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Developing new green solvents in designing chemical products and processes or successfully employing the already existing ones is one of the key subjects in green chemistry and is especially important in organometallic chemistry, which is an interdisciplinary field. Can we advantageously also use unconventional reaction media in place of current harsh organic solvents for polar organometallic compounds? This microreview critically analyses the state of the art with regard to this topic and showcases recent developments and breakthroughs that are becoming new research directions in this field. Because metals cover a vast swath of the Periodic Table the content is organised into three sections discussing the reactivity of organometallic compounds of s-, p- and d-block elements in unconventional solvents.

Some lipophilic fluoro-substituted N-benzoyl-2-aminobenzothiazole antibacterial agents have been evaluated for their activity in the presence of cyclodextrins (CDs) containing aqueous solutions where CDs are adopted as solubilizing excipients for improving the poor water solubility of these compounds. For such purpose both the natural β-CD and one of FDA/EMA approved CDs for parenteral use (i.e. HP-β-CD) have been employed. The solubility rank order observed was accounted for by thermal analysis (Differential Scanning Calorimetry) and FT-IR spectroscopy. The most promising compound was subjected to further NMR spectroscopic studies and molecular modelling simulations to verify the interactions between the guest molecule and the CD cavity. The assessment of the antibacterial activity of such compounds against selected Gram positive and Gram negative bacterial strains clearly showed that their antimicrobial effectiveness may, quite in all instances, be positively affected by complexation with β-CD and HP-β-CD. These results, which are in some ways in contrast with those already reported in the literature, are herein discussed on the basis of plausible mechanisms. Moreover, this investigation also reveals that the described methodology of complexing both lipophilic and hydrophilic antimicrobial agents with CDs may be an useful approach to enhance their effectiveness as well as a promising strategy to overcome even the microbial resistance problem.

The first structure of an aromatic bis(trifluoroborate) dipotassium salt, elucidated by the combination of crystallography, DFT calculations, topological and non-covalent interaction analysis, discloses a 3D network undergoing spontaneous self-assembly thanks to the massive participation of weak intra- and intermolecular interactions for which fluorine atoms proved to play a leading role. peerReviewed

o-Tolyl-substituted tetrahydrofurans undergo highly regioselective ring opening with the concomitant formation of new C–C bonds as the result of a lateral lithiation reaction. This reaction provides a new method for the synthesis of functionalised primary alcohols and can be run directly in protic eutectic mixtures as benign reaction media at 0 °C and under air, competitively with protonolysis.

An efficient regioselective mono-functionalization of 3,5-dimethyl-1,2,4-triazoles (I) is achieved by lithiation under optimized conditions and quenching with a variety of electrophiles.

The ortho lithiation-trapping sequence of phenylaziridines is described. This methodology, which counts on the ability of the aziridino group to act as a directed metalation group (DMG), provides an easy access to functionalized arylaziridines as well as to phthalans and phthalides. The importance of the aziridine N-substituent in this DoM reaction is stressed as well. [reaction: see text]

This account focuses on the generation of α-lithiated aryloxiranes such as lithiated oxazolinylaryloxiranes, lithiated styrene-, phenylpropylene- and stilbene oxides, their chemical and configurational stability, reactivity and some synthetic applications. It is shown how these special reagents have been successfully employed for the preparation of a variety of substances including polysubstituted aryloxiranes, highly strained heterocyclic compounds, α,β-epoxy-y-butyrolactones, cyclopropane carboxylates.

The stereospecific lithiation of diastereomeric phenylpropylene oxides has been studied as well as the trapping reaction with electrophiles. The reduction of the cis-alpha-benzoylpropylene oxide to give prevalently the anti-epoxy alcohol has been investigated as well.

The α-lithiation reaction of optically active oxazolinyloxiranes has been investigated. The trapping reaction with D2O, MeI and acetone affording substituted oxazolinyloxiranes proved that the corresponding lithiated species are configurationally unstable. A stereoconvergency was observed in the case of lithiation–deuteration sequence of two oxazolinyloxiranes.

A new simple and stereoselective synthesis of 5-isoxazolidinones based on the reaction of lithiated 2-isopropyl-2-oxazolines with nitrones is described. A chiral version of such a methodology allows the preparation of highly enantioenriched 5-isoxazolidinones which are useful precursors for the synthesis of beta-amino acids

[reaction: see text] The stereoselective synthesis of novel alpha-epoxy-beta-amino acids is described by a route that combines the chemistry of oxazolinyloxiranyllithiums with that of nitrones. The intermediate trioxadiazadispiro[2.0.4.3]undecanes 4 have been isolated and converted by hydrolysis into epoxy-5-isoxazolidinones 5 which can be transformed into the alpha-epoxy-beta-amino acids 8 by N-O reduction.

