On the Dichotomic Reactivity of Lithiated Styrene Oxide: A Computational and Multinuclear Magnetic Resonance Investigation

Abstract

A multinuclear magnetic resonance investigation, supported by density functional theory calculations, has been synergically used to investigate the configurational stability, reactivity and aggregation states of alpha-lithiated styrene oxide in THF at 173 K. NMR studies on alpha-lithiated [alpha,beta-(13)C(2)]styrene oxide (also in an enantiomerically enriched form) proved that in THF this oxiranyllithium is mainly present as a solvated monomeric species in equilibrium with a complex mixture of stereoisomeric dimeric aggregates, as well as with bridged and tetrameric aggregates. The fact that some C(alpha)-Li bonds are partially broken in some stereoisomers reduces their symmetry and complicates the NMR spectra: two diastereoisomers each having a pair of diastereotopic carbon atoms slowly inverting at the lithium atom in absence of tetramethylethylenediamine (TMEDA) have been detected. A ((13)C,(7)Li)-HMQC experiment to correlate (7)Li and (13)C resonances of the various aggregates has been performed for the first time. From natural bond analysis, the monomeric aggregate was proven to have a lower carbenoid character with respect to bridged O-coordinated dimeric aggregates. The employment of suitable experimental conditions in terms of concentration, temperature and the presence or not of TMEDA are crucial to mitigate at the best the "carbene-like" reactivity of lithiated styrene oxide toward intermolecular C-Li insertions, eliminative dimerisation reactions and ring-opening reactions. A two-step mechanism for the deprotonation of styrene oxide by sBuLi in THF has been proposed and discussed as well as competitive side reactions.