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International audience; Bioorthogonal click and release reactions are powerful tools for chemical biology allowing, for example, the selective release of drugs in biological media, including inside animals. Here we developed two new families of iminosydnone mesoionic reactants that allow a bioorthogonal release of electrophilic species under physiological conditions. Their synthesis and reactivities as dipoles in cycloaddition reactions with strained alkynes have been studied in detail. Whereas the impact of the pH on the reaction kinetics was demonstrated experimentally, theoretical calculations suggest that the newly designed dipoles display reduced resonance stabilization energies compared to previously described iminosydnones explaining their higher reactivity. These mesoionic compounds react smoothly with cycloalkynes under physiological, copper-free reaction conditions to form a click pyrazole product together with a released alkyl-or aryl-isocyanate. With kinetic constants up to 1000 M-1 s-1 , this click and release reaction is among the fastest described to date, and represents the first bioorthogonal process allowing the release of isocyanate electrophiles inside living cells offering interesting perspectives in chemical biology.

Herein, we describe a methodology for iminosydnone chlorination and we demonstrate the high beneficial effect of this modification on the reactivity of these mesoionic dipoles in strain-promoted cycloaddition reactions. Exploiting their reaction with cyclooctynes, we used these new iminosydnones for bioorthogonal release of amide, urea and sulfonamide containing drugs. Notably, drugs containing a terminal amide function were released for the first time with good kinetic constants.

The synthesis of quaternary homoallylic halides and trichloroacetates from cyclopropylcarbinols, as reported by Marek in 2020 (J. Am. Chem. Soc. 2020, 142, 5543-5548), is one of the few reported examples of stereospecific nucleophilic substitution involving chiral bridged carbocations. However, for the phenyl-substituted substrates the stereoselectivity of the reaction is poor and a mixture of diastereomers is obtained. In order to understand the nature of the intermediates involved in this transformation and explain the loss of selectivity for certain substrates, we have performed a Density Functional Theory investigation of the reaction mechanism at the DLPNO-CCSD(T)/Def2TZVPP level of theory. Our results indicate that cyclopropylcarbinyl cations are stable intermediates in this reaction, while bicyclobutonium structures are high-energy transition structures and as such are not involved, regardless of the substitution pattern on the substrate. Instead, multiple rearrangement pathways of cyclopropylcarbinyl cations have been located, including rotations around their π-bonds and ring openings to homoallylic cations. Importantly, the relative energies of these homoallylic cations and of the activation barriers to reach them are correlated to the nature of the substituents. While direct nucleophilic attack on the chiral cyclopropylcarbinyl cation is kinetically favored for most systems, the rearrangements become competitive with nucleophilic attack for the phenyl-substituted systems, leading to a loss of selectivity through a mixture of rearranged carbocation intermediates. As such, it appears that stereospecific reactions of chiral cyclopropylcarbinyl cations depend on the ability of these cations to access homoallylic structures, from which selectivity is not guaranteed.

The reactions of elemental sulfur (S8) and polysulfides with nucleophiles are relevant to organic synthesis, materials science and biochemistry, but the mechanisms by which they operate are still unknown. Depending on the nature of the nucleophile, a broad variety of products can be obtained, but reaction pathways are difficult to probe experimentally due to the inherent thermodynamic and kinetic instability of polysulfide intermediates. Using Density Functional Theory (DFT) calculations at the ωB97X-D/aug-cc-pV(T+d)Z/SMD(MeCN) // ωB97X-D/ aug-cc-pVDZ/SMD(MeCN) level of theory, this study provides the first comprehensive picture of the mechanisms behind the reaction of elemental sulfur and polysulfides with cyanide and phosphines, which quantitatively generate the monosulfide products thiocyanate and phosphine sulfides, respectively. These reactions are prototypical of the reactions of sulfur with other strong nucleophiles including carbon-based systems, and are important for the titration and detection of elemental sulfur in complex media. We first demonstrate that from the two mechanisms proposed more than 60 years ago, the Foss-Bartlett hypothesis is the most likely nucleophilic decomposition pathway for polysulfide intermediates containing good leaving groups. Importantly, we find that unimolecular decomposition pathways, either cyclizative or not, have the lowest activation barriers for most polysulfide intermediates. This indicates that such intermediates are fleeting in solution as they rapidly decompose even without an external nucleophile. Other bimolecular decomposition pathways were also located, including scrambling reactions and attack on thiosulfoxides, which are less likely to intervene as they require larger concentrations of highly-reactive intermediates. Overall, our results indicate that for long polysulfides intramolecular cyclization is the most favorable decomposition pathway. For short polysulfides, a mixture of unimolecular decomposition and nucleophilic attack can be expected, in addition to scrambling pathways if the concentration of nucleophile is low. Overall, this work highlighted the various pathways available for polysulfide decomposition, allowing the study of polysulfide reactivity in more complex settings such as organic transformations or biochemical pathways.

