On the Solution Behaviour of Benzyllithium⋅(−)-Sparteine Adducts and Related Lithium Organyls - A Case Study on Applying7Li,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.