Amine- and ether-chelated aryllithium reagents-structure and dynamics

Crystal, Solution, and Computational Study of the Structure of Ortho-Lithium N,N-Diisopropyl-P,P-Diphenylphosphinothioic Amide

The first crystal structure of an ortho-lithium phosphinothioic amide complexed with tetramethylethylenediamine 12 is reported. The complex consists of a spirane in which the spiro-lithium is N,N- and C,S-chelated by the diamine and organophosphorus ligands, respectively. The analogous ortho anion 14 obtained by Sn(IV)/Li transmetallation in THF has also been synthesized. Nuclear magnetic resonance study of both anions showed that they exist as monomers in solution and are involved in dynamic processes including the restricted rotation around the P-N bond. 14 is converted at room temperature by nucleophilic cyclization to the dearomatized anion 15, which evolves after a few hours to the benzophosphindole sulfide 16. Density functional theory calculations supported the aggregation state in solution and were used to explore the conformational space of anion 12, the mechanism of ortho-lithiation directed by P(X)-N (X=O, S) groups, and the mechanism of formation of 15.

"Buttressing Effect" in the Halogen-Lithium Exchange in ortho-Bromo-N,N-dimethylanilines and Related Naphthalenes

Non-covalent interactions such as coordination of an organolithium reagent by a directing group and steric repulsion of substituents strongly affect the halogen-lithium exchange process. Here we present the manifestation of the "buttressing effect" - an indirect interaction between two substituents issued by the presence of a third group - and its influence on the ease and selectivity of the bromine-lithium exchange and the reactivity of formed aryllithiums. The increase of the size of the "buttressing" substituent strongly affects the conformation of a NMe2 group, forcing it to hinder ortho-bromine and thus slowing down the exchange. In naphthalene substrates bearing two bromines, this suppresses regioselectivity of the reaction. The "buttressing effect" forces formed aryllithiums to deaggregate, thus boosting their reactivity. This facilitates the decomposition via protolisys by ethereal solvents even at low temperatures and in some cases initiates fast Wurtz-Fittig coupling.

NMR detection of the strained metallacycles in organolithiums: theoretical study

For the first time through quantum chemistry methods, the effective use of 1JCLi spin-spin coupling constants as descriptors for assessing the formation of strained metallacycles is demonstrated. Both acyclic organolithiums and 3- to 7-membered metallacycles are examined. 80 organolithium compounds, including both monomeric and dimeric species, with ligands containing fluorine, nitrogen, oxygen, and carbon (in the form of carbanions), are tested. In general, the 1JCLi values below 12 Hz for monomeric species and below 6 Hz for dimeric species serve as clear indicators of strained monomeric metallacycle formation (for 6Li nuclei). The primary contributor to the overall 1JCLi value is the Fermi-contact term, which correlates directly with the carbon-lithium interatomic distance and allows to distinguish between dimers and monomers.

Choice of computational protocol for carbon-lithium spin-spin coupling constants 1 JCLi

In this work, we tested various computational schemes for calculations of ¹ J_CLi constants with a high accuracy. On the example of six organolithium reagents (phenyllithium monomer and dimer, monomer s-butyllithium, monomers of 1- and 2-lithionaphthalenes, and a methyllithium tetramer), the following aspects are discussed: (i) the role of a model system geometry, (ii) influence of solvent effects, and (iii) the choice of a functional and basis set. Practical recommendations for calculation of ¹ J_CLi with an accuracy ±2 Hz are formulated.

The Hancock Alkaloids (-)-Cuspareine, (-)-Galipinine, (-)-Galipeine, and (-)-Angustureine: Asymmetric Syntheses and Corrected 1H and 13C NMR Data

The asymmetric syntheses of all members of the Hancock alkaloid family based upon a 2-substituted N-methyl-1,2,3,4-tetrahydroquinoline core are delineated. The conjugate addition of enantiopure lithium N-benzyl- N-(α-methyl- p-methoxybenzyl)amide to 5-( o-bromophenyl)- N-methoxy- N-methylpent-2-enamide is used to generate the requisite C-2 stereogenic center of the targets, while an intramolecular Buchwald-Hartwig coupling is used to form the 1,2,3,4-tetrahydroquinoline ring. Late-stage diversification completes construction of the C-2 side chains. Thus, (-)-cuspareine, (-)-galipinine, (-)-galipeine, and (-)-angustureine were prepared in overall yields of 30%, 28%, 15%, and 39%, respectively, in nine steps from commercially available 3-( o-bromophenyl)propanoic acid in all cases. Unambiguously corrected ¹H and ¹³C NMR data for the originally isolated samples of (-)-cuspareine, (-)-galipinine, and (-)-angustureine are also reported, representing a valuable reference resource for these popular synthetic targets.