The synthesis of quaternary homoallylic halides and trichloroacetates from cyclopropylcarbinols, as reported by Marek in 2020 (J. Am. Chem. Soc. 2020, 142, 5543-5548), is one of the few reported examples of stereospecific nucleophilic substitution on chiral non-classical carbocations. However, for the phenyl-substituted substrates the stereoselectivity of the reaction is lost and a mixture of diastereomers is obtained. In order to understand the nature of the intermediates in-volved in this transformation and explain the loss of selectivity for certain substrates, we have performed a Density Func-tional Theory investigation of the reaction mechanism at the ωB97X-D/Def2TZVPP and M06-2X/Def2TZVPP levels of theory. Our results indicate that non-classical cyclopropylcarbinyl cations are stable intermediates in this reaction, while bicyclobutonium structures are high-energy transition structures and as such are not involved, regardless of the substitu-ent pattern on the substrate. Instead, multiple rearrangement pathways of cyclopropylcarbinyl cations have been located, including rotations around their π-bonds and ring openings to classical homoallylic cations. Importantly, the relative energies of these homoallylic cations and of the activation barriers to reach them are correlated to the nature of the sub-stituents. While direct nucleophilic attack on the chiral cyclopropylcarbinyl cation is kinetically favored for most sys-tems, the rearrangements become competitive with nucleophilic attack for the phenyl-substituted systems, leading to a loss of selectivity through a mixture of rearranged carbocation intermediates. As such, it appears that stereospecific reac-tions of chiral non-classical cyclopropylcarbinyl cations depend on the ability of these cations to access classical homoallylic structures, from which selectivity is not guaranteed.

Two similar tridentate directing groups derived from glycine and 8-aminoquinoline were shown to enable the palladium-catalyzed anti-Markovnikov hydrofunctionalization of 4-pentenylamine with drastically different efficiencies. A computational investigation into the origin of the reactivity difference between these isomeric, carbonyl-transposed auxiliaries suggests that protonation state, thus charge of the substrate-metal complex prior to nucleopalladation is key. These investigations have culminated in a directing group design that can undergo Pd-catalyzed hydrofunctionalization under relatively mild conditions, as low as room temperature.

We describe the first thiourea-catalyzed C-F bond activation. The use of a thiourea catalyst and Ti(OiPr)4 as a fluoride scavenger allows the amination of benzylic fluorides to proceed in moderate to excellent yields. Preliminary results with S- and O-based nucleophiles are also presented. DFT calculations reveal the importance of hydrogen bonds between the catalyst and the fluorine atom of the substrate to lower the activation energy during the transition state.

A photoinitiated anti -hydropentafluorosulfanylation of terminal alkynes using SF 5 Cl and (TMS) 3 SiH as the hydrogen atom donor is reported. This transformation generates selectively ( Z ) - (1-alken-1-yl)pentafluoro-λ 6 -sulfanes ( Z:E : >85:15), thus allowing the preparation of this previously unknown geometrical isomer. DFT calculations highlight that the selectivity is due to the intrinsic preference of SF 5 -substituted vinylic radicals to adopt a cis geometry, and to increased steric contacts during the transition structures leading to the minor ( E )-products.

