Trichloroacetonitrile as an efficient activating agent for the: Ipso -hydroxylation of arylboronic acids to phenolic compounds

Article abstract of DOI:10.1039/c9ob01568j

A metal-free and base-free Cl₃CCN mediated method was developed for the ipso-hydroxylation of aryl boronic acids to their corresponding phenols, which was promoted by a key unstable Lewis adduct intermediate. This transformation has broad functional group tolerance, and late-stage functionalization was successful as well. After simple investigation, two pathways (radical/ionic mechanism) were suggested, and the beneficial action of blue light needs to be further studied.

Full text of DOI:10.1039/c9ob01568j

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DOI: 10.1039/C9OB01568J
ARTICLE
Trichloroacetonitrile as an efficient activating agent for ipso-
hydroxylation of arylboronic acids to phenolic compounds
Yuanding Fang,b,c Rong Zhao,b,c Yuan Yao,c Yang Liu,c Denghu Chang,b,c Ming Yaoa and Lei Shi*a,b,c,d
Received 00th January 20xx,
Accepted 00th January 20xx
A metal-free and base-free Cl3CCN mediated method was developed for the ipso-hydroxylation of aryl boronic acids to
corresponding phenols, which promoted by key unstable Lewis adduct intermediate. This transformation has broad
functional group tolerance, and late-stage functionalization were successful as well. After simple investigation, two
pathways (radical/ionic mechanism) were suggested, and the beneficial action of blue light needs to be further studied.
DOI: 10.1039/x0xx00000x
Introduction
a. Previous work
OH
Ph
OH
B
Lewis acid and base interactions are widespread phenomena in
organic chemistry, which was defined as the Lewis acid-base
theory by Gilbert N. Lewis as early as 1923.1 Lewis acid (LA)
species contains an empty orbital, while a Lewis base (LB)
contains an unbonded electron pair. A dative bond is formed
between the electron acceptor and donator to form a Lewis
adduct. A classic and valuable example of Lewis adduct is boron
trifluoride diethyl etherate (BF3•Et2O),2 widely used as an easy-
to-handle Lewis acid catalyst. Notably, one of the most
extensively investigated kind of Lewis adducts is the “frustrated
Lewis pairs” (FLP),3 consist of sterically demanding Lewis donors
and acceptors, which remains “unquenched” reactivities and
act as both Lewis acids and bases in reactions. Recently, Niu et
al. made use of the uncommon in situ unfrustrated Lewis adduct
of boronic acid and hydroxylamine (B and N atoms) for
aminations of boronic acids, with an activating reagent
trichloroacetonitrile (CCl3CN) (Scheme 1).4 We wondered if
there are other practical methods based on in situ Lewis adduct
intermediates of boronic acids and oxygen-containing
compounds.
Cl3CCN
OH
OH
O
Ph
+
H
N
H
Ph
N
Ph
Ph
B
NH
Ph
H
O
HN
CCl3
b. This work
OH
LA
Ph
OH
B
Cl3CCN
OH
OH
O
tBu
+
Ph
B
H
O
O
O
tBu
LB
Lewis adduct
HN
CCl3
?
Scheme 1 ipso-Functionalization of aryl boronic acids promoted by Lewis
adduct.
R
Nu
NH
CCl3
Cl3CCN
2014
Jeffrey S. Arnold, Eur. J. Org. Chem.
, 4925.
O
R
OH
OH
O
H
N
H
R1
Ooi, Chem
O
R2
N
R1
R4 R3
2016
, 1, 802.
HO OH
R
1961
G. B. Payne, J. Org. Chem.
, 26, 659.
The CCl3CN had been proved could transform the hydroxyl
group into the trichloroacetimidates,5,6 which is a good leaving
group to achieve multiple types of reactions and construct
diverse products (Scheme 2). Payne first developed the
epoxidation of alkenes by peroxy trichloroacetimidic acid
generated in situ from hydrogen peroxide and
O
HO
R1
R2 R3
2017
Ooi, Synlett
, 28, 1291.
