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International audience; Regioselective access to heterohelicenes through the 1,3-dipolar cycloaddition of sydnones with arynes is described. Novel access to sydnones and poly(hetero)aromatic aryne precursors allowed the introduction of chemical diversity over multiple positions of the helical scaffolds. The origins of the unconventional regioselectivity during the cycloaddition steps was systematically investigated using density functional theory (DFT) calculations, unveiling the key features that control this reactivity, namely, face-to-face ($\pi \cdot \cdot \cdot \pi$) or edge-to-face (C−H$\cdot \cdot \cdot \pi$) interactions, primary orbital interactions and distortion from coplanarity in the transition structures (TSs) of the transformation. From the library of 24 derivatives synthesized, a pyridyl containing derivative displayed reversible, red-shifted, pH-triggered chiroptical switching properties, with CPL-sign reversal. It is found that protonation of the helicene causes a change of the angle between the electric and magnetic dipole moments related to the S$_1$ → S$_0$ transition, resulting in this rare case of reversible CPL sign inversion upon application of an external stimulus.

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 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.

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.

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.

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.

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.