Crystal Structures of the Chiral Diamine (R,R)-TMCDA with the Commonly Used Alkyllithium Bases Methyllithium, iso-Propyllithium, and sec-Butyllithium

Schlenk's Legacy-Methyllithium Put under Close Scrutiny

Commercially available stock solutions of organolithium reagents are well-implemented tools in organic and organometallic chemistry. However, such solutions are inherently contaminated with lithium halide salts, which can complicate certain synthesis protocols and purification processes. Here, we report the isolation of chloride-free methyllithium employing K[N(SiMe₃ )₂ ] as a halide-trapping reagent. The influence of distinct LiCl contaminations on the ⁷ Li-NMR chemical shift is examined and their quantification demonstrated. The structural parameters of new chloride-free monomeric methyllithium complex [(Me₃ TACN)LiCH₃ ], ligated by an azacrown ether, are assessed by comparison with a halide-contaminated variant and monomeric lithium chloride [(Me₃ TACN)LiCl], further emphasizing the effect of halide impurities.

A monomeric (trimethylsilyl)methyl lithium complex: synthesis, structure, decomposition and preliminary reactivity studies

Monomeric organolithium (LiR) complexes could provide enhanced Li-C bond reactivity and suggest mechanisms for a plethora of LiR-mediated reactions. They are highly sought-after but remain a synthetic challenge for organometallic chemists. In this work, we report the synthesis and characterisation of a monomeric (trimethylsilyl)methyl lithium complex, namely [Li(CH₂SiMe₃)(κ³-N,N',N''-Me⁶Tren)] (1), where Me⁶Tren is a tetradentate neutral amine ligand. The structure of 1 was comprehensively examined by single-crystal X-ray diffraction, variable temperature NMR spectroscopy and electron absorption spectroscopy. Complex 1 decomposes via ligand C-H and C-N activations to produce a Li amide complex 2. Preliminary reactivity studies of 1 reveal CO insertion and C-H activation reaction patterns.

A monomeric methyllithium complex: synthesis and structure

Methyllithium (MeLi) is the parent archetypal organolithium complex. MeLi exists as aggregates in solutions and solid states. Monomeric MeLi is postulated as a highly reactive intermediate and plays a vital role in understanding MeLi-mediated reactions but has not been isolated. Herein, we report the synthesis and structure of the first monomeric MeLi complex enabled by a new hexadentate neutral amine ligand.

Delocalized Bonding in Li2X2 Rings: Probing the Limits of the Covalent and Ionic Bonding Models

In spite of the highly ionic character of most lithium-element bonds, the bonding within Li₂X₂ rings presents similarities with that found in analogous transition metal systems. They obey simple framework electron counting rules that allow us to predict whether they will form a regular ring or a squeezed one with short Li-Li or X-X distances. A combined computational and structural database analysis discloses the orbital conditions that determine the framework electron counting rules. These systems probe the borderline between the covalent and ionic bonding models since, paradoxically, a non-negligible covalent contribution of the two Li atoms (formally Li⁺ ions in the ionic model) to the Li-X framework bonding favors them approaching each other within bonding distance.

The structure-defining incorporation of chloride in methyllithium dimers

Two polymeric solid-state structures of pmdta-stabilized methyllithium with co-existing tetrameric and dimeric units were obtained, linked by the structure-giving “contaminant” LiCl, incorporated only in dimeric entities.

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.

200 Years of Lithium and 100 Years of Organolithium Chemistry

The element lithium has been discovered 200 years ago. Due to its unique properties it has emerged to play a vital role in industry, esp. for energy storage, and lithium‐based products and processes support sustainable technological developments. In addition to the many uses of lithium in its inorganic forms, lithium has a rich organometallic chemistry. The development of organometallic chemistry has been hindered by synthetic problems from the start. When Wilhelm Schlenk developed the basic principles to handle and synthesize air‐ and moisture‐sensitive compounds, the road was open to further developments. After more information was available about the stability and solubility of such compounds, they started to play an essential role in other fields of chemistry as alkyl or aryl transfer reagents.

Exploring the solid state and solution structural chemistry of the utility amide potassium hexamethyldisilazide (KHMDS)

The coordination chemistry of the important potassium amide KHMDS has been explored both in the solid state and in solution.

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.

Characterization of cyclopentyllithium and cyclopentyllithium tetrahydrofuran complex

The solid-state structures of unsolvated, hexameric cyclopentyllithium and tetrameric cyclopentyllithium tetrahydrofuran solvate were determined by single-crystal X-ray diffraction. Cyclopentyllithium easily crystallized in hydrocarbon solvents. Solution-state structural analyses of cyclopentyllithium and cyclopentyllithium-tetrahydrofuran complexes in toluene-d8 were also carried out by diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses and other one- and two-dimensional NMR techniques. The solution-state studies suggest that unsolvated cyclopentyllithium exists as hexamer and tetramer equilibrating with each other. Upon solvation with tetrahydrofuran, cyclopentyllithium exists mostly as a tetrahydrofuran tetrasolvated tetramer.

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.