Scheme 2
Research progress of trichloroacetonitrile.
trichloroacetonitrile, besides, discovered that CCl3CN could
increase the rate of the reactions in 1961.7a,7b This payne-type
oxidation has been classically utilized for glycosylation
reactions.7c,7d This kind of reactions were developed by
researchers in the past decade, the role of trichloroacetonitrile
had been deeply studied. Ooi’s group developed a strategy for
the direct catalytic asymmetric introduction of structurally
a. Hubei Key Laboratory of Drug Synthesis and Optimization, Jingchu University of
Technology, Jingmen, 448000, China.
E-mail: lshi@hit.edu.cn
b. Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen, 518055,
China.
c. MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion
and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of
Technology, Harbin 150001, China.
d. Beijing National Laboratory for Molecular Sciences Beijing 100190, China.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
diverse
non-protected
amino
groups
by
using
trichloroacetonitrile with the hydroxylamines in 2016,8 they
speculated an unstable electrophilic O-imino hydroxylamine
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DOI: 10.1039/C9OB01568J
Table 1 Optimization of reaction conditionsa
OH
OH
B
conditions
OH
1a
2a
O
N
O
N+ C-
Cl
O
N
3a
3b
3c
4a
Entry
Oxidant
Atmosphere
Solvent
Light
Activator
Yieldb (%)
1
TBHPc
TBHPc
TBHPc
TBHPc
TBHPc
TBHPc
TBHPc
TBHPc
TBHPc
TBHPc
4a
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
N2
MeCN
MeCN
THF
blue LED
dark
Cl3CCN
Cl3CCN
Cl3CCN
Cl3CCN
Cl3CCN
Cl3CCN
3a
78%
37%
59%
N.R.
trace
44%
73%
75%
67%
35%
N.R.
67%
70%
N.R.
46%
96%
N.R.
50%f
80%g
2
3
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
blue LED
--
4
DMF
5
DCM
6
toluene
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
7
8
3b
9
3c
10
11
12
13
14
15
16
17
18
19
--
Cl3CCN
Cl3CCN
Cl3CCN
Cl3CCN
Cl3CCN
Cl3CCN
Cl3CCN
--
d
H2O2
TBHPe
--
TBHPc
TBHPc
--
TBHPc
TBHPc
O2
O2
O2
O2
Cl3CCN
a Reaction conditions: 1a (0.6 mmol), oxidant (1.0 equiv.), nitriles (1.5 equiv.), solvent (5 mL), blue LED, 12 h, r.t. b Isolated yields. c TBHP was 5.5 M in
decane.d H2O2 was 30% in H2O. e TBHP was 70% in H2O, f 60h. g 40 ℃, 12 h.
was generated. Next year, they reported a method using by Ainley and Challenger in 1930.13 The initial low yields and
aqueous hydrogen peroxide and trichloroacetonitrile in situ undesirable side reactions were improved by various reaction
generated peroxy trichloroacetimidic acid, which could act as a conditions: (1) employing transition-metal catalysts, for
competent electrophilic oxygenating agent for the direct α- instance Cu,14a-14c Pd,14d Ru,14e and Ag;14f (2) using
hydroxylation of oxindoles.9
stoichiometric amounts of oxidants such as tert-butyl
Aryl boronic acids have been widely used in synthetic hydroperoxide (TBHP),15a,15b 3-chloroperoxybenzoic acid
chemistry, materials chemistry and pharmaceutical chemistry (MCPBA),15c oxone,15d,15e hydroxylamine,15f amine oxide,15g
on account of its relative stability to water and air, and their ozone,15h PhI(OAc)2,15i,15j benzoquinone,15k NaClO2,15l sodium
wide range of availability.10 On the other hand, as a kind of sulfite15m and sodium ascorbate,15n which are also carried out
cheap, attractive and readily available oxygen-containing with H2O2 in several reports.15o-15s (3) electrochemical
compounds, peroxides are widely used for delivering oxygen to reactions;16 (4) aerobic organocatalysis conditions;17 and (5)
various functional groups.11 Based on the interest of in situ photocatalytic reactions.18 However, most of the existing
unfrustrated Lewis adduct intermediates containing boron and protocols are not free from limitations such as hazardous metal
oxygen, we focused on the ipso-hydroxylation of arylboronic contamination, long reaction time, high temperature, tedious
acids by peroxides as the benchmark reaction to explore the workup and expensive reagents. Therefore, establishing of mild
function of Lewis acid and base interactions.
