Selective carboxylation of reactive benzylic C–H bonds by a hypervalent iodine(III)/inorganic bromide oxidation system

Article abstract of DOI:10.3762/BJOC.14.94

An oxidation system comprising phenyliodine(III) diacetate (PIDA) and iodosobenzene with inorganic bromide, i.e., sodium bromide, in an organic solvent led to the direct introduction of carboxylic acids into benzylic C–H bonds under mild conditions. The unique radical species, generated by the homolytic cleavage of the labile I(III)–Br bond of the in situ-formed bromo-λ³-iodane, initiated benzylic carboxylation with a high degree of selectivity for the secondary benzylic position.

Full text of DOI:10.3762/BJOC.14.94

Selective carboxylation of reactive benzylic C–H bonds by a
hypervalent iodine(III)/inorganic bromide oxidation system
Toshifumi Dohi*1, Shohei Ueda1, Kosuke Iwasaki1, Yusuke Tsunoda1, Koji Morimoto1
and Yasuyuki Kita*2
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2018, 14, 1087–1094.
1College of Pharmaceutical Sciences, Ritsumeikan University, 1-1-1
Nojihigashi, Kusatsu, Shiga 525-8577, Japan. Tel: +81-77-561-4908,
and 2Research Organization of Science and Technology, Ritsumeikan
University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan. Tel &
Fax: +81-77-561-5829
doi:10.3762/bjoc.14.94
Received: 13 February 2018
Accepted: 26 April 2018
Published: 16 May 2018
Email:
This article is part of the Thematic Series "Hypervalent iodine chemistry in
organic synthesis".
Toshifumi Dohi* - td1203@ph.ritsumei.ac.jp; Yasuyuki Kita* -
kita@ph.ritsumei.ac.jp
Guest Editor: T. Wirth
* Corresponding author
© 2018 Dohi et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
carboxylic acids; C–H activation; iodine; oxygenation; radicals
Abstract
An oxidation system comprising phenyliodine(III) diacetate (PIDA) and iodosobenzene with inorganic bromide, i.e., sodium
bromide, in an organic solvent led to the direct introduction of carboxylic acids into benzylic C–H bonds under mild conditions.
The unique radical species, generated by the homolytic cleavage of the labile I(III)–Br bond of the in situ-formed bromo-λ3-iodane,
initiated benzylic carboxylation with a high degree of selectivity for the secondary benzylic position.
Introduction
The oxidative activation of a C(sp3)–H bond in organic mole- coupling strategy have been reported over the past few years,
cules to directly install various functional groups and new car- involving the promising catalytic activities of metal complexes
bon–carbon networks is a topic of interest for researchers [13,14]. On the other hand, reports aimed at realizing efficient
engaged in modern synthetic chemistry [1-8]. Benzylic oxida- and selective metal-free C(sp3)–H transformations are rather
tion is of particular interest because it is a convenient direct ap- limited; however, investigations by several research groups are
proach to arylcarbonyl compounds; it has a long history of still ongoing [15-30].
research and development, and thus is included among the well-
investigated C(sp3)–H transformations [9-12]. To widen the Hypervalent iodine reagents are now widely accepted as a safe
scope, recent studies and reaction systems have been further replacement for certain heavy-metal oxidizers, such as lead,
elaborated to include elegant C–H coupling methodologies. mercury, and thallium-based salts, due to their low toxicities,
Several important researches that provide a new benzylic C–H high stabilities, operational simplicities, and many other user-
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Beilstein J. Org. Chem. 2018, 14, 1087–1094.
friendly characteristics [31,32]. By virtue of their wide array of synthetic chemists. Significant advances for realizing such
reactivity patterns, the controllable radical and single-electron- transformations have been made over the last decade, however,
transfer (SET) reactivities [33-37] allow selective activation of strategies that do not utilize a directing group to facilitate the
the benzylic C(sp3)–H bond for oxidative functionalization and activation of the benzylic C(sp3)–H bond are rare [21,57-62].
