Bidentate Ru(II)-NC Complexes as Catalysts for α-Alkylation of Unactivated Amides and Esters

Article abstract of DOI:10.1002/cctc.201901319

Five Ru(II)-NC complexes were tested as catalysts for α-alkylation of unactivated amides using alcohols as alkylating agents, and complex {(C₅H₄N)-(C₆H₄)}RuCl(CO)(PPh₃)₂ (1) showed the highest activity. With 0.5 mol% catalyst loading, a series of α-alkylated amides were isolated at 80 °C within 6 hours. Furthermore, under similar conditions, complex 1 was also active for α-alkylation of unactivated esters with alcohols, and the reaction time was shortened to 1.5 hours. The catalytic performance of 1 is comparable to the best reported catalyst.

Full text of DOI:10.1002/cctc.201901319

Accepted Article
Title: Bidentate Ru(II)-NC Complexes as Catalysts for α-Alkylation of
Unactivated Amides and Esters
Authors: Dawei Gong, Bowen Hu, Weiwei Yang, and Dafa Chen
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ChemCatChem
Bidentate Ru(II)-NC Complexes as Catalysts for α-Alkylation of
Unactivated Amides and Esters
Dawei Gong,‡[a] Bowen Hu,‡[a] Weiwei Yang[a], and Dafa Chen*[a]
[a] D. Gong, Prof. Dr. B. Hu, Prof. Dr. W. Yang, Prof. Dr. D. Chen
MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage,
School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001,
People's Republic of China.
‡These authors contributed equally to this work.
*E-mail: dafachen@hit.edu.cn.
Supporting
information
for
this
article
is
available
on
the
WWW
underhttps://doi.org/10.1002/xxxx
Five Ru(II)-NC complexes were tested as catalysts for α-alkylation of unactivated amides using
alcohols as alkylating agents, and complex {(C5H4N)-(C6H4)}RuCl(CO)(PPh3)2 (1) showed the
highest activity. With 0.5 mol% catalyst loading, a series of α-alkylated amides were isolated at 80
oC within 6 hours. Furthermore, under similar conditions, complex 1 was also active for
α-alkylation of unactivated esters with alcohols, and the reaction time was shortened to 1.5 hours.
The catalytic performance of 1 is comparable to the best reported catalyst.
Introduction
Carboxylic acid derivatives are regarded as versatile building blocks in organic chemistry and
medicinal chemistry, and α-C-alkylation of carboxylic acid derivatives is widely used in organic
synthesis.[1] For this transformation, the deprotonated carboxylic acid derivatives generated by the
addition of strong bases, are always used as nucleophilic reagents traditionally, to react with toxic
alkyl halides, thus leading to the formation of stoichiometric amount of inorganic salt as waste.[1,2]
In recently years, transition-metal catalyzed borrowing hydrogen has been developed as one of
the most fundamental processes for α-alkylation of the carbonyl compounds.[3] This strategy uses
primary alcohols as alkylating agents and water is released as the only by-product. Compared to
methyl ketones, the α-alkylation of unactivated amides and esters is more challenging, due to the
lower acidity of their α-H, and the readily occurrence of side reactions for esters. In 2010, Ishii’s
group reported the first catalytic α-alkylation of unactivated esters catalyzed by [IrCl(cod)]2/PPh3
(Scheme 1).[4] Later in 2013, Huang et al. developed the α-alkylation of unactivated amides using
an Ir-pincer complex (Scheme 1).[5] Since then, several other transition-metal catalysts, including
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Ir,[6,7] Ru,[8-10] Co,[11] Ni[12] and Mn[13-15] complexes, have been developed, most of which are based
on pincer-type ligands. For earth-abundant base metal catalysts, harsh reaction conditions and high
catalyst loadings were needed for good conversion in most of the cases, and sometimes the
substrates were limited.[11-15] So far, the best catalyst for α-alkylation of unactivated amides and
esters is the PCN-Ir complex discovered by Huang and coworkers (Scheme 1).[6,7] For amide
o
alkylation, the reactions proceeded well at 80 C with 0.5 mol% catalyst,[7] and when the
o
substrates were esters, 60 C was even enough.[6] Some other noble metal catalysts, such as the
NNN-Ru complex developed by Ryu’s group,[8] and the PNP-Ru[9] and NHC-Ru(p-cym)[10]
complexes developed by Gnanaprakasam et al, were not as effective as Huang’s catalyst, and 140
oC were needed for amide alkylation (Scheme 1).
