Photocatalytic alkene reduction by a B12-TiO2 hybrid catalyst coupled with C-F bond cleavage for: Gem -difluoroolefin synthesis

Article abstract of DOI:10.1039/c7cc04377e

Photocatalytic syntheses of gem-difluoroolefins were performed using the B₁₂-TiO₂ hybrid catalyst during the CC bond reduction of α-trifluoromethyl styrenes with C-F bond cleavage at room temperature under nitrogen. The gem-difluoroolefins were used as synthetic precursors for fluorinated cyclopropanes.

Full text of DOI:10.1039/c7cc04377e

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Photocatalytic alkene reduction by B12-TiO2 hybrid catalyst
coupled with C-F bond cleavage for gem-difluoroolefin synthesis
Received 00th January 20xx,
Accepted 00th January 20xx
Hui Tian, Hisashi Shimakoshi* Kenji Imamura, Yoshihito Shiota, Kazunari Yoshizawa, and Yoshio
Hisaeda*
DOI: 10.1039/x0xx00000x
www.rsc.org/
such as styrene derivatives and alkylacrylates. The Co-H
complex should work as a hydrogen radical donor and the
Photocatalytic syntheses of gem-difluoroolefins were performed
reaction was initiated by hydrogen radical attack on the
substrate alkene.3d During the alkene reduction by the B12-
using the B12-TiO2 hybrid catalyst during C=C bond reduction of
trifluoromethyl styrenes with C-F bond cleavage at room
α-
TiO2, a unique
derivatives reduction was observed, and
afforded a radical coupling product, hydrogenated product, and
C=C bonded product by changing the -substituent from H or
α
-substituent dependency of the styrene
temperature under nitrogen. The gem-difluoroolefins were used
α
-substituted styrenes
as synthetic precursors for fluorinated cyclopropanes.
α
The carbon-carbon double bond (C=C) reduction is an
important reaction in synthetic chemistry for the production of
versatile fine chemicals.1 To achieve the reaction, a number of
reagents and catalysts has been developed in recent years. The
transition metal hydrides are some of the useful reagents for
such reactions both on laboratory and industrial scales.2
Hydride transfer from a metal hydride to an olefin produces a
reduced product. To perform the reaction, high-pressure H2 gas
was generally required to form the intermediate metal hydride
species. A special reaction vessel is also needed for the
reaction. Recently, we synthesized the hybrid catalyst of the
corrinoid derivative (B12)-titanium oxide (TiO2) in which the
B12 complexes are immobilized on the TiO2 surface by
carboxylic group coordination.3 The catalyst consisted of a
cobalt complex and semiconductor as the reaction center and
photosensitizer, respectively, and reacts under mild conditions,
CH3 to COOH, and Br as shown in Fig. 1. For the development
of the substrate scope, we now report the fourth type of
reaction.
α
-Trifluoromethyl styrene
(
1
)
having
a
trifluoromethyl (CF3) group on the C=C bond produced the
gem-difluoroolefin by C-F bond cleavage in the CF3 group.
Since gem-difluoroolefin is a useful synthetic intermediate for a
variety of fluorinated compounds,5 their preparation methods
were explored.6 We now report the unique alkene reduction
coupled with C-F bond cleavage to form gem-difluoroolefin
catalyzed by the B12-TiO2.
The corrinoid derivative, cyanoaquacobyrinic acid,3a was
used as the B12 moiety in the hybrid catalyst and the B12-TiO2
(TiO2: anatase, AMT-600, average surface area: 52 m2/g,
TAYCA Co., Ltd.) was prepared by a previously reported
method.3a The content of the B12 complex on the surface of
TiO2 was 9.60x10-6 mol/g and the apparent surface coverage by
i.e. under normal pressure at room temperature.
