcal mol(-1) for enantiomerization. The theoretical calculations provide an energy-minimum structure for the lithium compound 2 with coordination of the carbonyl oxygen to lithium, with an antiperiplanar arrangement of the C,Li and S,CH3 bonds, and with relative like configuration of the two stereocenters--in perfect agreement with the conclusions from the IR- and NMR-spectroscopic measurements!

Synthesis of Enantiomerically Enriched Allenes by (−)-Sparteine-Mediated Lithiation of Alkynyl Carbamates

[reaction: see text]. The alpha-deprotonation of alkynyl carbamates 3 with the chiral base (-)-sparteine (4)/n-butyllithium, transmetalation with ClTi(O(i)()Pr)3, and subsequent substitution with an aldehyde results in the formation of enantioenriched 4-hydroxyallenyl carbamates 11. Stereoselection is determined by dynamic resolution of the lithium/(-)-sparteine complexes by selective crystallization.

On the structure of sulfur-stabilized allyllithium compounds in solution

A combination of density functional calculations (B3LYP/6-31+G(d) level of theory) and experimental investigations (NMR and cryoscopic measurements) lead to structural assignments in solution for a series of three sulfur stabilized allyllithium compounds 1-3. All three lithium species are monomers in THF under the experimental conditions studied here and exist exclusively in an endo conformation. Increasing the oxidation state of sulfur (thiol --> sulfoxide --> sulfone) causes a change in the solution state structure of the allyllithium compounds. In the case of 1-thiophenylallyllithium 1, a fast equilibrium between two eta(1)-species is present with the equilibrium favoring the eta(1) C(alpha)-Li contact ion pair. The preference for this conformation can be attributed to the charge stabilizing properties of the sulfur substituent. For the lithiated sulfoxide 2, this equilibrium is frozen on the NMR time scale and two different lithium species (a eta(1) C(alpha)-Li and a eta(1) C(gamma)-Li contact ion pair), each possessing an intramolecular Li-O contact, coexist in d(8)-THF. In the case of the lithiated sulfone 3, several solvated conformations are in rapid equilibrium with each other on the NMR time scale in solution. The presence of two chelating oxygen atoms allows the lithium to form a OLiO scissor-like contact ion pair that competes with the eta(1) C(alpha)-Li and the eta(1) C(gamma)-Li contact ion pairs also calculated for compound 3.

Dianion aggregates derived from lithiation of N-silyl allylamine

[reaction: see text] Three different N-silyl-protected allylamines, i.e., N-TMS, N-TBDMS, and N-TIPS allylamine, were lihtiated by reaction with excess n-butyllithium. Crystallization of the resulting dianions and X-ray structure determination yields three uniquely different aggregates.