Low‐Temperature 13C‐NMR Spectra of 6Li‐ and 13C‐Labelled Sulfur‐ and Selenium‐Substituted Organolithium Derivatives

The 13C‐NMR spectra of 19 different, singly, doubly, and triple 13C‐labelled α‐sulfur‐ and α‐selenium‐substituted 6Li‐derivatives generated from methyl and phenyl thioethers, thioacetals, trithio‐orthoesters and from their selenium analogues have been recorded in ethereal solutions (tetrahydrofuran (THF), 2‐methyltetrahydrofuran (MTHF) at temperatures between −30° and −110°. The effects of H/Li‐exchange upon chemical shifts and coupling constants, as well as the values and multiplicities of Li, 13C‐coupling are interpreted in vie wof crystal structures of some of the same compounds. In two thirds of the cases studied, the H‐decoupled 13C‐NMR signals observed below −80° were triplets, proving that the C‐atoms are bonded to single 6Li‐atoms. This is compatible either with monomeric or with dimeric, heteroatom‐bridged structures. The direct 1H, 13C‐ and 13C, 13C‐coupling constants (1J) decrease, the 13C, 77Se‐coupling constants increase upon lithiation. More striking is that the geminal coupling 13C‐S‐13C (2J) is too small to be observed in the non‐metalated species, while it ranges from 3.7 to 7.5 Hz in the lithiated derivatives. These observations may be interpreted as resulting from delocalization of electron density from the carbonionic center towards the adjacent heteroatom.