ETHYLENATION OF SECONDARY AND TERTIARY ALKYLLITHIUMS .2. ITS KINETICS AND NATURE OF ACTIVE SPECIES

The degree of etheration by dimethyl ether at its boiling point (–24.8 ˚) has been measured for n-heptane solutions of isopropyl-, sec-butyl-, and n-butyllithium over a range of ether partial pressures. The primary alkyllithium is appreciably more strongly etherated than the secondary ones. n-Butyllithium approaches a limit of four ether molecules coordinated with a tetrameric organolithium aggregate. The ethylenation of isopropyl-, sec-butyl-, and t-butyllithium at –24.8˚ is of the first order with respect to organolithium tetramer and of the first order with respect to ethylene. With diethyl ether in 5.7- to 113-fold excess over [RLi] in n-pentane, the ethylenation of isopropyl- and sec-butyllithium is of second order in ether. With the less hindered ether, tetrahydrofuran, the rate of ethylenation of t-butyllithium at -40.8˚ in n-pentane passes through a maximum and later becomes inversely proportional to [THF], Triethylamine and N-methylpyrrolidine produce a first-order catalysis of ethylenation, over a concentration range similar to that in which diethyl ether behaves in a second-order manner. 1,4-Dimethoxybutane is a strong first-order catalyst at low concentrations, but was not examined at high [ether]/[RLi] ratios. The activation parameters of the ethylenations observed lie in the range ΔH‡ = 9.0-11.5 kcal/mole and ΔS‡ = – 41.5 to –25.4 eu/mole. The implications of these facts for diethyl ether catalysis are discussed in terms of a transition state (Chart I) containing one molecule of ethylene, four units of RLi, and (2 + n) molecules of ether, where n = 0 or 1 and represents the prevalent degree of etheration over the range of conditions examined. The model predicts reaction orders which may vary with the case and the conditions. If RLi-initiated polymerization of styrene is a carbanion reaction, ethylenation is probably not one. © 1969, American Chemical Society. All rights reserved.