6
798
It is noteworthy that the hydrogenation rates were signi®cantly dierent for trans- and cis-
stilbenes, with the trans isomer being more easily hydrogenated (entries 5 and 6). Hydrogenation
of cis-stilbene under the same conditions as that of entry 5 in Table 2 aorded hydrogenated
product (dibenzyl) in 59% yield, along with isomerized compound (trans-stilbene, 9%) and the
.
recovered cis-stilbene (32%). The reaction of cis-stilbene with the NaBH /NiCl 6H O/moist
4
2
2
alumina system did not proceed and cis-stilbene was recovered quantitatively in the absence of
NaBH . The cis±trans isomerization may suggest that half-hydrogenated species
during the reaction.
1,12
is formed
4
The hydrogenation is applicable not only to cyclic alkene (entry 8) but also to linear alkene
entry 9) in a quantitative yield of corresponding alkanes.
We have found that the insoluble material's color changed from white to black. The phenomenon
(
2
may reveal that nickel boride is formed in the same way as P±2 Ni . Therefore, the hydrogenation
.
with the NaBH /NiCl 6H O/moist alumina system may proceed with the same mechanism as the
4
2
2
2
hydrogenation with P±2 Ni . The hydrogen source of products may come from NaBH since no
4
.
hydrogen gas is used in the NaBH /NiCl 6H O/moist alumina system. Thus the reaction may
4
2
2
start with the formation of nickel boride on an alumina surface, followed by alkenes. These can
be hydrogenated and catalyzed by nickel boride to produce the corresponding alkanes.
In summary, it can be emphasized that the reaction is clean, and set-up of the reaction and
work-up of the product are straightforward. From an economical and environmental point of
view, the use of hexane as the solvent might also be favorable. The hydrogenation may take place
on the alumina surface; further work on the selective hydrogenation of alkynes and dienes, for
example, using this system, is now underway in our laboratory.
Acknowledgements
The authors thank Dr. Takeshi Takeda, Tokyo University of Agriculture and Technology.
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1
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3
4
5
6
7
8
9
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