Cyanation of unactivated aryl chlorides and aryl mesylates catalyzed by palladium and hemilabile MOP-type ligands

Article abstract of DOI:10.1055/s-0034-1379483

Palladium-catalyzed cyanation of aryl halides and pseudo halides with potassium hexacyanoferrate is described employing the hemilabile, bulky, and electron-rich MOP-type ligands. When the mixture of t-BuOH and H₂O was used as the solvent and K₂CO₃ as the base, the MOP-type ligands showed high efficiency for the palladium-catalyzed cyanation. The effect of ligand structure was studied in detail, and 2-di-tert-butylphosphino-2′-isopropoxy-1,1′-binaphthyl was the more effective for the cyanation. The catalyst system allows the cyanation of unactivated aryl chlorides, and even aryl mesylates to occur in good yields. Furthermore, the reactivity of different arylated reagents in the catalytic system was found to be: ArBr > ArCl >> ArOMs > ArOSO₂Im > ArOSO₂NMe₂.

Full text of DOI:10.1055/s-0034-1379483

2938▌
LETTER
lCettyeranation of Unactivated Aryl Chlorides and Aryl Mesylates Catalyzed by
Palladium and Hemilabile MOP-Type Ligands
Cyanation of Unactivated Aryl Chlorides
Yahui Tu,a Yi Zhang,b Sheng Xu,*a Zhaoguo Zhang,b Xiaomin Xie*b
a
School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237,
P. R. of China
b
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. of China
Fax +86(21)54748925; E-mail: xiaominxie@sjtu.edu.cn
Received: 09.07.2014; Accepted after revision: 28.09.2014
solved cyanide ion.7 Typically, this was achieved by the
Abstract: Palladium-catalyzed cyanation of aryl halides and pseu-
use of nonpolar solvents such as toluene or xylene for
do halides with potassium hexacyanoferrate is described employing
KCN or NaCN or the use of insoluble cyanide salts
the hemilabile, bulky, and electron-rich MOP-type ligands. When
the mixture of t-BuOH and H2O was used as the solvent and K2CO3
[Zn(CN)2].8 Another approach is to slowly release cya-
as the base, the MOP-type ligands showed high efficiency for the nide ions from the transformation of organic compounds
silylcyanide,9
acetone
cyanohydrin,7
palladium-catalyzed cyanation. The effect of ligand structure was
(trimethyl
Bu3SnCN10). In addition, coupling with noncoordinating
organic cyanides followed by conversion to the corre-
sponding benzonitriles have also been reported.11 Recent-
ly, an attractive improvement was made by Beller’s
group.12 Nontoxic, cheap, and easy-to-handle potassium
hexacyanoferrate(II) {K4[Fe(CN)6]} was introduced as a
cyanide source in the cyanation reactions. Afterwards, im-
provement of this protocol was reported continually, such
as, development of novel catalyst systems,13 application
of challenging substrates like aryl chlorides,14 tosylates,15
and mesylates.16
studied in detail, and 2-di-tert-butylphosphino-2′-isopropoxy-1,1′-
binaphthyl was the more effective for the cyanation. The catalyst
system allows the cyanation of unactivated aryl chlorides, and even
aryl mesylates to occur in good yields. Furthermore, the reactivity
of different arylated reagents in the catalytic system was found to
be: ArBr > ArCl >> ArOMs > ArOSO2Im > ArOSO2NMe2.
Key words: palladium, monophosphine ligands, cyanation, aryl
chlorides, aryl mesylates
Benzonitriles are important building blocks of natural
products, pharmaceuticals, agrochemicals, and dyes;
moreover, nitriles are versatile intermediates in synthetic
organic chemistry which can be easily converted into oth-
er functional groups, such as carboxylic acids, amides,
amines, ketones, and nitrogen-containing heterocycles.1
Therefore, the introduction of cyanide groups into aryl
compounds is a very important step in the development of
a variety of well-known compounds used in pharmacy,
complex chemistry, and polymer chemistry.2 Among var-
ious synthetic methods,1,3 the transition-metal-catalyzed
cyanation of aryl halides or pseudohalides is the more di-
rect and versatile transformation.4 Although nickel,5 cop-
per,4b and palladium4a complexes are known to be
effective catalysts for this transformation, palladium cata-
lyst systems are more advantageous. In general, palladium
catalysts are more tolerant towards functional groups, less
sensitive to air and humidity compared to nickel catalytic
systems, and can be tuned more easily for activating aryl
C–X bonds compared to copper catalysts.4a Hence, a lot of
effort has been devoted in developing efficient palladium
catalysts for the cyanation.
