Structural and rate studies of the 1,2-additions of lithium phenylacetylide to lithiated quinazolinones: Influence of mixed aggregates on the reaction mechanism

The 1,2-addition of lithium phenylacetylide (PhCCLi) to quinazolinones was investigated using a combination of structural and rate studies. Li-6, C-13, and F-19 NMR spectroscopies show that deprotonation of quinazolinones and phenylacetylene in THF/pentane solutions with lithium hexamethyldisilazide affords a mixture of lithium quinazolinide/PhCCLi mixed dimer and mixed tetramer along with PhCCLi dimer. Although the mixed tetramer dominates at high mixed aggregate concentrations and low temperatures used for the structural studies, the mixed dimer is the dominant form at the low total mixed aggregate concentrations, high THF concentrations, and ambient temperatures used to investigate the 1,2-addition. Monitoring the reaction rates using F-19 NMR spectroscopy revealed a first-order dependence on mixed dinner, a zeroth-order dependence on THF, and a half-order dependence on the PhCCLi concentration. The rate law is consistent with the addition of a disolvated PhCCLi monomer to the mixed dimer. Investigation of the 1,2-addition of PhCCLi to an O-protected quinazolinone implicates reaction via trisolvated PhCCLi monomers.