Experimental and Theoretical Study of Tunable 1,3-Lithium Shift of Propargylic/Allenylic Species, Transmetallation, and Pd-Catalyzed Cross-Coupling Reactions

The highly selective tuning of the isomerization from 1-arylalka-1,2-dien-1-yllithium to 1-arylalka-1,2-dien-3-yllithium has been realized in the deprotonation of 1-arylalk-1-yne (conditions A and B) and carbolithiation of 1-arylbut-3-en-1-yne with alkyllithium (conditions C and D). Subsequent transmetallation and Pd-catalyzed Negishi coupling reactions afforded 1,1-diaryl or 1,3-diaryl allenes with high selectivity. Deuterium-labeling cross experiments indicated that an intermolecular lithiation process occurred in both 1,3-lithium shift conditions (conditions B and D). 1-Arylalka-1,2-diene was confirmed experimentally to be the intermediate. A computational study at the B3LYP level for the isomerization indicated that the acidity of H at the 3-position is higher than that of the H at the 1-position of 1-phenyl-1,2-butadiene. Under conditions B, iPr(2)NH acts as a proton carrier to finish the 1,3-lithium shift. The overall activation barrier for the rate-determining step in the solvated models is approximate to 21.0 kcal mol(-1), indicating that the isomerization is reasonable at room temperature. For the isomerization under conditions D, DFT calculations indicated that the addition of TMEDA (tetramethylethylenediamine) and HMPA (hexarnethylphosphoramide) changes the global minimum of the system; among the possible mechanisms (P1-P5) considered, the mechanism catalyzed by dilithiated species (P5) is the most probable one. The overall activation barriers for isomerization in THF and TMEDA solvated models are 22.6 and 19.7 kcal mol(-1), respectively, proving that the isomerization may proceed at RT in THF or at -78 degrees C with TMEDA, due to the fact that the solvation of the additives may increase the concentration of 1-phenyl-1,2-butadienyllithium monomer by a deaggregation effect.