Reductive cleavage of amides to alcohols and amines catalyzed by well-defined bimetallic molybdenum complexes

Article abstract of DOI:10.1002/chem.201202857

Triple bonds do it! The molybdenum-catalyzed Ci-N bond cleavage of organic amides with hydrosilanes to produce alcohols and amines has been investigated. This work complements previously established protocols that lead to the cleavage of the Ci-O bond. Modified triply bonded dimolybdenum(III) alkoxides have been found to be crucial for tuning the selectivity to Ci-N bond cleavage (see figure). Copyright

Full text of DOI:10.1002/chem.201202857

COMMUNICATION
DOI: 10.1002/chem.201202857
Reductive Cleavage of Amides to Alcohols and Amines Catalyzed by Well-
Defined Bimetallic Molybdenum Complexes
Sebastian Krackl, Chika I. Someya, and Stephan Enthaler*[a]
The selective reduction of carboxylic acid derivatives is
one of the most important transformations in academic and
industrial research. Such transformations afford countless
building blocks for use in, for example, the production of
pharmaceuticals, agrochemicals, or fine chemicals. Among
those carboxylic acid derivatives, the reduction of amides is
of great interest and is still a challenging task in organic
chemistry.[1] Besides reduction with metal hydrides (e.g.,
NaBH4, LiAlH4), the application of transition-metal cata-
lysts offers an efficient and versatile strategy.[1,2] Indeed, var-
amides. Applying ruthenium catalysts and molecular hydro-
gen as the reductant, excellent selectivities were
achieved.[5,6] Although the reductive cleavage of C N bonds
À
in amides applying molecular hydrogen is currently the pre-
ferred route, the application of hydrosilanes is a desirable
choice, because of the mild reaction conditions and practical
simplicity.[7] However, to date, no protocol has been realized
À
for C N bond cleavage in the presence of catalysts and hy-
À
drosilanes, and only cleavage of the C O bond has been re-
ported.[3]
À
ious catalysts are efficient for the cleavage of the C O bond
under reductive conditions to obtain the corresponding
amine (Figure 1). Currently, well-established methodologies
Recently, some of us investigated the ability of low-
valent, bimetallic dimolybdenum hexaalkoxides (Scheme 1)
as catalyst precursors in reduction chemistry applying hydro-
Figure 1. General pathways for the reduction of amides.
Scheme 1. Synthesis of dimolybdenum complexes 4 and 5.
are based on the combination of transition metals and hy-
drosilanes as the reductants.These procedures are able to re-
alize excellent yields and chemoselectivities, which under-
lines their high applicability.[3] On the other hand the reduc-
silanes as reductants. Excellent performance was observed
for the deoxygenation of sulfoxides[8] and the reduction of
tertiary amides.[9] Interestingly, in the case of the amide re-
À
À
tive cleavage of the C N bond of organic amides is more so-
duction, cleavage of the C O bond was mainly observed
and the corresponding amines were obtained in excellent
phisticated and generates primary alcohols and amines as
products, which are excellent synthons in organic chemistry
(Figure 1). It is worth noting that the procedure for a reduc-
yields. With regard to these results we wondered if it would
À
be possible to realize the C N bond cleavage with hydrosi-
À
tive cleavage of the C N bond will also be more useful for
lanes as reductants by modifying the ligand sphere of the di-
protection/deprotection chemistry or peptide chemistry.
However, only a few protocols have been described so far,
that accomplish this transformation with stoichiometric
amounts of reductants.[4] Recently, the groups of Milstein
molybdenum alkoxides. Based on this idea, we present
herein our initial results in the molybdenum-catalyzed C N
À
cleavage of tertiary amides.
Initially, pyrazoline ligands 1 and 2 were chosen as inter-
esting motifs for the modification of dimolybdenum
hexaalkoxide 3 in order to introduce greater steric bulk and
À
and Ikariya reported on the first catalytic C N cleavage of
to add Lewis basic sites to stabilize the complex (Scheme 1)
[a] Dr. S. Krackl, C. I. Someya, Dr. S. Enthaler
Technische Universitꢀt Berlin
[10]
.
