Improving the Halogen–Magnesium Exchange by using New Turbo‐Grignard Reagents

Recent Advances in Halogen-Metal Exchange Reactions

ConspectusThe halogen-metal exchange reaction is a very powerful method for preparing functionalized organometallic reagents in the fields of organic and organometallic chemistry. Since its inception, significant interest has been directed toward the on-demand development of new halogen-metal exchange reactions, primarily through the upgrading of exchange reagents. The enduring quest for optimal reactivity, superior functional group compatibility, and innovative synthetic applications of exchange reagents remains a fundamental objective. In the past several years, the emergence of some significant discoveries in halogen-metal exchange reactions has proclaimed a renaissance to this field. This Account outlines the latest advances within the domain contributed by the Knochel group, including the main points as follows.The stereoretentive I/Li exchange on stereodefined secondary alkyl iodides was developed for the synthesis of nonstabilized chiral secondary alkyllithium reagents. This provided a straightforward method to access chiral organolithium reagents, which can be trapped by various electrophiles or transmetalated with other metals such as copper, zinc, and magnesium, thus enabling the stereoselective synthesis of a series of functionalized compounds and natural products.Faster halogen-magnesium and halogen-zinc exchanges in toluene were realized using a novel kind of exchange reagent complexed with lithium alkoxide. These highly efficient exchange reactions are much faster than traditional ones and performed in an industrially friendly solvent. These advantages are of great value in practical synthesis, paving the way for new developments in this evolving area.Halogen-lanthanide exchanges and their novel applications in organic synthesis were established. These new exchanges introduced the lanthanide metals into halogen-metal exchange reactions for the first time, thereby opening new avenues in synthetic chemistry. Building on these achievements, a comparative analysis of the exchange reaction rates by kinetic study has quantified the relationship between the electronegativity of metals and the rates of halogen-metal exchanges.Br/Na exchange in continuous flow was achieved using a hexane-soluble exchange reagent, 2-ethylhexylsodium. This approach effectively circumvented the poor solubility of the organosodium reagent, which has proven to be of significant practical value and greatly enhanced the synthetic utility of the organosodium reagent in organic synthesis.These remarkable breakthroughs as mentioned above are fueled mainly by upgrading the exchange reagents, resulting in the development of new halogen-metal exchange reactions and innovative applications in organic synthesis. Given the importance of halogen-metal exchanges in synthetic chemistry, the pursuit of other types of exchange reactions, particularly those involving new metals, will be in continuous demand. This Account provides a timely summary of recent progress and will undoubtedly inspire further advances to drive this research field forward.

A General Method to Access Sterically Encumbered Geminal Bis(boronates) via Formal Umpolung Transformation of Terminal Diboron Compounds

General methods for the preparation of geminal bis(boronates) are of great interest due to their widespread applications in organic synthesis. While the terminal gem-diboron compounds are readily accessible, the construction of the sterically encumbered, internal analogues has remained a prominent challenge. Herein, we report a formal umpolung strategy to access these valuable building blocks. The readily available 1,1-diborylalkanes were first converted into the corresponding α-halogenated derivatives, which then serve as electrophilic components, undergoing a formal substitution with a diverse array of nucleophiles to form a series of C-C, C-O, C-S, and C-N bonds. This protocol features good tolerance to steric hindrance and a wide variety of functional groups and heterocycles. Notably, this strategy can also be extended to the synthesis of diaryl and terminal gem-diboron compounds, therefore providing a general approach to various types of geminal bis(boronates).

Synthesis of the aminocyclopentenediol fragment of queuosine by way of the stereoselective addition of an organometallic reagent to a N-t-butanesulfinyl glycosylamine

An innovative, concise synthesis of the aminocyclopentenediol fragment of queuosine is reported. The synthesis is based on the stereocontrolled addition of a vinylGrignard·LiCl reagent to a t-butanesulfinyl L-ribofuranosylamine, followed by dehydrodeoxygenation to generate a second vinyl group and ring-closing metathesis to form the five-membered ring scaffold of the natural product. This approach has the potential for the development of a larger scale synthesis.

