868
K. Tanemura et al.
phase or organic-solid phase using organic solvents such as benzene or
CH2Cl2,[4] however, there are few examples on the use of PEGDME as
solvents for this purpose as far as we know.[5–7] We supposed that interesting
reactivities might be expected in PEGDME.
In this article, we report that aldehydes and ketones react with NaBH4 in
PEGDME, which have an average molecular weight of 250 (PEG250DME),
to give the corresponding alcohols in good to excellent yields. Furthermore,
selective reduction of aldehydes in the presence of ketones at room tempera-
ture (rt) is described.
First, we examined the reactions of a variety of aldehydes with NaBH4 in
PEG250DME. The results are summarized in Table 1. In all cases, the reactions
proceeded at room temperature within 0.5 h to give the corresponding alcohols
in good yields (entries 1 and 3–6). On the other hand, reduction of ketones
required heating at 60 or 1008C to afford the corresponding alcohols in good
yields (entries 9–13). In the case of the reactive ketone, 4-tert-butylcyclohex-
anone (6), the longer reaction time (9 h) at room temperature was necessary
(entry 7). The difference of reactivities between aldehydes and ketones
makes chemoselective reduction of aldehydes possible. When this method
was applied to the reduction of a,b-unsaturated ketones, small amounts of
1,4-addition products as well as 1,2-addition products were yielded at room
temperature under high dilution conditions (entries 14, 16, and 17). When the
reactions were conducted at normal concentrations (conditions of footnote a
in Table 1), large amounts of polymeric materials were obtained together with
the desired products. The ratios of 1,2- and 1,4-addition products were almost
the same as those of the other reported methods.[8] When PEG500DME was
employed as a solvent, the reactions proceeded more slowly and gave the same
products in almost the same yields (entries 2, 8, and 15).
Next, we examined the chemoselective reduction of aldehydes in the
presence of ketones in PEG250DME under high dilution conditions. The
results are shown in Table 2. Indeed, aldehydes were reduced selectively in
the presence of ketones in each case. The concentration of the solution is an
important factor. When the reactions were conducted at normal concen-
trations, aldol-type dehydration products and small amounts of polymeric
materials were obtained together with the reduced alcohols. For example,
alcohols 18 (58%) and 25 (16%), a,b-unsaturated ketone 32 (14%), and
small amounts of polymeric materials were obtained from the reaction of
aldehyde 4 and ketone 11 with NaBH4 in PEG250DME.
When the reduction with NaBH4 was carried out in conventional protic
solvents such as MeOH, EtOH, and H2O, a large excess of the reducing