Mechanistic insight into stereoselective carbolithiation

Alkali Metal Dihydropyridines in Transfer Hydrogenation Catalysis of Imines: Amide Basicity versus Hydride Surrogacy

Catalytic reduction of a representative set of imines, both aldimines and ketimines, to amines has been studied using transfer hydrogenation from 1,4-dicyclohexadiene. Unusually, this has been achieved using s-block pre-catalysts, namely 1-metallo-2-tert-butyl-1,2-dihydropyridines, 2-tBuC5 H5 NM, M(tBuDHP), where M=Li-Cs. Reactions have been monitored in C6 D6 and tetrahydrofuran-d8 (THF-d8 ). A definite trend is observed in catalyst efficiency with the heavier alkali metal tBuDHPs outperforming the lighter congeners. In general, Cs(tBuDHP) is the optimal pre-catalyst with, in the best cases, reactions producing quantitative yields of amines in minutes at room temperature using 5 mol % catalyst. Supporting the experimental study, Density Functional Theory (DFT) calculations have also been carried out which reveal that Cs has a pathway with a significantly lower rate determining step than the Li congener. In the postulated initiation pathways DHP can act as either a base or as a surrogate hydride.

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

The title compound N,N,N',N'-tetra-methyl-ethanedi-amine, 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 mol-ecule is generated by a crystallographic center of symmetry and the conformation of the di-amine is anti-periplanar. To investigate the inter-molecular inter-actions, a Hirshfeld surface analysis was performed. It showed that H⋯H (van der Waals) inter-actions dominate with a contact percentage of 92.3%.

Aminopotassiation by Mixed Potassium/Lithium Amides: A Synthetic Path to Difficult to Access Phenethylamine Derivates

Insights gained from a comparison of aminometalation reactions with lithium amides, potassium amides and mixed lithium/potassium amides are presented. A combination of structural characterization, DFT calculations and electrophile reactions of aminometalated intermediates has shown the advantages of using a mixed metal strategy. While potassium amides fail to add, the lithium amides are uncontrollable and eliminated, yet the mixed K/Li amides deliver the best of both systems. Aminopotassiation proceeds to form the alkylpotassium species which has enhanced stability over its lithium counterpart allowing for its isolation and thereby its further characterization.

Computational Insight into 1,2-Diamine, -Diether, and -Amino Ether Chiral Ligand-Mediated Carbolithiation: A Case of Enantioinduction Reversal

trans-1,2-Cyclohexanediamine, -diether, and -amino ether were compared as chiral inducers in the asymmetric intramolecular carbolithiation of olefinic aryllithiums. Switching from diamine to ethereal ligands inverts the sense of asymmetric induction. This reversal of stereoselectivity was investigated through density functional theory calculations. High enantioselectivities observed with diether and amino ether ligands arise from favorable weak interactions between the ligand and the substrate. The relative efficiency of the three ligands and sense of stereoinduction for the most efficient diether and amino ether ligands prove to be foreseeable by modeling the reaction with the parent achiral 1,2-bidentate additives and comparing the diastereomeric transition states stemming from the two half-chair conformations of their lithium chelate.

Regio- and stereoselective intermolecular carbolithiation reactions

Carbolithiation of alkenes provides a powerful tool for the construction of up to two novel carbon–carbon or carbon–heteroatoms bonds in a regio- and stereo-controlled manner. The synthetic potentiality of this reaction is highlighted.

Kinetically controlled asymmetric synthesis of silicon-stereogenic methoxy silanes using a planar chiral ferrocene backbone

We present a mechanistic proposal for the desymmetrisation of dimethoxy silanes with alkyllithiums. The stereochemical pathway is highly defined by the coordination of the metal lithium, which suppresses the reversible interconversion of the various pentavalent intermediates. The predicted kinetic control of the stereoinduction is verified by the experiments using a planar chiral ferrocene backbone.

