Structure and reactivity of lithium diisopropylamide in the presence of N,N,N',N'-tetramethylethylenediamine

Crystal structure of N,N,N',N'-tetra-methyl-ethane-di-amine

N,N,N′,N′-tetra­methyl­ethanedi­amine, C₆H₁₆N₂, crystallizes in the monoclinic crystal system in the space group P2₁/c. For the investigation of the conformation, quantum chemical methods were used and for inter­molecular inter­actions, a Hirshfeld surface analysis was performed.

The title compound N,N,N′,N′-tetra­methyl­ethanedi­amine, C₆H₁₆N₂, is a bidentate amine ligand commonly used in organolithium chemistry for deaggregation. Crystals were grown at 243 K from n-pentane solution. The complete mol­ecule is generated by a crystallographic center of symmetry and the conformation of the di­amine is anti­periplanar. To investigate the inter­molecular inter­actions, a Hirshfeld surface analysis was performed. It showed that H⋯H (van der Waals) inter­actions dominate with a contact percentage of 92.3%.

Deprotonative Metalation of Methoxy-Substituted Arenes Using Lithium 2,2,6,6-Tetramethylpiperidide: Experimental and Computational Study

The reaction pathways of lithium 2,2,6,6-tetramethylpiperidide (LiTMP)-mediated deprotonative metalation of methoxy-substituted arenes were investigated. Importantly, it was experimentally observed that, whereas TMEDA has no effect on the course of the reactions, the presence of more than the stoichiometric amount of LiCl is deleterious, in particular without an in situ trap. These effects were corroborated by the DFT calculations. The reaction mechanisms, such as the structure of the active species in the deprotonation event, the reaction pathways by each postulated LiTMP complex, the stabilization effects by in situ trapping using zinc species, and some kinetic interpretation, are discussed herein.

Monodentate coordination of the normally chelating chiral diamine (R,R)-TMCDA

After isolating an unusual binuclear, but monosolvated NaHMDS complex [{(R,R)-TMCDA}·(NaHMDS)₂]_∞ which polymerises via intermolecular electrostatic NaMe_HMDS interactions, further (R,R)-TMCDA was added to produce the discrete binuclear amide [{κ²-(R,R)-TMCDA}·(NaHMDS)₂{κ¹-(R,R)-TMCDA}], whose salient feature is the unique monodentate coordination of one of the chiral diamine ligands.

Solid-State and Solution Structures of Glycinimine-Derived Lithium Enolates

A combination of crystallographic, spectroscopic, and computational studies was applied to study the structures of lithium enolates derived from glycinimines of benzophenone and (+)-camphor. The solvents examined included toluene and toluene containing various concentrations of tetrahydrofuran, N,N,N',N'-tetramethylethylenediamine (TMEDA), (R,R)-N,N,N',N'-tetramethylcyclohexanediamine [(R,R)-TMCDA], and (S,S)-N,N,N',N'-tetramethylcyclohexanediamine [(S,S)-TMCDA]. Crystal structures show chelated monomers, symmetric disolvated dimers, S4-symmetric tetramers, and both S6- and D3d-symmetric hexamers. (6)Li NMR spectroscopic studies in conjunction with the method of continuous variations show how these species distribute in solution. Density functional theory computations offer insights into experimentally elusive details.

Synthetic and Structural Studies of Mixed Sodium Bis(trimethylsilyl)amide/Sodium Halide Aggregates in the Presence of η(2)-N,N-, η(3)-N,N,N/N,O,N-, and η(4)-N,N,N,N-Donor Ligands

