Lithium diisopropylamide: solution kinetics and implications for organic synthesis

A strategy for the controllable generation of organic superbases from benchtop-stable salts

Organic superbases are a distinct class of strong base that enable numerous modern reaction applications. Despite their great synthetic potential, widespread use and study of superbases are limited by their air sensitivity and difficult preparation. To address this, we report air-stable carboxylate salts of BTPP and P2-t-Bu phosphazene superbases that, when added to solution with an epoxide, spontaneously generate freebase. These systems function as effective precatalysts and stoichiometric prereagents for superbase-promoted addition, substitution and polymerization reactions. In addition to improving the synthesis, shelf stability, handling and recycling of phosphazenes, this approach enables precise regulation of the rate of base generation in situ. The activation strategy effectively mimics manual slow addition techniques, allowing for control over a reaction's rate or induction period and improvement of reactions that require strong base but are also sensitive to its presence, such as Pd-catalyzed coupling reactions.

Potassium Hexamethyldisilazide (KHMDS): Solvent-Dependent Solution Structures

Solution structures of potassium hexamethyldisilazide [KHMDS] and labeled [15N]KHMDS were examined using a number of analytical methods including 29Si NMR spectroscopy and density functional theory computations. A combination of 15N-29Si couplings, 29Si chemical shifts, and the method of continuous variations reveals dimers, monomers, and ion pairs. Weakly coordinating monofunctional ligands such as toluene, N,N-dimethylethylamine, and Et3N afford exclusively dimers. 1,3-Dioxolane, THF, dimethoxyethane, hexamethylphosphoramide, and diglyme provide dimers at low ligand concentrations and monomers at high ligand concentrations. N,N,N',N'-Tetramethylethylenediamine and N,N,N',N'-tetramethylcyclohexanediamine provide exclusively dimers at all ligand concentrations at ambient temperatures and significant monomer at -80 °C. Studies of 12-crown-4 ran into technical problems. Equimolar 15-crown-5 forms a dimer, whereas excess 15-crown-5 affords a putative ion pair. Whereas equimolar 18-crown-6 also affords a dimer, an excess provides a monomer rather than a solvent-separated ion pair. [2.2.2]cryptand affords what is believed to be a contact-ion-paired cryptate. Solvation was probed using largely density functional theory (DFT) computations. Thermally corrected energies are consistent with lower aggregates and higher solvates at low temperatures, but the magnitudes of the computed temperature dependencies were substantially larger than the experimentally derived data.

Palladium-Catalyzed Allylation of Endocyclic 1-Azaallyl Anions

Endocyclic 1-azaallyl anions engage allyl acetates in a palladium-catalyzed allylation followed by reduction to give unprotected 2-(hetero)aryl-3-allylpiperidines and 2-allyl-3-arylmorpholines, products not easily accessible by other means. The allyl group is then readily transformed into a variety of functional groups. Preliminary studies on the asymmetric variant of the reaction using an enantiomerically pure BI-DIME-type ligand provide the product with moderate enantioselectivity. Computational studies suggest that energy barriers of inner-sphere reductive elimination and outer-sphere nucleophilic substitution are almost the same, which makes both of them possible reaction pathways. In addition, the inner-sphere mechanism displays an enantiodiscriminating C-C bond forming step, while the outer-sphere mechanism is much less selective, which combined to give the asymmetric variant of the reaction moderate enantioselectivity.

Comprehensive Mechanistic Analysis of Palladium- and Nickel-Catalyzed α,β-Dehydrogenation of Carbonyls via Organozinc Intermediates

Introducing degrees of unsaturation into small molecules is a central transformation in organic synthesis. A strategically useful category of this reaction type is the conversion of alkanes into alkenes for substrates with an adjacent electron-withdrawing group. An efficient strategy for this conversion has been deprotonation to form a stabilized organozinc intermediate that can be subjected to α,β-dehydrogenation through palladium or nickel catalysis. This general reactivity blueprint presents a window to uncover and understand the reactivity of Pd- and Ni-enolates. Within this context, it was determined that β-hydride elimination is slow and proceeds via concerted syn-elimination. One interesting finding is that β-hydride elimination can be preferred to a greater extent than C-C bond formation for Ni, more so than with Pd, which defies the generally assumed trends that β-hydride elimination is more facile with Pd than Ni. The discussion of these findings is informed by KIE experiments, DFT calculations, stoichiometric reactions, and rate studies. Additionally, this report details an in-depth analysis of a methodological manifold for practical dehydrogenation and should enable its application to challenges in organic synthesis.