coupling reactions. Initially, the SET oxidation ability of Furthermore, only a limited number of transition-metal-free
pentavalent iodine reagents, especially o-iodoxybenzoic acid methods have been reported; successful examples include the
(IBX), in benzylic oxidations was recognized for displaying the Wohl–Ziegler-type conditions [21], the sodium bromate system
new reactivities of hypervalent iodine reagents toward [57] for the conversion of benzylmethyl groups, and the use of
C(sp3)–H bonds [38,39]. By exploiting the radical behavior of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) [58] or cat-
trivalent iodine reagents discovered previously [40,41], the acti- alytic tetrabutylammonium iodide with tert-butyl hydrogen
vation of trivalent iodine reagents, e.g., phenyliodine(III) diac- peroxide for reactions with a large excess of aromatic hydro-
etate (PIDA), phenyliodine(III) bis(trifluoroacetate) (PIFA), and carbons [59]. Other than these excellent examples of metal-free
iodosobenzene, has since become a popular choice for benzylic methods, two protocols using a hypervalent iodine reagent were
oxidations, which further expanded the scope and availability of reported, both of which include the formation of benzyl radi-
methods for direct C–H functionalization and several coupling cals during the key initial reaction step. Togo and co-workers
reactions [42-50]. As such, we reported aqueous benzylic oxida- developed a reaction system consisting of stoichiometric
tions using polymeric iodosobenzene in the presence of inor- amounts of PIDA with catalytic amounts of molecular iodine
ganic bromide and montmorillonite-K10 [51]. In addition, a and p-toluenesulfonamide for the benzylic acetoxylation and
radical C–H activation strategy, using nonaqueous hypervalent benzoyloxylation of alkylbenzenes, where an in situ-generated
iodine(III)/inorganic bromide systems that can work in organic sulfonamidyl radical is the essential radical mediator that effec-
solvents, was developed for the novel synthesis of lactones via tively abstracts the benzylic hydrogen [49]. More recently,
the intramolecular oxidative cyclization of aryl carboxylic acids Maruoka et al. succeeded in the photolytic benzylic C–H bond
at the benzyl carbon under transition-metal-free conditions [52]. oxygenation of alkylbenzenes initiated by the decomposition of
Based on our previous research and general interest in the PIFA to form the trifluoroacetoxy radical under visible light ir-
unique reactivity of hypervalent iodine(III)–Br bonds [53-56], radiation [50].
we report the results of our extensive study and optimization of
our radical C–H activation strategy for the intermolecular oxi- Our approach for the generation of radical species for the
dative coupling between the benzylic secondary C–H bond and benzylic carboxylation using a hypervalent iodine reagent relies
the O–H group of carboxylic acids (Scheme 1).
on the unique reactivity of the hypervalent iodine(III)–bromine
bond with the following mechanistic principles: As illustrated in
Scheme 2, ligand exchange at the iodine(III) center of the
phenyliodine(III) dicarboxylate with the bromide ion can gradu-
ally produce the corresponding bromo-λ3-iodane in the first step
[63,64]. This unstable hypervalent iodine(III) species subse-
quently decomposes by facile homolytic cleavage of the
I(III)–Br bond, generating the iodanyl and bromo radicals
[51,52]. It appears that these radicals can then selectively
abstract the benzylic hydrogen atom of organic substrates, even
in the presence of a wide variety of functional groups, such as
electrophilic aromatic rings, non-acidic carbonyl groups, and
Scheme 1: Hypervalent iodine(III)-induced benzylic C–H functionaliza-
tion for oxidative coupling with carboxylic acids.
Results and Discussion
Benzylic C–H carboxylation can provide a convenient route to suitable oxygen, nitrogen, and sulfur functionalities. Carbony-
benzyl esters from non-functionalized aromatic hydrocarbons, loxy radicals derived from typical hypervalent iodine(III)
and thus has attracted continuous interest in the community of carboxylates by photolysis and other conditions [65-68] are
Scheme 2: Radical reactivities of the I(III)–Br bond generated from PIDA.
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Beilstein J. Org. Chem. 2018, 14, 1087–1094.
known to undergo irreversible decarboxylation [69,70]. There- we then examined other inorganic bromides, among which sodi-
fore, the formation of carbonyloxy radicals must be avoided to um bromide was the most promising, and product 2a was ob-
prevent non-productive side reactions and achieve the desired tained in 76% yield (Table 1, entry 6). By adding extra acetic
benzylic C–H transformations for an extended series of acid, the benzylic acetoxylation was further improved to
carboxylic acids.