The first catalytic intermolecular desymmetrization of azetidines was reported by Sun and coworkers in 2015 using a BINOL-derived phosphoric acid catalyst (J. Am. Chem. Soc. 2015, 137, 5895–5898). To uncover the mechanism of the reaction and the origins of the high enantioselectivity, Density Functional Theory (DFT) calculations were performed at the B97D3/6-311+G(2d,2p)/SMD(toluene)//B97D3/6-31G(d,p)/CPCM(toluene) level of theory. Comparison of four possible activation modes confirms that this reaction proceeds through the bifunctional activation of the azetidine nitrogen and the thione tautomer of the 2-mercaptobenzothiazole nucleophile. Upon thorough conformational sampling of the enantiodetermining transition structures (TSs), a free energy difference of 2.0 kcal mol−1 is obtained, accurately reproducing the experimentally measured 88% e.e. at 80 °C. This energy difference is due to both decreased distortion and increased non-covalent interactions in the pro-(S) TS. To uncover the true origins of selectivity, the TSs optimized with the full catalyst were compared to those optimized with a model catalyst through steric maps. It is found that the arrangements displayed by the substrates are controlled by strict primary orbital interaction requirements at the transition complex, and their ability to fit into the catalyst pocket drives the selectivity. A general model of selectivity for phosphoric acid-catalyzed azetidine desymmetrizations is proposed, which is based on the preference of the nucleophile and benzoyl group to occupy empty quadrants of the chiral catalyst pocket., The origins of selectivity in azetidine desymmetrizations have been determined computationally. Comparison of structures with model and full catalysts provided key details missed by typical analyses of the stereodetermining transition structures.

The relative rates of arylation of primary alkylamines with different Pd-NHC catalysts have been measured, as have the relative rates of arylation of the secondary aniline product in an attempt to understand the key ligand design features necessary to have high selectivity for the monoarylated amine product. As the substituents on the N-aryl ring of the NHC increase in size, selectivity for monoarylation increases and this is further enhanced by chlorinating the back of the NHC ring. Computations have been performed on the catalytic cycle of this transformation in order to understand the selectivity obtained with the different catalysts.

We report a general and rapid chemoselective Kumada-Tamao-Corriu (KTC) cross-coupling of aryl bromides in the presence of chlorides or triflates with functionalized Grignard reagents at 0 °C in 15 min by using Pd-PEPPSI-IPentCl (C4). Nucleophiles and electrophiles (or both) can contain Grignard-sensitive functional groups (-CN, -COOR, etc.). Control experiments together with DFT calculations suggest that transmetallation is rate limiting for the selective cross-coupling of Br in the presence of Cl/OTf with functionalized Grignard reagents. One-pot sequential KTC/KTC cross-couplings with bromo-chloro arenes have been demonstrated for the first time. We also report the one-pot sequential KTC/Negishi cross-couplings using C4 showcasing the versatility of this methodology.

Alkenes bearing four carbon-based groups are ubiquitous motifs in chemical sciences due to their various applications from medicinal to materials chemistry, and as chemical platforms for the synthesis of complex, chiral molecules. As such, tremendous research efforts are currently ongoing in order to develop general procedures for the challenging stereoselective synthesis of all-carbon tetrasubstituted alkenes, especially for acyclic structures. Since classical approaches to carbon–carbon double bonds are not suitable for the high steric demand around tetrasubstituted alkenes, a variety of unique approaches to access these privileged functional groups have been developed in recent years. This review article highlights the most significant developments in the field from 2007 to 2020, with an emphasis on the mechanisms and remaining limitations of these contemporary methods. Specifically, recent advances in internal alkyne carbofunctionalizations, in multicomponent couplings or other cross-couplings from nucleophilic or electrophilic alkenyl partners, and in the development of miscellaneous methods, are discussed.

The capture of reactive intermediates is important for the elucidation of reaction mechanisms. We report the first observation of electrochemically generated, short-lived radical cations of carbazole (

The first approach to pyrazole-containing helicenes via sydnone-aryne [3 + 2]-cycloaddition is described. An unprecedented regioselectivity in the cycloaddition step toward the more sterically constrained product was observed in the presence of extended aromatic scaffolds. DFT calculations enabled understanding the origin of this unexpected selectivity.

