Palladium catalyzed domino C-H activation strategy: An access to 9- fluorenones

Article abstract of DOI:10.1016/j.tet.2018.11.055

Palladium catalyzed domino C-H functionalization reaction of arylaldehyde with dihaloarene has been developed to access 9-flourenone molecules. Bidentate ligand assisted strategy, single step reaction, high yield and excellent functional group tolerance make this method concise and effective for the synthesis of 9-flourenone. In addition, proposed method has been successfully employed to synthesise Tilorone in gram scale.

Full text of DOI:10.1016/j.tet.2018.11.055

Accepted Manuscript
Palladium catalyzed domino C-H activation strategy: An access to 9- fluorenones
Anuj Patel, Mahommadumar Shaikh, Kishor Chikhalia
PII:
S0040-4020(18)31424-8
https://doi.org/10.1016/j.tet.2018.11.055
TET 29966
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 6 September 2018
Revised Date: 22 November 2018
Accepted Date: 24 November 2018
Please cite this article as: Patel A, Shaikh M, Chikhalia K, Palladium catalyzed domino C-H activation
strategy: An access to 9- fluorenones, Tetrahedron (2018), doi: https://doi.org/10.1016/j.tet.2018.11.055.
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ACCEPTED MANUSCRIPT
Graphical abstract:

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Palladium Catalyzed Domino C-H activation strategy: An access to 9-
Fluorenones
Anuj Patela, Mahommadumar Shaikha, Kishor Chikhaliab*
a Department of chemistry, Gujarat University, Ahmedabad, Gujarat, India 38009
b Department of chemistry, Veer Narmad South Gujarat University, Surat, Gujarat, India 395007
Chikhalia_kh@yahoo.com
Abstract:
Palladium catalyzed domino C-H functionalization reaction of arylaldehyde with dihaloarene has been
developed to access 9-flourenone molecules. Bidentate ligand assisted strategy, single step reaction, high
yield and excellent functional group tolerance make this method concise and effective for the synthesis of
9-flourenone. In addition, proposed method has been successfully employed to synthesise Tilorone in
gram scale.
Key words: Domino C-H activation, 9-Fluorenone, Palladium catalyst, Tilorone synthesis.
Introduction:
Flourenone is an important class of organic compound found as a core structure in various
bioactive, optical and electronic material (Scheme-1)1–5. The most applicable synthetic routes of
flourenone mainly include Friedel-Craft acylation6–9, oxidation of flourenols or flourenenes10–12, Pschorr
cyclization13, remote metalation14–17, intramolecular dehydro Diels-Alder cyclisation18–20. However, these
synthetic protocols have some limitations such as prefunctionalization, unavailable starting material and
multistep synthetic route.
Scheme 1: Natural products, pharmaceutical drugs and photoelectric materials containing the fluorenone
framework.
Denglibsin
PF
Tilorone
(Natural product)
(Electron-injection material)
(Interferon inducer)
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PFO – F (1%)
HIQF1
Sensitized dye for solar cell
Electrophorescent material
In recent scenario, transition metal catalyzed C-H bond functionalization has emerged as promising tool
to construct C-C bond leading to the development of some synthetic strategy to construct flourenone
framework. For instance, TM-catalyzed intramolecular cyclization of 2-arylbenzonitrile, o-halobiaryl and
carbon monoxide(CO), 2-arylbenzaldehyde, biaryl-2-carboxylic acid and different diaryl ketone
derivatives are effective methodologies to access flourenone (Scheme 2A)21–26. Furthermore, the directing
group assisted arylation of aryl aldoxime ether, benzamide, aryl nitrile and benzylamine with iodo arene,
simple arene and aryl boronic acid followed by cyclization have also been well studied for the synthesis
of flourenone (Scheme 2B)7,24,27,28. Nevertheless, the probability of employing ample, versatile, weakly
coordinating arylaldehyde along with cheap dihaloarene as substrates to synthesise flourenone remains
unprecedented till date. On the other hand sequential insertion of Palladium into formyl C-H bond
followed by insertion into dihaloarene can access flourenone in single step. Moreover, the proposed
transformation was found more sensitive toward ligand and proceed smoothly with bidentate ligand.
