An HF and DFT ab initio study on the mechanism of ortho-directed lithiation of hydric and nonhydric aromatic compounds incorporating aggregation and discrete solvation;The role of N,N,N',N'-tetramethylethane-1,2-diamine (TMEDA)in lithiation reactions

摘要

An HF and DFT ab initio study was set up to decipher the roles of aggregation and solvation in the orthodirected lithiation of aromatics (hydric and nonhydric),as well as to shed light on the much debated question of precomplexation in the mechanism of lithiation.Ab initio (HF/6-31-G~*) calcualtions on the lithiation of nonhydric aromatics have uncovered several competitive routes operating as a function of the aggregation state of the organolithium base used.Specifically,two competitive routes were found for the lithiation of the anisole model 2 by organolithium dimers 1-dim,namely the so-called cyclic-dimer and open-dimer routes,whereas,for organolithium tetramers 1-tert,the coresponding cyclic route is the only one oeprative,and for monomers 1-mon,several optional routes seem to be available.Precomplexation is,in all cases,a requirement.According to the computational data presented,the mysterious rate acceleration experimentally observed for lithiations carried out in TMEDA can be assigned to an aggregation effect on the intermediate open-dimer species,which subsidiarily give rise to several so-called s-monomer routes,of which the dimerization-driven s-monomer route s-m_(3b) is the one having the lowest energy barrier.The relevant species characteristic of both the open-dimer and s-monomer routes are the so-called open dimers,i.e.,high-energy intermediates ,resulting from cleavage-induced associative complexation of the aromatic substrate upon the fuly solvated organolithium dimer.DFT calculations (B3LYP/6-31+G*) also revealed that the perilithiation (i.e.,Li at C(8) of 1-naphthol mlodel 3 is a slow proces taking place preferentially through the opendimer route.