A general and efficient aldehyde decarbonylation reaction by using a palladium catalyst

Article abstract of DOI:10.1039/c2cc31144e

A facile decarbonylation reaction of aldehydes has been developed by employing Pd(OAc)₂. A wide variety of substrates are decarbonylated, without using any exogenous ligand for palladium as well as CO-scavenger.

Full text of DOI:10.1039/c2cc31144e

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COMMUNICATION
A general and efficient aldehyde decarbonylation reaction by using a
palladium catalystw
Atanu Modak, Arghya Deb, Tuhin Patra, Sujoy Rana, Soham Maity and Debabrata Maiti*
Received 15th February 2012, Accepted 23rd February 2012
DOI: 10.1039/c2cc31144e
A facile decarbonylation reaction of aldehydes has been developed
by employing Pd(OAc)2. A wide variety of substrates are
decarbonylated, without using any exogenous ligand for palladium
as well as CO-scavenger.
Scheme 1 Palladium catalysed decarbonylation of aldehydes.
Methods for removal of functional groups from organic
are successfully decarbonylated, without using any exogenous
synthesis of natural products.1,2 In this context, metal mediated
ligand for palladium as well as CO-scavenger (Scheme 1).
molecules are immensely important in chemistry, including
In order to realize a Pd-catalysed decarbonylation reaction,
decarbonylation of aldehydes has drawn considerable attention
over the decades3–5 since such processes enable provisional use of
we have first studied 2-naphthaldehyde as substrate by varying
palladium source and solvent.36 It has been observed that a
the beneficial features of the –CHO functionality such as regio-
selectivity in Diels–Alder reaction6 and domino oxa-Michael-aldol
catalytic amount of Pd(OAc)2 in cyclohexane or dichloroethane
reaction.7 An aldehyde can be utilized as an in situ CO source for
medium in the presence of molecular sieves (MS) in air (or under
a N2 atmosphere) can provide the best yield of the decarbonylated
a desired carbonylation reaction and, consequently, an external
product.36 Note that a variety of solvents including N-methyl-2-
pyrrolidone, DMF, dimethylaniline, butyronitrile and decalin can
also bring about the desired decarbonylation reaction in good
yield. Although we have carried out majority of our reactions in
cyclohexane as the solvent, these high boiling solvents may be
used if needed. Interestingly, use of an exogenous ligand decreases
the yield of the decarbonylated product notably.36 Further no
CO-scavenger is required for the efficient catalysis. Inspired
by these initial results, we have explored the scope of this
decarbonylation reaction with various aldehydes.
source of hazardous CO gas can be avoided.8,9 Most importantly,
recent reports unambiguously established that alka(e)nes are
biosynthesized via decarbonylation of fatty aldehydes.10,11 Hence,
an effective and versatile synthetic decarbonylation process can
be beneficial for the generation of fuel grade alkanes and should
be an attractive alternative to existing expensive hydrogenation
methodologies.10
Various metal-based systems have been investigated successfully
for decarbonylation reactions including those with Ru,12 Ir,13
Rh14–19 and Pd20–26 at 180–280 1C with limited substrate scope.
However, very few of the existing decarbonylation catalysts are
easily available and almost all of them are costly. Often, the
decarbonylations are done either under high temperature or with
a chemical scavenger to remove the evolved CO.27 In total
synthesis, a stoichiometric amount of Rh-catalyst or long
reaction time is often employed for crucial decarbonylation of
aldehyde functionalities.28–35 To the best of our knowledge, only
two examples of heterocyclic aldehydes have been reported till
date.13,20 It would, therefore, be highly desirable to develop an
efficient catalytic process for decarbonylation of aldehydes, which
can overcome the aforesaid drawbacks of the prior art processes.
In this context the present report demonstrates a simple and
efficient method for practical decarbonylation reaction by using
palladium acetate as the precatalyst. A wide variety of aldehydes
The non-functional fused-ring aryl aldehydes are decarbonylated
efficiently with 3–8 mol% catalyst loading (Table 1, entries 1a–1e).
Naphthaldehydes (entries 1a and 1b), 9-anthraldehyde (entry 1c),
9-phenanthrenealdehyde (entry 1d) and pyrene-1-aldehyde
(entry 1e) have all resulted in decarbonylated products in
excellent yields.37 Desired mesitylene was also obtained from
sterically demanding ortho-disubstituted aryl aldehyde (entry 1g).
It should be noted that previous attempts were unsuccessful to
form naphthalene from 2-naphthaldehyde.20,26 Additionally,
it is found that with a reflux condenser a 5 mmol scale reaction
of naphthaldehyde can be successfully carried out.36
We have also investigated the decarbonylation reactions of
benzaldehydes containing a wide variety of functional groups
(Table 2). Nitro-substituted analogues construct the desired
products in good yields (entries 2a and 2j). The current
method may thus find application in various nitro-containing
natural products and in drugs synthesis. It may be noted that the
recent report of rhodium14 catalysed decarbonylation reactions
of 4-nitrobenzaldehyde produced nitrobenzene in 12% yield
only. Tolerance of cyano group (entries 2b and 2k) may extend
Department of Chemistry, Indian Institute of Technology Bombay,
Powai, Mumbai 400076, India. E-mail: dmaiti@chem.iitb.ac.in
w Electronic supplementary information (ESI) available: Detailed
description of experimental procedures, characterization of all compounds.
See DOI: 10.1039/c2cc31144e
c
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Chem. Commun., 2012, 48, 4253–4255 4253

