REDUCTION OF CARBONYL COMPOUNDS
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can be used as an effective catalyst for this goal in the presence of isopropyl
alcohol as a source of active hydride and solvent under microwave
irradiation.
Alumina was activated by two different methods: by heating it in a
quartz vessel at 400ꢀC for 24 h[7] or by microwave irradiation in a commer-
cial microwave oven (800W) for 5 min. The two kinds of activated alumina
obtain show almost the same catalytic activity in the reduction reactions.
The reducing system was easily prepared by addition of activated alumina to
a suspension of KOH in isopropyl alcohol. Several reduction reactions were
performed, at various powers and with different amount of potassium
hydroxide and activated alumina, in order to find the most adequate con-
dition for this reaction under microwave irradiation. The required mole
ratio of KOH to substrate 0.5 : 1, weight ratio of activated alumina to sub-
strate 2.5 : 1, and the maximum power output of 200W were found to be the
optimal conditions.
The alumina/KOH system was cleanly and rapidly applied to the
reduction of variety of ketones and aldehydes in isopropyl alcohol with a
short time of microwave irradiation. The results are shown in Table 1. Thus
the reduction of aromatic, alicyclic, and aliphatic ketones (Entries 1–13),
aromatic and aliphatic aldehydes (Entries 14–26) afforded the correspond-
ing alcohols in excellent yields. Position and electronic properties of sub-
stituents on the benzene ring do not influence appreciably the reactivities of
aromatic aldehydes and ketones.
The successful reduction of acyloins (Entry 27) to their diols might
extend the utility of the present simple procedure. By this method absolute
selectivity is observed for the reduction of carbonyl functionality in the
presence of a nitro group (Entries 15,16).
Selective 1,2reduction of ꢁ,ꢂ-unsaturated carbonyl compound vs. 1,4
reduction is an important achievement for the synthesis of allyl alcohols
from easily available ꢁ,ꢂ-unsaturated carbonyl compounds. Posner
et al.[6,10] have applied ꢀ-Al2O3 in the MPV reduction of unsaturated
carbonyl compounds by isopropyl alcohol. However, rather high tempera-
tures were needed (up to 300ꢀC) and only low yields of alcohol were
obtained due to the occurrence of several side reactions. We have studied
the reduction of cinnamaldehyde with our system. The reduction is highly
regioselective and only cinnamyl alcohol is isolated with high yield (Entry
28).
It is previously proved that strong bases are frequently added as pro-
moters in H-transfer reactions since often they exert a beneficial effect on
reaction rates.[11,12] Also, in our experiments, it was found that the presence
of KOH in reaction media is essential, since the reaction in the absence of
KOH, furnished only a trace of the product.