Carbon-Nitrogen Bond Formation Using Sodium Hexamethyldisilazide: Solvent-Dependent Reactivities and Mechanisms

The solvent-dependent reactivity of sodium hexamethyldisilazide (NaHMDS) toward carbon-centered electrophiles reveals reactions that are poorly represented or unrepresented in the literature, including direct aminolysis of aromatic methyl esters to give carboxamides, nitriles, or amidines, depending on the choice of solvent. SNAr substitutions of aryl halides and opening of terminal epoxides are also examined. A combination of 1H and 29Si nuclear magnetic resonance (NMR) spectroscopic studies using [15N]NaHMDS, kinetic studies, and computational studies reveals the complex mechanistic basis of the preferences for simple aryl carboxamides in toluene and dimethylethylamine and arylnitriles or amidines in tetrahydrofuran (THF). A prevalence of dimer- and mixed dimer-based chemistry even starting from the observable NaHMDS monomer in THF solution is notable.

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.

Mechanistic Study of the Copper(II)-Mediated Site-Selective O-Arylation of Glycosides with Arylboronic Acids

Glycosides having multiple free OH groups have been shown to undergo site-selective O-arylations in the presence of arylboronic acids and copper(II) acetate. Herein, a mechanistic analysis of these Chan-Evans-Lam-type couplings is presented based on reaction kinetics, mass spectrometric analysis of reaction mixtures, and substituent effect studies. The results establish that the formation of a substrate-derived boronic ester accelerates the rate-determining transmetalation step. Intramolecular transfer of the aryl group from the boronic ester is ruled out in favor of a pathway in which the key pre-transmetalation assembly is generated from a boronic ester, a copper complex, and a second equivalent of arylboronic acid.

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.

Sodium Isopropyl(trimethylsilyl)amide: A Stable and Highly Soluble Lithium Diisopropylamide Mimic

The preparation, structure, physical properties, and reactivities of sodium isopropyl(trimethylsilyl)amide (NaPTA) are described. The solubilities at room temperature range from n-heptane (0.55 M), n-hexane (0.60 M), toluene (0.65 M), MTBE (1.7 M), Et₃N (3.2 M), and THF (>6.0 M). The half-life to destruction in neat THF is >1 year at 25 °C and 7 days at 70 °C, which compares favorably to 2.5 months and 1.5 days, respectively, for LDA in neat THF. This study focuses on NaPTA in THF. ²⁹Si NMR spectroscopy shows exclusively a mixture of cis and trans stereoisomeric dimers in 0.10-12 M THF in hexane. Density functional theory (DFT) computations suggest that the pK_b is intermediate between dimeric sodium diisopropylamide (NaDA) and dimeric sodium hexamethyldisilazide (NaHMDS). Metalations of arenes, epoxides, ketones, hydrazones, alkenes, and alkyl halides show higher reactivities than LDA (k_NaPTA/LDA = 1-30). While the rates of arene metalation are high, the lower pK_b of NaPTA limits the substrates. Metalation of pseudoephedrate-based carboxamides to form disodiated Myers enolates solves several challenging technical problems.

Ketone Enolization with Sodium Hexamethyldisilazide: Solvent- and Substrate-Dependent E-Z Selectivity and Affiliated Mechanisms

