DFT Studies on Molecular and Electronic Structures of Cationic Carbene Complexes [L2(η5-C5H5)Fe=CR5]+ (L = CO, PH3, dhpe, PPh3; R = H, F, CH3)

For the title complexes we discuss the results of DFT calculations for (i) the molecular and electronic structures, (ii) the rotational barriers of the carbenes around the Fe=Ccarb bond (ΔErot), and (iii) the binding energies of the carbenes (De). Where available, the calculated properties of the Fe=Ccarb bonds are compared with previous theoretical and experimental data of some prototypical carbene complexes classified as Fischer- or Schrock-type compounds. It is shown that the rotational barriers of the carbenes, the Fe−Ccarb bond distances and bond strengths are sensitive to the carbene substituents and to the ligands L attached to iron. For complexes with given L the values of ΔErot diminish in the order: CH2 > CF2 > CMe2 and an inverse ordering is obtained for the decrease of the Fe=Ccarb bond distance. The ΔErot of dimethylcarbene are close to those of Fischer-type compounds, while ΔErot of methylidene approach values typical for Schrock-type carbenes. The replacement of the CO ligand by poorer π-acceptor ligand increases the values of ΔErot in the order: CO CMe2 > CF2. The properties of the investigated compounds are traced back to the character of the Fe → Ccarb π-backbonding interactions and their competitions with the Fe → L and R → Ccarb π- interactions. It is also shown that the PH3 ligand can only be considered with caution as a good model for the PPh3 ligand in computational studies

Reaction between Azidyl Radicals and Alkynes: A Straightforward Approach to NHâ 1,2,3â Triazoles

Reaction between nitrogenâ centered radicals and unsaturated Câ C bonds is an effective synthetic strategy for the construction of nitrogenâ containing molecules. Although the reactions between nitrogenâ centered radicals and alkenes have been studied extensively, their counterpart reactions with alkynes are extremely rare. Herein, the first example of reactions between azidyl radicals and alkynes is described. This reaction initiated an efficient cascade reaction involving interâ /intramolecular radical homolytic addition toward a Câ C triple bond and a hydrogenâ atom transfer step to offer a straightforward approach to NHâ 1,2,3â triazoles under mild reaction conditions. Both the internal and terminal alkynes work well for this transformation and some heterocyclic substituents on alkynes are compatible. This mechanistically distinct strategy overcomes the inherent limitations associated with azide anion chemistry and represents a rare example of reactions between a nitrogenâ centered radicals and alkynes.Get radical! Although the reactions between nitrogenâ centered radicals and alkenes have been studied extensively, their counterpart reactions with alkynes are extremely rare. Herein, the first example of reactions between azidyl radicals and alkynes is described (see scheme).Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137186/1/chem201504515.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137186/2/chem201504515-sup-0001-misc_information.pd

Inverting Small Molecule-Protein Recognition by the Fluorine Gauche Effect: Selectivity Regulated by Multiple H→F Bioisosterism

Fluorinated motifs have a venerable history in drug discovery, but as C(sp3 )@F-rich 3D scaffolds appear with increasing frequency, the effect of multiple bioisosteric changes on molecular recognition requires elucidation. Herein we demonstrate that installation of a 1,3,5-stereotriad, in the substrate for a commonly used lipase from Pseudomonas fluorescens does not inhibit recognition, but inverts stereoselectivity. This provides facile access to optically active, stereochemically well-defined organofluorine compounds (up to 98% ee). Whilst orthogonal recognition is observed with fluorine, the trend does not hold for the corresponding chlorinated substrates or mixed halogens. This phenomenon can be placed on a structural basis by considering the stereoelectronic gauche effect inherent to F@C@C@X systems (s!s*). Docking reveals that this change in selectivity (H versus F) with a common lipase results from inversion in the orientation of the bound substrate being processed as a consequence of conformation. This contrasts with the stereochemical interpretation of the biogenetic isoprene rule, whereby product divergence from a common starting material is also a consequence of conformation, albeit enforced by two discrete enzymes

Contra-thermodynamic E → Z isomerization of cinnamamides via selective energy transfer catalysis

A bio-inspired, photocatalytic E → Z isomerization of cinnamides is reported using inexpensive (−)-riboflavin (vitamin B2) under irradiation at λ = 402 nm. This operationally simple transformation is compatible with a range of amide derivatives including –NR2, –NHSO2R and N(Boc)2 (up to 99:1 Z:E). Selective energy transfer from the excited state photocatalyst to the starting E-isomer ensures that directionality is achieved: The analogous process with the Z-isomer is inefficient due to developing allylic strain causing chromophore deconjugation. This is supported by X-ray analysis and Stern-Volmer photo-quenching studies. Preliminary validation of the method in manipulating the conformation of a simple model Leu-enkephalin penta-peptide is disclosed via the incorporation of a cinnamamide-based amino acid

