P.P. Jana et al. / Journal of Molecular Catalysis A: Chemical 289 (2008) 57–60
59
Table 1
Reduction of ␣,-unsaturated carbonyl compound by N,N-dimethylammine borane and by [Ni(bpy)Cl2] as catalyst (10 mol%) at 70 ◦
C
Entry number
Substrate
Time (h)
Ratio of A:B:C (as in Scheme 1)
Isolated yield (%)
1
2
3
4
5
R1 = C6H5, R2 = H
9
8
9
9
9
50:0:50
0:100:0
0:100:0
0:100:0
0:100:0
94
86
88
98
74
R1 = C6H5, R2 = C6H5
R1 = C10 H7, R2 = C6H5
R1 = p-OMe-C6H4, R2 = CH3
R1 = p-OMe-C6H4, R2 = p-OMe-C6H4
Table 2
Reduction of ␣,-unsaturated carbonyl compounds with N,N-dimethylammine borane
Entry number
Substrate
Time (h)
Ratio of A:B:C (as in Scheme 1)
Isolated yield (%)
1
2
3
4
5
6
R
R
R
R
R
R
1 = C6H5, R2 = H
1 = C6H5, R2 = C6H5
1 = p-OMe-C6H4, R2 = p-OMe-C6H4
1 = C6H5, R2 = C6H5
9
8
9
9
6
6
25:0:75
0:0:100
0:100:0
0:0:100
0:100:0
0:100:0
85
80
96
71
96a
94a
1 = C10 H7, R2 = C6H5
In all cases 1 mol% of Pd(TMEDA)Cl2 at 70 ◦C was used unless stated.
a
1 mol% of [Pd(bpy)Cl2] catalyst was used.
bonyl compounds by N,N-dimethylammine borane in the presence
of copper(II) salts are studied in the expectation that a copper
hydride complex will be formed and would catalyse the reduc-
tion reactions. It is found that the reaction of cinnamaldehyde with
N,N-dimethylammine borane with CuCl2 gives the corresponding
unsaturated alcohol in poor yield (∼40%). The reaction is shown
in Scheme 4. The reaction is found to be limited to aromatic ␣,-
unsaturated aldehydes. To confirm that copper(I) complexes are
formed in such reactions, copper(II)nitrate is allowed to react with
N,N-dimethylammine borane in the presence of triphenyl phos-
phine. This reaction led to the formation of [Cu(PPh3)2NO3]3H2O
with evolution of hydrogen gas. The complex [Cu(PPh3)2NO3]3H2O
can be alternatively prepared by the reaction of copper(II) nitrate
with excess of triphenyl phosphine in alcohol without the borane
[24].
N,N-dimethylammine borane in the presence of [Ni(bpy)Cl2] or
[Pd(TMEDA)Cl2] gives 3-phenylpropanol. For this reaction only
1 mol% of [Pd(TMEDA)Cl2] catalyst is required as against 10 mol%
of nickel catalyst. The reaction of ␣,-unsaturated ketone with
N,N-dimethylammine borane in presence of [Ni(bpy)Cl2] cata-
lyst, only the carbon–carbon double bond is reduced. So it is a
good catalyst for the hydrogenation of the carbon–carbon double
bond of ␣,-unsaturated ketones. A control experiment showed
that 1,5-bis-(4-methoxyphenyl)penta-1,4-dien-3-one (1 mmol) on
reaction with N,N-dimethylammine borane (3 mmol) in the pres-
ence of [Ni(bpy)Cl2] (10 mol%, 70 ◦C, 6 h), only one double bond
is reduced to give 1,5-bis-(4-methoxyphenyl)pent-1-en-3-one,
whereas this reaction with [Pd(bpy)Cl2] led to reduction of
1,5-bis-(4-methoxyphenyl)penta-1,4-dien-3-one in an unspecific
manner. The reduction of ␣,-unsaturated ketones with N,N-
dimethylammine borane in the presence of [Pd(TMEDA)Cl2] is
found to be substrate dependent. In the catalytic reactions by
[Pd(TMEDA)Cl2] either saturated carbonyl compound or a mix-
ture of saturated carbonyl compound and saturated alcohol is
formed. As far as the selectivity is concerned, both Ni and Pd has
advantages as well as disadvantages. Multiple numbers of prod-
ucts are formed in the case of [Pd(TMEDA)Cl2] as catalyst, whereas
[Ni(TMEDA)Cl2] gives selective products. However, [Pd(bpy)Cl2]
is more effective and selective also. These reactions are limited
to ␣,-unsaturated carbonyl compounds; substrates such as ace-
tophenone, cinnamyl esters, cyclo-octenes are not reactive towards
all the catalysts studied here, except [Pd(bpy)Cl2]. [Pd(bpy)Cl2] acts
as catalyst for reduction of acetophenone, benzophenone and cin-
namyl ester to the corresponding alcohols in varying yields from
3.5. Comparison of reduction reactions by amine borane with
different metal catalysts
Significant differences are observed in the reactivity between
[Ni(bpy)Cl2] and [Pd(TMEDA)Cl2]. For example the reaction
of cinnamaldehyde with N,N-dimethylammine borane in the
presence of a catalytic amount of [Pd(TMEDA)Cl2] gives satu-
rated:unsaturated alcohols in a 75:25 ratio; whereas a similar
reaction with [Ni(bpy)Cl2] as catalyst gives saturated:unsaturated
alcohols in a 50:50 ratio. The catalytic reduction reactions by
[Pd(bpy)Cl2] are comparable in selectivity with [Ni(bpy)Cl2];
but the former catalyst is more active, giving
a high yield
under mild conditions. The reaction of cinnamyl alcohol with
Scheme 4.