Variable Ca-Caryl Hapticity and its Consequences in Arylcalcium Dimers

The dimeric β-diketiminato calcium hydride, [(Dipp BDI)CaH]2 (Dipp BDI = HC{(Me)CN-2,6-i-Pr2 C6 H3 }2 ), reacts with ortho-, meta- or para-tolyl mercuric compounds to afford hydridoarylcalcium compounds, [(Dipp BDI)2 Ca2 (μ-H)(μ-o-,m-,p-tolyl)], in which dimer propagation occurs either via μ2 -η1 -η1 or μ2 -η1 -η6 bridging between the calcium centers. In each case, the orientation and hapticity of the aryl units is dependent upon the position of the methyl substituent. While wholly organometallic meta- and para-tolyl dimers, [(Dipp BDI)Ca(m-tolyl)]2 and [(Dipp BDI)Ca(p-tolyl)]2 , can be prepared and are stable, the ortho-tolyl isomer is prone to isomerization to a calcium benzyl analog. Computational analysis of this latter process with density functional theory (DFT) highlights an unusual mechanism invoking the generation of an intermediate dicalcium species in which the group 2 centers are bridged by a toluene dianion formed by the formal attachment of the original hydride anion to the initially generated ortho-tolyl substituent. Use of a more sterically encumbered aryl substituent, {3,5-t-Bu2 C6 H3 }, facilitates the selective formation of [(Dipp BDI)Ca(μ-H)(μ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)], which can be converted into the unsymmetrically-substituted σ-aryl calcium complexes, [(Dipp BDI)Ca(μ-Ph)(μ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)] and [(Dipp BDI)Ca(μ-p-tolyl)(μ-3,5-t-Bu2 C6 H3 )Ca(Dipp BDI)] by reaction with the appropriate mercuric diaryl. Conversion of [(Dipp BDI)Ca(H)(Ph)Ca(Dipp BDI)] to afford [{{(Dipp BDI)Ca}2 (μ2 -Cl)}2 (C6 H5 -C6 H5 )], comprising a biphenyl dianion, is also reported. Although this latter transformation is serendipitous, AIM analysis highlights that, in a related manner to the ortho-tolyl to benzyl isomerization, the requisite C-C coupling may be facilitated in an "across dimer" fashion by the experimentally-observed polyhapto engagement of the aryl substituents with each calcium.

Synthesis and reactivity of alkali metal aluminates bearing bis(organoamido)phosphane ligand

In this study, we report a group of alkali metal aluminates bearing bis(organoamido)phosphane ligand. The starting complex {[PhP(NtBu)2]AlMe2}Li·OEt2 (1) was prepared by stepwise deprotonation of the parent PhP(NHtBu)2 by nBuLi and AlMe3. Further derivatization of aluminate 1 was performed by the virtual substitution of lithium -{[PhP(NtBu)2]AlMe2}K (2), methyl substituents - {[PhP(NtBu)2]AlH2}Li·THF (3), modification of steric bulk and induction effects on the phosphorus atom - {[tBuP(N-2,6-iPr2C6H3)2]AlMe2}Li·(OEt2)2 (4), and phosphorus atom oxidation state {[Ph(Y)P(NtBu)2]AlMe2}Li (Y = O (5), S (6), Se (7), Te (8)). The structure causing non-covalent interactions in 1-4 were evaluated with the help of theoretical calculations and topological analysis ranging from π-electron system-metal to agostic interactions of various types. The further reactions of 1 with various nucleophiles were found to be a versatile tool for the preparation of iminophosphonamides via the formation of P-E bond (E = Si, Ge, Sn, Pb, P, and C) and followed by P(III) → P(V) tautomeric shift.

Six-Step Synthesis of (±)-Lysergic Acid

This article describes a concise synthesis of lysergic acid from simple aromatic precursors. The successful strategy relies on the coupling, dearomatization, and cyclization of a halopyridine with a 4-haloindole derivative in 6 total synthetic steps from commercial starting materials. In addition to highlighting the advantages of employing dearomative retrosynthetic analysis, the design is practical and anticipated to enable the synthesis of novel neuroactive compounds as exemplified by the synthesis of a novel natural product derivative, 12-chlorolysergic acid.