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Table 1 Pd-catalysed decarbonylation of non-functional aryl aldehydesa
base sensitive functional groups including ester, keto and
acid are also successful (entries 2f–2h) to generate the corre-
sponding decarbonylated products. It has been found that
3-vinylbenzaldehyde undergoes decarbonylation reaction to
produce styrene in 79% yield (entry 2l). Such an observation is
interesting from the perspective of Heck reaction. One can run the
decarbonylation reaction in a high boiling solvent such as in
decalin, since similar yields of the desired decarbonylated products
are obtained with both cyclohexane and decalin (entries 2g and 2j).
As mentioned before, only two discrete heterocyclic examples
are known in the literature.13,20 Various five and six membered
rings with single heteroatom afforded good to excellent yields of
the desired products, even with unprotected nitrogen hetero-
cycles (Table 3, entries 3e and 3j). In spite of the acidic nature of
a-H in five membered heterocycles (entries 3a, 3e and 3l), the
desired decarbonylated products are obtained without further
conversion. Thus an electrophile may be easily introduced in the
3-position despite its lower reactivity via simple blocking of the
reactive 2-position with a formyl group (entries 3a, 3e and 3l).
Similarly for indole and benzothiophene derivatives, irrespective
of low reactivity at the 2-position,39 easy insertion of electro-
philes will be possible by having a 3-CHO group which can be
decarbonylated effectively by the present method (entries 3j and 3k).
Chromene can be synthesized via decarbonylation as the key
step (entry 3c).7,36
a
Aldehyde (0.5 mmol), cyclohexane (1.3 mL), MS (150 mg), and
b
Pd(OAc)2 (8 mol%), 140 1C. 12 mol% Pd, 120 1C. c 3 mol% Pd, 140 1C.
d
e
f
g
12 mol% Pd, 110 1C. GC yield. 16 mol% Pd, 140 1C. 24%
recovered starting materials. 27% recovered starting materials.
h
Furthermore, heterocyclic aldehydes with two heteroatoms
produced the desired decarbonylated products in good yields
(entries 3n–3p). Successful decarbonylation of pyridine carbalde-
hydes, irrespective of the positions of the –CHO (entries 3f–3h)
functionality, further reveals the generality and independency
of the current methodology.
Table 2 Pd-catalysed decarbonylation of functional aldehydesa
Table 3 Pd-catalysed decarbonylation of heterocyclic aldehydesa
a
Aldehyde (0.5 mmol), cyclohexane (1.3 mL), MS (150 mg), and
b
Pd(OAc)2 (8 mol%), 140 1C. 8 mol% Pd, 120 1C. c 10 mol% Pd, 110 1C.
d
f
g
12 mol% Pd, 110 1C. e GC yield. 5 mol% Pd, 140 1C. 12 mol% Pd,
h
100 1C. Decalin (1.3 mL) as solvent, 8 mol% Pd, 140 1C.
the scope for effective use of this group in synthetic chemistry.
Chlorobenzene can be obtained from chlorobenzaldehyde in
good yield (entry 2c). Unfortunately, only 28% bromobenzene is
obtained from 4-bromobenzaldehyde and only 5% decarbonyl-
ation occurs with 4-iodobenzaldehyde.36 Phenol is generated
from 4-hydroxybenzaldehyde (entry 2e) after successful imple-