Preparation of [18F]fluoroarenes with nucleophilic [18F]fluoride for positron emission tomography (PET) molecular imaging ­research is a challenging chemical endeavor. Advances in radiofluorination have soared in the last decade, broadening the availability of potential [18F]fluoroarenes. In this Synpacts article, we highlight the ­recent development from our laboratory of a practical radiofluorination of anilines via N-arylsydnone intermediates to afford [18F]fluoroarenes. Further, we emphasize the utility of this methodology towards peptide labeling applications by preparing an 18F-labeled neuropeptide.1 Introduction2 Nucleophilic Radiofluorination of N-Arylsydnones3 Substrate Scope and Applications4 Conclusion

NK and DOH acknowledge support from the University of St Andrews, Engineering and Physical Sciences Research Council (EPSRC, Grant No.: EP/L017911/1), and the EPSRC UK National Mass Spectrometry Facility at Swansea University. This work was also supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the FRQNT Centre in Green Chemistry and Catalysis (CGCC), and the Université Laval. In recent years, the highly polar C-F bond has been utilised in activation chemistry despite its low reactivity to traditional nucleophiles, when compared to other C-X halogen bonds. Paquin's group has reported extensive studies on the C-F activation of benzylic fluorides for nucleophilic substitutions and Friedel-Crafts reactions, using a range of hydrogen bond donors such as water, triols or hexafluoroisopropanol (HFIP) as the activators. This study examines the stereointegrity of the C-F activation reaction through the use of an enantiopure isotopomer of benzyl fluoride to identify whether the reaction conditions favour a dissociative (SN1) or associative (SN2) pathway. [2H]-Isotopomer ratios in the reactions were assayed using the Courtieu 2H NMR method in a chiral liquid crystal (poly-γ-benzyl-L-glutamate) matrix and demonstrated that both associative and dissociative pathways operate to varying degrees, according to the nature of the nucleophile and the hydrogen bond donor. Publisher PDF

Density functional theory methods were used to elucidate the activation mode and origin of stereoselectivity in chiral phosphoric acid-catalyzed intramolecular oxetane desymmetrizations. Computed enantioselectivities are in excellent agreement with experiment. An unexpected, distortion-driven activation mode was observed, instead of the usual “bifunctional activation”. This mode is favored for only some intramolecular oxetane openings, highlighting an exception to known models. Stereoselectivity in these reactions can be explained by the balance of favorable noncovalent interactions of the substrates with both the aryl substituents and phosphoric acid functionality of the catalysts.

A practical method for radiofluorination of anilines with [18F]fluoride via N-arylsydnone intermediates is described. These precursors are stable, easy to handle and facilitate direct and regioselective 18F-labeling to prepare [18F]fluoroarenes. The value of this methodology is further highlighted by successful application to prepare an 18F-labeled neuropeptide.

We report that the addition of a catalytic amount of trifluoroacetic acid (TFA) shortens the induction period associated with the 1,1,1,3,3,3-hexafluoroisopropoanol (HFIP)-promoted Friedel-Crafts reaction of benzylic fluorides. This faster initiation is due to TFA’s strong hydrogen-bond donation capability, not its Bronsted acidity. The improved reaction conditions were applied to a set of substrates, demonstrating how they could improve yields and reliability of this transformation.

Sulfonamide derivatives have been used in pharmaceutics for decades. Here we report a new approach to release sulfonamides efficiently using a bioorthogonal reaction of sulfonyl sydnonimines and dibenzoazacyclooctyne (DIBAC). The second-order rate constant of the cycloaddition reaction can be up to 0.62 M−1 s−1, and the reactants are highly stable under physiological conditions. Most significantly, we also discovered the mutual orthogonality between the sydnonimine–DIBAC and benzonorbornadiene–tetrazine cycloaddition pairs, which can be used for selective and simultaneous liberation of sulfonamide and primary amine drugs.