Crystal structure of a carborane endo/exo-dianion and its use in the synthesis of ditopic ligands for supramolecular frameworks

The crystal structure of a monocarba-closo-dodecaborate endo/exo-dianion is reported, featuring a delocalized endohedral charge and a sigma-type C–[Li] moiety.

Structural Revision of the Hancock Alkaloid (-)-Galipeine

The ¹H and ¹³C NMR data of synthetic samples of (S)-N(1)-methyl-2-[2'-(3″-hydroxy-4″-methoxyphenyl)ethyl]-1,2,3,4-tetrahydroquinoline, the originally proposed structure of the Hancock alkaloid (-)-galipeine, do not match those of the natural product. Herein, the preparation of the regioisomer (S)-N(1)-methyl-2-[2'-(3″-methoxy-4″-hydroxyphenyl)ethyl]-1,2,3,4-tetrahydroquinoline is reported, the ¹H and ¹³C NMR data of which are in excellent agreement with those of (-)-galipeine. Comparison of specific rotation data enables assignment of the absolute (S)-configuration of the alkaloid, and together, these data engender the structural revision of (-)-galipeine to (S)-N(1)-methyl-2-[2'-(3″-methoxy-4″-hydroxyphenyl)ethyl]-1,2,3,4-tetrahydroquinoline.

Reactivity of 1,3-Disubstituted Indoles with Lithium Compounds: Substituents and Solvents Effects on Coordination and Reactivity of Resulting 1,3-Disubstituted-2-Indolyl Lithium Complexes

Reactivity of 1,3-disubstituted indolyl compounds with lithium reagents was studied to reveal the substituents and solvent effects on coordination modes and reactivities resulting in different indolyl lithium complexes. Treatment of 1-alkyl-3-imino functionalized compounds 1-R-3-(R'N═CH)C₈H₅N [R = Bn, R' = Dipp (HL¹); R = Bn, R' = ^tBu (HL²); R = CH₃OCH₂, R' = Dipp (HL³); Dipp = ⁱPr₂C₆H₃] with Me₃SiCH₂Li or ⁿBuLi in hydrocarbon solvents (toluene or n-hexane) produced 1,3-disubstituted-2-indolyl lithium complexes [η¹:(μ₂-η¹:η¹)-1-Bn-3-(DippN═CH)C₈H₄NLi]₂ (1), {[η¹:(μ₃-η¹:η¹:η¹)-1-Bn-3-(^tBuN═CH)C₈H₄N][η²:η¹:(μ₂-η¹:η¹)-1-Bn-3-(^tBuN═CH)C₈H₄N][η¹:(μ₂-η¹:η¹)-1-Bn-3-(^tBuN═CH)C₈H₄N]Li₃} (2), and [η¹:η¹:(μ₂-η¹:η¹)-1-CH₃OCH₂-3-(DippN═CH)C₈H₄NLi]₂ (3), respectively. The bonding modes of the indolyl ligand were kept in 1 by coordination with donor solvent, affording [η¹:(μ₂-η¹:η¹)-1-Bn-3-(DippN═CH)C₈H₄NLi(THF)]₂ (4). The trinuclear complex 2 was converted to dinuclear form with a change of bonding modes of the indolyl ligand by treatment of 2 with donor solvent THF, producing [η¹:(μ₂-η¹:η¹)-1-Bn-3-(^tBuN═CH)C₈H₄NLi(THF)]₂ (5). X-ray diffraction established that compounds 1, 3, 4, and 5 crystallized as dinuclear structures with the carbanionic sp² carbon atoms of the indolyl ligands coordinated to lithium ions in a μ₂-η¹:η¹ manner, while compound 2 crystallized as a trinuclear structure and the carbanionic atoms of the indolyl moieties coordinated to lithium ions in μ₂-η¹:η¹ and μ₃-η¹:η¹:η¹ manners. When the lithiation reaction of HL¹ with 1 equiv of ⁿBuLi was carried out in THF, the monomeric lithium complex {η¹:η¹-1-Bn-3-(DippN═CH)-2-[1'-Bn-3'-(DippNCH)C₈H₅N]C₈H₄NLi(THF)} (6) having coupled indolyl moieties was obtained. The compound 6 can also be prepared by the reaction of 1 with 0.5 equiv of HL¹ with a higher isolated yield. Accordingly, the lithium complexes [η¹:η⁴-1-Bn-3-^tBuN═CH-2-(1'-Bn-3'-^tBuNCHC₈H₅N)C₈H₄NLi(L)] (L = THF, 7a; L = Et₂O, 7b) with the coupled indolyl moieties in η⁴ mode were isolated by treatment of HL² with 2 in THF or Et₂O. All complexes were characterized by spectroscopic methods, and their structures were determined by X-ray diffraction study.

