Lithium Amino Alkoxide-Evans Enolate Mixed Aggregates: Aldol Addition with Matched and Mismatched Stereocontrol

Lithiated Oppolzer Enolates: Solution Structures, Mechanism of Alkylation, and Origin of Stereoselectivity

Camphorsultam-based lithium enolates referred to colloquially as Oppolzer enolates are examined spectroscopically, crystallographically, kinetically, and computationally to ascertain the mechanism of alkylation and the origin of the stereoselectivity. Solvent- and substrate-dependent structures include tetramers for alkyl-substituted enolates in toluene, unsymmetric dimers for aryl-substituted enolates in toluene, substrate-independent symmetric dimers in THF and THF/toluene mixtures, HMPA-bridged trisolvated dimers at low HMPA concentrations, and disolvated monomers for the aryl-substituted enolates at elevated HMPA concentrations. Extensive analyses of the stereochemistry of aggregation are included. Rate studies for reaction with allyl bromide implicate an HMPA-solvated ion pair with a ⁺Li(HMPA)₄ counterion. Dependencies on toluene and THF are attributed to exclusively secondary-shell (medium) effects. Aided by density functional theory (DFT) computations, a stereochemical model is presented in which neither chelates nor the lithium gegenion serves roles. The stereoselectivity stems from the chirality within the sultam ring and not the camphor skeletal core.

Cooperative Stereoinduction in Asymmetric Photocatalysis

Stereoinduction in complex organic reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chemical reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochemical reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Brønsted acid strategy for highly enantioselective [2 + 2] photocycloadditions of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. These results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochemical reactions.

CeCl3 /n-BuLi: Unraveling Imamoto's Organocerium Reagent

CeCl₃(thf) reacts at low temperatures with MeLi, t‐BuLi, and n‐BuLi to isolable organocerium complexes. Solvent‐dependent extensive n‐BuLi dissociation is revealed by ⁷Li NMR spectroscopy, suggesting “Ce(n‐Bu)₃(thf)_x” or solvent‐separated ion pairs like “[Li(thf)₄][Ce(n‐Bu)₄(thf)_y]” as the dominant species of the Imamoto reagent. The stability of complexes Li₃Ln(n‐Bu)₆(thf)₄ increases markedly with decreasing Ln^III size. Closer inspection of the solution behavior of crystalline Li₃Lu(n‐Bu)₆(thf)₄ and mixtures of LuCl₃(thf)₂/n‐BuLi in THF indicates occurring n‐BuLi dissociation only at molar ratios of <1:3. n‐BuLi‐depleted complex LiLu(n‐Bu)₃Cl(tmeda)₂ was obtained by treatment of Li₂Lu(n‐Bu)₅(tmeda)₂ with ClSiMe₃, at the expense of LiCl incorporation. Imamoto's ketone/tertiary alcohol transformation was examined with 1,3‐diphenylpropan‐2‐one, affording 99 % of alcohol.

Wittig Rearrangements of Boron-Based Oxazolidinone Enolates

[2,3]-Sigmatropic rearrangements (Wittig rearrangements) of α-alkoxy oxazolidinone enolates are described. Whereas alkali metal enolates fail, owing to facile deacylation, boron enolates generated from di-n-butylboron triflate and triethylamine rearranged in good yields and high selectivities with exceptions noted. IR and NMR spectroscopies show the boron is chelated by the α-alkoxy group rather than the more distal oxazolidinone carbonyl in the complex and enolate. The rearrangement product contains a boron alkoxide that remains unchelated by either carbonyl. Optimization was guided by density functional theory computations, suggesting that valine-derived oxazolidinones would be superior to the phenylalanine-derived analogues.

Use of chiral auxiliaries in the asymmetric synthesis of biologically active compounds: A review

This review article describes the use of some of the most popular chiral auxiliaries in the asymmetric synthesis of biologically active compounds. Chiral auxiliaries derived from naturally occurring compounds, such as amino acids, carbohydrates, and terpenes, are considered essential tools for the construction of highly complex molecules. We highlight the auxiliaries of Evans, Corey, Yamada, Enders, Oppolzer, and Kunz, which led to remarkable progress in asymmetric synthesis in the last decades and continue to bring advances until the present day.

Pseudophedrine-Derived Myers Enolates: Structures and Influence of Lithium Chloride on Reactivity and Mechanism

The structures and reactivities of pseudoephedrine-derived dianionic Myers enolates are examined. A combination of NMR and IR spectroscopic, crystallographic, and computational data reveal that the homoaggregated dianions form octalithiated tetramers displaying S4-symmetric Li8O8 cores and overall C2 symmetry. Computational and isotopic labeling studies reveal strong N-Li contacts in the carboxamide enolate moiety. The method of continuous variations proves deceptive, as octalithiated tetrameric homoaggregates afford hexalithiated trimeric heteroaggregates. A lithium diisopropylamide-lithium enolate mixed aggregate is found to be a C2-symmetric hexalithiated species incorporating two enolate dianions and two lithium diisopropylamide (LDA) subunits. Structural and rate studies show that lithium chloride has little effect on the dynamics of the enolate homoaggregates but forms adducts of unknown structure. Rate studies of alkylations indicate that the aging of the aggregates can have effects spanning orders of magnitude. The LiCl-enolate adduct dramatically accelerates the reaction but requires superstoichiometric quantities owing to putative autoinhibition. Efforts and progress toward eliminating the requisite large excess of LiCl are discussed.

Structures and Reactivities of Sodiated Evans Enolates: Role of Solvation and Mixed Aggregation on the Stereochemistry and Mechanism of Alkylations

Oxazolidinone-based sodiated enolates (Evans enolates) were generated using sodium diisopropylamide (NaDA) or sodium hexamethyldisilazide (NaHMDS) in the presence of N,N,N',N'-tetramethylethylenediamine (TMEDA), ( R,R)- trans- N,N,N',N'-tetramethylcyclohexanediamine [( R,R)-TMCDA], or ( S,S)-TMCDA. 13C NMR spectroscopic analysis in conjunction with the method of continuous variations (MCV), x-ray crystallography, and density functional theory (DFT) computations revealed the enolates to be octahedral bis-diamine-chelated monomers. Rate and computational studies of an alkylation with allyl bromide implicate a bis-diamine-chelated-monomer-based transition structure. The sodiated Evans enolates form mixed dimers with NaHMDS, NaDA, or sodium 2,6-di- tert-butylphenolate, the reactivities of which are examined. Stereoselective quaternizations, aldol additions, and azaaldol additions are described.