and base-free oxidation system to overcome these drawbacks
Among the synthetic methods developed to prepare for ipso-hydroxylation of aryl boronic acids is very much
phenols, which are important structural units in required.
pharmaceuticals, agrochemicals, and polymers etc.,12 oxidation
of aryl boronic acid is the most straightforward reaction, and Results and discussion
has attracted considerable interest. The first reported oxidation
of aryl boronic acids and its derivatives to phenols was realized
Initially, phenylboronic acid (1a) was employed as a model
substrate to optimize reaction conditions (Table 1). When a
2 | J. Name., 2012, 00, 1-3
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mixture of 1a (0.6 mmol), TBHP (1.0 equiv) and Cl3CCN (1.5 Table 2 Substrate scope of ipso-hydroxylation of substituVteiedw AarrtyicllbeoOrnolniniec
equiv) was stirred under blue LED and air conditions at room acids to corresponding phenolsa,b
DOI: 10.1039/C9OB01568J
temperature, to our delight, the desired phenol (2a) was
obtained in 78% yield after 12 h (Table 1, entry 1). In the
OH
OH
B
TBHP/Cl3CCN
O2/blue LED
absence of blue LED, low yield of 2a was observed (Table 1,
entry 2), which shows that the irradiation with visible light plays
an important role for high conversion of the aryl boronic acids.
The effect of solvents on the reaction was examined
subsequently (Table 1, entries 3-6), and MeCN was found to be
the most effective solvent. As the significant activating reagent,
a range of nitriles and isocyanides such as benzonitrile (3a), 2-
isocyano-2-methylpropane (3b), and chloroacetonitrile (3c)
were examined (Table 1, entries 7-9). Although other nitriles or
isocyanides could afford moderate yields, it was found that
Cl3CCN still promoted this reaction most efficiently, and was
necessary for smooth progress of ipso-hydroxylation (Table 1,
entry 10). In the course of optimization of oxidant, TBHP was
found to be the optimal oxidant (Table 1, entries 11-14). The
screening of the amount of Cl3CCN and TBHP indicated that 1.5
equiv of Cl3CCN and 1.0 equiv of TBHP were the most suitable
for the reaction (Table S1). Interestingly, the reaction gave low
yield under an inert N2 environment (Table 1, entry 15), which
indicated that molecular oxygen in air may promote the
conversion of 1a. Therefore, the atmosphere was replaced with
O2 environment, then the reaction occurred to give the desired
phenol 2a in high yield (Table 1, entry 16). But when we
replaced the TBHP with oxygen, there was no phenol 2a been
detected (Table 1, entry 17). Subsequently, to figure out if
Cl3CCN was necessary in this reaction, we prolonged reaction
time to 60 h and the yield was still undesirable. (Table 1, entry
18). At last, to determine the existence of thermal effects, we
heated the reaction to 40 ℃ and the best yield didn’t achieve
(Table 1, entry 19).
OH
OH
R
R
OMe
OH
OH
OH
2a
, 96%
2b
2c
2d
, 45%
, 38%
, 97%
MeO
OH
OH
OH
OH
OH
MeO
MeS
O
2e
, 95%
O
2f
2g
2k
2h
, 98%
, 61%
, 57%
OH
OH
OH
2i
, 82%
2j
2l
, 98%
, 89%
, 85%
OH
OH
OH
OH
F
Cl
Br
2m
, 55%
OH
2n
2o
2p
, 85%
, 98%
OH
, 88%
OH
OH
OH
I
F3C
F3CO
O2N
2q
2u
, 34%
2r
2s
2t
, 65%
OH
OH
, 66%
, 48%
, 75%
, 79%
OH
OH
O
O
NC
2v
2w
2x
, 31%
, 85%
OH
O
O
H
With the optimal reaction condition in hand, we turned our
attention to examine the scope of the reaction (Table 2). A wide
range of functional groups were tolerant in this transformation,
such as sulfide, halide, nitro, nitrile, aldehyde, and carboxyl, etc.