Ketone enolization by sodium hexamethyldisilazide (NaHMDS) shows a marked solvent and substrate dependence. Enolization of 2-methyl-3-pentanone reveals E-Z selectivities in Et₃N/toluene (20:1), methyl-t-butyl ether (MTBE, 10:1), N,N,N',N″,N″-pentamethyldiethylenetriamine (PMDTA)/toluene (8:1), TMEDA/toluene (4:1), diglyme (1:1), DME (1:22), and tetrahydrofuran (THF) (1:90). Control experiments show slow or nonexistent stereochemical equilibration in all solvents except THF. Enolate trapping with Me₃SiCl/Et₃N requires warming to -40 °C whereas Me₃SiOTf reacts within seconds. In situ enolate trapping at -78 °C using preformed NaHMDS/Me₃SiCl mixtures is effective in Et₃N/toluene yet fails in THF by forming (Me₃Si)₃N. Rate studies show enolization via mono- and disolvated dimers in Et₃N/toluene, disolvated dimers in TMEDA, trisolvated monomers in THF/toluene, and free ions with PMDTA. Density functional theory computations explore the selectivities via the E- and Z-based transition structures. Failures of theory-experiment correlations of ionic fragments were considerable even when isodesmic comparisons could have canceled electron correlation errors. Swapping 2-methyl-3-pentanone with a close isostere, 2-methylcyclohexanone, causes a fundamental change in the mechanism to a trisolvated-monomer-based enolization in THF.

Organic Sonochemistry: A Chemist's Timely Perspective on Mechanisms and Reactivity

Sonochemistry, the use of sound waves, usually within the ultrasonic range (>20 kHz), to boost or alter chemical properties and reactivity constitutes a long-standing and sustainable technique that has, however, received less attention than other activation protocols despite affordable setups. Even if unnecessary to underline the impact of ultrasound-based strategies in a broad range of chemical and biological applications, there is considerable misunderstanding and pitfalls regarding the interpretation of cavitational effects and the actual role played by the acoustic field. In this Perspective, with an eye on mechanisms in particular, we discuss the potentiality of sonochemistry in synthetic organic chemistry through selected examples of past and recent developments. Such examples illustrate specific controlling effects and working rules. Looking back at the past while looking forward to advancing the field, some essentials of sonochemical activation will be distilled.

Reactions of Sodium Diisopropylamide: Liquid-Phase and Solid-Liquid Phase-Transfer Catalysis by N,N,N',N″,N″-Pentamethyldiethylenetriamine

Sodium diisopropylamide (NaDA) in N,N-dimethylethylamine (DMEA) and DMEA-hydrocarbon mixtures with added N,N,N',N″,N″-pentamethyldiethylenetriamine (PMDTA) reacts with alkyl halides, epoxides, hydrazones, arenes, alkenes, and allyl ethers. Comparisons of PMDTA with N,N,N',N'-tetramethylethylenediamine (TMEDA) accompanied by detailed rate and computational studies reveal the importance of the trifunctionality and κ2-κ3 hemilability. Rate studies show exclusively monomer-based reactions of 2-bromooctane, cyclooctene oxide, and dimethylresorcinol. Catalysis with 10 mol % PMDTA shows up to >30-fold accelerations (kcat > 300) with no evidence of inhibition over 10 turnovers. Solid-liquid phase-transfer catalysis (SLPTC) is explored as a means to optimize the catalysis as well as explore the merits of heterogeneous reaction conditions.

α-C-H/N-H Annulation of Alicyclic Amines via Transient Imines: Preparation of Polycyclic Lactams

Polycyclic lactams are prepared in a single operation from o-toluamides and cyclic amines in a process that involves transient cyclic imines, species that are conveniently obtained in situ from the corresponding lithium amides and simple ketone oxidants. Imines thus generated, such as 1-pyrroline and 1-piperideine, engage lithiated o-toluamides in a facile annulation process. Undesired side reactions such as imine deprotonation and o-toluamide dimerization are suppressed through the judicious choice of reaction conditions.

TIPS-Diazoacetone Aldol Addition: Mechanistic Aspects and Contribution to the Synthesis

Aldol addition of α-triisopropylsilyl-α-diazoacetone (TIPS-diazoacetone), promoted by excess lithium diisopropylamide (LDA), was developed and applied to the synthesis of original C-TIPS diazoaldols, C-TIPS diazoketols, and a related Mannich-type product. An unprecedented mechanistic pathway has been proposed, involving a lithiated triazene intermediate resulting from the nucleophilic addition of LDA on the diazo moiety, supported by experimental results and DFT calculations.

