10.1002/cssc.201801770
ChemSusChem
COMMUNICATION
Having established the substrate scope of different alkynes,
various alkenes and carbonyl-containing compounds were
tested under optimal conditions. We were pleased to find that
chemoselective reduction of alkene derivatives to the
corresponding alkanes could be achieved in 84-99% yields. We
found that a wide range of electron-deficient alkenes with
various functional groups was well tolerated under optimized
conditions (Figure 2b, 3a-3k). Several electron-withdrawing
alkenes were compatible and even with low catalyst loading
affording the desired products smoothly. 1,4-Dicarbonyl alkene
derivatives and chalcone derivatives led to products (4a-4i).
Chemoselective reduction of chromone was observed in good
yield without disturbing the carbonyl group (4g). The
hydrogenation of two double bonds led to ketone 4h in 98%
yield using an excess of glucose. Pleasingly, a 2-oxindole
derivative and progesterone with trisubstituted double bond were
reduced selectively to 4k and 4j. Afterwards, we explored the
scope of the reaction with terminal alkenes under the developed
reaction conditions. The corresponding ethyl arenes 4l and 4m
were isolated with 85-93% yields. Having described the
reduction of alkenes, we examined various carbonyl containing
compounds under standard reaction conditions (Figure 2c). The
desired products were observed for the reduction of
benzaldehyde derivatives to their corresponding benzyl alcohols
(6a, 6b). The aliphatic 2-(benzyloxy)acetaldehyde was
selectively reduced to alcohol (6c) in 75% yield without cleavage
of the benzyl group. A cinnamaldehyde derivative underwent a
cascade reduction leading to the formation of 6d in 88% yield.
Finally, the keto group in isatin (5e) can be selectively reduced
to give 3-hydroxyoxindole (6e) with 85% yield.
Afterwards, the rhodium complex is coordinated with the acyclic
form of the monosaccharide via the carbonyl group. In the next
step, a rhodium hydride complex is formed via oxidation of the
aldehyde of the carbohydrate. This complex is coordinated to
the substrate to form intermediate 7. The following transfer of
hydrogen from rhodium to the substrate gave complex 8.
Protonation of complex 8 by acetic acid led to the product of
hydrogenation and regeneration of the active catalyst. Using
DMF or DMA as solvent, we obtained cis-isomers as major
products. This selectivity was probably raised by the
coordination of the solvent to the rhodium in complex 8. The
introduction of bulkier ligands such as DMF or DMA instead of
acetate induced cis-configuration of the alkenes. The transfer
hydrogenation led to oxidation of glucose to gluconic acid, which
is an important commercial product with wide application.
In conclusion, we have developed
a
catalytic transfer
hydrogenation using biomass derived carbohydrates as reagent.
Stereoselective and chemoselective hydrogenation of alkynes,
alkenes and carbonyl compounds was demonstrated. Various
functional groups were tolerated and hydrogenated products
were obtained with up to 99% yield and notable stereoselectivity.
The synthesis of stereoisomers by variation of reaction
conditions was demonstrated. Developed method allows the
application of various renewable carbohydrates and lignin in the
hydrogenation. Our discovery provides an operationally simple
method for transfer hydrogenation.
Acknowledgements
Having established the substrate scope of the transfer
hydrogenation reaction, we conducted a series of control
experiments to gain more mechanistic details. When the
reaction was carried out without catalyst or glucose, we did not
observe any desired product formation. When we carried out the
reaction with δ-gluconolactone under standard reaction
conditions, we observed trace amounts of the desired product
(see Supporting information for the details). It is clear, that the
reaction undergoes through abstraction of hydrogen from the
carbonyl group of monosaccharides. Afterwards, we performed
the transfer hydrogenation of 1a using DMF as solvent. Again, in
absence of catalyst or glucose, product formation was not
detected. This indicates that rhodium is essential for the transfer
hydrogenation. Furthermore, DMF cannot be utilized as source
of hydrogen. Afterwards, we performed the hydrogenation using
hydrogen, formic acid and iso-propanol instead of glucose.
Using hydrogen, we obtained 2a in 55% yield with an E/Z ratio of
2:1. Transfer hydrogenation using formic acid gave 2a in 74%
with E/Z ratio 4:1. The highest yield of 80% of 2a was observed
using iso-propanol as hydrogen source. However, the
stereoselectivity of the reaction was lower (E/Z ratio is 2:1) and
the formation of over-reduced product (1,2-diphenylethane) was
detected. This indicates that the developed transfer
hydrogenation conditions using glucose as a renewable reagent
are superior to commonly used conditions for transfer
hydrogenation.
This work was supported by the Max-Planck-Gesellschaft. A.P.A.
acknowledges the support of the Ministry of Education and
Science of the Russian Federation (agreement number
02.a03.21.0008). We thank Dr. S. Brand for the preparation of
potato starch
Keywords: Biomass • Hydrogenation• Green chemistry •
Carbohydrates • Rhodium
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Based on preliminary investigations and control experiments, a
plausible mechanism has been proposed in Figure 2d. Initially,
active cationic Cp*Rh(OAc)2 is formed by ligand exchange.
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