Reaction of Aldehydes and Ketones with Dichloroisopropoxyborane. Comparison of the Reducing Characteristics of Isopropoxyborane Derivatives

Abstract

Recently, we reported that boron triisopropoxide reduces simple aldehydes and ketones at 25 C or in refluxing THF under a slow stream of nitrogen. This is the first example that boron alkoxides react with carbonyl compounds in a Meerwein-Ponndorf-Verley (MPV) type fashion. In general, the reaction of boron triisopropoxide is much milder than that of aluminum triisopropoxide. Boron triisopropoxide only can reduce aliphatic aldehydes and ketones among the carbonyl compounds, whereas the aluminum reagent reduces most carbonyl compounds irrespective of its structure. Therefore, the reagent might serve as a comparative partner of aluminum triisopropoxide. However, side reactions are significant under the reaction conditions and hence yields become lower as the reaction goes on. Furthermore, in these reactions the acetone formed should be removed from the equilibrium mixture by a slow stream of nitrogen: otherwise, the yields are often quite low. In these respects, we need to develop other boron alkoxide derivatives, which can overcome such difficulties. In the course of such efforts, we found that dichloroisopropoxyborane appears to be one of possible candidates. We wish now to report the reducing characteristics of dichloroisopropoxyborane in the reaction of selected organic compounds containing representative functional groups. In Table 1, the reactivity of dichloroisopropoxyborane toward various aldehydes is summarized and compared to those of boron triisoproxide. As compared in the Table, the reactivity of dichloroisopropoxyborane is found to be quite different from that of boron triisoproxide. Thus, dichloroisopropoxyborane reduced both aliphatic and aromatic aldehydes readily to the corresponding alcohols within 1 or 3 h at 25 C, whereas boron triisoproxide showed a much lower reactivity toward aliphatic aldehydes and actually no reactivity toward aromatic ones. In addition to that, the side reactions, which are significant in the reaction of aldehydes with boron triisoproxide under a stream of nitrogen, were not accompanied in this reaction even in the presence of acetone formed. Dichloroisopropoxyborane shows also quite similar trend in the reduction of ketones, as shown in Table 2. The reactivity of the reagent toward ketones is much stronger than that of boron triisoproxide. Simple ketones were readily reduced, but some hindered alkyl and aryl ketones very slowly and hence could not be employed for this purpose under these reaction conditions. However, the reagent is absolutely inert toward other organic functional groups, such as acid chlorides, carboxylic acids, amides, esters, nitriles, etc. Therefore, we will explore in more detail the possibility achieving a selective reduction of particular organic functional groups. It is generally accepted that the MPV type reactions using boron triisopropoxide proceeds via a complex in which both the carbonyl compound and the reducing alcohol are bound to the boron ion. Thus, the carbonyl is activated upon coordination to B(III), followed by a hydride transfer from the alcoholate to the carbonyl group via a six-membered transition state. From this mechanistic point of view, the coordinating capacity of boron atom to the carbonyl oxygen