Although palladium-catalyzed cyanation is well-estab-
lished with aryl iodides,17 bromides,10,13b,18 and triflates;19
aryl chlorides and aryl mesylates offer potential advantag-
es due to their wider availability and lower cost. However,
due to the high bond energy of the aryl C–Cl bond, espe-
cially the aryl C–O bond of aryl mesylates,20 only a few
successful cyanations of these substrates have been re-
ported. Jin and coworkers reported the first palladium-cat-
alyzed cyanation of aryl chlorides at 150 °C with Zn(CN)2
as the cyanating source.21 The use of bulky and electron-
rich phosphines as the ligand allowed the palladium-cata-
lyzed cyanation of aryl chlorides to be conducted under
milder conditions.8d,14a,b,22 However, utilizing potassium
ferrocyanide as the cyanide source required harsh reaction
conditions (i.e., temperatures ranging from 140–160 °C).
Significant advances were reported by the research groups
of Huang15 and Kwong,14e,16 who utilized the mixture of
organic/aqueous solvent to enable cyanide ion transfer
from K4[Fe(CN)6] under milder conditions. In these cata-
lytic systems, the effect of the ligand structure was very
significant. However, the impact of the structural features
of the ligands on the catalytic activity for the cyanation
have not been explored in detail.
The main problem of palladium-catalyzed cyanation is the
catalyst deactivation by coordination of dissolved cyanide
ions in the reaction mixture.6 This issue can be circum-
vented by careful control of the concentration of the dis-
Compared to the other palladium-catalyzed coupling re-
actions of the unactive electrophiles (such as Suzuki cou-
pling, Buchwald–Hartwig amination), until now, only a
few phosphine ligands have been demonstrated to be effi-
cient for the cyanation with K4[Fe(CN)6]14c,16 (Figure 1).
The hemilabile MOP-type ligands [2-(dialkyl-phosphi-
SYNLETT 2014, 25, 2938–2942
Advanced online publication: 21.10.2014
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DOI: 10.1055/s-0034-1379483; Art ID: st-2014-w0578-l
© Georg Thieme Verlag Stuttgart · New York

LETTER
Cyanation of Unactivated Aryl Chlorides
2939
no)-2′-alkoxy-1,l′-binaphthyls] are bulky monodentate yield (Table 1, entry 9). When n-BuOH or t-AmOH was
phosphines with a binaphthyl skeleton and a tunable used as the solvent, tert-butylbenzene was formed as the
alkoxy group at the adjacent position of phosphorus at- major byproduct due to reduction of the aryl chloride (Ta-
om.23 These ligands can coordinate to palladium centers ble 1, entries 10 and 11). In a mixture of acetonitrile and
through both the phosphorous and oxygen atoms.24 The water, the cyanation occurred in a moderate yield of 57%
coordination of the oxygen is considered to be a hemila- (Table 1, entry 12). However, water–dioxane or toluene–
bile interaction, and we hypothesized that this additional water solvent mixtures were not found to be effective (Ta-
hemilabile coordination site could stabilize the active pal- ble 1, entries 13 and 14). An optimal ratio of t-BuOH–
ladium center and inhibit cyanide binding that typically water was found to be 1:1 as increasing or decreasing the
leads to catalyst deactivation. Herein, we report the effi- water amount would decrease the yield (Table 1, entries
ciency of the MOP-type ligands in palladium-catalyzed 16 and 17).
cyanation of ‘inert’ aryl chlorides and aryl mesylate with
potassium ferrocyanide.