Mild, Pd-catalyzed stannylation of radioiodination targets

Trialkylstannanes are versatile precursors for chemical transformations, including radiolabeling with a variety of halogens, particularly iodine. In the present work a convenient, Pd-mediated stannylation method is presented that can be performed in an open flask. The method is selective for aryl iodides allowing selective stannylations in the presence of other halogen atoms. The reaction conditions are mild, making the method compatible with chemically sensitive bioactive compounds.

Directed ortho-lithiation: observation of an unexpected 1-lithio to 3-lithio conversion of 1-lithio-naphthyllithium compounds with an ortho-directing 2-(dimethylamino)methyl group

Regioselectivity is an important aspect in the design of organic protocols involving Directed ortho-Lithiation (DoL) of arenes, in particular with those arenes containing heteroatom substituents as directing groups. The DoL of 2-[(dimethylamino)methyl]naphthalene (dman) that proceeds with low regioselectivity was revisited by varying both the nature of the lithiating reagent (either n-BuLi or t-BuLi) and/or the solvent (pentane or diethyl ether); the 3-deuterated substrate, 3-Ddman, was also investigated as a substrate to compare to that of dman. The 3-lithio regioisomer exists as tetranuclear [2-(Me2NCH2)C10H6Li-3]4, 1, both in the solid state (X-ray) and in solution (NMR). The 1-lithio regioisomer, 2a, is insoluble; in the presence of additional coordinating solvents (Et2O) or ligands (dman), it exists as dinuclear [2-(Me2NCH2)C10H6Li-1]2·L (coordinated L = Et2O: 2b, dman: 2c) in apolar solvents. Heating solutions of 2c in toluene-d8 (to 90 °C) induced a surprisingly clean and quantitative 1-lithio to 3-lithio conversion of the 1-lithio-naphthalene isomer. This type of reaction is rare in organolithium chemistry and has obvious significant implications for the design of regioselective DoL protocols; this thus represents the synthetically useful protocol for the DoL of dman in a one-pot/two-step process in toluene solution. The results of the use of 3-Ddman in these reactions gives strong credence to a mechanism involving formation of the heteroleptic species [(2-(Me2NCH2)C10H6-1)(2-(Me2NCH2)C10H6-3)Li2]·[dman], A, as the key intermediate. Intramolecular trans-lithiation takes place with A; dman becomes selectively lithiated at its 3-position, while the formerly 1-lithio-naphthalene fragment, acting as a highly unusual ortho-lithiating reagent, is converted into the N-coordinated amine, dman. In this intramolecular DoL process, free dman can be considered to act as a catalyst.

Formula weight prediction by internal reference diffusion-ordered NMR spectroscopy (DOSY)

Formula weight (FW) information is important to characterize the composition, aggregation number, and solvation state of reactive intermediates and organometallic complexes. We describe an internal reference correlated DOSY method for calculating the FW of unknown species in different solvents with different concentrations. Examples for both the small molecule (DIPA) and the organometallic complex (aggregate 1) yield excellent correlations. We also found the relative diffusion rate is inversely proportional to the viscosity change of the solution, which is consistent with the theoretical Stokes-Einstein equation. The accuracy of the least-squares linear prediction r(2) and the percentage difference of FW prediction are directly related to the density change; greater accuracy was observed with decreasing density. We also discuss the guidelines and other factors for successful application of this internal reference correlated DOSY method. This practical method can be conveniently modified and applied to the characterization of other unknown molecules or complexes.

