Rearrangement mechanisms for azoxypyridines and azoxypyridine <i>N</i>-oxides in the 100% H₂SO₄ region - The Wallach rearrangement story comes full circle

Buncel, Erwin,Keum, Sam-Rok,Rajagopal, Srinivasan,Cox, Robin A. Canadian Science Publishing 2009 Canadian journal of chemistry Vol.87 No.8

<P> Kinetic studies of the Wallach rearrangements of four azoxypyridines, four azoxypyridine N-oxides, and one azoxypyridine N-methiodide have been carried out in the 100% H2SO4 acidity region. For all of the β-isomers in the study the reactions proceeded at a spectrally measurable rate, and the log observed rate constants were found to be linear functions of the log H3SO4<SUP>+ </SUP> concentration, as previously found for azoxybenzene itself, suggesting that the reaction mechanism for these substrates is the same as that previously deduced for axozybenzene, i.e., a general-acid-catalysis A-SE2 process. For the α-azoxypyridines no reaction could be observed at all. The two α-azoxypyridine N-oxides in the study did react, albeit very slowly, but for these two compounds the log observed rate constants were not linear functions of the log H3SO4<SUP>+ </SUP>concentration, but were instead found to be linear in the H0 acidity function, which is known for the 100% H2SO4 acidity region. It follows that the reaction mechanism for these α-isomers is a different one, presumably an A1 process. This mechanism was proposed back in 1963 for azoxybenzene, but has never actually been observed for any substrate before the work reported in this study. Thus, the Wallach rearrangement story can be said to have come full circle. </P>

Theoretical and Experimental 31P NMR and ESI-MS Study of Hg2+ Binding to Fenitrothion

In Sun Koo,Dildar Ali,Kiyull Yang,Gary W. vanLoon,Erwin Buncel 대한화학회 2009 Bulletin of the Korean Chemical Society Vol.30 No.6

We present the theoretical and experimental results of 31P NMR and low energy CID MS/MS study of Hg2+ binding to fenitrothion (FN). The calculated 31P NMR chemical shifts order for FN with Hg2+ complex is in good agreement with experimental 31P NMR chemical shifts order. The experimental and theoretical 31P NMR study of organophosphorus pesticide with Hg2+ gives to important information for organophosphorus pesticide metal complexes.ESI-MS and low energy CID MS/MS experiments of Hg2+-FN complexes combined with accurate mass measurements give insight into the metal localization and allow unambiguous identification of fragments and hydrolysis products.

Theoretical Study of 31P NMR Chemical Shifts for Organophosphorus Esters, Their Anions and O,O-Dimethylthiophosphorate Anion with Metal Complexes

Yong Park,In Sun Koo,Dildar Ali,Erwin Buncel,양기열,David M. Wardlaw 대한화학회 2008 Bulletin of the Korean Chemical Society Vol.29 No.11

Ab initio and density functional theory (DFT) studies along with gauge-including atomic orbitals (GIAO) have been carried out on 31P NMR chemical shifts for a series of organophosphorus esters, replacing (RO)P=O by (RS)P=O, (RO)P=S and (RS)P=S functionalities, and for O,O-dimethylthiophosphorate ion (PA−) complexed with metal ions (Ag+, Hg2+). Ab initio and DFT results are in good agreement with experimental 31P NMR chemical shifts. It is shown that the major contribution of 31P NMR chemical shifts derives from the total paramagnetic tensor and variation of d orbital population at P atom by dπ-pπ bond back-donation.

SFRP Synthesis of Acenaphthylene Oligomers and Block Copolymers. Potential Light Harvesting Structures

Dildar Ali,Peter M. Kazmaier,Julian M. Dust,Zaheer Ahmed,Erwin Buncel 대한화학회 2011 Bulletin of the Korean Chemical Society Vol.32 No.7

Azo-acenaphthylene oligomers with repeating acenaphthylene units “n” up to 4, 5, 7, 17 and 19 have been prepared successfully using nitroxide mediated Stable Free Radical Polymerization (SFRP). Azoacenaphthylene oligomers, reversibly end-capped by the stable nitroxide 2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO), were further reacted via radical addition to 4-(naphthalenemethoxy)styrene monomer for diblock co-polymer formation. Characterization of the oligomers and diblock co-polymers was accomplished using MALDI-MS supported by GPC (Gel Permeation Chromatography) and ^1H NMR spectrometry. MALDI-MS afforded definitive results by providing an inter-peak interval of 152 (m/z), corresponding to acenaphthylene monomer, and inter-peak interval of 260 (m/z) for the naphthalenemethoxystyrene monomer unit in block copolymers. Our study opens the way to control the number of repeat units in the oligomers. Further these oligomers can be tailored with various monomers for the formation of block copolymers.