The 6,6-quinolone scaffolds on which viridicatin-type fungal alkaloids are built are frequently found in metabolites that display useful biological activities. Here we report in vitro and computational analyses leading to the discovery of a hemocyanin-like protein AsqI from the Aspergillus nidulans aspoquinolone biosynthetic pathway that forms viridicatins via a conversion of the cyclopenin-type 6,7-bicyclic system into the viridicatin-type 6,6-bicyclic core through elimination of carbon dioxide and methylamine through methyl isocyanate., Viridicatin is a fungal alkaloid. Here, the authors identify and characterize the cyclopenase that catalyzes the last step of its biosynthesis in Aspergillus nidulans, the conversion of cyclopenin to viridicatin, and find that the reaction proceeds via an unusual elimination mechanism.

Experimental and theoretical evidence is reported for a rare type I dyotropic rearrangement involving a [1,2]-alkene shift, leading to the regio- and stereospecific ring contraction of bromocycloheptenes. This reaction occurs under mild conditions, with or without a Lewis acid catalyst. DFT calculations show that the reaction proceeds through a nonclassical carbocation-anion pair, which is crucial for the low activation barrier and enantiospecificity. The chiral cyclopropylcarbinyl cation may be a transition state or an intermediate, depending on the reaction conditions.

The nucleophilic substitution of a benzylic fluoride by morpholine promoted by alcohols in neutral conditions has been studied extensively by DFT calculations and structure–activity analysis. This C F activation strategy is possible through hydrogen-bond donation from the alcohol. The experiments demonstrate that this reaction is very tolerant of variations on the promoting alcohol, yet some activators are significantly weaker. These results were supported by DFT calculations, which indicate that three OH groups around the fluorine atom at the transition-state are better to activate the C F bond, but only two of them can originate from a single triol molecule, at least for the triol under study. A revised mechanistic explanation has been proposed and provides a better understanding of experimental results. Finally, the use of a catalytic amount of triol was also investigated.

The Friedel-Crafts benzylation of arenes using benzyl alcohols activated in situ with XtalFluor-E is described. A wide range of 1,1-diarylmethanes and 1,1,1-triarylmethanes were prepared under experimentally simple and mild conditions, without the need for a transition metal or a strong Lewis acid. Notably, the reactivity observed demonstrates the potential of XtalFluor-E to induce C-OH bond ionization and SN1 reactivity of benzylic alcohols.

Experimental and computational studies pertaining to the Fischer azaindolization reaction are reported. These studies explain why pyridylhydrazines are poorly reactive in Fischer indolization reactions, in addition to the origin of hydrazine substituent effects. Additionally, an interrupted variant of Fischer azaindolization methodology is disclosed, which provides a synthetic entryway into fused azaindoline scaffolds.

The geometries, stabilities, and 1,3-dipolar cycloaddition reactivities of 24 mesoionic azomethine ylides and imines were investigated using density functional theory calculations at the M06-2X/6-311+G-(d,p)/M06-2X/6-31G-(d) level. The computed structures highlight how the commonly used "aromatic" resonance form should be replaced by two more accurate resonance structures. Stabilities of the dipoles were assessed by various homodesmotic schemes and are consistent with these compounds being nonaromatic. The activation free energies with ethylene or acetylene range from 11.8 to 36.6 kcal/mol. Within each dipole type, the predicted cycloaddition reactivities correlate with the reaction energies and the resonance stabilization energies provided by the various substituents. Endocyclic (X) heteroatoms increase the reactivity of the 1,3-dipoles in the order of O > NH ≅ S, whereas exocyclic (Y) substituents increase it in the order of CH2 > NH > O > S. Distortion/interaction analysis indicated that the difference in reactivity between differently substituted 1,3-dipoles is driven by distortion, whereas the difference between azomethine ylides and imines is related to lower interaction energies of imines with the dipolarophiles.

Activation of the C–F bond of benzylic fluorides was achieved using 1,1,1-tris(hydroxymethyl)propane (2) as a hydrogen bond-donating agent. Investigations demonstrated that hydrogen bond-donating solvents are promoting the activation and hydrogen bond-accepting ones are hindering it. However, the reaction is best run under highly concentrated conditions, where solvents cannot interfere with the interaction between the organofluorine compound and the triol. Various benzylic fluorides react with secondary amines or anilines to form benzylic amines in good yields.

