Enamines as Surrogates of Alkyl Carbanions for the Direct Conversion of Secondary Amides to α-Branched Ketones

Article abstract of DOI:10.1002/adsc.201801443

A direct transformation of secondary amides into α-branched ketones with enamines as soft alkylation reagents was developed. In this reaction, enamines serve as surrogates of alkyl carbanions, rather than the conventional enolates equivalents in the Stork's reactions, which allowed for the easy introduction of alkyl groups with electrophilic functional groups. In the presence of 4 ? molecular sieves, the method can be extended to the one-pot coupling of secondary amides with aldehydes to yield ketones. (Figure presented.).

Full text of DOI:10.1002/adsc.201801443

Title: Enamines as Surrogates of Alkyl Carbanions for the Direct
Conversion of Secondary Amides to α-Branched Ketones
Authors: Yong-Peng Liu, Shu-Ren Wang, Ting-Ting Chen, Cun-Cun
Yu, Ai-E Wang, and Peiqiang Huang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801443
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10.1002/adsc.201801443
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Enamines as Surrogates of Alkyl Carbanions for the Direct
Conversion of Secondary Amides to -Branched Ketones
Yong-Peng Liu,a Shu-Ren Wang,a Ting-Ting Chen,a Cun-CunYu,a Ai-E Wang*,a,b and
Pei-Qiang Huang*,a,b
a
Department of Chemistry and Key Laboratory for Chemical Biology of Fujian Province, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China
E-mail: aiewang@xmu.edu.cn; pqhuang@xmu.edu.cn
State Key Laboratory of Applied Organic Chemistry Lanzhou University, Lanzhou 730000, China
b
E-mail: aiewang@xmu.edu.cn; pqhuang@xmu.edu.cn
Received:((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
this respect, Charette's and our group have
independently reported the direct transformations of
secondary amides into ketones by addition of
Grignard/organozinc reagents,[7f] and organocerium
reagents,[7g] respectively (Scheme 1a). By addition of
Grignard reagents, we have also accomplished the
transformations of tertiary amides into ketones
(Scheme 1b).[8] The reactions demonstrated
exceptional functional group tolerance, allowing the
facile addition of readily available organometallics
reagents to a variety of amides. However, some
limitations of the organometallic reagents must be
taken into account when planning synthetic
transformations: (1) the spontaneous decomposition
of alkyl organometallics, particularly bulky
secondary alkyl organometallics, via -elimination or
by proto-demetalation; (2) the difficulties in the
preparation of secondary alkylmetallic reagents,
particularly those bearing functional groups. To avoid
the limitation of using alkylmetallic nucleophiles,
syntheses of ketones by addition of mild nucleophiles
are intriguing alternatives. In this respect, we have
recently demonstrated that both alkenes and arenes
could serve as effective nucleophiles for the direct
transformation of secondary amides into enones7l and
aryl ketones.[7m]
Abstract: A direct transformation of secondary amides
into -branched ketones with enamines as soft
alkylation reagents was developed. In this reaction,
enamines serve as surrogates of alkyl carbanions, rather
than the conventional enolates equivalents in the
Stork’s reactions, which allowed for the easy
introduction of alkyl groups with electrophilic
functional groups. In the presence of 4 Å molecular
sieves, the method can be extended to the one-pot
coupling of secondary amides with aldehydes to yield
ketones.
Keywords: amides; alkylation; carbanions; enamines;
ketones
Ketones are a class of versatile compounds in organic
chemistry, which can be converted into a large
variety of functionalized organic molecules across a
wide array of fundamental transformations. Moreover,
ketones are common structural motifs found in
numerous
biologically
active
molecules,
pharmaceuticals, functional materials, and natural
product.[1]
Nucleophilic addition of organometallics to
carboxylic acids and esters is a fundamental method
for the synthesis of ketones.[2] This approach,
however, suffers from over-addition, resulting in the
formation of tertiary alcohol as the main products.[3]
To overcome this problem, indirect methods based on
employment of chelating amides such as N-methoxy-
N-methylamides (Weinrebamides)[4] and thioesters
such as S-(2-pyridyl)thiolates[5] as reliable acylating
reagents have been developed. The direct
transformation of ordinary amides into ketones,
however, remains challenging, because of the high
stability of amides.
Scheme 1. Direct Conversion of Secondary and Tertiary
Amides to Ketones with Organometallic Reagents.