Notably, bidentate ligands were frequently reported to act as a directing group for rapid progress of C-H
activation reaction29–33
.
Scheme 2: Approaches towards the synthesis of Flourenone
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The C-H functionalization, bidentate ligands are widely investigated skeletons mainly for two purposes
(i) is to bring catalyst closer to the reaction site and (ii) to act as directing group (Figure 1). Considering
the prior role of bidentate ligand, domino coupling of dihaloarene, effectiveness of palladium to active
formyl C-H bond and our continuous efforts in C-H functionalization reactions34–37 we herein report the
promising way to synthesise flourenone molecules.
Figure 1: C-H functionalization through (i) Bidentate ligand working as directing group (ii) Bidentate
moiety working as ligand only
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Previous work: Directing group assisted
C-H functionalization
This work: Bidentate ligand assisted
C-H functionalization
Result and discussion:
Initially, we have examined several bidentate ligands for the reaction of benzaldehyde 1a and 2-
bromoiodobenzene 2a in the presence of Pd(OAc)2 and KHCO3 under N2 gas in tAmylOH (Table 1). The
reaction was found to proceed with picolinamide ligand L1, L3 and L4 in 50%, 10% and 60% yield
respectively to afford desired 9-flourenone (3a). It was observed that the position of pyridyl groups seems
crucial for the difference in the reactivity of L1 and L3. The yield of 3a was increased to 80% in case of
quinolinylamide ligand L2 and decreased to 15% and 20% for picolinate ligand L5 and amide ligand L6
respectively. Pyrrolidinylamide ligand L7 was found most fruitful for the proposed reaction to afford
desired 9-flourenone (3a) in 85% yield. In addition, other ligands like BINAP, bipyridine, PPh3 and
pyridine were also found ineffective and gave around 10% yield for this reaction.
Table 1: Investigation of ligand
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After ligand, we have examined different Pd catalyst, base as well as solvent for further
optimization. Screening study mentioned in Table-2 reveals that the proposed transformation could not
proceed through Pd(0) catalyst such as Pd(PPh3)4 and Pd2(dba)3 (entry no. 1-4). However, Pd(II) catalysts
were found to purvey good to excellent yield among them Pd(OAc)2 was most effective (entry no. 5-7). In
addition to this role of base was found to be important for desired transformation in remarkable yield. It
was noted that Potassium bicarbonate (KHCO3) was found versatile among other tested bases including
NaHCO3, Cs2CO3, Na2CO3, K2CO3 and KOAc (entry no. 7-12). Moreover, tertiary amylalcohol solvent
system could transform the reaction effectively (entry no. 15). Other polar solvents were not effective to
give remarkable yield (entry no. 13, 14) whereas nonpolar solvent decreases the yield notably (entry no.
16-18). In addition, proposed transformation appeared with trace yield when performed under O2 gas
(entry no. 19). In summary combination of Pd(OAc)2 catalyst, KHCO3 base in tAmOH solvent was
chosen as optimized catalytic system.
Table 2: Optimization of reaction conditiona
Yield
Entry
Catalyst
Base
Solvent
(%)b
1
Pd(PPh3)4
NaHCO3
DMF
Trace
2
Pd(PPh3)4
KHCO3
DMF
Trace
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3
4
Pd2(dba)3
KHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
Na2CO3
K2CO3
t-AmylOH
Trace
Trace
42
Pd2(dba)3
PdCl2
DMF
DMF
5
6
Pd(dppf)Cl2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
DMF
37
7
DMF
65
8
DMF
20
9
DMF
28
10
11
12
13
14
15
16
17
18
c19
KOAc
DMF
15
NaOAc
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
DMF
18
DMF
70
iPrOH
DMSO
tAmylOH
Toluene
Dioxane
DCE
70
55
85
47
50
45
tAmylOH
Trace
aReaction condition: benzaldehyde (1.0 mmol), 2-bromoiodobenzene (2.0 equiv), Pd catalyst (10.0
mol%), L7(15.0 mol%) and base (5.0 equiv) in solvent (5.0 ml) at 120°C for 24 h under N2 atmosphere.
bIsolated yield. CUnder O2 gas.
