1,3-Li/H Shift of 1-Aryl-1,2-alkadienyl Reagents: An Experimental and Theoretical Study

A mechanism involving the exchange of equilibrated protons initiated by a catalytic amount of a proton donor, the most effective of which appears to be di-isopropylamine was proposed. The thermodynamic potential of the rearrangement was first examined computationally, since reversion of the stability of the two isomers as a function of chelation. The greater stability of 2- or 2-Li with respect to the 1- analogues strongly suggests that 1, 3-shift should be observed under all conditions and it non-reproducibility has to be linked to kinetic factors. The elucidation of the transposition mechanism would enlarge its scope, allowing a generalization to variously substituted propargyl reagents. The thermodynamic stability of the two allenyllithium isomers, it must be possible to predict the feasibility of this rearrangement, as promotion by proton donor should be general behavior.