Gaviglio and Doctorovich
SCHEME 1. Generation of Coordinated Diimide
Useful methods for the generation of this transfer hydrogena-
tion agent in situ include hydroxylamine and ethyl acetate,10
oxidation of hydrazine with one of several oxidants,11,12
decomposition of an azodicarboxylic acid,13 thermal decomposi-
tion of an anthracene-diimide adduct,14 and elimination of HX
from an acid hydrazide.12
Diimide is also capable of reacting with itself to form the
disproportionation products, hydrazine and nitrogen. Therefore,
the common procedures for reduction with this unstable reducing
agent involve in situ generation of diimide in the presence of
the molecule to be reduced. In general, diimide reacts with
greater stereospecificity than most hydrogenation catalysts.7,12,15
This stereospecificity is believed to derive from a concerted
dihydrogen transfer mechanism.7,12,16 Presently, this species is
widely known as a cis-specific reducing agent.7,12,17 Conse-
quently, diimide has great synthetic utility due to its ability to
stereospecifically reduce olefinic bonds.7,12,18 Symmetrical
multiple bonds such as carbon-carbon multiple bonds are
reduced much more readily by diimide than are unsymmetrical
unsaturated bonds of the type of Ct N, CdO, NdO, SdO.7,12,19
Also, diimide does not produce hydrogenolysis of single bonds
such as S-S or C-Br.12 Heteroatom bonds like N-N, N-O,
and O-O, which often suffer reductive cleavage under catalytic
hydrogenation conditions, remain intact during diimide reduc-
tions. On the other hand, as it was previously mentioned, it is
highly unstable in its free state but can be stabilized by
coordination to metal complex fragments.5,8,9
SCHEME 2. Proposed Catalytic Cycle
3-
3-
5
reactionof[Fe(CN) H2O] ,producedbyaquationof[Fe(CN)5NH3]
(2), with an excess of NH2OH. In the absence of olefin,
coordinated diimide disproportionates spontaneously, producing
N2 and NH3.
However, in the present work we will show that in the
presence of a double or triple C-C bond, diimide acts as a
reducing agent, producing the corresponding saturated C-C
bond in a syn stereospecific arrangement (Scheme 2).
This reduction can be carried out under mild conditions
(atmospheric pressure, temperature range ) 25-100 °C, and
pH ) 9), in aqueous solutions or solvent mixtures, and without
the use of hydrogen gas. The reduced product can be obtained
quantitatively in all cases by adding the appropriate amount of
hydroxylamine. Although this environmentally attractive method
is ideal for water-soluble olefins, it can be extended to
nonsoluble ones by the use of solvent mixtures. It is important
to remark that very few methods7,20 are known which are
efficient for the reduction of olefins, mild, and can be carried
out in water without the use of H2 gas.
Compared with traditional procedures using finely divided
metals and high hydrogen pressures, homogeneous catalytic
reduction of alkenes and alkynes in aqueous solutions, which
generates no organic solvent waste, is environmentally and
economically attractive.
The present work discusses the homogeneous catalytic
reduction of alkenes and alkynes in aqueous solution and solvent
mixtures under mild conditions, by in situ generation of the
diimide complex [Fe(CN)5(N2H2)]3- (1).
As shown in Scheme 1, our previously reported work5a
describes how coordinated diimide (1) can be generated by the
(9) (a) Barney, B. M.; McClead, J.; Lukoyanov, D.; Laryukhin, M.; Yang,
T.; Dean, D. R.; Hoffman, B. M.; Seefeldt, L. C. Biochemistry 2007, 46, 6784–
6794. (b) McKenna, C. E.; Simeonov, A. M.; Eran, H.; Bravo-Leerabhandh, M.
Biochemistry 1996, 35, 4502–4514.
Results and Discussion
(10) Wade, P. A.; Amin, N. V. Synth. Commun. 1982, 12, 287–291.
(11) (a) Kondo, K.; Murai, S.; Sonoda, N. Tetrahedron Lett. 1977, 42, 3727–
3730. (b) Furst, A.; Berlo, R. C.; Hooton, S. Chem. ReV. 1965, 65, 51–68. (c)
Corey, E. J.; Mock, W. L.; Pasto, D. J. Tetrahedron Lett. 1961, 11, 347–352.
(12) Back, R. A. ReV. Chem. Intermed. 1984, 5, 293–323.
Catalytic Behavior in Aqueous Solutions. The diimide-
bound complex [Fe(CN)5(N2H2)]3- (1) resulted from aquation
of the catalyst precursor [Fe(CN)5(NH3)]3- (2) and subsequent
reaction of the catalyst [Fe(CN)5(H2O)]3- with an excess of
NH2OH.5a Compound 1 was studied as an in situ reducing agent
of unsaturated substrates. Results for the reduction of maleic
acid (3) under different reaction conditions are shown in
Table 1.
Except for entries 7 and 8, in all cases the reactions were
carried out up to the complete consumption of 1, easily followed
thanks to its characteristic orange color (λmax ) 440 nm, ε ≈
4500 M-1 cm-1) which bleaches upon consumption.
Upon treatment of 3 at 25 °C with a catalytic amount of 2 (8
mol %), 38% yield of succinic acid (4) was obtained (Table 1,
entry 1). Remarkably, performing the reaction at 60 °C and at
100 °C resulted in formation of 4 in higher yield, 60% (entry
2) and 68% (entry 3), respectively; showing that at higher
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5380 J. Org. Chem. Vol. 73, No. 14, 2008