title: On the solution behaviour of benzyllithium·(-)-sparteine adducts and related lithium organyls--a case study on applying 7Li,15N{1H} HMQC and further NMR methods, including some investigation into asymmetric synthesis.
abstract: 2D (7)Li,(15)N heteronuclear shift correlation through scalar coupling has successfully been applied to several lithium organyls consisting of polydentate N ligands such as N,N,N',N'-tetramethylethylenediamine (tmeda), N,N,N',N',N''-pentamethyldiethylentriamine (pmdta) and (-)-sparteine. Structural insights on the conformation of benzyllithium·pmdta (5) in a toluene solution and the strength of ion pairing in combination with PGSE NMR measurements, (1)H,(1)H-NOESY and (1)H,(7)Li-HOESY experiments are presented. By studying in detail the formation of 5 in solution, a transient species has been observed for the first time and assigned to a pre-complex of nBuLi and pmdta. In addition, the solution behaviour of the complex formed between benzyllithium and (-)-sparteine (8) has been studied by PGSE and multinuclear NMR spectroscopy. The straightforward synthesis and first applications in asymmetric lithiations are also reported, which show that the new system benzyllithium·(-)-sparteine (8) provide poorer enantioselective induction than the classical nBuLi·(-)-sparteine (6). The results were supported by deprotonation experiments confirming that the formation of 8 relies on two relevant factors, namely temperature and lithiating reagent. The existence of 8 may thus interfere with the asymmetric induction when the system nBuLi· (-)-sparteine is used in the enantioselective deprotonations of N-Boc-N-(p-methoxyphenyl)-benzylamine conducted in toluene.
authors.affiliation: Laboratory of Organic Chemistry, University of Almería, Carretera de Sacramento s/n 04120 Almería, Spain.
authors.forename: María
authors.initials: M
authors.lastname: Casimiro
authors.forename: Pascual
authors.initials: P
authors.lastname: Oña-Burgos
authors.forename: Jens
authors.initials: J
authors.lastname: Meyer
authors.forename: Steffen
authors.initials: S
authors.lastname: Styra
authors.forename: Istemi
authors.initials: I
authors.lastname: Kuzu
authors.forename: Frank
authors.initials: F
authors.lastname: Breher
authors.forename: Ignacio
authors.initials: I
authors.lastname: Fernández