Effects of the covalent linker groups on the spin transport properties of single nickelocene molecules attached to single-walled carbon nanotubes

The understanding of how the spin moment of a magnetic molecule transfers to a carbon nanotube, when the molecule is attached to it, is crucial for designing novel supramolecular spin devices. Here we explore such an issue by modeling the spin transport of a single-walled carbon nanotube grafted with one nickelocene molecule. In particular we investigate how the electron transport becomes spin-polarized depending on the specific linking group bonding nickelocene to the nanotube. We consider as linkers both aziridine and pyrrolidine rings and the amide group. Our calculations show that, at variance with aziridine, both pyrrolidine and amide, do alter the sp(2) character of the binding site of the nanotube and thus affect the transmission around the Fermi level. However, only aziridine allows transferring the spin polarization of the nickelocene to the nanotube, whose conductance at the Fermi level becomes spin-polarized. This suggests the superiority of aziridine as a linker for grafting magnetic molecules onto carbon nanotubes with efficient spin filtering functionality.     

Synthesis of heterocycles using amphoteric molecules

Amphoteric molecules contain functional groups that are normally regarded as incompatible. We have shown that reagents of this class, particularly unprotected aziridine aldehydes and alpha-boryl aldehydes, enable rapid synthesis of complex heterocycles and facilitate the development of new synthetic processes that are characterized by high bond-forming efficiency.     

Reversible Deactivation Radical Polymerization of Monomers Containing Activated Aziridine Groups

Polymers bearing activated aziridine groups are attractive precursors to α‐substituted‐β‐amino‐functionalized materials due to the enhanced reactivity of the pendant aziridine functionalities toward ring‐opening by nucleophiles. Two aziridine‐containing styrenic monomers, 2‐(4‐vinylphenyl)aziridine (VPA) and N‐mesyl‐2‐(4‐vinylphenyl)aziridine (NMVPA), were polymerized under a variety of reversible deactivation radical polymerization conditions. Low‐catalyst‐concentration atom transfer radical polymerization (LCC‐ATRP) and reversible addition‐fragmentation chain‐transfer (RAFT) polymerization were ineffective at producing well‐defined polymers from VPA due to side reactions between the aziridine functionalities and the agents controlling the polymerizations (catalysts or chain transfer agents). PolyVPA produced under nitroxide‐mediated polymerization (NMP) conditions had narrow molecular weight distribution at low to moderate conversions of monomer, but branched and eventually cross‐linked polymers were formed at higher conversions due to ring‐opening reactions of the aziridine groups. Most of these undesirable side reactions were eliminated by attaching a methanesulfonyl (mesyl) group to the aziridine nitrogen atom, and well‐defined linear homopolymers with targeted molecular weights were realized from NMVPA under RAFT and NMP conditions; however, side reactions between the aziridine groups and the catalyst in LCC‐ATRP still occured and the polymerization was uncontrolled using this technique.

     

Preparation and reactivity of versatile alpha-amino ketones

Many challenges of chemoselectivity arise from the requirement to manipulate incompatible functional groups. Synthetic methods that do not rely on protecting groups are of strategic significance to chemical synthesis. Particularly valuable are molecules with reactive functionalities that are kinetically stabilized against inter- or intramolecular reactions with each other. We have developed a simple access to molecules that contain both ketone and N-H aziridine functionalities. These compounds were found to undergo highly selective reduction and carbonyl addition reactions, making them versatile precursors to complex amines.     

The conversion of an aziridine plus an iminium salt to a 1,2-diamine

The conversion of an aziridine to a 1,2-diamine using lithium iodide and an iminium salt is discussed. We have found that when the aziridine is substituted by only alkyl groups, it is the less substituted carbonnitrogen bond that is broken; whereas, when the aziridine is substituted by a phenyl group at either the nitrogen or the carbon, it is the more substituted carbonnitrogen bond that is broken. For a 2,3-disubstituted aziridine, the reaction sequence goes with net retention of stereochemistry.     