Table 1 Optimization of Reaction Conditions for Coupling of p-tert-
Butyl-Chlorobenzene (1a) with Potassium Ferrocyanidea
OMe
t-Bu
t-Bu
PCy2
MeO
PCy2
Pd (2 mol%), L1 (8 mol%)
PCy2
+
K4[Fe(CN)6]⋅3H2O
i-Pr
i-Pr
i-Pr
i-Pr
Me2N
base, solvent, 100 °C, 6 h
Cl
CN
i-Pr
i-Pr
1a
2a
DavePhos
BrettPhos
X-Phos
Entry Pd Source Base
Solvent
Conv. Yield
(%)b (%)b
Me
N
PCy2
Oi-Pr
PCy2
OMe
1
2
Pd(dba)2
Pd(dba)2
Cs2CO3
Cs2CO3
Cs2CO3
t-BuOH
–
–
i-PrO
MeO
t-BuOH–H2O
t-BuOH–H2O
18
25
75
99
trace
17
68
96
Cy2P
3c Pd(OAc)2
4c Pd(OAc)2
5c Pd(OAc)2
CM-Phos
RuPhos
Sphos
Na2CO3 t-BuOH–H2O
Figure 1 Structures of bulky phosphine ligands applied in the palla-
K2CO3
K2CO3
K3PO4
KOAc
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
t-BuOH–H2O
t-BuOH–H2O
t-BuOH–H2O
t-BuOH–H2O
t-BuOH–H2O
n-BuOH–H2O
t-AmOH–H2O
MeCN–H2O
dioxane–H2O
toluene–H2O
H2O
dium-catalyzed cyanation
6
Pd(OAc)2
trace trace
We initially examined the feasibility of using the MOP-
type as the ligands in the palladium-catalyzed cyanation
of aryl chloride with potassium ferrocyanide (Table 1).
The cyanation of electronically neutral p-tert-butyl-chlo-
robenzene did not occur when Pd(dba)2 was used as the
precatalyst with Cs2CO3 and t-BuOH (Table 1, entry 1).
On the basis that K4[Fe(CN)6] was soluble in water and
water can promote the dissociation of cyanide ion, the sol-
vent was replaced by a mixture of t-BuOH and water
(1:1), however, no coupled product was obtained. Gener-
ation of a Pd(0) species in situ by addition of PhB(OH)2 to
Pd(OAc)2 provided the desired 4-(tert-butyl)benzonitrile
in 17% yield (Table 1, entry 3).25 The poor catalyst activ-
ity of Pd(dba)2 is likely due to the binding of the electron-
poor dibenzylideneacetone (dba) to the palladium center.
Replacing the base with other carbonate bases, gratifying-
ly, increased the cyanation yield significantly. When
Na2CO3 was used as the base, the coupling product was
obtained in 68% yield, and the yield increased to 96% in
the case of K2CO3 as the base (Table 1, entry 5). However,
only trace of aryl nitriles was obtained without the addi-
tion of PhB(OH)2 (Table 1, entry 6). K3PO4 and KOAc
were inferior in the reaction and provided lower yields of
the desired product (Table 1, entries 7 and 8). Reducing
the loading of K2CO3 led to a decrease of the coupling
7c Pd(OAc)2
8c Pd(OAc)2
9c,d Pd(OAc)2
10c Pd(OAc)2
11c Pd(OAc)2
12c Pd(OAc)2
13c Pd(OAc)2
14c Pd(OAc)2
15c Pd(OAc)2
16c Pd(OAc)2
17c Pd(OAc)2
23
8
16
trace
83
86
99
76
65
10
10
33
27
34
57
3
trace
11
t-BuOH–H2O (2:1) 97
t-BuOH–H2O (1:2) 94
74
81
a Reaction conditions: aryl chloride (1.0 mmol), K4[Fe(CN)6]·3H2O
(0.5 equiv), precatalyst (2 mol%), L1 (8 mol%), base (1.0 equiv), sol-
vent (4.0 mL).
b Conversion and yields determined by GC analysis of the crude reac-
tion mixture.
c Additive: PhB(OH)2 (5 mol%).
d K2CO3 (0.25 equiv).
