Organometallic exchange reactions. X. Cross-association of tert butyllithium. Kinetics of tert butyllithium dissociation
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Intermolecular exchange leading to equilibration of mixtures of t-butyllithium (I) and trimethylsilyl-methyllithium (II) have been studied using 7Li nuclear magnetic resonance. The exchange is rate controlled by dissociation of t-butyllithium tetramers. The first-order rate constant in cyclopentane at 20 is 1 105 sec1; the enthalpy of activation is 24 6 kcal/mol. The dissociation is about 20 times more rapid in toluene at 20. Addition of small quantities of triethylamine to the cyclopentane solutions causes the exchange to become much more rapid; equilibration is achieved in less than 15 min. Mass spectra of samples recovered from cyclopentane solutions made up by mixing I synthesized from 6Li with I synthesized from 7Li exhibit a slow approach to equilibration of the lithium isotopes among the tetramers, commensurate with the nmr results. Mass spectra of samples recovered from mixtures of I with lithium t-butoxide, LiO-t-C4H9, in benzene solution demonstrate the existence of mixed organolithium species in the vapor phase. Similar results were obtained for samples derived from mixtures of I with ethyllithium and with II. 1970, American Chemical Society. All rights reserved.
