Solvent effects on the aggregation state of lithium dialkylaminoborohydrides

Aggregation and Solvation of Sodium Hexamethyldisilazide: Across the Solvent Spectrum

We report solution structures of sodium hexamethyldisilazide (NaHMDS) solvated by >30 standard solvents (ligands). These include: toluene, benzene, and styrene; triethylamine and related trialkylamines; pyrrolidine as a representative dialkylamine; dialkylethers including THF, tert-butylmethyl ether, and diethyl ether; dipolar ligands such as DMF, HMPA, DMSO, and DMPU; a bifunctional dipolar ligand nonamethylimidodiphosphoramide (NIPA); polyamines N,N,N',N'-tetramethylenediamine (TMEDA), N,N,N',N″,N″-pentamethyldiethylenetriamine (PMDTA), N,N,N',N'-tetramethylcyclohexanediamine (TMCDA), and 2,2'-bipyridine; polyethers 12-crown-4, 15-crown-5, 18-crown-6, and diglyme; 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane ([2.2.2] cryptand); and tris[2-(2-methoxyethoxy)ethyl]amine (TDA-1). Combinations of 1H, 13C, 15N, and 29Si NMR spectroscopies, the method of continuous variations, X-ray crystallography, and density functional theory (DFT) computations reveal ligand-modulated aggregation to give mixtures of dimers, monomers, triple ions, and ion pairs. 15N-29Si coupling constants distinguish dimers and monomers. Solvation numbers are determined by a combination of solvent titrations, observed free and bound solvent in the slow exchange limit, and DFT computations. The relative abilities of solvents to compete in binary mixtures often match that predicted by conventional wisdom but with some exceptions and evidence of both competitive and cooperative (mixed) solvation. Crystal structures of a NaHMDS cryptate ion pair and a 15-crown-5-solvated monomer are included. Results are compared with those for lithium hexamethyldisilazide, lithium diisopropylamide, and sodium diisopropylamide.

Mixed aggregates of 1-methoxyallenyllithium with lithium chloride

A combined computational and ¹³C NMR study was used to investigate the formation of mixed aggregates of 1-methoxyallenyllithium and lithium chloride in tetrahydrofuran (THF) solution. The observed and calculated chemical shifts, as well as the calculated free energies of mixed aggregate formation (MP2/6-31+G(d)), are consistent with the formation of a mixed dimer as the major species in solution. Free energies of mixed dimer, trimer, and tetramer formation were calculated by using the B3LYP and MP2 methods and the 6-31+G(d) basis set. The two methods generated different predictions of which mixed aggregates will be formed, with B3LYP/6-31+G(d) favoring mixed trimers and tetramers in THF solution, and MP2/6-31+G(d) favoring mixed dimers. Formation of the sterically unhindered mixed dimers is also consistent with the enhanced reactivity of these compounds in the presence of lithium chloride. The spectra are also consistent with some residual 1-methoxyallenyllithium tetramer, as well as small amounts of higher mixed aggregates. Although neither computational method is perfect, for this particular system, the calculated free energies derived using the MP2 method are in better agreement with experimental data than those derived using the B3LYP method.

A computational study of lithium cuprate mixed aggregates

Lithium dialkylcuprates may potentially form mixed aggregates with many species in solution. Those include excess alkyllithium used to prepare the cuprate, lithium halide, and lithium cyanide from cuprate preparation and from coupling reactions with alkyl halides, higher order cuprates, and species resulting from incomplete cuprate reactions. The M06 DFT method was used to elucidate the structures and energies of formation of potential mixed aggregates. A comparison was made to available experimental data.

A computational study of lithium ketone enolate aggregation in the gas phase and in THF solution

The aggregation state of several lithium enolates were calculated in the gas phase and in THF solution by the B3LYP DFT and MP2 methods. The gas phase free energies of aggregate formation were underestimated by the DFT calculations, compared to those obtained by the G3MP2 method, although DFT did correctly predict the hexamer to be the major gas phase species. The DFT calculations correctly predicted the tetramer to be the major species in THF, while MP2 underestimated the stability of the tetramer relative to the dimer.

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.

Novel diborane-analogue transition structures for borane reactions with alkyl halides

Ab initio and DFT calculations were performed to examine the mechanisms of reduction of alkyl halides and formaldehyde by borane. With alkyl halides, the optimized transition structure geometry resembled diborane, with a pair of hydrogen atoms bridging the boron and carbon atoms by three-center-two-electron bonds. A similar transition structure was found for the reduction of formaldehyde, although it was not the lowest-energy transition structure. Solvation by dimethyl ether or dimethyl sulfide disrupted this bridging with chloromethane, while both ligands dissociated from borane during the reduction of formaldehyde. The high calculated activation free energies of alkyl halide reduction are consistent with their observed lack of reactivity with borane.

Gas-Phase Reactions of Lithium Dimethylaminoborohydride and Related Species

[reaction: see text] Ab initio and density functional theory (DFT) calculations were used to examine the mechanisms of reduction and amination of chloromethane by lithium dimethylaminoborohydride (LAB) in the gas phase. For comparison, the amination of chloromethane by lithium dimethylamide and the reduction by borane, diborane, and borohydride ions were also examined. The reduction of chloromethane by LAB occurred most readily from a conformation that allowed coordination of the lithium atom to the chloride leaving group, and the most favorable amination pathway occurred by a backside attack of the nitrogen nucleophile on chloromethane.

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.