Benzyl bromides as aroyl surrogates in substrate directed Pd catalysed o-aroylation

Article abstract of DOI:10.1039/c5ra03836g

An oxidative cross-coupling between directing substrates and benzyl bromides via the combined effect of oxidants TBHP and NMO, catalysed by Pd(ii) has been investigated. Benzyl bromides served as efficient aroyl surrogates in this substrate directed C-H f

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Benzyl bromides as aroyl surrogates in substrate directed Pd catalysed
o-aroylation†
Ahalya Behera, Wajid Ali, Srimanta Guin, Nilufa Khatun, Prakash R. Mohanta and Bhisma K. Patel*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
An oxidative crossꢀcoupling between directing substrates and benzyl bromides via the combined effect of oxidants TBHP and NMO,
catalysed by Pd(II) has been investigated. Benzyl bromides served as efficient aroyl surrogates in this substrate directed C−H
functionalisation. The in situ generated benzaldehyde originating from benzyl bromide is the active aroylating source
.
10
50 an orthoꢀaroylated product (1a) (37%) as expected. It may be
mentioned here that in an auxillary assisted Pd catalysed C−C
bond forming reaction benzyl bromide served as the benzylating
agent for the sp2 and sp3 C−H bonds.9 Herein we illustrate the
first report on the use of benzyl bromides as aroylating agent in
55 Pdꢀcatalysed substrate directed process.10
Introduction
In last two decades, an extensive exploration of Pdꢀcatalysed
regioselective C–H functionalisation has been investigated to
transform simple precursors into products of significant
15 importance, particularly for the synthesis of complex molecules
and natural products.1,2 Most of these Pdꢀcatalysed reactions have
been intended for the C−C bond forming processes involving sp,
sp2 or sp3 hybridised C−H’s as a mutual or a crossꢀcoupling
partners.2d,g,3 The classical synthesis of aryl ketones mainly rely
20 on the FriedelꢀCrafts acylation of aromatics in the presence of
corrosive AlCl3 or by the oxidation of corresponding secondary
alcohols.4 Compared to the classical FriedelꢀCrafts acylation, the
direct introduction of carbonyl functionality into aromatic motifs
via C−H bond cleavage is more ecoꢀfriendly alternative. The
25 acylation of 2ꢀphenylpyridines via orthoꢀC−H bond activation has
been previously reported by our group and others using
aldehydes,5a αꢀoxocarboxylic acids,5b alcohols,5c αꢀdiketones,5d
toluenes,5e,f benzylamines5g and alkenes5h,i / alkynes5i as aroyl
surrogates. The process of aroylation is of significant importance
30 because aryl ketones are useful in pharmaceuticals, fragrance, dye
Scheme 1 Differential reactivities of benzyl bromides towards
directing substrates.
Results and Discussion
60
With the finding of benzyl bromide serving as a new aroyl
surrogate we probed the effect of aroylating agent, catalysts,
oxidants, additives, and solvents to obtain the best yield of the oꢀ
aroylated product. Increasing the quantity of benzyl bromide
65 from 2 to 3 equiv., the yield of the product (1a) improved up to
48% (Table 1, entry 2). The oxidant Nꢀmethyl morpholine Nꢀ
oxide (NMO) is reported to be quite effective in converting
benzyl halides to their corresponding aldehydes, which we
presume to be the active aroylating agent.11 Taking cues from this
70 report, when 1 equiv. of NMO was used in the reaction, the yield
of the desired product improved to 55% (Table 1, entry 3). A
further improvement in the yield (66%) was observed when the
quantity of NMO was increased to 2 equiv. (Table 1, entry 4).
Since the other oxidant NMO (2 equiv.) is also used along with
75 the primary oxidant TBHP (4 equiv.), thus the reaction is
expected to proceed with a lesser quantity of TBHP. With this in
mind, keeping all other parameters as such, a reaction was
performed with a combination of TBHP (2 equiv.) and NMO (2
equiv.). The reaction furnished a comparable yield of 62% for
80 (1a) (Table 1, entry 5). Interestingly, under a TBHP free
condition, using NMO as the sole oxidant, formation of the
and
agrochemical
industries.6
However,
the
crossꢀ
dehydrogenative coupling (CDC) strategy of converting sp2 C−H
bonds of aryl moiety into C−C bonds are challenging.7 Generally,
most of the developed approaches on regioselective C−H
35 activation involve suitable catalysts, functionalised partners and
oxidants.8 In search of an alternative aroyl source, we envisaged
that benzyl bromides which under an oxidative condition can be
converted into benzaldehyde may serve as the aroylating source.