The
the B12 complex was 1.8x10-11 mol/cm2.
styrene ( ) reduction by the B12-TiO2 under UV light irradiation
(black light,
α-Trifluoromethyl
immobilized B12 complex accepts an electron from the
conduction band (CB) of TiO2 to form the Co(I) species by UV
light irradiation. As the Co(I) form of B12 showed a
supernucleophilicity,4 various B12-dependent enzyme inspired
reactions were conducted by the B12-TiO2 such as
isomerization,3b dechlorination,3a,3c and alkene reduction.3d In
the alkene reductions, the cobalt-hydrogen complex (Co-H
complex) was postulated to be the intermediate and the B12-
TiO2 showed a high reactivity for conjugated alkene reduction
1
λ
max=365 nm) was conducted at room temperature,
in which the B12-TiO2 (10 mg, B12 content=9.6x10-8 mol) was
COOH
CN
F
X =CF3
CoIII
HOOC
F
N
=
N
Co
N
N
CꢀF Bond cleavage
X
HOOC
C
H2O
OOC
X
X = ꢀH, CH3
O
Ph
Ph
eꢀ
O
X
UV
CoIII
eꢀ
Radical coupling
eꢀ
eꢀ
CO2H
Ph
X = ꢀCO2H
X =Br
TiO2
h+
CoI
H
Hydrogenation
Ph
MeOH
MeO
MeO
H+
Ph
CoIII
B12ꢀTiO2
C=C Bond formation
suspended in MeOH under anaerobic conditions (ESI†).
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b
Fig. 1 Photocatalytic reduction of styrene derivatives by the B12-TiO2.
substrate. Each volume % of MeOH was added to CH3CN. c Content
For the reaction, MeOH was used as a solvent and a hole of Pt is 0.15 wt%. d In the dark. e In the presence of DMPO, 8x10-1 M.
(hVB+) consumer of the TiO2 valence band (VB).7 The results
for the reduction of
hydrogenated product,
1
α
are summarized in Table 1. The
-trifluoromethyl ethylbenzene ( ), was
3
obtained with a 70% yield in MeOH and a 29% yield of the
defluorinated product, -difluoro- -methylstyrene ( ), was
β
,
β
α
2
also produced (entry 1 in Table 1). Without MeOH or light
irradiation, the reaction did not proceed (entries 2 and 8 in
Table 1). The B12 was also necessary for the reaction (entries 6
and 7 in Table 1). The product ratio was dependent on the
protic solvent amount. With the decreasing amount of MeOH
(2.3 v%), the ratio of the gem-olefin (2) production was
increased to 3.8, while the substrate conversion decreased to
72% (entry 3 in Table 1). Since a suitable amount of MeOH
(hVB+ consumer) was required for the reaction, a moderate yield
of
2 was obtained in CH3CN containing 17 v% of MeOH
(entry 4 in Table 1).‡
The reaction was applied to other trifluoromethyl styrene
Table
2
Reduction of α-trifluoromethyl-p-methylstyrene (4) by
derivatives as shown in Eq. 1. From the variety of
p
-substituted
16), the corresponding
17) were obtained in
several reagents
Entry
α
-trifluoromethyl styrenes (
4
,
7,
10
,
13,
Reagent
Solvent
Conv.
(%)a
92
Yield of
5 (%)a
68
Yield of
6 (%)a
14
gem-difluoroolefins (
5
,
8,
11
,
14
,
1b
2c
3d
a
B12-TiO2
Pd-C/H2
CH3CN+MeOH
MeOH
moderate yields. The B12-TiO2 also catalyzed the reduction of
-difluoromethyl styrene 19 to yield -fluoro-
α
(
)
β
α-
>99
67
0
>99
11
methylstyrenes with 45% of 20 and 24% of 21 as shown in Eq.
2. It was noted that the reaction by the B12-TiO2 proceeds
under mild conditions without H2 gas and a special reaction
vessel. The B12 inspired reaction may become one of useful
methods for the gem-difluoroolefin synthesis.
NaBH4/CoCl2 CH3CN+MeOH
55
Conversion of substrate and the yield of product were based on the
b
c
initial concentration of the substrate.
