A simple method for the reduction of carboxylic acids to aldehydes or alcohols using H2 and Pd/C

Full text of DOI:10.1021/jo991112f

8962
J . Org. Chem. 1999, 64, 8962-8964
and their derivatives using hydrogen and a catalyst.10
A Sim p le Meth od for th e Red u ction of
For example, in a Rosenmund type reduction a couple of
the major problems are the stereochemical lability of the
acyl chlorides and the difficulties in their preparation in
the presence of acid labile groups or protections.11
Following our interest in the use of [1,3,5]triazine
derivatives in organic synthesis,12 we discovered that the
activated ester of N-Boc amino acids with 2-chloro-4,6-
dimethoxy[1,3,5]triazine (1) can be reduced into the
corresponding aldehydes in good yields with H2 on
catalytic Pd/C.
Treating a carboxylic acid 3 with the complex formed
by 2-chloro-4,6-dimethoxy[1,3,5]triazine (1) and N-meth-
ylmorpholine in DME (adduct 2 in Scheme 1), the
corresponding activated ester 4 is quantitatively formed.13
A solution of this ester can be directly treated with H2 (1
atm pressure) at room temperature in the presence of
Pd catalyst (Pd/C 10%) to give the corresponding alde-
hyde 5. After filtration of the catalyst and acidic work
up to separate the [1,3,5]triazine byproducts, the alde-
hyde can be recovered practically pure by evaporation of
the solvent or purified by standard methods. Conse-
quently, aldehydes are obtained from carboxylic acids
using a catalytic process and friendly reaction conditions.
This process, although simple, will require optimization
of the solvent, reaction time, and H2 pressure.
Ca r boxylic Acid s to Ald eh yd es or Alcoh ols
Usin g H2 a n d P d /C
Massimo Falorni,† Giampaolo Giacomelli,
Andrea Porcheddu, and Maurizio Taddei*
Dipartimento di Chimica, Universita` di Sassari,
ViaVienna 2 07100 Sassari Italy
Received J uly 12, 1999
Aldehydes are versatile compounds in organic synthe-
sis. Despite their intrinsic benefits, there are relatively
few methods for their preparation.1 A common approach
to obtain aldehydes is in fact the oxidation of primary
alcohols2 or the reduction of carboxylic acids and their
derivatives.3 This last transformation is particularly
useful for the preparation of N-protected R-amino alde-
hydes4 that are valuable intermediates for the synthesis
of biologically active compounds.5 Several methods em-
ployed for the preparation of protected amino aldehydes
make use of complex metal hydrides as the reducing
agents and esters or amides as the starting material (for
example DIBAL-H on methyl esters6 and LiAlH4 on
particularly reactive amides7).
All these approaches suffer from several disadvan-
tages: metal hydrides are generally highly reactive or
expensive, some problems may occur during the separa-
tion of the aldehyde from the reaction mixture after the
quench of the metal hydride, and the reaction conditions
can produce sometimes racemization of the formed al-
dehyde. Consequently, a practicable alternative for the
large-scale preparation of amino aldehydes is the reduc-
tion of the carboxylic acid to alcohol followed by reoxi-
dation of the alcohol to aldehyde.8
The best results for conversion and isolated yields of
the aldehyde were obtained using DME or THF in the
formation of the activated ester and EtOH in the reduc-
tive step.
On the other hand, when the reaction was carried out
for longer times, the alcohol obtained from over-reduction
of the aldehyde was the main component of the reaction
mixture.
For example, in the case of the activated ester formed
from acid 3i, after 3 h of reduction at room temperature,
we observed the formation of the aldehyde 5i with a
conversion of about 85%. On waiting one additional hour
for the complete disappearance of the activated ester, we
observed the formation of the corresponding alcohol 6i
that, after additional 4 h of reduction, became the
predominant product. Nevertheless, as alcohol does not
react with 4 in the absence of a proper activation,14 we
did not observe the formation of the symmetric ester as
often happens in Rosenmund type reductions.15
Except for the classical Rosenmund catalytic hydroge-
nation of acyl chlorides and some related reactions,9 few
reports describe the direct reduction of carboxylic acid
* Corresponding author. E-mail: mtad@ssmain.uniss.it.
† Deceased March 1999.
