General and Mild Cobalt-Catalyzed C-Alkylation of Unactivated Amides and Esters with Alcohols

Article abstract of DOI:10.1021/jacs.6b06448

The borrowing hydrogen or hydrogen autotransfer methodology is an elegant and sustainable or green concept to construct carbon-carbon bonds. In this concept, alcohols, which can be obtained from barely used and indigestible biomass, such as lignocellulose, are employed as alkylating reagents. An especially challenging alkylation is that of unactivated esters and amides. Only noble metal catalysts based on iridium and ruthenium have been used to accomplish these reactions. Herein, we report on the first base metal-catalyzed α-alkylation of unactivated amides and esters by alcohols. Cobalt complexes stabilized with pincer ligands, recently developed in our laboratory, catalyze these reactions very efficiently. The precatalysts can be synthesized easily from commercially available starting materials on a multigram scale and are self-activating under the basic reaction conditions. This Co catalyst class is also able to mediate alkylation reactions of both esters and amides. In addition, we apply the methodology to synthesize ketones and to convert alcohols into aldehydes elongated by two carbon atoms.

Full text of DOI:10.1021/jacs.6b06448

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Communication
General and Mild Cobalt-Catalyzed C-Alkylation
of Unactivated Amides and Esters with Alcohols
Nicklas Deibl, and Rhett Kempe
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06448 • Publication Date (Web): 04 Aug 2016
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General and Mild Cobalt-Catalyzed C-Alkylation of
Unactivated Amides and Esters with Alcohols
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Nicklas Deibl and Rhett Kempe*
Anorganische Chemie II−Katalysatordesign, Universität Bayreuth, 95440 Bayreuth, Germany
Supporting Information Placeholder
However, alkylations of these substrate classes have proved
ABSTRACT: The borrowing hydrogen or hydrogen
autotransfer methodology is an elegant and sustainable or
green concept to construct carbon-carbon bonds. In this
concept, alcohols, which can be obtained from barely used
and indigestible biomass, such as lignocellulose, are
employed as alkylating reagents. An especially challenging
alkylation is that of unactivated esters and amides. Only
noble metal catalysts based on iridium and ruthenium have
been used to accomplish these reactions. Herein, we report
on the first base metal catalyzed α-alkylation of unactivated
amides and esters by alcohols. Cobalt complexes stabilized
with pincer ligands, recently developed in our laboratory,
catalyze these reactions very efficiently. The precatalysts can
be synthesized easily from commercially available starting
materials on a multigram scale and are self-activating under
the basic reaction conditions. This Co catalyst class is also
able to mediate alkylation reactions of both esters and
amides. In addition, we apply the methodology to synthesize
ketones and to convert alcohols into aldehydes elongated by
two carbon atoms.
to be challenging. Amides have a comparably low CH-acidic
nature due to resonance stabilization and esters can readily
undergo side reactions. To date, both α-alkylations of
activated4 and unactivated5 amides and of activated6 and
unactivated7 esters with alcohols have only been reported
using iridium or ruthenium catalysts (Scheme 1).
Scheme 1. Recent methods for the alkylation of
unactivated amides and esters with alcohols and the
work using Co catalysts, stabilized by PN5P ligands,
described herein. R1 = i-Pr, Cyclohexyl; R2 = 4-CF3-
C6H4, NH-C3H5, Methyl
α-Alkylation of carbonyl compounds is a fundamental
method for the construction of carbon-carbon bonds.1 In the
course of such
a transformation, a base is used to
deprotonate the carbonyl compound and the anion is
trapped with a reactant which bears a leaving group, such as
a halide. The borrowing hydrogen (BH) or hydrogen
autotransfer (HA) concept is an elegant and operationally
easy method for C-C bond formation using alcohols as the
electrophile.2 Alcohols are especially appealing building
blocks, since they can be obtained from indigestible,
abundantly available and barely used biomass, such as
lignocellulose. 3 A transition metal complex is used to oxidize
the alcohols to the corresponding carbonyl compound and a
The substitution of noble metals by earth-abundant and
inexpensive base metals is a key challenge in transition
metal-mediated catalysis. Cobalt complexes have recently
been reported as catalysts in key reaction steps of the BH/HA
concept, such as hydrogenation (olefins,8 ketones,9
13
CO2
carboxylic
acids,10
nitriles,11
esters,12
)
and
dehydrogenation.14 Our group recently reported C-
alkylations based on BH/HA15 and on the sustainable
syntheses of N-heterocycles based on PN5P-stabilized Ir-
complexes.3c,16 Using the same ligand class, we also reported
on the first Co-catalyzed alkylation of aromatic amines by
alcohols.17 Related PN3P-pincer ligands were introduced by
Haupt and coworkers18 and the broad applicability of this
ligand class was demonstrated by Kirchner and coworkers.19
Herein, we report on the first α-alkylation of unactivated
amides and esters by alcohols applying base metal catalysts.
