Visible light-promoted dihydroxylation of styrenes with water and dioxygen

Article abstract of DOI:10.1039/c7cc06745c

An efficient visible light promoted metal-free dihydroxylation of styrenes with water and dioxygen has been developed for the construction of vicinal alcohols. The protocol was operationally simple with a broad substrate scope. The mechanistic studies demonstrated that one of the hydroxyl groups came from water and the other one came from molecular oxygen. Additionally, the β-alkyoxy alcohols could also be obtained using a similar strategy.

Full text of DOI:10.1039/c7cc06745c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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DOI: 10.1039/C7CC06745C
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Visible Light-Promoted Dihydroxylation of Styrenes with Water and Dioxygen
Bo Yang and Zhan Lu*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient visible light promoted metal-free dihydroxylation of
styrenes with water and dioxygen has been developed for the
construction of vicinal alcohols. The protocol was operationally
simple with broad substrate scope. The mechanistic studies
demonstrated that one of the hydroxyl group came from water
and the other one came from molecular oxygen. Additionally, the
ß-alkyoxy alcohols could also be obtained using a similar strategy.
The vicinal alcohols motifs are vastly existed in natural and artificial
products that have emerged biologically active properties (Scheme
1).1 Dihydroxylation of alkenes to achieve 1,2-diols derivatives is one
of the most powerful and straightforward methods.2 There are
tremendous metal-catalyzed or metal-free or enzyme-catalyzed
processes capable of making 1,2-dioxylation products.3 However,
the directly construction of free 1,2-diols is still limited. Among
these cases, peroxides, hypervalent iodines, periodate salt and
selenium dioxide are usually used as stoichiometric oxidants which
have explosive characteristic or produce stoichiometric wastes
(Scheme 2a).
Scheme 2. Dihydroxylation of alkenes
conduct olefin dihydroxylation. It should be noted that the oxygen
atom of water could be partially incorporated into the alcohols in
the presence of Lewis acid. However, these approaches require the
utilization of transition metal complexes and/or harsh conditions.
Furthermore, the intrinsic limitations of transition metal catalyzed
processes are associated with the limited functional group
tolerances. Additionally, it is difficult to selectively oxidize the aryl
Scheme 1. 1,2-Diol motif in natural and artificial products.
Dioxygen is generally considered as an ideal oxidant, and also a alkenes and aliphatic alkenes in transition metal catalyzed processes.
clean and cheap oxygen source (Scheme 2b). 4 Beller group 5
The development of efficient, mild and green approaches to address
these issues is highly desirable. To the best of our knowledge, the
selective use of readily available dioxygen and water as oxygen
sources for direct dihydroxylation of alkenes under mild conditions
has not been well explored.
developed an osmium-catalyzed dihydroxylation of olefins using 1
bar of dioxygen which was used as the terminal oxidant to
regenerate the osmium species and the water afforded the oxygen
sources. Jiang and co-workers6 reported a facile palladium-catalyzed
dihydroxylation of alkenes under 8 atm of dioxygen at 100 oC, in
which dioxygen played a role of oxidant. The oxygen atom in
Visible light, as a safe and abundant energy source, has emerged
as a powerful tool to construct organic molecules under mild
conditions.9 We are particularly interested in visible light-promoted
alkene difunctionalization and dioxygen oxidation. 10 Here, our
strategy is aimed to use the abundant and environmentally friendly
molecular oxygen and water as oxygen sources to directly achieve
the dihydroxylation of alkenes utilizing visible light (Scheme 2c).
Based on the previously reported literatures11, the excited state
of Acr+Mes generated by absorbing visible light could oxidize the
aryl alkene to its radical cation. We envisoned whether the radical
cation could be trapped by water and dioxygen directly to achieve
the dihydroxylation of alkenes. However, a number of challenges
products came from acetic acid or water. Paine and co-workers7 8
,
developed an iron-oxygen oxidant generated from dioxygen and an
FeII-benzilate complex of a facile N3 ligand which could be used to
a.Department of chemistry, Zhejiang University, Hangzhou 310027, China
b.E-mail: luzhan@zju.edu.cn
†Electronic Supplementary Information (ESI) available: *details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1. Optimization of reaction conditionsa
Table 2.Substrate scope of alkenesa
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DOI: 10.1039/C7CC06745C
Entry
X
Sat. aq
Solvent
Yield of 2a
(mol%)
(%)
27-57
75
40
19
24
0
40
63
87
92
0
0
1
2
3
4
5
6
7
8
5
5
5
5
5
5
5
5
5
3
3
/
H2O
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
MeCN
MeCN
MeCN
MeCN
MeCN
KH2PO4
K2HPO4
K3PO4
K2CO3
NaOH
NaHCO3
K2HPO4
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
9
10b
11b,c
12b
13b,d
14b,e
15b,e,f
3
3
3
MeCN
MeCN
0
93
MeCN
94(87g)
Reaction conditions: 1a (0.3 mmol), Acr+MesClO4 (5 mol%), and solvent/saturated
a
-
aqueous (v/v, 11/1, 6 mL) were stirred for 3 h under O2 balloon. Yields determined by 1
H
NMR spectra with mesitylene as an internal standard after treated with PPh3 (1 equiv.)
