Fluorine... and pi...alkali metal interactions control in the stereoselective amide enolate alkylation with fluorinated oxazolidines (Fox) as a chiral auxiliary: an experimental and theoretical study

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

NMR spectroscopic studies reveal camphorsultam-derived sodium enolates known as Oppolzer enolates reside as monomers in neat THF and THF/HMPA solutions and as dimers in toluene when solvated by N,N,N',N'-tetramethylethylenediamine (TMEDA) and N,N,N',N'',N''-pentamethyldiethylenediamine (PMDTA). Density functional theory (DFT) computations attest to the solvation numbers. Rate studies show analogy with previously studied lithiated Oppolzer enolates in which alkylation occurs through non-chelated solvent-separated ion pairs. The origins of the selectivity trace to transition structures in which the alkylating agent is guided to the exo face of the camphor owing to stereoelectronic preferences imparted by the sultam sulfonyl moiety. Marked secondary-shell solvation effects are gleaned from the rate studies.

Detrifluoroacetylative in Situ Generation of Free 3-Fluoroindolin-2-one-Derived Tertiary Enolates: Design, Synthesis, and Assessment of Reactivity toward Asymmetric Mannich Reactions

The discovery of detrifluoroacetylative in situ generation of a new type of fluorinated amide enolates derived from 3-fluoroindolin-2-one and their asymmetric Mannich additions with sulfinylaldimines bearing fluoroalkyl groups is reported, which afforded α-fluoro-β-(fluoroalkyl)-β-aminoindolin-2-ones containing C-F quaternary stereogenic centers with excellent yields and high diastereoselectivities.

Mechanism of Lithium Diisopropylamide-Mediated Ortholithiation of 1,4-Bis(trifluoromethyl)benzene under Nonequilibrium Conditions: Condition-Dependent Rate Limitation and Lithium Chloride-Catalyzed Inhibition

Lithiation of 1,4-bis(trifluoromethyl)benzene with lithium diisopropylamide in tetrahydrofuran at -78 °C occurs under conditions at which the rates of aggregate exchanges are comparable to the rates of metalation. Under such nonequilibrium conditions, a substantial number of barriers compete to be rate limiting, making the reaction sensitive to trace impurities (LiCl), reactant concentrations, and isotopic substitution. Rate studies using the perdeuterated arene reveal odd effects of LiCl, including catalyzed rate acceleration at lower temperature and catalyzed rate inhibition at higher temperatures. The catalytic effects are accompanied by corresponding changes in the rate law. A kinetic model is presented that captures the critical features of the LiCl catalysis, focusing on the influence of LiCl-catalyzed re-aggregation of the fleeting monomer that can reside above, at, or below the equilibrium population without catalyst.

Lithium diisopropylamide-mediated lithiation of 1,4-difluorobenzene under nonequilibrium conditions: role of monomer-, dimer-, and tetramer-based intermediates and lessons about rate limitation

Lithiation of 1,4-difluorobenzene with lithium diisopropylamide (LDA) in THF at -78 °C joins the ranks of a growing number of metalations that occur under conditions in which the rates of aggregate exchanges are comparable to the rates of metalation. As such, a substantial number of barriers vie for rate limitation. Rate studies reveal that rate-limiting steps and even the choice of reaction coordinate depend on subtle variations in concentration. Deuteration shifts the rate-limiting step and markedly alters the concentration dependencies and overall rate law. This narrative is less about ortholithiation per se and more about rate limitation and the dynamics of LDA aggregate exchange.

Method of continuous variation: characterization of alkali metal enolates using ¹H and ¹⁹F NMR spectroscopies

The method of continuous variation in conjunction with (1)H and (19)F NMR spectroscopies was used to characterize lithium and sodium enolates solvated by N,N,N',N'-tetramethylethyldiamine (TMEDA) and tetrahydrofuran (THF). A strategy developed using lithium enolates was then applied to the more challenging sodium enolates. A number of sodium enolates solvated by TMEDA or THF afford exclusively tetramers. Evidence suggests that TMEDA chelates sodium on cubic tetramers.

Direct asymmetric syntheses of chiral aldehydes and ketones via N-acyl chiral auxiliary derivatives including chiral Weinreb amide equivalents

This article covers N-acyl chiral auxiliary-based approaches to the asymmetric synthesis of enantiopure aldehydes and ketones. The use of diastereoisomerically pure N-acyl derivatives of chiral auxiliaries (including chiral Weinreb amide equivalents) and their conversion to the corresponding enantiopure aldehydes and ketones in a single synthetic operation by treatment with a hydride reducing agent or an organometallic reagent, respectively, are highlighted.

25th anniversary article: Design of polymethine dyes for all-optical switching applications: guidance from theoretical and computational studies

All-optical switching--controlling light with light--has the potential to meet the ever-increasing demand for data transmission bandwidth. The development of organic π-conjugated molecular materials with the requisite properties for all-optical switching applications has long proven to be a significant challenge. However, recent advances demonstrate that polymethine dyes have the potential to meet the necessary requirements. In this review, we explore the theoretical underpinnings that guide the design of π-conjugated materials for all-optical switching applications. We underline, from a computational chemistry standpoint, the relationships among chemical structure, electronic structure, and optical properties that make polymethines such promising materials.

Lithium diisopropylamide-mediated ortholithiation of 2-fluoropyridines: rates, mechanisms, and the role of autocatalysis

Lithium diisopropylamide (LDA)-mediated ortholithiations of 2-fluoropyridine and 2,6-difluoropyridine in tetrahydrofuran at -78 °C were studied using a combination of IR and NMR spectroscopic and computational methods. Rate studies show that a substrate-assisted deaggregation of LDA dimer occurs parallel to an unprecedented tetramer-based pathway. Standard and competitive isotope effects confirm post-rate-limiting proton transfer. Autocatalysis stems from ArLi-catalyzed deaggregation of LDA proceeding via 2:2 LDA-ArLi mixed tetramers. A hypersensitivity of the ortholithiation rates to traces of LiCl derives from LiCl-catalyzed LDA dimer-monomer exchange and a subsequent monomer-based ortholithiation. Fleeting 2:2 LDA-LiCl mixed tetramers are suggested to be key intermediates. The mechanisms of both the uncatalyzed and catalyzed deaggregations are discussed. A general mechanistic paradigm is delineated to explain a number of seemingly disparate LDA-mediated reactions, all of which occur in tetrahydrofuran at -78 °C.

Mechanism of lithium diisopropylamide-mediated substitution of 2,6-difluoropyridine

Treatment of 2,6-difluoropyridine with lithium diisopropylamide in THF solution at -78 degrees C effects ortholithiation quantitatively. Warming the solution to 0 degrees C converts the aryllithium to 2-fluoro-6-(diisopropylamino)pyridine. Rate studies reveal evidence of a reversal of the ortholithiation and a subsequent 1,2-addition via two monomer-based pathways of stoichiometries [ArH*i-Pr(2)NLi(THF)](double dagger) and [ArH*i-Pr(2)NLi(THF)(3)](double dagger). Computational studies fill in the structural details and provide evidence of a direct substitution without the intermediacy of a Meisenheimer complex.