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Reduction O 0220 Reaction of Aldehydes and Ketones with Dichloroisopropoxyborane. Comparison of the Reducing Characteristics of Isopropoxyborane Derivatives. -Tri(isopropoxy)borane is a less reducing agent. -(CHA*, J. S.; YU, S. J.; ROH, M. Y.; PARK, S. J.; YI, J. E.; KWON, O. O.; Bull. Korean Chem.

In the previous communication, 1 we reported that boron triisopropoxide attacked aryl epoxides slowly but regioselectively to produce the anti-Markovnikov ring-opened reduction products in the Meerwein-Ponndorf-Verley (MPV) type reduction procedure. In the course of our successive efforts to devise new unique MPV type reagents, we found Al-fluorodiisobutylalane (DIBAF), a new MPV type reagent, reacts readily with both aromatic and aliphatic epoxides to produce the corresponding alcohols in an essentially perfect regioselectivity. This paper described such a regioselective MPV type reduction of epoxides.As listed in Table 1, DIBAF reduced both aliphatic and aromatic epoxides examined in hexane 2 -THF in 24 h at 25 o C. However, it is noteworthy that the reagent attacks aromatic epoxides more readily than aliphatic ones. DIBAF shows substituted alcohol is produced as a sole product by trapping of β-hydrogen from isobutyl group of the reagent at the site best able to accommodate a carbonium ion.Furthermore, the most interesting example should be found in the reaction of trans-β-methylstyrene oxide. Thus, the reaction yield 1-phenyl-2-propanol in a 100% selectivity. It is rather surprising that the reagent discriminates between the phenyl group-and the alkyl group-attacked carbon site.In the mechanistic point of view, the reaction of epoxides with DIBAF seems to involve the formation of a coordination complex, in which the aluminum atom bound to the epoxy oxygen, and followed by the hydride transfer via a cyclic transition state as in a usual reaction of carbonyl compounds with a MPV reagent.

Lithium cyanoaluminum hydride (LCAH) was prepared by the metal cation exchange reaction of sodium cyanoaluminum hydride with lithium chloride in tetrahydrofuran. The reducing characteristics of LCAH were explored systematically by the reaction with selected organic compounds containing representative functional groups under the standardized conditions (tetrahydrofuran, 0 o C). The reducing ability of LCAH was also compared with of the sodium derivative, sodium cyanoaluminum hydride (SCAH). Generally, the reducing behavior of LCAH resembles that of SCAH closely, but the reactivity of LCAH toward representative organic functional groups appeared to be stronger than that of SCAH. Thus, the regent reduces carbonyl compounds, epoxides, amides, nitriles, disulfides, carboxylic acids and their acyl derivatives to the corresponding alcohols or amines, at a relatively faster rate than that of SCAH. The cyano substitution, a strong election-withdrawing group, diminishes the reducing power of the parent metal aluminum hydrides and hence effects the alteration of their reducing characteristics.