| We have investigated the effects of intramolecular amine and ether chelation for a series of ortho-substituted aryllithium reagents using low-temperature, multinuclear NMR techniques developed in this group for studying the solution structures of organolithium reagents. Our series included the classic ortho-metalation product of N,N-dimethyl-benzylamine, ortho -[(dimethylamino)methyl]phenyllithium, which exists as three isomeric chelated dimers. In this system and its derivatives, we measured activation barriers of -8 kcal/mol for nitrogen-lithium decoordination and ∼13 kcal/mol for the dissociation of the dimers to a monomeric species. Our work further addressed the profound effects of both chelation ring size and amine versus ether intramolecular chelation on the aryllithium solution structures in THF. Most dimeric compounds are chelated; whereas, the monomeric structures are not. Furthermore, we believe that the relative strength of chelation and the stabilities of the aggregated solution structures are correlated. The coordination to lithium by the chelating groups was strongest in the five-membered-ring series for the nitrogen systems and in the six- and seven-membered-ring series for oxygen. The reactivity of the aryllithium reagents was also investigated in the addition to chalcone and the metalations of thiophenes. The five-ring chelates gave mostly 1,2-addition to chalcone, while the six and seven-ring chelates gave ∼75% and ∼50% 1,2-addition products, respectively. The metalation reactions remarkably all gave similar rates for the aryllithium series, but slight rate increases were observed in the cases where higher concentrations of non-chelated monomers were present.;We attempted to determine the rotational barriers in a series of conformationally constrained carbanions, differing only in the chalcogen substituent at the carbanion center in chiral organolithium reagents by dynamic NMR spectroscopy. Our goal was frustrated by insoluble synthetic problems. However, we successfully determined the phenylthio rotation barrier in a cyclic silicon- and sulfur-stabilized organolithium reagent to be 10.0 kcal/mol in THF and 10.6 kcal/mol in THF/HMPA solution. Its acyclic analog had a rotation barrier of ≥13 kcal/mol. |