In recent years, considerable progress been
achieved for the direct conversion of amides into
functional groups with lower oxidation states.[6-7] In
1
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10.1002/adsc.201801443
Advanced Synthesis & Catalysis
Enamines are mild C-nucleophiles and they
Scheme 2. Our Proposal for the Conversion of Secondary
Amides to Ketones by Reaction with Enamines.
usually serve as stable yet more nucleophilic
synthetic equivalents of enols and enolates (S-1),[9]
thanks to the pioneering work of G. Stork.[10]
Enamines are also well recognized as key
intermediates in several powerful organocatalytic
enantioselective reactions.[11] As part of our ongoing
effort to develop amide-based C–C bond forming
reactions, we have recently disclosed a new strategy
to construct allylamines through reductive
alkenylation of secondary amides with enamines
serving as surrogates of alkene carbanions (S-2).[7q]
Encouraged by these findings, we wish to report
herein that enamines may also be employed as
surrogates of secondary alkyl carbanions (S-3) in the
reaction with secondary amides for ketone synthesis
(Figure 1).
Firstly, the addition of enamine 1a derived from
isobutyl aldehyde to N-isopropylbenzamide 2a was
selected as a model reaction. To our delight,
successive treatment of secondary amide 2a with
Tf2O (1.1 equiv), 2-F-Pyr. (1.2 equiv) and enamine
(2.5 equiv) at 0 °C, followed by hydrolysis with
aqueous HCl solution provided the expected ketone
product 4aa in a yield of 82% (Table 1). When the
reaction mixture was quenched under less acidic
conditions (with H2O or sat. NaHCO3), some -
ketoaldehydes 3aa was isolated, which was consistent
with our consumption. Further optimization showed
that the amount of enamine can be decreased to 2.0
equiv without much influence on the yield.
Table 1. Preliminary Studies on the Reaction of N-
Isopropylbenzamide 2a with Enamine 1a.[a]
Enamine
(equiv)
Figure 1. Enamines as classic synthetic equivalents of
enolate (S-1), as the unconventional surrogates of alkene
carbanions S-2 and as secondary alkyl carbanions S-3.
Entry
Hydrolysis
Yiled (%)[b]
1
2
3
4
5
2.5
2.5
2.5
2.0
3.0
3 M HCl
H2O
sat. NaHCO3
3 M HCl
3 M HCl
82
60[c]
70[d]
81
We envisioned that addition of steric hindered
enamines 1 derived from -substituted aldehydes to
Tf2O activated secondary amides would yield -
iminoiminium salts B, which could be converted to -
ketoaldehydes 3 under acidic hydrolysis (Scheme 2).
As the nonenolizable aldehyde group of -
ketoaldehydes is quite easily lost upon alkaline and
acidic conditions,[12] it is expectable that -
ketoaldehydes 3 could undergo decarbonylation to
give ketone 4, thus forging mild enamines 1 as
surrogates of highly reactive secondary alkyl
carbanions (S-3).
82
[a] Reaction conditions: amide 2a (1.00 mmol), Tf2O (1.10
mmol), 2-F-Pyr. (1.20 mmol), CH2Cl2 (0.2 M), 0 °C, 20
min.; enamine 1a, 0 °C to rt, 3 h; quenched and stirred for
5-6 h. [b] Isolated yield. [c] 15% of 3aa was isolated. 8%
[d]
of 3aa was isolated.
As can be seen from Table 2, with enamine 1a as
the alkylation reagent a wide array of secondary
amides can be converted to the corresponding ketones
in good to modest yields. For benzamide derivatives,
those bearing electron-donating groups on the phenyl
ring (entries 2-5) gave higher yields than having
electron-withdrawing groups (entries 6-15). The
reaction is widely functional
group tolerant
and
highly chemoselective. Not only halogen, nitro,
cyano groups, but also ester, ketone and aldehyde
groups, which are sensitive and labile under Tf2O
activation, are largely compatible with the current
process to furnish the desired ketones in good yields.
It should be noted that the reactions took place
chemoselectively even in the presence of tertiary
amide group to give the corresponding ketone 4oa
(entry 15). The steric hindered 2-methylbenzamide 2c,
2
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10.1002/adsc.201801443
Advanced Synthesis & Catalysis
however, is inert to the reaction (entry 3). The
reaction could proceed smoothly with thiophene-2-
carboxamide 2p. Variation of N-substituents on the
secondary aroyl amides from isopropyl to other
substituents such as tert-butyl, benzyl and phenyl
failed to give the desired products (entries 17-19).