With this in mind, an initial trial reaction was performed between
40 2ꢀphenylpyridine (1) (0.5 mmol) and benzyl bromide (a) (1
mmol) in the presence of Pd(OAc)2 (5 mol%) and oxidant TBHP
o
(5−6 M in decane) (4 equiv.) in chlorobenzene (2 mL) at 120 C
(Table 1, entry 1). The reaction led to the exclusive formation of
___________________________________________
45 Department of Chemistry, Indian Institute of Technology Guwahati, 781
039, Assam, India. Fax no. +91-3612690762; E-mail: patel@iitg.ernet.in
†Electronic supplementary information (ESI) available: 1H and 13C NMR
spectra For ESI or other electronic format see DOI: 10.1039/xxxxxx.
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desired product (1a) was not observed (Table 1, entry 6)
suggesting the essential requirement of TBHP in bringing about
the transformation. Next, various other salts of Pd such as PdCl2
found to be superior compared to other inorganic bases such as
Cs2CO3 (71%), NaHCO3 (73%) and Na2CO3 (56%). Finally, the
optimum condition was the use of 2ꢀphenyl pyridine (1) (0.5
(56%), PdBr2 (53%), Pd(PPh3)2Cl2 (39%), Pd(OCOCF3)2 (43%) 40 mmol), benzyl bromides (a) (1.5 mmol), Pd(OAc)2 (5 mol%),
TBHP (2 equiv.), Nꢀmethyl morpholine Nꢀoxide (2 equiv.),
K2CO3 (1.5 equiv.) in chlorobenzene (2 mL) at 120 C (Table 1,
entry 21).
5
(Table 1, entries 7−10), were examined to check their efficacy,
from which Pd(OAc)2 was found to be the superior (Table 1,
entry 5). When PdCl2 and PdBr2were used as the catalysts along
with the formation of the desired oꢀaroylated product, traces of oꢀ
brominated product was also observed. It may be mentioned here
o
Scheme 2 Substrate scope for the oꢀaroylationa,b
10 that in an analogous reaction reported by Wu’s group (using
benzyl chloride and PdCl2) exclusive formation of the
corresponding oꢀchlorinated product was observed with no
formation of oꢀacylated product.10 However the oꢀbrominated
product so generated in our case is converted to its corresponding
15 oꢀacylated product on prolonging the reaction time. A two fold
increase in the catalyst loading had no significant effect on the
product yield (Table 1, entry 11). The effects of different oxidants
for this transformation were also screened as shown in Table 1.
All the primary oxidants such as 70% aq. TBHP (59%), DTBP
20 (45%), K2S2O8 (05%) and H2O2 (00%) were found to be inferior
compared to the use of a decane solution of TBHP (Table 1,
entries 12−15). Among various solvents tested such as THF
(00%), dioxane (30%), DMSO (27%), DMF (52%), DCE (49%)
(Table 1, entries 16−20), chlorobenzene (62%) was found to be
25 the most appropriate (Table 1, entry 5).
Table 1 Screening of the reaction conditionsa,b
Sl.
No
Catalyst
(mol%)
Oxidant
(equiv.)
Additive
(equiv.)
Solvent
Yield
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
PdCl2 (5)
PdBr2 (5)
Pd(PPh)3Cl2 (5)
Pd(OCOCF3)2 (5) TBHP (2)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (2)
ꢀ
TBHP (2)
TBHP (2)
TBHP (2)
ꢀ
ꢀ
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
37
48c
55c
66c
62c
00c
56c
53c
39c
43c
64c
59c,d
45c
05c
NMO (1)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
Pd(OAc)2 (10)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
TBHP (2)
TBHP (2)
DTBP (2)
K2S2O8 (2)
15
16
17
18
19
20
21
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
H2O2 (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
TBHP (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
NMO (2)
PhCl
THF
Dioxane
DMSO
DMF
00c
00c
30c
27c
52c
49c
79c,e
aReaction conditions: 1−6 (0.5 mmol), a−k (1.5 mmol), Pd(OAc)2 (5
mol%), TBHP (5−6 M in decane) (2 equiv.), NMO (2 equiv.), K2CO3 (1.5
equiv.) at 120 oC in chlorobenzene (2 mL), time 8−17 h. bYield of isolated
pure product reported.
