Interactions of aziridines with nickel complexes: oxidative-addition and reductive-elimination reactions that break and make C-N bonds

Reaction of the N-tosylaziridines (p-CH(3)C(6)H(4)SO(2))NCH(2)CHR (1a, R = H; 1b, R = Me; 1c, R = n-Bu; 1d, R = i-Pr) with (bpy)Ni(cod) (2; bpy = 2,2'-bipyridine; cod = 1,5-cyclooctadiene) or (bpy)NiEt(2) (3) results in elimination of cod or butane from 2 and 3, respectively, and oxidative addition of an aziridine C-N bond to give the azametallacyclobutane complexes (bpy)Ni(NTosCHRCH(2)) (4a, R = H; 4b, R = Me; 4c, R = n-Bu; 4d, R = i-Pr) as maroon solids in 50-70% isolated yields. The structure of 4b exhibits a puckered four-membered azametallacycle containing a pyramidal nitrogen and with Ni-N(1) = 1.911(5) A; the tosyl group on N and the methyl substituent on the adjacent C are disposed in an anti conformation. The monodeuterated aziridine syn-(p-CH(3)C(6)H(4)SO(2))NCHDCH-n-Bu (1e) reacts with either 2 or 3 to give (bpy)Ni[NTosCH(n-Bu)CHD] (4e) in 60-65% yield, having an anti arrangement of the methine and methylene protons in the azametallacycle, and indicates that >95% inversion of stereochemistry has occurred at the methylene carbon during the oxidative-addition reaction. When the azametallacyclobutane complexes 4a-e are exposed to oxygen, oxidatively induced reductive elimination ensues, giving the free aziridines in 30-60% isolated yields. In the oxidation of 4e, the product aziridine is spectroscopically identical to its parent, 1e, indicating the elimination that forms the C-N bond also proceeds with inversion of stereochemistry (approximately 92% by (1)H NMR) at the methylene carbon.     

Selektive Entfernung störender Ionen aus polarographischen Lösungen mittels Aluminium

Zusammenfassung Die Konzentration von Metallionen, die die polarographische Analyse stören, kann mittels Aluminiummetall als Reduktionsmittel vermindert werden. Eine für viele polarographische Analysen ausreichende Herabsetzung der Konzentration von Cu²⁺-, Pb²⁺-, Cd²⁺-, Ni²⁺-, Co²⁺- und Zn²⁺-Ionen auf wenigo /ml ist möglich. Es werden Verfahren angegeben, die eine Ausfällung der ententsprechenden Metalle durch Aluminium aus salzsauren oder essigsauren Lösungen mit oder ohne Zusatz von Hg-Ionen bewirken. Die Möglichkeiten der Verfahren werden an Cu–(Ni, Co, Zn, Fe, Mn); Cd–(Ni, Zn); Pb–Zn; Zn–Mn-Lösungen aufgezeigt. Die Genauigkeit der Analysenverfahren, die besonders für Proben mit kleinem Volumen (1–2 ml) sowie Serienanalysen geeignet sind, liegt bei ±4%. Das Verhalten der verschiedenen Ionen wird durch den Einfluß der Wasserstoffüberspannung erklärt.     

Increased complexity in aesthetic field theory

We continue the program of looking for increased complexity within aesthetic field theory. We study a solution with five planar maxima and minima. Another solution in which we counted 19 planar maxima and minima is also studied. This latter solution was obtained by modifying our previous principles by allowing for an arbitrariness associated with the integration path in conjunction with the equation _jk:1 ⁱ =0.Supported by a research grant from the University of North Dakota.     

A method to bypass the lack of solutions in MinSup problems under quasi-equilibrium constraints

MinSup problems with constraints described by quasi-equilibrium problems are considered in Banach spaces. The solutions set of such problems may be empty even in very good situations, so the aim of this paper is twofold. First, we determine appropriate regularizations (called inner regularizations) which allow to reach the value of the original problem. Then, among these regularizations we identify those which allow to bypass the lack of exact solutions to these problems by a suitable concept of “viscosity” solution whose existence is then proved under reasonable assumptions.     

