A photoinduced cross-dehydrogenative-coupling (CDC) reaction between aldehydes and N-hydroxyimides by a TiO2-Co ascorbic acid nanohybrid under visible light irradiation

Article abstract of DOI:10.1039/c7nj03651e

In this study, we performed a visible light-mediated aerobic photo-cross dehydrogenative coupling (CDC) reaction between aldehydes and N-hydroxyimides using TiO₂-AA-Co as a photocatalyst for the synthesis of active esters. The synergistic and selective effects of the cobalt ascorbic acid complex (Co-AA) and TiO₂ nanoparticles on the visible-light photocatalytic activity were explored. The method possesses some advantages such as environmentally friendly conditions, easy work-up procedure, reusability, and scalability.

Full text of DOI:10.1039/c7nj03651e

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A photoinduced cross-dehydrogenative-coupling
(CDC) reaction between aldehydes and
Cite this:DOI: 10.1039/c7nj03651e
N-hydroxyimides by a TiO –Co ascorbic acid
nanohybrid under visible light irradiation†
2
Received 23rd September 2017,
Accepted 1st December 2017
Fahimeh Feizpour,
Maasoumeh Jafarpour * and Abdolreza Rezaeifard
*
DOI: 10.1039/c7nj03651e
rsc.li/njc
In this study, we performed a visible light-mediated aerobic photo-
cross dehydrogenative coupling (CDC) reaction between aldehydes hyde and N-hydroxyimide can be employed for the synthesis of
and N-hydroxyimides using TiO –AA–Co as a photocatalyst for the active esters with high atomic efficiency and does not require
synthesis of active esters. The synergistic and selective effects of additional synthetic steps. Therefore, development of methods
the cobalt ascorbic acid complex (Co–AA) and TiO nanoparticles on for the cross-dehydrogenative coupling (CDC; or oxidative cross-
The cross-dehydrogenative coupling reaction between an alde-
2
2
the visible-light photocatalytic activity were explored. The method coupling) is of importance in modern organic chemistry and
possesses some advantages such as environmentally friendly condi- represents a promising approach for reducing the by-products
7
–26
tions, easy work-up procedure, reusability, and scalability.
as well as the number of synthesis steps.
In the present study, we performed a visible light-mediated
Quite recently, we improved the visible light photocatalytic aerobic photo-cross dehydrogenative coupling (CDC) reaction
properties of TiO by functionalization with a Co–ascorbic acid between aldehydes and N-hydroxyimides using TiO –AA–Co as
2
2
complex. The modified TiO
2
nanoparticles improved significantly a photocatalyst for the synthesis of active esters (Scheme 1),
the selective aerobic benzylic C–H oxidation of alcohols and which could be one of the straightforward alternatives to tradi-
hydrocarbons as well as epoxidation of various olefins in ethyl tional coupling that produces minimum by-products.
1
,2
1
acetate. Our results revealed that the efficiency of oxidation
The study was initiated by screening of the solvent nature,
is dependent on the NHPI amount, which acts as a free radical temperature, catalyst and NHPI amounts and oxidant nature
3
,4
oxidation co-catalyst. During the examination of NHPI con- under visible light (Fig. 1). 4-Chloro benzaldehyde was used as a
1
centration in the aerobic benzylic C–H oxidation of alcohols,
substrate to optimize the conditions. For the conversion of 4-chloro
in some cases we observed the pertinent carboxylic acid and benzaldehyde to the corresponding active ester, we observed that an
N-hydroxyesters as by-products. This observation induced us to excellent isolated yield of 97% (100% conversion) could be realized
develop a novel aerobic photocatalytic system to access N-hydroxy- using TiO
esters, which are of extreme interest and widely used in organic of aldehyde : NHPI in ethyl acetate (1 mL) for 45 min at 60 1C under
synthesis as reactive acylating reagents. air and visible light. Replacing the air with O did not change the
2
–AA–Co (0.06 mol%) as a catalyst with a 1 : 0.5 molar ratio
2
These active esters have been employed profusely for the reaction yield and selectivity; nevertheless, other oxidants such as
s
generation of the amide bond in peptides and proteins via H O (10% yield), UHP (90% yield), Oxone (30% yield) and TBAOX
2
2
5
N-acylation. The traditional approach involving the acylation (60% yield) reduced considerably the yield and selectivity of the
of alcohols with activated carboxylic acids or coupling of products as given in parentheses. Air was preferred as an ideal
N-hydroxyimides with carboxylic acids suffers from serious oxidant because it is easy, convenient and economical to use.
problems during the separation of by-products, particularly in
industrial scale-up. Thus, the development of effective and are also important determinants of the success of the reaction.
environmentally benign methods for this transformation under
The amount of catalyst and the molar ratio of aldehyde : NHPI
6
mild reaction conditions is needed.
