Structure, bonding, and solvation of lithium vinylcarbenoids

[reaction: see text] Molecular modeling was used to determine the structure of lithium vinylcarbenoids in the gas phase and in THF solution. Solvent effects were modeled by microsolvation with explicit THF ligands on each of the lithium atoms. The carbenoid geometries are dependent on the heteroatom and on solvation. The calculations predict 1-chlorovinyllithium and 1-bromovinyllithium to be a mixture of monomer and dimer at 200 K and mostly monomer at higher temperatures, whereas the 1-fluoro-, 1-methoxy-, and 1-dimethylaminovinyllithium are predicted to be dimeric in solution.     

Structure, bonding, and solvation of dilithiodiamines

Computational methods were used to determine the structure of dilithiodiamines and the effects of solvation by ethereal solvents. Solvation was examined by the use of microsolvation with explicit dimethyl ether or THF ligands and by the combined use of microsolvation and the IEFPCM continuum solvent model. It was determined that each of the compounds studied exists exclusively as a bridged intramolecular dimer, both in the gas phase and in solution. Thermodynamic properties were calculated at 200 and 298 K to estimate the effect of temperature on the cyclization energies. Infrared spectroscopy was used to confirm the proposed intramolecular dimer structures.     

Density functional study of the lithiation of cyclic vinyl ethers in solution

Lithiation of three cyclic vinyl ethers—2,3-dihydrofuran, 3,4-dihydro-2H-pyran, and 2,3-dihydrooxepin in ethereal solution are investigated at the density functional theory level of B3LYP/6-31++G(d,p). Several solvation models were used, including the microsolvation model, the continuum models such as the Onsager model, the polarized continuum models, and the isodensity polarized continuum model, and the mixed discrete-continuum model. Both the microsolvation and the mixed discrete-continuum model gave results consistent with experiments. Theoretical calculations also indicate that lithiation of 2,3-dihydrooxepin undergoes allylic lithiation concomitant with ring opening.     

A computational study of lithium enolate mixed aggregates

Ab initio calculations were performed to examine the formation of mixed dimer and trimer aggregates between the lithium enolate of acetaldehyde (lithium vinyloxide, LiOV) and lithium chloride, lithium bromide, and lithium amides. Gas-phase calculations showed that in the absence of solvation effects, the mixed trimer 2LiOV.LiX is the most favored species. Solvation in ethereal solvents was modeled by a combination of specific coordination of dimethyl ether ligands on each lithium and "dielectric solvation" (DSE, dielectric solvation energies), immersion of each molecule in a cavity within a continuous dielectric having the dielectric constant of THF at room temperature. DSE is less important for aggregates (reduced dipoles or quadrupoles) than monomers (dipoles) and is also reduced for the coordinatively solvated species. Both solvation terms reduce the exothermicity of aggregation. In many cases, lithium salts that are three- rather than four-coordinate have significant populations at room temperature. The strongly basic lithium amides prefer mixed aggregates with weaker bases than homoaggregates. The computational results are consistent with the limited experimental data available.     

Ketone enolization with lithium dialkylamides: the effects of structure,solvation, and mixed aggregates with excess butyllithium

The effects of lithium dialkylamide structure, mixed aggregate formation, and solvation on the stereoselectivity of ketone enolization were examined. Of the lithium dialkylamides examined, lithium tetramethylpiperidide (LiTMP) in THF resulted in the best enolization selectivity. The stereoselectivity was further improved in the presence of a LiTMP-butyllithium mixed aggregate. The use of less polar solvents reduced the enolization stereoselectivity. Ab initio calculations predict LDA and LiTMP to form mixed cyclic dimers in ethereal solvents. The calculations also predict LiTMP-alkyllithium mixed aggregates to competitively inhibit the formation of less stereoselective LiTMP-lithium enolate mixed aggregates.     

A computational study of oxiranyllithium

[reaction: see text] Computational methods were used to determine the structure, bonding, and aggregation states of oxiranyllithium in the gas phase and in THF solution, at 200 and 298 K. THF solvation was modeled by microsolvation with explicit THF ligands, forming a supermolecule that includes the oxiranyllithium aggregate and its first solvation shell. Because oxiranyllithium has a chiral center, two diastereomeric dimers were formed, the RR and the RS, along with their enantiomers. Similarly, three diastereomers of the tetramer were formed, the RRRR, RRRS, and RRSS and their enantiomers. Oxiranyllithium was found to exist predominantly as the tetramer in the gas phase, while the dimer was the dominant species in THF solution. The relative concentrations of the different stereoisomers were calculated from equilibrium constants.     