Mechanistic insights into rhodium-catalyzed enantioselective allylic alkylation for quaternary stereogenic centers

Installing quaternary stereogenic carbon is an arduous task of contemporary importance in the domain of asymmetric catalysis. To this end, an asymmetric allylic alkylation of α,α-disubstituted aldehydes by using allyl benzoate in the presence of Wilkinson's catalyst [Rh(Cl)(PPh₃)₃], (R)-BINOL–P(OMe) as the external ligand, and LiHMDS as the base has been reported to offer high enantioselectivity. The mechanistic details of this important reaction remain vague, which prompted us to undertake a detailed density functional theory (SMD_(THF)/B3LYP-D3) investigation on the nature of the potential active catalyst, energetic features of the catalytic cycle, and the origin of high enantioselectivity. We note that a chloride displacement from the native Rh-phosphine [Rh(Cl)(PPh₃)₃] by BINOL–P(OMe) phosphite and an ensuing MeCl elimination can result in the in situ formation of a Rh-phosphonate [Rh(BINOL–P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O)(PPh₃)₃]. A superior energetic span (δE) noted with such a Rh-phosphonate suggests that it is likely to serve as an active catalyst. The uptake of allyl benzoate by the active catalyst followed by the turnover determining C–O bond oxidative addition furnishes a Rh-π-allyl intermediate, which upon interception by (Z)-Li-enolate (derived from α,α-disubstituted aldehyde) in the enantiocontrolling C–C bond generates a quaternary stereogenic center. The addition of the re prochiral face of the (Z)-Li-enolate to the Rh-bound allyl moiety leading to the R enantiomer of the product is found to be 2.4 kcal mol⁻¹ more preferred over the addition through its si face. The origin of the stereochemical preference for the re face addition is traced to improved noncovalent interactions (NCIs) and less distortion in the enantiocontrolling C–C bond formation transition state than that in the si face addition. Computed enantioselectivity (96%) is in very good agreement with the experimental value (92%), so is the overall activation barrier (δE of 17.1 kcal mol⁻¹), which is in conformity with room temperature reaction conditions.

The origin of high enantioselectivity in the formation of quaternary stereogenic carbon.

Mechanism and origins of selectivity in the enantioselective oxa-Pictet-Spengler reaction: a cooperative catalytic complex from a hydrogen bond donor and chiral phosphoric acid

Enantioselective additions to oxocarbenium ions are high-value synthetic transformations but have proven challenging to achieve. In particular, the oxa-Pictet-Spengler reaction has only recently been rendered enantioselective. We report experimental and computational studies on the mechanism of this unusual transformation. Herein we reveal that this reaction is hypothesized to proceed through a self-assembled ternary hydrogen bonding complex involving the substrate, chiral phosphate ion, and a urea hydrogen-bond donor. The computed transition state reveals C2-symmetric grooves in the chiral phosphate that are occupied by the urea and substrate. Occupation of one of these grooves by the urea co-catalyst tunes the available reactive volume and enhances the stereoselectivity of the chiral phosphate catalyst.

Structure, Reactivity, and Synthetic Applications of Sodium Diisopropylamide

The 60-year history of sodium diisopropylamide (NaDA) is described herein. We review various preparations, solvent-dependent stabilities, and solution structures. Synthetic applications of NaDA reported to date are framed by a mechanism-driven approach, emphasizing selectivities when appropriate. We conclude with examples beyond metalation in which NaDA plays a central role and a few thoughts on where future applications could be focused.

Disodium Salts of Pseudoephedrine-Derived Myers Enolates: Stereoselectivity and Mechanism of Alkylation

Pseudoephedrine-derived dianionic Myers enolates were generated using sodium diisopropylamide (NaDA) in THF solution. The reactivities and selectivities of the disodium salts largely mirror those of the dilithium salts but without the requisite large excesses of inorganic salts (LiCl) or mandated dilute solutions. The disodium salts require careful control of temperature to preclude deleterious aggregate aging effects traced to changes in the aggregate structure and intervening O-alkylations. Structural studies and density functional theory (DFT) computations show a dominant highly symmetric polyhedron quite different from the lithium analogue. No enolate-NaDA mixed aggregates are observed with excess NaDA. Rate studies show an alkylation mechanism involving an intervening tetramer-monomer pre-equilibrium followed by rate-limiting alkylation of tetrasolvated monomers. DFT computations were conducted to explore the possible influences on stereochemistry. A crystal deriving from samples aged at ambient temperature contains six dianionic subunits and two monoanionic (alkoxide-only) subunits. A new preparation of concentrated solutions of NaDA in THF solution is described.

