ORGANOBORANES. XXVIII. CONVENIENT PROCEDURES FOR THE SYNTHESIS OF BORINANE

Article abstract of DOI:10.1016/0022-328X(83)85036-0

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-tatrahydrofuran complex (BH3*THF) or the borane-dimethyl sulfide complex (BH3*SMe2, 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 (Et3N) led to selective complexation with borinane.By cooling the reaction mixture to -78 deg C, it was now possible to crystallize out the uncomplexed 9-BBN almost quantitatively (98percent).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 (BF3*OEt2).Finally, the two dialkylboranes were readily separated by fractional distillation, using a specially designed reaction setup.Distillation at about 70-80 deg 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 BH3*THF or BH3*SMe2 converts two moles of borinane into three, without the formation of a by-product.