School of Chemistry - Research Publications

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Author: O'Hair, Richard × Author: Khairallah, George × Author: LI, JIAWEI ×

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    Dimethylcuprate-Mediated Transformation of Acetate to Dithioacetate
    Li, J ; Khairallah, GN ; O'Hair, RAJ (AMER CHEMICAL SOC, 2015-01-26)
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    Decarboxylation versus Acetonitrile Loss in Silver Acetate and Silver Propiolate Complexes, [RCO2Ag2(CH3CN)n]+ (where R = CH3 and CH3CC; n=1 and 2)
    Li, J ; Khairallah, GN ; O'Hair, RAJ (CSIRO PUBLISHING, 2015)
    Gas-phase experiments using collision-induced dissociation in an ion trap mass spectrometer have been used in combination with density functional theory (DFT) calculations (at the B3LYP/SDD6–31+G(d) level of theory) to examine the competition between decarboxylation and loss of a coordinated acetonitrile in the unimolecular fragmentation reactions of the silver acetate and silver propiolate complexes, [RCO2Ag2(CH3CN)n]+ (where R = CH3 and CH3C≡C; n = 1 and 2), introduced into the gas-phase via electrospray ionisation. When R = CH3, loss of acetonitrile is the sole reaction channel observed for both complexes (n = 1 and 2), consistent with DFT calculations, which highlight that the barriers for decarboxylation 2.18 eV (n = 2) and 1.96 eV (n = 1) are greater than the binding energies of the coordinated acetonitriles (1.60 eV for n = 2; 1.64 eV for n = 1). In contrast, when R = CH3C≡C, decarboxylation is the main fragmentation pathway observed for both complexes (n = 1 and 2), with loss of acetonitrile only being a minor product channel. This is consistent with DFT calculations, which reveal that the barriers for decarboxylation are 1.17 eV (n = 2) and 1.16 eV (n = 1), which are both below the binding energies of the coordinated acetonitriles (1.55 eV for n = 2; 1.56 eV for n = 1). The barrier for decarboxylation of [CH3C≡CCO2Ag2]+ is 1.22 eV, which is less than the 2.06 eV reported for decarboxylation of [CH3CO2Ag2]+ (Al Sharif et al. Organometallics, 2013, 32, 5416). The observed ease of decarboxylation of silver propiolate complexes in the gas-phase is consistent with the recently reported use of silver salts in metal catalysed decarboxylative C–C and C–X bond forming reactions of propiolic acids.
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    Copper mediated decyano decarboxylative coupling of cyanoacetate ligands: Pesci versus Lewis acid mechanism
    Li, J ; Khairallah, GN ; Steinmetz, V ; Maitred, P ; O'Hair, RAJ (ROYAL SOC CHEMISTRY, 2015)
    A combination of gas-phase ion trap multistage mass spectrometry (MS(n)) experiments and density functional theory (DFT) calculations have been used to examine the mechanisms of the sequential decomposition reactions of copper cyanoacetate anions, [(NCCH2CO2)2Cu](-), introduced into the gas-phase via electrospray ionization. Gas phase IR spectroscopy, used to probe the coordination mode of the cyanoacetate ligands, revealed that the initial precursor ions are bound to the Cu via the carboxylate, [NCCH2CO2CuO2CCH2CN], 1. Multistage collision-induced dissociation (CID) of 1 gave sequential losses of CO2 and ethene. DFT calculations suggest that the lowest energy pathways for sequential decarboxylation involve Lewis acid mechanisms in which the binding of the cyanoacetate ligand sequentially rearranges from O to N: [NCCH2CO2CuO2CCH2CN](-) → [NCCH2CO2CuNCCH2CO2](-) → [NCCH2CO2CuNCCH2](-) + CO2 and [NCCH2CO2CuNCCH2](-) → [O2CCH2CNCuNCCH2](-) → [CH2CNCuNCCH2](-) + CO2. Loss of ethene involves sequential rearrangement of the binding of the cyanomethyl carbanion ligands from N to C: [CH2CNCuNCCH2](-) → [NCCH2CuNCCH2](-) → [NCCH2CuCH2CN](-). CH2=CH2 loss then proceeds via a 1,2-dyotropic rearrangement to form [NCCuCH2CH2CN](-) followed by β-cyanide transfer. This study highlights the rich mechanistic possibilities for metal mediated decarboxylation reactions involving ambidentate carboxylate ligands.