Reactivity of Xantphos-Type Rhodium Complexes Towards SF4: SF3 Versus SF2 Complex Generation

S−F-bond activation of sulfur tetrafluoride at [Rh(Cl)(^tBuxanPOP)] ( 1 ; ^tBuxanPOP=9,9-dimethyl-4,5-bis-(di-tert-butylphosphino)-xanthene) led to the formation of the cationic complex [Rh(F)(Cl)(SF₂)(^tBuxanPOP)][SF₅] ( 2 a ) together with trans-[Rh(Cl)(F)₂(^tBuxanPOP)] ( 3 ) and cis-[Rh(Cl)₂(F)(^tBuxanPOP)] ( 4 ) which both could also be obtained by the reaction of SF₅Cl with 1 . In contrast to that, the conversion of SF₄ at the methyl complex [Rh(Me)(^tBuxanPOP)] ( 5 ) gave the isolable and room-temperature stable cationic λ⁴-trifluorosulfanyl complex [Rh(Me)(SF₃)(^tBuxanPOP)][SF₅] ( 6 ). Treatment of 6 with the Lewis acids BF₃ or AsF₅ produced the dicationic difluorosulfanyl complex [Rh(Me)(SF₂)(^tBuxanPOP)][BF₄]₂ ( 8 a ) or [Rh(Me)(SF₂)(^tBuxanPOP)][AsF₆]₂ ( 8 b ), respectively. Refluorination of 8 a was possible with the use of dimethylamine giving [Rh(Me)(SF₃)(^tBuxanPOP)][BF₄] ( 9 ). A reaction of 6 with trichloroisocyanuric acid (TClCA) gave the fluorido complex [Rh(F)(Cl)(SF₂)(^tBuxanPOP)][Cl] ( 2 b ) together with chloromethane and SF₅Cl.     

Synthesis of Intramolecular P/Al-Based Frustrated Lewis Pairs via Aluminum-Tin-Exchange and their Reactivity toward CO2

Frustrated Lewis pairs (FLPs) composed of acidic alane and basic phosphane functions, separated by a xanthene linker, can be prepared through the corresponding Me₃Sn derivative and methyl aluminum compounds with elimination of Me₄Sn. This way MeClAl-, Cl₂Al- and (C₆F₅)₂Al- moieties could be introduced and the resulting FLPs are stabilized by a further equivalent of the alane precursors. In contact with the FLPs CO₂ is bound via the C atom at the phosphane functions and the two O atoms at the Al centers. The residues at the latter determine the binding strength. Hence, in case of MeClAl CO₂ capture occurs at higher pressure and under ambient conditions CO₂ is released again, while for Cl₂Al and (C₆F₅)₂Al CO₂ binding becomes irreversible. The results of DFT calculations rationalize these findings by the high thermodynamic stabilization in case of more electronegative residues, which concomitantly lead to higher barriers, and in case of (C₆F₅)₂Al further stabilization is achieved through a low reorganization energy.     

Synthesis and spin-trapping properties of a new spirolactonyl nitrone

Spin trapping using electron paramagnetic resonance (EPR) spectroscopy is commonly employed for the identification of transient radicals in chemical and biological systems. A spirolactonyl-nitrone with rigid H-bond acceptor, 7-oxa-1-azaspiro[4.4]non-1-en-6-one 1-oxide, CPCOMPO, has been synthesized and characterized, and its spin-trapping properties were investigated. The rate of formation of CPCOMPO-O2H was determined using competition kinetics between the superoxide/hydroperoxyl radical (O2*-/HO2*) trapping by CPCOMPO and the spontaneous dismutation of this radical in aqueous media. The rate constant of 60 M-1.s-1 is the highest rate constant thus far observed at neutral pH for any nitrones using the kinetic method employed. Decay kinetics were also experimentally investigated for CPCOMPO-O2H. The effect of rigid H-bond acceptor on the stability of the CPCOMPO-O2H were computationally rationalized and compared to that of EMPO-O2H, which has flexible H-bond acceptor, and results show the need of a "loose" H-bond acceptor for improved adduct stability.     

Infinite Classes of Dihedral Snarks

Flower snarks and Goldberg snarks are two infinite families of cyclically 5–edge–connected cubic graphs with girth at least five and chromatic index four. For any odd integer k, k > 3, there is a Flower snark, say J _k , of order 4k and a Goldberg snark, say B _k , of order 8k. We determine the automorphism groups of J _k and B _k for every k and prove that they are isomorphic to the dihedral group D _4k of order 4k. Research performed within the activity of INdAM–GNSAGA with the financial support of the Italian Ministry MIUR, project “Strutture Geometriche, Combinatoria e loro Applicazioni”.