Optimization of Organolithium Reactions

A strategy for the controllable generation of organic superbases from benchtop-stable salts

Organic superbases are a distinct class of strong base that enable numerous modern reaction applications. Despite their great synthetic potential, widespread use and study of superbases are limited by their air sensitivity and difficult preparation. To address this, we report air-stable carboxylate salts of BTPP and P2-t-Bu phosphazene superbases that, when added to solution with an epoxide, spontaneously generate freebase. These systems function as effective precatalysts and stoichiometric prereagents for superbase-promoted addition, substitution and polymerization reactions. In addition to improving the synthesis, shelf stability, handling and recycling of phosphazenes, this approach enables precise regulation of the rate of base generation in situ. The activation strategy effectively mimics manual slow addition techniques, allowing for control over a reaction's rate or induction period and improvement of reactions that require strong base but are also sensitive to its presence, such as Pd-catalyzed coupling reactions.

Lithiated Oppolzer Enolates: Solution Structures, Mechanism of Alkylation, and Origin of Stereoselectivity

Camphorsultam-based lithium enolates referred to colloquially as Oppolzer enolates are examined spectroscopically, crystallographically, kinetically, and computationally to ascertain the mechanism of alkylation and the origin of the stereoselectivity. Solvent- and substrate-dependent structures include tetramers for alkyl-substituted enolates in toluene, unsymmetric dimers for aryl-substituted enolates in toluene, substrate-independent symmetric dimers in THF and THF/toluene mixtures, HMPA-bridged trisolvated dimers at low HMPA concentrations, and disolvated monomers for the aryl-substituted enolates at elevated HMPA concentrations. Extensive analyses of the stereochemistry of aggregation are included. Rate studies for reaction with allyl bromide implicate an HMPA-solvated ion pair with a ⁺Li(HMPA)₄ counterion. Dependencies on toluene and THF are attributed to exclusively secondary-shell (medium) effects. Aided by density functional theory (DFT) computations, a stereochemical model is presented in which neither chelates nor the lithium gegenion serves roles. The stereoselectivity stems from the chirality within the sultam ring and not the camphor skeletal core.

Application of Biobased Solvents in Asymmetric Catalysis

The necessity of more sustainable conditions that follow the twelve principles of Green Chemistry have pushed researchers to the development of novel reagents, catalysts and solvents for greener asymmetric methodologies. Solvents are in general a fundamental part for developing organic processes, as well as for the separation and purification of the reaction products. By this reason, in the last years, the application of the so-called green solvents has emerged as a useful alternative to the classical organic solvents. These solvents must present some properties, such as a low vapor pressure and toxicity, high boiling point and biodegradability, and must be obtained from renewable sources. In the present revision, the recent application of these biobased solvents in the synthesis of optically active compounds employing different catalytic methodologies, including biocatalysis, organocatalysis and metal catalysis, will be analyzed to provide a novel tool for carrying out more ecofriendly organic processes.

Regioselective Functionalization of Ester-, Amide-, Carbonate-, and Carbamate-Substituted 2-Phenyl-2-oxazolines with Mixed Lithium-Magnesium Amides

We have prepared novel highly functionalized benzene derivatives by regioselective metalation of ester-, amide-, carbamate-, and carbonate-substituted 2-phenyl-2-oxazolines with mixed lithium-magnesium amides followed by reaction with different electrophiles. While a complementary metalation site can be accessed by using different bases, steric and electronic effects promoted by the aromatic ring substituents also play an important role in reaction regioselectivity. Computational calculations of the aromatic hydrogen pK_a values have helped us to rationalize the metalation preference by the complex-induced proximity effect concept.

Enantioselective, Organocatalytic Strategy for the Oxazolomycin Core: Formal Synthesis of (+)-Neooxazolomycin

A concise, organocatalytic, enantioselective route to the γ-lactam core of the oxazolomycins was developed. Key steps include a Lewis base-catalyzed, Michael proton transfer-lactamization organocascade, a one-pot N-methylation and diastereoselective α-alkylation, a diastereotopic group-selective reduction, a substrate-directed allylic hydroxylation, and a lanthanide-mediated organolithium addition to append the side chain. A formal synthesis of (+)-neooxazolomycin via interception of a Kende intermediate, accessed in 10 steps (previously 24 steps from α-d-glucose), enabled confirmation of the relative and absolute stereochemistry.