Regio- and Diastereoselective Carbometalation Reaction of Cyclopropenes

The various facets of the chemistry of cyclopropane derivatives, the smallest carbocycle, are amazingly diverse and continue to fascinate theoreticians, synthetic or structural chemists having interest in fundamental physical, medicinal chemistry, and natural product synthesis. The challenges generated by this intriguing cyclic arrangement of only three tetravalent carbons represent a wide area of the chemical spectrum. From fundamental aspects of bonding through the synthesis of highly strained molecules, the understanding of the mode of action in biological systems to the selective cleavage into acyclic substrates makes the chemistry of these small rings fascinating. Therefore, efficient routes to prepare differently polysubstituted cyclopropanes have always been of a primordial importance. In the past decade, we and others have expanded the scope of the carbometalation reaction of cyclopropenes as a broad and general method to the formation of stereodefined cyclopropane derivatives. Although cyclopropenes, with their even higher strain energy, easily undergo addition reactions of organometallic reagents, their carbometalation reactions generate new regio-, diastereo-, and enantioselectivity issues that needed to be addressed. These various stereochemical aspects accompanied our research from its origins to today, and we are proposing in this Account, a didactic overview of the different ways by which cyclopropenes can lead to the formation of polysubstituted cyclopropanes or open-products possessing several stereogenic centers as a single regio- and diastereomer.We initially launched our research campaign on the chemistry of these strained three-membered rings by the regio- and diastereoselective copper-catalyzed carbomagnesiation of enantiomerically enriched cyclopropenyl carbinols. The directing alcohol governed both the regio- and diastereoselectivity of the addition and also served as a good leaving group as it undergoes a selective 1,2-elimination reaction to provide enantioenriched alkylidenecyclopropanes in excellent yields and enantiomeric excesses. Then, we turned our attention to the regio- and stereoselective synthesis of stereodefined tri- and tetrasubstituted cyclopropanes through the diastereoselective addition to sp²- monosubstituted cyclopropenyl ester derivatives. With the aim to further expand this concept to the formation of penta- and hexa-substituted cyclopropanes as single isomer, we had first to design the preparation of the required 1,2-disubstituted cyclopropenes that would control the regioselective addition of the organometallic derivatives. The synthesis of penta- and hexa-substituted cyclopropanes was then reported for the first time as a single regio- and diastereomer. It should be noted that the in situ formed cyclopropyl-metal intermediate is configurationally stable and can be subsequently functionalized with pure retention of the configuration by addition of electrophiles. Then, the enantioselective-catalyzed carbometalation reaction of achiral cyclopropenes allowed the synthesis of several new classes of cyclopropane derivatives in high enantiomeric ratios. Finally, by combining the regio- and diastereoselective carbometalation reaction of a cyclopropene with a subsequent reaction of the resulting cyclopropylmetal species, a selective carbon-carbon bond cleavage was observed to lead to the preparation of acyclic substrates possessing several stereocenters including a quaternary carbon stereogenic center. Our original vision of using strain within an embedded double bond in a three-membered ring has provided new routes to the stereoselective synthesis of polysubstituted cyclopropanes and has been extremely successful, as it represents a current new tool for the synthesis of persubstituted cyclopropanes as a single diastereomer.

In Situ Grignard Metalation Method for the Synthesis of Hauser Bases

The in situ Grignard Metalation Method (iGMM) is a straightforward one‐pot procedure to quickly produce multigram amounts of Hauser bases R₂N‐MgBr which are valuable and vastly used metalation reagents and novel electrolytes for magnesium batteries. During addition of bromoethane to a suspension of Mg metal and secondary amine at room temperature in an ethereal solvent, a smooth reaction yields R₂N‐MgBr under evolution of ethane within a few hours. A Schlenk equilibrium is operative, interconverting the Hauser bases into their solvated homoleptic congeners Mg(NR₂)₂ and MgBr₂ depending on the solvent. Scope and preconditions are studied, and side reactions limiting the yield have been investigated. DOSY NMR experiments and X‐ray crystal structures of characteristic examples clarify aggregation in solution and the solid state.

Lithium, Magnesium, and Zinc Centers N,N'-Chelated by an Amine-Amide Hybrid Ligand