Factors influencing prenylation of an aromatic organolithium

An apparently routine metal–halogen exchange (MHE) reaction gave variable low yields under typical conditions. 13C and 7Li NMR studies suggested a diversity of organolithiums were formed in the MHE reaction leading to a plethora of products on subsequent electrophilic aromatic substitution with geranyl bromide. A reversal of reagent addition simplified the 7Li NMR spectrum and the product distribution, in addition to increasing the isolated yield of the desired product from a variable 5-40% to a consistent 65%.

13C INEPT diffusion-ordered NMR spectroscopy (DOSY) with internal references

13C INEPT Diffusion-ordered NMR spectroscopy (DOSY) with an internal reference system was developed to study the aggregation state of THF-solvated LDA dimeric complex. Six components are clearly identified in the diffusion dimension, and their DOSY-generated 13C INEPT spectrum slices agree extremely well with their respective INEPT spectra. The correlation between log D and log FW of the linear least-squares fit to reference points of all components is exceptionally high: (r = 0.9985).

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.

One-Dimensional, Cofacial Porphyrin Polymers Formed by Self-Assembly ofmeso-Tetrakis(ERE Donor) Zinc(II) Porphyrins

A series of meso-tetrakis-(ERE donor) zinc(II) porphyrins nZn (ERE donor = 4-R-3,5-bis[(E)-methyl]phenyl; 1Zn: E = NMe2, R = Br; 2Zn: E = NMe2, R = H; 3Zn: E = OMe, R = Br; 4Zn: E = OMe, R = H) have been synthesized in excellent yields. As a result of the combination of a Lewis acidic site and eight Lewis basic sites within one molecule, monomeric molecules of nZn self-assemble to form one-dimensional porphyrin polymers [nZn](infinity) in the solid state, as confirmed for 1Zn and 3Zn by X-ray crystallography. The coordination environment around the zinc(II) ions in these polymers is octahedral. They are ligated by four equatorial nitrogen atoms of the porphyrin and two apical E atoms (E = N, O) provided by the EBrE donor groups of adjacent nZn molecules. Complexes 2Zn and 4Zn did not form single crystals, but solid-state UV/Vis analysis points to the formation of similar structures. Solution UV/Vis and 1H NMR spectroscopy indicated that interactions between 1Zn and 2Zn monomers in the polymers are stronger than between 3Zn and 4Zn monomers. Interestingly, they also revealed that the presence of a neighboring bromine atom in the EBrE donor groups has a considerable influence on the coordination properties of the benzylic N or O atoms. The zinc(II) ions of the porphyrins most likely adopt only hexacoordination in the solid state, owing to the unique predisposition of Lewis acidic and basic sites in the nZn molecules. Several parameters of the aggregates, for example, the interplanar separation between porphyrins and the zinc-zinc distances, change as a function of the coordinating E groups. The high degree of modularity in their synthesis makes these zinc(II) porphyrins an interesting new entry in noncovalent multiporphyrin assemblies.

Quantitative Data on the Effects of Alkyl Substituents and Li–O and Li–N Chelation on the Stability of Secondary α-Oxy-Organolithium Compounds

A DeltaG(eq) stability scale of secondary alpha-oxy-organolithium compounds was established from measurements of tin-lithium exchange equilibria in THF, and the quantitative effects of substituents at the anionic center on carbanion stability are presented. A new lead-lithium exchange equilibrium reaction was also investigated and shown to be a very useful alternative for the determination of the relative stability of the more sterically hindered organolithium compounds. Alkyl groups adversely affect the stability of organolithium compounds when attached to the carbon bearing the negative charge, but the extent of this effect is highly dependent on the nature of the rest of the substituents attached to the anionic center. Quantitative data on the stabilization imparted to organolithium compounds by Li-O and Li-N chelation have been determined for a variety of systems. The formation of four- and five-membered chelate rings leads to a considerable stabilization of the organolithium compound, while chelation through the formation of six-membered rings affords no extra stabilization to this type of organometallics. Multinuclear NMR experiments carried out on several alpha-oxy-organolithium compounds to determine their aggregation state are supportive of these species being monomers in THF solution.