Reactivity of activated versus nonactivated 2-(bromomethyl)aziridines with respect to sodium methoxide: a combined computational and experimental study

The difference in reactivity between the activated 2-bromomethyl-1-tosylaziridine and the nonactivated 1-benzyl-2-(bromomethyl)aziridine with respect to sodium methoxide was analyzed by means of DFT calculations within the supermolecule approach, taking into account explicit solvent molecules. In addition, the reactivity of epibromohydrin with regard to sodium methoxide was assessed as well. The barriers for direct displacement of bromide by methoxide in methanol are comparable for all three heterocyclic species under study. However, ring opening was found to be only feasible for the epoxide and the activated aziridine, and not for the nonactivated aziridine. According to these computational analyses, the synthesis of chiral 2-substituted 1-tosylaziridines can take place with inversion (through ring opening/ring closure) or retention (through direct bromide displacement) of configuration upon treatment of the corresponding 2-(bromomethyl)aziridines with 1 equiv of a nucleophile, whereas chiral 2-substituted 1-benzylaziridines are selectively obtained with retention of configuration (via direct bromide displacement). Furthermore, the computational results showed that explicit accounting for solvent molecules is required to describe the free energy profile correctly. To verify the computational findings experimentally, chiral 1-benzyl-2-(bromomethyl)aziridines and 2-bromomethyl-1-tosylaziridines were treated with sodium methoxide in methanol. The presented work concerning the reactivity of 2-bromomethyl-1-tosylaziridine stands in contrast to the behavior of the corresponding 1-tosyl-2-(tosyloxymethyl)aziridine with respect to nucleophiles, which undergoes a clean ring-opening/ring-closure process with inversion of configuration at the asymmetric aziridine carbon atom.     

o-nitrobenzenesulfonamides in nucleoside synthesis: efficient 5'-aziridination of adenosine

5'-Aziridinoadenylates of the form 1 and a related nitrogen mustard variant have been constructed using a novel variation of the Mitsunobu reaction. Such molecules allow conversion of biological methyltransferases into nucleoside transferases, thus providing powerful tools for investigating S-adenosyl-l-methionine (SAM)-dependent methylation. We present here a highly effective synthesis of such molecules that is amenable to aziridine diversification as well as elaboration of the base moiety so as to afford "bumped" cofactor mimics compatible with "hole"-bearing mutant proteins.     

Strained enamines as versatile intermediates for stereocontrolled construction of nitrogen heterocycles

This contribution assesses the synthetic utility of molecules that impose conformational constrains onto aziridine-derived enamines. Synthetically versatile [3.1.0] and [4.1.0] bicyclic enamines have been prepared by intramolecular oxidative cycloamination of aziridine-containing olefins. This process is initiated by N-bromosuccinimide followed by base-mediated elimination of HBr to afford highly strained exo-bicyclic enamines. In addition, intramolecular aziridine addition to aldehyde functionality was found to afford the [3.1.0] and [4.1.0] bicyclic hemiaminals. These routes highlight possibilities for chemoselective oxidative transformations of aziridine-containing precursors without nitrogen protection/deprotection steps. The resulting products provide straightforward synthetic entries into a wide range of pyrrolidine- and piperidine-containing heterocycles that are positioned toward subsequent transformations via aziridine ring opening.     

Stereoselective recognition of an aziridine with a Co(III) complex: a potential transition-state analogue for catalytic epoxidation

Reversible and stereoselective coordination of 1-(R)-phenyl-2-(S)-methylaziridine to (S,S)L-Co(III) and (R,R)L-Co(III) (where L represents a salen ligand obtained from diamino cyclohexane and 2,4-di-tert-butyl salicylaldehyde) has been investigated. 1H NMR data indicate that the aziridine binds about three times more tightly to (S,S)L-Co(III) than to (R,R)L-Co(III). Crystal structures of two molecules of the aziridine coordinated to the Co(III) complexes have been determined. These structures together with molecular mechanics computation provide insight into the origin of stereoselective recognition of the aziridine to the Co(III) complexes.     