When n-hexane solutions of an excess of sodium bis(trimethylsilyl)amide (NaHMDS) are combined with cesium halide (halide = Cl, Br, or I) in the presence of the tetradentate donor molecule [tris[2-(dimethylamino)ethyl]amine] (Me6TREN), the isolation and characterization of a series of sodium amide/sodium halide mixed aggregates was forthcoming. Cesium halide was employed because it efficiently reacted with NaHMDS to produce a molecular, soluble source of sodium halide salt (which was subsequently captured by an excess of NaHMDS) via a methathetical reaction. These mixed sodium amide/sodium halide complexes are formally sodium sodiates, are deficient in halide with respect to the amide, and have the general formula [{Na5(μ-HMDS)5(μ5-X)}{Na(Me6TREN)}] [where X = Cl (1), Br (2), or I (3)]. The influence of the donor ligand was studied for the NaI/NaHMDS system, and when n-hexane solutions of this composition were treated with tridentate donors such as N,N,N',N″,N″-pentamethyldiethylenetriamine (PMDETA) or N,N,N',N'-tetramethyldiaminoethyl ether (TMDAE), solvent-separated ion-pair cocomplexes [Na5(μ-HMDS)5(μ5-I)](-)[Na3(μ-HMDS)2(PMDETA)2](+) (4) and [Na5(μ-HMDS)5(μ5-I)](-)[Na(TMDAE)2](+) (5) were isolated. However, upon reaction with bidentate proligands such as the chiral diamine (R,R)-N,N,N',N'-tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA] or N,N,N',N'-tetramethylethylenediamine (TMEDA), neutral complexes [Na4(μ-HMDS)3(μ4-I)(donor)2] [donor = (R,R)-TMCDA (6) and TMEDA (7)] were produced. To illustrate the generality of the latter reaction with other halides, [Na4(μ-HMDS)3(μ4-Br)(TMEDA)2] (8) was also prepared by employing NaBr in the synthesis instead of NaI.

Synthetically important alkali-metal utility amides: lithium, sodium, and potassium hexamethyldisilazides, diisopropylamides, and tetramethylpiperidides

Most synthetic chemists will have at some point utilized a sterically demanding secondary amide (R2 N(-) ). The three most important examples, lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS), lithium diisopropylamide (LiDA), and lithium 2,2,6,6-tetramethylpiperidide (LiTMP)-the "utility amides"-have long been indispensible particularly for lithiation (Li-H exchange) reactions. Like organolithium compounds, they exhibit aggregation phenomena and strong Lewis acidity, and thus appear in distinct forms depending on the solvents employed. The structural chemistry of these compounds as well as their sodium and potassium congeners are described in the absence or in the presence of the most synthetically significant donor solvents tetrahydrofuran (THF) and N,N,N',N'-tetramethylethylenediamine (TMEDA) or closely related solvents. Examples of hetero-alkali-metal amides, an increasingly important composition because of the recent escalation of interest in mixed-metal synergic effects, are also included.

Structure determination using the method of continuous variation: lithium phenolates solvated by protic and dipolar aprotic ligands

The method of continuous variation (MCV) was used in conjunction with (6)Li NMR spectroscopy to characterize four lithium phenolates solvated by a range of solvents, including N,N,N',N'-tetramethylethylenediamine, Et2O, pyridine, protic amines, alcohols, and highly dipolar aprotic solvents. Dimers, trimers, and tetramers were observed, depending on the precise lithium phenolate-solvent combinations. Competition experiments (solvent swaps) provide insights into the relative propensities toward mixed solvation.

A hetero-alkali-metal version of the utility amide LDA: lithium-potassium diisopropylamide

Designed to extend the synthetically important alkali-metal diisopropylamide [N(i)Pr(2); DA] class of compounds, the first example of a hetero-alkali-metallic complex of DA has been prepared as a partial TMEDA solvate. Revealed by an X-ray crystallographic study, its structure exists as a discrete lithium-rich trinuclear Li(2)KN(3) heterocycle, with TMEDA only solvating the largest of the alkali-metals, with the two-coordinate lithium atoms being close to linearity [161.9(2)°]. A variety of NMR spectroscopic studies, including variable temperature and DOSY NMR experiments, suggests that this new form of LDA maintains its integrity in non-polar hydrocarbon solution. This complex thus represents a rare example of a KDA molecule which is soluble in non-polar medium without the need for excessive amounts of solubilizing Lewis donor being added.