In accordance with a previously established protocol, we
Department of Chemistry
were able to synthesize disubstituted complexes [{Mo(1-H)-
(OtBu)2}2] (4) and [{Mo(2-H)(OtBu)2}2] (5) in 48–75% yield
by direct protolysis of
(Scheme 1).[8,11] 1H, 19F, and
Cluster of Excellence, “Unifying Concepts in Catalysis”
Str. des 17. Juni 115/C2, 10623 Berlin (Germany)
Fax : (+49)3031429732
G
ACHTUNGTRENNUNG
3
13C NMR investigations of these diamagnetic complexes
confirmed the desired double substitution and successful co-
ordination to the triple bond (see Supporting Information).
E-mail: stephan.enthaler@tu-berlin.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202857.
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15267

Table 1. Reduction of amide 6 with complexes 4 and 5 as precatalysts
Moreover, the obtained complexes were investigated with
single-crystal X-ray diffraction analysis revealing isomor-
phous molecular structures of triply-bonded complexes with
and different silanes as reducing reagents.[a]
À
a staggered-ligand arrangement and typical Mo Mo bond
lengths of 2.260 and 2.254 ꢂ for 4 and 5, respectively
(Figure 2). Interestingly, the oxygen O2 of the acetyl or ben-
Pre-
catalyst
Silane
Solvent Select. Conv. [%] (Yield [%])
7 [%][b] 6 h[c]
24 h[c]
1
2
3
4
5
6
7
8
3
4
5
4
4
4
4
4
4
4
4
4
PhSiH3
PhSiH3
PhSiH3
Ph2SiH2
Ph3SiH
Et3SiH
toluene
toluene
toluene
toluene
toluene
toluene
<1
62
60
97
–
>99 (62)
>99 (60)
73 (59)
<1 (<1)
9 (9)
15 (15)
63 (54)
27 (<1) >99 (<1)
>99 (62)
>99 (60)
>99 (78)
5 (5)
16 (15)
32 (30)
>99 (81)
78
>99
>99
>99
>99
<1
78
Me2PhSiH toluene
(EtO)3SiH toluene
Me2ClSiH toluene
ACHTUNGTRENNUNG
9
10
11[d]
12[d]
Ph2SiH2
Ph2SiH2
Ph2SiH2
diglyme
diglyme
decaline
77 (60)
51 (32)
73 (22)
>99 (78)
48
24
>99 (68)[e]
>99 (32)[e]
[a] Reaction conditions: A solution of silane (828 mmol, 2.5 equiv) in the
stated solvent (2.0 mL) was added dropwise over a period of 1 h with a
syringe pump to a stirred solution containing the precatalyst (13 mmol,
4.0 mol%), substrate 6 (331 mmol) in the stated solvent (2.0 mL), 1118C.
[b] Selectivity refers to 7. [c] The yields of 7 and the deoxygenated side
product as well as the conversion were determined by GC (internal
standard n-dodecane). Ethanol was detected qualitatively. The yield is
given in brackets. [d] T=1308C. [e] Reduced yield due to unknown side
product.
Figure 2. ORTEP presentation of complex 4. Thermal ellipsoids are
drawn at the 50% probability level. Solvent molecules (toluene), hydro-
gen atoms, and ellipsoids of carbon atoms are omitted for clarity.
zoyl function of the ligand coordinates to the same molyb-
denum atom as the O1 atom and does not bridge the triple
bond as observed for other bidentate ligands.[11a] Further de-
tails on bond lengths, bond angles and other crystallographic
data are described in the Supporting Information. Advanta-
geously, the synthesized complexes are, to some extent,
more stable to oxygen and moisture compared with the
parent compound 3, which potentially makes them resistant
to unwanted catalyst deactivation.
level throughout the reaction seemed to be essential, since
the reaction with one-time addition of 2.5 equivalents of the
phenylsilane afforded a reduced amount of 7 (37%). More-
over, the analogous reaction with complex 5 generates 7 in a
similar yield (60%) (Table 1, entry 3). The obtained results
prompted us to focus ongoing studies on complex 4. We
next examined the influence of the substitution level of the
silane by testing different reducing reagents (Table 1, en-
tries 4–9). By increasing the number of phenyl substituents
À
Having suitable complexes in hand, we were interested in
their catalytic abilities for the reduction of organic amides.