To investigate the scope of this domino functionalization, we studied various aryl/heteroaryl
aldehyde with different dihaloarene precursors under optimized reaction condition. During the study of
substrate scope, it was observed that o-bromoiodobenzene (2a) was found more fruitful than any other
dihaloarene with a remarkable yield up to 85% (entry no. 1-5). After dihaloarene, we shifted our attention
toward arylaldehyde. Desired 9-flourenone derivatives obtained in good to excellent quantity with both
electron donating and withdrawing group. Phenol, nitro, ether, halogen were found tolerant toward
optimized condition and the corresponding 9-flourenone were obtained. Yield of the product increases
when aryl aldehyde substituted with electron donating group and decreased with electron withdrawing
group. In addition, sterically hindered arylaldehyde as well as heteroaryl aldehydes also found tolerant
toward optimized condition and appeared with remarkable yield.
Table 3: Substrate scopea
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Entry
1
2
3
Yieldb
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O
OHC
9
80%
2a
O
O
1c
3h
O
CHO
11
82%
2a
1e
3c
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aReaction condition: arylaldehyde (1.0 mmol), dihaloarene (2.0 equiv), Pd(OAc)2 (10.0 mol%), L7 (15.0
mol%) and KHCO3 (5.0 equiv) in tert-amylalcohol (5.0 ml) at 120°C for 24 h under N2 atmosphere.
bIsolated yield.
To demonstrate the synthetic utility of present protocol, we tried to scale up the synthesis of
Tilorone on gram scale through the proposed strategy. Tilorone is well-known orally active, broad-
spectrum antiviral drug4 which is generally synthesised in multistep synthetic strategies38. We could able
to synthesised Tilorone through an intermediate 2,7-dihydroxy-fluoren-9-one (3x) which was synthesized
by reaction of 3-hydroxybenzaldehyde with 4-bromo-3-iodo phenol under optimized conditions in 74%
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yield. Synthesised 3u on treatment with 2-chloro-N,N-diethylethylamine hydrochloride in presence of
KOH in toluene/H2O at refluxing temperature for 8.0 hours offered Tilorone in 85% yield.
Scheme 3: Two step synthesis of Tilorone
A plausible mechanism for the synthesis of fluorenone through described strategy is outlined in
Figure 2. We proposed that the reaction proceeds with the transient generation of palladium-ligand
complex (A) from Pd(OAc)2 and ligand (L7), which on reaction with KHCO3 to give intermediate (B)
with removal of KOAc, H2O and CO2. After generation of intermediate (B) C-H palladation occurs on
aryl aldehyde (1) to form organo-palladium complex (C) and liberate KHCO3. Oxidative addition of
intermediate (C) with dihaloarene (2a) to generate Pd(IV) complex (D) which undergoes reductive
elimination to generate the arylated compound (2A) and complex (E). Palladium-ligand complex (A)
regenerates through complex (E) on reaction with liberated AcOH and removal of HI followed by
oxidative addition of 2A to give intermediate (G). Finally reductive elimination gives the desire product
with elimination of HCl molecule and regenerate the palladium-ligand complex (A).
Figure 2: Plausible mechanism
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Conclusion:
In conclusion, we successfully synthesized flourenone molecules through ligand directed Pd(II)
catalyzed domino coupling reaction of arylaldehyde with dihaloarene under C-H activation route. The
12

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desired fluorenones were synthesized in good to excellent yield with remarkable functional group
tolerance. Synthesis of an antiviral agent Tilorone has been successfully carried out in gram scale through
intermediate 3u which was synthesised by present method and hence we hope that the proposed work will
become a useful tool for the synthesis of other potent molecules. We further hope that present work will
provide some important insights into C-H functionalization and become an advantageous methodology
for the synthesis of cyclic ketone with excellent functional group tolerance.
Acknowledgement:
Authors are thankful to NFDD centre Rajkot, IIT Ropar and Oxygen healthcare Pvt.ltd for the spectral
analysis. Authors express their gratitude to department of Chemistry, Gujarat University to provide
necessary lab facilities.