DFT study of the effect of sigma-ligands on the structure of ester enolates in THF, as models of the active center in the anionic polymerization of methyl methacrylate.

A Density Functional Theory (DFT) study was carried out on structures of the lithium ester enolate of methyl isobutyrate (MIB-Li) in THF solution, in the presence of TMEDA, dimethoxyethane (DME), crown ether 12-crown-4, and cryptand-2,1,1, as electron donor ligands (sigma-ligands). Both specific solvation with THF and/or ligand molecules and nonspecific solvation by the solvent continuum were taken into account. The possibility of ligand-separated ion pair formation was analyzed for each of the ligands, including THF alone. In most cases peripherally solvated dimers are the most stable species. Only in the presence of cryptand-2,1,1 was a ligand-separated triple ion pair, (MIB-Li-MIB)(-)(THF)(2),Li(2,1,1)(1)(+), shown to be comparable in stability to the THF-solvated dimer, (MIB-Li)(2)(THF)(4). These results are in agreement with experimental NMR data on the structure of MIB-Li in the presence of DME, 12-crown-4, and cryptand-2,1,1. An upfield shift of the (13)C NMR signal of the alpha-carbon of MIB-Li observed in the presence of cryptand-2,1,1, originally attributed to a ligand-separated monomer, MIB(-),Li(2,1,1)(+), was well reproduced by Hartree--Fock calculated NMR shifts for the predicted ligand-separated triple ion pair.     

Solvent and temperature effects on the reduction and amination reactions of electrophiles by lithium dialkylaminoborohydrides

The influence of temperature and solvent effects on the reduction and amination mechanisms of iodomethane by lithium N,N-diisopropylaminoborohydride (iPr-LAB) was examined in varying concentrations of THF and dioxane. The reactions of benzyl chloride and trimethylsilyl chloride with iPr-LAB in THF were also studied. The amination of iodomethane is favored over reduction at low and room temperatures in pure THF and with increasing the amount of dioxane in THF. At higher temperatures, the reduction reaction appears to compete with the amination. In dioxane solvent, however, iodomethane yields exclusively the amination product regardless of temperature. On the other hand, reduction by iPr-LAB to the aminoborane is the only product observed in THF when benzyl chloride and trimethylsilyl chloride are used. To understand the solvent effects on the product distribution, ab initio and density functional theory (DFT) calculations were used to examine the mechanisms of reduction and amination of chloromethane and bromomethane by lithium dimethylaminoborohydride (LAB) in THF and dioxane. The results of these calculations show that the relative reaction barrier heights are significantly affected by the nature of the coordinated solvent molecule and thus lend support to the experimental observations.     

A theoretical investigation into the inversion barrier of dipole-stabilized alpha-aminoorganolithiums

[reaction: see text] Results from density functional theory calculations (B3LYP/6-31+G) suggest that inversion of the monomer of 2-lithio-N-formylpyrrolidine (2) in coordinating ethereal solvent occurs with an activation barrier of 15.7 kcal/mol, while the inversion of the monomer in a noncoordinating hydrocarbon solvent is considerably slower. However, aggregation into a trimer in hydrocarbon solvent restores the low inversion barrier. This study suggests that solvation and aggregation may influence the mechanism and rate of racemization of dipole-stabilized alpha-aminoorganolithiums.     

