General Papers ARKIVOC 2007 (xiv) 134-140
Results and Discussion
Boron alkoxides such as BIP and BSB are generated in situ from borane and the corresponding alcohols in THF using Schlenk techniques at room temperature (~ 27 0C) within 45 min. The alcohols are used in excess, to take their role in the alkoxide generation reaction and later in the actual reduction.
The reduction reaction of aldehydes and ketones was examined first in the presence of BIP, and then BSB in THF at 27oC under a slow stream of nitrogen. Preliminary experiments showed that the most suitable reduction time was about 6 hours. The alcohols as the reduction product were analyzed by GC-FID and confirmed by GC-MS. It was found that aliphatic aldehydes and ketones can easily be reduced to the corresponding alcohols by the catalytic activity of boron alkoxides in the presence of a secondary alcohol, while aromatic aldehydes and ketones can not be reduced under the same conditions. The selected carbonyl compounds and the yields of the reduction products are summarized in Table 1.
The reaction mechanism for the homogeneous MPV reaction involves a cyclic six-membered transition state.12 Scheme 2 shows the proposed mechanism for the carbonyl compounds in the presence of BIP. First, the carbonyl compound is coordinated to the boron of the boron alkoxide. The reaction proceeds by hydride- transfer to the carbonyl compound from the sec-alcohol, which is bound to the boron center as an alkoxide. Since the reduction is reversible, the acetone was removed from the medium by a slow stream of nitrogen. The removal of the acetone from the reaction solution leads to the progress of reaction to the right hand side. BSB also reduces carbonyl compounds with a similar mechanism; the 2-butanone produced was removed by the same procedure. The removal of 2-butanone (b.p. 810C) is more difficult than acetone (b.p. 560C). Therefore, as shown in Tables 1 and 2, the yields of the alcohol products from reduction are greater than those using BSB.
Although aromatic carbonyl compounds are known to be reduced to alcohols by the MPV reaction using 2-propanol and aluminum iso-propoxide, they were found to be resistant to MPV reduction using boron alkoxides instead of metal alkoxides. Mixtures of aliphatic and aromatic aldehydes and ketones were also examined for reduction in the present of boron alkoxides. While the aliphatic carbonyl compounds were reduced, the aromatic carbonyl compounds were not. This can be explained on the basis of electronegativity difference between Al (e.n. 1.5) and B
(e.n. 2.0). The electron attraction from alkoxy (-OR) groups by the boron in the boron alkoxide is stronger than that by Al in the aluminum alkoxide, and the electron density on the boron atom is higher than on the aluminum. The boron alkoxides are weaker Lewis acids than aluminum alkoxides. The attraction of electrons from the carbonyl groups of aldehydes and ketones is weaker for boron alkoxides than aluminum alkoxides. Aluminum alkoxides reduce most carbonyl compounds, irrespective of structure, as described previously. In the presence of BIP and BSB only aliphatic aldehydes and ketones can be reduced to the corresponding alcohols. This characteristic of boron alkoxides is very important in relation to selective reduction process by the MPV reactions. ISSN 1424-6376 Page 136 ©ARKAT USA, Inc.
