2-Iodo-N-isopropyl-5-methoxybenzamide as a highly reactive and environmentally benign catalyst for alcohol oxidation

Article abstract of DOI:10.3762/bjoc.14.82

Several N-isopropyliodobenzamides were evaluated as catalysts for the oxidation of benzhydrol to benzophenone in the presence of Oxone (2KHSO₅·KHSO₄·K₂SO₄) as a co-oxidant at room temperature. A study on the substituent effect of the benzene ring of N-isopropyl-2-iodobenzamide on the oxidation revealed that its reactivity increased in the following order of substitution: 5-NO₂ < 5-CO₂Me, 3-OMe < 5-OAc < 5-Cl < H, 4-OMe < 5-Me < 5-OMe. The oxidation of various benzylic and aliphatic alcohols using a catalytic amount of the most reactive 5-methoxy derivative successfully resulted in moderate to excellent yields of the corresponding carbonyl compounds. The high reactivity of the 5-methoxy derivative at room temperature is a result of the rapid generation of the pentavalent species from the trivalent species during the reaction. 5-Methoxy-2-iodobenzamide would be an efficient and environmentally benign catalyst for the oxidation of alcohols, especially benzylic alcohols.

Full text of DOI:10.3762/bjoc.14.82

2-Iodo-N-isopropyl-5-methoxybenzamide as a highly reactive
and environmentally benign catalyst for alcohol oxidation
Takayuki Yakura*, Tomoya Fujiwara, Akihiro Yamada and Hisanori Nambu
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 971–978.
Graduate School of Medicine and Pharmaceutical Sciences,
University of Toyama, Sugitani, Toyama 930-0194, Japan
doi:10.3762/bjoc.14.82
Received: 13 February 2018
Accepted: 18 April 2018
Published: 30 April 2018
Email:
Takayuki Yakura* - yakura@pha.u-toyama.ac.jp
* Corresponding author
This article is part of the Thematic Series "Hypervalent iodine chemistry in
organic synthesis".
Keywords:
hypervalent iodine; iodobenzamide; organic catalysis; oxidation;
oxone
Guest Editor: T. Wirth
© 2018 Yakura et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Several N-isopropyliodobenzamides were evaluated as catalysts for the oxidation of benzhydrol to benzophenone in the presence of
Oxone® (2KHSO5·KHSO4·K2SO4) as a co-oxidant at room temperature. A study on the substituent effect of the benzene ring of
N-isopropyl-2-iodobenzamide on the oxidation revealed that its reactivity increased in the following order of substitution:
5-NO2 < 5-CO2Me, 3-OMe < 5-OAc < 5-Cl < H, 4-OMe < 5-Me < 5-OMe. The oxidation of various benzylic and aliphatic alco-
hols using a catalytic amount of the most reactive 5-methoxy derivative successfully resulted in moderate to excellent yields of the
corresponding carbonyl compounds. The high reactivity of the 5-methoxy derivative at room temperature is a result of the rapid
generation of the pentavalent species from the trivalent species during the reaction. 5-Methoxy-2-iodobenzamide would be an effi-
cient and environmentally benign catalyst for the oxidation of alcohols, especially benzylic alcohols.
Introduction
The development of an efficient and environmentally benign produce hazardous waste. Recently, hypervalent iodine oxidants
organic synthesis is required for minimizing material use, have been widely employed for oxidation in organic synthesis
energy consumption, and environmental pollution in the pro- [1-9] because they are nonmetallic, less toxic, and easy to
duction of both bulk and fine chemicals. Oxidation is a funda- handle, and they allow mild reaction conditions in most cases.
mental and frequently used transformation in organic synthesis. Pentavalent iodine reagents such as Dess–Martin periodinane
Heavy metal-based oxidants such as chromium(VI), lead(IV), (DMP, 1) [10] and 2-iodoxybenzoic acid (IBX, 2) [11] are well
and mercury(II) have been extensively used for this purpose for known as representative environmentally benign oxidants for
a long time. However, these oxidants are highly toxic and alcohol oxidation (Figure 1). However, despite the utility and
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Figure 1: Structures of pentavalent iodine oxidants 1 and 2, and iodine catalysts 3–13.