The substituted aryl boronic acids bearing electron donating or
withdrawing substituents at para-position were smoothly
oxidized to the corresponding phenols in good to excellent
yields (2f, 2n, 2t, and 2u). Boronic acids with methoxyl
substituent at meta- and para-position afforded corresponding
phenols in high yields (2e, 2f). However, ortho-methoxyl (2d)
and other ortho-substituted boronic acids were examined, and
low to moderate yields were obtained (2b, 2d, 2v, 2x, and 2y),
possibly due to the effect of steric hindrance on interaction of
Lewis adduct and substrates. Also, α-naphthylboronic acid 2l
was more efficiently converted into corresponding naphthol
than β-naphthylboronic acid 2m. In addition, bromo and iodo
substituted boronic acids were tolerated providing the desired
COOH
HO
H
H
HO
2y
2z
, 28%
, 57%
aReaction conditions: arylboronic acids (0.6 mmol), TBHP (5.5M in decane
1.0 equiv.), Cl3CCN (1.5 equiv.), MeCN (5 mL), blue LED, r.t., O2, 12 h.
bIsolated yields.
OH
HO
Ph
OH
OH
B
B
Ph
2.79
3.36
4.03
tBu
O
2.71
O
O
tBu
O
NH
Cl3C
NH
Cl3C
OH
OH
OH
OH
Ph
B
B
1.77
NH
3.49
O
Ph
3.87
2.64
CCl3
Ph
O
HN
NH
Ph
Cl3C
NH
product (2p, 2q), which could be used for subsequent practical Scheme 3 The optimized geometries of aryl boronic acids coordinated by
functionalization. Noteworthily, oxidation-sensitive trichloroimidate.
substituents (2g, 2w) and substituents bearing active hydrogen
(2v, 2y) were also tolerated in the conditions without suffering
To identify which O atom in TBHP could be coordinated to B
overoxidation and got good yields. Furthermore, boronic acid atom in aryl boronic acid during ipso-hydroxylation, we applied
derivative of lithocholic acid was successfully converted into DFT calculations at M062x/6-31G(d) level used Gaussian
corresponding phenol (2z).
Program.19 benzyl group could be more stable than O atom in
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DOI: 10.1039/C9OB01568J
light on
light off
d)
80
60
40
20
0
80%
c)
53%
45%
b)
26%
20%
a)
0
2
4
6
8
10
time/h
90
80
70
60
50
40
30
20
10
0
-10 -20 -30 -40 -50 -60 -70 -80 -90 -100
f1 (ppm)
Scheme 7 Intermediate experiment verification of 11B NMR (128 MHz, CDCl3)
spectra: a) 1a; b) reaction in standard conditions after 10 min; c) after 1 h; d)
after 8 h. ■1a; ▲III. (see Scheme 8).
Scheme 4 The light on/off experiments of ipso-hydroxylation aryl boronic
acid.
coordinating to B atom (scheme 3 C-D), which was compatible
with Niu’s suppose. On the other side, in our system, it shows
that both of O atoms in TBHP can coordinate to B atom with the
distances between O and B atoms of 2.71 and 2.79 Å,
respectively (scheme 3 A-B). However, the former is 7.6
kcal/mol lower in energy than the later, indicating that the O
atom adjacent to tBu-group is more favourable to be
coordinated to B atom during the reaction.
In order to figure out the role of blue LED in this reaction, we
conducted the light/dark experiment (Scheme 4). The result
shows significant improvement of reaction yield (detected by
GC-MS) in blue LED, which means the blue LED does promoted
the reaction. Then the consequence of UV-vis absorption
spectra (Scheme S2) illustrated the compound (blue line)
bathochromic slightly shift up to the visible range in λ = 225 nm
and 275 nm.
the key Lewis adduct intermediate might promote ipso-
hydroxylation via two pathways, radical and ionic mechanism
(Scheme 6). The path A described the directed oxidation of
phenylboronic acid to phenol by TBHP. Another possible
pathway (path B), which described the combination of TBHP,
Cl3CCN and phenylboronic acid transformed to the highly
unstable Lewis adduct intermediate (III). Although the detailed
process was not fully understood, the trichloroacetimidates
intermediate and aryl boronic acid were speculated to form a
Lewis adduct, which was similar to Niu’s report.4 Finally,
interaction of Hydroxyl radical (IV) and arylboronic acid
produced a carbon central free radical (V), then the reaction of
IV and V formed to the product (2a).20 Noteworthily, the
hydroxyl radical might be induced by photolysis of TBHP, and O2
could boost the production of hydroxyl radical (path C).21
Eventually, in order to verify the reaction mechanism, we
explored further by 11B NMR spectroscopy in real-time
monitoring the progress of the reaction in standard conditions.
