SN2 vs. E2 on quaternary centres: an application to the synthesis of enantiopure ?2,2-amino acidsElectronic supplementary information (ESI) available: experimental details, spectroscopic characterization of all compounds, crystal structure data and tables of electronic energies, enthalpies, entropies, Gibbs free energies and coordinates for the conformations of structures. See http://www.rsc.org/suppdata/cc/b4/b400282b/