Organoboranes: XXVIII. Convenient procedures for the synthesis of borinane

Abstract

Convenient syntheses of the six-ring boracyclane, borinane, have been developed. Hydroboration of 1,4-pentadiene with two molar equivalents of 9-borabicyclo-[3.3.1]nonane (9-BBN) in a suitable solvent, followed by reaction of the resulting trialkylborane with one molar equivalent of the borane-tetrahydrofuran complex (BH 3·THF) or the borane-dimethyl sulfide complex (BH 3·SMe 2, BMS), leads to the cyclization of the pentadiene moiety, forming borinane with the regeneration of two molar equivalents of 9-BBN. Complete separation of the two dialkylboranes by fractional crystallization from solvents such as THF, 1,2-dimethoxyethane (DME), 1,3-dioxolane, n-hexane, n-pentane and hexane plus dioxolane was unsuccessful. Treating the reaction mixture in hexane with the requisite amount of triethylamine (Et 3N) led to selective complexation with borinane. By cooling the reaction mixture to −78°C, it was now possible to crystallize out the uncomplexed 9-BBN almost quantitatively (98%). Alternatively, borinane could be precipitated selectively from the hexane reaction mixture as its bis-adduct with either N, N, N′, N′-tetramethylethylenediamine (TMED) or 1,4-diazabicyclo[2.2.2]octane (DABCO). Free borinane was readily liberated from its amine adducts by treatment with boron trifluoride etherate (BF 3·OEt 2). Finally, the two dialkylboranes were readily separated by fractional distillation, using a specially designed reaction setup. Distillation at about 70–80°C and 0.01 mm pressure led to the clean distillation of borinane, leaving the 9-BBN, as a residue, readily recycled in subsequent preparations. Alternatively, once free borinane has been obtained, it can be used to hydroborate 1,4-pentadiene in place of 9-BBN. Treatment of the product with BH 3·THF or BH 3·SMe 2 converts two moles of borinane into three, without the formation of a by-product.