Lewis Base Assisted Lithium Brønsted Base Catalysis: A New Entry for Catalytic Asymmetric Synthesis of β2,2-Amino Acids

A new catalytic system comprising chiral Ag complex and Li aryloxide/bisphosphine oxide is developed for the synthesis of β^2,2-amino acids via direct asymmetric Mannich-type reaction of 4-subsituted isoxazolidin-5-ones. The Mannich adduct is a direct precursor of β-peptidic compounds otherwise difficult to obtain.

Sodium Diisopropylamide-Mediated Dehydrohalogenations: Influence of Primary- and Secondary-Shell Solvation

Eliminations of alkyl halides by sodium diisopropylamide (NaDA) in tetrahydrofuran (THF)/hexane or THF/N,N-dimethylethylamine (DMEA) solutions are facile and complementary to analogous reactions of lithium diisopropylamide in THF. Rate studies show a dominance of monomer-based metalations and prevalent secondary-shell solvation effects overlaid on primary-shell effects. 1-Halooctanes exclusively undergo elimination rather than substitution. Rate and isotopic labeling studies on 1-bromooctane reveal an E2-like elimination pathway via trisolvated NaDA monomer. By contrast, 1-chlorooctane is eliminated via disolvated monomer through a carbenoid mechanism. exo-2-Norbornyl chloride and bromide are also eliminated via disolvated monomer; a syn E2 mechanism is inferred for these substrates. The cis- and trans-4-tert-butylcyclohexyl bromides show a preference for the elimination of the cis isomer (k_cis/ax/k_trans/eq = 10). Rate and isotopic labeling studies are consistent with a trans-diaxial E2 elimination via trisolvated monomer for the cis isomer and a carbenoid mechanism via disolvated monomer for the trans isomer. Vicinal haloethers show substrate-dependent reactivities, affording alkynes and enol ethers. trans-1-Bromo-2-methoxycyclohexane provides enol ether 1-methoxycyclohexene, while trans-1-bromo-2-methoxycyclooctane provides dimeric products consistent with fleeting cycloocta-1,2-diene (cyclic allene), which was fully characterized as two conformers.

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.

Lithium Diisopropylamide Catalyzed Allylic C-H Bond Alkylation with Styrenes

Allylic substitution reactions, a well-established approach for new bond construction, often need transition-metal catalysts and stoichiometric amounts of organometallic reagents, strong bases, or oxidants. Lithium diisopropylamide (LDA), a widely used and commercially available Brønsted base, is herein reported to catalyze the allylic C-H bond addition of 1,3-diarylpropenes to styrenes. Preliminary mechanism studies have provided a solid structure of the π-allyllithium intermediate and revealed the unique catalytic roles of LDA and its conjugate acid diisopropylamine.

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.

Case for Lithium Tetramethylpiperidide-Mediated Ortholithiations: Reactivity and Mechanisms

Rate and mechanistic studies of ortholithiations by lithium 2,2,6,6-tetramethylpiperidide focus on four arenes: 1,4-bis(trifluoromethyl)benzene, 1,3-bis(trifluoromethyl)benzene, 1,3-dimethoxybenzene, and 4,4-dimethyl-2-phenyl-2-oxazoline. Metalations occur via substrate-dependent combinations of monosolvated monomer, disolvated monomer, and tetrasolvated dimer (triple ions). Density functional theory computational studies augment the experimental data. We discuss the challenges presented by shifting dimer-monomer proportions in determining the observable reaction orders and our mathematical treatment of such shifting in reactant structure.

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

Building on structural and mechanistic studies of lithiated enolates derived from acylated oxazolidinones (Evans enolates) and chiral lithiated amino alkoxides, we found that amino alkoxides amplify the enantioselectivity of aldol additions. The pairing of enantiomeric series affords matched and mismatched stereoselectivities. The structures of mixed tetramers showing 2:2 and 3:1 (alkoxide-rich) stoichiometries are determined spectroscopically. Rate and computational studies provide a viable mechanistic and stereochemical model based on the direct reaction of the 3:1 mixed tetramers, but they raise unanswered questions for the 2:2 mixed aggregates.