Data from equation 1.
[
1
]=4x10-3 M, Pd-C=5 mg under 1 atom of H2 at room temperature.
d
Reaction time 8 h.
[1
]=4x10-3 M, [NaBH4]=4x10-3 M, [CoCl2]=4x10-4
Table 1 Photocatalytic reduction of
under UV light irradiationa
α-trifluoromethylstyrene (1)
M in 6 mL CH3CN +23v% MeOH under N2 at room temperature.
Reaction time 24 h.
To investigate the advantage of the B12-TiO2 catalyst, the
reduction of
other reagents. The usual hydrogenation by the Pd-C catalyst
under hydrogen gas gave only the reduced product ( ) (entry 2
in Table 2). Though the conventional hydride reagent,
NaBH4, afforded the gem-difluoroolefin ( ) in the presence of
CoCl2 (entry 3 in Table 2),‡‡ a large amount of inorganic
wastes was formed after the reaction. In contrast, the B12-TiO2
system affords the gem-difluoroolefin by consuming MeOH as
an electron source without any waste, and the catalyst could be
recovered by simple filtration (entry 1 in Table 2).3
α-trifluoromethyl styrene was carried out using
Entry
Catalyst
Solvent
Conv. Yield Yield Ratio
6
(%)
of 2
(%)
29
of 3
(%)
70
of
2/3
0.41
1
2
B12-TiO2
B12-TiO2
B12-TiO2
MeOH
>99
0
5
CH3CN
0
0
0
3b
CH3CN+MeOH
(2.3 v%)
CH3CN+MeOH
(17 v%)
CH3CN+MeOH
(23 v%)
72
57
15
3.8
4b
5b
B12-TiO2
B12-TiO2
91
95
65
52
24
32
2.7
1.6
The proposed reaction mechanism by the B12-TiO2 is shown
in Fig. 2. The Co(III) form of B12 on the TiO2 surface was first
reduced to Co(I) via Co(II) by UV light irradiation with
absorption maxima at 390 nm and 470 nm in the diffuse
reflectance (DR) UV-VIS spectra, respectively (Fig. S1(a),
ESI†). The Co(I) species of B12 reacts with a proton from the
MeOH to form the cobalt-hydrogen complex, Co(III)-H, as an
intermediate. Though 1-trifluoromethylvinyl compounds are
easily attacked by nucleophiles and underwent nucleophilic
6
TiO2
MeOH
0
Trace
0
0
0
0
0
0
0
0
0
0
0
0
0
7c
8d
9 e
Pt-TiO2
B12-TiO2
B12-TiO2
MeOH
MeOH
CH3CN+MeOH
(23 v%)
0
a UV irradiation (black light,
[B12-TiO2]=10mg (B12, 1.60x10-5 M), [
λ
max=365 nm, 1.76 mWcm-2). Conditions:
1
]=4x10-3 M, solvent 6 mL under
addition-elimination reaction (SN2’reaction) to yield the gem
-
N2 at room temperature. Reaction time 24 h. Conversion of substrate
and the product yield were based on the initial concentration of the
difluoroolefin,8 direct reaction of nucleophilic Co(I) species of
B12 to the substrate alkene was not observed by cyclic
2 | J. Name., 2012, 00, 1-3
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voltammetric study.# As we previously reported, the Co(III)-H Fig. 2 Photocatalytic reduction of styrene derivatives by the B12-TiO2
complex of the B12 derivative has a radical character and a
hydrogen radical should attack at the
derivatives avoiding the steric hindrance of the phenyl group to
form the
-radical intermediate and the Co(II) species.3c,9
Actually, the reaction was completely inhibited by adding the
spin-trapping reagent, 5,5-dimethyl-1-pyrroline- -oxide
β-position of the styrene
α
N
(DMPO) (entry 9 in Table 1). The diffuse reflectance (DR)
UV-VIS spectra also showed the existence of the Co(II) state of
the B12 complex with
S1(b), ESI ). Due to an electron-withdrawing property of the
CF3 group, the -radical intermediate could be reduced by TiO2
to form the carbanion intermediate (CB-).## It is known that the
carbanion having a CF3 group at the -position easily allowed
-fluorine elimination to form the gem-difluoroolefin and
fluoride ion.10 § The following protonation of the CB- should
form the hydrogenated product. Therefore, the product
selectivity was dependent on the amount of a protic solvent as
shown in Table 1. Actually, -trifluoromethyl styrene (24
having a CF3 group at the -position afforded no defluorinated
λmax=470 nm during the reaction (Fig.