in one pot. b 6 h. c Without light. d Under N2. e Under air. f 0.1 M. g Isolated yield.
had to be addressed in developing such a transformation. For
instance, the aryl alkenes are easily oxidized to ketones or aldehydes
under dioxygen atmosphere employing visible light.12 Furthermore,
the radical cations of simple alkenes are prone to react with itself to
afford [4+2] cycloaddition products (Scheme 2c).13
Initially, we chose α-methyl styrene 1a as a model substrate to
explore the dihydroxylation of alkenes using dioxygen and pure
water. To our disappointed, various solvents, photosensitizers and
additives were examed, only a small amount of desired product 2a
was obtained accompanied by the formation of acetophenone and
[4+2] oxidative cycloaddition reaction. The reproducibility of these
reactions was also disastrous (Table 1, entry 1). We speculated that
the intermediate might be unstable under the reaction conditions.
Given the fact that photocatalyst Acr+Mes was easily decomposed
under the moderate to strong basic conditions in the presence of
dioxygen. As a result, we used the weak basic and acidic solutions as
alternatives to pure water to test the model reaction.
a
-
Standard conditions: alkenes (0.3 mmol), Acr+MesClO4 (3 mol%), saturated NaHCO3
(0.25 mL) in a solution of MeCN (2.75 mL) under the irradiation of 8 W blue LEDs under
air for 6 h. Isolated yield after reduction workup with PPh3 (1 equiv.). 12 h. 14 h.
Saturated aqueous KH2PO4 was used.
b
c
d
were established as following: 3 mol% of Acr+MesClO4 , MeCN/sat.
NaHCO3 (v/v, 11/1) (0.1 M) under air irradiated by 8 W blue LEDs for
6 h and then reduction workup with triphenylphosphine (1 equiv.).
With the optimized reaction conditions in hand, a range of
alkenes were subjected to yield the corresponding vicinal alcohols
(Table 2). When aryl alkenes bearing both electron-donating and
electron-withdrawing groups were used as substrates, the
corresponding diols were obtained in moderate to excellent yield
(2a-2k). The ortho-, meta- and para-substituted chloro styrenes with
different steric hindrance furnished the transformations producing
the desired products in 51%, 76%, and 90% yields, respectively (2d-
2f). It was noteworthy that a number of styrenes bearing functional
groups, such as aldehyde, amide, ester and alcohols, performed well
under the standard conditions to afford the corresponding vicinal
alcohols in moderate yields (2i-2k, 2n). Importantly, no obvious
side-products were formed with the recoveries of the alkenes.
The α-substituents on styrenes could be not only 1o- alkyl group
but also more sterically hindered 2o- and 3o- alkyl groups (2l-2q).
The 1,1-diaryl alkene was allowed for the transformations delivering
the vicinal alcohol in 77% yield (2r). The 1-methyleneindanecould
be transformed into corresponding alcohol (2s) in 46% yield using
saturated aqueous KH2PO4 solutions instead of saturated aqueous
NaHCO3 solutions. A majority of mono-substituted aryl alkenes
-
To investigate our proposal, we first used the saturated aqueous
KH2PO4 solutions as nucleophiles in the presence of 5 mol%
-
Acr+MesClO4 and O2 with acetone as solvent to examine the
dihydroxylations of alkenes (Table 1). To our delight, 75% yield of 2a
was obtained with 100% conversion of alkene after reduction
workup with triphenyl phosphine in one pot (entry 2). Various
saturated aqueous solutions were screened, the yields were lower
(entries3-7). It was worth to note that although only 40% yield of 2a
was obtained in the presence of saturated aqueous NaHCO3solution
(entry 7), the side-products were rare. The yield of 2a was
significantly improved to 87% yield in acetonitrile using a saturated
aqueous NaHCO3 solution as nucleophiles (entries 8 and 9). The
reaction using 3 mol% of catalyst was carried out for 6 h to afford 2a
in 92% yield (entry 10). No desired product 2a was obtained without
light, photocatalyst or dioxygen which demonstrated that the light,
photocatalyst and dioxygen were all essential to the reactions
(entries 11-13). The 2a could also be obtained under air in 93% yield
(entry 14). The reaction was also performed smoothly in0.1 M,
yielding 2a in 87% isolated yield (entry 15). The standard conditions
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performed well under the standard conditions to afford the
corresponding alcohols in 48-79% yields (2t-2y). Notably, the 1,2-
disubstituted alkenes could also be transformed into the desired
products in 66% yield with 1.9/1 dr. Moreover, the trisubstituted
alkene was also a suitable partner to producing 61% yield of 2aa. It
should be noted that 2ab could be isolated in 46% yield through
selective oxidation of aryl alkene. The relatively complex molecules
derived from DL-Menthol could be converted to 2ac in 51% yield.