Reactivity of Lithium β-Ketocarboxylates: The Role of Lithium Salts

Lithium β-ketocarboxylates 1(COOLi), prepared by the reaction of lithium enolates 2(Li⁺) with carbon dioxide, readily undergo decarboxylative disproportionation in THF solution unless in the presence of lithium salts, in which case they are indefinitely stable at room temperature in inert atmosphere. The availability of stable THF solutions of lithium β-ketocarboxylates 1(COOLi) in the absence of carbon dioxide allowed reactions to take place with nitrogen bases and alkyl halides 3 to give α-alkyl ketones 1(R) after acidic hydrolysis. The sequence thus represents the use of carbon dioxide as a removable directing group for the selective monoalkylation of lithium enolates 2(Li⁺). The roles of lithium salts in preventing the disproportionation of lithium β-ketocarboxylates 1(COOLi) and in determining the course of the reaction with bases and alkyl halides 3 are discussed.

Sodium Diisopropylamide in Tetrahydrofuran: Selectivities, Rates, and Mechanisms of Arene Metalations

Sodium diisopropylamide (NaDA)-mediated metalations of arenes in tetrahydrofuran (THF)/hexane or THF/Me₂NEt solutions are described. A survey of >40 benzenoid- and pyridine-based arenes with a range of substituents demonstrates the efficacy and regioselectivity of metalation. Metalations of activated disubstituted arenes and selected monosubstituted arenes are rapid at -78 °C. Rate studies of 1,3-dimethoxybenzene and related methoxylated arenes show exclusively monomer-based orthometalations with two or three coordinated THF ligands. Rate studies of the isotopic exchange of benzene and monosubstituted arenes with weakly activating groups reveal analogous di- and trisolvated monomer-based metalations. Cooperative inductive, mesomeric, steric, and chelate effects are discussed.

Lithium Hexamethyldisilazide-Mediated Enolization of Highly Substituted Aryl Ketones: Structural and Mechanistic Basis of the E/Z Selectivities

Enolizations of highly substituted acyclic ketones used in the syntheses of tetrasubstituted olefin-based anticancer agents are described. Lithium hexamethyldisilazide (LiHMDS)-mediated enolizations are moderately Z-selective in neat tetrahydrofuran (THF) and E-selective in 2.0 M THF/hexane. The results of NMR spectroscopy show the resulting enolates to be statistically distributed ensembles of E,E-, E,Z-, and Z,Z-enolate dimers with subunits that reflect the selectivities. The results of rate studies trace the preference for E and Z isomers to tetrasolvated- and pentasolvated-monomer-based transition structures, respectively. Enolization using LiHMDS in N,N-dimethylethylamine or triethylamine in toluene affords a 65:1 mixture of LiHMDS-lithium enolate mixed dimers containing E and Z isomers, respectively. Spectroscopic studies show that condition-dependent complexation of ketone to LiHMDS occurs in trialkylamine/toluene. Rate data attribute the high selectivity exclusively to monosolvated-dimer-based transition structures.

Sodium Diisopropylamide in Tetrahydrofuran: Selectivities, Rates, and Mechanisms of Alkene Isomerizations and Diene Metalations

Sodium diisopropylamide in tetrahydrofuran is an effective base for the metalation of 1,4-dienes and isomerization of alkenes. Dienes metalate via tetrasolvated sodium amide monomers, whereas 1-pentene is isomerized by trisolvated monomers. Facile, highly Z-selective isomerizations are observed for allyl ethers under conditions that compare favorably to those of existing protocols. The selectivity is independent of the substituents on the allyl ethers; rate and computational data show that the rates, mechanisms, and roles of sodium-oxygen contacts are substituent-dependent. The competing influences of substrate coordination and solvent coordination to sodium are discussed.

Aggregation and Solvation of n-Butyllithium

Solution characterizations and ligand binding constants were determined for n-butyllithium in hydrocarbon and ethereal solvents using diffusion-ordered NMR. In hydrocarbon solvents, n-butyllithium exists primarily as an octamer at -40 °C and deaggregates to a hexamer when the temperature is increased. In the presence of THF or diethyl ether, n-butyllithium exists predominantly as a tetra-solvated tetramer and deaggregates to a tetra-solvated dimer in the presence of a large excess or neat THF. The ligand binding constants for the tetra-solvated tetramers were measured using ¹H NMR/DOSY titration.

