Direct Amidation of Aldehydes with Primary Amines under Mild Conditions Catalyzed by Diolefin-Amine-RhI Complexes

Full text of DOI:10.1002/cctc.201200495

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DOI: 10.1002/cctc.201200495
Direct Amidation of Aldehydes with Primary Amines under
Mild Conditions Catalyzed by Diolefin-Amine–RhI
Complexes
Rafael E. Rodrꢀguez-Lugo, Mꢁnica Trincado,* and Hansjçrg Grꢂtzmacher*[a]
The development of methods for the simple and eco-
nomical syntheses of key functional groups is of fun-
damental importance. Aldehydes play an important
role as synthetic precursors because they are easily
obtained through the carbonylation reactions of hy-
drocarbons.[1,2] Despite the importance of amido
compounds, there are relatively few practical meth-
ods available for their efficient and environmentally
benign direct synthesis.[3–7] Although the catalyzed
oxidative amidation of aldehydes has been investi-
gated for several decades, major challenges still
remain, including: 1) undesired imine formation
when primary amines are used in the reactions with
aldehydes; 2) most of the currently used methods
employ strongly oxidizing agents that lead to low
chemoselectivities; and 3) these methods have limit-
ed functional-group tolerance. Few methods allow
the conversion of aliphatic aldehydes and their corre-
sponding amides are obtained in low-to-moderate
yields.[5,6c,k,o,q] Only recently, Chan and co-workers re-
ported a metal-promoted amidation reaction with
Scheme 1. Amidation of aldehydes promoted by RhI–amido complex 2 as the active cat-
alyst. Air-stable amine complex 1 serves as a catalyst precursor and is in equilibrium with
iminoiodinanes that tolerated a broad range of ali-
phatic aldehydes.[6l]
complex 2 in the presence of base.
In a previous work, we reported that the RhI com-
plex of the ligand bis(5-H-dibenzo[a,d]cyclohepten-5-
yl)-amine (trop2NH), complex 1, is an easily accessible and stor-
able precursor complex of amido complex 2 upon the addition
of base (Scheme 1). This latter complex is a highly efficient cat-
alyst for hydrogenation reactions[8] and, especially, for dehydro-
genative coupling reactions (DHCs) in the presence of a hydro-
gen acceptor (A).[9] In these reactions, a primary alcohol is cou-
pled with either water, MeOH, or primary amines to afford car-
boxylic-acid derivatives. Computations have shown that these
reactions proceed step-wise according to Equations (1) and
(2).[9a,b]
RCH¼O þ HX þ A ! RCOðXÞ þ AH2
ðX ¼ OH, OMe, NHR0Þ
ð2Þ
Both reactions are promoted by 1,2-addition reactions
across the polar RhꢀN bond and follow the established princi-
ples of cooperating metal–ligand catalysis.[10] Computational
analysis indicated that the DHC step (2) is a fast metal-cata-
lyzed reaction, as shown in Scheme 1.
The addition of water (X=OH)[11] or an amine (X=NHR’)
across the RꢀN bond in amide 2 leads to compound 3, which
binds the aldehyde through a NꢀH···O hydrogen bond in inter-
mediate I. Intramolecular attack of the nucleophile (X) on the
activated carbonyl group leads to a hemiacetal/aminal com-
plex (II), which is dehydrogenated to afford the carboxylic-acid
derivative and hydride complex 4. According to theoretical cal-
culations, this reaction sequence is almost barrier-less.[9b] The
hydride complex (4) is converted into amide complex 2 by re-
action with hydrogen acceptor A, thereby closing the catalytic
cycle. Consequently, complex 2 should be a very efficient cata-
lyst for the direct synthesis of amides from aldehydes and pri-
mary amines under mild and oxidant-free conditions. Herein,
we report the conditions under which this goal can be ach-
RꢀCH2ꢀOH þ A ! RCH¼O þ AH2
ð1Þ
[a] R. E. Rodrꢀguez-Lugo, Dr. M. Trincado, Prof. Dr. H. Grꢁtzmacher
Department of Chemistry and Applied Biosciences
ETH-Hçnggerberg
8093 Zꢁrich (Switzerland)
Fax: (+41)44-633-1032
E-mail: hgruetzmacher@ethz.ch
trincado@inorg.chem.ethz.ch
Supporting information for this article, including full experimental de-
tails, procedures for the catalytic studies, and characterization data, is
available on the WWW under http://dx.doi.org/10.1002/cctc.201200495.