Scheme 1. α-Alkylation of unactivated amides and esters with alcohols.
Recently, our group has reported several transition metal complexes for borrowing hydrogen
reactions,[16] including α-alkylation of ketones.[16c-d] As an extension, herein we present a bidentate
Ru(II)-NC complex {(C5H4N)-(C6H4)}RuCl(CO)(PPh3)2 (1), which is active for the
dehydrogenative reaction from primary alcohols to carboxylic acids,[17] for α-alkylation of
unactivated amides and esters with alcohols (Scheme 1). With low catalyst loading (0.5 mol%),
complex 1 showed excellent conversions and functional group tolerance under relatively mild
o
conditions (80 C) in short reaction times (6 h for amides and 1.5 h for esters). It is worth noting
that complex 1 is easily synthesized from a commercially cheap ligand in high yield, and its
efficiency is comparable to the best Ir catalyst reported up to date.[6,7]
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Results and Discussion
Complexes 1-5 were known complexes and synthesized as previously described by us.[17]
Initially, the reaction of benzyl alcohol with N,N-dimethylacetamide (3 eq.) was chosen as the
model reaction, t-BuOK (1.5 eq.) was selected as the base, and complexes 1-5 were tried as the
catalysts, respectively (Table 1). After heated at 80 oC for 6 h, complexes 1 and 2 showed similar
catalytic performances (94% for 1 and 93% for 2), while complexes 3-5 were less active (Table 1,
Entries 1-5). The catalytic performances of 1–5 are not related to the 31P{1H} and (CO) values.
In the absence of catalyst, no conversion was detected (Table 1, Entry 6). Thus, complex 1 was
chosen as the catalyst for further investigation. When the reaction was carried out under air, the
product was a mixture of unsaturated amide and saturated amide, with a ratio of about 68:32
(Table 1, Entry 7). In order to explore the effect of bases, KOH, NaOH and t-BuONa were then
tested in standard conditions, while they all gave poor conversions (Table 1, Entries 8-10). The
amounts of t-BuOK and N,N-dimethylacetamide were also important for this reaction. For t-BuOK,
0.5, 1 or 2 equivalents were not as good as 1.5 equivalents (Table 1, Entries 11-13). When the
ratios of benzyl alcohol to N,N-dimethylacetamide were 1:1, 1:2 and 1:4, the yields of
N,N-dimethyl-3-phenylpropanamide were 81%, 87% and 93%, respectively (Table 1, Entries
14-16), not as good as that of 1:3 ratio. Notably, even if the temperature was reduced to 60 oC, the
result was also satisfied when the reaction time was prolonged to 18 h (85%) (Table 1, Entry 17).
Table 1. Optimization of reaction conditions for alkylation of amides with primary
alcohol
O
Base
0.5 mol% [Ru]
toluene
O
N
OH
N
+
N
o
80 C 6h
PPh
Ru
3
PPh
BF
4
PPh
PPh
3
3
3
PPh
Ru
3
Cl
N
Cl
Cl
Cl
N
N
CO
N
Ru
Ru
Ru
CO
CO
CO
CO
CO
PPh
3
PPh
PPh
PPh
3
3
3
PPh
3
1
2
3
5
4
Entry
[Ru]
Base
Amide
3.0 eq
3.0 eq
3.0 eq
3.0 eq
3.0 eq
3.0 eq
3.0 eq
3.0 eq
Yield [%][a]
1
2
3
4
5
6
7
8
1
2
3
4
5
-
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
KOH (1.5 eq.)
94
93
71
74
32
n.d.
32[b]
<5
1
1
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9
1
1
1
1
1
1
1
1
1
NaOH (1.5 eq.)
t-BuONa (1.5 eq.)
t-BuOK (0.5 eq.)
t-BuOK (1.0 eq.)
t-BuOK (2.0 eq.)
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
t-BuOK (1.5 eq.)