Cobalt complexes stabilized with PN5P-ligands (Scheme 1,
bottom) are efficient catalysts for both reactions. The
subsequent condensation reaction with
a
CH-acidic
compound yields an unsaturated intermediate that is
reduced by the catalyst with the hydrogen obtained from the
oxidation step. Only water is released in these reactions,
rendering them green or sustainable apart from the use of
alcohols as an alkylating agent. Derivatives of carboxylic
acids, such as esters and amides, are valuable intermediates
and products both in industry and academia. An elegant
approach to modify ordinary and broadly available amides
and esters is the α-alkylation by alcohols. Here, even solvents
commonly used and other amides and esters can be
converted into more sophisticated and valuable products.
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precatalysts can be synthesized in two steps in almost
alkylation (Table 3). The screening product 5a was isolated in
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quantitative yield beginning with commercially available
starting materials on a gram scale and become activated
under the basic reaction conditions.9a,17a In addition, we
describe the application of the alkylation of amides to
synthesize unsymmetrically substituted ketones and to
convert alcohols into aldehydes, which are extended by two
carbon atoms.
83 % yield. Methyl substituents in the ortho, meta and para-
positions of the phenyl ring furnished 5b-d in again very
good yields (80-85 %, respectively) and application of 1-
naphthyl methanol gave 5e in 78 % yield. Methoxy-
substituted benzyl alcohols gave the corresponding products
in excellent yields (89 % and 86 % for 5f and 5g,
respectively). When 4-chlorobenzyl alcohol was subjected to
the reaction conditions, partial dechlorination was observed
by GC analysis and the products were inseparable by column
chromatography. However, the change to t-BuONa (instead
of t-BuOK) under otherwise identical reaction conditions
furnished 5h as the only product in 76 % isolated yield. 3-
and 4-pyridine methanol also proved to be challenging
alcohols and low conversions were obtained under the
standard conditions. However, with the diverse Co-complex
library as a toolbox, the application of 1f with higher catalyst
loading (5 mol%) gave the alkylation products 5i in
acceptable 70 % and 5j in good 81 % yield.
Table 1. Co complexes used herein to identify the
best reaction conditions
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Complex R1
R2
X
N
N
N
N
N
N
1a
1b
1c
1d
1e
1f
H
i-Pr
i-Pr
i-Pr
Me
Ph
4-CF3-C6H4 i-Pr
NH-C3H5
i-Pr
Table 2. Catalyst screening for amide and ester
alkylation
Me
H
Cy
1g
1h
1i
i-Pr CH
i-Pr CH
Me
H
Ph
CH
Cy = cyclohexyl.
