Process for the preparation of triarylborane

#1. Field of the Invention

#The present invention is directed to an improved process for preparing triarylboranes by reacting an alkali metal, an organohalide, and an orthoborate ester and for the recovery of the borane.

#2. Description of the Prior Art

#A schematic representation of a typical method for practicing the process of the present invention is shown in the drawing.

#The preferred triarylboranes contemplated by the present invention include those of the formula R.sub.3 -B wherein the R's are the same or different and are aryl or substituted aryl groups having 6 to 12 carbon atoms, e.g. phenyl, orthotolyl, paratolyl, naphthyl, methoxy paraphenyl, paraaminophenyl, biphenyl, chlorophenyl and bromophenyl. Triphenylborane and the sodium isopropoxide adduct thereof are of particular interest.

#Alkali metals which are operable in the present process include lithium, potassium, etc. with sodium being preferred. The rate and efficiency of the reaction with alkali metal depends at least in part upon the surface area of the alkali metal. Preferably, the metal is in a suspension of particles of 1-5.mu. in the reaction solvent, but particles up to and exceeding 100.mu. may be utilized along with certain activators including the lower alkyl alcohols, i.e. those having 1 to 6 carbon atoms, e.g. methanol, ethanol and isopropanol. The activator is preferably employed when the reaction is conducted at lower temperatures and may be introduced directly into the reaction mixture or as a solution in the reaction solvent. If the reaction is conducted at temperatures of 100.degree. C. or higher, e.g. in a methylcyclohexane solvent the sodium can be introduced in the molten state directly to the reaction medium. Since the instant reaction is highly exothermic, the cooling requirements can represent an excessive economic penalty if the reaction is conducted too rapidly. Therefore, it is desirable to control the reaction rate at the desired operating conditions to reduce the cooling requirements. This object can be accomplished without adversely affecting the yield by continuously metering the sodium dispersion to the reaction or by staged addition of the sodium dispersion.

#A wide variety of compounds may be employed as the inert organic solvent in the present process. The reactants should be relatively soluble in the solvent at reaction temperature and should not react with the solvent. Compounds such as benzene which increase electron transfer (transfer agents) can be included in the medium especially at lower reaction temperatures. The suitability of certain solvents can vary depending upon the method of conducting the process. For example, in a process wherein the arylating agent is prepared and then reacted with the orthoester, solvents such as toluene, xylene, and cumene tend to be less desirable because they react with the arylating agent. However, when the orthoborate ester is present the arylating agent preferentially reacts with the ester thereby rendering solvents such as toluene essentially inert. Preferably the solvent has a boiling point at atmospheric pressure near the desired reaction temperature to facilitate heat removal via reflux of the solvent. Suitable solvents include singly or in mixture branched or unbranched alkanes, having 5-8 carbon atoms, e.g. pentane, hexane, heptane, octane and 3-methylpentane; cycloalkanes having 5-8 carbon atoms, e.g. cyclohexane, methylcyclohexane, cyclooctane, cyclopentane; alkenes having 5-8 carbon atoms and cycloalkenes having 5-8 carbon atoms wherein unsaturation does not react with the alkali metal, the organohalide or the orthoester. Examples of the foregoing include hexene and octene. Other suitable inert organic solvents will be apparent to one skilled in the art in view of the foregoing consideration. Cyclohexane is preferred because it boils at a preferred reaction temperature and thus permits effective heat removal and because it provides high yield to the desired products when certain esters are employed as is discussed hereinbelow.