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

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.