Visible-light-promoted oxidation/condensation of benzyl alcohols with dialkylacetamides to cinnamides

Article abstract of DOI:10.1039/c8ob02938e

Oxidative cross-coupling reactions of benzyl alcohols with N,N-dialkylacetamides were developed only employing oxygen as the terminal oxidant, efficiently providing a new, novel protocol for the construction of multifunctionalized cinnamides with the synergistic effects of KOH, organic photocatalyst eosin Y, and visible light irradiation at room temperature. A broad substrate scope and mild reaction conditions are the prominent features of this transformation.

Full text of DOI:10.1039/c8ob02938e

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Visible-light-promoted oxidation/condensation of
benzyl alcohols with dialkylacetamides to
cinnamides†
Cite this: Org. Biomol. Chem., 2019,
17, 449
Received 25th November 2018,
Accepted 10th December 2018
Tianlong Yang,a Maojian Lu,a Zhaowei Lin,a Mingqiang Huanga and
a,b
DOI: 10.1039/c8ob02938e
Shunyou Cai
*
rsc.li/obc
Oxidative cross-coupling reactions of benzyl alcohols with N,N-di-
However, this method usually requires multiple steps and
alkylacetamides were developed only employing oxygen as the term- expensive or toxic reagents. To address these issues, various
inal oxidant, efficiently providing a new, novel protocol for the con- alternative methods for cinnamide syntheses including the
struction of multifunctionalized cinnamides with the synergistic strategies of C–H bond functionalizations are disclosed in pre-
effects of KOH, organic photocatalyst eosin Y, and visible light vious literature.4
irradiation at room temperature. A broad substrate scope and mild
In 2010, Tsuji and co-workers developed a pioneering meth-
reaction conditions are the prominent features of this transformation. odology for the preparation of a variety of α,β-unsaturated
amides through Pd-catalyzed intermolecular addition of for-
Unsaturated amides are useful building blocks of numerous
mamides to terminal alkynes, in which the unsaturated
organic compounds, which are prevalent in many bioactive
amides carrying a terminal methylene moiety were recorded as
natural products, functional materials, and pharmaceutical
the major products (Scheme 2b).4a Other synthetic studies
drugs.1 They are able to frequently function as anticancer
aimed at straightforward functionalization of the inert sp2
agents, histone deacetylase inhibitors, as well as antitumor
C–H bond of the formamides were subsequently stimulated by
this work. For example, a practical methodology to assemble
α,β-unsaturated amides via the strategy of oxidative coupling of
carboxylic acids with N,N-dialkylformamides was realized in
agents, and gelatinase inhibitors (Scheme 1).2 Given the sig-
nificance of these utilities, the development of convenient pro-
tocols for the generation of cinnamides has captured consider-
able attention in the synthetic community. Traditionally, the
direct amidation of cinnamic acids with amines is the most
frequent synthetic route (Scheme 2a).3
the presence of
(Scheme 2c).4f
a CuCl2 and tBuOOH catalytic system
In spite of the efficiency and generality of these reactions,
they remain not enough for green chemistry because the estab-
Scheme 1 Cinnamide moiety in biologically active molecules.
aKey Laboratory of Modern Analytical Science and Separation Technology of Fujian
Province, School of Chemistry, Chemical Engineering and Environment, Minnan
Normal University, Zhangzhou, 363000, China. E-mail: caishy05@mnnu.edu.cn
bKey Laboratory of Chemical Genomics, School of Chemical Biology and
Biotechnology, Peking University, Shenzhen Graduate School, Shenzhen, 518055
China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8ob02938e
Scheme 2 Diverse synthetic strategies to α,β-unsaturated amides.
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lished protocols always require the use of toxic metal salts yield of the desired cinnamide 3a was observed in the solvent
and/or strong oxidants under harsh reaction conditions. of N,N-dimethylacetamide (DMA) at room temperature in the
Therefore, discovering convenient, mild, and straightforward presence of an organic dye-type eosin Y (0.01 equiv.) and
methods for α,β-unsaturated amide synthesis with simple sub- NaOH (1.0 equiv.) under an air balloon atmosphere for 8 h
strate structures, less employment of transition-metal catalysts (entry 1). This result convincingly supported the feasibility of
and oxidants, and a step- and atom-economical process is still the directed oxidation/condensation of benzyl alcohols with
not fully developed.