Lithiated N -sulfonyloxazolinylaziridines 6a and 7a , generated by deprotonation of the corresponding aziridines 6 and 7 with sec -BuLi/TMEDA at −98°C in THF, were found to be chemically and configurationally stable to be stereospecifically captured by electrophiles, while warming up to rt resulted in the formation of oxazolinylazirine 15 . In contrast, lithiation of N -phenyloxazolinylaziridines 8 and 9 led to oxazolinylenamine 18 . Tricyclic aziridines 10 and 11 resulted from an intramolecular addition of the aziridinyllithium 6a to the phenyl ring of the benzenesulfonyl group.

4,4-Dimethyl-2-(3-chloro-2-methyl-1-propenyl)-2-oxazoline has been synthesized and deprotonated with LDA in THF to give the corresponding lithiated species, which has been found to react α-regioselectively with NH4Cl and alkyl halides, and γ-regioselectively with carbonyl compounds to afford α-protonated (or α-alkyl-substituted) regioisomers and vinyl epoxides, respectively. The Z diastereoselectivity of both the protonation and the alkylation reactions was usually found to increase with the temperature. Ab initio calculations, performed on both the naked lithium salt and the corresponding solvated form, support the observed regioselectivity.

— Aryl substituted oxazolinyloxiranes 1a – g have been found to undergo smooth isomerization into oxazolinyl substitutedaryl alkanones 2a – g upon treatment with LDA in Et 2 Oat−98°C, warming to room temperature and quenching with satd aq.NH 4 Cl. A mechanism involving an oxazoline-assisted ortho lithiation of the aryl group ( cis to the oxazoline system) of oxiranes 1 is proposed. © 2002 Published by Elsevier Science Ltd. Oxiranes are versatile synthetic intermediates. 1 Theirreactions generally involve cleavage of the strainedoxirane ring and include a wide range of nucleophilicring openings and acid- and base-induced isomeriza-tions. 2 The chemistry of oxiranes under basic condi-tions has been extensively investigated, including: (a) -deprotonation to give an oxiranyl anion, which, inturn, can be captured by an electrophile, dimerize togenerate alkenediols, ring open to give an enolate or acarbene, or undergo a C H insertion reaction; 3 and (b) -elimination to furnish an allylic alcohol.

Stereospecific β-lithiation of β-aryl-substituted oxazolinyloxiranes is described. The trapping reaction of such reactive intermediates with carbonyl compounds gave α,β-epoxy-γ-butyrolactones after deblocking of the oxazoline moiety. This methodology has been also extended to the synthesis of optically active α,β-epoxy-γ-butyrolactones.

Several allyl heteroarylalkyl ethers have been synthesized and then deprotonated with nBuLi in THF at −78°C to give lithium derivatives. The lithium-bearing terminus was either the α- or the α′-carbon atom, depending on the associated proton acidity. In the absence of an external electrophile, a sigmatropic rearrangement occurs, generating a new C−C bond. Heteroarylalkyl homoallylic alcohols and allyl heteroarylalkylic alcohols were obtained as products of stereoselective [2,3]- or [1,2]-Wittig rearrangements, respectively. Homoallylic alcohols were obtained in high yields and with fairly good enantiomeric enrichments when the reactions were carried out in toluene with (−)-sparteine as the external chiral ligand.

2-Chloromethyl-2-oxazoline converts cleanly into trans-1,2,3-tris(oxazolinyl)cyclopropane upon treatment with strong bases such as LDA or KN(SiMe(3))(2). Deprotonation of the above cyclopropane followed by the addition of electrophiles allows the preparation of more functionalized tris(oxazolinyl)cyclopropanes.

This Microreview focuses on the regio- and stereoselective lithiation of small-ring heterocycles such as aryl-substituted oxiranes, aziridines, azetidines, oxetanes, and tetrahydrofurans highlighting some useful selected synthetic applications extracted from recent literature. It will be shown, in particular, that regio- and stereoselectivity and the configurational stability of the above lithiated species are controlled by a subtle interplay of factors. Aggregation, complexation phenomena, dynamics at the heteroatom of the heterocyclic ring, substitution in both the aryl and the heterocyclic ring, solvent polarity, temperature, and ligands play a crucial role. The ability of the afore-mentioned heterocycles as important directed metalation groups for the arene ortho-functionalization is also discussed as well as the carbene-like nature exhibited by some lithiated epoxides and aziridines.

