Visible-light-mediated photocatalytic cross-coupling of acetenyl ketones with benzyl trifluoroborate

Article abstract of DOI:10.1039/c9ob02624j

In this report, we describe a simple visible light-triggered Barbier-type reaction by employing acetenyl ketones with benzyl trifluoroborates. Through a radical-radical cross-coupling process, this photocatalytic protocol furnished a wide range of tertiary propargyl alcohols. Mechanistic investigation indicated that proton-coupled electron transfer (PCET) might be involved in the photochemical transformations.

Full text of DOI:10.1039/c9ob02624j

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DOI: 10.1039/C9OB02624J
COMMUNICATION
Visible-Light-Mediated Photocatalytic Cross-Coupling of Acetenyl
Ketones with Benzyl Trifluoroborate
Received 00th January 20xx,
Accepted 00th January 20xx
Lingchun Zhanga#, Yanle Chub#, Peizhi Maa#, Shujuan Zhaoa, Qiaoyan Lia, Boya Chena, Xuejiao Honga,
Jun Suna*
DOI: 10.1039/x0xx00000x
In this report, we describe a simple visible light-triggered Barbier-
With the development of green chemistry, visible-light-
type reaction by employing acetenyl ketones with benzyl mediated photocatalysis has been widely recognized as a
trifluoroborates. Through a radical cross-coupling process, this benign and promising tool for building new chemical bonds14.
photocatalytic protocol furnished
a wide range of tertiary In recent years, tremendous achievements have been made to
propargyl alcohols. Mechanistic investigation indicated that facilitate the hydroxyl-containing compounds or other
proton-coupled electron transfer (PCET) might be involved in the interesting transformations from carbonyl derivatives by
photochemical transformations.
taking advantage of the reactive ketyl radicals via
photocatalytic single electron transfer (SET)15. Generally, in
these reactions, acids or acids generated in situ were always
required to trigger a proton coupling electron transfer (PCET)
event, which was critical to umpolung the polarity of carbonyl
species because their reduction potentials are too negative to
be reduced. Remarkable examples with this well-developed
synthetic strategy in the Barbier-type reactions have been
described by Reuping16, Ngai17, Knowles18, et.al., thus making
PECT as a useful tool to accomplish the conversion of carbonyl
to hydroxyl group in photocatalytic synthesis. Despite of these
progresses, in respect to these reports, transforming
conjugated acetenyl ketones to their propynol derivatives has
scarcely been explored.
Propynols have always been an important class of structural
units because of their extensive existence in natural products,
pharmaceuticals and biologically-active chemicals1 (Figure 1).
Besides, they represent one of the synthetically versatile
building blocks and intermediates as the unsaturated C-C triple
bond and free hydroxyl group permit a variety of further
transformations, including nucleophilic substitution2, selective
reduction3 and oxidation4. Traditional methods to access these
interesting motifs mainly relied on the alkynylation of
aldehydes/ketones, in which process stoichiometric hazardous
organometallics, such as the pyrophoric lithium or zinc
reagents, are often involved5. Besides, transition-metal (e.g.,
Ag6, Zn7, Cu8, Rh9) mediated nucleophilic additions of terminal
alkynes to carbonyl compounds have attracted much attention
to achieve such transformations, but high cost and toxicity
issues severely limited its applications. Although metal-free
technologies have emerged as an elegant alternative to
circumvent the aforementioned drawbacks, strong bases
(BuOK10, CsOH11, KOH12, ect) are required to serve as
promoters, which might introduce the enone isomers13
(Scheme 1, a). As a consequence, searching for a convenient
protocol to access propargyl alcohols continues to be
constantly pursued.
O
COOH
OH
N
HO
Me
H
Me
H
H
H
HO
H
O
OH
Cicaprost
O
Dimethisterone
Mifepristone
Figure 1. Representative biologically-active propynol-contained
compounds.
(a) Reported synthetic methods for propargylic alcohols
OH
R1
(i) Organometallics
(ii) TM (Ag, Zn, Cu, Rh)
O
R2
R3
+
R1 R2
(iii) Strong base
R3
a. Department of pharmacy, Henan Provincial People’s Hospital, Zhengzhou,
450000, P. R. China. E-mail: Jun_84@163.com
(b)This study: a visible-light-mediated synthetic strategy
b. Department of pharmacy, The Second Affiliated Hospital of Zhengzhou University,
450000, P. R. China.