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science,
University of Birjand, Birjand, 97179-414, Iran. E-mail: mjafarpour@birjand.ac.ir,
rrezaeifard@birjand.ac.ir; Fax: +98 5632202065; Tel: +98 5632202065
†
Electronic supplementary information (ESI) available: Synthesis and character-
ization of catalyst, experimental details and spectral data of all products. See DOI:
0.1039/c7nj03651e
1
Scheme 1 Synthesis of active esters.
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Fig. 1 (i) Screening of solvent [0.06 mol% catalyst, 0.5 mmol NHPI, temp. 60 1C], (ii) temperature [0.06 mol% catalyst, 0.5 mmol NHPI, EtOAc],
iii) solvent amount [temp. 60 1C, 0.06 mol% catalyst, 0.5 mmol NHPI], (iv) NHPI amount [temp. 60 1C, 0.06 mol% catalyst, 1 mL EtOAc], (v) catalyst
amount [temp. 60 1C, 0.5 mmol NHPI, 1 mL EtOAc] in air and visible light and (vi) oxidant nature on the cross coupling reaction of 4-chlorobenzaldehyde
1 mmol) and NHPI in the presence of TiO –AA–Co after 45 min.
(
(
2
For example, when 0.02 or 0.12 mol% catalyst was used versus aldehydes were suitable substrates toward photo-CDC reactions,
.06 mol% under the optimized conditions, the conversion reac- aliphatic ones such as acetaldehyde failed to produce any product
tion decreased from 100 to 80 and 60%, respectively. Likewise, under the optimized conditions.
when the ratio of aldehyde : NHPI was varied from the optimized After successful results of photo-CDC reaction with NHPI as
: 0.5 ratio to, for example, 1 : 0.8 and 1 : 1, in both cases, a coupling partner, we replaced NHPI with NHSI under the
0
1
the yields decreased to 60%. It is noteworthy that by varying optimal reaction conditions. It produced the corresponding
the aldehyde : NHPI ratio from the optimized ratio of 1 : 0.5 the NHSI ester in a comparable yield and selectivity, albeit in a
corresponding carboxylic acid as a by-product was obtained.
longer reaction time (Table 2). The lower reactivity of NHSI than
With an improved procedure for successful ester formation NHPI might be attributed to the higher bond dissociation energy
in hand, we then focused on the evaluation of the substrate (BDE) of the NO–H bond in the former during the generation of
2
7
scope. First, a set of structurally diverse aldehydes was coupled the reactive N–O radical.
with NHPI under air and visible light (Table 1). A broad range Next, we screened the photocatalytic effect of TiO
2
at the
of NHPI esters were successfully formed with moderate to core of the title nanohybrid on the photo-CDC efficiency as we
excellent yield using this protocol (Table 1). It was observed investigated previously on the oxidation of some organic com-
1
,2
that the electronic properties of the substituents on the aromatic pounds. For this purpose, we replaced the title nanohybrid
aldehydes displayed effects on the reaction efficiency. When the with other nanosized oxometals and their nanocomposites. The
phenyl substrate was substituted with electron donor groups product yield and selectivity reduced under standard conditions
such as 4-Me and 4-OMe (entries 3b and 3c), as well as halogens (Table 3).
(
3d, 3e, and 3g), excellent yields were observed (90–94%).
Nevertheless, with electron-withdrawing substituents such as nanohybrid was further supported by performing the CDC
NO (3f, 3h), moderate yields of 40–46% were obtained. reactions of NHPI with aryl aldehydes under UV and visible
2
The photocatalytic property of TiO at the core of the title
–
2
Similarly, the heterocyclic aromatic aldehydes (3m and 3n) light as well as in the dark (Table 4). The reactions accelerated
exhibited low yields (35 and 41%), which may be a consequence under light radiation particularly UV light confirming once
of the electron-withdrawing effect of the intermediates or again the photocatalytic effect of the TiO2 core on the CDC
transition states of the reaction. It is also apparent that the efficiency of the TiO –AA–Co nanohybrid.
2
2
6,28
steric bulkiness in the molecule retards reactivity and produces
Taking into account the data published in the literature,
the pathway for the formation of oxidative cross-coupling pro-
a lower product (3g–3k).