Substitution effects in intramolecular aziridine-allylsilane cyclizations

We report here the synthesis of an aziridine tethered to a substituted allylsilane via a substituted tether. These tether-substituted aziridine-allylsilanes cyclize differently upon treatment with BF₃·OEt₂ than tether-unsubstituted aziridine-allylsilanes and provide a 6-endo-type product. We show that steric interactions between the N-substituent of the aziridine and substitution on the tether can control the product distribution.     

Alkylative Aziridine Ring-Opening Reactions

In this study, the highly strained three-membered aziridine ring was successfully activated as the aziridinium ion by alkylation of the ring nitrogen with a methyl, ethyl or allyl group, which was followed by ring opening with external nucleophiles such as acetate and azide. Such alkylative aziridine ring opening provides an easy route for the synthesis of various N-alkylated amine-containing molecules with concomitant introduction of an external nucleophile at either its α- or β-position.     

A synthesis of 6-azabicyclo[3.2.1]octanes. The role of N-substitution

The intramolecular cyclization reactions of aziridines with pi-nucleophiles can be a useful route to a number of heterocyclic and carbocyclic ring systems. We were particularly interested in the use of this cyclization reaction for the synthesis of 6-azabicyclo[3.2.1]octanes. We report here the development of a new synthesis of the aziridine necessary for the aziridine--pi-nucleophile cyclization. We also report on the cyclization of aziridines with three different substitutions on the aziridine nitrogen. We have found that N-diphenylphospinyl and N-H aziridines, while participating in the initial ring-opening reaction, do not lead to the desired bicyclic ring systems. In contrast, a nosyl group on the aziridine nitrogen leads efficiently to the bicyclic ring system and can be readily deprotected.     

Mechanism of amination of aziridines and oxiranes. 3. Quantum-chemical investigation of the effects of specific solvation

The characteristics of the associates formed by the aziridine and ethylene oxide molecules in protic solvents were studied by the nonempirical SCF MO-LCAO quantum-chemical method in the STO-3G basis set. It was shown that the main reacting particles in the acid-catalyzed ammonolysis reactions are ethylene oxide-ammonium cation and protonated aziridine-solvent complexes.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1036–1041, August, 1986.The authors are grateful to M. V. Bazilevskii for discussing the results.     

Nucleophilic ring opening and the formation of saturated heterocycles. 11. Quantum-chemical investigation of the reactivities of aziridine and 2-methylaziridine in aminolysis reactions

The aminolysis of aziridine and 2-methylaziridine in the gas phase was investigated by the MNDO method. Using the Germer solvatone model, modified for the MNDO computational scheme, we studied the effect of a polar medium on the ammonolysis of aziridine and 2-methylaziridine. We showed that the most probable mechanism for the investigated reactions is an A_c2 mechanism; however, in polar media the realization of the A_a2 mechanism is possible in the case of sterically hindered amines. We found that the determining factor for the nucleophilic opening of aziridine rings is the three-dimensional structure of the nucleophile rather than its basicity.See [1] for communication 10.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7. pp. 905–912, July, 1991.We thank Yu. V. Puzanov for supplying us with sets of programs for the quantum-chemical calculations. We especially thank S. G. Koldobskii for his assistance in discussing the results of our calculations and providing us with an interpretation of the solvatone method for MNDO.     

Studies on the ring opening reactions of 3-oxa-1-azabicyclo[3.1.0]hexan-2-ones. Synthesis of aminomethyl oxazolidinones and aziridinyl ureas

The reaction of fused ring aziridines, 3-oxa-1-azabicyclo[3.1.0]hexan-2-ones, with amine nucleophiles can provide either an aminomethyl oxazolidinone or an aziridinyl urea. The amine, reaction solvent, and aziridine substitution have been examined with the aid of computational studies to identify reaction conditions that provide a single product. Polar solvents provided only the aminomethyl oxazolidinone products. Formation of aziridinyl ureas required control of aziridine substitution, solvent, and reactant stoichiometry.     