The synthesis and structure of lithium, magnesium, and zinc complexes N,N'-chelated by a hybrid amine-amido ligand ([2-(Me₂NCH₂)C₆H₄NR]^-, abbreviated as L^NR, where R = H, SiMe₃, or Bn) are reported. The reaction of the least sterically demanding L^NH with various magnesium sources gives the hexameric imide [L^NMg]₆ (4) by the elimination of n-butane from L^NHMgⁿBu (2) or by the reaction of L^NHLi (1) with MeMgBr. [L^NH]₂Mg (3) is obtained through the addition of 0.5 equiv of ⁿBu₂Mg or Mg[N(SiMe₃)₂]₂ to L^NH₂ and with 1 equiv of ⁿBu₂Mg reacting to 2. Both L^NHMgN(SiMe₃)₂ (6) and isostructural L^NHZnN(SiMe₃)₂ (16) have been prepared using two different approaches: monodeprotonation of L^NH₂ by Zn/Mg[N(SiMe₃)₂]₂ in a 1:1 ratio or ligand substitution of 2 or L^NHZnEt (12) by 0.5 equiv of Sn[N(SiMe₃)₂]₂. The reactions of 2 or 3 with 1 provide the heterotrimetallic complex [L^NH]₄Li₂Mg (5). Benzyl- or trimethylsilyl-substituted anilines [L^N(SiMe₃)H (7) and L^N(Bn)H (8)] with 0.5 equiv of ⁿBu₂Mg allow the formation of homoleptic bis(amides) of the [L^N(R)]₂Mg type (10 and 11). Nevertheless, only the silylated secondary amine 7 is able to provide the heteroleptic n-butylmagnesium amide L^N(SiMe₃)MgⁿBu (9) upon reaction with an equimolar amount of ⁿBu₂Mg. Similarly, 12, [L^NH]₂Zn (13), L^N(R)ZnEt (17 and 18), and [L^N(R)]₂Zn [R = SiMe₃ (19) and Bn (20)] were prepared by the monodeprotonation of L^NH₂ or L^N(R)H using Et₂Zn in the corresponding stoichiometric ratio. L^NHZnI was prepared by the nucleophilic substitution of an ethyl chain in 12 by molecular iodine. A heterometallic complex, [L^NH]₄Li₂Zn (14), analogous to 5 was prepared from 12 or 13 with 1 or 2 equiv of 1, respectively.

Reliable Functionalization of 5,6‐Fused Bicyclic N‐Heterocycles Pyrazolopyrimidines and Imidazopyridazines via Zinc and Magnesium Organometallics

DFT‐calculations allow prediction of the reactivity of uncommon N‐heterocyclic scaffolds of pyrazolo[1,5‐a]pyrimidines and imidazo[1,2‐b]pyridazines and considerably facilitate their functionalization. The derivatization of these N‐heterocycles was realized using Grignard reagents for nucleophilic additions to 5‐chloropyrazolo[1,5‐a]pyrimidines and TMP₂Zn ⋅ 2 MgCl₂  ⋅ 2 LiCl allowed regioselective zincations. In the case of 6‐chloroimidazo[1,2‐b]pyridazine, bases such as TMP₂Zn ⋅ MgCl₂  ⋅ 2 LiCl, in the presence or absence of BF₃  ⋅ OEt₂, led to regioselective metalations at positions 3 or 8. Subsequent functionalizations were achieved with TMPMgCl ⋅ LiCl, producing various polysubstituted derivatives (up to penta‐substitution). X‐ray analysis confirmed the regioselectivity for key functional heterocycles.

Nickel-Catalyzed Intermolecular Asymmetric Addition of Aryl Iodides across Aldehydes

Enantioenriched alcohols comprise much of the framework of organic molecules. Here, we first report that chiral nickel complexes can catalyze the intermolecular enantioselective addition of aryl iodides across aldehydes to provide diverse optically active secondary alcohols using zinc metal as the reducing agent. This method shows a broad substrate scope under mild reaction conditions and precludes the traditional strategy through the pre-generation of organometallic reagents. Mechanistic studies indicate that an in situ formed arylnickel, instead of an arylzinc, adds efficiently to aldehydes, forming a new C-C bond and a chiral nickel alkoxide that may be turned over by zinc powder.

Aryl Boronic Esters Are Stable on Silica Gel and Reactive under Suzuki-Miyaura Coupling Conditions

A wide range of aryl boronic 1,1,2,2-tetraethylethylene glycol esters [ArB(Epin)s] were readily synthesized. Purifying aryl boronic esters by conventional silica gel chromatography is generally challenging; however, these introduced derivatives are easily purified on silica gel and isolated in excellent yields. We subjected the purified ArB(Epin) to Suzuki-Miyaura couplings, which provided higher yields of the desired biaryl products than those obtained using the corresponding aryl boronic acids or pinacol esters.

Exploring Short and Efficient Synthetic Routes Using Titanocene(III)-Catalyzed Reactions: Total Synthesis of Natural Meroterpenes with Trisubstituted Unsaturations

The stereo- and regioselective total syntheses of OMe derivatives of the scarce bioactive meroterpenoids makassaric acid (1) and fascioquinol B (2) have been accomplished. The synthetic sequences are based on the following three efficient and selective catalytic reactions: Cu-catalyzed addition of Grignard compounds to an epoxide; a regioselective Barbier-type reaction, catalyzed by Cp₂TiCl; and regio- and stereoselective bioinspired cyclization, also catalyzed by Cp₂TiCl. These three key processes allow us to obtain the main skeletons of 1 and 2 in a few steps. The valuable synthetic proposal shown in this work provides fast access to scarce, structurally complex meroterpenes with promising biological activities, which are a sustainable source for later studies and applications in medicine.

