13C‐NMR.‐Spektroskopie von Organolithiumverbindungen bei tiefen Temperaturen. Strukturinformation aus der 13C, 6Li‐Kopplung
Authors: Seebach Dieter , Hässig Robert , Gabriel Josef 
Published: December 2004

Low‐Temperature 13C‐NMR. Spectroscopy of Organolithium Derivatives. ‐ 13C, 6Li‐Coupling, a Powerful Structural Information

The 13C‐NMR. spectra of thirteen lithiated hydrocarbons (1c–13c. Table 2) and of eighteen a‐halo‐lithium carbenoids (14c–31c, Table 3) have been recorded in donor solvent (R2O, R3N) mixtures at temperatures down to −150°. The organolithium species were generated from singly or doubly 13C‐labelled precursors by H/6Li‐ or Br/6‐exchange. ‐ 13C, 6Li‐Coupling was observed of all species but those which supposedly contain contact ion pair C,Li‐bonds (benzylic and acetylenic derivatives). The multiplicities of the signals are correlated with the degree of aggregation in solution: the triplets of the halocarbenoids must arise from monomers or heteroatom‐bridged oligomers, the quintuplets of butyl‐, cyclopropyl‐, bycyclo[1.1.0]butyl‐, vinyl‐, and phenyllithium from dimers with planar arrangement of two Li‐ and two C‐atoms, as known from crystal structures (Scheme 3). All 13C, 6Li‐couplings are temperature‐dependent, dynamic processes cause them to disappear above ca. −70° (Fig. 1–4). ‐ Types of organolithium compounds are categorized according to the change of chemical shift δΔ (H, Li) upon H/Li‐substitution, according to the 13C, 6Li‐coupling constants ranging from 0 to 17 Hz, and according to the multiplicities which indicate the aggregation: type A are Li‐derivatives of alkanes and cycloalkanes, type B are s̀‐bonded vinyl, aryl, and alinyl derivatives, type C are a‐heterosubstituted (RS, hetero=halogen) organolithium compounds, and type D are π‐bonded allylic and benzylic systems (Table 5). The C,Li‐distances in the crystal structures of representatives of all four classes are within the small range of 2.18–2.28 Å (cf. Scheme 3). ‐ Some surprising observations and their interpretations and consequences are: (a) butyllithium solutions in THF, THF/TMEDA, and dimethyl ether contain increasing amounts of dimer upon cooling, the equilibrium (tetramer · 4 THF)+4 THF ⇌ 2 (dimer · 4 THF) being shifted to the right (Fig. 1 and Scheme 4); thus, more of a different species is present at low temperatures, with the accompanying changes in reactivity; (b) mixed higher aggregates are formed upon addition of butyllithium to bicyclobutyllithium; these are broken up to dimers upon addition of TMEDA (Tetramethylethylene‐diamine) (Fig. 2 and Scheme 5); (c) the solid state, the calculated gas‐phase and the solution species of phenyllithium all have dimeric structures, and so do vinyl and cyclopropyl lithium derivatives; the 13C‐deshielding observed upon replacement of H by Li on sp2‐ and sp‐C‐atoms is related to a polarization of the π‐electrons (Table 3, Fig. 3 and Scheme 6); (d) the spectra of halo‐lithium carbenoids show three striking features as compared to the C,H‐compound: deshielding of up to 280 ppm (Table 3), strong decrease of the coupling constant with 1H‐ and 13C‐nuclei attached to the carbenoid C‐atom (Table 4), and a structure‐independant, almost constant, large 13C, 6Li‐coupling constant of 17 Hz (Table 3); as shown in Scheme 7, these effects might be the consequence of a reduced degree of hybridization of the carbenoid C‐atom. ‐ The preparation of the labelled compounds and the generation of solutions of the organolithium compounds for NMR. measurements are described in full detail.