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 2938–2942

2940
Y. Tu et al.
LETTER
Next, the structural influence of ligands on the catalytic ordination of the oxygen atom with palladium, which sta-
activity for the cyanation was studied (Scheme 1). Com- bilizes the active palladium center and increases the
pared to X-Phos, the series of 2-dialkyl phosphino-2′- electron density to facilitate the oxidative addition of aryl
alkoxyl-1,1′-binaphthyl ligands all showed higher catalyt- chloride. Di-tert-butyl{2′-isopropoxy-[1,1′-binaphthalen]-
ic activity for the palladium-catalyzed cyanation of elec- 2-yl}phosphane (L1) provided the highest conversions
tronically neutral aryl chlorides. Among this type of and yields (Scheme 1). Decreasing the loading of the li-
ligands, both the substituents on the phosphorus atom and gands led to slightly reduced yields of the desired cyana-
oxygen atom affected the catalytic activity. Overall, in- tion product. Therefore, the optimized conditions of the
creasing the steric profile of the dialkylphosphino group palladium-catalyzed cyanation were identified as:
or alkoxy group led to higher catalyst activity. Ligands L1 Pd(OAc)2 (2 mol%), L1 (8 mol%), PhB(OH)2 (5 mol%),
and L4 with an iso-propoxy group showed high efficiency K3PO4 (1.0 equiv) as the base, and the mixture of t-BuOH
for the cyanation. The conversion of aryl chloride in- and H2O (1:1) as the solvent at 100 °C.
creased to 99%, while the yield of the cyanation increased
Under the optimized reaction conditions, we explored the
further to 96% when the steric hindrance of substituents
reactivity of different kinds of arylation reagents in the
on the phosphorus was increased. Under these reaction
catalytic system (Scheme 2). The comparative rate studies
conditions, DavePhos with a dimethylamino group at the
showed that aryl bromides and aryl chloride have very
adjacent position of phosphorus atom showed higher ac-
high reactivity in the Pd/MOP catalyst system. The con-
tivity than that of XPhos. This can be attributed to the ad-
version of 4-tert-butylbromobenzene (2a) and corre-
ditional coordination of the hemilabile amino group. The
sponding chloride reached 99% and 92% in one hour,
high catalytic activity of the bulky MOP-type ligands may
respectively, while the conversion rate of the phenol de-
be attributed to both the bulky nature of the ligands which
rivatives were slower. This is in accordance to the higher
facilitates the reductive elimination and the hemilabile co-
aryl C–O bond strength relative to aryl C–X bonds.
The results revealed that the oxidative addition of the aryl
C–X or C–O bond may be the rate-determining step
t-Bu
t-Bu
during the catalytic cycle. Gratifyingly, the unactive aryl
mesylate was also transformed into the aryl nitrile in good
yield (83%) after 16 hours, and in the case of the
sulfamates, more inert electrophiles, the catalytic system
also showed some catalytic activity. Aryl imidazolesul-
fonates which show high reactivity in palladium-cata-
lyzed Suzuki–Miyaura couplings reactions,26 showed
moderate selectivity to give the product in 73% yield after
24 hours.
Pd(OAc)2(2 mol%), ligand (8 mol%)
PhB(OH)2 (5 mol%)
+
K4[Fe(CN)6]⋅3H2O
K2CO3, t-BuOH–H2O (1:1)
100 °C, 6 h
Cl
CN
1a
2a
Pt-Bu2
Oi-Pr
Pt-Bu2
Pt-Bu2
The scope of the palladium-catalyzed cyanation of aryl
chlorides was then explored, and the results are given in
OBn
OMe
L1
L2
L3
conv.: 99%; yield: 96% conv.: 66%; yield: 57% conv.: 45%; yield:40%
a conv.: 99%; yield: 93%
b conv.: 99%; yield: 86%
PCy2
i-Pr
PCy2
NMe2
PCy2
OBn
PCy2
i-Pr
Oi-Pr
i-Pr
L4
L5
DavePhose
X-Phos
conv.: 36%
yield: 19%
conv.: 86%
yield: 79%
conv.: 36%
yield: 32%
conv.: 99%
yield: 87%
Scheme 1 The effect of the structure of ligands on the palladium-cat-
alyzed coupling of p-tert-butyl-chlorobenzene (1a) with potassium
ferrocyanide. Reagents and conditions: aryl chloride (1.0 mmol),
K4[Fe(CN)6]·3H2O (0.5 equiv), Pd(OAc)2 (2 mol%), ligand (8
mol%), PhB(OH)2 (5 mol%), K2CO3 (1.0 equiv), t-BuOH–H2O (1:1)
(4.0 mL). Conversion and yields determined by GC analysis of
a
the crude reaction mixture. Pd(OAc)2 (2 mol%), L1 (6 mol%).
b Pd(OAc)2 (2 mol%), L1 (4 mol%).