The reaction of aziridine with perhalogenated nitriles

Reaction of one or two molecules of aziridine with the cyano groups of trichloroacetonitrile and trifluoroacetonitrile was observed. The resulting aziridinyl amino derivatives represent novel functional groups. Intramolecular cyclization of a bis-aziridinyl structure involving one aziridinyl group was observed. New synthetic routes to imidazolines and imidazolidines are a result of these rearrangments.     

The mechanism and regioselectivity of the ene reactions of nitroso compounds: a theoretical study of reactivity,regioselectivity, and kinetic isotope effects establishes a stepwise path involving polarized diradical intermediates

A theoretical study of the mechanisms of ene reactions of nitroso compounds has been completed, using UB3LYP, CASPT2, UCCSD(T) and UQCISD(T) methods. Stepwise paths through polarized diradical intermediates are always preferred. These intermediates have unusual properties, involving high rotational barriers about formally single bonds, which permit them to maintain stereochemical relationships. The diradicals may exchange the RNO moiety between the two ends of the alkene via an aziridine N-oxide. The aziridine N-oxide cannot be accessed directly from reactants and cannot lead directly to ene products. It is therefore an innocent by-stander in the way proposed by Singleton for the aziridinium imide in the ene reactions of triazolinediones. A detailed analysis of the electronic structure of the polarized diradicals is given. The kinetic isotope effects measured in a Stephenson isotope effect test have been reproduced. These kinetic isotope effects are consistent with a mechanism in which partitioning of the polarized diradical between cyclization to an aziridine N-oxide and H-abstraction to ene product takes place, and in which the formation of the polarized diradical is to some extent reversible. Finally, calculated regioselectivities reproduce those observed experimentally.     

Direct control of electron transfer to the surface-CO bond on a Pt/TiO2 catalytic diode

We study CO adsorption on a multilayer catalytic diode in which electron transfer at the metal-semiconductor (Pt/TiO(2)) junction is controlled by an applied external voltage. The multilayer diode structure enhances infrared absorption signals from CO molecules adsorbed on the small area Pt surface. We find that the diode behaves like a Schottky junction and that changes in electron transfer at the junction are directly correlated with reversible shifts in the vibrational frequency of adsorbed CO. Infrared polarization and incidence angle dependent studies show that the magnitude of vibrational frequency shift varies with orientation of the molecules being probed and increases with proximity to the Pt/TiO(2) interface. The results demonstrate the ability to control the metal-adsorbate bond through external electronic modifications of a metal-support junction. The catalytic diode can potentially provide control of the surface chemical bond by an external voltage, providing a new approach for investigations in heterogeneous catalysis, sensors, and plasmonic devices.     

Kinetic Study of 1-Substituted Aziridines for Ring-Opening Polymerization Initiated with 3-Hydroxy-1-propane Sulfonic Acid Sultone

Abstract

Kinetics of the cationic polymerization of 1-substituted aziridines, such as 1-methoxycarbonylmethyl aziridine, 1-methoxycarbonylethyl aziridine, and 1-benzyl aziridine, initiated with 3-hydroxy-1-propane sulfonic acid sultone have been investigated, and the results are compared with the results of the polymerization of 1-β-cyanoethyl aziridine. The course of polymerization of 1-methoxycarbonylmethyl aziridine involved a termination reaction due to the reaction between the growing endgroup and the imino group in the polymer chain. On the other hand, the polymerizations of 1-methoxycarbonylethyl aziridine and 1-benzyl aziridine were terminated by a backbiting reaction with the formation of a piper-adinium ring on the polymer end. The propagation and termination constants were obtained at different temperatures, and the enthalpies of activation (ΔH*) and the entropies of activation (ΔS*) for this polymerization were calculated.