Palladium-catalyzed synthesis of fluoreones from bis(2-bromophenyl)methanols

Article abstract of DOI:10.1039/c7ob02895d

A palladium-catalyzed synthesis method of fluorenones has been developed. A variety of bis(2-bromophenyl)methanols could undergo the reaction smoothly in the presence of Pd(OAc)₂, affording a series of fluorenones in moderate to good yields (two steps). Mechanistic studies reveal that the reaction might be triggered by oxidation of alcohol followed by intramolecular reductive coupling.

Full text of DOI:10.1039/c7ob02895d

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Palladium-catalyzed Synthesis of Fluoreones from Bis(2-
bromophenyl)methanols
Qian Gaoa and Senmiao Xu*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
palladium-catalyzed synthesis of fluorenones has been or excessive amount of oxidants, regioselectivity issue, and
developed. A variety of bis(2-bromophenyl)methanols could requirement of excessive amount of strong acids in some
undergo reaction smoothly in the presence of Pd(OAc)2, affording cases. In addition, the formation of a preinstalled carbonyl
a series of fluorenones in moderate to good yield (two steps). group from corresponding alcohol requires at least one
Mechanistic studies reveal the reaction might be triggered by equivalent of oxidants in some examples.
oxidation of alcohol followed by intramolecular reductive
coupling.
Palladium-catalyzed reductive coupling of aryl halides has
emerged as a useful approach to access biaryls due to the mild
reaction conditions and high functionality tolerance.17
Reducing reagents are usually necessary to regenerate Pd(0)
species. Hydroquinone,18 formate salt,19 hydrogen gas,20
amines,21 and zinc22 and even alcohols23 are commonly used
for this purpose. Therefore, we hypothesized that formation of
a carbonyl group from the secondary alcohol facilitated by an
intramolecular reductive coupling of C-X bonds would provide
fluorenones by combination of distinctive redox abilities of
Pd(II) and Pd(0) species (Scheme 2). We herein disclose the
palladium-catalyzed synthesis of fluorenones from bis(2-
halophenyl)methanols via an oxidation of alcohol/reductive
coupling of C-Br bonds sequence for the first time. We
demonstrate the current method is also amendable to gram-
scale synthesis of fluorenone with reduced catalyst loading.
Fluorenone and its derivatives have gained increasing
attention due to their importance in biomedical and
photoelectric materials.1 As a result, the construction of this
scaffold has received considerable interests. Conventional
methods include intramolecular Friedel-Crafts reaction of
biaryl carboxylic acids,2 Pschorr cyclization of 2-amino diaryl
ketones,3 oxidation of fluorenol,4 and oxidation of fluorene.5
However, most of the aforementioned methods encounter
some limitations such as multi-step procedures and
prefunctionalization as well as involvement of hazardous
oxidants. On the other hand, transition-metal-catalyzed
synthesis of fluorenones has been studied extensively over the
past decades. A number of them were accomplished by
intramolecular C-C bond forming reactions via C-H
functionalization, including palladium catalyzed C-H arylation
of o-halo diaryl ketones,6 cyclocarbonylation of o-halobiaryls,7
oxidative coupling of diaryl ketones via dual C-H activation,8
arylation of o-biaryl nitriles,9 imines,10 and aldehydes,11 radical
cyclization,12 rhodium-catalyzed acylation of biarylcarboxylic
acids,13 and gold-catalyzed reactions.14 Besides, palladium-
catalyzed tandem oxidative C-H/C-H coupling/Friedel-Crafts
acylation15 and other intermolecular pathways16 also provide
complementary approaches to fluorenone scaffolds. However,
some of the above mentioned catalytic methods still suffer
from several limitations such as involvement of stoichiometric
oxidation of alcohol by Pd(II)
O
OH
sole catalyst
•
• scalable
H
easy operation
•
X
X
reductive coupling of C-X bonds by Pd (0)
Scheme 1 Our strategy to the synthesis of fluorenones.