The reaction between benzyl alcohol (2a) and N,N-
dimethylacetamide (3a) to give 5a was thoroughly
investigated to find broadly applicable reaction conditions
for the Co-catalyzed amide alkylation proposed (Tables 1 and
2). Starting with catalyst 1c (Table 1, 5 mol%), common
reaction parameters, such as solvent, base, base amount,
substrate ratio and temperature, were investigated (see the
Supporting Information [SI] for details). Afterwards, a
screening of the Co complexes 1a-1i (2.5 mol%) was applied
in the synthesis of the model compound 5a (Table 2). While
precatalysts 1a-1d (entries 1-4) gave unsatisfying yields, 1e
and 1f (entries 5 and 6) gave the highest yield of the
alkylation product. Alcohol conversion was quantitative for
both precatalysts. Complex 1e is less expensive (i-Pr moieties
of the P-atom) and possesses very good solubility in thf or
dioxane at RT and is, therefore, very convenient to handle as
a stock solution. Thus, 1e was selected eventually. Most
interestingly, 1g-1i, which are based on a pyridine core, and
CoCl2 failed to catalyze the reaction (entries 7-10). In
summary, the reaction can be conducted with 2.5 mol% 1e in
thf at 100 °C (closed system) with 1.2 equiv t-BuOK as the
base and a twofold excess of amide with respect to the
alcohol. Notably, these conditions are milder than those for
the Ir-catalyzed approach reported by Huang and coworkers
(toluene, 120 °C, 2 equiv. base, 2 mol% Ir).5a Mechanistic
investigations of the model reaction (Table 2) indicate that
alcohol oxidation is rate-limiting and reduction of the double
bond is comparably fast. The key to a selective reaction is a
low concentration of the unsaturated intermediate (N,N-
dimethylcinnamamide) since it undergoes multiple side
reactions with educts and products (see SI for details). The
metal base (metal-to-Co ratio 2 : 1) is used to activate the
dichloro complexes via double deprotonation of the ligand
and removal of chloro ligands (salt elimination).9a The
double deprotonation option is a unique feature of the ligand
class used herein. Taking note of these optimized conditions,
we started to explore the substrate scope of the amide
Yield [%]c
Entry
[Co] Precatalyst
5a
7
6a
13
1
2
1a
1b
1c
1d
1e
1f
49
25
41
69
74
0
43
50
60
42
45
31
3
4
5
6
7
1g
1h
1i
8
9
10
0
24
0
0
CoCl2
0
0
a
Reaction conditions:
Benzyl alcohol (1 mmol), N,N-di-
methylacetamide (2 mmol), t-BuOK (1.2 mmol), THF (4 mL),
[Co] (0.025 mmol), 20 h at 100 °C (oil bath temperature);
b Benzyl alcohol (1 mmol), tert-butyl acetate (2 mmol),
toluene (1 mL), [Co] (0.02 mmol), 20 h at 70 °C (oil bath
temperature) c Determined by GC with dodecane as an
internal standard.
When aliphatic (branched and linear) alcohols were used
in this reaction (products 5k-n), the highest yields of up to
93 % were obtained. In order to C-alkylate a secondary amide
the reaction conditions had to be adjusted and 5o was
obtained in 55 % yield when the reaction was run in 1,4-
dioxane at 120 °C with 1.5 equiv t-BuOK and 5 mol% 1e.
Further variation of the amide moiety gave 5p-r in good
yields (66-77 %, respectively) and the very interesting N-
morpholino amides 5s-v (vide infra) in very good yields, even
on a higher scale (10 mmol, > 2 gram for 5t,u). The latter
compounds exhibit a similar reactivity to Weinreb amides
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(R-CO-N(Me)OMe) which failed to react. Taking note of the
chlorine-substituted product 6j free of side products (72 %
1
2
3
4
good performance of the Co catalysts for the amide
alkylation, we focused on acetates as the coupling partner
(Table 4, top).
Table 3. Product scope for amide alkylationa
yield), t-BuONa had to be used as the base (t-BuOK: 48 %
isolated yield).
Table 4. Product scope for ester alkylationa
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a Reaction conditions: Alcohol (1 mmol), tert-butyl acetate (4
mmol), t-BuOK (1.5 mmol), toluene (1 mL), 1d (5 mol%), 4 h
at 80 °C (oil bath). Yields are of isolated products. b t-BuONa
was used as a base.
The use of 4-fluorobenzyl alcohol gave an excellent yield
(6k, 82 %). Ester 6l could be obtained in 58 % yield using 3-
phenyl-1-propanol.
Table 5. Derivatization of amide substratesa
a
Reaction conditions: Alcohol (1 mmol), amide (2 mmol),
t-BuOK (1.2 mmol), 1e (0.025 mmol) and THF (4 mL) were
heated for 24 h at 100 °C (oil bath) in a closed system. Yields
are of isolated products. b t-BuONa was used as a base. c 1f (5
mol%) was used. 10 mmol scale. 5 mmol scale. 1,4-
dioxane, 120 °C, 1.5 eq t-BuOK.
d
e
f
The reaction between benzyl alcohol and tert-butyl acetate
was investigated to find suitable reaction conditions (Table 1,
synthesis of 6a; see the SI for details). A catalyst screening
identified precatalyst 1d (entry 4) to be the most active for
this transformation. The peak of product yield was obtained
when the amount of tert-butyl acetate (4) was increased to
four equiv. When the reaction was conducted in neat tert-
butyl acetate, the yield dropped. In summary, the reaction
should be run in toluene at 80 °C with 1.5 equiv. of t-BuOK,
four equiv. of tert-butyl acetate and complex 1d (5 mol%).
tert-Butyl acetate undergoes fast transersterification and the
equilibrium is shifted to the alkylated tert-butyl esters 6 with
the consumption of the primary alcohol (see SI for details).