A computational study on the mechanism for the chemical fixation of nitric oxide leading to 1,2,3-oxadiazole 3-oxide

The chemical fixation of nitric oxide (NO) reacting with alkynyllithium to produce 5-methyl-3-oxide-1,2,3-oxadiazole has been investigated by using ab initio (U)MP2 and DFT/(U)B3LYP methods. The solvent effect was assessed using the combination of microsolvation model with explicit THF ligands on lithium and continuum solvent model based on the SCRF/CPCM method at the (U)B3LYP/6-31G* level. Our results reveal that the overall reaction is stepwise and considered to include two processes. In process 1, the nitrogen atom in nitric oxide at first attacks the C(1) atom in alkynyllithium to afford the intermediate 5. In process 2, after another nitric oxide reacted with the intermediate 5 to produce 8a, we found that two pathways are involved. For path 1, the O(2) atom at first attacks the C(2) atom to form a five-membered ring geometry, and then lithium can rotate around the N(1)-O(1) bond, leading to the product 5-methyl-3-oxide-1,2,3-oxadiazole followed addition of water. However, for path 2, lithium atom rotates first around the N(1)-O(1) bond, and then the product 5-methyl-3-oxide-1,2,3-oxadiazole is also generated by addition of water. Our calculations indicate that path 1 is more favorable than path 2 in the gas phase, while both of them exist possibly in THF solvent. The overall reaction is exothermic.     

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.     

Structures and aggregation states of fluoromethyllithium and chloromethyllithium carbenoids in the gas phase and in ethereal solvent

Using high-level quantum mechanical calculations and various models to account for solvation effects, monomers and dimers of fluoromethyllithium and chloromethyllithium carbenoids are studied in the gas phase and in dimethyl ether solvent. A combination of explicit microsolvation and a continuum reaction field is required to account fully for the structural and energetic effects of solvation. One important effect of solvent is the stabilization of charge-separated structures in which the lithium-halogen distance is much greater than in the gas-phase structures. At the most complete level of theory the 173 K standard-state free energy of dimerization of fluoromethyllithium in dimethyl ether is predicted to be -0.9 kcal mol(-)(1), while that for chloromethyllithium in the same solvent is predicted to be 3.7 kcal mol(-)(1). This suggests that, under typical experimental conditions, dimers of chloroalkyllithiums will not be observed, while dimers of fluoroalkyllithiums may contribute to the equilibrium population at a detectable level.     

Ab initio analysis of lithium dimethylaminoborohydride

Ab initio calculations were used to determine the equilibrium geometries and energies of lithium dimethylaminoborohydride. Relative energies of the monomeric and dimeric species were calculated in the gas phase and for the dimethyl ether microsolvated molecules. The most stable structure was a dimer in which the lithium and boron atoms were bridged by two hydrogen atoms, similar to the three-center two-electron bonds in diborane. This hydrogen bridging was maintained in the lithium dimethylaminoborohydride bis(dimethyl ether) microsolvate.     

A theoretical study on nBuLi/lithium aminoalkoxide aggregation in hexane and THF

Theoretical calculations on aggregation of nBuLi/lithium aminoalkoxide superbases, such as nBuLi/LiDMAE (LiDMAE = Me(2)N(CH(2))(2)OLi) and nBuLi/LiPM (LiPM = Li-N-methyl-2-pyrrolidine methoxide) in gas phase and solution are reported. The combination of equimolar amounts of each component in hexane induced unusual reactivity of the resulting superbase, which remains misunderstood. In order to elucidate the corresponding reaction mechanisms, it is imperative to get a deeper insight into the energetics of aggregation and the effect of the medium on equilibrium constants. In the present study, we compute and compare the stability of (nBuLi)(n), (LiPM)(n), and equimolecular mixed aggregates (nBuLi:LiPM)(n) in gas phase, hexane, and THF. Calculations have been carried out at the density functional theory level (B3LYP/6-31G(d)) using continuum and discrete continuum models of solvation. Higher-level calculations (MP2/aug-ccpVQZ) have been done in some cases for test purposes. Enthalpic and entropic contributions have been discussed and were shown to play an opposite role in hexane (or gas phase) and THF. The characteristics of LiPM and mixed nBuLi/LiPM solutions are found to be significantly different from those of nBuLi solutions. These calculations are in accordance with experimental data in both hexane and THF. Further comparison of theoretical and experimental results for gas-phase Li(+)-THF and Li(+)-DME complexes has enabled a discussion on computational errors for entropic contributions in THF. The value for the release of a THF solvent molecule is proposed to be DeltaS approximately 23 eu. These results provide new insights to the aggregation of organolithium compounds in solution and will be useful for the investigation of other systems.     