The unbranched secondary alkanamides are also
suitable substrates for the reaction (entries 20-21),
while the branched ones failed to give the products
(entries 22-23).
min.; enamine 1a (2.50 mmol), 0 °C to rt, 3 h; quenched by
3 M HCl and stirred for 5-6 h. [b] Isolated yield. [c] Enamine
[d]
[e]
1a (2.00 mmol).
15% of 4ga was isolated.
The
[f]
reaction was complex and no desired product obtained.
Tf2O was added dropwise at –78 °C for 30 min. [g] SM was
[h]
recovered.
Amide 2a (10.0 mmol), enamine 1a (25.0
mmol), 0 °C to rt, 5 h.
Besides pyrrolidine enamine of isobutyl aldehyde
1a, enamines derived from other -substituted
aldehydes could also react smoothly with Tf2O
activated amide 2a to afford the corresponding ketone
products 4ab-af (Table 3). In general, enamines from
alkyl substituted aldehydes gave higher yields than
those from aryl substituted aldehydes. It should be
noted that the reaction could proceed with
functionalized enamines 1d and 1f to introduce the
alkyl groups with ester and halogen groups, which is
hard with organometallics as the alkylation reagents.
Aside from pyrrolidine, morpholine enamines could
also serve as competent alkylation reagents.
To explore the practical utility of this
transformation in organic synthesis, a gram scale
preparation of ketone 4aa from amides 2a (1.63 g,
10.0 mmol) with enamine 1a (3.13 g, 25.0 mmol) as
the alkylation reagent was conducted, which afforded
the desired ketone 4aa in 64% yield (entry 1, Table 2).
Table 2. Scope of Amides.[a]
Yield
(%)[b]
Entry Amides
R1
Ph
R2
Product
82[c]/
64[h]
78[c]
1
i-Pr
2a
4aa
2
3
4
p-MeC6H4
o-MeC6H4
p-MeOC6H4
3,4,5-
(MeO)3C6H4
p-FC6H4
p-ClC6H4
m-BrC6H4
p-CF3C6H4
p-NCC6H4
p-O2NC6H4
i-Pr
i-Pr
i-Pr
2b
2c
2d
4ba
4ca
4da
[c,g]
-
80[c]
85[c]
Table 3. Scope of Enamines.[a]
5
i-Pr
2e
4ea
6
7
8
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
63
51[d]
81
71
80
2f
2g
2h
2i
4fa
4ga
4ha
4ia
4ja
4ka
9
10
11
2j
2k
79
p-
12
i-Pr
i-Pr
77
2l
4la
MeC(O)C6H4
13
14
p-HC(O)C6H4
74
75
2m
2n
4ma
4na
p-MeO2CC6H4 i-Pr
p-
i-Pr
15
66[f]
45
2o
4oa
Et2NC(O)C6H4
16
17
2-Thienyl
Ph
i-Pr
t-Bu
2p
2q
4pa
4aa
[g]
-
18
Ph
Bn
trace[e]
2r
4aa
[e]
19
20
21
Ph
n-C10H21
Ph(CH2)2
Ph
i-Pr
i-Pr
-
2s
2t
2u
4aa
4ta
4ua
51
54
[a] Reaction conditions: amide 2a (1.00 mmol), Tf2O (1.10
mmol), 2-F-Pyr. (1.20 mmol), CH2Cl2 (0.2 M), 0 °C, 20
min.; enamines 1b and 1a'(2.0 mmol), 1c-f (2.5 mmol),
0 °C to rt, 3 h; quenched by 3 M HCl and stirred for 5-6 h.
[b] E/Z geometry undetermined.
22
23
c-Hex
i-Pr
i-Pr
trace[e]
2v
4va
[d]
-
2w
4wa
[a]
Reaction conditions: amides (1.00 mmol), Tf2O (1.10
mmol), 2-F-Pyr. (1.20 mmol), CH2Cl2 (0.2 M), 0 °C, 20
3
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10.1002/adsc.201801443
Advanced Synthesis & Catalysis
Having successfully implemented our synthetic
complex molecules. Further investigations on the
synthetic applications of this method are in progress.
plan, we next explored the possibility to undertake a
one-pot decarbonyl coupling of aldehydes with
amides. The Tf2O activated amide 1a was thus
reacted with enamines generated in situ from
aldehydes 5 and pyrrolidine, in the presence of 4 Å
molecular sieves. As can be seen from Table 4,
comparable yields were obtained for alkyl substituted
aldehydes, while much lower yields were afforded for
aryl substituted ones. It is noteworthy that the
reaction of cyclohexanone-4-carboxaldehyde 5g
proceeded chemoselectively to form enamine of
aldehyde, allowing for the introduction of 4-
oxocyclohexyl group.