Azaaldol condensation of a lithium enolate solvated by N,N,N',N'-tetramethylethylenediamine: dimer-based 1,2-addition to imines

The lithium enolate of tert-amylacetate solvated by N,N,N',N'-tetramethylethylenediamine (TMEDA) is shown to be a doubly chelated dimer. Adding the dimeric enolate to 4-fluorobenzaldehyde-N-phenylimine affords an N-lithiated β-amino ester shown to be monomeric using (6)Li and (15)N NMR spectroscopies. Rate studies using (19)F NMR spectroscopy reveal reaction orders consistent with a transition structure of stoichiometry [(ROLi)2(TMEDA)2(imine)](‡). Density functional theory computations explore several possible dimer-based transition structures with monodentate and bidentate coordination of TMEDA. Supporting rate studies using trans-N,N,N',N'-1,2-tetramethylcyclohexanediamine showing analogous rates and rate law suggest that TMEDA is fully chelated.

catena-Poly[sodium-μ2-(N,N,N',N'-tetra-methyl-ethane-1,2-diamine)-κ(2) N:N'-sodium-bis-[μ2-bis-(trimethyl-sil-yl)aza-nido-κ(2) N:N]]

The title compound, [Na2(C6H18NSi2)2(C6H16N2)] n , was found to consist of dimeric [Na(NSiMe3)2] units with crystallographically imposed centrosymmetry based upon four-membered NaNNaN rings. The dimers are bridged by N,N,N',N'-tetra-methyl-ethylenediamine ligands, which act in an unusual extended non-chelating coordination mode. This gives a one-dimensional coordination polymer that extends parallel to the a-axis direction.

Structural influence of steric factors and donor functions on lithium silylamides in non-coordinating solvents

We herein present the synthesis and crystallographic characterisation of lithium silylamides displaying different coordination numbers and aggregation states according to the number of N- and O-donor functions in the starting material, (aminomethyl) substituted silazane ligands. The dimeric dimethyl-(N-lithio-tert-butylamino)-piperidinomethyl)-silane and dimethyl-(N-lithio-iso-propylamino)-piperidinomethyl)-silane, with three-coordinate lithium centres, were prepared by deprotonation of the corresponding silazane with (n)BuLi. Using the tridentate silazane (1R,2R)-N(1)-[{(tert-butylamino)-dimethylsilyl}methyl]-N(1),N(2),N(2)-trimethylcyclohexane-1,2-diamine a mixed "dimer" of lithium silylamide and lithium silanolate with four-coordinate lithium centres was obtained. Additionally, a monomeric lithium silylamide was synthesised using the tridentate [bis(methoxyethyl)aminomethyl] side arm.

Computational studies of lithium diisopropylamide deaggregation

Density functional theory computations [MP2/6-31G(d)//B3LYP/6-31G(d)] on the deaggregation of lithium diisopropylamide (LDA) dimer solvated by two tetrahydrofuran ligands to give the corresponding trisolvated monomer show eight structurally distinct minima. The barriers to exchange are comparable to those expected from experimental studies showing rate-limiting deaggregations. The role of conformational isomerism in deaggregation and the extent that deaggregation rates dictate LDA reactivity under synthetically important conditions are considered.

Experimental and theoretical study of tunable 1,3-lithium shift of propargylic/allenylic species, transmetallation, and Pd-catalyzed cross-coupling reactions