A Kinetic Study on Nucleophilic Displacement Reactions of Aryl Benzenesulfonates with Potassium Ethoxide: Role of K⁺ Ion and Reaction Mechanism Deduced from Analyses of LFERs and Activation Parameters

Um, Ik-Hwan,Kang, Ji-Sun,Shin, Young-Hee,Buncel, Erwin American Chemical Society 2013 Journal of organic chemistry Vol.78 No.2

<P>Pseudofirst-order rate constants (<I>k</I><SUB>obsd</SUB>) have been measured spectrophotometrically for the nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates <B>4a</B>–<B>f</B> and Y-substituted phenyl benzenesulfonates <B>5a</B>–<B>k</B> with EtOK in anhydrous ethanol. Dissection of <I>k</I><SUB>obsd</SUB> into <I>k</I><SUB>EtO<SUP>–</SUP></SUB> and <I>k</I><SUB>EtOK</SUB> (i.e., the second-order rate constants for the reactions with the dissociated EtO<SUP>–</SUP> and ion-paired EtOK, respectively) shows that the ion-paired EtOK is more reactive than the dissociated EtO<SUP>–</SUP>, indicating that K<SUP>+</SUP> ion catalyzes the reaction. The catalytic effect exerted by K<SUP>+</SUP> ion (e.g., the <I>k</I><SUB>EtOK</SUB>/<I>k</I><SUB>EtO<SUP>–</SUP></SUB> ratio) decreases linearly as the substituent X in the benzenesulfonyl moiety changes from an electron-donating group (EDG) to an electron-withdrawing group (EWG), but it is independent of the electronic nature of the substituent Y in the leaving group. The reactions have been concluded to proceed through a concerted mechanism from analyses of the kinetic data through linear free energy relationships (e.g., the Brønsted-type, Hammett, and Yukawa–Tsuno plots). K<SUP>+</SUP> ion catalyzes the reactions by increasing the electrophilicity of the reaction center through a cyclic transition state (TS) rather than by increasing the nucleofugality of the leaving group. Activation parameters (e.g., Δ<I>H</I><SUP>‡</SUP> and Δ<I>S</I><SUP>‡</SUP>) determined from the reactions performed at five different temperatures further support the proposed mechanism and TS structures.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/joceah/2013/joceah.2013.78.issue-2/jo302373y/production/images/medium/jo-2012-02373y_0009.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jo302373y'>ACS Electronic Supporting Info</A></P>

Dissection of Activation Parameters in the Bell-Shaped α-Effect Following Solvent Modulation (DMSO–H₂O Media)

Um, Ik-Hwan,Kang, Ji-Sun,Kim, Min-Young,Buncel, Erwin American Chemical Society 2013 Journal of organic chemistry Vol.78 No.17

<P>This paper comprises results of our investigation of the α-effect phenomenon for the reaction of <I>O-p</I>-nitrophenyl thionobenzoate (PNPTB) with butane-2,3-dione monoximate (Ox<SUP>–</SUP>, α-nucleophile) and <I>p</I>-chlorophenoxide (<I>p</I>-ClPhO<SUP>–</SUP>, normal-nucleophile) in DMSO–H<SUB>2</SUB>O mixtures of varying compositions at 15.0 °C, 25.0 °C, and 35.0 °C. The reactivity of Ox<SUP>–</SUP> and <I>p</I>-ClPhO<SUP>–</SUP> increases significantly as the DMSO content in the medium increases, although the effects of medium on reactivity are not the same for the reactions with Ox<SUP>–</SUP> and <I>p</I>-ClPhO<SUP>–</SUP>. Ox<SUP>–</SUP> exhibits the α-effect in all solvent compositions and temperatures. The α-effect increases up to 50 mol % DMSO and then decreases thereafter, resulting in a bell-shaped α-effect profile. Dissection of the activation parameters (i.e., Δ<I>H</I><SUP>⧧</SUP> and <I>T</I>Δ<I>S</I><SUP>⧧</SUP>) has revealed that the bell-shaped α-effect behavior is due to entropy of activation differences rather than enthalpy terms, although the enthalpy term controls almost entirely the solvent dependence of the reaction rate. Differences in the transition-state (TS) structures for the reactions with Ox<SUP>–</SUP> (a six-membered cyclic TS) and <I>p</I>-ClPhO<SUP>–</SUP> (an acyclic TS) are consistent with the entropy-dependent α-effect behavior.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/joceah/2013/joceah.2013.78.issue-17/jo401415f/production/images/medium/jo-2013-01415f_0008.gif'></P>

Theoretical and Experimental ³¹P NMR and ESI-MS Study of Hg²⁺ Binding to Fenitrothion