Experimental Section
General procedure for the direct transformation of
secondary amides into ketones with enamines:
Trifluoromethanesulfonic anhydride (Tf2O, 1.10 mmol)
was added dropwise to a solution of amide 2 (1.00 mmol)
and 2-fluoropyridine (1.20 mmol) in dichloromethane (5.0
mL) at 0 ºC. After the mixture was stirred for 20 min,
enamine 1 was added dropwise at 0 °C. The resulting
mixture was warmed to rt and stirred for 3 h. The reaction
was quenched with aqueous HCl solution (3.0 M, 5.0 mL)
and stirredat rt for 5-6 h. The organic layer was separated
and the aqueous phase was extracted with diethyl ether (3
x 10 mL). The combined organic layers were washed with
brine (3 x 3 mL), dried over anhydrous MgSO4, filtered
and concentrated under reduced pressure. The residue was
purified by flash column chromatography on silica gel
(elution: hexane/EtOAc) to afford the desired ketone.
Table 4. One-pot Coupling of Secondary Amides with
Aldehydes.[a]
General procedure for the one-pot coupling of
secondary amides with aldehydes: Pyrrolidine (3.50
mmol) was added dropwise to a solution of -substituted
aldehydes 5 (3.50 mmol) and 4 Å molecular sieves (3.50 g,
1.00 g/mmol) in dichloromethane (5.0 mL) at 0 ºC. The
mixture was warmed to rt and stirred for
2 h.
Trifluoromethanesulfonic anhydride (Tf2O, 1.10 mmol)
was added dropwise to a solution of N-isopropylbenzamide
2a (163 mg, 1.00 mmol) and 2-fluoropyridine (1.20 mmol)
in dichloromethane (5.0 mL) at 0 ºC and stirred for 20 min.
The mixture of activated amide was syringed dropwise to
the mixture of enamines at 0 °C and the resulting mixture
was warmed to rt and stirred for 3 h. The reaction was
quenched by aqueous HCl solution (3.0 M, 5.0 mL) and
stirred at rt for 5-6 h. The organic layer was separated and
the aqueous phase was extracted with diethyl ether (3 x 10
mL). The combined organic layers were washed with brine
(3 x 3 mL), dried over anhydrous MgSO4, filtered and
concentrated under reduced pressure. The residue was
purified by flash column chromatography on silica gel
(elution: hexane/EtOAc) to afford the desired ketone.
Acknowledgements
[a] Reaction conditions: amide 2a (1.00 mmol), Tf2O (1.10
mmol), 2-F-Pyr. (1.20 mmol), CH2Cl2 (0.2 M), 0 °C, 20
min.; enamines generated in situ from aldehydes 5 (3.50
mmol) and pyrrolidine (3.50 mmol) in the presence of 4 Å
molecular sieve (1.0 g/mmol), 0 °C to rt, 3 h; quenched by
3 M HCl and stirred for 5-6 h.
Financial support of this work: the National Key R&D Program
of China (grant No. 2017YFA0207302), the National Natural
Science Foundation of China (21332007 and 21672176), the
PCSIRT of Ministry of Education, and the Natural Science
Foundation of Fujian Province of China (2014J01062).
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In conclusion, we have disclosed a new strategy
for the direct conversion of secondary amides to -
branched ketones with enamines as soft alkylation
reagents. In this transformation, enamines serve as
surrogates of secondary alkyl carbanions (S-2), rather
than the conventional enolates equivalents in the
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the traditional carbanion-based methods performed
under strongly basic reaction conditions, the current
conversion is performed in moderate acidic
conditions, which might be useful in the synthesis of
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10.1002/adsc.201801443
Advanced Synthesis & Catalysis
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5
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Advanced Synthesis & Catalysis
COMMUNICATION
Enamines as Surrogates of Alkyl Carbanions for
the Direct Conversion of Secondary Amides to -
Branched Ketones
Adv. Synth. Catal.Year, Volume, Page – Page
Yong-Peng Liu, Shu-Ren Wang, Ting-Ting Chen,
Cun-Cun Yu, Ai-E Wang* and Pei-Qiang Huang*
6
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