mentation of this method. Reactions in the presence of different
a
Aldehyde (0.5 mmol), cyclohexane (1.3 mL), MS (150 mg), and
b
Pd(OAc)2 (8 mol%); GC yield; 16 mol% Pd.
c
c
4254 Chem. Commun., 2012, 48, 4253–4255
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Table 4 Pd-catalysed decarbonylation of alkanes and alkenylaldehydesa
Notes and references
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Scheme 2 Decarbonylation of phenylglyoxal hydrate.
With the tremendous challenge of minimization of b-H
eliminated and homo-coupled side products38 finally we have
concentrated on simple alkanes and alkenylaldehydes
(Table 4). Moderate to high yields of the decarbonylated
products are obtained with alkenylaldehydes (entries 4a–4f).
It is observed that b-disubstitution leads to higher yield with
the least homo coupled side product (entries 4d–4f), whereas
a-substitution leads to a lesser conversion (entry 4c). The
decarbonylated product of an alkenyl aldehyde can therefore
be used in further olefin metathesis to get a higher number of
carbon containing hydrocarbons.40 In the case of trans-a-
methylcinnamaldehyde (entry 4c), the formation of 1 : 1 cis-
and trans-product reveals cis–trans isomerization at elevated
reaction temperature. Interestingly, no trace of the b-H
eliminated side product is observed for simple alkyl aldehydes
(entries 4g–4i). It should be noted that previous reactions were
unsuccessful in decarbonylating cinnamaldehyde (entry 4a)
and octanal.26
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26 Konopka, Jessica, Green Decarbonylation of Aldehydes using
Palladium(II) Acetate, 2011. Pell Scholars and Senior Theses.
Paper 2070. http://escholar.salve.edu/pell_theses/2070.
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36 See ESIw for detailed description.
37 Although a trace of homo-coupled product (o2%) is obtained in
entry 1b, only decarbonylated products are obtained in other cases.
38 Metal-Catalyzed Cross-Coupling Reactions, ed. A. de Meijere and
F. Diederich, Wiley-VCH, Weinheim, Germany, 2nd edn, 2004.
39 J. March, Advanced Organic Chemistry, John Wiley & Sons,
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Exploring phenylglyoxal hydrate as substrate (Scheme 2), we
find that with increasing catalyst loading, aldehydes containing
a-carbonyl center lead to decarbonylated product(s) in a chain
fashion to provide benzaldehyde and benzene.
In summary, we have developed an efficient decarbonylation
reaction by using Pd(OAc)2 as the precatalyst.41 A number of
substrates are successfully decarbonylated, without using any
exogenous ligand for palladium as well as CO-scavenger.
This activity is supported by DST (R/S1/IC–24/2011) and
BRNS (2011/20/37C/13/BRNS), India.
40 N. Fontan, M. Dominguez, R. Alvarez and A. R. de Lera, Eur. J.
Org. Chem., 2011, 6704–6712.
41 A provisional patent on this work has been filed, application no.
3280/MUM/2011.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4253–4255 4255