Copper Catalyzed Regioselective and Stereospecific Aziridine Opening with Pyridyl Grignard Nucleophiles

Copper catalyzed regioselective and stereospecific coupling between aziridines and in situ generated pyridine Grignard reagents is reported. This method provides β-pyridylethylamines with diverse structures and functionalities from aziridines and iodopyridines. β-Pyridylethylamines are potential scaffolds for the synthesis of biologically active compounds often found in pharmaceuticals. The synthesis of challenging chiral dihydroazaindoles was also achieved through mild one-pot reaction conditions via aziridine opening followed by nucleophilic cyclization.

Preparation of Primary and Secondary Dialkylmagnesiums by a Radical I/Mg‐Exchange Reaction Using s Bu 2 Mg in Toluene

The treatment of primary or secondary alkyl iodides with sBu₂Mg in toluene (25–40 °C, 2–4 h) provided dialkylmagnesiums that underwent various reactions with aldehydes, ketones, acid chlorides or allylic bromides. 3‐Substituted secondary cyclohexyl iodides led to all‐cis‐3‐cyclohexylmagnesium reagents under these exchange conditions in a highly stereoconvergent manner. Enantiomerically enriched 3‐silyloxy‐substituted secondary alkyl iodides gave after an exchange reaction with sBu₂Mg stereodefined dialkylmagnesiums that after quenching with various electrophiles furnished various 1,3‐stereodefined products including homo‐aldol products (99 % dr and 98 % ee). Mechanistic studies confirmed a radical pathway for these new iodine/magnesium‐exchange reactions.

D. dos Santos AC, Kohlhepp SV, Mendoza A. Direct Addition of Grignard Reagents to Aliphatic Carboxylic Acids Enabled by Bulky turbo ‐Organomagnesium Anilides

The synthesis of ketones through addition of organometallic reagents to aliphatic carboxylic acids is a straightforward strategy that is limited to organolithium reagents. More desirable Grignard reagents can be activated and controlled with a bulky aniline‐derived turbo‐Hauser base. This operationally simple procedure allows the straightforward preparation of a variety of aliphatic and perfluoroalkyl ketones alike from functionalized alkyl, aryl and heteroaryl Grignard reagents.

Copper-Free Alternatives to Access Ketone Building Blocks from Grignard Reagents

Grignard reactions are an efficient way to form carbon-carbon bonds with widespread applications in large-scale processes. Classically, the electrophiles of choice to form ketones from Grignard reagents are acid chlorides. The reactions are typically catalyzed by additives such as CuCl to increase selectivity and yields. This work was focused on the use of acetic anhydride as an alternative to acetic chloride in the synthesis of 4-fluoro-2-(trifluoromethyl)acetophenone (3), a useful intermediate for the synthesis of active agricultural ingredients. The use of acetic anhydride as an electrophile not only equals but also surpasses acetic chloride in yield and selectivity, while also being more tolerable toward higher reaction temperatures. Furthermore, the reaction was performed in the absence of any additive, making it a highly attractive process for large-scale production. Computational mechanistic studies suggest that this advantageous behavior can be ascribed to the superior complexation of carboxylic acid anhydrides in the transition state.

Recent total syntheses of anthraquinone-based natural products

The scientific community has found deep interest in anthraquinone-based compounds due to their therapeutic properties and challenging structural elements. Various architecturally beautiful natural products have been successfully synthesized in recent decades utilizing two main strategies: either an early-stage synthesis of the anthraquinone and further elongation of the system, or a late-stage introduction of the anthraquinone ring moiety. Select syntheses of complex anthraquinone monomers and dimers within the past 20 years are described with an emphasis on the retrosynthetic disconnections that shape the anthraquinone-installation strategy.

Organoarsenic probes to study proteins by NMR spectroscopy

Arsenical probes enable structural studies of proteins. We report the first organoarsenic probes for nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy to study proteins in solutions. These probes can be attached to irregular loop regions. A lanthanide-binding tag induces sizable pseudocontact shifts in protein NMR spectra of a magnitude never observed for small paramagnetic probes before.

