Copper mediated decyano decarboxylative coupling of cyanoacetate ligands: Pesci versus Lewis acid mechanism

First published online 24 Apr 2015A combination of gas-phase ion trap multistage mass spectrometry (MSn) 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[double bond, length as m-dash]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.Jiawei Li, George N. Khairallah, Vincent Steinmetz, Philippe Maitre and Richard A. J. O'Hai

Gas-phase structure and reactivity of the keto tautomer of the deoxyguanosine radical cation

International audienceGuanine radical cations are formed upon oxidation of DNA. Deoxyguanosine (dG) is used as a model, and the gas-phase infrared (IR) spectroscopic signature and gas-phase unimolecular and bimolecular chemistry of its radical cation, dG˙+, A, which is formed via direct electrospray ionisation (ESI/MS) of a methanolic solution of Cu(NO3)2 and dG, are examined. Quantum chemistry calculations have been carried out on 28 isomers and comparisons between their calculated IR spectra and the experimentally-measured spectra suggest that A exists as the ground-state keto tautomer. Collision-induced dissociation (CID) of A proceeds via cleavage of the glycosidic bond, while its ion–molecule reactions with amine bases occur via a number of pathways including hydrogen-atom abstraction, proton transfer and adduct formation. A hidden channel, involving isomerisation of the radical cation via adduct formation, is revealed through the use of two stages of CID, with the final stage of CID showing the loss of CH2O as a major fragmentation pathway from the reformed radical cation, dG˙+. Quantum chemistry calculations on the unimolecular and bimolecular reactivity are also consistent with A being present as a ground-state keto tautomer

Linking structure to reactivity for guanine radical cations found in oxidized DNA. The gas-phase model systems 9-methylguanine, deoxyguanosine and the dGdC base pair

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