3.0 eq
3.0 eq
3.0 eq
3.0 eq
3.0 eq
1.0 eq
2.0 eq
4.0 eq
3.0 eq
<5
<5
9
10
11
12
13
14
15
16
17
78
70
81
87
93
85[c]
[a] Reaction conditions: benzyl alcohol (1 mmol), base, amide, toluene (3 mL), Ar,
o
catalyst (0.5 mol%), 80 C, 6 h; the yield was calculated by GC using dodecane (0.5
mmol) as the internal standard. [b] Under air. [c] 60 oC, 18 h.
According to the above results, the conditions in Entry 1 of Table 1 were selected for substrate
expansion, and a series of α-alkylated N,N-dimethylacetamide derivatives were isolated (Table 2).
From Entries 1-5, the substituents at para position of benzyl alcohols did not influence the
transformation dramatically. For 4-fluorobenzyl alcohol and 4-chlorobenzyl alcohol, the
conversions
were
both
above
95%,
and
the
corresponding
products
3-(4-fluorophenyl)-N,N-dimethylpropanamide and 3-(4-chlorophenyl)-N,N-dimethylpropanamide
were isolated in 94% and 89% yields, respectively (Table 2, Entries 2 and 3). Electron-donating
groups, such as methyl and methoxy, also gave high conversions and isolated yields (Table 2,
Entries 4 and 5). When there was a substituent at meta position of benzyl alcohols, no matter if it
was electron-rich or electron-deficient, the conversions were slightly decreased. A conversion of
87% and an isolated yield of 80% were emerged when 3,4-dimethoxybenzyl alcohol was treated
with N,N-dimethylacetamide (Table 2, Entry 6). Other mono-meta substituted benzyl alcohols,
gave conversions of 80-83% and isolated yields of 72-77% (Table 2, Entries 7-10). Ortho
derivatives, such as 2-methylbenzyl alcohol and 2-methoxybenzyl alcohol, were also highly active,
and the corresponding products could be isolated in the yields above 80% (Table 2, Entries 11 and
12). Chain alcohol was much less effective, for instance, N,N-dimethyl-6-phenylhexanamide was
isolated in 53% yield (Table 2, Entry 13). 2-Naphthalenemethanol showed 91% conversion,
similar to benzyl alcohol (Table 2, Entry 14). When N,N-dimethylacetamide was replaced by
N,N-dimethylpropionamide, no conversion was detected.
Table 2. Substrate scope for alkylation of N,N-dimethylacetamide with primary alcohols using
precatalyst 1
1.5 eq t-BuOK
O
O
0.5 mol% 1
+
R
OH
R
N
N
toluene
80 C 6h
o
Entry
Alcohol
Product
Conversion [%][a]
Isolated yield [%]
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1
2
3
4
5
6
94%
99%
95%
92%
92%
87%
87%
94%
89%
85%
84%
80%
7
8
83%
80%
82%
80%
77%
73%
72%
75%
9
10
11
12
13
14
90%
89%
62%
91%
83%
84%
53%
83%
[a] Reaction conditions: benzyl alcohol (1 mmol), t-BuOK (1.5 eq), N,N-dimethylacetamide (3.0
eq), toluene (3 mL), Ar, catalyst (0.5 mol%), 80 oC, 6 h; the conversion was calculate by GC using
dodecane (0.5 mmol) as the internal standard.
As mentioned in Entry 7 of Table 1, if the reaction of benzyl alcohol with
N,N-dimethylacetamide was carried out under air using 1 as the catalyst, a mixture of
N,N-dimethyl-3-phenylpropanamide and (E)-N,N-dimethylcinnamamide would be produced with
a ratio of about 32:68. Therefore, controlling the oxygen level was essential for avoiding the
unsaturated amide by-product. Interestingly, in the absence of transition metal catalyst, benzyl
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alcohol was converted completely to (E)-N,N-dimethylcinnamamide with an isolated yield of 90%
(Scheme 2). Transition metal catalysts or photocatalysts were necessary for such transformation at
room temperature reported in previous literatures.[18]
Scheme 2. Transformation of benzyl alcohol with N,N-dimethylacetamide under air without metal catalyst.