provide an 86% yield of product 2a. Since most of the sodium
bromide was present as a precipitate in the flask, the reaction
Based on these considerations, we performed an optimization was found to work even with catalytic amounts of the bromide
study using 1-ethyl-4-methoxybenzene (1a) as a model sub- activator (Table 1, entry 8). Other reaction factors, such as sol-
strate for the oxidative C–H coupling of the benzyl group with vent, concentration, reaction time, and reagent quantities, were
acetic acid derived from PIDA (Table 1). Reactions using PIDA screened and, eventually, the reaction conditions of entries 6
with finely powdered inorganic bromide in degassed dichloro- and 7, which consisted of 1.2 equiv of PIDA with 2 equiv of so-
methane containing 0.1 M of the substrate were first examined dium bromide in dichloromethane (0.1 M of the reaction sub-
at room temperature. The use of potassium bromide [52] strate) at 40 °C, were determined to be the best in terms of prod-
afforded modest yields of the carboxylation product 2a uct yield. No reaction was observed in the absence of sodium
(Table 1, entry 1). Interestingly, a dramatic influence was ob- bromide (Table 1, entry 9) and other representative hypervalent
served when altering the bromide source to other types; the use iodine(III) reagents, such as PIFA and PhI(OH)OTs, and
of lithium bromide or organic bromides, e.g., bromotrimethylsi- pentavalent Dess–Martin periodinane and IBX, were inferior for
lane and tetraethylammonium bromide, instead of the potas- this carboxylation when compared to PIDA.
sium salt, were unsuccessful in forming the carboxylate 2a
(Table 1, entries 2–4). The reason for this behavior was thought We then examined the reactivity of different benzyl groups
to be because lithium bromide or organic bromides in combina- under the optimized reaction conditions for the radical activa-
tion with PIDA generated the electrophilic ‘Br+’ species [71] tion system for C–H acetoxylations (Table 2). When the reac-
and molecular bromine [72], or hypobromite and bisacetoxy tions were performed using ethylbenzene (1b) and its deriva-
bromate(I) [73], respectively, rather than the desired bromo tives without an electron-donating group (1c–e), the corre-
radical. As a result, bromination at the aromatic ring of sub- sponding benzyl bromides were mainly obtained along with a
strate 1a occurred when LiBr was used (Table 1, entry 2), while small amount of the target C–H acetoxylation product; this by-
no reaction was observed in the other two trials using organic product formation might imply the intermediacy of these
bromides (Table 1, entries 3 and 4). At the boiling temperature organic bromides before the production of benzyl acetates.
of dichloromethane, ca. 40 °C, the reaction time was shorter Hence, the reaction system was modified to include zinc(II)
(Table 1, entry 5). Aiming to improve the yield of the reaction, acetate [74] for substrates 1b–d and benzyl acetates 2b–d were
Table 1: Reaction optimization of benzylic C–H acetoxylation using PIDA.
entry
bromide
conditions
yield of 2a (%)
1
potassium bromide
lithium bromide
rt, 6 h
55
11a
nd
nd
57
76
86c
72
nd
2
rt, 20 h
3
bromotrimethylsilane
tetraethylammonium bromide
potassium bromide
sodium bromide
sodium bromide
sodium bromided
none
rt, 20 h
4
rt, 20 h
5
40 °C, 4 h
40 °C, 4 h
40 °C, 3 h
40 °C, 3 h
40 °C, 3 h
6
7b
8b
9
aMainly bromination of the aromatic ring was observed. bAcetic acid (20 equiv) was added. cThe reaction was performed on a 10 mmol scale.
dCatalytic amounts of sodium bromide were added (0.5 equiv). nd: not determined.
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Beilstein J. Org. Chem. 2018, 14, 1087–1094.
Table 2: Substrate screening for benzylic C–H acetoxylation by the PIDA/NaBr system.a
entry
substrate
product
time
yield of 2 (%)
1b–e
2b–e
1
2
3
4
1b (R1 = H)
1c (R1 = Ph)
2b (R1 = H)
2c (R1 = Ph)
20 h
20 h
20 h
48 h
55b
91b
65b
ndc
1d (R1 = Br)
2d (R1 = Br)
1e (R1 = CO2CH3)
2e (R1 = CO2CH3)
5
6
20 h
62
68
1f
2f
4 h
1g
2g
1h–j
2h–j
7
8
9
1h (R2 = R3 = H)
1i (R2 = R3 = F)
2h (R2 = R3 = H)
2i (R2 = R3 = F)
4 h
4 h
4 h
90
93
45
1j (R2 = Ph, R3 = H)
2j (R2 = Ph, R3 = H)
10
4 h
nd
1k
2k
aConditions: PIDA (1.2 equiv), sodium bromide (2 equiv), and acetic acid (20 equiv) in dichloromethane (0.1 M of substrate 1) at 40 °C. nd: not deter-
mined. bZinc acetate (1 equiv) was added for the conversion of the benzyl bromide byproduct to the desired benzyl acetates 2b–d. cThe correspond-
ing benzyl bromide was obtained in 60% yield.