DCE
PhCl
aReaction conditions: arenes (1) (0.5 mmol), solvent (2 mL) at 120 oC
bYields of isolated pure product. c1.5 mmol of benzyl bromide. daq.
30 TBHP. eK2CO3 (1.5 equiv.).
45
The optimised conditions were then implemented for the oꢀ
aroylation of 2ꢀphenyl pyridine (1) using various substituted
benzyl bromides and the results are summarised in Scheme 2.
Benzyl bromides having electronꢀdonating groups such as pꢀMe
The catalytic activity of Pd is often hampered in acidic medium.
Since an equivalent amount of HBr is liberated in the medium
during the conversion of benzyl bromide to active aroylating
35 species, it was decided to perform the reaction in the presence of
a mild base. The use of K2CO3 (79%) (Table 1, entry 21) was
50 (b), p-tBu (c) and m-Me (d) when treated with 2ꢀphenylpyridine
2
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65 (1a) (< 10%) and isolation of TEMPO ester (E) in 67% yield
suggests a radical pathway. Taking cues from these experimental
results and related literature reports,5i,12 a proposed mechanism is
shown in Scheme 3. In the presence of oxidants NMO11 and
TBHP benzyl bromide is first oxidised to benzaldehyde (A). This
70 in situ generated benzaldehyde (A) is then converted to benzoyl
radical (B) with the aid of TBHP. Subsequent ligation of this
radical species (B) with the chelated complex of Pd(II) (C) gives
either a dimeric Pd(III) or Pd(IV) intermediate (D). Finally, C−C
bond formation via reductive elimination of Pd formed complex
75 (D) afforded the oꢀaroylated product (1a) and releasing Pd(II)
catalyst in the medium to maintain the catalytic cycle.
(1) afforded corresponding oꢀaroylated products (1b−1d) in
moderate to good yields. Benzyl bromides substituted with
moderately electronꢀwithdrawing groups such as pꢀBr (e), pꢀCl
(f) and pꢀF (g) served as less effective acylating agent giving their
oꢀaroylated products (1e), (1f) and (1g) in 67%, 55% and 43%
yields respectively which are slightly less compared to benzyl
bromides possessing electronꢀdonating substituents. However, the
influence of electronꢀwithdrawing substituents such as −Cl (h),
−F (i) and −CF3 (j) when present at the mꢀposition had no effect
5
10 on the products (1h−1j) yield. To further explore the scope of this
protocol, aroylation of other 2ꢀaryl pyridines were carried out
with a set of substituted benzyl bromides. The reactivity of
substituted benzyl bromides with 2ꢀ(pꢀtolyl)pyridine (2) were
found to be similar to that of (1). A slight increase in the product
15 yields (2a−2c and 2e−2j) with 2ꢀ(pꢀtolyl)pyridine (2) could be
attributed due to the better chelation of the metal catalyst with the
electron rich pꢀtolyl ring in (2). 2ꢀIodo benzyl bromide (k) when
treated with (2), a lower yield of 41% of (2k) was observed while
its reaction with (1) is completely unproductive. The nonꢀ
20 reactivity of 2ꢀiodo benzyl bromide (k) with (1) could be due to
the electronic and steric hindrance imparted by the bulky oꢀiodo
substituent. However due to the better chelation ability of (2)
leads to the formation of product (2k) in 41% yield. Similarly
when 2ꢀ(4ꢀmethoxyphenyl)pyridine (3) was treated with
25 benzylbromides (b) and (f) provide the acylated product in 87%
and 66% yield respectively (3b and 3f). 2ꢀ(4ꢀ
Bromophenyl)pyridine (4), a moderately deactivated substrate
when treated with benzyl bromides (a and b) provided
corresponding oꢀaroylated products (4a) and (4b) in 62% and
30 69% yields respectively as shown in Scheme 2. Furthermore,
electron neutral (a) and electron deficient benzyl bromides (e and
g) provided oꢀaroylated products (5a), (5e) and (5g) in moderate
yields when reacted with 2ꢀ(4ꢀchlorophenyl)pyridine (5). Lesser
reactivity of directing arenes possessing electronꢀwithdrawing
35 groups –Br (4) and −Cl (5) as compared to electron neutral –H (1)
and electron rich –CH3 (2) and –OCH3 (3) analogues may be due
to their poor chelating ability with the metal catalyst.