Having pinpointed the reaction conditions, we explored the
substrate scope of this reaction (Table 4). The application of
benzyl and methylbenzyl alcohols and 1-naphthyl methanol
gave the ester alkylation products 6a-e in 63-76 % isolated
yields, respectively. The use of electron-rich methoxybenzyl
alcohols furnished the corresponding products in good yields
as well (77 and 70% for 6f,g, respectively). The application of
methanol bearing heteroaromatic substituents (furyl,
pyridyl) also gave the C-alkylation products, albeit in lower
yields (55 % for 6h and 48 % for 6i). In order to obtain the
a
Reaction conditions: THF, –78 °C, R’-Li (2.5-3 eq) or
diisopropylaluminum hydride (DIBAL-H, 1.15-1.30 eq), 1 h
reaction time. Yields are of isolated products. PMP = para-
methoxyphenyl, p-Tol = para-tolyl, Bu = butyl.
Finally, we became interested in exploring the applicability
of amide alkylation products synthesized via Co catalysis to
expand the scope of the BA/HA methodology16a,20 (Table 5).
N-morpholino amides 5t-v were converted into the
corresponding ketones 7a-d using alkyl and aryl Li reagents
and only mono-addition was observed (as opposed to esters).
The reaction of 5t and 5u with diisobutyl aluminumhydride
(DIBAL-H) at –78 °C gave aldehydes 7e and 7f in 95 and 73 %
yield, respectively.
In summary, we report on the first base metal catalyzed C-
alkylation of unactivated amides and esters by alcohols. The
reaction is catalyzed most efficiently by PN5P-stabilized Co-
complexes developed in our laboratory. These catalysts are
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(6) (a) Grigg, R.; Whitney, S.; Sridharan, V.; Keep, A.; Derrick, A.
easy to synthesize on a large scale from commercially
available starting materials. The catalysts are self-activating
under the reaction conditions needed to accomplish the
alkylations. The method is characterized by mild reaction
conditions and good functional group tolerance. A key to a
broad substrate scope is also the library of easily accessible
PN5P-Co catalysts. Amide alkylation products were obtained
in up to 93 % isolated yields with catalyst loadings nearly the
same as those reported for Ir, but under milder reaction
conditions and applying less base. The demanding ester
alkylation reaction gave the corresponding products in
moderate to good yields. So far, different catalyst classes
have been applied to α-alkylate amides and esters. Finally,
further transformations of the amide alkylation products into
compounds with other functional groups (ketone, aldehyde)
showcase the value of the products obtained by this method.
Eight novel compounds out of 40 examples have been
synthesized.
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures and spectroscopic data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(10) Korstanje, T. J.; van der Vlugt, J. I.; Elsevier, C. J.; de Bruin, B.
Science 2015, 350, 298-302.
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Milstein, D. J. Am. Chem. Soc. 2015, 137, 8888-8891.
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Posner, Y.; Shimon, L. J.; Ben David, Y.; Milstein, D. Angew. Chem.
Int. Ed. 2015, 54, 12357-12360.
AUTHOR INFORMATION
Corresponding Author
kempe@uni-bayreuth.de
Notes
The authors declare no competing financial interests.
(13) (a) Federsel, C.; Ziebart, C.; Jackstell, R.; Baumann, W.; Beller,
M. Chem. Eur. J. 2012, 18, 72-75. (b) Jeletic, M. S.; Mock, M. T.; Appel,
A. M.; Linehan, J. C. J. Am. Chem. Soc. 2013, 135, 11533-11536.
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Lett. 2016, 18, 300-303.
ACKNOWLEDGMENT
We thank the Deutsche Forschungsgemeinschaft, DFG, KE
756/23-2 28-1, for financial support.
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OH
O
OH
O
R
+
+
R'
NR2
R'
O
N
N
6
7
HN
N
NH
8
Co
(R)2P
P(R)2
9
Cl Cl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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40
41
42
43
44
45
46
47
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49
50
51
52
53
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55
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59
60
O
O
t-BuOK
R'
NR2
R'
O
+ H2O
+ H2O
40 examples
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