The substrates bis(2-halophenyl)methanols of our
proposed reaction could be readily synthesized according to
literature procedures.24 To test our hypothesis, we initially
examined reaction of bis(2-bromophenyl)methanol 1a in the
presence of 10 mol % of PdCl2 and 2.5 equivalents of Cs2CO3 in
toluene at 110 ˚C for 16 h, affording the desired product 2a in
20% isolated yield (Table 1, entry 1). In contrast, no product
was observed when bis(2-chlorophenyl)methanol was used as
the substrate under otherwise identical reaction conditions
(Table 1, entry 2). The additive quaternary ammonium salt was
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found to affect the reactivity significantly (Table 1, entries 3- substrates (2e and 2f). Interestingly, substrate 1j bearing an
6).23a When tetrabutylammonium iodide (TBAI) was used unprotected OH group did not adversely affect the reaction,
(Table 1, entry 5), fluorenone 2a was obtained in 45% yield. producing the corresponding fluorenone 2j in 42% isolated
Other bases such as K2CO3 and K3PO4 gave inferior results yield. The current method also tolerates substrates bearing
compared to Cs2CO3 under otherwise identical reaction one heteroarene such as thiophene and pyridine, giving the
conditions (Table 1, entries 7 and 8 vs entry 6). Further corresponding thiofluorenone and azafluorenones (2p
scrutiny of catalyst revealed that Pd(OAc)2 was optimal, which 42-64% yield.24
delivered 2a in 70% yield (Table 1, entry 12). A decreased yield
-2r) in
O
Br OH Br
was observed when base and TBAI were reduced to 2
equivalents (Table 1, entry 13). The effect of phosphine ligands
were also investigated (Table 1, entries 14-17). However, no
further enhancement of yield of 2a was observed. Therefore,
we chose entry 12 in Table 1 as our optimal reaction
conditions to determine substrate scope.
Pd(OAc)2 (10 mol %)
Cs2CO3, nBu4NI
toluene, 110 C, 16 h
R1
R2
R2
R1
1
2
O
O
O
O
F
F
F
2b, 43%
2c, 68%
2d, 41%
2a, 70%
O
O
O
Table 1 Optimization of palladium-catalyzed synthesis of
fluorenone 2a from bis(2-bromophenyl)methanol 1aa
O
Cl
CF3
Cl
CO2Me
2e, 25% (49%)b
2f, 35% (61%)b
2h, 25%
2g
, 52%
O
O
O
O
Me
OH
OMe
OMe
OMe
Me
Yieldb (%)
2i, 42%
2j, 42%
2l, 74%
2k, 64%
Entry
Catalyst
Additive (x equiv)
O
O
O
O
1
PdCl2
None
20
n.d.
36
2c
3
PdCl2
TBAF (2.5)
TBAC (2.5)
TBAB (2.5)
TBAI (2.5)
TBAI (2.5)
TBAI (2.5)
TBAI (2.5)
TBAI (2.5)
TBAI (2.5)
TBAI (2.5)
TBAI (2.5)
TBAI (2.0)
TBAI (2.5)
TBAI (2.5)
TBAI (2.5)
TBAI (2.5)
S
O
OMe
OMe
O
PdCl2
Cl
2p
, 64%
2m
, 65%
2o
, 54%
O
2n, 35%
4
PdCl2
33
O
5
PdCl2
42
6
PdCl2
45
N
N
7d
8e
9f
PdCl2
15
2q, 55%
2r
, 42%
PdCl2
44
aUnless otherwise noted, all the reactios were performed with 1 (0.20 mmol), Pd(OAc)2 (0.02 mmol), Cs2CO3
(0.50 mmol), and nBu4NI (0.50 mmol) in toluene (2.0 mL) at 110 C for 18 h; bYields in the brackets using
PdCl2 (0.02 mmol) as the catalyst.
PdCl2
trace
38
10g
11
12
13h
14
15
16
17
PdCl2
PdBr2
55
Scheme 2 Substrate scope of palladium-catalyzed synthesis of
Pd(OAc)2
Pd(OAc)2
Pd(PPh3)2Cl2
PdCl2/dppf
PdCl2/xantphos
PdCl2/X-phos
70
Fluorenones.