OH
O
R1
#Lingchun Zhang, Yanle Chu and Peizhi Ma contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Photocatalyst
Visible-light
R1
+
Ph
BF3K
R2
Ph
R2
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Scheme 1. Strategies for the synthesis of propargyl alcohols.
a
Barbier-type reaction of benzyl trifluoroborates with
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DOI: 10.1039/C9OB02624J
different easily prepared acetenyl ketones, which allowed for
the syntheis of tertiary propargyl alcohols in a visible-light-
promoted process (Scheme 1, b).
The harmfulless, stable organo-trifluoroborates are widely
used Suzuki-Miyaura coupling or addition reagents in terms of
their exquisite features such as easy preparation, purification,
handling and storage19. Notably, photocatalysis offered a
milder and greener process to convert trifluoroborates into
Our initial attempt was commenced with acetenyl ketone 1a
and potassium benzyl trifluoroborate under the irradiation of
36 W blue LEDs. Delightfully, by using catalytic amount of
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6 (2.0 mol%) and triethylamine (3
eq) in MeCN, desired product 2a was isolated in 21% yield
under nitrogen atmosphere after 24 hours of irradiation (Table
1, entry 1). Subsequently extensive screening of solvents
identified acetone as the optimal medium, providing 2a in
yield to 34% (entries 2-7). The investigation of additives
disclosed the presence of NH4OAc could improve the yield of
2a to 72% (entries 8-13). Weaker catalytic performances were
observed when Ir[dF(CF3)(ppy)]2(dtbbpy)PF6 was replaced by
other photocatalysts (entries 14-18). To gain more details on
this transformation, sets of comparative experiments were
carried out. As seen in entry 19, treatment of reaction system
under air atmosphere resulted in a sharp decrease on yield
(21%), maybe due to the radical quenching effect of oxygen.
Moreover, the visible-light-promoted nature of this protocol
was confirmed by experiments performing in the absence of
light irradiation or photocatalyst, in which no product 2a was
afforded (entries 20-21).
synthetically useful radicals. Pioneering studies by
20
Monlander19a,
,
Chen21, Akita22 have successfully
demonstrated a wide range of cross-coupling reactions by
Table 1. Optimization of reaction parameters.
Additive (3 eq)
PC (2 mol %)
OH
Ph
O
+
Ph
BF3K
Ph
Solvent (0.1 M)
N2, hv, r.t.
Ph
Ph
Ph
2a
1a
Entrya
Photocatalyst (PC)
Additive
Solvent
Yield (%)b
1
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir(ppy)2(dtbbpy)PF6
Et3N
Et3N
MeCN
MeOH
21
27
2
3
Et3N
Acetone
1,2-DCE
DMSO
34
4
Et3N
None
None
13
5
Et3N
Table 2. The investigation of the substrate scope.a,b
6
Et3N
THF
O
OH
R1
NH4OAc (3 eq)
PC (2 mol %)
7
Et3N
DMF
None
29
R1
+
Ph
BF3K
Acetone (0.1 M)
N2, hv, r.t.
R2
R2
Ph
8
HCl
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
2
1
9
HCO2H
CF3CO2H
NH4Cl
34
Ph
R
N
X
Ph
OH
Ph
10
11
12
13
14
15
16
17
18
19 c
20 d
21
30
OH
Ph
OH
OH
R
Ph
Ph
Ph
Ph
57
R = 2-naph, 2h, 61%
R = 1-naph, 2i, 58%
R = H, 2a, 72%
2l, 66%
X = O, 2j, 48%
X = S, 2k, 43%
R = p-OMe, 2b, 70%
R = o-OMe, 2c, 64%
R = m-Me, 2d, 56%
R = p-t-Bu, 2e, 71%
R = p-Cl, 2f, 53%
NH4OAc
HCO2NH4
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
NH4OAc
72
HO
Ph
OH
Ph
Ph
66
Ph
OH
Ph
Ph
R = p-CF3, 2g, 41%
2m, 36%
2n, 49%
2o, 54%
12
Ph
Ph
Ph
OH
Ir(ppy)2(bpy) PF6
25
Ph
R = p-OMe, 2q, 73%
R = m-Me, 2r, 44%
R = p-F, 2s, 68%
Ph
OH
OH
Ph
Ru(bpz)3(PF6)2
23
R
2p, 24%
2t, 39%
Acr-Mes
17
Ph
Ph
Ph
Ph
OH
Ph
Ph
R'
Ph
eosin Y
Trace
21
R
OH
3
OH
OH
R
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
-
2u, 45%
R = 4-OMePh,
2v, 53%
R = cyclopropyl, R’ = Ph,
2w, 48%
2z, 19%
R = cyclohexyl, R’ = Ph,
2x, 65%
R = Ph, R’ = 4-MePh, 2y,
63%
None
None
aUnless otherwise stated, reactions were conducted with acetenyl ketones (0.1
mmol), benzyl trifluoroborate (0.15 mmol), NH4OAc (0.3 mmol), 2 mol% of PC
(Ir[dF(CF3)ppy)2](dtbbpy)PF6), acetone (1.0 ml), nitrogen atmosphere, 36 W blue
LEDs, irradiated under room temperature for 24 hours. bIsolated yields.