Light Directs Monomer Coordination in Catalyst-Free Grignard Photopolymerization

π-Conjugated polymers can serve as active layers in flexible and lightweight electronics and are conventionally synthesized by transition-metal-mediated polycondensation at elevated temperatures. We recently reported a photopolymerization of electron-deficient heteroaryl Grignard monomers that enables the catalyst-free synthesis of n-type π-conjugated polymers. Herein, we describe an experimental and computational investigation into the mechanism of this photopolymerization. Spectroscopic studies performed in situ and after quenching reveal that the propagating chain is a radical anion with halide end groups. DFT calculations for model oligomers suggest a Mg-templated S_RN1-type coupling, in which Grignard monomer coordination to the radical anion chain avoids the formation of free sp² radicals and allows C-C bond formation with very low barriers. We find that light plays an unusual role in the reaction, photoexciting the radical anion chain to shift electron density to the termini and thus enabling productive monomer binding.

Nickel-Mediated Alkoxycarbonylation for Complete Carbon Isotope Replacement

Many commercial drugs, as well as upcoming pharmaceutically active compounds in the pipeline, display aliphatic carboxylic acids or derivatives thereof as key structural entities. Synthetic methods for rapidly accessing isotopologues of such compounds are highly relevant for undertaking critical pharmacological studies. In this paper, we disclose a direct synthetic route allowing for full carbon isotope replacement via a nickel-mediated alkoxycarbonylation. Employing a nickel^II pincer complex ([(N₂N)Ni-Cl]) in combination with carbon-13 labeled CO, alkyl iodide, sodium methoxide, photocatalyst, and blue LED light, it was possible to generate the corresponding isotopically labeled aliphatic carboxylates in good yields. Furthermore, the developed methodology was applied to the carbon isotope substitution of several pharmaceutically active compounds, whereby complete carbon-13 labeling was successfully accomplished. It was initially proposed that the carboxylation step would proceed via the in situ formation of a nickellacarboxylate, generated by CO insertion into the Ni-alkoxide bond. However, preliminary mechanistic investigations suggest an alternative pathway involving attack of an open shell species generated from the alkyl halide to a metal ligated CO to generate an acyl Ni^III species. Subsequent reductive elimination involving the alkoxide eventually leads to carboxylate formation. An excess of the alkoxide was essential for obtaining a high yield of the product. In general, the presented methodology provides a simple and convenient setup for the synthesis and carbon isotope labeling of aliphatic carboxylates, while providing new insights about the reactivity of the N₂N nickel pincer complex applied.

Selective functionalization of the 1H-imidazo[1,2-b]pyrazole scaffold. A new potential non-classical isostere of indole and a precursor of push-pull dyes

We report the selective functionalization of the 1H-imidazo[1,2-b]pyrazole scaffold using a Br/Mg-exchange, as well as regioselective magnesiations and zincations with TMP-bases (TMP = 2,2,6,6-tetramethylpiperidyl), followed by trapping reactions with various electrophiles. In addition, we report a fragmentation of the pyrazole ring, giving access to push-pull dyes with a proaromatic (1,3-dihydro-2H-imidazol-2-ylidene)malononitrile core. These functionalization methods were used in the synthesis of an isostere of the indolyl drug pruvanserin. Comparative assays between the original drug and the isostere showed that a substitution of the indole ring with a 1H-imidazo[1,2-b]pyrazole results in a significantly improved solubility in aqueous media.

Synthesis of 7-Chloroquinoline Derivatives Using Mixed Lithium-Magnesium Reagents

We have prepared a library of functionalized quinolines through the magnesiation of 7-chloroquinolines under mild conditions, employing both batch and continuous flow conditions. The preparation involved the generation of mixed lithium-magnesium intermediates, which were reacted with different electrophiles. Mixed lithium-zinc reagents allowed the synthesis of halogenated and arylated derivatives. Some of the synthesized 4-carbinol quinolines have shown interesting antiproliferative properties, their hydroxyl group being a suitable amino group bioisostere. We also report a two-step approach for optically active derivatives.

Practical one-pot amidation of N-Alloc-, N-Boc-, and N-Cbz protected amines under mild conditions

A facile one-pot synthesis of amides from N-Alloc-, N-Boc-, and N-Cbz-protected amines has been described. The reactions involve the use of isocyanate intermediates, which are generated in situ in the presence of 2-chloropyridine and trifluoromethanesulfonyl anhydride, to react with Grignard reagents to produce the corresponding amides. Using this reaction protocol, a variety of N-Alloc-, N-Boc-, and N-Cbz-protected aliphatic amines and aryl amines were efficiently converted to amides with high yields. This method is highly effective for the synthesis of amides and offers a promising approach for facile amidation.