Koo, In-Sun,Ali, Dildar,Yang, Ki-Yull,vanLoon, Gary W.,Buncel, Erwin Korean Chemical Society 2009 Bulletin of the Korean Chemical Society Vol.30 No.6

We present the theoretical and experimental results of $^{31}P$ NMR and low energy CID MS/MS study of $Hg^{2+}$ binding to fenitrothion (FN). The calculated $^{31}P$ NMR chemical shifts order for FN with $Hg^{2+}$ complex is in good agreement with experimental $^{31}P$ NMR chemical shifts order. The experimental and theoretical $^{31}P$ NMR study of organophosphorus pesticide with $Hg^{2+}$ gives to important information for organophosphorus pesticide metal complexes. ESI-MS and low energy CID MS/MS experiments of $Hg^{2+}$-FN complexes combined with accurate mass measurements give insight into the metal localization and allow unambiguous identification of fragments and hydrolysis products.

Effect of modification of the electrophilic center on the α effect

Um, Ik-Hwan,Lee, Ji-Youn,Bae, Sun-Young,Buncel, Erwin Canadian Science Publishing 2005 Canadian journal of chemistry Vol.83 No.9

<P> We report on a nucleophilic study of esters R-C(=X)-Y-Ar in which the electrophilic center has been modified by replacing O by S in the leaving group or carbonyl center: 4-nitrophenyl acetate (1), S-4-nitrophenyl thioacetate (2), 4-nitrophenyl benzoate (3), and O-4-nitrophenyl thionobenzoate (4). The studies include O<SUP>–</SUP> and S<SUP>–</SUP> nucleophiles as well as α nucleophiles in H2O at 25.0 ± 0.1 °C. The sulfur nucleophile (4-chlorothiophenoxide, 4-ClPhS<SUP>–</SUP>) exhibits significant enhanced reactivity for the reactions with thiol and thione esters 2 and 4 compared with their oxygen analogues 1 and 3. On the contrary, the common nucleophile OH<SUP>–</SUP> is much less reactive towards 2 and 4 compared with 1 and 3. The effect of changing both the electrophilic center and the nucleofugic center on the reactivity of the other oxygen nucleophiles is not so significant: 4-chlorophenoxide (4-ClPhO<SUP>–</SUP>) is four to six times more reactive in the reactions with thiol and thione esters 2 and 4 compared with their oxygen analogues 1 and 3. The α effects exhibited by butan-2,3-dione monoximate (Ox<SUP>–</SUP>) and HOO<SUP>–</SUP> are strongly dependent on the nature of the electrophilic center of the substrates, indicating that the difference in the ground-state solvation energy cannot be fully responsible for the α effect. Our results clearly emphasize the strong dependence of the α effect on the substrate structure, notably, the nature of the electrophilic center. The impact of change in the nucleofuge (1→2) and the electrophilic center (3→4) on reactivity indicates that α nucleophiles will need to be “purpose built” for decontamination and nucleophilic degradation of specific biocides.Key words: α effect, nucleophilicity, nucleofuge effect, electrophilicity, polarizability. </P>

Evidence for a Catalytic Six-Membered Cyclic Transition State in Aminolysis of 4-Nitrophenyl 3,5-Dinitrobenzoate in Acetonitrile: Comparative Brønsted-Type Plot, Entropy of Activation, and Deuterium Kinetic Isotope Effects

Um, Ik-Hwan,Kim, Min-Young,Bae, Ae-Ri,Dust, Julian M.,Buncel, Erwin American Chemical Society 2015 Journal of organic chemistry Vol.80 No.1

SFRP Synthesis of Acenaphthylene Oligomers and Block Copolymers. Potential Light Harvesting Structures

Ali, Dildar,Ahmed, Zaheer,Dust, Julian M.,Kazmaier, Peter M.,Buncel, Erwin Korean Chemical Society 2011 Bulletin of the Korean Chemical Society Vol.32 No.7

Azo-acenaphthylene oligomers with repeating acenaphthylene units "n" up to 4, 5, 7, 17 and 19 have been prepared successfully using nitroxide mediated Stable Free Radical Polymerization (SFRP). Azo-acenaphthylene oligomers, reversibly end-capped by the stable nitroxide 2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO), were further reacted via radical addition to 4-(naphthalenemethoxy)styrene monomer for diblock co-polymer formation. Characterization of the oligomers and diblock co-polymers was accomplished using MALDI-MS supported by GPC (Gel Permeation Chromatography) and $^1H$ NMR spectrometry. MALDI-MS afforded definitive results by providing an inter-peak interval of 152 (m/z), corresponding to acenaphthylene monomer, and inter-peak interval of 260 (m/z) for the naphthalenemethoxystyrene monomer unit in block copolymers. Our study opens the way to control the number of repeat units in the oligomers. Further these oligomers can be tailored with various monomers for the formation of block copolymers.