The highly selective tuning of the isomerization from 1-arylalka-1,2-dien-1-yllithium to 1-arylalka-1,2-dien-3-yllithium has been realized in the deprotonation of 1-arylalk-1-yne (conditions A and B) and carbolithiation of 1-arylbut-3-en-1-yne with alkyllithium (conditions C and D). Subsequent transmetallation and Pd-catalyzed Negishi coupling reactions afforded 1,1-diaryl or 1,3-diaryl allenes with high selectivity. Deuterium-labeling cross experiments indicated that an intermolecular lithiation process occurred in both 1,3-lithium shift conditions (conditions B and D). 1-Arylalka-1,2-diene was confirmed experimentally to be the intermediate. A computational study at the B3LYP level for the isomerization indicated that the acidity of H at the 3-position is higher than that of the H at the 1-position of 1-phenyl-1,2-butadiene. Under conditions B, iPr(2)NH acts as a proton carrier to finish the 1,3-lithium shift. The overall activation barrier for the rate-determining step in the solvated models is approximately 21.0 kcal mol(-1), indicating that the isomerization is reasonable at room temperature. For the isomerization under conditions D, DFT calculations indicated that the addition of TMEDA (tetramethylethylenediamine) and HMPA (hexamethylphosphoramide) changes the global minimum of the system; among the possible mechanisms (P1-P5) considered, the mechanism catalyzed by dilithiated species (P5) is the most probable one. The overall activation barriers for isomerization in THF and TMEDA solvated models are 22.6 and 19.7 kcal mol(-1), respectively, proving that the isomerization may proceed at RT in THF or at -78 degrees C with TMEDA, due to the fact that the solvation of the additives may increase the concentration of 1-phenyl-1,2-butadienyllithium monomer by a deaggregation effect.

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.

Stereoselective alkylations of chiral nitro imine and nitro hydrazone dianions. Synthesis of enantiomerically enriched 3-substituted 1-nitrocyclohexenes

Dianions of chiral nitro imines (generated by a combination of LDA and s-BuLi) underwent diastereoselective alkylation with methyl, butyl, isopropyl, allyl, and methallyl iodides. In contrast to the behavior of simple metalloenamines, the most selective auxiliary contained no coordinating groups but did possess a large steric difference between the two substituents. The yield and selectivity of the alkylations were improved by the addition of HMPA or DMPU. The use of (S)-1-naphthylethylamine as the auxiliary afforded the R absolute configuration of the alkylation products. This stereochemical outcome could be rationalized by simple steric approach controlled alkylation in a conformationally fixed, internally coordinated dianion. A SAMP nitro hydrazone gave poorer yields and selectivities.

Anionic Snieckus-Fries rearrangement: solvent effects and role of mixed aggregates

Lithiated aryl carbamates (ArLi) bearing methoxy or fluoro substituents in the meta position are generated from lithium diisopropylamide (LDA) in THF, n-BuOMe, Me2NEt, dimethoxyethane (DME), N,N,N',N'-tetramethylethylenediamine (TMEDA), N,N,N',N'-tetramethylcyclohexanediamine (TMCDA), and hexamethylphosphoramide (HMPA). The aryllithiums are shown with (6)Li, (13)C, and (15)N NMR spectroscopies to be monomers, ArLi-LDA mixed dimers, and ArLi-LDA mixed trimers, depending on the choice of solvent. Subsequent Snieckus-Fries rearrangements afford ArOLi-LDA mixed dimers and trimers of the resulting phenolates. Rate studies of the rearrangement implicate mechanisms based on monomers, mixed dimers, and mixed trimers.

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.

Lithium diisopropylamide-mediated reactions of imines, unsaturated esters, epoxides, and aryl carbamates: influence of hexamethylphosphoramide and ethereal cosolvents on reaction mechanisms

Several reactions mediated by lithium diisopropylamide (LDA) with added hexamethylphosphoramide (HMPA) are described. The N-isopropylimine of cyclohexanone lithiates via an ensemble of monomer-based pathways. Conjugate addition of LDA/HMPA to an unsaturated ester proceeds via di- and tetra-HMPA-solvated dimers. Deprotonation of norbornene epoxide by LDA/HMPA proceeds via an intermediate metalated epoxide as a mixed dimer with LDA. Ortholithiation of an aryl carbamate proceeds via a mono-HMPA-solvated monomer-based pathway. Dependencies on THF and other ethereal cosolvents suggest that secondary-shell solvation effects are important in some instances. The origins of the inordinate mechanistic complexity are discussed.

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.