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The use of simple inorganic bases (KOH or K2CO3) yielded
a mixture of the amide, imine, benzyl alcohol (BnOH, as the
product of a transfer-hydrogenation reaction of PhCHO), and
the benzyl ester (PhCOOBn, as the product of a Tischenko-type
transformation). The reactions with NaHCO3 (Table 1, entry 4)
or triethylamine (Table 1, entry 6) as a base exclusively afforded
the imine, PhHC=NBn. Fortunately, when the reaction was per-
formed with an excess of K3PO4, the selective formation of
benzyl benzamide (7a) was observed in very high yield
(>90%, Table 1, entry 8). As a control, the reaction was per-
formed without a metal catalyst and it failed to generate the
amide product.
To shed some light on the surprising effect of K3PO4, which
was suspended in the reaction mixture, two simple experi-
ments were performed. In the first one, benzylamine and ben-
zaldehyde were reacted together in dry THF at room tempera-
ture in the absence of K3PO4 and, as expected, quantitative for-
mation of the imine PhCH=NꢀBn was observed within a few
minutes. In the second experiment, a two-fold excess of K3PO4
was added to the mixture and about 60% imine was rapidly
formed. Remarkably, no further conversion was achieved, even
after 4 h. The addition of 0.2 mol% of compound 1 and meth-
ylmethacrylate (MMA) as a hydrogen acceptor to this mixture
led to the formation of benzyl benzamide (7a) in about 30%
yield, whilst the amount of imine remained unaltered. These
experiments show that 1) K3PO4 inhibits the imine formation to
some extent and 2) it does not promote the hydrolysis of the
imine back into the aldehyde and the primary amine because
the addition of the catalyst only converts the unreacted alde-
hyde and amine but does not lead to a quantitative formation
of the amide. Finally, we confirmed that the use of isolated
RhI–amido complex 2 as the “true” catalyst showed compara-
ble selectivity and activity. Because the cationic precursor com-
plex (1) is air-stable and easy to handle, and sufficient amounts
of complex 2 are formed in the deprotonation/protonation
equilibrium under the basic reaction conditions, complex
1 was used as the catalyst precursor in all subsequent experi-
ments (Scheme 3).
Scheme 2. Amidation of alcohols or aldehydes catalyzed by amido- or
amine-diolefin–RhI complexes.
ieved. We began our investigations by studying the coupling
reaction between benzaldehyde and benzylamine. This model
reaction was tested under previously reported reaction condi-
tions (Scheme 2).[9b] In this experiment, the only observed
product was the condensation product, that is, the corre-
sponding imine, PhCH=NꢀBn.
The hemiaminal is a key intermediate from which water is
eliminated in a fast reaction to give the undesired imine.
Therefore, we anticipated that its deprotonation by a base
would stabilize this intermediate sufficiently to make its metal-
catalyzed dehydrogenation coupling to benzamide an efficient
and competitive process. Indeed, when one equivalent of po-
tassium tert-butoxide (KOtBu) is added, complex 1 promotes
the DHC reaction, thereby leading to a good ratio of amide-
7a/imine (16:1, Table 1, entry 1). Unfortunately, the reaction
does not proceed cleanly and further undesired by-products,
besides the imine, are formed, including benzyl alcohol (BnOH)
and benzyl benzoate (PhCOOBn), both of which are detected
in significant amounts. Based on this observation, we tested
the effect of different bases on the activity and selectivity of
the catalytic system (Table 1).
Table 1. Screen of various bases in the catalyzed amidation of benzalde-
hyde with [Rh(OTf)(trop2NH)(PPh3)] (1) as the catalyst precursor.[a]
Under the optimized reaction conditions, a screen of a wide
range of primary amines and aldehydes (Table 2 and 3) was
performed to explore the scope and limitations of this catalytic
system. In general, the DHC reaction between benzaldehyde
and a range of differently functionalized primary amines af-
forded their corresponding amides in good-to-excellent yields
(Table 2). The reactions proceeded cleanly, with the exception
of the isopropylamine, where benzyl benzoate was obtained
Entry Base (equiv) Amide[b] [%] Imine[b] [%] BnOH[b] [%] PhCO2Bn[b] [%]
1
2
3
4
5[c]
6
7
8
9[d]
KOtBu (1.1) 15
–
5
–
23
–
95
–
13
–
–
61
93
–
–
18
–
–
–
–
KOH
2
4
K2CO3 (1.1)
NaHCO3 (1.1)
K2CO3 (1.1)
NEt3 (1.1)
K2CO3 (2.1)
K3PO4 (2.1)
K3PO4 (2.1)
–
–
–
>99
68
>99
1
5
65
12
95
–
–
–
[a] Reaction conditions: Benzaldehyde (0.65 mmol, 1 equiv), benzylamine
(1 mmol, 1.5 equiv), MMA (2 mmol, 3 equiv), compound 1 (0.2 mol%),
THF, RT, 3 h. [b] Yield determined by gas chromatography/flame-ioniza-
tion detector, with n-decane as an internal standard. [c] Addition of water
(10 equiv). [d] Reaction in the absence of compound 1.