We report that the addition of a catalytic amount of trifluoroacetic acid (TFA) shortens the induction period associated with the 1,1,1,3,3,3-hexafluoroisopropoanol (HFIP)-promoted Friedel-Crafts reaction of benzylic fluorides. This faster initiation is due to TFA’s strong hydrogen-bond donation capability, not its Bronsted acidity. The improved reaction conditions were applied to a set of substrates, demonstrating how they could improve yields and reliability of this transformation.

The origins of the high enantioselectivity of chiral phosphoric acid-catalyzed oxetane desymmetrizations were investigated by density functional theory (DFT) calculations. Distortion of the catalyst structure, caused by steric crowding in the catalyst pocket of one enantiomeric transition state, is the main cause for stereochemical preference. A general model was developed to assist in the rational design of new catalysts for related transformations.

A Friedel–Crafts benzylation of arenes with benzyl fluorides has been developed. The reaction produces 1,1-diaryl alkanes in good yield under mild conditions without the need for a transition metal or a strong Lewis acid. A mechanism involving activation of the CF bond through hydrogen bonding is proposed. This mode of activation enables the selective reaction of benzylic CF bonds in the presence of other benzylic leaving groups.

A Friedel-Crafts benzylation of arenes with benzyl fluorides has been developed. The reaction produces 1,1-diaryl alkanes in good yield under mild conditions without the need for a transition metal or a strong Lewis acid. A mechanism involving activation of the C-F bond through hydrogen bonding is proposed. This mode of activation enables the selective reaction of benzylic C-F bonds in the presence of other benzylic leaving groups.

[3880-03-3] C2H6F3NS (MW 133.14) InChI = 1S/C2H6F3NS/c1-6(2)7(3,4)5/h1-2H3 InChIKey = SAVQQRYWWAGSQW-UHFFFAOYSA-N (deoxofluorinating agent) Alternate names: dimethylaminosulfurtrifluoride; Me-DAST; (N,N-dimethylamino)sulfur trifluoride. Physical Data: mp −78.7 °C; bp 117.5 °C; d 1.364 g mL−1 at 25 °C. Solubility: Soluble in chlorinated, hydrocarbon, and ethereal solvents; reacts violently with water; avoid alcohols and ketone-containing solvents (e.g., acetone). Preparative Method: the reagent is prepared by the reaction of N,N-dimethyltrimethylsilylamine and sulfur tetrafluoride.1 Form supplied in: colorless liquid; few suppliers. Purification: distillation of the reagent prior to use is recommended. Handling, Storage, and Precautions: dimethylaminosulfur trifluoride is extremely toxic, highly corrosive, and flammable. Manipulate with great caution, under a fume hood. Storage in plastic bottles between 2–8 °C (although storage at room temperature is possible),1 and under an inert atmosphere is preferential.

International audience; It was discovered that the presence of water as a cosolvent enables the reaction of activated alkyl fluorides for bimolecular nucleophilic substitution reactions. DFT calculations show that activation proceeds through stabilization of the transition structure by a stronger F···H$_2$O interaction and diminishing C–F bond elongation, and not simple transition state electrostatic stabilization. Overall, the findings put forward a distinct strategy for C–F bond activation through H-bonding.

A simple and effective method for stereocontrolled preparation of 1,1-diaryl-2-fluoroethenes is reported. First, 1-aryl-1-bromo-2-fluoroethenes are generated using an addition/elimination reaction ...

An addition/elimination reaction of organolithium reagents to silylated β,β-difluorostyrene derivatives followed by a bromination/desilicobromination reaction provides a simple and effective synthetic approach to a wide range of bromofluoroalkenes (up to >97/3). In addition, the bromofluoroalkenes can be used in Pd-catalyzed transformations giving access to both tri- and tetrasubstituted fluoroalkenes.

An addition/elimination reaction of organolithium reagents to silylated beta,beta-difluorostyrene derivatives followed by a bromination/desilicobromination reaction provides a simple and effective synthetic approach to a wide range of bromofluoroalkenes (up to97/3). In addition, the bromofluoroalkenes can be used in Pd-catalyzed transformations giving access to both tri- and tetrasubstituted fluoroalkenes.