Consequences of correlated solvation on the structures and reactivities of RLi-diamine complexes: 1,2-addition and alpha-lithiation reactions of imines by TMEDA-solvated n-butyllithium and phenyllithium

Sparteine-Free, Highly Stereoselective Construction of Complex Allylic Alcohols Using 1,2-Metallate Rearrangements

Stereotriads bearing allylic alcohols are privileged structures in natural products, and new methods accessing these in a stereoselective fashion are highly sought after. Toward this goal, we found that the use of chiral polyketide fragments allows for performing the Hoppe-Matteson-Aggarwal rearrangement in the absence of sparteine with high yields and diastereoselectivities, rendering this protocol a highly valuable alternative to the Nozaki-Hiyama-Takai-Kishi reaction. The switch of directing groups in most cases resulted in the reversed stereochemical outcome, which could be explained by conformational analysis on density functional theory level and a Felkin-like model.

Scalable Birch reduction with lithium and ethylenediamine in tetrahydrofuran

[Figure: see text].

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.

Investigation of the Deprotonative Generation and Borylation of Diamine-Ligated α-Lithiated Carbamates and Benzoates by in Situ IR spectroscopy

Diamine-mediated α-deprotonation of O-alkyl carbamates or benzoates with alkyllithium reagents, trapping of the carbanion with organoboron compounds, and 1,2-metalate rearrangement of the resulting boronate complex are the primary steps by which organoboron compounds can be stereoselectively homologated. Although the final step can be easily monitored by ¹¹B NMR spectroscopy, the first two steps, which are typically carried out at cryogenic temperatures, are less well understood owing to the requirement for specialized analytical techniques. Investigation of these steps by in situ IR spectroscopy has provided invaluable data for optimizing the homologation reactions of organoboron compounds. Although the deprotonation of benzoates in noncoordinating solvents is faster than that in ethereal solvents, the deprotonation of carbamates shows the opposite trend, a difference that has its origin in the propensity of carbamates to form inactive parasitic complexes with the diamine-ligated alkyllithium reagent. Borylation of bulky diamine-ligated lithiated species in toluene is extremely slow, owing to the requirement for initial complexation of the oxygen atoms of the diol ligand on boron with the lithium ion prior to boron-lithium exchange. However, ethereal solvent, or very small amounts of THF, facilitate precomplexation through initial displacement of the bulky diamines coordinated to the lithium ion. Comparison of the carbonyl stretching frequencies of boronates derived from pinacol boronic esters with those derived from trialkylboranes suggests that the displaced lithium ion is residing on the pinacol oxygen atoms and the benzoate/carbamate carbonyl group, respectively, explaining, at least in part, the faster 1,2-metalate rearrangements of boronates derived from the trialkylboranes.

Ligand Binding Constants to Lithium Hexamethyldisilazide Determined by Diffusion-Ordered NMR Spectroscopy

We report the direct measurement of ligand-binding constants of organolithium complexes using a ¹H NMR/diffusion-ordered NMR spectroscopy (DOSY) titration technique. Lithium hexamethyldisilazide complexes with ethereal and ester donor ligands (THF, diethyl ether, MTBE, THP, tert-butyl acetate) are characterized using ¹H NMR and X-ray crystallography. Their aggregation and solvation states are confirmed using diffusion coefficient-formula weight correlation analysis, and the ¹H NMR/DOSY titration technique is applied to obtain their binding constants. Our work suggests that steric hindrance of ethereal ligands plays an important role in the aggregation, solvation, and reactivity of these complexes. It is noteworthy that diffusion methodology is utilized to obtain binding constants.

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.

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 Use of (-)-Sparteine/Organolithium Reagents for the Enantioselective Lithiation of 7,8-Dipropyltetrathia[7]helicene: Single and Double Kinetic Resolution Procedures

The effect of organolithium reagent (RLi: R=nBu, iPr, sBu, tBu), solvent (diethyl ether, diethyl ether/THF and MTBE), and stoichiometry on the (-)-sparteine-mediated silylation of 7,8-dipropyltetrathia[7]helicene shows that, unusually, substantially more than 0.5 equivalent of RLi (R=iPr, sBu, tBu) and a large excess of (-)-sparteine (R=nBu, sBu) is often needed to achieve substantial conversions and good ee values. With nBuLi, however, just one equivalent of the organolithium reagent is sufficient to obtain high conversions. Our best results were obtained using the convenient tBuLi/(-)-sparteine adduct with which the need for a high (-)-sparteine/RLi ratio can be avoided. Single- and double-kinetic resolution (KR) procedures give enantiopure samples of 2-trimethylsilyl- and 2,13-di(trimethylsilyl)-7,8-dipropyltetrathia[7]helicene and two-step double-KR combining (-)-sparteine/sBuLi and chiral formamides affords the synthetically valuable 2-formyl-7,8-dipropyltetrathia[7]helicene. This is the first use of (-)-sparteine for the enantioselective lithiation of helicenes and the first report of tBuLi outperforming sBuLi in a (-)-sparteine-mediated procedure.