Experimental Section:
General information
Palladium catalysts, ligands and dihaloarenes were purchased from sigma-aldrich and directly
used without further purification. All reactions were monitored by thin-layer chromatography using
Merck 60 F254 precoated silica gel plates (0.25 mm thickness). Column chromatography was performed
1
13
using 60-120 and 200-400 mess silica from Spectrochem PVT.LTD. Mumbai, India. H and C NMR
spectra were recorded on a Bruker AM400 (400 MHz for 1H NMR, 100 MHz for 13C NMR) instrument.
NMR spectra were processed with the MestReNova program. Data for 1H NMR are reported as chemical
shift (δ ppm), multiplicity (s=singlet, d=doublet, t=triplet, q=quartet, m=multiplet), coupling constant
(Hz), integration, and assignment. Data for 13C NMR are reported as chemical shift. HRMS spectra were
recorded on XEVO G2-XS QTOF spectrophotometer. All liquid reactants and solvents were distilled
before use.
General procedure for the synthesis of 9-fluorenone:
A mixture of arylaldehyde (0.1 g, 1 mmol), dihaloarene (0.565 g, 2 mmol), Pd(OAc)2 (0.022 g,
10.0 mol%), N-phenylpicolinamide (L7, 0.019 g, 15.0 mol%) and potassium hydrogen carbonate (0.5 g, 5
mmol) in tert-amyl alcohol (5.0 ml) was taken in 100.0 ml round bottom flask under N2 atmosphere and
stirred for 120°C for 24 h. Progress of the reaction was monitored continuously by TLC with ethyl acetate
: hexane (2:3) eluent system. After completion of reaction, crude was poured into crushed ice and then
filter the reaction mixture. Filtrate then extracted with ethyl acetate (3times). Organic layer was separated,
dried (over anhydrous Na2SO4) and evaporated under reduced pressure and purified by column
chromatography to obtain desired product.
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Characterization data of compounds 3a, 3b, 3c, 3d, 3e, 3f, 3g, 3h, 3i, 3j, 3k, 3l, 3v and 3x were found
exactly similar as reported in the literature (References of above compound are mentioned in
Supplementary data).
3-Hydroxy-1-methoxy-6-methyl-9H-fluoren-9-one (3m)
Yellow Powder: 0.284 g, 80 % isolated yield; 1H NMR (400 MHz, CDCl3): δ 9.45 (s, 1H), 7.64(d, J= 7.3
MHz, 1H), 7.57(s,1H), 7.24(d, J= 7.5 MHz, 1H), 6.91(s,1H), 6.39(s,1H), 3.81(s,3H), 2.35(s,3H); 13C
NMR (100 MHz, CDCl3): δ 197.15, 165.31, 163.56, 147.30, 146.22, 145.91, 134.40, 129.49, 125.84,
123.88, 114.48, 103.26, 99.26, 50.79, 21.21; FT-IR (KBr, cm-1) 3080, 2950, 1752, 1220, 1050. Anal.
Calcd. For C15H12O3: C: 83.10; H: 3.92. Found: C: 83.11; H: 3.91. mp 135-137 °C. HRMS-ESI (m/z)
calcd for C15H12O3 [M + H]+ 241.0864, found 241.0862.
3-Hydroxy-1,6-dimethyl-9H-fluoren-9-one (3n)
1
Yellow Powder: 0.282 g, 83 % isolated yield; H NMR (400 MHz, CDCl3): δ 9.45 (s, 1H), 7.63-7.57(m,
2H), 7.24(d, J= 7.1 MHz, 1H), 7.10(s,1H), 6.69(s,1H), 2.53(s,3H), 2.35(s,3H); 13C NMR (100 MHz,
CDCl3): δ 200.02, 162.26, 147.30, 149.91, 144.29, 142.19, 134.40, 129.49, 125.84, 123.88, 122.72,
117.01, 108.02, 21.21, 20.73; FT-IR (KBr, cm-1) 3090, 3020, 2970, 2950, 1750, 1215. Anal. Calcd. For
C15H12O2: C: 80.34; H: 5.39. Found: C: 80.37; H: 5.38. mp 123-124 °C. HRMS-ESI (m/z) calcd for
C15H12O2 [M + H]+ 225.0915, found 225.0917.