Barbier Continuous Flow Preparation and Reactions of Carbamoyllithiums for Nucleophilic Amidation

An ambient temperature continuous flow method for nucleophilic amidation and thioamidation is described. Deprotonation of formamides by lithium diisopropylamine (LDA) affords carbamoyllithium intermediates that are quenched in situ with various electrophiles such as ketones, allyl bromides, Weinreb and morpholino amides. The nature of the reactive lithium intermediates and the thermodynamics of the metalation were further investigated by ab initio calculations and kinetic experiments.

Evans Enolates: Structures and Mechanisms Underlying the Aldol Addition of Oxazolidinone-Derived Boron Enolates

The soft enolization of an acylated oxazolidinone using di-n-butylboron triflate (n-Bu₂BOTf) and trialkylamines and subsequent aldol addition was probed structurally and mechanistically using a combination of IR and NMR spectroscopies. None of the species along the reaction coordinate show a penchant for aggregating. Complexation of the acylated oxazolidinone by n-Bu₂BOTf was too rapid to monitor, as was the subsequent enolization with Et₃N (triethylamine). The preformed n-Bu₂BOTf·Et₃N complex, displaying muted Lewis acidity and affiliated tractable rates, reveals a rate-limiting complexation via a transition structure with a complicated counterion. n-Bu₂BOTf·i-Bu₃N bearing a hindered amine shifts the rate-limiting step to proton transfer. Rate studies show that the aldol addition with isobutyraldehyde occurs as proffered by others.

Sodium Diisopropylamide: Aggregation, Solvation, and Stability

The solution structures, stabilities, physical properties, and reactivities of sodium diisopropylamide (NaDA) in a variety of coordinating solvents are described. NaDA is stable for months as a solid or as a 1.0 M solution in N,N-dimethylethylamine (DMEA) at -20 °C. A combination of NMR spectroscopic and computational studies show that NaDA is a disolvated symmetric dimer in DMEA, N,N-dimethyl-n-butylamine, and N-methylpyrrolidine. Tetrahydrofuran (THF) readily displaces DMEA, affording a tetrasolvated cyclic dimer at all THF concentrations. Dimethoxyethane (DME) and N,N,N',N'-tetramethylethylenediamine quantitatively displace DMEA, affording doubly chelated symmetric dimers. The trifunctional ligands N,N,N',N″,N″-pentamethyldiethylenetriamine and diglyme bind the dimer as bidentate rather than tridentate ligands. Relative rates of solvent decompositions are reported, and rate studies for the decomposition of THF and DME are consistent with monomer-based mechanisms.

Lithium Diisopropylamide: Nonequilibrium Kinetics and Lessons Learned about Rate Limitation

The kinetics of lithium diisopropylamide (LDA) in tetrahydrofuran under nonequilibrium conditions are reviewed. These conditions correspond to a class of substrates in which the rates of LDA aggregation and solvation events are comparable to the rates at which various fleeting intermediates react with substrate. Substrates displaying these reactivities, by coincidence, happen to be those that react at tractable rates on laboratory time scales at -78 °C. In this strange region of nonlimiting behavior, rate-limiting steps are often poorly defined, sometimes involve deaggregation, and at other times include reaction with substrate. Changes in conditions routinely cause shifts in the rate-limiting steps, and autocatalysis is prevalent and can be acute. The studies are described in three distinct portions: (1) methods and strategies used to deconvolute complex reaction pathways, (2) the resulting conclusions about organolithium reaction mechanisms, and (3) perspectives on the concept of rate limitation reinforced by studies of LDA in tetrahydrofuran at -78 °C under nonequilibrium conditions.