An Iterative Divergent Approach to Conjugated Starburst Borane Dendrimers

Using a new divergent approach, conjugated triarylborane dendrimers were synthesized up to the 2nd generation. The synthetic strategy consists of three steps: 1) functionalization, via iridium catalyzed C-H borylation; 2) activation, via fluorination of the generated boronate ester with K[HF2 ] or [N(nBu4 )][HF2 ]; and 3) expansion, via reaction of the trifluoroborate salts with aryl Grignard reagents. The concept was also shown to be viable for a convergent approach. All but one of the conjugated borane dendrimers exhibit multiple, distinct and reversible reduction potentials, making them potentially interesting materials for applications in molecular accumulators. Based on their photophysical properties, the 1st generation dendrimers exhibit good conjugation over the whole system. However, the conjugation does not increase further upon expansion to the 2nd generation, but the molar extinction coefficients increase linearly with the number of triarylborane subunits, suggesting a potential application as photonic antennas.

Computer-generated "synthetic contingency" plans at times of logistics and supply problems: scenarios for hydroxychloroquine and remdesivir

A computer program for retrosynthetic planning helps develop multiple "synthetic contingency" plans for hydroxychloroquine and also routes leading to remdesivir, both promising but yet unproven medications against COVID-19. These plans are designed to navigate, as much as possible, around known and patented routes and to commence from inexpensive and diverse starting materials, so as to ensure supply in case of anticipated market shortages of commonly used substrates. Looking beyond the current COVID-19 pandemic, development of similar contingency syntheses is advocated for other already-approved medications, in case such medications become urgently needed in mass quantities to face other public-health emergencies.

The interpretation of small molecule diffusion coefficients: Quantitative use of diffusion-ordered NMR spectroscopy

Measuring accurate molecular self-diffusion coefficients, D, by nuclear magnetic resonance (NMR) techniques has become routine as hardware, software and experimental methodologies have all improved. However, the quantitative interpretation of such data remains difficult, particularly for small molecules. This review article first provides a description of, and explanation for, the failure of the Stokes-Einstein equation to accurately predict small molecule diffusion coefficients, before moving on to three broadly complementary methods for their quantitative interpretation. Two are based on power laws, but differ in the nature of the reference molecules used. The third addresses the uncertainties in the Stokes-Einstein equation directly. For all three methods, a wide range of examples are used to show the range of chemistry to which diffusion NMR can be applied, and how best to implement the different methods to obtain quantitative information from the chemical systems studied.

Isoxazole mediated synthesis of 4-(1H)pyridones: improved preparation of antimalarial candidate GSK932121

A new synthesis of the antimalarial clinical candidate GSK932121 is described. This approach has two key reactions, the selective acylation of an unprotected 3-hydroxymethyl-5-methyl isoxazole and the reductive N-O bond cleavage of the previously functionalized isoxazole derivative, to give the 4-(1H)pyridone ring present in the final structure. The complete synthesis consists of 5 steps (versus 10 steps in previously published reports) and has enabled the preparation of the material in kilogram scale to support clinical studies.

Lithium Hexamethyldisilazide-Mediated Enolization of Acylated Oxazolidinones: Solvent, Cosolvent, and Isotope Effects on Competing Monomer- and Dimer-Based Pathways

Lithium hexamethyldisilazide (LiHMDS)-mediated enolization of (+)-4-benzyl-3-propionyl-2-oxazolidinone in THF-hydrocarbon mixtures shows unusual sensitivity to the choice of hydrocarbon cosolvent (hexane versus toluene) and to isotopic labeling. Four mechanisms corresponding to monosolvated monomers, trisolvated dimers, octasolvated monomers, and octasolvated dimers were identified. Even under conditions in which the LiHMDS monomer was the dominant observable form, dimer-based metalation was significant. The mechanism-dependent isotope and cosolvent effects are discussed in the context of ground-state stabilization and transition-state tunneling.