obtained in moderate to excellent yields after prolonged reac- the pseudo acetal 2g (Table 2, entry 6). Diphenylmethane and
tion times (Table 2, entries 1–4). Furthermore, iterative oxida- its derivatives were very good substrates for our system and
tive coupling at the aromatic and benzylic C–H position using were less sensitive to the electronic effects of the aromatic ring
hypervalent iodine chemistry is possible, and 1-ethyl-4-me- substituents than in other previously described compounds,
thoxy-3-thiocyanatobenzene (1f), prepared from 1-ethyl-4- showing higher reactivity and chemoselectivity of the benzylic
methoxybenzene (1a) through the hypervalent iodine(III)-in- position (see ref. [51]). The reactions of substrates 1h–j
duced aromatic cation radical coupling with thiocyanate proceeded without the use of zinc(II) acetate (see Table 2,
[75,76], similarly acetoxylated under the standard reaction entries 7–9 versus entry 2).
conditions without zinc(II) acetate (Table 2, entry 5). The
acetoxylation at the activated benzyl carbon adjacent to an The installation of other carboxylic acids, such as propionic
oxygen atom proceeded smoothly as shown by the formation of acid, cyclohexyl carboxylic acid, pivalic acid, and benzoic acid,
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Beilstein J. Org. Chem. 2018, 14, 1087–1094.
were also possible by simply replacing PIDA with iodosoben- mechanism assumes the formation of a benzyl cation, resulting
zene (Scheme 3). Here, the addition of 3 Å molecular sieves from the SET oxidation of the benzyl radical, probably by the
was essential for removing water derived from the iodosoben- iodanyl radicals (path B).
zene and to suppress the benzylic oxidation forming aryl ke-
tones [52]. Note that the successful coupling of a range of sec- A significant point of interest in this study is understanding the
ondary and tertiary carboxylic acids now supports the direct and relative reactivity of the iodanyl radical versus the bromo
selective C–H bond activation at the benzyl position by radical. To gain more information about this, we confirmed the
avoiding the formation of carbonyloxy radicals, which are chemoselectivity of our reaction system towards different
susceptible to decarboxylation. In addition, it was revealed that classes of benzyl carbon atoms. To this end, the clear direction
more acidic benzoic acids were also suitable substrates for our to the secondary benzylic position in substrates 1a–j [51], and
method. However, even more acidic acids, such as trifluoro- the lack of benzylic acetoxylation of the tertiary carbon in sub-
acetic acid and methanesulfonic acid, were not effectively intro- strate 1k (see Table 2) are noteworthy. As bromo radicals are
duced by our benzylic C–H carboxylation procedures.
known to readily abstract hydrogen atoms at the tertiary carbon
centers [79], the results suggest a predominant participation of
the iodanyl radical as the hydrogen atom abstractor for the
selective secondary benzylic C–H bond activation in our reac-
tion. It seems that the iodanyl radical is more reactive than the
bromo radical and a large-size effect of the hypervalent iodine
species establishes the selectivity over the electronically favored
tertiary radical formation. A similar trend in the secondary-pref-
erential hydrogen abstraction at the sp3 carbons was recently re-
ported by Maruoka and co-workers for the C–H oxidations of
unreactive alkanes by iodanyl radicals [80]. Based on these ob-
servations, the reaction mechanism via path A that involves the
formation of benzyl bromides (X = Br) seems to be more rea-
sonable for our benzylic C–H carboxylation system based on
Scheme 3: Benzylic C–H carboxylations by the iodosobenzene/NaBr
system.
We believe that the reaction mechanism involves a benzyl the hypervalent iodine(III) reagent/inorganic bromide combina-
radical formation as the initiating step (Scheme 4). Either the tion. This mechanistic course was also partially supported by
iodanyl radical or the bromo radical may cause the H-atom the control experiment, whereby one of the separately prepared
abstraction generating the benzyl radical, and the resulting bromides, 2h’, was gradually transformed into the correspond-
benzyl radical is trapped by the persistent bromo radical, giving ing acetate 2h in good yield under the radical C–H acetoxyla-
rise to the observed benzyl bromide intermediate (path A). Al- tion reaction conditions (Scheme 5).
ternatively, the benzyl radical formed in situ couples with the
iodanyl radical to give the pseudo halide, benzyl-λ3-iodane, Conclusion
which is more reactive to nucleophilic substitution by a In conclusion, we have described the optimization and scope of
carboxylic acid [77,78]. Another possible course of the reaction an oxidation system for benzyl C–H carboxylation that utilizes
Scheme 4: Outline of the proposed reaction mechanism for the PIDA/NaBr system.
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Beilstein J. Org. Chem. 2018, 14, 1087–1094.
HRMS (MALDI): [M + Na]+ calcd for C12H13NO3SNa,
274.0507; found, 274.0508.