Scheme 2 The reactivity of different kinds of arylation reagents in
the Pd/MOP catalytic system
Synlett 2014, 25, 2938–2942
© Georg Thieme Verlag Stuttgart · New York

LETTER
Cyanation of Unactivated Aryl Chlorides
2941
Scheme 3.27 The unactivated aryl chlorides were coupled the bath temperature was increased to 120 °C, electroni-
to give the aromatic nitriles in good yields. The products cally neutral and electron-rich aryl mesylates were cya-
of benzonitrile (2b) and 2-naphthonitrile (2c) were isolat- nated in good yield (Scheme 4).27 The coupling reaction
ed in 78% and 83% yields, respectively. Furthermore, the of p-methoxyphenyl (5d) or p-(N-Boc)aminophenylmeth-
electron-rich aryl chlorides were also reacted in high anesulfonate (5f) afforded the products in 85% and 78%,
yields. 3,4-(Methylenedioxy)chlorobenzene (1e) and respectively. Similar to the reactivity of aryl chlorides, an
N-Boc-4-chloroaniline (1f) afforded the coupling prod- increase in the steric hindrance of the substrate led to a de-
ucts in 81% and 80%, respectively. Even the unprotected crease in yield. The cyanation of o-methylphenylmeth-
4-chloroaniline (1g) was cyanated to obtain the desired anesulfonate (5h) generated the product in 53% yield.
aryl nitriles in 50% yield, and no amination product, dia- However, the cyanation of the electron-deficient aryl
rylamine, was detected. Reaction of o-tolylchloride (1h) mesylates 5l–n occurred in moderate yield, and was ac-
provided a diminished yield of 53% which is likely due to companied by the formation of the corresponding phenols
the steric hindrance near the C–Cl bond. Moreover, the due to competitive hydrolysis.
catalytic system tolerated heterocyclic substrates. Reac-
tion of 3-chloropyridine (1j) proceeded smoothly to give
Pd(OAc)2 (2 mol%), L1 (8 mol%)
CN
OMs
PhB(OH)2 (5 mol%)
the desired product in 76% yield. The cyanation of 6-chlo-
roquinoline (1k) was also successful and afforded the
product in 62% yield.
+
K4[Fe(CN)6]⋅3H2O
K2CO3, t-BuOH–H2O (1:1)
R
R
120 °C, 24 h
5
2
Pd(OAc)2 (2 mol%), L1 (8 mol%)
CN
Cl
CN
PhB(OH)2 (5 mol%)
CN
CN
CN
+
K4[Fe(CN)6]⋅3H2O
K2CO3, t-BuOH–H2O (1:1)
R
R
100 °C, 6 h
MeO
2
1
2a, 80%
2b, 78%
CN
2c, 83%
2d, 85%
CN
CN
CN
CN
CN
O
CN
MeO
O
CN
2a, 83%
2b, 78%
CN
2c, 83%
2d, 81%
OMe
NHBoc
CN
2e, 71%a
2f, 78%
2h, 53%
2i, 85%
O
O
CN
O
O
CN
CN
Ph
NH2
NHBoc
MeOOC
2l, 58%
NC
NC
2g, 50%
2h, 53%
2e, 81%
NC
2f, 80%
2m, 45%
2n, 43%
CN
N
OMe
Scheme 4 Palladium-catalyzed cyanation of aryl mesylates. Re-
agents and conditions: aryl chloride (1.0 mmol), K4[Fe(CN)6]·3H2O
(0.5 equiv), Pd(OAc)2 (2 mol%), L1 (8 mol%), PhB(OH)2 (5 mol%),
K3PO4 (1.0 equiv), t-BuOH–H2O (1:1, 4.0 mL), 120 °C, isolated
yield. a Pd(OAc) (3 mol%) was used.
N
NC
2j, 71%a
2k, 62%a
2i, 38%
Scheme 3 Palladium-catalyzed the cyanation of aryl chlorides. Re-
agents and conditions: aryl chloride (1.0 mmol), K4[Fe(CN)6]·3H2O
(0.5 equiv), Pd(OAc)2 (2 mol%), L1 (8 mol%), PhB(OH)2 (5 mol%),
K3PO4 (1.0 equiv), t-BuOH–H2O (1:1, 4.0 mL), 100 °C, isolated
yield. a Pd(OAc) (4 mol%) was used.