Synthesis and Pharmacological Evaluation of a Selected Library of New Potential Anti-inflammatory Agents Bearing the γ-Hydroxybutenolide Scaffold: a New Class of Inhibitors of Prostanoid Production through the Selective Modulation of Microsomal Prostaglandin E Synthase-1 Expression

As a part of our drug discovery effort, recently we clarified the molecular basis of phospholipase A2 (PLA2) inactivation by petrosaspongiolide M (PM), an interesting metabolite belonging to a marine sesterterpene family, containing in its structural architecture a gamma-hydroxybutenolide moiety and showing potent anti-inflammatory activity. In the attempt to expand structural diversity as well as to simplify crucial synthetic features of the parent compound, we decided to develop a selected library based on the densely functionalized gamma-hydroxybutenolide scaffold. The synthesized products were tested for their ability to inhibit PLA2 enzymes as well as to modulate the expression of inducible cyclooxygenase 2 (COX-2) and microsomal prostaglandin E synthase 1 (mPGES-1), two key enzymes highly involved in the inflammatory event, in order to discover new promising anti-inflammatory agents with better pharmacological profiles. This led us to the discovery of a promising inhibitor (4e) of prostanoid production acting by in vitro and in vivo selective modulation of microsomal prostaglandin E synthase 1 expression.

Density Functional Study of Lithium Hexamethyldisilazide (LiHMDS) Complexes: Effects of Solvation and Aggregation

The title compound, lithium hexamethyldisilazide (LiHMDS), has been studied using modern quantum-chemical methods in the form of the B3LYP approach. Monomers, dimers, trimers, and tetramers, microsolvated with up to four THF molecules have been considered. The choice of model complex is seen to be important-for instance, the simpler water molecule is shown to be an inappropriate substitute for the THF solvent. Calculated lithium NMR shieldings are reported, but by themselves, they seem to be insufficient for unequivocal assignments of the different species. The energetics of aggregation and solvation have been studied. Temperature effects are seen to be important, and the degrees of solvation and aggregation are higher at 0 K than at 298 K. The highest degree of THF solvation for the monomer and dimer is found to be three (0 K) and two (298 K), respectively. The highest possible degree of aggregation for unsolvated LiHMDS is four. However, in nonpolar solvents, formation of the LiHDMS dimer from the trimer is thermodynamically preferred. The pathway is likely to involve an intermediate tetramer. In THF solution, di-solvated monomers and dimers are the most likely species.

Enzyme inhibitors from marine invertebrates

Marine invertebrates are rich sources of small molecules with unique chemical skeletons and potent bioactivities. Historically, such compounds were discovered mainly through the use of assays for phenotype-oriented activities, such as cytotoxicity or antimicrobial effects. More recently, target-oriented searches for bioactive substances, as exemplified by enzyme inhibitors, have become much more common, given a growing need for small-molecule inhibitors essential for studies of complex processes at the interface of chemistry and biology. In this review, selected enzyme inhibitors from marine invertebrates are presented.

Mooloolabenes A-E, norsesterterpenes from the Australian sponge Hyattella intestinalis

Five new norsesterterpenes, mooloolabenes A-E (1-5), and the new sesterterpene mooloolaldehyde (6), related to the scalarane family of compounds, were isolated from an acetone extract of the Australian sponge Hyattella intestinalis. Structural elucidation, including relative stereochemical assignment, was based on spectroscopic analysis. All compounds tested showed cytotoxic activity against the P388 cell line.

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.

Lithiated Imines: Solvent-Dependent Aggregate Structures and Mechanisms of Alkylation

We describe efforts to understand the structure and reactivity of lithiated cyclohexanone N-cyclohexylimine. The lithioimine affords complex solvent-dependent distributions of monomers, dimers, and trimers in a number of ethereal solvents. Careful selection of solvent provides exclusively monosolvated dimers. Rate studies on the C-alkylations reveal chronic mixtures of monomer- and dimer-based pathways. We explore the factors influencing reactants and alkylation transition structures and the marked differences between lithioimines and isostructural lithium dialkylamides with the aid of density functional theory calculations.