Lithium Salts of Conformationally Constrained and Restricted Chiral Allylic α‐Sulfonyl Carbanions − A Joint Study of their Structures, Dynamics, and Stereoselectivities

Abstract The structures of the lithium salts of the chiral bicyclic allylic α‐sulfonyl carbanions 3 − 5 , each possessing a norbornane skeleton and a tert ‐butyl group at the S atom, have been studied by 1 H, 13 C, 6 Li, and 6 Li, 1 H HOESY NMR spectroscopy, cryoscopy, and X‐ray crystal structure analysis. Because of their relatively high endo ‐ exo isomerization barriers, the Cα−S endo and exo diastereomers of 3 − 5 could be observed by NMR spectroscopy at −30 °C to −50 °C in [D 8 ]THF. The endo diastereomer is the preferred equilibrium species under these conditions, as shown by 1 H, 1 H HOESY experiments. Carbanion salt 3 has endo ‐ exo isomerization barriers of Δ G # 270 = 13.1±0.1 kcal/mol and 12.6±0.1 kcal/mol, while the 7‐benzhydrylidene‐substituted carbanion salt 5 has barriers of Δ G # 288 = 13.5±0.1 kcal/mol and 13.3±0.1 kcal/mol. Cryoscopy and 6 Li NMR spectroscopy of 5 in THF at −100 °C to −108 °C revealed the formation of dimers and monomers in a ratio of approximately 2:1. NMR spectroscopy of 3 − 5 at −90 °C to −105 °C allowed observation of the dimers and monomers of which the anions have endo conformations and also of which the anions adopt exo conformations. The NMR spectroscopic results for 3 − 5 are compatible with monomeric and dimeric CIPs, featuring planar allylic moieties and allylic stabilization by delocalization of the negative charge. 6 Li, 1 H‐HOESY examination of the mixture of the monomers and dimers of endo ‐ 5 and exo ‐ 5 in [D 8 ]THF at room temperature gave only evidence for coordination of the Li atom to the O atom(s) in the CIPs. The NMR spectroscopic results for 3 were corroborated by X‐ray crystal structure analysis of the monomer exo ‐ 3· PMDETA, which features (i) an essentially planar anionic C(2) atom, (ii) the exo conformation, (iii) the typical Cα−S conformation, also allowing for a stabilizing n C −σ * S t Bu interaction, and (iv) a single O−Li bond, but no C−Li bond. Upon treatment of the endo and exo sulfones 9 , 10 , and 12 with n BuLi at very low temperatures, the corresponding endo and exo carbanion salts endo ‐ 3 − 5 and exo ‐ 3 − 5 , respectively, were selectively generated as mixtures of monomers and dimers, the reactions of which with electrophiles were studied. Deprotonation of the exo and endo sulfones with n BuLi proceeds stereoselectively, the exo sulfone preferentially giving the endo anion and vice versa. The diastereomeric endo and exo carbanion salts 3 − 5 each react with reactive electrophiles at the anionic C(2) atom syn to the sulfonyl O atoms, giving the corresponding substituted endo and exo sulfones, respectively, with significant degrees of asymmetric induction. Reactions of the endo and exo diastereomers of 3 − 5 with CF 3 COOD and MeOCH 2 I were faster than their endo ‐ exo isomerization and approached kinetic quenching, while those with MeI and allyl iodide were slower, approaching the Curtin−Hammett limit and preferentially giving the exo sulfones. Deprotonation‐deuteration experiments of the 7‐oxa‐sulfone endo ‐ 11 showed that the corresponding 7‐oxa‐substituted carbanion salts endo ‐ 6 and exo ‐ 6 not only can be generated at low temperatures but may also, despite their tendency to rearrange, be converted into the corresponding 7‐oxa‐sulfones on treatment with reactive electrophiles.