†
α
under UV light irradiation.
α
β
β
)
β
product but the reduced product (25) as shown in Eq. 3.
To confirm the mechanism, the source of the hydrogen in
the products was determined using CH3OD as the solvent. One
and two deuterium atoms were incorporated into the 2-d1 and 3-
Fig. 3 ESR spectra observed during the reaction by the B12-TiO2;
[1
]=9.4x10-3 M, [DMPO]=1 M, 2 mg of B12-TiO2 under N2 at room
d
2, respectively, as confirmed by MS analyses (Eq. 4, Fig. S4,
ESI ). When the reaction was carried out in the presence of a
spin-trapping reagent, DMPO, ESR signals of two DMPO
trapping species were observed during the reduction of by the
†
temperature in MeOH (a) and MeOD (b).
1
To help understanding of the reduction of α-trifluoromethyl
B12-TiO2 during UV light irradiation. The ESR signal was
ascribed to the hydrogen radical trapped DMPO, DMPO-H
styrene, we performed DFT calculations using the B3LYP
functional combined with the 6-31G** basis. All calculated
energies and optimized structures are summarized in the
(
g
=2.009,
centered radical trapped DMPO, DMPO-C (
G,
H=21.7 G) as shown in Fig. 3(a).§§ The obtained ESR
A
N=16.6 G,
A
H=22.5 G), combined with the carbon-
g
=2.009, N=15.0
A
Supplementary Information (Fig. S66, ESI
intermediate (CB-) undergoes
-fluorine elimination with the
activation barrier of 22.0 kcal/mol (Fig. S66a, ESI ). This
†). The carbanion
A
parameters were similar to the reported values.11 Furthermore,
the DMPO-H signal disappeared using CH3OD as shown in
Fig. 3(b). These results suggested that the Co-H complex is
formed during the reaction and provides a hydrogen radical.
These ESR experiments supported our proposal that the
Co(III)-H complex of the B12 derivative could have a hydrogen
radical character and catalyze the alkene reduction by a radical
mechanism.3d,9
β
†
reaction step for the dissociation of the C-F bond is computed
to be endothermic by 8.0 kcal/mol. TS1 is a transition state,
which connects CB- and 2-F-. The transition structure involves
a C-F bond of 2.597 Å. The elongated bond distance of
transition-state structure corresponds to the cleavage of the C-F
bond. The C-CF bond distance is decreased to 1.337 Å in TS1
from 1.441 Å in CB- to 1.333 Å in 2-F-, whereas the C-CPh
bond distance is increased to 1.476 Å in TS1 from 1.423 Å in
CB- to 1.490 Å in 2-F-
Finally we utilized the gem-difluoroolefins as synthetic
intermediates for the fluorinated compounds. The gem
difluorocyclopropanes (26 27 28) were synthesized from the
reaction of the gem-difluoroolefins ( 14) and chloroform in
.
-
,
,
2, 5,
an aq. 40% NaOH solution using the phase-transfer catalyst,
+
benzyltriethylammonium chloride (PhCH2NEt3 Cl-), in good
yields as shown in Eq. 5. The gem-difluorocyclopropane is an
important compound in various areas.13,14
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and D. Gryko, Chem. Soc. Rev., 2015, 44, 3391-3404; (d) H.