Binding of molecular magnesium hydrides to a zirconocene-enyne template

An enyne-zirconium complex stabilizes molecular magnesium hydride ('MG''H IND. 2') and even a molecular hydride, n'C IND. 4''H IND. 9''MG'H. These systems feature magnesium olefin "pi" complexation.Deutsche ForschungsgemeinschaftNRW Forschungsschule “Molecules and Materials

DFT Studies on Molecular and Electronic Structures of Cationic Carbene Complexes [L2(η5-C5H5)Fe=CR5]+ (L = CO, PH3, dhpe, PPh3; R = H, F, CH3)

For the title complexes we discuss the results of DFT calculations for (i) the molecular and electronic structures, (ii) the rotational barriers of the carbenes around the Fe=Ccarb bond (ΔErot), and (iii) the binding energies of the carbenes (De). Where available, the calculated properties of the Fe=Ccarb bonds are compared with previous theoretical and experimental data of some prototypical carbene complexes classified as Fischer- or Schrock-type compounds. It is shown that the rotational barriers of the carbenes, the Fe−Ccarb bond distances and bond strengths are sensitive to the carbene substituents and to the ligands L attached to iron. For complexes with given L the values of ΔErot diminish in the order: CH2 > CF2 > CMe2 and an inverse ordering is obtained for the decrease of the Fe=Ccarb bond distance. The ΔErot of dimethylcarbene are close to those of Fischer-type compounds, while ΔErot of methylidene approach values typical for Schrock-type carbenes. The replacement of the CO ligand by poorer π-acceptor ligand increases the values of ΔErot in the order: CO CMe2 > CF2. The properties of the investigated compounds are traced back to the character of the Fe → Ccarb π-backbonding interactions and their competitions with the Fe → L and R → Ccarb π- interactions. It is also shown that the PH3 ligand can only be considered with caution as a good model for the PPh3 ligand in computational studies

Mild C–F activation in perfluorinated arenes through photosensitized insertion of isonitriles at 350 nm

Authors thank the Westfälische Wilhelms-University of Münster for continuous financial support.Fluorinated compounds have become important in the fields of agrochemical industry, pharmaceutical chemistry and materials sciences. Accordingly, various methods for their preparation have been developed in the past. Fluorinated compounds can be accessed via conjugation with fluorinated building blocks, via C–H fluorination or via selective activation of perfluorinated compounds to give the partially fluorinated congeners. Especially the direct activation of C–F bonds, one of the strongest σ‐bonds, still remains challenging and new strategies for C–F activation are desirable. Herein a method for the photochemical activation of aromatic C–F bonds is presented. It is shown that isonitriles selectively insert into aromatic C–F bonds while aliphatic C–F bonds remain unaffected. Mechanistic studies reveal the reaction to proceed via the indirect excitation of the isonitrile to its triplet state by photoexcited acetophenone at 350 nm. Due to the relatively mild light used, the process shows high functional group tolerance and various compounds of the class of benzimidoyl fluorides are accessible from aryl isonitriles and commercially available perfluorinated arenes.PostprintPeer reviewe

Asymmetric Addition of Allylsilanes to Aldehydes – A Cr/Photoredox Dual Catalytic Approach Complementing the Hosomi–Sakurai Reaction

The allylation of aldehydes is a fundamental transformation in synthetic organic chemistry. Among the multitude of available reagents, especially allylsilanes have been established as preferred allyl source. As initially reported by Hosomi & Sakurai, these non-toxic and highly stable reagents add to carbonyls via an open transition state upon Lewis acid activation. Herein, we report a general strategy to access a variety of valuable homoallylic alcohols in opposite chemo- and diastereoselectivity to the established Hosomi–Sakurai conditions by switching to photocatalytic activation in combination with a closed transition state (Chromium catalysis). Moreover, this dual catalytic approach displays a straightforward way to introduce excellent levels of enantioselectivity and its mild conditions allow for a broad substrate scope including chiral boron-substituted products as a highlight. To emphasize the synthetic utility, our method was applied as the key step in the synthesis of a bioactive compound and in the late-stage functionalization of steroid derivatives. Detailed mechanistic studies and DFT calculations hint towards an unprecedented photo-initiated chain being operative