versatility of these oxidants, they still have several drawbacks: Results and Discussion
both are potentially explosive, DMP is moisture-sensitive, and We evaluated several 2-iodobenzoic acid derivatives including
IBX is insoluble in common organic solvents. To overcome esters and amides as catalysts for alcohol oxidation in the
these drawbacks, IBX analogs [12-22] and several iodoxyarene presence of Oxone®, and we found that the simply modified de-
derivatives [23-38] have been developed, and the stabilization rivative, N-isopropylamide 13, exhibited excellent catalytic
of IBX by combining it with benzoic and isophthalic acids properties at room temperature [67]. Interestingly, the reactivi-
(SIBX) [39] has been reported. Nevertheless, from a green ty of 13 was much higher than that of 3 at room temperature
chemistry viewpoint, pentavalent iodine oxidants are not ideal but was lower at 70 °C. However, the reactivity of 13 itself was
because oxidation reactions require a stoichiometric amount of not so high: the oxidation of benzhydrol (14a) with 0.3 equiv of
the oxidant that produces an equimolar amount of organoiodine 13, 2.5 equiv of Oxone®, and 1 equiv of Bu4NHSO4 in
waste. The catalytic use of pentavalent iodine species is an MeNO2/H2O (8:3) completed in 12 h at room temperature
effective method for reducing the use of iodine compounds and (25 °C) to produce a 98% yield of benzophenone (15a, Table 1,
the produced waste [40-44]. 2-Iodobenzoic acid (3) [45,46] and entry 1) [67]. Therefore, enhancement of the reactivity of the
its derivatives such as 4–7 [20,47-51] and 2-iodobenzenesul- catalyst was desired for practical use. To develop a more reac-
fonic acid (8) and its derivatives 9–11 [52-57] have been de- tive catalyst, we evaluated several types of N-isopropyliodo-
veloped as catalysts for the oxidation of alcohols in the pres- benzamides (Table 1). Although most of the previously re-
ence of Oxone® (2KHSO5·KHSO4·K2SO4) as a co-oxidant. In ported iodoarene catalysts based on pentavalent iodine com-
these reported systems, high temperatures (40–70 °C) are often pounds have a carboxyl or sulfoxyl group at the ortho-position
required to generate potentially explosive pentavalent iodine to the iodine atom, Zhdankin and colleagues reported that both
compounds in situ except for the reactions involving multisub- 2-iodobenzenesulfonic acid (8) and 4-iodobenzensulfonic acid
stituted benzoic acids and benzenesulfonic acid (4–6 and 10) (12) exhibited high reactivity in their RuCl3–Oxone®
which can be performed at room temperature [20,48-51,57]. In co-oxidant system [58,59]. Therefore, we expected that the re-
contrast, the use of a combination of a catalytic amount of activity of 4-iodobenzamide 16 (Figure 2) would be similar to
RuCl3 and a stoichiometric amount of Oxone® as the co-oxida- that of 13. However, the oxidation of 14a with 16 under the
tion system does not require heating despite the use of iodo- above conditions did not complete even after 24 h, and the yield
benzene and 4-iodobenzenesulfonic acid (12) as catalysts of 15a and of recovered 14a was 30% and 67%, respectively
[58,59]. As part of our study on the development of multifunc- (Table 1, entry 2). These results indicate that the reactivity of
tionalized organocatalysts based on hypervalent iodine chem- the iodobenzamides depends on the ortho-relationship of the
istry [60-66], we found that N-isopropyl-2-iodobenzamide (13), iodine atom to the amide group. Therefore, we then investigat-
when utilized as a catalyst with Oxone® at room temperature, ed N-isopropyl-2-iodobenzamides that have an additional func-
appears to be a promising catalyst for efficient and environmen- tional group on the benzene ring. Based on the results of our
tally benign alcohol oxidation reactions [67]. Herein, we report studies on phenol oxidation [60,62] and Ishihara’s [52] and
our efforts on improving the reactivity of 2-iodobenzamide Moorthy’s [20,48] studies on alcohol oxidation, an electron-do-
catalysts.