Preparation of aminomethyl functionalised silanes via an α-lithiated amine: from their synthesis, stability and crystal structures to stereochemical issues

The preparation of aminomethyl functionalised silanes based on the α-lithiated amine, (1R,2R)-N,N,N',N'-tetramethylcyclohexane-1,2-diamine [(R,R)-TMCDA] is reported. This methodology can be applied for the synthesis of mono-aminomethyl substituted systems, but most remarkably also for di- and trifunctionalised compounds. The trapping of the lithiated amine is accompanied by transmetallation reactions resulting in the formation of (silylmethyl)silanes depending on the reaction temperature. The zinc(II) halide complexes of the mono-functionalised systems show the formation of exclusively one configuration of the stereogenic nitrogen atom, in which the spatially more demanding substituent exhibits the pseudo-equatorial position. The di- and trifunctionalised systems feature high sensitivity towards Si-C bond cleavage under re-formation of the (R,R)-TMCDA fragment.

Mechanistic insight into stereoselective carbolithiation

This article addresses the mechanistic features of asymmetric carbolithiation of β-methylstyrenes. While often the presence of functional groups is required to obtain high enantioselectivities in carbolithiation reactions, simple β-methylstyrene also gives high selectivities in (-)-sparteine-mediated addition of alkyl lithium compounds. Computational studies on the carbolithiation of β-methylstyrene with (-)-sparteine show that the observed selectivities are the result of repulsion effects in the diastereomeric transition states between the (-)-sparteine⋅alkyl lithium adduct and the β-methylstyrene, upon approximation of the two reactants. In contrast, for the ortho-amino β-methylstyrene (E)-benzyl(2-propenylphenyl)amine (4) X-ray structure analyses of intermediate lithium amides indicate a carbolithiation mechanism in which one side of the double bond is shielded by the amide moiety, leaving only one side free for approach of the chiral alkyl lithium adduct.

Solvent-free mesityllithium: solid-state structure and its reactivity towards white phosphorus

The donor-free mesityllithium was prepared from the reaction of MesBr with n-BuLi in diethyl ether at -78 degrees C. The solid-state structure of unsupported mesityllithium consists of C(2)Li(2)-rings composed of two LiMes units, which interact with adjacent dimers [LiMes](2), forming a polymeric infinite chain along the crystallographic c-axis (monoclinic space group, P2(1)/n). The structure of donor-free mesityllithium reveals short contacts between the C atoms of the mesityl rings and the lithium atoms of neighbouring [LiMes](2) units. The structure determination of LiMes was performed by X-ray powder diffraction. In addition we have investigated the reaction of LiMes with Me(3)SnCl and P(4) for our understanding of the reactivity of donor-free mesityllithium. The heterogeneous reaction of donor-free mesityllithium with Me(3)SnCl produces conveniently the stannylated mesitylene Me(3)SnMes (triclinic, space group P1). White phosphorus reacts with three equivalents of unsupported mesityllithium in benzene to give Li(3)P(4)Mes(3). In this context it should be noted that a tetraphosphide with an identical LiP-core as in Li(3)P(4)Mes(3) had been formed in the 1:3 reaction of P(4) with the silanide Li[SitBu(3)]. The tetraphosphide Li(3)P(4)Mes(3) was analyzed using X-ray crystallography (monoclinic, space group C2/c).

(1R,2R)-N,N'-Diisobutyl-N,N'-dimethyl-cyclo-hexane-1,2-diamine

The title compound, C₁₆H₃₄N₂, is a chiral diamine with fixed R configuration at both stereogenic carbon centres of the cyclo­hexane backbone. Due to their different substituents, the two N atoms also become stereogenic. In the crystal structure, the configuration at one of the two nitro­gen centres is fixed, with the free electron pair pointing inward and the isobutyl group in a trans position towards the cyclo­hexane backbone resulting in an R configuration. The isobutyl group at the second N atom, however, is disordered with 75% S configuration and 25% R configuration. In both cases, the isobutyl group is arranged in a trans position towards the cyclo­hexane backbone.

2-[(2-Hydr-oxy-2,2-diphenyl-ethyl)(meth-yl)amino]-N,N-dimethyl-ethanaminium bromide

The title compound, C₁₉H₂₇N₂O⁺·Br⁻, is the hydro­bromide of the trapping product of lithia­ted N,N,N′,N′-tetramethylethylenediamine (TMEDA) with benzophenone. Thereby, the N atom of the NMe₂ group is selectively protonated and the respective trapping product represents a potential tridentate ligand with one O and two N donor atoms. The H atoms at N (H2N) and O (H1O) are involved in hydrogen bonds with the Br⁻. The mol­ecular structure shows all donor atoms to be arranged on one side of the mol­ecule, thus indicating a potential threefold coordination of a Lewis acid.

Crystal structures of n-BuLi adducts with (R,R)-TMCDA and the consequences for the deprotonation of benzene

Combinations of organolithium compounds and diamine bases have become a powerful tool in synthetic chemistry. Because of the structure-reactivity relationship, the elucidation of reaction mechanisms of these reagents is strongly connected with the structural determination of intermediate species. In mixtures of the diamine TMCDA (N,N,N',N'-tetramethylcyclohexane-1,2-diamine) and n-butyllithium, two different structures, the dimeric [n-BuLi x (R,R)-TMCDA]2 and the aggregate [(n-BuLi)2 x (R,R)-TMCDA]2, can be isolated, depending on the n-BuLi/TMCDA ratio. Thereby, [(n-BuLi)2 x (R,R)-TMCDA]2 is a rare example of an organolithium compound with a ladder arrangement of the central four-membered Li-C-Li-C rings. Two isomers of the ladder structure are formed in the crystal by changing from the enantiomerically pure to racemic TMCDA. As n-BuLi/TMCDA mixtures are also able to deprotonate benzene, these structures give hint to possible mechanisms. Supported by theoretical studies, transition states based on the dimer, the ladder structure, and a hypothetical monomer are discussed.