As a model substrate, we chose the acetyl-protected dihydro
dibenzoazepine derivative 6. It is worth noting that 6 was
chosen, because the azepine core is a key structural motif in
synthetic and medicinal chemistry as well as natural prod-
ucts and the deprotection of such compounds often requires
harsh conditions.[12] Initially, the precatalyst 4 (4 mol%) and
substrate 6 were dissolved in toluene followed by slow addi-
tion of phenylsilane (2.5 equiv), as the reducing agent,
through a syringe pump (over 1 h) under reflux (Table 1,
entry 2). After 6 h an excellent conversion of >99% was
on the silicon, an increased selectivity for the C N cleavage
was noticed. For example, with diphenylsilane (Table 1,
entry 4) the selectivity is increased to 78% after 24 h. Fol-
lowing this trend, the reaction of triphenylsilane under the
À
same conditions gave the C N cleavage product exclusively,
but with a dramatically reduced conversion of 5% (Table 1,
entry 5).
Other tertiary hydrosilanes, namely Et3SiH and
Me2PhSiH, gave higher, but still unsatisfying yields of 15
and 30%, respectively, after 24 h, although the high selectiv-
ity was retained (Table 1, entries 6 and 7). The most active
tertiary silane was (EtO)3SiH with an excellent selectivity of
>99% and a yield of 81% of 7 (Table 1, entry 8). Moreover,
the influence of the reaction solvent and temperature were
studied. Applying diglyme under the same reaction condi-
tions resulted in an increased yield of 7 after 24 h over en-
tries 5–9 and comparable results to entry 4 (Table 1,
entry 10). Higher temperatures resulted in a decrease in re-
activity and selectivity (Table 1, entries 11 and 12). Further-
more, the generation of 7 was studied over time. A presen-
À
observed. The desired C N bond cleaved compound 7 was
formed as the major product (62%) accompanied by the
À
C O bond cleaved product 8 (36%). Cleary, modification of
3 with ligand 1 showed potential for the reductive cleavage
À
of the C N bond, whereas the parent complex Mo
G
(3) produced mainly the deoxygenation product 8 and only
small amounts of 7 were detected (Table 1, entry 1). It is
worth noting that keeping the silane concentration at a low
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Reductive Cleavage of Amides
COMMUNICATION
Table 2. Reduction of different amides applying 4 as precatalyst and
tation of the time-dependent yield for the reaction of 6 with
Ph2SiH2 in toluene is given in Figure 3, showing significantly
faster generation of 7 compared with 8.
Ph2SiH2 (2.5 equiv) as reducing reagent.[a]
Substrate
Product
Conv.
[%][b]
Yield
[%][b]
1
2
6
9
7
7
>99
78
>99
75
3
4
10
12
11 >99
64[c]
13
15
86
98
72
Figure 3. Time-dependent yield for the reaction of 6 with Ph2SiH2 in tol-
uene catalyzed by 4 (Table 1, entry 3). Yields were determined by GC-
MS with an internal standard (n-dodecane).
5
6
14
16
63
34
To evaluate the scope and applicability of the observed
reaction, we investigated various organic amides with diphe-
nylsilane as the reductant and 4 (4 mol%) as the precatalyst
in toluene (Table 2, entries 1–10). First, the reduction of a
series of tertiary heterocyclic carboxamides was examined
17 >99
À
(Table 2, entries 1–5). High yields of the corresponding C N
cleavage products were obtained. On the other hand, non-
ACHTUNGTRENNUNGheterocyclic amides were converted in only moderate yields
7
8
18
20
19
89
29
31
with deoxygenation representing the main reaction pathway
(Table 2, entries 6–10). Less sterically demanding substitu-
ents on the nitrogen resulted in decreased amounts of the
desired product. Secondary amides were also investigated,
however, only poor yields were observed.
19 >99
À
The mechanism for such C N bond cleavage reactions is
currently unknown (Scheme 2). Nevertheless, based on pre-
viously reported mechanistic assumptions[13] and our experi-
mental observations,[8] we assume that 4 serves as a precata-
À
lyst for Si H bond activation, while no reactivity was moni-
tored for complex 4 in the presence of amides. An earlier
low-temperature NMR study with complex 3 revealed the
9
21
23
22 >99
24 >99
25[d]
À
formation of a Mo H species by oxidative addition of
10
5[d]
PhSiH3 to dinuclear molybdenum complexes.[8] Interestingly,
the obtained results suggest unsymmetrical addition, mean-
ing the silyl residue and the hydride are connected to one
molybdenum, while two alkoxides bridge the molybdenum–
molybdenum double bond. Unfortunately, attempts to iso-
late the species failed so far because of instability; hence the
structure of the bimetallic complex is currently unknown.[9]
Due to the hemilability of the acetyl or benzoyl function
of the ligands, a free coordination site can be generated and
allow coordination of the carboxamide. By coordination
through the amide oxygen, activation of the amide carbon
[a] Reaction conditions:
equiv) in toluene (2.0 mL) was added dropwise over a period of 1 h with
syringe pump to stirred solution containing the precatalyst 4
A solution of diphenylsilane (828 mmol, 2.5
a
a
(13 mmol, 4 mol%) and the respective substrate (331 mmol) in toluene
(2.0 mL). [b] The yields of the C N cleavage products (sec. amine) and
deoxygenated side product as well as the conversion were determined by
GC (internal standard n-dodecane). The corresponding alcohols were de-
tected qualitatively. [c] Isolated yield. [d] Yield was determined by
1H NMR spectroscopy.