nating group at the para-position to the iodine should be ex-
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Table 1: Oxidation of benzhydrol (14a) to benzophenone (15a) catalyzed by 13 and 16–25.
entrya
iodobenzamide
time (h)
yield (%)b
recovery of 14a (%)b
1c
2
13
16
17
18
19
20
21
22
23
24
25
12
24
6
98
30
97
98
98
98
96
98
23
98
99
–
67
–
3
4
10
18
19
23
46
48
13
23
–
5
–
6
–
7
–
8
–
9
75
–
10
11
–
aAll reactions were performed on a 0.5 mmol scale. bIsolated yield. cRef. [67]
When 14a was oxidized with 23, the reaction was very slow and
yielded only 23% of 15a and 75% of recovered 14a even after
48 h (Table 1, entry 9) [68]. Since the 5-methoxy derivative 17
was the most reactive, we then investigated the reactivities of
24 and 25, which have methoxy groups at the meta- and ortho-
positions to the iodine atom, and found that 4-methoxyamide 24
exhibited the same reactivity as nonsubstituted 13 (Table 1,
entry 10). On the other hand, oxidation with 3-methoxyamide
25 was much slower than that with 13 and needed 23 h to com-
plete (Table 1, entry 11). Although the ortho- and the para-
methoxy groups should show a similar electronic effect on the
iodine atom, the lower reactivity of 25 might be explained by
the steric hindrance around the iodine atom of 25. Conse-
quently, the reactivities of the 2-iodobenzamides 17–22, 24,
Figure 2: Structures of the catalysts 16–25.
pected to enhance the reactivity of iodoarene catalysts. The in- and 25 decreased in the following order of substitution:
vestigation began with the 5-substituted 2-iodobenzamides 5-OMe (17) > 5-Me (18) > H (13), 4-OMe (24) > 5-Cl (19) >
17–22 (Figure 2). Oxidation of 14a with 5-methoxy-2-iodo- 5-OAc (20) > 5-CO2Me (21), 3-OMe (25) > 5-NO2 (22).
benzamide 17 was much faster than that with 13 and was com-
pleted within 6 h to produce 15a in 97% yield (Table 1, With a highly reactive catalyst in hand, we examined the oxida-
entry 3). The 5-methyl derivative 18 exhibited a slightly higher tion of various secondary alcohols 14b–f and primary alcohols
reactivity than 13 (Table 1, entry 4). The introduction of an 14g–k with 0.3 equiv of 17 in the presence of 2.5 equiv of
electron-withdrawing group such as a chloro, acetoxy, Oxone® and 1 equiv of Bu4NHSO4 in an 8:3 mixture of
methoxycarbonyl, or nitro group at the 5-position led to a de- MeNO2 and water at room temperature. These results as well as
crease in reactivity (Table 1, entries 5–8). These results are in those obtained from a similar oxidation using 13 as a catalyst
good agreement with the reported studies [20,48,52,60,62]. To are summarized in Table 2. The secondary benzylic alcohols
confirm the importance of the electron-withdrawing 14b–e were oxidized with 17 in much shorter reaction times
carboxyamide group at the ortho-position, we examined the ox- than those oxidized with 13 to give the corresponding ketones
idation of 14a with 4-iodophenoxyacetic acid (23); the results 15b–e in good to excellent yields (Table 2, entries 1–4). Oxida-
showed excellent reactivity for phenol oxidations [60-64]. tion of the aliphatic secondary alcohol 14f with 17 required a
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Table 2: Oxidation of various alcohols 14b–k with 17.a
entry
1
alcohol
carbonyl compound
time (h)b
9 (15)
yield (%)b,c
95 (74)
14b
14c
15b
15c
2
3
7 (17)
86 (71)
62 (70)
11 (14)
14d
14e
15d
4
5
6
10 (20)
36 (30)
10 (20)
98 (97)
64 (74)
89 (82)
15e
15f
14f
14g
26g
26h
7
24 (24)
83 (74)
14h
8
9
16 (16)
9 (20)
96 (89)
85 (90)
14i
14j
26i
26j
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Beilstein J. Org. Chem. 2018, 14, 971–978.