aUnless otherwise stated, reactions were conducted with 1a (0.1 mmol), benzyl
trifluoroborate (0.15 mmol), photocatalyst (2.0 mmol%), solvent (0.1M),
nitrogen atmosphere, 36 W blue LEDs, irradiated under room temperature for
24 hours. bIsolated yields. cReaction performed under air atmosphere. dReaction
performed without light irradiation.
With the optimal conditions in hand, our efforts were
shifted to assess the substrate scope. Generally, all the
investigated acetenyl ketones were well-matched with the
mild conditions, providing a wide range of tertiary propargyl
using alkyl trifluoroborates as radical precursors with diverse
electronphiles under visible-light-promoted photocatalytic
conditions. Within this context, we presented our discovery on
2 | J. Name., 2012, 00, 1-3
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alcohols within 24 hours (Table 2). Firstly, various diphenyl
ketones were employed to evaluate the efficiency of this
strategy. Gladly, the Barbier-type reaction process was feasible
and afforded corresponding products in moderate to good
yields (2a-2g, 41-72%). Notably, both electron-rich (methyl,
methoxy, tert-Butyl) and electron-poor (F, CF3) substituents on
the benzene ring of R1 were compatible with the current
conditions despite of the substitution patterns. Alternatively,
replacing R1 with other aromatic groups, including poly- and
hetero-cyclic aromatics, led to the formation of propargyl
alcohols bearing naphthyl (2h, 2i) or oxygen (2j), sulfur (2k),
nitrogen (2l) atoms with acceptable yields. Subsequently, the
reaction scope was extended to alkyl-linked ketone substances.
As listed in Table 2, the benzyl moiety could be successfully
installed onto carbonyl group to deliver alcohol adducts 2m-2o,
further demonstrating the generality of this photocatalytic
protocol. Remarkably, in addition to linear or cyclic alkyl,
unsaturated functional group such as alkenyl was also proved
to be competent substituent, although the corresponding
product was obtained in a low yield (2p, 24%). Encouraged by
these satisfying results, we next examined the reactivity of
acetenyl ketones by variation of the acetylenic bond-
connected R2. For instances, functional group with different
properties on phenyl ring was as well tolerated (2q-2s).
Changing R2 with types of alkyl also was amenable, which
coupled effectively with benzyl trifluoroborate to produce 2t-
2w in a range of yields from 39% to 65%. Additionally, reaction
using a substituted benzyl trifluoroborate as coupling partner
was as well tested, which furnished the desired product with
moderate yield (2y). Besides the derivatives of benzyl, other
types of aliphatic or aryl trifluoroborates were applied to this
synthetic strategy, which, unfortunately, failed to afford the
target adducts. Otherwise, without further optimization, α, β
unsaturated chalcone was subjected to the employed
conditions, to our satisfactory, desired 2z could be obtained,
but in a low isolated yield (19%).
View Article Online
DOI: 10.1039/C9OB02624J
Figure 3. Stern-Volmer fluorescence quenching experiments.
OH
O
NH4OAc (3 eq)
TEMPO (2 eq),
PC (2 mol%)
Acetone,N2, LEDs, rt
N
O
Ph
Ph
, 0%
Ph
+
+
Ph
BF3K
(eqn 1)
(eqn 2)
Ph
Ph
Ph
Ph
Ph
Ph
1a
2a
A
, 13%
O
OH
BnBF3K (1.5 eq)
NH4OAc (3 eq)
PC (2 mol%)
Ph
Ph
Ph
2a
+
Ph
+
Ph
Acetone,N2, LEDs, rt
OH
1a
B
C
72%
PC = Ir[dF(CF3)(ppy)]2(dtbbpy)PF6
B and C, detected by GC-MS
Scheme 2. Control experiments
In order to gain more insights into this protocol, the redox
potential of 1a was measured by voltammetric cycle to be -1.7
V against SCE in MeCN (Figure 2), which is far beyond the
reduction range of E1/2 red[IrIII/IrII] (-1.37 V vs SCE in MeCN)23,
therefore indicating a PCET event might be involved in the
reaction process. Besides, we also measured the Stern-Volmer
fluorescence quenching experiments of the reactants. As seen
in Figure 3, the result suggested the excited photocatalyst was
more likely initially quenched by benzyl trifluoroborate than
acetenyl ketones. (Please see SI for details)
Moreover, sets of controlled experiments were carried out
by employing 1a as model substrate (Scheme 2). When
conducted in the presence of radical trapping agent TEMPO,
the cross-coupling reaction was totally repressed and
meanwhile afforded alkyl-TEMPO adduct A in 13% yield (eqn
1). Additionally, we also detected the formation of B and C by
GC-MS in the standard reaction system (eqn 2). This result
provided a useful evidence for the generation of ketyl and
benzyl radicals in the Barbier-type reaction process.