Solution structures of lithium amino alkoxides used in highly enantioselective 1,2-additions

Lithium ephedrates and norcarane-derived lithium amino alkoxides used to effect highly enantioselective 1,2-additions on large scales have been characterized in toluene and tetrahydrofuran. The method of continuous variations in conjunction with (6)Li NMR spectroscopy reveals that the lithium amino alkoxides are tetrameric. In each case, low-temperature (6)Li NMR spectra show stereoisomerically pure homoaggregates displaying resonances consistent with an S4-symmetric cubic core rather than the alternative D2d core. These assignments are supported by density functional theory computations and conform to X-ray crystal structures. Slow aggregate exchanges are discussed in the context of amino alkoxides as chiral auxiliaries.

Synthesis and kinetic resolution of N-Boc-2-arylpiperidines

An efficient kinetic resolution of 2-arylpiperidines was developed using a chiral base.

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.

Structure determination using the method of continuous variation: lithium phenolates solvated by protic and dipolar aprotic ligands

The method of continuous variation (MCV) was used in conjunction with (6)Li NMR spectroscopy to characterize four lithium phenolates solvated by a range of solvents, including N,N,N',N'-tetramethylethylenediamine, Et2O, pyridine, protic amines, alcohols, and highly dipolar aprotic solvents. Dimers, trimers, and tetramers were observed, depending on the precise lithium phenolate-solvent combinations. Competition experiments (solvent swaps) provide insights into the relative propensities toward mixed solvation.

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.

Lithium phenolates solvated by tetrahydrofuran and 1,2-dimethoxyethane: structure determination using the method of continuous variation

The method of continuous variation in conjunction with (6)Li NMR spectroscopy was used to characterize lithium phenolates solvated by tetrahydrofuran and 1,2-dimethoxyethane. The strategy relies on the formation of ensembles of homo- and heteroaggregated phenolates. The symmetries and concentration dependencies of the heteroaggregates attest to the aggregation numbers of the homoaggregates. The structurally diverse phenols afford substrate- and solvent-dependent combinations of lithium phenolate monomers, dimers, trimers, tetramers, and pentamers. We discuss the refinement of protocols for characterizing O-lithiated species. Computational studies examine further the substituent and solvent dependencies of aggregation.

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.

Unusual reactivity patterns of 1,3,6,8-tetraazatricyclo-[4.4.1.1(3,8)]-dodecane (TATD) towards some reducing agents: synthesis of TMEDA

N,N,N,N-tetramethylethylenediamine (TMEDA) can be synthesized by simple reduction of 1,3,6,8-tetraazatricyclo-[4.4.1.1.(3,8)]dodecane (TATD), an aminal cage type amine, with formic acid. The aminal can be converted to TMEDA in high yield very easily and in a very short time. We comment on the scope and limitations of the reduction of this aminal and propose a possible reaction mechanism.

Lithium diisopropylamide: solution kinetics and implications for organic synthesis

Lithium diisopropylamide (LDA) is a prominent reagent used in organic synthesis. In this Review, rate studies of LDA-mediated reactions are placed in the broader context of organic synthesis in three distinct segments. The first section provides a tutorial on solution kinetics, emphasizing the characteristic rate behavior caused by dominant solvation and aggregation effects. The second section summarizes substrate- and solvent-dependent mechanisms that reveal basic principles of solvation and aggregation. The final section suggests how an understanding of mechanism might be combined with empirical methods to optimize yields, rates, and selectivities of organolithium reactions and applied to organic synthesis.

Structure of n-Butyllithium in Mixtures of Ethers and Diamines: Influence of Mixed Solvation on 1,2-Additions to Imines

n-BuLi in diamine/dialkyl ether mixtures forms ensembles of hetero- and homosolvated dimers. Solutions in TMEDA/THF (TMEDA = N,N,N',N'-tetramethylethylenediamine) are not amenable to detailed investigation because of rapid ligand exchange. TMCDA/THF mixtures (TMCDA = trans-N,N,N',N'-tetramethylcyclohexanediamine) afford clean assignments for a mixture of homo- and heterosolvated dimers but demonstrate poor control over structure. TMCDA/tetrahydropyran (THP) mixtures and TMEDA/Et2O mixtures afford clean structural assignments as well as excellent structural control. Rate studies of the 1,2-addition of n-BuLi using TMCDA/THP mixtures reveal cooperative solvation in which both THP and TMCDA coordinate to lithium at the monomer- and dimer-based transition structures. The two mechanisms are affiliated with markedly different stereochemistries of the 1,2-addition to imines. The results show strong parallels with previous investigations of 1,2-additions in TMEDA/Et2O mixtures.

