Amination of [60]fullerene by ammonia and by primary and secondary aliphatic amines--preparation of amino[60]fullerene peroxides

Ammonia and aliphatic amines react readily in the oxygen-rich regions of the Cs symmetric fullerene peroxides C60(O)(OOtBu)4 (1) and C60(OH)(Br)(OOtBu)4 (2 c). Michael addition-type hydroamination of the 1,4-diene moiety on the central skew-pentagon was observed when 1 was treated with ammonia or with nonbulky primary amines, while sterically demanding primary amines opened the epoxy moiety to form vicinal aminohydroxy fullerene compounds with the amino group on the central pentagon. In 2 c the bromo group was replaced under similar conditions by ammonia and primary amines. Cyclic secondary amines showed different reaction patterns, forming hydrogenation products or aminoketal-fullerenes when treated with 1 and 2 c, respectively. Single-electron transfer (SET) is the key step in all the proposed mechanisms. The compounds were characterized by their spectroscopic data, and in addition, three single-crystal X-ray structures were obtained.     

Transition-metal-free trifluoromethylaminoxylation of alkenes

No transition metal! Fluorinated hypervalent-iodine reagents react with TEMPONa in the presence of an alkene under mild conditions to give the corresponding perfluoroalkylaminoxylation products. These radical addition/trapping reactions occur with high stereoselectivity using commercially available reagents, and the product alkoxyamines are readily transformed into the corresponding alcohols.     

Energies for cyclic and acyclic aggregations of adamantane and diamantane units sharing vertices,edges, or six‐membered rings

Diamondoids are hydrocarbons having a carbon scaffold comprised from polymer‐like composites of adamantane cages. This article describes computed total energies and “SWB‐tension” energies (often referred to as “strain” energies) for species having n adamantane or diamantane units sharing pairwise: one carbon atom (spiro‐[n]adamantane or spiro‐[n]diamantane); one C? C bond (one‐bond‐sharing‐[n]adamantane or one‐bond‐sharing‐[n]diamantane); or one chair‐shaped hexagon of carbon atoms (1234‐helical‐cata‐[n]diamantanes). Each of the five investigated polymer‐like types is considered either as an acyclic or a cyclic chain of adamantane‐ or diamantane‐unit cages. With increasing n values, SWB‐tension energies for acyclic aggregates are found to increase linearly, while the net SWB‐tension energies of cyclic aggregates often go thru a minimum at a suitable value of . In all five cases, a limiting common energy per unit ( ) is found to be approached by both cyclic and acyclic chains as , as revealed from plots of versus 1/n for acyclic chains and of versus 1/n² for cyclic chains. © 2015 Wiley Periodicals, Inc.     

Spectroscopic and computational investigations of stable radical anions of triosmium benzoheterocycle clusters

The variable temperature (1)H and (13)C NMR and EPR spectra of the stable radical anions [Os(3)(CO)(9)(micro(3)-eta(2)-L)(micro-H)] (LH=phenanthridine, 1; 5,6-benzoquinoline, 2), and [Os(3)(CO)(10)(micro(3)-eta(2)-L)(micro-H)] (LH=quinoxaline, 3) are reported. The radical anions 1(-), 2(-), and 3(-) can be prepared by both exhaustive electrolysis and partially by chemical reduction with cobaltocene and with sodium dispersion (only with sodium dispersion in the case of 3(-)). DFT calculations on 1-3 reveal that the LUMO for the electron-deficient compounds 1 and 2 involves significant contributions from both the heterocyclic ligand and the two metal atoms bridged by the ligand and the micro-hydride. The character of this orbital rationalizes the previously observed regioselective reactions of these complexes with nucleophiles. In contrast, the LUMO for the electron precise 3 involves only ligand-based orbitals. Partial chemical reduction of 1 and 2 requires an excess of either cobaltocene or sodium, and their (1)H and (13)C NMR spectra reveal selective line broadening of those proton resonances that are predicted by DFT calculations to bear the greatest amount of free spin density. The variable temperature behavior of the partially chemically reduced species of 1 and 2 indicates that electron transfer between the reduced/unreduced cluster pair and between the cobaltocene/cobaltocenium pair occurs on the NMR timescale. The radical anions of 1 and 2 prepared by exhaustive electrolysis show an EPR signal at room temperature, while the NMR signals are uniformly broadened. Compound 3 appears to be partially reduced by sodium at room temperature and shows uniformly broadened (1)H NMR resonances at room temperature that sharpen significantly at -80 degrees C. The temperature dependence of the spectra are discussed in terms of the effects of relative electron nuclear relaxation processes, chemical exchange, and the results of the DFT calculations.     

