Scope of the reaction of organoboranes with organolithiums as a method of preparation of lithium organoborates. Stability of lithium organoborates

Abstract

The scope and limitations of the reaction of organoboranes with a representative series of organolithiums as a method for the preparation of lithium organoborates have been examined by spectroscopic means (i.r., ¹H and ¹³C n.m.r.). The results indicate that, although the complexation reaction is highly general, the following limitations exist. In addition to the known limitation that hindered alkyl-lithiums with β-hydrogen atoms tend to give triorganoborohydrides in their reaction with triorganoboranes, charge-delocalized organolithiums derived from carbon acids whose pK_a are lower than ca. 20, such as cyclopentadienyl-lithium, indenyl-lithium, and α-lithioacetophenone, do not form lithium organoborates to any detectable extent under the conditions used. Equilibration of an organoborate with an appropriate organoborane or organolithium followed by treatment with iodine and g.l.c. analysis of the organic products provides a generally applicable and highly sensitive method for determining the stability of the organoborate with respect to dissociation which is often difficult to examine by spectroscopic methods. This method has been applied to examination of the stability of several representative lithium organoborates.