Gram-scale reaction could be conducted in 71% yield (eq. 1).
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DOI: 10.1039/C7CC06745C
The further derivatizations were shown in Scheme 3. The high-
value-added ß-hydroxyazide 3 could be obtained by nucleophilic
attack the cyclic sulfites which was generated from the reaction
ofthionyl chloride and 2f. The 2f could also be transformed into 4 by
simple oxidation of pyridinium chlorochromate. The oxirane 5 could
be prepared in 61% yield. Additionally, the protection of ß-hydroxy
group in 2f by TsCl could deliver 6 in 71% yield, followed by NaN3
nucleophile-attacking to afford the ß-azido alcohol 7 in 90% yield.
Scheme 4.Mechanistic studies
reaction of α-methyl styrene with methanol under the atmosphere
O2 afforded the hydroxyetherification product 13a in 63% yield.
Various styrenes such as ortho-, meta-, and para-methyl styrenes
could be transformed into corresponding products in moderate to
good yields. Moreover, a variety of alcohols, such as 1o-, 2o-, 3o-alkyl
alcohols were suitable partners delivering the hydroxyetherfication
products (13b-13h, in Table 3).
Table 3.Dioxygenation of alkenes
A plausible mechanism is proposed in Scheme 5. The styrene 1a
(Eox = 1.91 V vs SCE)[15] could be oxidized to its radical cation 14 by
Scheme 3.The transformations of vicinal alcohols
To gain further insight into the mechanism of the dihydroxylation
of alkenes, control experiments were conducted as illustrated in
Scheme 4. No vicinal alcohol 2a was detected when 1.5 equiv.
TEMPO was added under standard conditions using 1a as substrate
(eq. 2). The results indicated that a radical pathway maybe involved
in the transformations. To further demonstrate the existence of
radical intermediate, 8 was used as a substrate to be performed
under standard conditions (eq. 3). Interestingly, 37% yield of 9 was
isolated with 40% recovery of 8. No ring-opening dihydroxylation of
8 was observed by 1H NMR analysis. This phenomenon maybe
attributed to the fact that the radical cation generated from alkene
was quickly oxygenated by dioxygen resulting in no 10 was detected.
To elucidate where are the two oxygen atoms come from, a H218O
isotopic labeling experiment was conducted (eq. 4). The alkene 1a
was conducted under dry O2 atmosphere using the saturated
NaHCO3 solution freshly prepared from H218O and NaHCO3. 11 was
confirmed by HRMS (ESI) ([M+Na+] 177.0772). As expected, one of
the oxygen atom of vicinal alcohol 11 originated from the water and
another oxygen atom derived from molecular oxygen. In addition, to
clearly demonstrate one of which oxygen atom of vicinal alcohol 11
come from H2O and which one come from dioxygen, 1a was
performed under the standard conditions without reduction workup
to afford 12 which was in accordance with the previously reported
literature (eq. 5).14 The result clearly shows that the oxygen atom of
α-hydroxyl of 11 originated from air, the other oxygen atom come
from H2O.
the excited state of photosensitizer (II, Ered*
= 2.06 V vs
SCE)[ 16 ]generated through absorbing visible light under the
irradiation of 8 w blue LEDs. The radical cation 14 was quickly
oxygenated by molecule oxygen under air to afford 15 which was
nucleophilic attacked by water to generate peroxy radical species
16.[17] The 16 was reduced by III (Mes-Acr•) to form 17 and
regenerated the photocatalyst I through SET process, respectively.
The another possible pathway which could not be exclusively ruled
out is that 16
Scheme 5. A proposed mechanism for dihydroxylation of alkenes
also could be reduced by 1a to provide 17 which got a proton to
deliver 12. Moreover, 12 had been isolated before reduction workup,
which is another proof that the ß-oxygen atom originated from
dioxygen. Additionally, the photocatalyst could also be regenerated
by dioxygen. It was noteworthy that the product 12 could exist
stably under the standard conditions.
In conclusion, we have developed a mild condition to achieve the
dihydroxylation of aryl alkenes with water and dioxygen from air.
The operationally simple protocol features a broad substrate scope,
mild conditions and great functional group tolerance. The
To probe the H2O-nuclephilic attack to radial cation, alcohols were
used instead of H2O. Under the slightly optimization conditions, the
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mechanistic experiments suggest that the α-oxygenatom and the ß-
oxygen atom are originated from the molecular oxygen and water,
respectively. Additionally, the ß-alkyoxy alcohols could also be
obtained using a similar strategy. The new difunctionalizations of
alkenes will be explored in our laboratory.
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We thank the National Science Foundation of China
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financial support.
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