Lithium Hexamethyldisilazide-Mediated Enolization of Acylated Oxazolidinones: Solvent, Cosolvent, and Isotope Effects on Competing Monomer- and Dimer-Based Pathways

Lithium hexamethyldisilazide (LiHMDS)-mediated enolization of (+)-4-benzyl-3-propionyl-2-oxazolidinone in THF-hydrocarbon mixtures shows unusual sensitivity to the choice of hydrocarbon cosolvent (hexane versus toluene) and to isotopic labeling. Four mechanisms corresponding to monosolvated monomers, trisolvated dimers, octasolvated monomers, and octasolvated dimers were identified. Even under conditions in which the LiHMDS monomer was the dominant observable form, dimer-based metalation was significant. The mechanism-dependent isotope and cosolvent effects are discussed in the context of ground-state stabilization and transition-state tunneling.

KOtBu-Catalyzed lithiation of PMDTA and the direct functionalization of bridged alkenes under mild conditions

A practical preparation of the reagent PMDTALi using a super base system under mild conditions has been developed.

Conformational Polymorphism of Lithium Pinacolone Enolate

A metastable, polymorphic hexameric crystal structure of lithium pinacolone enolate (LiOPin) is reported along with three preparation methods. NMR-based structural characterization implies that the lithium pinacolate hexamer deaggregates to a tetramer in toluene but retains mainly the hexameric structure in nonaromatic hydrocarbon solvents such as cyclohexane. Moreover, the presence of a small amount of lithium aldolate (LiOA) dramatically influences the aggregation state of LiOPin by forming a mixed aggregate with a 3:1 ratio (LiOPin₃·LiOA).

The Two Alternative Rate-Determining Steps in Benzylic Lithiation Reactions of Esters and Carbamates

Lithiation reactions of tertiary benzylic esters and carbamates have been studied. Kinetic methodology revealed that a two-step reaction pathway should be considered for these reactions, where either the lithium precomplexation and/or the proton transfer steps can be rate determining. Kinetic isotopic effects were evaluated by comparison of the lithiations of the corresponding protio/deutero substrates, and the results obtained support the notion that lithium precomplexation is taking place on the reaction pathway and that it is the rate-determining step in this transformation.

Alkali-Controlled C-H Cleavage or N-C Bond Formation by N2-Derived Iron Nitrides and Imides

Formation of N-H and N-C bonds from functionalization of N2 is a potential route to utilization of this abundant resource. One of the key challenges is to make the products of N2 activation reactive enough to undergo further reactions under mild conditions. This paper explores the strategy of "alkali control," where the presence of an alkali metal cation enables the reduction of N2 under mild conditions, and then chelation of the alkali metal cation uncovers a highly reactive species that can break benzylic C-H bonds to give new N-H and Fe-C bonds. The ability to "turn on" this C-H activation pathway with 18-crown-6 is demonstrated with three different N2 reduction products of N2 cleavage in an iron-potassium system. The alkali control strategy can also turn on an intermolecular reaction of an N2-derived nitride with methyl tosylate that gives a new N-C bond. Since the transient K(+)-free intermediate reacts with this electrophile but not with the weak C-H bonds in 1,4-cyclohexadiene, it is proposed that the C-H cleavage occurs by a deprotonation mechanism. The combined results demonstrate that a K(+) ion can mask the latent nucleophilicity of N2-derived nitride and imide ligands within a trimetallic iron system and points a way toward control over N2 functionalization.

Preparation of an ion with the highest calculated proton affinity: ortho-diethynylbenzene dianion

Owing to the increased proton affinity that results from additional negative charges, multiply-charged anions are shown as a route to preparing powerful ‘superbases’.

Owing to the increased proton affinity that results from additional negative charges, multiply-charged anions have been proposed as one route to prepare and access a range of new and powerful “superbases”. Paradoxically, while the additional electrons in polyanions increase basicity they serve to diminish the electron binding energy and thus, it had been thought, hinder experimental synthesis. We report the synthesis and isolation of the ortho-diethynylbenzene dianion (ortho-DEB^2–) and present observations of this novel species undergoing gas-phase proton-abstraction reactions. Using a theoretical model based on Marcus–Hush theory, we attribute the stability of ortho-DEB^2– to the presence of a barrier that prevents spontaneous electron detachment. The proton affinity of 1843 kJ mol^–1 calculated for this dianion superbase using high-level quantum chemistry calculations significantly exceeds that of the lithium monoxide anion, the most basic system previously prepared. The ortho-diethynylbenzene dianion is therefore the strongest base that has been experimentally observed to date.