Table 2: Oxidation of various alcohols 14b–k with 17.a (continued)
10
36 (36)
59 (58)
14k
26k
aAll reactions were performed on a 0.5 mmol scale. bThe results obtained for the oxidation using 13 are shown in parentheses [67]. cIsolated yield.
slightly longer reaction time than that with 13 (Table 2,
entry 5). The primary alcohols 14g–k were converted into the
corresponding carboxylic acids 26g–k in moderate to excellent
yields (Table 2, entries 6–10). However, the reaction times of
the oxidations of 14h, 14i, and 14k with 17 were similar to
those involving 13. These results may be due to the slow oxida-
tion of the aldehydes to carboxylic acids [69]. The catalyst 17
was stable under the oxidation conditions and it was recovered
in 67–92% after reductive treatment.
The next objective was to investigate the oxidation mechanism
of 2-iodobenzamide catalysts. Zhdankin and colleagues re-
ported the oxidation of alcohols to their corresponding carbon-
yl compounds using several 2-iodoxybenzamides [23]. There-
fore, we tried to confirm the formation of 2-iodoxybenzamide
29 from 2-iodobenzamide 17 in an oxidation reaction
(Scheme 1). Iodoarene 17 was treated with 2.5 equiv of Oxone®
and 1 equiv of Bu4NHSO4 in a 4:1 mixture of acetonitrile-d3
and D2O at a low concentration (0.01 M) at room temperature,
and the reaction was monitored by 1H NMR spectroscopy. The
reaction profiles for 17 as well as for 2-iodobenzoic acid (3) are
summarized in Figure 3. In the reaction involving 17, the
Figure 3: Reaction profile of the oxidation of (a) iodobenzamide 17
and (b) 2-iodobenzoic acid (3) with Oxone® in the presence of
Bu4NHSO4 in a 4:1 mixture of CD3CN and D2O: monovalent iodine de-
rivatives 17 and 3 (blue), trivalent iodine derivatives 27 and 28 (green),
and pentavalent iodine derivatives 29 and 2 (red).
amount of pentavalent iodine derivative 29 gradually increased
with a decrease in the amount of 17 and only a small amount of
trivalent iodine derivative 27 was observed during the reaction.
The ratio of 17 to trivalent 27 and pentavalent 29 was deter-
mined to be 38:9:53 after 36 h. In the oxidation of 3, monova-
lent 3 was consumed at almost the same rate as the oxidation of
17. However, the formation of pentavalent 2 was much slower
than that of 29, and a considerable amount of trivalent 28
remained during the reaction. After 36 h, the oxidation of 3
Scheme 1: Oxidation of the monovalent iodine derivatives 17 and 3 to
the pentavalent iodine derivatives 29 and 2 using Oxone®.
generated 27% of 2 with 35% unreacted 3 and 38% trivalent 28.