To expand the scope of this approach, the alkylation of esters was then explored. As showed in
Table 3, reactions of benzyl alcohol (1 mmol) and its derivatives with tert-butyl acetate (2.5 eq)
o
were implemented at 80 C in toluene under Ar atmosphere. In the presence of t-BuOK (1.2 eq)
and complex 1, most of the alkylated products were obtained in good yields after 1.5 h. For benzyl
alcohols bearing electron-donating substituents or electron-withdrawing substituents at the para
positions, the conversions were excellent (90-96%), and comparable to that of benzyl alcohol
(97%) (Table 3, Entries 1-6). These compounds were also isolated in satisfied yields (69-85%).
Similar to amide alkylation, meta-substituted benzyl alcohols gave slightly lower conversions and
isolated yields (Table 3, Entries 7-11). When a substituent was introduced to the ortho position of
benzyl alcohol, the conversions were decreased clearly. For example, 2-methylbenzyl alcohol and
2-methoxybenzyl alcohol only resulted in moderate conversions (60% and 62%, respectively), and
tert-butyl 3-(o-tolyl)propanoate and tert-butyl 3-(2-methoxyphenyl)propanoate were isolated in
low yields (36% and 35%, respectively) (Table 3, Entries 12 and 13). The reduced efficiency
might be caused by the steric hindrance. And the large difference between conversions and yields
was mainly due to the formation of transesterification products, which were not discovered in
amide alkylation. 2-Naphthalenemethanol showed 89% conversion and 79% yield (Table 3, Entry
14). For 4-phenyl-1-butanol, transesterification was the primary reaction. It should also be noted
that benzyl acetate was not suitable for such transformation.
Table 2. Substrate scope for alkylation of tert-butyl acetate with primary alcohols using precatalyst 1
O
t-BuOK
0.5 mol% [Ru]
toluene
O
O
OH
+
O
o
80 C 1.5h
Entry
1
Alcohol
Product
Conversion [%][a]
97%
Isolated yield [%]
85%
2
3
96%
96%
82%
81%
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4
5
6
7
92%
90%
89%
85%
79%
72%
69%
65%
8
9
85%
84%
85%
82%
70%
70%
68%
69%
10
11
12
13
14
60%
62%
89%
36%
35%
79%
[a] Reaction conditions: benzyl alcohol (1 mmol), t-BuOK (1.2 eq), tert-butyl acetate (2.5 eq),
o
toluene (3 mL), Ar, catalyst (0.5 mol%), 80 C, 1.5 h; the conversion was calculated by GC using
dodecane (0.5 mmol) as the internal standard; difference between conversion and yield was mainly
due to the formation of transesterification product (for entry 1, the transesterification product was
benzyl acetate).
In order to study the mechanism, some control experiments were carried out. Excess of Hg
(Hg:Ru = 400:1) did not influence the conversion of amide alkylation obviously, suggesting the
reaction is homogeneous (Scheme 3, eqn (1)). Then different equivalents of PPh3 were added, 5
mol% PPh3 lowered the conversion to 68%. When the amount was increased to 10 mol%, the
conversion was reduced to 32% (Scheme 3, eqn (2)). The results indicated that PPh3 dissociation
is involved in the rate-determining step. When the reaction of benzyl alcohol with
N,N-dimethylacetamide was stopped in 3 hours under standard conditions, a conversion of 77%
was
found,
with
the
ratio
of
N,N-dimethyl-3-phenylpropanamide
to
(E)-N,N-dimethylcinnamamide about 96:4. (E)-N,N-dimethylcinnamamide was then treated with
benzyl alcohol under catalytic conditions, and after 6 hours, it was consumed completely, and a
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72% GC yield of N,N-dimethyl-3-phenylpropanamide was observed (Scheme 3, eqn (3)). The
results suggested that (E)-N,N-dimethylcinnamamide is the critical intermediate, consistent with
the previously reported outcome.[15]
Scheme 3. Control experiments.
Together with the experimental results and the related literatures,[8-13, 17] a possible mechanism
is proposed in Scheme 4. Firstly, 1 reacts with base and alcohol to form a Ru-OR intermediate A.
-Hydride elimination of A generates the intermediate B and aldehyde, which reacts with amide
or ester to form ,-unsaturated product under basic condition. The unsaturated product then
reacts with B to form C, with the hydride transfer to the C=C double bond. The targeted product is
finally made by the reaction of C with alcohol, accompanied by the regeneration of A, and the
catalytic cycle is finished.