This Frontier highlights and analyses, from the authors’ perspective, the impact on organic synthesis of some recent contributions from the organolithium field, also discussing the opportunities that are on the horizon.

The lithiation of 2-bromophenyloxiranes (I) followed by a reaction with enaminones (II) leads to diastereomeric isochromanes (III) and (IV), which are readily separable by column chromatography.

Lithiated 2,4,4-trimethyl-2-oxazoline 2a and 2-chloromethyl-4,4-dimethyl-2-oxazoline 2b react smoothly with a number of nitrones 3 to produce α,β-unsaturated oxazolines 6 and 7 highly stereoselectively.

Optically pure chloromethyloxazolines have been prepared, lithiated and then reacted with benzophenone to famish stereoselectively oxazolinyl oxiranes and formyl oxiranes. The stereoselection observed has been related to the geometry of the intermediate lithium azaenolates. Calculations have been carried out in order to evaluate the relative stability of the E and Z forms of the lithium azaenolates. The reaction of a lithium azaenolate with other ketones leading to oxazolinyl epoxides is also reported. (C) 2001 Elsevier Science Ltd. All rights reserved

Lithiooxiranes 3a and 3b, generated by deprotonation of oxiranes 2a and 2b with s-BuLi at -100 degrees C in Et(2)O, were found to be chemically very stable. trans-Lithiooxirane 3a was also configurationally stable and reacted stereospecifically with electrophiles to give 4a--k. In contrast, cis-lithiooxirane 3b was found to be configurationally much less stable and reacted with electrophiles affording mixtures of diastereomers 4, 7, and 8. After only a very short reaction time, 3b too reacted with electrophiles highly stereospecifically. Deprotonation--deuteration and deprotonation--alkylation of chiral oxazolinyloxiranes 12a and 12b to give oxiranes 12c and 12d were also examined. Semiempirical and ab initio calculations were carried out in an effort to explain the observed stereochemistry.

Boron azaenolates 2 and 8 from the optically active chloromethyloxazolines 1 and 7 have been found to couple with ketones in a highly diastereoselective fashion, which depends markedly upon the geometry (Z) of the azaenolates and the nature of the ligands which are present on the boron atom, in good accordance with semiempirical calculations (AM1).

Lithiated chloromethyl derivatives 2a-d , available by deprotonation of 2-(chloromethyl)-1,3-benzothiazole 1a , -1,3-oxazoline 1b , -pyridine 1c and -quinoline 1d , react with carbonyl compounds to give chlorohydrins 3a-g and then epoxides 4a-g upon treatment with NaOH/ i PrOH. The same chlorohydrins 3a-g could be converted into heterosubstituted chloroalkenes 5a-g with very high E stereoselection upon reaction with MeSO/ 2 ClEt 3 N Download full-size image

2-Oxiranyl and 2-aziridinyl thiazoles 3 and 7 have been prepared by lithiation of thiazoles 1a and 1b and reaction with α-halogenocarbonyl compounds [1a, 1c-d] and α-halogenoimines [1b], respectively.

Deprotonation of oxazolinyl oxirane 1a with sec-BuLi/TMEDA at −100 °C in Et2O furnished oxazolinyl oxiranyllithium 1b, which could be trapped with electrophiles to give oxiranes 1c-g. The reaction of 1b with aldehydes produced diastereomers syn (2a-d) and anti (3a-d). Oxiranyllithium 1i from trans-1-oxazolinyl-2-p-tolyl epoxy ethane 1h was found to be configurationally stable while oxiranyllithium 11, generated from the cis isomer 1k, was not. Oxazolinyl epoxides 1d, 1j and 1m could be deblocked to acyl oxiranes 5a-e through oxazolidines 4a-e.

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.

alpha-Lithiated epoxides, long considered "fleeting" intermediates in the reactions of epoxides with strong bases, have nowadays proven to be key synthons for asymmetric synthesis. In this study, the solution and the solid state structure of an alpha-lithiated aryloxirane, namely a-lithiated ortho-trifluoromethyl styrene oxide (1-Li), were determined. Single crystal X-ray diffraction analysis of 1-Li performed at 100 K applying the X-TEMP-2 device revealed a self-assembled heterochiral dimeric structure with a rare central six-membered (O-Li-C)(2) planar core, which is unprecedented in Li/oxygen carbenoids. Multinuclear magnetic resonance (H-1, C-13, F-19, Li-7) studies suggested that 1-Li exists in THF solution as a mixture of two interconverting diastereomeric dimeric aggregates, each one featuring a single s-contact between lithium and a carbon atom. Line shape analysis provided activation parameters for both the dynamic interconversion of the two dimers and the enantiomerisation of 1-Li, which proved to be mostly entropy controlled. The structural assignment in solution was supported by density functional theory computations through the investigation of conformers of monomeric and dimeric complexes of 1-Li featuring different degrees of specific solvation. A mechanism based on the equilibration of six-membered homo-and heterochiral dimers was proposed to explain the configurational instability exhibited by 1-Li in THF.