A plausible mechanism for this photocatalytic protocol was
demonstrated in Scheme 3 on basis of previous relative
literatures and the mechanistic studies. Initially, a visible-light-
induced SET was occurred between benzyl trifluoroborate (Ered
= +1.10 V)24 and excited IrIII* (E red[IrIII*/IrII] = +1.21 V vs SCE in
MeCN) to afford a benzyl radical I and a reducing IrII species,
which was consistent with the Stern-Volmer fluorescence
quenching experiments. According to the above analysis, the
potential of IrII is insufficient to reduce the carbonyl of
acetenyl ketones to their radical species. As a consequence,
we considered a hydrogen-bond (II) was formed between the
acetenyl ketones and NH4OAc, which was believed to lower
Figure 2. Cyclic voltammogram of substrate 1a.
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the reduction barrier of carbonyl15a. Thus, through a PCET
event, the radical intermediate III was generated, and
meanwhile the ground state IrIII was released to re-catalyze
the reaction. Finally, a radical-radical cross-coupling of I and III
furnished the target propargyl alcohol.
General procedure for the visible-light-promoted cross-coupling
DOI: 10.1039/C9OB02624J
reactions: Acetenyl ketone (0.1 mmol), potassium trifluoroborates
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(0.15 mmol) and Ir[dF(CF3)(ppy)]2(dtbbpy)PF6 (2.0 mmol%) were
added to a transparent glass tube equipped with a magnetic stir
bar. Then the tube was covered with a rubber septum and sealed
with a coiled seal film. By evacuating air with N2 for three times at -
78 oC, dried and degassed acetone (1mL) was added. Subsequently,
the reaction system was treated under the irradiation of blue LEDs
(36 W) at room temperature for 24 hours. Monitored the reaction
process with TLC till completed, the reaction mixture was filtered
and the residue was washed with acetone (2 mL). The resulting
filter liquor was concentrated under reduced pressure and purified
by column chromatography to afford propargyl alcohols.
Ph
BF3K
Ph
O
I
IrIII*
SET
H
O
H
H
N
H
O
IrII
O
Blue
R1
LEDs
NH4OAc
- H
R1
R2
R2
PCET
hv
II
IrIII
OH
R1
R1
HO
Conflicts of interest
There are no conflicts to declare
Ph
R2
radical-radical
cross-coupling
R2
coupling product
Ph
III
Scheme 3. Proposed mechanism
Notes and references
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Conclusions
In conclusion, we have developed a visible-light-triggered
photocatalytic cross-coupling strategy that facilitated the
reaction of acetenyl ketones with benzyl trifluoroborate. The
efficiency of this protocol was highlighted by a wide range of
substrate scope, thus providing a feasible way to access types
of tertiary propargyl alcohols. Mechanistic studies indicated
the transformations proceeded via a radical pathway, in which
a PCET event was considered critical for the umpolung
coupling process.
Experimental
General methods: All reagents were purchased from commercial
suppliers such as Aldrich, Energy Chemical Chemicals and Aladdin.
All were used as received. Unless other noted, all photochemical
reactions were carried out in a 10 ml, flame-dried glass tube under
nitrogen atmosphere. H NMR (400 MHz) and 13C NMR (101 MHz)
were recorded on Bruker AV-400 instrument in CDCl3 with TMS as
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Organic & Biomolecular Chemistry
Page 6 of 6
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DOI: 10.1039/C9OB02624J
Contents entry
OH
R1
O
NH4OAc (3.0 eq)
PC (2.0 mol%)
R1
+
Ph
BF3K
LEDs, N2, rt
R2
Ph
R2
R1, R2 = alkyl, aryl
via PCET-involved
radical-radical cross-coupling
A visible-light-mediated photocatalytic strategy for the radical-radical cross-coupling of
acetenyl ketones with benzyl trifluoroborate.