Scheme 4. Proposed reaction mechanism.
Conclusions
In summary, a series of bidentate Ru(II)-NC complexes (1-5) were applied for α-alkylation of
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N,N-dimethylacetamide with alcohols using borrowing hydrogen strategy, and 1 showed excellent
o
conversions and functional group tolerance under relatively mild conditions (80 C) with a low
catalytic loading (0.5 mol%) within short time (6 hours). Furthermore, 1 was also highly effective
for α-alkylation of tert-butyl acetate, and a series of α-substituted tert-butyl acetates were isolated
in 1.5 hours with 0.5 mol% catalyst at 80 oC. Complex 1 can be synthesized from a commercially
cheap ligand 2-phenylpyridine, while its efficiency is competitive to the best noble catalyst, which
is an Ir complex based on a pincer type PCN ligand.[6,7] Thus, the results in this manuscript
provide an effective and green method for α-alkylation of unactivated amides and esters, using a
new type of Ru catalysts.
Experimental Section
General considerations
All manipulations were carried out under an inert argon atmosphere using a Schlenk line. Solvents
were distilled from appropriate drying agents under Ar before use. All reagents were purchased
from commercial sources. Liquid compounds were degassed by standard freeze-pump-thaw
procedures prior to use. Ru complexes 1-5 were prepared as previously described,[17] respectively.
1
1
The H and 13C{1H} NMR spectra were recorded on a Bruker Avance 400 spectrometer. The H
NMR chemical shifts were referenced to residual solvent as determined relative to Me4Si (δ = 0
ppm). The 13C chemical shifts were reported in ppm relative to the carbon resonance of CDCl3
(77.0 ppm) or d6-DMSO (39.5 ppm). High-resolution mass spectra (HR-MS) were recorded on a
Varian 7.0 T FTICR-MS instrument by the ESI technique.
General procedure for the alkylation of amides with primary alcohols:
Ruthenium catalyst (0.005 mmol), base (1.5 mmol), internal standard (dodecane, 0.5 mmol),
alcohol (1 mmol) and amide (3 mmol) were dissolved in toluene (3 mL) and the solution was
o
heated to 80 C for 6 h under an Ar atmosphere. The conversion was calculated by GC using
dodecane as the internal standard. The mixture was allowed to cool to room temperature and the
organic phase was evaporated under vacuum. Residue was purified by column chromatography on
silica gel (ethyl acetate/petroleum ether, v:v = 1:1) to give the corresponding product.
General procedure for the alkylation of tert-butyl acetate with primary alcohols:
Ruthenium catalyst (0.005 mmol), t-BuOK (1.2 mmol), internal standard (dodecane, 0.5 mmol),
alcohol (1 mmol) and tert-butyl acetate (2.5 mmol) were dissolved in toluene (3 mL) and the
solution was heated to 80 oC for 1.5 h under an Ar atmosphere. The conversion was calculated by
GC using dodecane as the internal standard. The reaction mixture was quenched with water (1 mL)
and extracted with diethyl ether (3 x 10 mL). The combined organic phases were dried over
Na2SO4. After evaporated, the residue was purified by column chromatography on silica gel (ethyl
acetate/petroleum ether, v:v = 100:1) to give the corresponding product.
Control experiments:
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Hg poisoning test: Hg (Hg:Ru = 400:1) was added into the optimum conditions used 1 as catalyst.
The conversion was unaffected.
Reaction of N,N-dimethylcinnamamide with benzyl alcohol: Ruthenium catalyst (0.005 mmol),
t-BuOK (1.5 mmol), internal standard (dodecane, 0.5 mmol), benzyl alcohol (1 mmol) and
N,N-dimethylcinnamamide (1 mmol) were dissolved in toluene (3 mL) and the solution was
heated to 80 oC for 6 h under an Ar atmosphere. The yield was calculated by GC using dodecane
as the internal standard.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 21871068
and 21672045), the China Postdoctoral Science Foundation (2016M591519), the Heilongjiang
Postdoctoral Foundation (LBH-Z16069), and the National Natural Science Foundation of
Heilongjiang (Nos. B2018005 and LC2018005).
Keywords: alkylation of amides • alkylation of esters • bidentate • Ru catalyst • borrowing
hydrogen
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