Scheme 3. Optimized conditions for the RhI-triggered direct amidation of
benzaldehyde.
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Table 2. Catalyzed amidation of benzaldehyde with primary amines by
using [Rh(OTf)(trop2NH)(PPh3)] (1) as the catalyst precursor.[a]
Table 3. Catalyzed amidation of various aldehydes with benzylamine by
using [Rh(OTf)(trop2NH)(PPh3)] (1) as the catalyst precursor.[a]
Entry
1
Product
T [8C]
t [h]
Yield[a] [%]
95 (86)
Entry
1
Product
Yield[b] [%]
99 (98)[c]
RT
2
2
3
4
ꢀ10
ꢀ10
RT
2
3
4
96 (70)
77 (64)[b]
99 (94)
2
3
4
5
78 (62)[d]
95 (60)
98 (60)
91 (74)
5
6
7
RT
0
3
4
6
99 (96)
92 (88)
85 (77)[c]
6
7
98 (80)
0
75 (75)[e]
[a] Yield determined by GC-FID with n-decane as an internal standard;
yield of the isolated analytically pure compound is given in parentheses.
[b] Benzyl benzoate is isolated in 18% yield. [c] An excess of refluxing am-
monia was used.
8
9
70 (65)[f]
71 (60)
as a by-product in about 20% yield (Table 2, entry 3). The in-
creased steric bulk of the amine perceptibly slowed down the
DHC reaction to an extent that the condensation reaction
started to compete kinetically. Notably, for a number of amines
(Table 2, entries 2, 3, 6, and 7), the reaction proceeded at tem-
peratures of 08C or below and the catalytic system remained
efficient. Under these conditions, the yield of the desired prod-
uct was improved whereas the formation of the imine consid-
erably decreased. When enantiomerically pure phenetylamine,
7b, was employed as the substrate, the reaction proceeded
without any loss of the chiral information (Table 2, entry 2).
The direct amidation of benzaldehyde with ammonia (NH3) to
give benzamide 7g was also performed successfully (Table 2,
entry 7).
[a] The reactions were performed at 08C for 3–6 h (for details, see the
Supporting Information). [b] Yield determined by GC-FID with n-decane
as an internal standard; yield of the isolated analytically pure compound
is given in parentheses. [c] The corresponding imine and octyl octanoate
were obtained as side-products. [d] The corresponding imine was formed
as the only side-product. [e] Of the remaining products, 14% correspond-
ed to the imine, 9% corresponded to amide 7i, and 2% was cinnamyl al-
cohol and hydrocinnamyl alcohol. [f] Amides 7i and 7n were formed in
15% and 10%, respectively.
corresponding unsaturated amides (7n and 7o, Table 3, en-
tries 7 and 8, respectively). The amidation of 4-methylthioben-
zaldehyde affords amide 7l with >90% conversion (isolated in
74% yield) and we have no indication that the thioether
moiety has a negative effect on the reaction, that is, the reac-
tion rate is not affected and the thioether group is not oxi-
dized. The catalyst also tolerates a highly reactive polychlori-
nated substrate and, in the reaction with chloral (Cl3CCHO),
amide 7m, in which one of the chlorine atoms has been re-
In a second set of experiments, aldehydes that contained
various functional groups were reacted with compound 6a as
the amine component (Table 3). Aliphatic aldehydes (Table 3,
entries 1, 2, and 6) are transformed into their corresponding
amides in good-to-excellent yields. The functional-group toler-
ance of this procedure is quite remarkable; for instance, a,b-
unsaturated aldehydes are conveniently transformed into their
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placed by a hydrogen atom, is obtained in quantitative yield
(Table 3, entry 6). Lastly, the reaction between pyridine-3-car-
baldehyde and tryptamine yields reserpine analogue 7p
(Table 3, entry 9) in good yield, which demonstrates that nitro-
gen-donor centers in the aldehyde and amine moieties are tol-
erated without deactivation of the catalyst.
Indeed, the DHC between aldehydes 5a/5h and amines 6a/
6c is not significantly hampered by a hydrogen atmosphere
and it gives a library of all four amides, 7a/7c/7h/7q
(Scheme 4, black bars) in approximately the same total yield as
in the reaction under an inert atmosphere (Scheme 4, gray
bars). Interestingly, the relative distribution of the amides is
strongly influenced by the presence or absence of H2. Whilst in
an inert atmosphere, amide 7c (from benzaldehyde and the
aliphatic isopropylamine, 6c) is the main product (about 50%),
under a hydrogen atmosphere, the coupling product between
benzaldehyde and benzylamine (6a) becomes the main com-
ponent (almost 60%) in the mixture. The coupling products
between aliphatic aldehyde 5h and both amines (6a and 6c)
are only influenced a little by the reaction atmosphere and
amides 7h and 7q are formed in about equal ratios and signif-
icantly smaller overall amounts. Notably, amides 7a/7c/7h/7q
are not cleaved under a hydrogen atmosphere[12] and, under
our reaction conditions, the coupling reaction between an al-
dehyde and an amine is irreversible.