In summary, the hemilabile and bulky MOP-type ligands
were found to be efficient ligands for the palladium-cata-
lyzed cyanation of unactivated aryl chlorides and aryl
mesylates with the nontoxic K4[Fe(CN)6]·3H2O as the cy-
anide source. The steric profile and the presence of a he-
milabile coordination contribute to the effectiveness of
these ligands. 2-Di-tert-butylphosphino-2′-isopropoxy-
1,1′-binaphthyl (L1) exhibited the highest efficiency for
the cyanation of both aryl chlorides and aryl mesylates.
The mild catalytic system tolerated electron-donating
substituents, such as free amino groups and heterocyclic
groups. In the catalytic system, the reactivity of different
arylated reagents was also obtained: ArBr > ArCl >>
ArOMs > ArOSO2Im > ArOSO2NMe2.
Phenol derivatives are complementary to aryl halides as
electrophiles in the transition-metal-catalyzed coupling
reactions, because phenols are easily available, and the
oxygen atom on the aromatic ring can be used to introduce
additional substituents via a number of pathways which
would enrich the coupling substrates.20 Among the phenol
derivatives, aryl mesylates are cheaper and more stable
than aryl triflates, so they are highly favorable as the sub-
strates of cyanation. Encouraged by the reactivity of dif-
ferent electrophiles (Scheme 2), the cyanation of aryl
mesylates was explored with our catalytic system. When
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 2938–2942

2942
Y. Tu et al.
LETTER
52, 10035. (d) Wang, B.; Zhao, R.; Chen, B.-C.;
Acknowledgment
Balasubramanian, B. ARKIVOC 2010, (vi), 47. (e) Yeung, P.
Y.; So, C. M.; Lau, C. P.; Kwong, F. Y. Org. Lett. 2011, 13,
648.
We thank the National Natural Science Foundation of China for
financial support.
(15) Zhang, J.; Chen, X.; Hu, T.; Zhang, Y.; Xu, K.; Yu, Y.;
Huang, J. Catal. Lett. 2010, 139, 56.
Supporting Information for this article is available online
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(27) General Procedures for the Cyanation of Aryl Chlorides
and Mesylates
An oven-dried Schlenk tube was evacuated and backfilled
with nitrogen. The Schlenk tube was charged with Pd(OAc)2
(4.5 mg, 0.02 mmol ), L1 (36.5 mg, 0.08 mmol), PhB(OH)2
(6.1 mg, 0.05 mmol), and t-BuOH (2 mL), and the mixture
was stirred for half hour at 50 °C. After cooling to r.t., aryl
chloride or mesylates (1.00 mmol), K4[Fe(CN)6]·3H2O
(211.2 mg, 0.50 mmol), K2CO3 (138.2 mg, 1.00 mmol), and
H2O (2 mL) were added. The septum was replaced with an
inside reflux condenser, and then the Schlenk tube was
placed in an oil bath preheated to 100 °C (120 °C for aryl
mesylates) with stirring for 6 h (24 h for aryl mesylates).
Then the reaction mixture was allowed to cool to r.t.,
extracted with CH2Cl2, and concentrated under reduced
pressure. The crude material was purified by column
chromatography on silica gel.
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Wan, Y. Eur. J. Org. Chem. 2008, 3524. (b) Grossman, O.;
Gelman, D. Org. Lett. 2006, 8, 1189.
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A.; Gotta, M.; Beller, M. Adv. Synth. Catal. 2009, 351, 643.
(b) Schareina, T.; Zapf, A.; Maegerlein, W.; Mueller, N.;
Beller, M. Tetrahedron Lett. 2007, 48, 1087. (c) Senecal, T.
D.; Shu, W.; Buchwald, S. L. Angew. Chem. Int. Ed. 2013,
4-(tert-Butyl)benzonitrile (2a)
The crude material was purified by column chromatography
on silica gel (eluting with PE–EtOAc, 20:1) to give the
compound as yellow oil (127.4 mg, 80%). 1H NMR (400
MHz, CDCl3): δ = 7.61–7.57 (m, 2 H), 7.50–7.46 (m, 2 H),
1.33 (s, 9 H). 13C NMR (100 MHz, CDCl3): δ = 156.4, 131.7,
126.0, 118.9, 109.1, 35.1, 30.8.
Synlett 2014, 25, 2938–2942
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