Recent advancements on the use of 2-methyltetrahydrofuran in organometallic chemistry

ABSTRACT

Since the introduction of 2-methyltetrahydrofuran as an useful alternative to the classical tetrahydrofuran, there has been a continuous interest in the synthetic community operating at academic and industrial towards it. In particular, the much higher stability that basic organometallic reagents display in 2-methyltetrahydrofuran makes it suitable for processes involving such sensitive species including asymmetric transformations. The easy formation of an azeotropic mixture with water, the substantial immiscibility with water, and the fact it derives from natural sources (corncobs or bagasse), allow to consider it in agreement with the Anastas' Geen Chemistry principles. In this minireview, selected examples of its employment in organometallic transformations ranging from carbanions to radical and transition metal-catalyzed processes are provided.

GRAPHICAL ABSTRACT

Regioselective desymmetrization of diaryltetrahydrofurans via directed ortho-lithiation: an unexpected help from green chemistry

An efficient functionalization of diaryltetrahydrofurans via a regioselective THF-directed ortho-lithiation is first described. This reaction can be successfully carried out in cyclopentyl methyl ether as a "greener" alternative to Et2O, with better results in terms of yield and selectivity and, surprisingly, also in protic eutectic mixtures competitively with protonolysis.

“The Great Beauty” of organolithium chemistry: a land still worth exploring

This Frontier highlights and analyses, from the authors’ perspective, some recent selected contributions from the organolithium field with an eye towards new challenging research directions.

Enediolate-dilithium amide mixed aggregates in the enantioselective alkylation of arylacetic acids: structural studies and a stereochemical model

A combination of X-ray crystallography, (6)Li, (15)N, and (13)C NMR spectroscopies, and density functional theory computations affords insight into the structures and reactivities of intervening aggregates underlying highly selective asymmetric alkylations of carboxylic acid dianions (enediolates) mediated by the dilithium salt of a C2-symmetric chiral tetraamine. Crystallography shows a trilithiated n-butyllithium-dilithiated amide that has dimerized to a hexalithiated form. Spectroscopic studies implicate the non-dimerized trilithiated mixed aggregate. Reaction of the dilithiated amide with the dilithium enediolate derived from phenylacetic acid affords a tetralithio aggregate comprised of the two dianions in solution and the dimerized octalithio form in the solid state. Computational studies shed light on the details of the solution structures and afford a highly predictive stereochemical model.

2-Methyltetrahydrofuran (2-MeTHF): a biomass-derived solvent with broad application in organic chemistry

2-Methyl-tetrahydrofuran (2-MeTHF) can be derived from renewable resources (e.g., furfural or levulinic acid) and is a promising alternative solvent in the search for environmentally benign synthesis strategies. Its physical and chemical properties, such as its low miscibility with water, boiling point, remarkable stability compared to other cyclic-based solvents such as THF, and others make it appealing for applications in syntheses involving organometallics, organocatalysis, and biotransformations or for processing lignocellulosic materials. Interestingly, a significant number of industries have also started to assess 2-MeTHF in several synthetic procedures, often with excellent results and prospects. Likewise, preliminary toxicology assessments suggest that the use of 2-MeTHF might even be extended to more processes in pharmaceutical chemistry. This Minireview describes the properties of 2-MeTHF, the state-of-the-art of its use in synthesis, and covers several outstanding examples of its application from both industry and academia.

Synthetic approaches to multifunctional indenes

The synthesis of multifunctional indenes with at least two different functional groups has not yet been extensively explored. Among the plausible synthetic routes to 3,5-disubstituted indenes bearing two different functional groups, such as the [3-(aminoethyl)inden-5-yl)]amines, a reasonable pathway involves the (5-nitro-3-indenyl)acetamides as key intermediates. Although several multistep synthetic approaches can be applied to obtain these advanced intermediates, we describe herein their preparation by an aldol-type reaction between 5-nitroindan-1-ones and the lithium salt of N,N-disubstituted acetamides, followed immediately by dehydration with acid. This classical condensation process, which is neither simple nor trivial despite its apparent directness, permits an efficient entry to a variety of indene-based molecular modules, which could be adapted to a range of functionalized indanones.