Methyllithium

All reagents were obtained from commercial suppliers and used without further purification. Methyl lithium (1.6 M in diethyl ether), n‐butyl lithium (2.5 M solution in hexanes), ethyl lithium (0.5 M in cyclohexane/benzene), phenyl lithium [1.9 M in di(n‐butyl)ether], 2‐thienyl lithium (1 M in THF/hexanes), (CH₃)₃SiCH₂Li (1 M in pentane), n‐butyl magnesium chloride (2 M in THF) and i‐PrMgCl₂Li (1.3 M in THF) were purchased from Sigma Aldrich. Concentrations of all organolithium reagents were established by titration with l‐menthol,^[17] and the concentration of Grignard reagents were determined by titrating against iodine.^[18]Infrared spectra were recorded on a Bruker Tensor 27 spectrometer, using an ATR (Attenuated total reflection) accessory. NMR spectra were recorded on a Bruker Avance Neo 400 spectrometer operating at 400.13 MHz for ¹H, 100.62 MHz for ¹³C and 376 MHz for ¹⁹F. All ¹³C and ¹⁹F spectra were proton decoupled. ¹H and ¹³C NMR spectra were referenced against the appropriate solvent signal. ¹⁹F NMR spectra were referenced against CFCl₃. Characterisation details, including ¹H, ¹⁹F and ¹³C{¹H} NMR spectra, for compounds 3a–q and 4a–s are included in the Supporting Information.