ChemInform Abstract: CHARACTERISTIC (13)C CHEMICAL SHIFTS AND (13)C‐H COUPLING CONSTANTS IN ORGANOLITHIUM COMPOUNDS

Carbanion Chemistry

“…In combination with a dimethylphenylsilyl substituent he stabilized the twisted propenyl lithium structure in which the Li is located in the alkene plane and a localized double bond is preferred 35 (Figure 8). As a consequence, the first example of 6 Li-13 C coupling in an allylic lithium is detected. This unique form is thought to be the transition state (presumably pyramidal) for rotation about the C 1 -C 2 bond.…”

“…They studied the monomer/dimer/tetramer equilibria of phenyl lithium as a function of solvent and temperature. The temperature dependence of the 13 C and 6 Li spectra in ether is shown in Figure 3. Coupling patterns established the state of aggregation.…”

“…20 Boersma and coworkers, 21 show a comparable sequence in an exemplary study of the solid-state and solution structure of 2-methoxyphenyl lithium:-tetramer (155.9 ppm), dimer (171.1 ppm) and monomer (175.6 ppm). Different aggregation states were generated by solvent changes and confirmed by 6 Li-13 C coupling patterns. A similar trend is seen for C 1 in vinyl lithium; tetramer (183.0 ppm, 2.26Å) 22 to dimer (190.8 ppm, ?).…”

“…35(c) Vicinal proton-proton coupling shows that relatively small substituents, including phenyl and methyl, prefer the syn conformation. 35(c), 39 Balzer and Berger 42 used 1 H-1 H and 1 H- 6 Li Overhauser effects to identify the endo and exo protons in 2-methyl-3-phenyl-allyl lithium as rotation around the C ipso -C 3 bond was slow with an activation energy of 42 kJ mol −1 .…”

“…With the substituent at the central carbon C 2 , the ligand arms are diastereotopic and show resolved resonances at low temperature. Line broadening techniques were used to identify three distinct processes -inversion of the tetrahedral lithiated carbon (from 2 site exchange broadening), dissociation of the carbon lithium bond (from 6 Li-13 C coupling collapse), and C 1 − −− → ← −− − C 3 allylic interchange (2 site exchange). With the substituent at the terminal carbon, C 1 , the allylic system was induced to form both exo and endo isomers.…”