Benzo[h]quinoline (6), a rigid directing system has similar
structural frame to that of 2ꢀphenyl pyridine (1) where the sp2
40 C−H at C10 can be aroylated. When (6) was reacted with benzyl
bromide (a), and those possessing electronꢀdonating groups such
as pꢀCH3 (b), pꢀtBu (c) all provided their respective oꢀaroylated
products (6a), (6b) and (6c) (Scheme 2). Similarly benzyl
bromides having electronꢀwithdrawing substituents such as –Br
45 (e), −Cl (f) and –F (g) also resulted in their corresponding
aroylated products (6e), (6f) and (6g) respectively but in slightly
lesser yields compared to their electronꢀdonating counterparts.
When moderately –Cl (h) or even strongly −CF3 (j) electronꢀ
withdrawing substituents are present in the meta position of
50 benzyl bromides, corresponding C10 aroylated products (6h) and
(6j) were obtained in 59% and 48% yields respectively. In order
to broaden the scope of this methodology when the present
conditions were applied to substrates having other directing
Scheme 3 Proposed mechanism for oꢀaroylated product from
benzyl bromide and 2ꢀphenyl pyridine.
80 Conclusion
This protocol demonstrates a convenient and efficient
synthesis of aryl ketones via Pdꢀcatalysed orthoꢀC−H activation
of 2ꢀphenyl pyridines and benzo[h]quinoline using commercially
available benzyl bromides as unconventional synthetic
85 equivalents of aroyl group (ArCO−). All the benzyl bromides
having activating or deactivating groups efficiently gave the
desired product with 2ꢀaryl pyridines and benzo[h]quinoline.
groups
such
as
2ꢀphenylbenzo[d]thiazole,
2,3ꢀ
General procedure for the synthesis of phenyl(2ꢀ(pyridinꢀ2ꢀ
yl)phenyl)methanone (1a)
55 diphenylquinoxaline and ketoxime ether, the reactions were
completely unproductive giving no desired acylated products.
90
A plausible mechanism for this palladium catalysed carboꢀ
acylation reaction of 2ꢀphenyl pyridine (1) with benzyl bromide
(a) via directed C−H bond activation is depicted in Scheme 3.
60 The detection of benzaldehyde (A) in the crude reaction mixture
A mixture of 2ꢀphenyl pyridine (77.5 mg, 0.5 mmol),
Pd(OAc)2 (5.6 mg, 5 mol%), benzyl bromide (256.5 mg, 1.5
mmol), Nꢀmethyl morpholine Nꢀoxide (NMO) (351 mg, 3 mmol,
2equiv. w.r.t. benzyl bromide), chlorobenzene (2 mL) were taken
suggests its intermediacy for this transformation. Furthermore, to 95 in an oven dried round bottom flask (10 mL). To this mixture,
deduce the nature of this coupling, a reaction between (1) and (a)
was performed in the presence of a radical quencher TEMPO
under otherwise identical conditions. The formation of product
K2CO3 (103.5 mg, 0.75 mmol) and decane solution of TBHP
(5−6 M) (600 µL, 3 mmol, 2 equiv. w.r.t. benzyl bromide) were
added. Then the flask was fitted with a condenser and heated for
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o
9 h in a preꢀheated oil bath maintained at 120 C. During this
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period, the progress of the reaction was monitored by TLC at
regular intervals. After completion of the reaction, it was cooled
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5
filtered. The ethyl acetate layer was washed successively with
water (3 x 5 mL). The organic layer was then dried over
anhydrous Na2SO4 and the solvent was evaporated under reduced
pressure. The crude product was purified over a column of silica
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10 afford phenyl(2ꢀpyridinꢀ2ꢀyl)phenyl methanone (1a) (102 mg,
79% yield).
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Acknowledgements
(9) (a) E. T. Nadres, G. I. F. Santos, D. Shabashov and O.
Daugulis, J. Org. Chem., 2013, 78, 9689.
B. K. P acknowledges the support of this research by the Science
15 and Engineering Research Board (SERB) (SB/S1/OCꢀ53/2013),
New Delhi, and the Council of Scientific and Industrial Research
(CSIR) (02(0096)/12/EMRꢀII). W.A., S. G. and N. K. thank
CSIR.
80 (10) While our manuscript was under preparation, an analogous
aroylation reaction was reported using benzyl chloride and a
cyclopalladated ferrocenylimine complex. G. Zhang, S. Sun, F.
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