À
the electrophilic carbon and the silyl residue to the oxygen
can occur. The Mo H intermediate transfers the hydride to
to form O-silylated N,O-acetal A.[3t,p] In the next step, a
À
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15269

S. Enthaler et al.
Scheme 2. Mechanistic proposal for molybdenum-catalyzed reduction of amides.
À
second equivalent of the Mo H intermediate transfers a hy-
dride to the carbon of A and the silyl residue to the nitrogen
Experimental Section
General experimental data, synthetic details, and analytical data (NMR
spectra, single crystal XRD) are given in the Supporting Information.
function.
À
As a consequence, the C N bond is cleaved to produce
General procedure for the reduction of amides: A solution of silane
(828 mmol, 2.5 equiv) in toluene (2.0 mL) was added dropwise over a
period of 1 h with a syringe pump to a stirred solution containing the pre-
catalyst 4 (13 mmol, 4 mol%) and 6 (331 mmol) in toluene (2.0 mL). At
the end of the reaction, the catalyst was removed by passing the reaction
mixture through a short path of alumina followed by elution with ethyl
acetate (2.0 mL). Furthermore, n-dodecane was added as an internal
standard. The reaction mixture was treated with aqueous HCl and the or-
ganic layer was dried over MgSO4. The mixture was analyzed by GC-MS
silyl ether B and silyl amine C, which afford the correspond-
ing alcohol and amine under workup conditions. In order to
study the transfer of the hydrides, labeling experiments were
performed with different substrates. For instance, the reduc-
tive cleavage of N,N-dibenzyl-4-iodobenzamide 18 (Table 2,
entry 7) with Ph2SiD2 as the reducing reagent showed the in-
corporation of two deuterium atoms in the corresponding
benzyl alcohol 25. It is worth noting that no exchange reac-
tions with the aromatic systems of the substrates or products
were detected.
À
and the C N cleavage products, deoxygenated products, and residual
starting material were either quantified by GC by comparison with a cali-
bration curve made with the pure compound or by NMR spectroscopy.
As was proposed for the reduction of amides to amines,
O-ACHTUNGTRENNUNGsilylated N,O-acetal A can also be transformed into the
iminium species D to avoid the deoxygenation pathway. A
second equivalent of the activated hydrosilane is transferred
to D to produce the corresponding amine E and siloxane
F.[3t]
In summary, we have demonstrated that triply-bonded di-
molybdenum complexes modified by ligands 1 and 2 could
Acknowledgements
This research was financially supported by the Cluster of Excellence
“Unifying Concepts in Catalysis” (sponsored by the Deutsche For-
schungsgemeinschaft and administered by the Technische Universitꢀt
Berlin) and a Kekulꢃ Ph.D. Fellowship (Fond der chemischen Industrie)
to S.K. The authors thank Dr. K. Junge (Leibniz Institute for Catalysis,
Rostock, Germany) for substrate donation. Furthermore, Prof. M. Driess
(Technische Universitꢀt Berlin) is thanked for supporting this work.
À
serve as precatalysts for the C N bond cleavage of amides
by applying a hydrosilane as the reductant. The correspond-
ing alcohols and amines were accessible under mild reaction
À
conditions. It is worth noting that the observed C N bond
cleavage of amides is in contrast to reported protocols based
on other transition metals and hydrosilanes, in which the
Keywords: amides · homogeneous catalysis · molybdenum ·
reductive cleavage
À
C O bond of the amide is cleaved. In general, the described
methodology may be of interest for protection/deprotection
chemistry or peptide chemistry. Future studies will be direct-
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Reductive Cleavage of Amides
COMMUNICATION
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Received: August 9, 2012
Published online: November 5, 2012
Chem. Eur. J. 2012, 18, 15267 – 15271
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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