Furthermore, cinnamaldehyde participated successfully in this ducts 3 and 4 may be proposed (Scheme 2). It is believed that
reaction to provide ester 3l with 62% yield. Although aromatic the photo excited Co(II) oxidized NHPI to a phthalimide N-oxyl
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Table 1 Photo-cross dehydrogenative coupling reaction between aldehydes
Table 2 Photo-cross dehydrogenative coupling (CDC) reaction between
a,b,c
a,b,c
and NHPI using TiO
2
–AA–Co
2
aldehydes and NHSI using TiO –AA–Co
a
Reaction conditions: aldehyde (1 mmol), NHSI (0.5 mmol), catalyst
0.06 mol%) in EtOAc (1 mL) under air and visible light conditions.
The products were identified by H NMR spectroscopy. The reported
(
b
1
c
yields are yields of isolated products.
2
Table 3 The comparison of catalytic activity of TiO –AA–Co with other
nanocomposites and nanooxometals in photo-CDC reaction between
aldehydes and NHPI
a
Reaction conditions: aldehyde (1 mmol), NHPI (0.5 mmol), catalyst
(
0.06 mol%) in EtOAc (1 mL) under air and visible light conditions.
The products were identified by H NMR spectroscopy. The reported
b
1
c
yields are yields of isolated products.
a
radical (PINO), a fairly stable but highly reactive free radical. This
radical was then intercepted by the aldehyde to form a hemi-
aminal radical which on oxidation furnished the ester product.
The efficiency of the present photo-CDC reaction could be
highlighted by its reusability and scalability. The catalyst was
easily separated by centrifugation and reused in five con-
sequent runs in the reaction of 4-chlorobenzaldehyde and
NHPI (Fig. S7, ESI†). A slight reduction in the product yields
was observed after the 5th run indicating the efficiency and
Entry
Catalyst
Conversion (%) (A, B)
Selectivity %
1
2
3
4
5
6
7
8
9
TiO
TiO
2
–AA–Co
–AA
100 (A)
25 (A)
100
100
50
75
80
50
83
80
80
50
85
55
75
46
—
2
AA–Co
60 (30, 30)
20 (15, 5)
50 (40, 10)
40 (20, 20)
30 (25, 5)
50 (40, 10)
50 (40, 10)
20 (10, 10)
35 (30, 5)
45 (25, 20)
60 (45, 15)
65 (30, 35)
o5
TiO
TiO
2
–Co
2
ZrO
2
Fe
Fe
Fe
2
O
2
O
2
O
3
3
3
–AA
–AA–Co
10
MoO
MoO
MoO
3
3
3
stability of the title nanocatalyst. Also, the leaching of the 11
catalyst was screened by centrifugation and decantation of
the reaction mixture. No catalytic activity was observed in the
–AA
–AA–Co
12
13
14
2
TiO –OA–Co
Co(OAc)
AA
2
filtrate solution and based on the ICP-AES results, the amount 15
16
No catalyst
—
—
of Co in the catalyst after recycling is comparable with that of
the fresh catalyst. Much evidence for this issue was observed by
comparing the FT-IR and TEM images of the used catalyst with
the fresh one (Fig. S8, ESI†). The structure, size and morphology
of the catalyst remained intact after recycling. In support
of the scalability of this method, we conducted the reaction on
a
Reaction conditions: aldehyde (1 mmol), NHPI (0.5 mmol), catalyst
0.06 mol%) in EtOAc (1 mL) at 60 1C after 45 min. Under air and visible
(
light (Fig. S6, ESI).
In conclusion, we have developed an aerobic photocatalytic
10 times scale, whereupon the product was obtained with almost cross-dehydrogenative coupling reaction of aldehydes with
the same yield.
N-hydroxyimides, which provides active esters in moderate to
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a
Table 4 Screening of the photocatalytic activity of TiO
2
–AA–Co
Notes and references
b
Entry
Catalyst
Condition
Yield (%)
Time (min)
1
M. Jafarpour, F. Feizpour and A. Rezaeifard, RSC Adv., 2016,
c
1
2
3
4
5
6
7
8
9
TiO
TiO
TiO
TiO
TiO
TiO
TiO
TiO
TiO
2
2
2
2
2
2
2
2
2
UV
40
30
95
50
25
97
30
15
35
45
45
30
45
45
45
90
90
90
6, 54649–54660.
–AA
–AA–Co
UV
UV
2
M. Jafarpour, F. Feizpour and A. Rezaeifard, Synlett, 2017,
d
235–238.
Visible light
Visible light
Visible light
Darkness
Darkness
Darkness
–AA
–AA–Co
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Conflicts of interest
There are no conflicts to declare.
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