Structure, bonding, and aggregation of selenium-containing organolithium species

1,3-Dioxo compounds can be prepared from selenium-mediated carbonylation of lithium enolates in the presence of carbon monoxide. Intermediates in this reaction include several organic species that contain both selenium and lithium. The first step in understanding the detailed reaction mechanism is to understand the structure of these intermediates. Like most organolithium compounds, these species can exist as aggregates in solution. The B3LYP density functional theory (DFT) method was used to examine the gas phase and THF solvated structures of these compounds. The calculations showed that each of the compounds forms dimers or higher aggregates in the gas phase. Aggregates are also formed in THF solution, although solvation favors lower aggregates as compared to the gas phase.     

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.     

Molecular structures of THF-solvated alkali-metal 2,2,6,6-tetramethylpiperidides finally revealed: X-ray crystallographic, DFT, and NMR (including DOSY) spectroscopic studies

The often studied THF solvates of the utility alkali-metal amides lithium and sodium 2,2,6,6-tetramethylpiperidide are shown to exist in the solid state as asymmetric cyclic dimers containing a central M(2)N(2) ring and one molecule of donor per metal to give a distorted trigonal planar metal coordination. DFT studies support these structures and confirm the asymmetry in the ring. In C(6)D(12) solution, the lithium amide displays a concentration-dependent equilibrium between a solvated and unsolvated species which have been shown by diffusion-ordered NMR spectroscopy (DOSY) to be a dimer and larger oligomer, respectively. A third species, a solvated monomer, is also present in very low concentration, as proven by spiking the NMR sample with THF. In contrast, the sodium amide displays a far simpler C(6)D(12) solution chemistry, consistent with the solid-state dimeric arrangement but with labile THF ligands.     

Electronic structure calculations on lithium battery electrolyte salts

New lithium salts for non-aqueous liquid, gel and polymeric electrolytes are crucial due to the limiting role of the electrolyte in modern lithium batteries. The solvation of any lithium salt to form an electrolyte solution ultimately depends on the strength of the cation-solvent vs. the cation-anion interaction. Here, the latter is probed via HF, B3LYP and G3 theory gas-phase calculations for the dissociation reaction: LiX <--> Li(+) + X(-). Furthermore, a continuum solvation method (C-PCM) has been applied to mimic solvent effects. Anion volumes were also calculated to facilitate a discussion on ion conductivities and cation transport numbers. Judging from the present results, synthesis efforts should target heterocyclic anions with a size of ca. 150 A(3) molecule(-1) to render new highly dissociative lithium salts that result in electrolytes with high cation transport numbers.     

Solvent-dependent mixed complex formation-NMR studies and asymmetric addition reactions of lithioacetonitrile to benzaldehyde mediated by chiral lithium amides

Lithioacetonitrile and a chiral lithium amide with an internally coordinating methoxy group form mixed dimers in diethyl ether (DEE) and in tetrahydrofuran (THF) according to NMR studies. Based on the observed (6)Li,(1)H heteronuclear Overhauser effects, in THF lithioacetonitrile is present in a mixed complex with the chiral lithium amide, and this complex has a central N-Li-N-Li core. In DEE, on the other hand, the acetonitrile anion bridges two lithiums of the dimer to form a central six-membered Li-N-C-C-Li-N ring. Gauge individual atomic orbital DFT calculations of the (13)C NMR chemical shifts of the DEE- and THF-solvated mixed dimers show good agreement with those obtained experimentally. Lithioacetonitrile complexed to the chiral lithium amide has been employed in asymmetric addition to benzaldehyde in both DEE and THF. In THF the product, (S)-3-phenyl-3-hydroxy propionitrile, is formed in 55 % ee and in DEE the R enantiomer is formed in 45 % ee. This change in stereoselectivity between solutions in DEE and THF was found to be general among a number of different chiral lithium amides, all with an internal chelating methoxy group.     

Aggregation of alkyllithiums in tetrahydrofuran

Density functional theory was used to examine the solvation number and aggregation state of several alkyllithium compounds in clusters with tetrahydrofuran molecules coordinated to each lithium atom. We then made the microsolvation approximation and approximated the bulk free energy of solvation by the free energy of clustering with solvent molecules in the gas phase. The trends in the computed results are in reasonable agreement with the available experimental data.