65
49
69
67
52
aunder otherwise noted, all the reactions were carried out
with 1a (0.20 mmol), catalyst (0.02 mmol), Cs2CO3 (0.50
mmol) and toluene (2.0 mL) under N2 atmosphere at 110 ˚C
for 16 h; byields of isolated products; cbis(2-
d
chlorophenyl)methanol was used instead of 1a; K2CO3 was
e
used instead of Cs2CO3; K3PO4 was used instead of Cs2CO3;
fDMSO as solvent; g1,4-dioxane as the solvent; h2.0
equivalents of Cs2CO3 was used.
Scheme 3 Application of current method.
To highlight utility of our method, a gram-scale synthesis
was performed. As illustrated in Scheme 3, reaction of 1l (10
mmol scale) in the presence of reduced catalyst loading (2
mol %) could afford corresponding fluorenone 2l in 46% yield
(1.1 grams). Reaction of fluorenone 2l with arylmagnesium
bromide gave corresponding fluorenols 3a and 3b in 80% and
82% yields, respectively.25 In the presence of AcOH and HCl, 3b
With a set of optimized reaction conditions in hand, the
substrate scope was then surveyed as illustrated in Scheme 2.
Most of bis(2-bromophenyl)methanols underwent the reaction
smoothly to give the corresponding products in reasonable
yields. The substrates with electron donating groups gave
superior results compared to those with electron withdrawing
groups (2k-m vs. 2b-h). It has to be noted that PdCl2
performed better than Pd(OAc)2 in some chlorinated
2 | J. Name., 2012, 00, 1-3
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could be further converted to 9,9’-spirobi[fluorene]
yield via a intramolecular Friedel-Crafts alkylation.25
4
in 92% diphenylmethanol
8
under otherwise standard reaction
conditions (Scheme 4, eq 2). The identical control experiments
We are interested in the mechanism of the reaction. Based of biarylmethyl methyl ether 10 did not give any methyl 9-
on the reaction sequence, there are four plausible scenarios, fluorenol (Scheme 4, eqs 3 and 4). These results clearly show
namely paths A-D as illustrated in Figure 1. In path A, the that direct palladium-catalyzed intramolecular Ullman reaction
reaction is initiated by Pd(II) in which the substrate 1a of bis(2-bromophenyl)methyl methyl ether 10 is not feasible.
undergoes ligand exchange with PdX2 to form a Pd(II) alkoxide Therefore, path B should not be possible. Notably, We found
species
along with the formation of the diaryl ketone
generated then could undergo reductive coupling to form whereas the major product was diphenyl ketone
fluorenone 1a with regeneration of PdX2.27 In path B, substrate yield) that was problay formed via reductive debromination
1a undergoes reductive coupling first to give fluorenol with (Scheme 4, eq 5). Therefore, two bromos in compounds are
concurrent formation of PdX2. Then intermediate could be essential to prohibit reductive debromination and make this
oxidized by PdX2 with assistance of base to form 2a coinciding reaction happen. Next, we examined the reaction of
with regeneration of Pd(0) for the next catalytic cycle. In the monobromo ketone under standard conditions, which gave a
alternative scenarios, the reaction could also take place via the 98% GC yield of product 2a (Scheme 4, eq 6). It seems that
intermediate o-bromo biaryl ketone that has been known to paths C and D are possible. Further experiments of reaction
form fluorenone by palladium catalysis.6a The C-H bond in
with deuterium substrates 1-d1 and 1-d1’ showed no
could be formed either by protonolysis of Ar-Pd bond (path C) deuterium incorporated product was observed (Scheme 4, eqs
I
, proceeding β-hydride elimination to generate Pd(0) the reaction of monobromo alcohol 12 under standard
. The in-situ conditions produced only 6% GC yield of fluorenone 2a
(63% GC
5
,
5
9
6
1
6
7
7
7
or reductive elimination of ArPdH species (path D).