After 8 h, it showed that 1a was consumed a lot and a new signal
peak at -10.5 ppm appeared. In addition, the amount of the new
compound increased over time (scheme 7). Based on the
literature,7a,22 we surmised the new signal may be intermediate
III (Scheme 6), furthermore, we detected it by GC-MS (see
supporting information).
Furthermore, the ipso-hydroxylation was partly suppressed
and 47% phenol was obtained, when 2,2,6,6-
tetramethylpiperidinooxy (TEMPO) as a radical inhibitor was
added to model reaction (Scheme 5). Therefore, we speculated
OH
standard condition
OH
B
+ TEMPO (2 eq.)
OH
2a
47% yield
1a
Scheme 5 Simply mechanism investigated.
Conclusions
tBuOH
HO OH
B
path A
path B
path C
O
OB(OH)2
Ph
O
H
I
tBu
Ph
II
In conclusion, a TBHP and Cl3CCN mediated, base-free ipso-
hydroxylation of aryl boronic acid has been developed. The
reaction was conducted under blue LED irradiation with oxygen,
and had no use for any exogenous base. A broad series of
functional groups were tolerated under the standard reaction
condition, and resulted corresponding phenols in moderate to
excellent yields, even the late-stage functionalization of the
derivative of natural product gave the desired product. After
the inhibition experiment for radical reaction, a multi of
pathway mechanism was suggested. Then, the light/dark
experiment demonstrated blue LED promoted the conversion
hydrolysis
NH
NH tBu
OH
B
Cl3CCN
OH
O
tBu
O
H
O
O
Ph
Cl3C
O
tBu
Cl3C
O
B
Ph
OH
HO OH
2a
V
III
hv, O2
OH
IV
OH
IV
B(OH)3
Scheme 6 Proposed mechanism for the ipso-hydroxylation of aryl boronic
acid reaction.
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7
(a) G. B. Payne, P. H. Deming and P. H. Williams, J. Org.
of the reaction. This methodology is a valuable exploration for
application of unstable Lewis adduct intermediates in organic
chemistry, and we detected it by 11B NMR spectroscopy and GC-
MS. Further investigations on the role of blue LED and practical
application of Lewis adduct are currently ongoing in our
laboratories.
View Article Online
Chem., 1961, 26, 659; (b) G. B. PaynDeO, IT: e10tr.1a0h3e9/dCr9oOnB, 011956682J,
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Experimental section
General procedure for the ipso-hydroxylation reaction
Aryl boronic acids (0.6 mmol), 5.5 M TBHP in decane (0.6 mmol,
1.0 equiv.) and Cl3CCN (0.9 mmol, 1.5 equiv.) were added to a
25 mL pressure tube under oxygen atmosphere. Dry MeCN (5
mL) was added subsequently. The mixture was then stirred
under blue LEDs until the starting material had been consumed
after 12 h. When the reaction was finished, the crude product
was purified by flash chromatography on silica gel (PE/EA).
Placed the reaction device at the center of a stir plate and
two parallel 15W blue LED lamps, make sure the fan always
working in the process of reacting to eliminated the effect of
the heat which generated from blue LED (Scheme S1).
13 A. D. Ainley and F. Challenger, J. Chem. Soc., 1930, 0, 2171.
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Conflicts of interest
There are no conflicts to declare.
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The authors are grateful for the financial support from the
National Natural Science Foundation of China (Grant No.
21871067), the Natural Science Foundation of Guangdong
Province (Grant No. 2018A030313038), Shenzhen Fundamental
Research Projects (Grant No. JCYJ20180306171838187), the
Open Project Program of Hubei Key Laboratory of Drug
Synthesis and Optimization, Jingchu University of Technology
(No. OPP2015ZD01), the Open Project Program of Beijing
National Laboratory for Molecular Sciences (BNLMS201819),
and Harbin Institute of Technology (Shenzhen) (Talent
Development Starting Fund from Shenzhen Government).
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