An experimental and theoretical study of surface segregation in a Fe-6at.%Si bicrystal

The analysis of segregation phenomena in bicrystals is an important step for the understanding of combined effects of the elementary diffusion processes involved in the segregation in polycristalline systems. The segregation of Si and P in a Fe-6at.%Si bicrystal with a (100) and (110) surface has been investigated by means of AES (Auger electron spectroscopy). For these experiments the technique of a linearly increased temperature has been applied. Significant differences between the segregation kinetics at the two surfaces of the sample have been found on the one hand for the maximum coverage of P and on the other hand for the high temperature behaviour of Si. Additionally, model calculations based on the KTBIM (kinetic tight binding Ising model) have been performed to qualitatively describe the experimental results. It is shown, that the striking differences between the segregation behaviour at the two differently oriented surfaces can be explained by different segregation energies of P, whereas Si plays a minor role due to its relatively small segregation energy.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

An Amphoteric Switch to Aromatic and Antiaromatic States of a Neutral Air‐Stable 25π Radical

Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open‐shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air‐ and water‐stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one‐electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single‐crystal X‐ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.     

Single‐Electron‐Transfer‐Induced Coupling of Arylzinc Reagents with Aryl and Alkenyl Halides

Arylzinc reagents, prepared from aryl halides/zinc powder or aryl Grignard reagents/zinc chloride, were found to undergo coupling with aryl and alkenyl halides without the aid of transition‐metal catalysis to give biaryls and styrene derivatives, respectively. In this context, we have already reported the corresponding reaction using aryl Grignard reagents instead of arylzinc reagents. Compared with the Grignard cross‐coupling, the present reaction features high functional‐group tolerance, whereby electrophilic groups such as alkoxycarbonyl and cyano groups are compatible as substituents on both the arylzinc reagents and the aryl halides. Aryl halides receive a single electron and thereby become activated as the corresponding anion radicals, which react with arylzinc reagents, thus leading to the cross‐coupling products.     

Peptide cation-radicals. A computational study of the competition between peptide N-Calpha bond cleavage and loss of the side chain in the [GlyPhe-NH2 + 2H]+. cation-radical

Cation‐radicals and dications corresponding to hydrogen atom adducts to N‐terminus‐protonated N_α‐glycylphenylalanine amide (Gly‐Phe‐NH₂) are studied by combined density functional theory and Møller‐Plesset perturbational computations (B3‐MP2) as models for electron‐capture dissociation of peptide bonds and elimination of side‐chain groups in gas‐phase peptide ions. Several structures are identified as local energy minima including isomeric aminoketyl cation‐radicals, and hydrogen‐bonded ion‐radicals, and ylid‐cation‐radical complexes. The hydrogen‐bonded complexes are substantially more stable than the classical aminoketyl structures. Dissociations of the peptide N? C_α bonds in aminoketyl cation‐radicals are 18–47 kJ mol^?1 exothermic and require low activation energies to produce ion‐radical complexes as stable intermediates. Loss of the side‐chain benzyl group is calculated to be 44 kJ mol^?1 endothermic and requires 68 kJ mol^?1 activation energy. Rice‐Ramsperger‐Kassel‐Marcus (RRKM) and transition‐state theory (TST) calculations of unimolecular rate constants predict fast preferential N? C_α bond cleavage resulting in isomerization to ion‐molecule complexes, while dissociation of the C_α? CH₂C₆H₅ bond is much slower. Because of the very low activation energies, the peptide bond dissociations are predicted to be fast in peptide cation‐radicals that have thermal (298 K) energies and thus behave ergodically. Copyright © 2003 John Wiley & Sons, Ltd.     

A density functional theory computational study of the role of ligand on the stability of CuI and CuII species associated with ATRP and SET‐LRP

Atom transfer radical polymerization (ATRP) and single electron‐transfer living radical polymerization (SET‐LRP) both utilize copper complexes of various oxidation states with N‐ligands to perform their respective activation and deactivation steps. Herein, we utilize DFT (B3YLP) methods to determine the preferred ligand‐binding geometries for Cu/N‐ligand complexes related to ATRP and SET‐LRP. We find that those ligands capable of achieving tetrahedral complexes with Cu^I and trigonal bipyramidal with axial halide complexes with [Cu^IIX]⁺ have higher energies of stabilization. We were able to correlate calculated preferential stabilization of [Cu^IIX]⁺ with those ligands that perform best in SET‐LRP. A crude calculation of energy of disproportionation revealed that the same preferential binding of [Cu^IIX]⁺ results in increased propensity for disproportionation. Finally, by examining the relative energies of the basic steps of ATRP and SET‐LRP, we were able to rationalize the transition from the ATRP mechanism to the SET‐LRP mechanism as we transition from typical nonpolar ATRP solvents to polar SET‐LRP solvents. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4950–4964, 2007     