(1) Larock, R. C. Comprehensive Organic Transformation; VCH
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64, 1972. Salvino, J . M.; Mervic, M.; Mason, H. J .; Kiesov, T.; Teager,
D.; Airey, J .; Labaudiniere, R. J . Org. Chem. 1999, 64, 1823. Wrodnigg,
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10.1021/jo991112f CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/04/1999

Notes
J . Org. Chem., Vol. 64, No. 24, 1999 8963
Sch em e 1
This methodology was applied to aliphatic acids and
N-Boc-amino acids. When the reaction was carried out
on aromatic carboxylic acids (3e,f), the main reaction
product was the alcohol, even at low temperature and
H2 pressure. The use of a poisoned Pd catalyst did not
improve the yields of the aldehyde.16 Nevertheless, as the
intermediate ester 4 is not consumed by the alcohol
formed during the reaction, we were able to isolate a
limited amount of the aldehyde at the end of the reaction.
The formation of the alcohol may happen also during
the reduction of N-Boc amino acids. We observed that
even in the case of similar substrates, different reaction
times were needed for a satisfactory conversion to alde-
hydes. For example, the transformation of N-Boc-valine
(3l) occurred after 3 h of reduction at room temperature,
whereas N-Boc-leucine (3i) could be successfully reduced
to aldehyde 5i exclusively at 0 °C for 2 h. For this reason
we suggest trying this procedure first on small amounts
of the substrate to find the correct conditions before
attempting the reduction of the whole material of inter-
est. We also recommend use of a low pressure of H2 (using
an H2 buret or a balloon) and eventually carrying out the
reaction at 0 °C to prevent the formation of the alcohol.
On the other hand, if the alcohol is desired, the
reduction under 5 atm of H2 gives quantitative yields of
the products.
Significant racemization of R-amino aldehydes did not
occur under these conditions, as revealed by the optical
rotation values of the products and their derivatives
(semicarbazones and dinitrophenyl hydrazones; see the
Supporting Information).
We think that the method described in this note can
be very useful for the preparation of aldehydes (and
alcohols) in large scale, as it does not use anhydrous
solvents, strong reaction conditions, or dangerous or
expensive reagents. The main drawback (in the case of
the aldehyde) is the necessity to monitor the reaction to
prevent the formation of the alcohol, but we are currently
trying to improve the conditions to avoid this trouble also.
Exp er im en ta l Section
All the solvents and the reagents (including Pd/C) were used
in the commercially available grade purity. The protected amino
acids were purchased and their purity was established before
utilization by melting point and optical rotation. Although
2-chloro-4,6-dimethoxy[1,3,5]triazine is commercially available,
we prepared it following a published procedure.17
Sch em e 2
Gen er a l P r oced u r e for th e Red u ction of Ca r boxylic
Acid s to Ald eh yd es. (S)-2-(ter t-Bu toxyca r bon yla m in o)-
p r op a n a l (5h ). To a solution of 2-chloro-4,6-dimethoxy[1,3,5]-
triazine (1) (0.93 g, 5.3 mmol) in DME (30 mL) cooled to 0 °C
was added N-methylmorpholine (0.56 g, 5.4 mmol). A white
precipitate was immediately formed and to this mixture a
solution of (S) N-Boc-alanine 3h (1.0 g,5.3 mmol), dissolved in
DME (10 mL), was slowly added. After stirring at 0 °C for 3 h,
the solid formed was filtered off on Celite and the solution
containing the activated ester transferred into a flask containing
Pd/C 10% (0.10 g, 0.1 mmol of active Pd) dispersed in EtOH (20
mL). The flask was connected with a buret containing H2. After
stirring at room temperature for 3 h (or until TLC analysis
showed the disappearance of the activated ester spot (Rf 0.6 with
eluent: AcOEt/hexane 4/6) the catalyst was filtered on Celite
and the solvent was evaporated and replaced with ethyl acetate
(50 mL). The solution was washed three times with 1 M HCl
(15 mL each) and three times with 5% NaHCO3 (15 mL each).
The formation of the alcohol was also observed upon
increasing the H2 pressure. When the reaction was
carried out in a Parr apparatus (3-5 atm of H2, room
temperature) on compounds 3f,j-i, the corresponding
alcohols 6f,j-i were obtained in high yield (Scheme 2).