Thus, it is clear that the pentavalent iodine species 2 and 29
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Beilstein J. Org. Chem. 2018, 14, 971–978.
were generated during the oxidation reactions of monovalent 3 pound 29. 5-Methoxy-2-iodobenzamide 17 promises to be an
and 17 and that the oxidation of trivalent 27 to pentavalent 29 is efficient and environmentally benign catalyst for oxidation of
much faster than that of 28 to 2 at room temperature. These alcohol, especially benzylic alcohols.
results also suggest that the formation of the pentavalent iodine
species from the corresponding trivalent iodine species might be Experimental
the rate-determining step in the catalytic hypervalent iodine oxi- Typical experimental procedure for the oxidation of second-
dation of 17 [70].
ary alcohols 14a–f: Secondary alcohol 14 (0.50 mmol) was
added to a solution of the catalyst (0.15 mmol) and Bu4NHSO4
On the basis of the above results, a plausible mechanism for the (170 mg, 0.50 mmol) in a mixture of MeNO2 (1.6 mL) and
oxidation catalyzed by the 2-iodobenzamides is shown in water (0.6 mL), followed by Oxone® (768 mg, 1.25 mmol) at
Scheme 2. Iodobenzamide A is readily oxidized by tetra-n- room temperature (25 °C). After 14 was completely consumed,
butylammonium peroxymonosulfate (Bu4NHSO5, 30), which is as indicated by TLC, the resulting mixture was diluted using
derived from Bu4NHSO4 and Oxone®, to pentavalent iodine EtOAc and was washed with water. The organic layer was then
species C at room temperature. The resultant C oxidizes alcohol washed with saturated aqueous Na2S2O3 and saturated aqueous
14 to ketone 15 or aldehyde 31 during its reduction to trivalent NaHCO3, dried over MgSO4, filtered, and concentrated under
iodine B. Aldehyde 31 is further oxidized with 30 with the reduced pressure. The residue was purified by silica gel column
assistance of A or C [67] to give carboxylic acid 26. Iodine B is chromatography to give pure ketone 15 and the catalyst.
re-oxidized with 30 to regenerate C. The oxidation proceeds at
room temperature because of the fast oxidation of trivalent B to Typical experimental procedure for the oxidation of prima-
pentavalent C.
ry alcohols 14g–k: Primary alcohol 14 (0.50 mmol) was added
to a solution of the catalyst (0.15 mmol) and Bu4NHSO4
(170 mg, 0.50 mmol) in a mixture of MeNO2 (1.6 mL) and
water (0.6 mL), followed by Oxone® (768 mg, 1.25 mmol) at
room temperature (25 °C). After 14 was completely consumed,
as indicated by TLC, the resulting mixture was diluted with
EtOAc, water, and saturated aqueous Na2S2O3. The organic
layer was then washed with saturated aqueous Na2S2O3, satu-
rated aqueous NaHCO3, and brine, dried over MgSO4, filtered,
and concentrated under reduced pressure. The residue was puri-
fied by silica gel column chromatography to give the catalyst.
The combined aqueous layers were acidified with 10% HCl and
extracted with EtOAc. The organic layer was washed with
brine, dried over MgSO4, filtered, and concentrated under
reduced pressure. The residue was purified by silica gel column
chromatography to give pure carboxylic acid 26.
Supporting Information
Scheme 2: Plausible reaction mechanism for the oxidation of alcohols
catalyzed by the 2-iodobenzamides.
Supporting Information File 1
Experimental details and the 1H and 13C NMR spectra of
the catalysts, the substrates, and the products.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-82-S1.pdf]
Conclusion
In summary, we have developed 2-iodo-N-isopropyl-5-
methoxybenzamide (17) as an efficient catalyst for the oxida-
tion of primary and secondary alcohols. The reaction of
benzylic and aliphatic alcohols 14 with a catalytic amount of 17
in the presence of Oxone® and Bu4NHSO4 at room tempera-
ture proceeded smoothly to provide good to excellent yields of
the corresponding carbonyl compounds 15 and 26. The higher
reactivity of 17 at room temperature results from the rapid oxi-
dation of trivalent iodine compound 27 to the pentavalent com-
Acknowledgments
This research was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas “Advanced Molecular
Transformations by Organocatalysts” from the Ministry of
Education, Culture, Sports, Science and Technology, Japan;
JSPS Core-to-Core Program, B. Asia-Africa Science Platforms;
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Beilstein J. Org. Chem. 2018, 14, 971–978.
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