3-Methoxy-1,5-dimethyl-9H-fluoren-9-one (3o)
1
Light yellow Powder: 0.284 g, 80 % isolated yield; H NMR (400 MHz, CDCl3): δ 7.57(s, 1H), 7.36-
7.34(m, 2H), 7.20(s, 1H), 6.75(s, 1H), 3.81(s, 3H), 2.54(s, 3H), 2.47(s, 3H); 13C NMR (100 MHz,
CDCl3): δ 200.64, 164.44, 147.85, 140.82, 139.91, 136.81, 135.66, 130.71, 128.49, 124.02, 123.03,
116.00, 104.46, 56.04, 20.73, 20.58; FT-IR (KBr, cm-1) 3020, 2960, 2870, 1740, 1030. Anal. Calcd. For
C16H14O2: C: 80.65; H: 5.92. Found: C: 80.63; H: 5.91. mp 127-130 °C. HRMS-ESI (m/z) calcd for
C16H14O2 [M + H]+ 239.1072, found 239.1071.
1-Chloro-6-fluoro-3-methoxy-9H-fluoren-9-one (3p)
Pale yellow Powder: 0.302 g, 80 % isolated yield; mp 183-187 0C; 1H NMR (400 MHz, CDCl3): δ 7.68(s,
13
1H), 7.46(s, 1H), 7.24(s, 1H), 7.13(d, J= 7.5 MHz, 1H), 6.95(s, 1H), 3.81(s, 3H); C NMR (100 MHz,
CDCl3): δ 193.28, 169.68 (d, JCF=152.3 Hz), 164.32, 148.34, 143.26, 137.77, 130.90, 128.49, 128.45,
114.74, 114.65, 109.78, 104.70, 56.04; FT-IR (KBr, cm-1) 3070, 1745, 1230, 1025, 820. Anal. Calcd. For
C14H8ClFO2: C: 64.02; H: 3.07. Found: C: 64.03; H: 3.06. HRMS-ESI (m/z) calcd for C14H8ClFO2 [M +
H]+ 263.0275, found 263.0272.
1-Ethoxy-6-methyl-9H-fluoren-9-one (3q)
0
Light brown Powder: 0.263 g, 74 % isolated yield; mp 97-99 C; 1H NMR (400 MHz, CDCl3): δ 7.63(s,
1H), 7.57(m, 2H), 7.41(d, J= 6.9 MHz, 1H), 7.24(d, J= 7.4 MHz, 1H), 7.13(d, J= 7.3 MHz, 1H), 4.06 (q, J
= 7.4 MHZ , 2H), 2.35(s, 3H), 1.39(t, J= 7.0 MHz, 3H); 13C NMR (100 MHz, CDCl3): δ 197.25, 160.05,
147.20, 146.36, 145.23, 135.27, 134.83, 129.11, 125.86, 123.54, 122.21, 115.15, 114.14, 64.32, 21.21,
13.83; FT-IR (KBr, cm-1) 3040, 2950, 2860, 1750, 1040. Anal. Calcd. For C16H14O2: C: 80.65; H: 5.92.
Found: C: 80.64; H: 5.91. HRMS-ESI (m/z) calcd for C16H14O2 [M + H]+ 239.1072, found 239.1074.
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1-Ethyl-6-hydroxy-9H-fluoren-9-one (3r)
0
1
Yellow Powder: 0.238 g, 70 % isolated yield; mp 116-117 C; H NMR (400 MHz, CDCl3): δ 9.45 (s,
1H), 7.54-7.50(m, 3H), 7.28(d, J= 7.3 MHz, 1H), 7.19(s, 1H), 6.88(d, J= 7.0 MHz, 1H), 2.93(q, J= 7.5,
13
2H), 1.31(t, J= 7.1 MHz, 3H); C NMR (100 MHz, CDCl3): δ 200.57, 163.43, 145.88, 145.55, 143.61,
133.07, 131.11, 129.33, 127.83, 127.49, 118.25, 115.87, 109.00, 26.80, 13.43; FT-IR (KBr, cm-1) 3050,
2970, 2850, 1745, 1200. Anal. Calcd. For C15H12O2: C: 80.34; H: 5.39. Found: C: 80.36; H: 5.38. HRMS-
ESI (m/z) calcd for C15H12O2 [M + H]+ 225.0915, found 225.0917.