Representative experimental procedure for
the benzylic C–H carboxylation by the
iodosobenzene/NaBr system
In a flame-dried two-necked round-bottomed flask, under
nitrogen, iodosobenzene (132 mg, 0.6 mmol) and finely
powdered sodium bromide (103 mg, 1.0 mmol) were subse-
quently added to a stirred solution of diphenylmethane (1h,
84 mg, 0.50 mmol) and the carboxylic acid (ca. 2.5 mmol) with
freshly-dried molecular sieves 3 Å (ca. 300 mg) in dry dichloro-
methane (5 mL). Then the mixture was vigorously stirred at
40 °C. After checking the reaction completion by TLC, satu-
rated aqueous sodium carbonate was added to the mixture and
the resulting solution was stirred for an additional 5 min. The
organic layer was separated, washed again with saturated
aqueous sodium carbonate, then with dilute aqueous sodium
thiosulfate, and was dried over anhydrous sodium sulfate. After
removal of the solvents, the residue was subjected to column
chromatography on silica gel (eluents: n-hexane/ethyl acetate)
to give the benzylic C–H carboxylation product in the indicated
yield; the physical and spectral data of the carboxylation prod-
ucts (3h, 4h, 5h, 6h) matched those previously reported (see
Supporting Information File 1).
Scheme 5: Reaction of benzyl bromide 2h’ under radical C–H
acetoxylation conditions.
the radical reactivity of a hypervalent iodine(III) reagent pro-
duced under suitable conditions. The mechanistic information
obtained in this study indicates that iodanyl radicals, generated
by the homolytic cleavage of the labile I(III)–Br bond of in situ
formed bromo-λ3-iodane, are the key initiators for this benzylic
carboxylation, and they show a high degree of selectivity
towards the secondary benzylic position under mild reaction
conditions.
Experimental
Representative experimental procedure for
the benzylic C–H acetoxylation by the PIDA/
NaBr system
In a flame-dried two-necked round-bottomed flask, under
nitrogen, phenyliodine(III) diacetate (PIDA, 193 mg, 0.6 mmol)
and finely powdered sodium bromide (103 mg, 1.0 mmol) were
subsequently added to a stirred solution of arylalkane 1
(0.50 mmol) and acetic acid (0.57 mL, ca. 10 mmol) in dry
dichloromethane (5 mL). Then the mixture was vigorously
stirred at 40 °C with or without zinc acetate (92 mg, 0.5 mmol).
After checking the reaction completion by TLC, saturated
aqueous sodium carbonate was added to the mixture and the re-
sulting solution was stirred for an additional 5 min. The organic
layer was separated, washed again with saturated aqueous sodi-
um carbonate, then with dilute aqueous sodium thiosulfate, and
was dried over anhydrous sodium sulfate. After removal of the
solvents, the residue was subjected to column chromatography
on silica gel (eluents: n-hexane/ethyl acetate) to give the benzyl
acetate 2 in the indicated yield; the physical and spectral data of
the reported compounds (2a–d, g–j) matched those of the
authentic samples (see Supporting Information File 1).
Supporting Information
Supporting Information File 1
Starting materials and Copies of 1H and 13C NMR spectra
of all products.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-14-94-S1.pdf]
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific
Research (A) to Y.K. from JSPS and a Grant-in-Aid for Encour-
agement of Young Scientists (A) and Scientific Research (C) to
T.D. from JSPS, and Ritsumeikan Global Innovation Research
Organization (R-GIRO) project. T.D. thanks the research fund
of the Asahi Glass Foundation.
ORCID® iDs
1-(4-Methoxy-3-thiocyanatophenyl)ethyl acetate (2f). Yield
62% (156 mg). Obtained as a colorless oil; 1H NMR (CDCl3,
400 MHz) δ 1.51 (d, J = 6.8 Hz, 3H), 2.05 (s, 3H), 3.89 (s, 3H),
5.80 (q, J = 6.8 Hz, 1H), 6.88 (d, J = 8.3 Hz, 1H), 7.32 (dd, J =
8.3, 1.9 Hz, 1H), 7.53 (d, J = 2.0 Hz, 1H) ppm; 13C NMR
(100 MHz, CDCl3) δ 21.3, 22.1, 56.3, 71.3, 110.3, 111.2, 113.3,
127.6, 128.7, 135.8, 156.0, 170.2 ppm; IR (KBr): 2984, 2158,
1741, 1603, 1499, 1370, 1291, 1242, 1208, 1062, 1023 cm−1;
Toshifumi Dohi - https://orcid.org/0000-0002-2812-9581
Yasuyuki Kita - https://orcid.org/0000-0002-2482-0551
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