Fig. 3. (a) Proposed mechanism for the cyclization of 1 by XantphosCu(i) through dinuclear intermediates, (b) deuterium labeling experiment and ¹H NMR spectrum after 24 h of reaction, showing the mixture of products (for reaction conditions see ESI Fig. S45†), (c) CSI-HRMS of the reaction mixture showing the peaks belonging to intermediates [XantphosCu]⁺ and B, together with the isotopic pattern of the peak corresponding to B (up experimental spectrum, down simulated spectrum; for detailed spectra and reaction conditions see ESI Fig. S47–S49†).