SELECTIVE REDUCTIONS .25. REMARKABLY FACILE REDUCTIVE OPENING OF CYCLIC ETHERS BY THE LITHIUM TRI-TERT-BUTOXYALUMINOHYDRIDE-TRIETHYLBORANE COMBINATION

Solutions of lithium tri-tert-butoxyaluminohydride, stable indefinitely in tetrahydrofuran, undergo rapid reaction in the presence of triethylborane to lose hydride and form 1-butanol (92% in 3 h at 25 °C) through reductive cleavage of the tetrahydrofuran molecule. The reaction is attributed to the rapid formation of a lithium triethylborohydride-aluminum fert-butoxide complex. The development led to an exploration of a method for generalizing this reductive cleavage of ethers. In tetrahydropyran, this type of reductive cleavage is much slower ' (24 h, 17%). Consequently, this was utilized as the solvent to explore the reductive cleavage of a variety of more reactive cyclic and acyclic ethers. The reaction is quite sensitive to the steric and electronic environment of the ether linkage. Epoxides, such as cyclohexene oxide, 1-methylcyclohexene oxide, etc., are instantly and quantitatively cleaved to their corresponding carbinols. Oxetane and tetrahydrofuran are readily cleaved, whereas the reductive cleavage of oxepane is quite sluggish. The utility of the lithium tri-tert-butoxyaluminohydride-triethylborane system is emphasized by the facile reductive cleavage of 2,5-dihydrofuran to ds-crotyl alcohol and 7-oxabi-cyclo[2.2.1]heptane to cyclohexanol. Dimethyl ether and methyl alkyl ethers are cleaved rapidly. The reductive cleavage of ethers bearing higher alkyl substituents is very slow. Thio analogues, such as dimethyl sulfide and tetrahydrothiophene, undergo very little reductive cleavage. © 1979, American Chemical Society. All rights reserved.