One-pot efficient transformation of N-Alloc-, N-Boc-, and N-Cbz protected amines to amides was achieved by using 2-chloropyridine and trifluoromethanesulfonyl anhydride as well as Grignard reagent and MgCl₂.

Synthesis of biaryl ketones by arylation of Weinreb amides with functionalized Grignard reagents under thermodynamic control vs. kinetic control of N,N-Boc2-amides

A highly efficient method for chemoselective synthesis of biaryl ketones by arylation of Weinreb amides (N-methoxy-N-methylamides) with functionalized Grignard reagents is reported. This protocol offers rapid entry to functionalized biaryl ketones after Mg/halide exchange with i-PrMgCl·LiCl under operationally-simple and practical reaction conditions. The scope of the method is highlighted in >40 examples, including bioactive compounds and pharmaceutical derivatives. Collectively, this transition-metal-free approach offers a major advantage over the recently established cross-coupling of amides by oxidative addition of N-C(O) bonds. Considering the utility of amide acylation reactions in modern synthesis, we expect that this method will be of broad interest.

Facile Access to Hetero-poly-functional Arenes and meta-Substituted Arenes via Two-Step Dimetalation and Mg/Halogen-Exchange Protocol

The Grignard reagent, iPrMgCl and its lithium chloride-enhanced 'turbo' derivative iPrMgCl⋅LiCl have been employed to investigate the single iodo/magnesium exchange reactions of the trisubstituted arenes, 2,5-diiodo-N,N-diisopropylbenzamide 1, 1,4-diiodo-2-methoxybenzene 2, and 1,4-diiodo-2-(trifluoromethyl)benzene 3. These three arenes themselves were initially prepared by a double ortho-, meta'-deprotonation of N,N-diisopropylbenzamide, anisole and (trifluoromethyl)benzene, respectively, using the sodium magnesiate reagent [Na₄ Mg₂ (TMP)₆ (nBu)₂ ] (where TMP is 2,2,6,6-tetramethylpiperidide), and subsequent electrophilic quenching with iodine/THF solution. Thus, by following a combined deprotonation and magnesium/halogen exchange strategy, the simple monosubstituted arenes can be converted to trisubstituted diiodoarenes, which can ultimately be transformed into the corresponding mono-magnesiated arenes, in THF at -40 °C, within seconds in good yields. The other functional group (OMe, NiPr₂ or CF₃ respectively) present on the di-iodoarenes helps direct the exchange reaction to the ortho position, whereas subsequent addition of different electrophiles permits the preparation of hetero-poly-functional-arenes, with three different substituents in their structure. Intriguingly, if water is used as the electrophile, a new and facile route to prepare meta-substituted arenes, which cannot be easily obtained by conventional processes, is forthcoming. In contrast to directed ortho-metalation (DoM) chemistry, this reaction sequence can be thought of as InDirect meta-Metalation (IDmM). The scope of the chemistry has been tested further by exposing the initial unreacted iodo-functionality at the meta-position to a second Mg/I-exchange reaction and subsequent functionalization.

Organophosphorus and Iron Catalysis: Good Partners for Hydrometalation of Olefins and Alkynes

The last decades have seen an impressive development of iron complexes involving organophosphorus ligands applied in homogeneous catalyzed hydrometalation of olefins and alkynes. Two main topics will be covered in this JOCSynopsis: (i) an overview of the achievements in the area of iron-catalyzed hydrophosphination and then (ii) hydrosilylation, hydroborylation, and hydromagnesiation reactions promoted by catalysts based on organophosphorus ligands and iron.

Chromium-Catalyzed Linear-Selective Alkylation of Aldehydes with Alkenes

We developed a chromium-catalyzed, photochemical, and linear-selective alkylation of aldehydes with alkylzirconium species generated in situ from a wide range of alkenes and Schwartz's reagent. Photochemical homolysis of the C-Zr bond afforded alkyl radicals, which were then trapped by a chromium complex catalyst to generate the alkylchromium(III) species for polar addition to aldehydes. The reaction proceeded with high functional group tolerance at ambient temperature under visible-light irradiation.