7 and 8). Additionally, in the presence of 2 equivalents of
MeOD, no deuterium incorporated product was observed
(Scheme 4, eq 9). Therefore, the pathways C and D could be
ruled out. On the basis of these results, our reaction is
probably triggered by palladium-catalyzed oxidation of
secondary alcohol, which facilitates an intramolecular
reductive coupling of C-Br bonds (Figure 1, path A).
O
O
Pd(OAc)2 (X mol %)
X = 10, 10% GC yield
X = 100, 92% GC yield
(1)
Cs2CO3, nBu4NI
toluene, 110 C, 16 h
Br Br
5
2a
O
O
O
OH
Pd(OAc)2 (10 mol %)
+
(2)
Cs2CO3, nBu4NI
Br Br
toluene, 110 C, 16 h
8
2a
9
5
93%
98%
OMe
OMe
Pd(OAc)2 (X mol %)
X = 10 and 100,
no product was observed
Cs2CO3, nBu4NI
toluene, 110 C, 16 h
(3)
(4)
Br Br
10
11
OMe
OMe
O
OH
Pd(OAc)2 (10 mol %)
+
+
Cs2CO3, nBu4NI
Br
Br
10
OH
toluene, 110 C, 16 h
11
not observed
O
9
8
O
Pd(OAc)2 (10 mol %)
Figure 1 Plausible reaction pathways: oxidation-reductive coupling
sequence (path A); reductive coupling-oxidation sequence (path B);
pro-todebromination-coupling sequence (path C); hydro-
debromination-coupling sequence (path D).
(5)
(6)
+
Cs2CO3, nBu4NI
toluene, 110 C, 16 h
Br
Br
Br
9 63% (GC yield)
12
2a, 6% (GC yield)
O
O
Pd(OAc)2 (10 mol %)
Cs2CO3, nBu4NI
toluene, 110 C, 16 h
To shed light on which one is most likely pathway, we
performed control experiments as illustrated in Scheme 4.
7
2a, 97% (GC yield)
OH
O
Pd(OAc)2 (10 mol %)
Reaction of dibromophenyl ketone
5 under standard reaction
D
(7)
no deuterated product was observed
no deuterated product was observed
Cs2CO3, nBu4NI
toluene, 110 C, 16 h
Br
conditions (10 mol % Pd(OAc)2) afforded corresponding
2a, 66%
1-d1, 95% D
product 2a in 10% GC yield, whereas 92% yield of product was
observed with 100 mol % of Pd(OAc)2 (Scheme 4, eq 1). These
two control experiments suggest that Pd(OAc)2 is able to
promote intramolecular Ullman coupling of bis(2-
bromophenyl)methanone whereas active catalyst could not be
regenerated after one catalytic cycle without a reducing
reagent such as alcohol. Indeed, 93% GC yield of fluorenone 2a
was observed upon addition of 1.0 equivalent of
OD
O
Pd(OAc)2 (10 mol %)
Cs2CO3, nBu4NI
toluene, 110 C, 16 h
(8)
(9)
Br
Br
1-d1', >95% D
2a, 68%
OH
O
Pd(OAc)2 (10 mol %)
no deuterated product was observed
Cs2CO3, nBu4NI
MeOD (2.0 equiv)
toluene, 110 C, 16 h
Br Br
1
2a, 46%
Scheme 4 Control experiments.
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Conclusions
We have developed
a simple, novel palladium-catalyzed
synthesis of fluorenones by combination of distinctive redox
abilities of Pd(0) and Pd(II). variety of bis(2-
A
bromophenyl)methanols underwent the reaction smoothly,
affording the corresponding fluorenones in moderate to good
yields via two steps. We demonstrate the present method is
also amendable to gram-scale synthesis with reduced catalyst
loading. Mechanistic studies revealed the plausible mechanism
might undergo oxidation of secondary alcohol first followed by
reductive coupling of C-Br bonds.
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Conflicts of interest
There are no conflicts to declare.
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C7OB02895D