Theoretical study on the lithium-halogen exchange reaction of 1,1-dihaloalkenes with methyllithium and the nucleophilic substitution reaction of the resulting alpha-halo alkenyllithiums.
Transition structures for the lithium-bromine exchange reaction of 1,1-dibromoalkenes with methyllithium have been located by both the B3LYP and the MP2 levels of theory with the 6-31+G basis set. The reaction with methyllithium dimer gave similar results with lower activation energies. These calculations predict both the kinetic and the thermodynamic stereoselectivity correctly. That is, the sterically more constrained bromine atom of 1,1-dibromoalkenes was predominantly reacted with alkyllithium (dimer) in the kinetic condition. The intramolecular substitution reaction of 4,4-dibromo-3-methyl-3-pentenol in the presence of methyllithium has been investigated. After deprotonation of the alcohol and the lithium-bromine exchange reaction, the intramolecular substitution reaction occurs to give dihydrofuran in a concerted manner. The intermolecular substitution of alpha-chloro alkenyllithium with methyllithium was also studied for comparison. The formation of the indene derivative from 3-(o-bromophenyl)-1,1-dibromo-1-propene in the presence of methyllithium occurs in a similar manner. The lithium-bromine exchange reaction of bromobenzene with methyllithium occurs in an S(N)2 mechanism and the solvent plays an important role.
Ando
Kaori
K
College of Education, University of the Ryukyus, Nishihara-cho, Okinawa 903-0213, Japan. ando@edu.u-ryukyu.ac.jp