Main group bimetallic partnerships for cooperative catalysis

Over the past decade s-block metal catalysis has undergone a transformation from being an esoteric curiosity to a well-established and consolidated field towards sustainable synthesis. Earth-abundant metals such as Ca, Mg, and Al have shown eye-opening catalytic performances in key catalytic processes such as hydrosilylation, hydroamination or alkene polymerization. In parallel to these studies, s-block mixed-metal reagents have also been attracting widespread interest from scientists. These bimetallic reagents effect many cornerstone organic transformations, often providing enhanced reactivities and better chemo- and regioselectivities than conventional monometallic reagents. Despite a significant number of synthetic advances to date, most efforts have focused primarily on stoichiometric transformations. Merging these two exciting areas of research, this Perspective Article provides an overview on the emerging concept of s/p-block cooperative catalysis. Showcasing recent contributions from several research groups across the world, the untapped potential that these systems can offer in catalytic transformations is discussed with special emphasis placed on how synergistic effects can operate and the special roles played by each metal in these transformations. Advancing the understanding of the ground rules of s-block cooperative catalysis, the application of these bimetalic systems in a critical selection of catalytic transformations encompassing hydroamination, cyclisation, hydroboration to C-C bond forming processes are presented as well as their uses in important polymerization reactions.

Organomagnesium Based Flash Chemistry: Continuous Flow Generation and Utilization of Halomethylmagnesium Intermediates

The generation of highly unstable chloromethylmagnesium chloride in a continuous flow reactor and its reaction with aldehydes and ketones is reported. With this strategy, chlorohydrins and epoxides were synthesized within a total residence time of only 2.6 s. The outcome of the reaction can be tuned by simply using either a basic or an acidic quench. Very good to excellent isolated yields, up to 97%, have been obtained for most cases (30 examples).

Insight into the Bonding and Aggregation of Alkyllithiums by Experimental Charge Density Studies and Energy Decomposition Analyses

In this Article, the organolithiums [((-)-sparteine)Li^tBu] (1), [(ABCO)Li^tBu]₂ (2), and [(ABCO)₂(LiⁱPr)₄] (3) are investigated by means of experimental and theoretical charge density determination to elucidate the nature of the Li-C and Li-N bonds. Furthermore, the valence shell charge concentrations (VSCCs) in the nonbonding region of the deprotonated C_α-atom will provide some insight on the localization of the carbanionic lone pair. Analysis of the electron density (ρ(r_BCP)), Laplacian (∇²ρ(r_BCP)), and the energy decomposition (EDA) confirmed that the Li-C/N bond exhibits astonishingly similar characteristics, to reveal an increasingly polar contact with decreasing aggregate size. This explains former observations on the incorporation of halide salts in organolithium reagents. Furthermore, it could be shown that the bonding properties of the ⁱPr group are similar to those of the ^tBu substituent. The accuracy of fit to all previously determined properties in organolithiums is remarkable.

Nickel-Catalyzed Asymmetric Addition of Aromatic Halides to Ketones: Highly Enantioselective Synthesis of Chiral 2,3-Dihydrobenzofurans Containing a Tertiary Alcohol

A highly enantioselective and straightforward synthetic procedure to chiral 3-hydroxy-2,3-dihydrobenzofurans has been developed by nickel/bisoxazoline-catalyzed intramolecular asymmetric addition of aryl halides to unactivated ketones, giving 2,3-dihydrobenzofurans with a chiral tertiary alcohol at the C-3 position in good yields and excellent enantioselectivities (up to 92% yield and 98% ee). The gram-scale reaction also proceeded smoothly without a loss of yield and enantioselectivity.

Disposable cartridge concept for the on-demand synthesis of turbo Grignards, Knochel-Hauser amides, and magnesium alkoxides

Magnesium organometallic reagents occupy a central position in organic synthesis. The freshness of these compounds is the key for achieving a high conversion and reproducible results. Common methods for the synthesis of Grignard reagents from metallic magnesium present safety issues and exhibit a batch-to-batch variability. Tubular reactors of solid reagents combined with solution-phase reagents enable the continuous-flow preparation of organomagnesium reagents. The use of stratified packed-bed columns of magnesium metal and lithium chloride for the synthesis of highly concentrated turbo Grignards is reported. A low-cost pod-style synthesizer prototype, which incorporates single-use prepacked perfluorinated cartridges and bags of reagents for the automated on-demand lab-scale synthesis of carbon, nitrogen, and oxygen turbo magnesium bases is presented. This concept will provide access to fresh organomagnesium reagents on a discovery scale and will do so independent from the operator’s experience in flow and/or organometallic chemistry.