Shimakoshi and Y. Hisaeda, ChemPlusChem., 2017, 82, 18-
29.
In summary, we have developed a new synthetic procedure
for the gem-difluoroolefin under mild conditions using the
green photo-driven catalyst, B12-TiO2, at room temperature.
The putative Co-H complex, which was indirectly detected by
the ESR spin-trapping technique, was thought to be an
5
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intermediate for the reaction.
The bioinspired reaction
6
7
catalyzed by the B12-TiO2 hybrid may open the door for a new
synthetic methodology in organic synthesis.15 On-going work
in our laboratory is focused on the application of the hybrid
catalyst to develop various organic syntheses with a green
procedure.
This study was partially supported by a Grant-in-Aid for
Scientific Research (C) (No. 23550125) from the Japan Society
for the Promotion of Science (JSPS), KAKENHI Grant Number
JP16H01035 in Precisely Designed Catalysts with Customized
Scaffolding and Grant Number JP16H04119, and a Grant from
the Iwatani Naoji Foundation (No. 16-4313).
8
9
H. Shimakoshi, Z. Luo, K. Tomita and Y. Hisaeda, J.
Organomet. Chem., 2017, 839, 71-77.
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Rev., 2003, 103, 1071-1098; M. Fedorynski, Chem. Rev.,
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14 For representative examples, see: (a) K. Ninomiya, K.
Tanimoto, N. Ishida, D. Horii, M. Sisido and T. Itoh, J.
Fluorine Chem., 2006, 127, 651-656; (b) T. Itoh, M. Kanbara,
M. Ohashi, S. Hayase, M. Kawatsura, T. Kato, K. Miyazawa,
Y. Takagi and H. Uno, J. Fluorine Chem., 2007, 128, 1112-
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Notes and references
‡ No further reduction of the gem-difluoroolefin (2) by the B12-
TiO2 catalyst did not proceed. When the 2 was used as substrate,
almost starting substrate was recovered.
‡‡ Without CoCl2, the reduction of 1 did not proceed. The Co(II)
salt increases the reactivity of the hydride reagent. See: S.-K.
Chung, J. Org. Chem., 1979, 44, 1014-1016; E. C. Ashby and J.
J. Lin, J. Org. Chem., 1977, 43, 2567-2572.
#
The Co(II)/Co(I) redox couple of the B12 complex, heptamethyl
cobyrinate perchlorate, did not change in the presence of 1 (Fig.
S2, ESI
†).
##
Conduction band (CB) redox potential of TiO2 in CH3CN is -
2.0 V vs. SCE and it is strong enough to reduce the benzyl
radical. See: M. Cherevatskaya, M. Neumann, S. Füldner, C.
Harlander, S. Kümmel, S. Dankesreiter, A. Pfitzner, K. Zeitler
and B. König, Angew. Chem. Int. Ed., 2012, 51, 4062-4066.
§ The number of F- formed by the B12-TiO2 catalyzed reaction
was quantified by the spectrophotometric determination using the
15 H. Shimakoshi and Y. Hisaeda, Angew. Chem. Int. Ed., 2015,
54, 15439-15443.
AlfusoneR reagent (DOJINDO) (Fig. S3, ESI
†
).
The
corresponding amount of F- toward the amount of the gem
-
difluoroolefin was detected.
§§ The trapped carbon centered radical may come from solvent
MeOH, not from the substrate radical since a similar DMPO-C
signal was observed without 1. (Fig. S5, ESI
†
). See: 7c, h+(TiO2)
+ CH3OH
→
•CH2OH+ TiO2 + H+
1
(a) P. J. Chirik, Acc. Chem. Res., 2015, 48, 1687-1695; (b) V.
K. Ahluwalia, Reduction in Organic Synthesis
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S. Izumi and Y. Hisaeda, Chem. Commun., 2009, 6427-6429;
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4 | J. Name., 2012, 00, 1-3
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