N,N-dialkylacetamides as a new potential reaction pathway to
Owing to remarkable benefits of visible-light-promoted cinnamides under mild conditions. The reaction efficiency
photoredox catalysis,5–8 there has been a recent surge in inter- and product yield could be readily improved by employing the
est to design and identify a variety of protocols available for amount of NaOH in excess, from which the use of 5 equiv. of
facile preparation of bioactive molecules using stable and in- NaOH could provide 3a in
a satisfactory 65% yield.
expensive substrates. Within this scenario, we anticipated that Interestingly, a switch of the base NaOH to KOH was demon-
an oxidation event taking place on a benzyl alcohol might be strated to be advantageous for the product yield (78% yield,
first initiated in the presence of a photocatalyst, visible light entry 4). So we took it that the significant promoting effect of
stimulation, and external oxidation, thereby converting it into the KOH additive principally came from its ability to decrease
a benzaldehyde,9 which would subsequently undergo conden- the oxidation potential of benzyl alcohols. However, further
sation with the N,N-dialkylacetamide to yield α,β-unsaturated increasing the loading of base KOH (entry 5) or photocatalyst
amides under alkaline conditions (Scheme 2). As far as we eosin Y (entry 6) could not improve the reactivity. Subsequent
know, this scenario has not been developed before. attempts with the utilizations of a range of bases including
Fortunately, a sequential oxidation/condensation process in a K2CO3, Cs2CO3, NaH, tBuOK, and 1,8-diazabicyclo[5.4.0]undec-
range of benzyl alcohols with N,N-dialkylacetamides was ulti- 7-ene (DBU) all resulted in complete inhibitions in reactivities
mately realized through careful identification of the optimal or complicated mixtures (entries 7–11). Other transition-metal-
reaction conditions. This method was able to operate efficien- lic photocatalysts, such as Ru(bpy)3Cl2 and Ir[dF(CF3)
tly at room temperature and avoid employing strong oxidants, ppy]2(dtbbpy)PF6 (Ir-cat.), were employed in place of eosin Y;
transition-metal catalysts, and unstable substrates.
competent reactivities were recorded in both cases under
In our preliminary investigation, benzyl alcohol 1a was otherwise identical reaction conditions (entries 12 and 13).
employed as the model substrate and blue LEDs as the light However, an organic dye-type eosin Y, compared with its tran-
source. As illustrated in Table 1, we initially found that 26% sition-metal-derived counterparts, appears to represent
a
superior alternative in terms of environmental friendliness
and cost. Finally, the solvent effect was also shown to be fairly
critical, since addition of tetrahydrofuran (THF), dimethyl-
formamide (DMF), or 1,2-dichloroethane (DCE) to DMA as the
co-solvent, under otherwise comparable reaction conditions,
was found to have a negative effect on the reaction efficiency
(entries 16–18). Collectively, these screening results estab-
lished a convenient and efficient visible-light-enabled photo-
catalytic oxidation/condensation of benzyl alcohols with di-
alkylacetamides relying on the synergistic effects of 5.0 equiv.
of KOH base and 1 mol% of the eosin Y photocatalyst under
an air balloon atmosphere at room temperature.
With the identified conditions in hand, the scope of the
benzyl alcohols with a series of functional groups was next
investigated and the results are described in Scheme 3. The
reactivity seems to be general, producing smoothly the
expected α,β-unsaturated amide 3 in moderate-to-good isolated
yields (54–87%) with high E/Z geometrical selectivity in each
case. Interestingly, the alcohols with a 2-naphthalenyl or
3-naphthalenyl substituent, respectively, were analyzed in par-
allel; the latter was demonstrated to be more efficient than the
former (28% yield vs. 76% yield). The benzyl alcohols bearing
a variety of frequently encountered electron-donating groups,
such as methyl (3d–3f), methoxyl (3g–3j), benzyloxy (3k), tri-
fluoromethoxy (3l), and tert-butyl (3m), on the aromatic ring
were found to be well tolerated and efficiently converted
into the corresponding products in various isolated yields
(54–87%). Most remarkably, the successful effective formation
Table 1 Optimization of the reaction conditionsa
[PC]
Base
(equiv.)