Mild Iodine−Magnesium Exchange of Iodoaromatics Bearing a Pyrimidine Ring with Isopropylmagnesium Chloride

[reaction: see text] Iodoaromatics bearing a reactive pyrimidine ring underwent a clean iodine-magnesium exchange with isopropylmagnesium chloride in the presence of bis[2-(N,N-dimethylamino)ethyl] ether to provide the corresponding Grignard reagents. The presence of bis[2-(N,N-dimethylamino)ethyl] ether prevented reduction of the pyrimidine ring and addition by isopropylmagnesium chloride. As a result, the newly formed reactive Grignard reagents were allowed to react with electrophiles in a highly selective manner to afford adducts in excellent yields.

Reactivity series for s-BuLi/diamine-mediated lithiation of N-Boc pyrrolidine: applications in catalysis and lithiation of N-Boc piperidine

Using competition experiments between a range of ligands and (-)-sparteine, a reactivity series for N-Boc pyrrolidine lithiation using s-BuLi/diamines has been constructed; the results indicate that the s-BuLi/(+)-sparteine surrogate complex is more reactive than s-BuLi/(-)-sparteine and this has been exploited in the selection of ligand pairs for ligand exchange catalytic asymmetric lithiation of N-Boc pyrrolidine and lithiation of N-Boc piperidine.

Addition of n-Butyllithium to an Aldimine: Role of Chelation, Aggregation, and Cooperative Solvation

Rate studies of the addition of n-BuLi in the presence of TMEDA to potentially chelating aldimines are consistent with a combination of monomer- and dimer-based mechanisms. Using mixtures of TMEDA and Et2O reveals cooperative solvation in which both Et2O and TMEDA coordinate to lithium at the monomer- and dimer-based transition structures. The four discrete mechanisms are affiliated with markedly different stereochemistries of the 1,2-addition.

Ring fission of chiral cyclic acetals plus intramolecular [4 + 2] cycloaddition: a sequential access to medium-size lactones. Application to the synthesis of carbasugars

A set of alpha,beta-unsaturated cyclic acetals were reacted with t-butyllithium in THF. A conjugate elimination took place, triggering a ketal ring cleavage when two eq. of TMEDA were added to the medium. The lithium alcoholate thus obtained could be trapped in situ by 2,2,2-trifluoroethyl acrylate. The resulting acryloyl (1Z,3E)-dienyl ether was then submitted to an intramolecular Diels-Alder cycloaddition under high pressure (12 kbar, 50 degrees C). Depending on the structure of the linkage between the diene and the acrylate, the expected ("fused") lactones were obtained with total endo- or exo-selectivities and high to complete diastereoselectivities. A remarkable inversion of selectivity, from endo to exo, with respect to the stereochemical elements of the tether could be observed in these cases. A five step diastereoselective transformation of two of the resulting nine-membered ring lactones into modified carbasugars was finally achieved in 23% overall yield.

An Experimental and Computational Study of the Enantioselective Lithiation of N-Boc-pyrrolidine Using Sparteine-like Chiral Diamines

The enantioselective lithiation of N-Boc-pyrrolidine using sec-butyllithium and isopropyllithium in the presence of sparteine-like diamines has been studied experimentally and computationally at various theoretical levels through to B3P86/6-31G*. Of the (-)-cytisine-derived diamines (N-Me, N-Et, N-(n)Bu, N-CH(2)(t)Bu, N-(i)Pr) studied experimentally, the highest enantioselectivity (er 95:5) was observed with the least sterically hindered N-Me-substituted diamine, leading to preferential removal of the pro-R proton i.e., opposite enantioselectivity to (-)-sparteine. The experimental result with the N-Me-substituted diamine correlated well with the computational results: at the B3P86/6-31G* level, the sense of induction was correctly predicted; the lowest energy complex of isopropyllithium/diamine/N-Boc-pyrrolidine also had the lowest activation energy (DeltaH++ = 11.1 kcal/mol, DeltaG++= 11.5 kcal/mol) for proton transfer. The computational results with the N-(i)Pr-substituted diamine identified a transition state for proton transfer with activation energies of DeltaH++= 11.7 kcal/mol and DeltaG++= 11.8 kcal/mol (at the B3P86/6-31G* level). Although comparable to (-)-sparteine and the N-Me-substituted diamine, these DeltaH++ and DeltaG++ values are at odds with the experimental observation that use of the N-(i)Pr-substituted diamine gave no product. It is suggested that steric crowding inhibits formation of the prelithiation complex rather than increasing the activation enthalpy for proton transfer in the transition state. Three other ligands (N-H and O-substituted as well as a five-membered ring analogue) were studied solely using computational methods, and the results predict that the observed enantioselectivity would be modest at best.