Metal Bridging for Directing and Accelerating Electron Transfer as Exemplified by Harnessing the Reactivity of AIBN

A new strategy for tuning the electron transfer between radicals and enolates has been developed. This method elicits the innate reactivity of AIBN with a copper catalyst and enables a cascade reaction with cinnamic acids. Electron paramagnetic resonance studies and control experiments indicate that the redox‐active copper species not only activates the radical by coordination, but also serves as a bridge to bring the radical and nucleophile within close proximity to facilitate electron transfer. By exploiting possible combinations of redox‐active metals and radical entities with suitable coordinating functional groups, this strategy should contribute to the development of a broad range of radical‐based reactions.     

A General Palladium‐Catalyzed Method for Alkylation of Heteroarenes Using Secondary and Tertiary Alkyl Halides

A general alkylation of heterocycles using a simple palladium catalyst is reported. Most classes of heterocycles, including indoles and pyridines, efficiently coupled with unactivated secondary and tertiary alkyl halides. An alkyl radical addition to neutral heteroarenes is most likely involved.     

Hydrogen‐Bonding Effect on Spin‐Center Transfer of Tetrathiafulvalene‐Linked 6‐Oxophenalenoxyl Evaluated Using Temperature‐Dependent Cyclic Voltammetry and Theoretical Calculations

The stable tetrathiafulvalene (TTF)‐linked 6‐oxophenalenoxyl neutral radical exhibits a spin‐center transfer with a continuous color change in solution caused by an intramolecular electron transfer, which is dependent on solvent and temperature. Cyclic voltammetry measurements showed that addition of 2,2,2‐trifluoroethanol (TFE) to a benzonitrile solution of the neutral radical induces a redox potential shift that is favorable for the spin‐center transfer. Temperature‐dependent cyclic voltammetry of the neutral radical using a novel low‐temperature electrochemical cell demonstrated that the redox potentials change with decreasing temperature in a 199:1 CH₂Cl₂/TFE mixed solvent. Furthermore, theoretical calculation revealed that the energy levels of the frontier molecular orbitals involved in the spin‐center transfer are lowered by the hydrogen‐bonding interaction of TFE with the neutral radical. These results indicate that the hydrogen‐bonding effect is a key factor for the occurrence of the spin‐center transfer of TTF‐linked 6‐oxophenalenoxyl.     

A comparative computational study of the homolytic and heterolytic bond dissociation energies involved in the activation step of ATRP and SET‐LRP of vinyl monomers

A quantum‐chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported. These model compounds include the dormant chloride, bromide, and iodide species for representative activated and nonactivated monomers containing electron‐withdrawing groups as well as for a nonactivated monomer containing an electron‐donor group. Two examples of sulfonyl and N‐halide initiators are also reported. The homolytic inner‐sphere electron‐transfer bond dissociation is known as atom transfer and is responsible for the activation step in ATRP. The heterolytic outer sphere single electron transfer bond dissociation is responsible for the activation step in single electron transfer mediated living radical polymerization (SET‐LRP). The results of this study demonstrated much lower bond dissociation energies for the outer sphere single electron transfer processes. These results explain the higher rate constant of activation, the higher apparent rate constant of propagation, and the lower polymerization temperature for both activated and nonactivated monomers containing electron‐withdrawing groups in SET‐LRP. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1607–1618, 2007     

An updated version of the computational package SIMPRE that uses the standard conventions for Stevens crystal field parameters

The crystal field approach used by SIMPRE is analyzed, verifying the exactness of the results concerning energy levels and magnetic properties calculated by the package. To coincide with the prevailing conventions, we reformulate the presentation of the crystal field parameters, so that the results are now, also from a formal point of view, strictly correct. New calculations are presented to test the influence of neglecting the excited J states, a common but critical approximation employed by SIMPRE. For that, we examine the case of Er(trensal) complex (H₃trensal = 2,2′,2″‐tris(salicylideneimino)triethylamine) where the influence of this approximation is found to be minimal. A patched version of the code, SIMPRE 1.1, and an updated version of the user manual are now available. Finally, we comment on “Software package SIMPRE – revisited,” which apparently revisits a software package without inspecting or using the code. © 2014 Wiley Periodicals, Inc.