The direct conversion of a carboxylic acid into the
corresponding alcohol by catalytic hydrogenation was
thus achieved.
(15) Rosenmund, K. W. Chem. Ber. 1918, 51, 585. For a critical
discussion on Rosenmund reduction, see: Mossettig, E.; Mozingo, R.
Org. Reactions 1948, 4, 362.
(16) We tried only Pd-BaSO4 5% and not Pd-C quinoline-S catalyst
as described: Rachlin, A. I.; Gurien, H.; Wagner, D. P. Org. Synth.
1971, 51, 8.

8964 J . Org. Chem., Vol. 64, No. 24, 1999
Notes
The ethereal layer was separated and dried over anhydrous Na2-
SO4 to give, after evaporation of the solvent, product 5h (0.80
g, 79% yield) practically pure for any further transformation of
the aldehyde group. An analytical sample was purified by
column chromatography on silica gel.
The identity of the product was determined by comparison of
1H NMR data, [R] values, and semicarbazone melting point with
those previously described in the literature.18
1H NMR (300 MHz, CDCl3) δ 9.53 (s-like, 1H), 5.05 (bs, 1H),
4.2 (m, 6 components visible, 1H), 1.42 (s, 9H), 1.32 (d, J ) 7
Hz, 3H). [R]D ) -25.0 (c ) 1, MeOH); lit.18 [R]D ) -25.52 (c )
1, MeOH). Semicarbazone: [R]D ) -24.3 (c ) 1, MeOH); lit.18
[R]D ) -23.82 (c ) 1, MeOH). Anal. Calcd for C9H18N4O3
(250.17): C, 46.95; H, 7.87; N, 24.34. Found: C, 46.77; H, 7.88;
N, 24.30.
Gen er a l P r oced u r e for th e Ca ta lytic Red u ction of Ca r -
b oxylic Acid s t o Alcoh ols. (S)-2-(ter t-Bu t oxyca r b on yl-
a m in o)-3-p h en yl-1-p r op a n ol (6j). A solution of the activated
ester 3j prepared as previously described was transferred into
a flask containing Pd/C 10% (0.10 g, 0.1 mmol of active Pd)
dispersed into DME (5 mL) and the flask connected with a Parr
apparatus filled with H2. The hydrogenation was carried at 5
atm of H2 for 3 h, then the flask was purged with N2 and the
catalyst was filtered off on Celite. The solvent was evaporated
and product 6j was isolated by column chromatography on silica
gel (eluent AcOEt/ hexane 1/1) to yield 1.2 g (85% yield).
The identity of the product was determined by comparison of
melting point, 1H NMR data and the [R] values with those
previously described in the literature.19,20
6j: mp 94-95 °C; lit.19 mp 94-96 °C; [R]D ) -20 (c ) 1,
MeOH); lit.20 [R]D ) -21.6. 1H NMR (300 MHz, CDCl3) δ 7.2
(m, 5H), 5.0 (bs, 1H), 4.4 (m, 4 components visible, 1H), 4.2 (m,
2H), 2.2 (m, 2H), 2.0 (bs, 1H), 1.45 (s, 9H).
Ack n ow led gm en t. This work was financially sup-
ported by MURST (Rome) within the project “Chimica
dei Composti Organici di Interesse Biologico Es Fin
1997”.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR data for
compounds 5d ,g-n and 6f,j-i; [R] values for compounds
5g-m and 6h -k ; and characterization of semicarbazones and
2,4 dinitrophenylhyrdrazones of all the aldehydes. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
(17) Cronin, J . S.; Ginah, F. O.; Murray, A. R.; Copp, J . D.Synth.
Commun. 1996, 26, 3491.
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46, 4797. Hamada, Y.; Shioiri, T.; Chem. Pharm. Bull 1982, 30, 1921.
Fehrentz, J . A.; Pothion, C.; Califano, J . C.; Coffet, A.; Martinez, J .
Tetrahedron Lett. 1995, 35, 9031. Zlatoidsky, P. Helv. Chim. Acta 1994,
77, 545. Zlatoidsky P. Tetrahedron Lett. 1995, 36, 7281
J O991112F
(19) Rubini, E.; Gilon, C.; Selinger, Z.; Choreu, M. Tetrahedron 1986,
42, 6032.
(20) Kokotos, G. Synthesis 1990, 299.