This Frontier highlights and analyses, from the authors’ perspective, the impact on organic synthesis of some recent contributions from the organolithium field, also discussing the opportunities that are on the horizon.

Lithiation of 2-(chloroethyl)benzothiazole 1a gives (benzothiazolylchloroethyl)lithium 1b . The reaction of 1b with ketones in the presence of (−)-sparteine leads to chlorohydrins 2a-c that cyclize to epoxides 3a-c , enantioselectively, the enantiomeric enrichment being dependent upon the solvent, the lithiating agent, the reaction time. The reaction with aldehydes furnishes chlorohydrins 2d-g and then epoxides 3d-g diastereo- and enantioselectively.

2-chloroalkylbenzothiazoles 1a-c react with alkoxides to give substitution and ring-expanded products 3 and 4. The substitution ring-enlargement competition much depends upon the solvent, the substitution reaction being preferred in alcohols and the ring-enlargement in DMF.

The reaction of ortho-lithiated aryloxiranes with various enaminones straightforwardly affords new functionalized isochromanes as mixtures of two epimeric stereoisomers in reasonable to very good yields (50–90%). The two diastereomers, which show a high structural variability, can be easily separated by column chromatography.

Review: new applications of organometallics as catalysts or reagents in organic syntheis; 108 refs.

α,β-epoxy aldehydes have been prepared by deblocking of oxazolinyl oxiranes, which in turn have been synthesized on treatment of 4,4-dimethyloxazolinylchloromethyllithium with aldehydes.

The two geometric isomers of 2-(tert-butyl)-4-phenyloxetane have, for the first time, been prepared starting from the commercially available 4,4-dimethyl-1-phenylpentane-1,3-dione. The latter was reduced with NaBH4 to give a mixture of diastereomeric syn and anti diols which were then stereospecifically cyclized into the corresponding oxetanes with an overall yield for the two steps of 69.6%. The newly synthesized stereoisomeric four-membered oxygenated heterocycles were separated by column chromatography on silica gel and fully spectroscopically characterized.

A two-step stereoselective chemoenzymatic synthesis of optically active ?-aryl-substituted oxygen heterocycles was developed, exploiting a whole-cell mediated asymmetric reduction of ?-, ? -, and ? -chloroalkyl arylketones followed by a stereospecific cyclization of the corresponding chlorohydrins into the target heterocycles. Among the various whole cells screened (baker's yeast, Kluyveromyces marxianus CBS 6556, Saccharomyces cerevisiae CBS 7336, Lactobacillus reuteri DSM 20016), baker's yeast was the one providing the best yields and the highest enantiomeric ratios (up to 95:5 er) in the bioreduction of the above ketones. The obtained optically active chlorohydrins could be almost quantitatively cyclized in a basic medium into the corresponding ?-aryl-substituted cyclic ethers without any erosion of their enantiomeric integrity. In this respect, valuable, chiral non-racemic functionalized oxygen containing heterocycles (e.g., (S)-styrene oxide, (S)-2-phenyloxetane, (S)-2-phenyltetrahydrofuran), amenable to be further elaborated on, can be smoothly and successfully generated from their prochiral precursors.

The first examples for utilization of oxetane as an directing metalation group are presented.

The first dynamic thermodynamic resolution of a racemic oxiranyllithium is described with selectivities of up to 82 : 18 from a selection of three types of chiral ligands (seven ligands in total). Both (−)-sparteine and its (+)-surrogate were surprisingly found to increase the enantiomerisation barrier.

Reductive ring cleavage of thiochromane and benzodihydrothiophene by the radicalanion 4,4′-di- tert -butylbiphenylide (LDBB) leads to organolithium compounds bearing a lithium arylthiolate group. The adducts of the latter with aldehydes and ketones undergo ring closure when treated under appropriate acidic conditions. The result is a two-pot one-carbon subsititutive ring expansion of the original sulfur heterocycle.

Trimethylbenzothiazoline 1 has been reductively lithiated by lithium 4,4′- di-t-butylbiphenylide (LDBB) leading to dianion 3, that has been trapped with electrophiles to give aminodisulfides 4, aminosulfide 8 and aminoalcohols 9.