3-Benzyloxy-1-chloro-6-fluoro-9H-fluoren-9-one (3s)
Orange Powder: 0.323 g, 77 % isolated yield; mp 191-194 0C; 1H NMR (400 MHz, CDCl3): δ 7.80(d, J=
7.6 MHz, 1H), 7.74(d, J= 7.2 MHz, 1H), 7.53(s, 1H), 7.42(s, 1H), 7.27-7.12(m, 7H), 5.20(s, 2H); 13C
NMR (100 MHz, CDCl3): δ196.21, 168.41(d, JCF=155.1 Hz), 164.61, 147.33, 145.14, 137.09, 131.50,
130.47, 128.32, 128.27, 128.17, 128.16, 126.75, 115.05, 114.84, 109.67, 107.76, 70.70; FT-IR (KBr, cm-
1) 3070, 2930, 1740, 1340, 1150. Anal. Calcd. For C20H13FO2: C: 78.94; H: 4.31. Found: C: 78.93; H:
4.32. HRMS-ESI (m/z) calcd for C20H13FO2 [M + H]+ 305.0978, found 305.0980.
11H-indeno[1,2-c]quinolin-11-one (3t)
Brown Powder: 0.316 g, 72 % isolated yield; 1H NMR (400 MHz, CDCl3): δ 9.05 (s, 1H), 8.90(d, J= 8.4
13
MHz, 1H), 8.15(d, J= 7.9 MHz, 1H), 7.84(d, J= 6.2 MHz, 1H), 7.77-7.53(m, 5H); C NMR (100 MHz,
CDCl3): δ 196.67, 150.57, 143.06, 142.28, 140.53, 139.97, 134.49, 133.47, 131.70, 129.80, 128.06,
126.75, 125.52, 124.85, 124.03, 122.21; FT-IR (KBr, cm-1) 3080, 2895, 1740, 1590, 1520, 1470, 1390.
Anal. Calcd. For C16H9NO: C: 83.10; H: 3.92; N: 6.06. Found: C: 83.11; H: 3.91; N: 6.05. mp 164-166
°C. HRMS-ESI (m/z) calcd for C16H9NO [M + H]+ 232.0762, found 232.0761.
Indeno[1,2-b]indol-10(9bH)-one (3u)
Light orange Powder: 0.333 g, 78 % isolated yield; 1H NMR (400 MHz, CDCl3): δ 7.94(s, 1H), 7.90(d, J=
7.4 MHz, 1H), 7.75-7.46(m, 4H), 7.22-7.16(m, 3H); 13C NMR (100 MHz, CDCl3): δ 202.12, 153.68,
137.14, 136.01, 135.21, 132.88, 127.04, 126.97, 125.28, 125.04, 124.73, 122.76, 119.69; FT-IR (KBr,
cm-1) 3390, 3020, 2950, 1755, 1150. Anal. Calcd. For C15H9NO: C: 82.18; H: 4.14; N: 6.39. Found: C:
82.16; H: 4.15; N: 6.40. mp 146-147 °C. HRMS-ESI (m/z) calcd for H9NO [M + H]+ 220.0762, found
15
220.0765.
10-Hydroxy-11H-benzo[b]fluoren-11-one (3w)
1
Yellow Powder: .340 g, 75 % isolated yield; H NMR (400 MHz, CDCl3): δ 8.11-8.08(m, 2H), 7.85-
7.71(m, 3H), 7.50-7.46(m, 4H), 2.08(s, 1H); 13C NMR (100 MHz, CDCl3): δ 198.46, 159.59, 144.16,
142.29, 135.81, 134.25, 134.14, 129.16, 128.55, 127.96, 127.13, 125.46, 125.42, 124.84, 121.68, 113.80,
109.70; FT-IR (KBr, cm-1) 3090, 3040, 2950, 1745, 1220. Anal. Calcd. For C17H10O2: C: 82.91; H: 4.09.
Found: C: 82.90; H: 4.08. mp 19-121 °C. HRMS-ESI (m/z) calcd for C17H10O2 [M + H]+ 247.0759, found
247.0761.
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