The dehydrogenative coupling reaction for the formation of
amides formally corresponds to an oxidation reaction when
using classical chemical terminology. Therefore, it was of spe-
cial interest to see whether the DHC reactions reported herein,
that is, the “oxidation” of aldehydes into their corresponding
amides, would also proceed under reducing conditions. Thus,
we investigated the amidation of a mixture of two different al-
dehydes, 5a and 5h, and two different amines, 6a and 6c,
under an atmosphere of argon or hydrogen (Scheme 4). In
In conclusion, this work describes a very simple and conven-
ient procedure for a dehydrogenative coupling reaction be-
tween various functionalized aldehydes and primary amines to
give their corresponding amides. In the presence of heteroge-
neously suspended K3PO4 as a base and methylmethacrylate
(MMA) as hydrogen acceptor, the reaction can be simply per-
formed as a one-pot procedure in which all of the reactants
and the catalyst are mixed in one batch in the reaction vessel.
The presence of K3PO4 hinders the rapid formation of imines,
which would otherwise become the predominant reaction
products. Imines are exclusively formed in the absence of the
catalyst and phosphate. Once formed, they are not hydrogen-
ated under the reaction conditions and remain unchanged.
The exact role of K3PO4 remains unknown at present. The DHC
reaction proceeds under remarkably mild conditions and the
active catalyst, a diolefin-amide–RhI complex, can be applied in
low concentrations (0.2 mol%). We assume that the metal
complex catalyzes both the formation of the hemiaminal and
its rapid dehydrogenation into the amide, whilst the non-cata-
lyzed reaction leads to imine formation. The activity of the cat-
alytic system is not significantly changed when the reaction is
performed under an atmosphere of hydrogen, which allows
the formal “oxidation” of aldehydes into amides, even under
reducing conditions. These reactions can be performed with
a mixture of different aldehydes and amines, which, in princi-
ple, allows the simple synthesis of libraries of amides. The se-
lectivity in these multicomponent DHC reactions is altered by
the presence or absence of hydrogen.
Scheme 4. Comparative parallel synthesis of amides catalyzed by diolefin-
amine–RhI complexes in the presence/absence of H2.
a previous work,[8] we demonstrated that: 1) amide [Rh(trop2N)-
(PPh3)] (2) reacts reversibly with H2 to afford its corresponding
amine–hydride, [Rh(H)(trop2NH)(PPh3)] and 2) a propionic-acid-
methyl-ester derivative, that is, the hydrogenated form of the
hydrogen acceptor (MMA, H2C=CMeCOOMe) used herein, is
dehydrogenated in a stoichiometric reaction.[9a] Consequently,
the active catalyst (2) may always be present in the set of pos-
sible hydrogenation and dehydrogenation equilibria, Equa-
tions (3)–(6).
½Rhðtrop2NÞðPPh3Þꢁ þ H2 Ð ½RhðHÞðtrop2NHÞðPPh3Þꢁ
ð3Þ
ð4Þ
Acknowledgements
½RhðHÞðtrop2NHÞðPPh3Þꢁ þ H2C¼CMeCOOMe Ð
½Rhðtrop2NÞðPPh3Þꢁ þ MeꢀCHMe-COOMe
This work was supported by the SNF, the DFG through the project
“Unconventional Approaches to the Activation of Dihydrogen”
(FOR1175), and by the ETH Zꢁrich.
½RhðHÞðtrop2NHÞðPPh3ÞꢁþRCH¼O Ð
½Rhðtrop2NÞðPPh3Þꢁ þ RꢀCH2ꢀOH
½Rhðtrop2NÞðPPh3Þꢁ þ H2NR0 Ð ½RhðNHR0Þðtrop2NHÞðPPh3Þꢁ ð6Þ
ð5Þ
Keywords: aldehydes · amidation · amines · olefins · rhodium
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S. S. Sohn, J. Am. Chem. Soc. 2007, 129, 13798.
[6] The metal-catalyzed conversion of aldehydes into amides; for nickel
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[11] A hydroxo–rhodium complex has been previously isolated and charac-
terized, see: S. P. Annen, V. Bambagioni, M. Bevilacqua, J. Filippi, A.
Marchionni, W. Oberhauser, H. Schoenberg, F. Vizza, C. Bianchini, H.
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[12] E. Balaraman, B. Gnanaprakasam, L. J. W. Shimon, D. Milstein, J. Am.
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Received: July 23, 2012
Published online on October 12, 2012
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