The smaller, the better? How the aggregate size affects the reactivity of (trimethylsilyl)methyllithium

An unexpected influence of the nature of stabilizing additives to alkyllithium compounds on an aggregate's reactivity was examined experimentally and theoretically.

Controlling the Coordination Sphere of Alkyllithiums Results in Selective Reactions with Allylic Amines

Described herein is a selective way to control the reaction of allylic amines with metalorganic bases depending on the amine handle as well as the metalorganic base is used. Depending on the number of coordinating groups within the amine handle either a selective carbometalation or deprotonation reaction can be performed. By changing the alkali metal within the base from lithium to either sodium or potassium, a change of chemoselectivity takes place and the reaction of piperidinoallylamine can be controlled.

Lithium Enolates in the Enantioselective Construction of Tetrasubstituted Carbon Centers with Chiral Lithium Amides as Noncovalent Stereodirecting Auxiliaries

Lithium enolates derived from carboxylic acids are ubiquitous intermediates in organic synthesis. Asymmetric transformations with these intermediates, a central goal of organic synthesis, are typically carried out with covalently attached chiral auxiliaries. An alternative approach is to utilize chiral reagents that form discrete, well-defined aggregates with lithium enolates, providing a chiral environment conducive of asymmetric bond formation. These reagents effectively act as noncovalent, or traceless, chiral auxiliaries. Lithium amides are an obvious choice for such reagents as they are known to form mixed aggregates with lithium enolates. We demonstrate here that mixed aggregates can effect highly enantioselective transformations of lithium enolates in several classes of reactions, most notably in transformations forming tetrasubstituted and quaternary carbon centers. Easy recovery of the chiral reagent by aqueous extraction is another practical advantage of this one-step protocol. Crystallographic, spectroscopic, and computational studies of the central reactive aggregate, which provide insight into the origins of selectivity, are also reported.

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.

A facile and convenient sequential homobimetallic catalytic approach towards β-methylstyrenes. A one-pot Stille cross-coupling/isomerization strategy

A one-pot approach towards β-methylstyrenes is reported. The transformation involves a Stille cross-coupling reaction of aryl halides with allyltributylstannane, followed by an in situ Pd-catalyzed double bond conjugative migration.

Enediolate-dilithium amide mixed aggregates in the enantioselective alkylation of arylacetic acids: structural studies and a stereochemical model

A combination of X-ray crystallography, (6)Li, (15)N, and (13)C NMR spectroscopies, and density functional theory computations affords insight into the structures and reactivities of intervening aggregates underlying highly selective asymmetric alkylations of carboxylic acid dianions (enediolates) mediated by the dilithium salt of a C2-symmetric chiral tetraamine. Crystallography shows a trilithiated n-butyllithium-dilithiated amide that has dimerized to a hexalithiated form. Spectroscopic studies implicate the non-dimerized trilithiated mixed aggregate. Reaction of the dilithiated amide with the dilithium enediolate derived from phenylacetic acid affords a tetralithio aggregate comprised of the two dianions in solution and the dimerized octalithio form in the solid state. Computational studies shed light on the details of the solution structures and afford a highly predictive stereochemical model.

Direct benzylic metalation of a phenethylamine derivative: potassium as the key to both generation and stabilization of a labile anion

t-BuLi-t-BuOK selectively metalates the benzylic position of 2-phenylethyldimethylamine under mild conditions without occurrence of β-elimination in the resulting metalated species. Theoretical and structural studies indicate that potassium is crucial for both the lowering of the barrier of the initial deprotonation step and the stabilization of the labile anion.

A diastereomerically enriched, dimeric organolithium compound and the stereochemical course of its transformations

A dimeric α-silylated ethyllithium compound is presented featuring stereogenic metalated carbon centres with defined configurations. It can be generated by diastereoselective deprotonation of the corresponding ethylsilane. Reaction with Me(3)SnCl proceeds under inversion and the transfer of the stereoinformation is possible with dr's of up to 97:3.

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.