Solvent
(v/v)
Yielde
(%)
Entry (x mol%)
1
2
3
4
5
6
7
8
Eosin Y (1.0)
NaOH (1.0)
NaOH (3.0)
NaOH (5.0)
KOH (5.0)
KOH (7.0)
KOH (5.0)
K2CO3 (5.0)
DMA
DMA
DMA
DMA
DMA
DMA
DMA
26
43
65
78
72
74
Trace
Trace
Decomp
Decomp
NR
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (2.0)
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
Cs2CO3 (5.0) DMA
9
NaH (5.0)
tBuOK (5.0)
DBU (5.0)
DMA
DMA
DMA
DMA
DMA
DMA
DMA
10
11
12
13b
14c
15d
16
17
18
[Ru(bpy)3Cl2] (1.0) KOH (5.0)
71
73
52
Trace
[Ir-Cat] (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
Eosin Y (1.0)
KOH (5.0)
KOH (5.0)
KOH (5.0)
KOH (5.0)
KOH (5.0)
KOH (5.0)
DMA/THF (1/1) Decomp
DMA/DMF (1/1) 32
DMA/DCE (1/1) Trace
a General reaction conditions: 1a ((0.24 mmol), photocatalyst
(1.0 mol%) and base (5 equiv.) in DMA (1.0 mL) at rt in air with 18 W
blue LEDs. b [Ir-Cat] = Ir[dF(CF3)ppy]2(dtbpy)PF6. c The reaction was
performed under a balloon oxygen atmosphere. d 0.1 mL of water was
added. e Yield of the isolated product.
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Scheme 5 Preparation of the cinnamonitrile 6.
Some control experiments were subsequently performed to
rationalize the potential reaction mechanism. As shown in
Scheme 6, when the reactions were carried out in the absence
of the air atmosphere, KOH, eosin Y, or visible light irradiation
only resulted in nearly inhibiting the intended reactivity, ascer-
taining definitely that generation of 3a needed all. Since the
transformation required oxygen participation, a radical trap-
ping experiment was thus carried out by introducing 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) into the reaction mix-
tures as a radical scavenger. The result showed that the desired
reactivity was inhibited by TEMPO to some extent, suggesting
involvement of radical intermediates in this reaction.
Furthermore, when benzyl methyl ether 7 was employed in
place of benzyl alcohol 1a to couple with dimethylacetamide
2a under standard reaction conditions, no anticipated cinna-
mide product 3a was observed at all. When benzaldehyde 8
was exposed to the optimal reaction conditions or only to alka-
line conditions, it was capable of affording the cinnamide
product 3a in good yield, implying strongly that benzaldehyde
might serve as a key intermediate in this oxidation/conden-
sation process.
Scheme 3 Substrate scope screenings on benzyl alcohols 1.
of 3m–3r with intact carbon-halogen bonds would afford a
good platform for further needed structural manipulations. It
should be mentioned that the heterocyclic moiety was also
accommodated in this oxidation/condensation process and
provided 3s in 62% yield. However, cinnamyl alcohol 1t was
demonstrated to be nearly inert under our conditions.
On the basis of the observed reactivities and above investi-
gations, even though the exact reaction mechanism is not
clear, a plausible mechanistic rationale for cinnamide for-
mation is illustrated in Scheme 7. Within visible light
irradiation, the photo-excited state species eosin Y would
induce an event of electron abstraction on the benzyl alcohol
1a, whose oxidation potential might be significantly decreased
under alkaline conditions, and thus gave rise to the highly
active radical intermediate A and radical anion eosin Y. This
radical anion eosin Y was able to be expediently oxidized by
the molecular oxygen to go back to the ground state.
Encouraged by the above results, we attempted to further
explore the substrate scope of diverse N,N-dialkylacetamides
under the optimal reaction conditions. As compiled in
Scheme 4, utilizing 1a as a representative substrate, similar
reactions were found to proceed smoothly with other dialkyl-
acetamide derivatives, except for the 2b, thus generating the
intended cinnamide 4 in different isolated yields (39–72%).
These findings, in conjunction with the aforementioned reac-
tivities, further indicated that the present protocol could be
widely applicable to substrates with varied functionalities.
Finally, it is interesting that with a switch to employment of
acetonitrile as the solvent under otherwise identical reaction
conditions, the benzyl alcohol 1a could also be converted into
the corresponding condensation product cinnamonitrile 6,
albeit with relatively low reactivity (Scheme 5).
Scheme 4 Substrate scope screenings on dialkylacetamides 2.
Scheme 6 Probes on control experiments.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The work was supported by the National Natural Science
Foundation of China (No. 21502086 and 41575118), Natural
Science Foundation of Fujian Province (No. 2015J05028),
Outstanding Youth Science Foundation of Fujian Province (No.
2015J06009), and Program for Excellent Talents of Fujian
Province.
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