General Method for the Synthesis of Substituted Cyclopentenones via α-Borylzirconacyclopentene Intermediates

Full text of DOI:10.1080/00304948.2019.1677998

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
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General Method for the Synthesis of Substituted
Cyclopentenones via α-Borylzirconacyclopentene
Intermediates
Ghassan Albarghouti & Saleh Rayyan
To cite this article: Ghassan Albarghouti & Saleh Rayyan (2020): General Method for the
Synthesis of Substituted Cyclopentenones via α-Borylzirconacyclopentene Intermediates, Organic
Preparations and Procedures International, DOI: 10.1080/00304948.2019.1677998
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ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
https://doi.org/10.1080/00304948.2019.1677998
EXPERIMENTAL PAPER
General Method for the Synthesis of Substituted
Cyclopentenones via a-Borylzirconacyclopentene
Intermediates
Ghassan Albarghouti and Saleh Rayyan
Department of Chemistry, Faculty of Science, Birzeit University, Ramallah-West Bank, Palestine
ARTICLE HISTORY Received 23 September 2018; Accepted 9 August 2019
The substituted cyclopentenone unit is a common scaffold in numerous natural prod-
ucts and pharmaceuticals.1 As well, the cyclopentene moiety is also an important func-
tional group in the prostaglandins (PGs).2 For example, prostaglandin analogs (PGAs)
have long been known to be involved in the control of renal function, hormone regula-
tion, and vaso- and broncho-dilatation.3 Moreover, these scaffolds are useful as agricul-
tural chemicals, perfumes and as general intermediates in organic synthesis.4 The
a,b-unsaturated carbonyl structure is crucial in such varied biological processes as the
inhibition of tumors, viruses and inflammation.5,6
Several methodologies for the synthesis of substituted cyclopentenones have thus
been reported;7,8 these have included the Nazarov cyclization,9 the Pauson-Khand reac-
tion10 and the use of metal-carbonyl complexes.11,12 However, given their significance,
there is still considerable room for improvement in the preparation of these com-
pounds. As background, we noted that using the reagent Cp2ZrCl2/2EtMgBr together
with 1-alkynylboronates13 leads to the regioselective formation of zirconacyclopentene
boronate intermediates of Type 3 (Scheme 1).14,15
Syntheses of cyclopentenone boronates are yet to be reported, but these structures
would be highly valued intermediates in organic synthesis and could be useful partners
in such cross-coupling reactions as the Suzuki- Miyaura coupling.16–20
Herein we report for the first time the synthesis of cyclopentenone boronic acids
under mild conditions using a simple work-up. The present method gives excellent
results and offers a direct method for the synthesis of functionalized cyclopente-
nones. A number of zirconacyclopentenylboronates were treated with oxalyl chloride
in THF at 0 ꢀC to yield the corresponding cyclopentenone boronic acids (Table 1,
Table 2).
Thus, alkyne boronate 2a prepared by borylation of 1-heptyne (Table 1)13 was chosen
as a model to test our hypothesis. The zirconacyclopentenylboronate Type 3 intermedi-
ate was prepared by reacting two equivalents of the Cp2ZrCl2/2EtMgBr reagent with the
1-alkynylboronate. We found an efficient system for the desired transformation to 4a
could be made from the combination of CuCl catalyst in the presence of N,N’-
CONTACT Ghassan Albarghouti
gdaghra@birzeit.edu
Department of Chemistry, Faculty of Science, Birzeit
University, Birzeit P.O. 14, Ramallah-West Bank, Palestine
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/uopp.
ß 2020 Taylor & Francis Group, LLC

2
G. ALBARGHOUTI AND S. RAYYAN
Scheme 1. Synthesis of zirconacyclopentenylboronates.
Table 1. Conditions for carbonylation of a-zirconacyclopentene boronates.a
Entry
Catalyst (10%)
Additive
% yieldb
1
–
–
0
2
3
4
5
6
7
8
9
CuCl
CuI
CuI
CuCl
CuCl (50 mol%)
CuBr.S(CH3)2
ZnCl2
–
–
15
Traces
DMPU
DMPU
DMPU
DMPU
DMPU
DMPU
DMPU
25
84 (75)c
60d
Traces
0
0
0
CoCl2
CrCl3
10
aAll reactions were performed with 0.2 mmol of the substrate.
bGC yield.
cIsolated yield.
dReduction of ethylated product was obtained as side-product.
Table 2. Access to substituted cyclopentenone boronic acids.a
aIsolated yields, 0.5 mmol scale.
bThe molecular structure of 2-(cyclohexylmethyl)-5-oxocyclopent-1-enylboronic acid was determined by
X-ray.
dimethyl-N,N’-trimethyleneurea (DMPU). In the absence of any catalyst and additive,
no product 4a was detected, and only the reduction of ethylated product was obtained
(Entry 1, Table 1).

ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
3
Figure 1. View of molecular structures of 4a and 4d.
When the a-zirconacyclopentene boronate intermediate was treated with oxalyl
dichloride in the presence of freshly prepared CuCl catalyst it gave 15% of product 4a.
With a catalytic amount of CuI catalyst, the a-borylcyclopentenone product 4a was only
detected in trace amounts. When the a-zirconacyclopentene boronate intermediate was
treated with oxalyl dichloride in the presence of a catalytic amount (10 mol%) of CuI
and DMPU, the yield of a-borylcyclopentenone product 4a was 25% (Entry 4). Fuller
conversion occurred in the presence of 10 mol% CuCl catalyst (Entry 5). Other metal
chloride catalysts did not show any conversion to 4a (Entries 7-10).
Our procedure was applied to several other alkyneboronates of Type 2 (Scheme 1). The
designed precursors were then subjected to the carbonylation conditions to provide the cor-
responding a-borylcyclopentenone products in a regioselective manner and in good yields
(Table 2). Under these conditions, acyclic, cyclic, aromatic and halogenated substituents
were tolerated.
1
All the prepared compounds were characterized by H NMR, 13C NMR, 11B NMR
and HRMS. X-ray crystal structures were recorded using single crystal X-ray analysis
for compounds 4a and 4d (Figure 1). The 11B NMR chemical shift observed in the
region (26.04-29.10 ppm) is expected for vinylboronate compounds.
The mechanism of this reaction is proposed to follow Cu(I)-catalyzed or -mediated
reactions of organozirconium compounds as described by Xi’s group.20,21 The a-boryl-
zirconacyclopentene intermediate undergoes transmetallation with CuCl followed by
coordination of a carbonyl oxygen from oxalyl chloride to form a seven membered
transition state. The cyclopentenone boronate moiety is generated upon intramolecular
nucleophilic attack and elimination of CuCl, CO gas and zirconocene dichloride.
Finally, using aqueous HCl during the workup gives the cyclopentenone product.
As an illustration of the preparation of a substituted cyclopentenone, cyclopentenone
boronic acid 4b was reacted with bromobenzene in the presence of Pd(OAc)2 catalyst,22
affording the corresponding coupling product 5 (Scheme 2) in good yield.
In conclusion, we have developed a method for the preparation of functionalized
cyclopentenone derivatives which can be obtained from readily available starting

4
G. ALBARGHOUTI AND S. RAYYAN
Scheme 2. Coupling of cyclopentenone boronic acid with bromobenzene.
materials. The introduction of the boronic acid functional group at the a-position of an
a,b-unsaturated carbonyl moiety would be very useful for further functionalization of
these scaffolds. We hope that this work will find direct application in the synthesis of
bioactive natural products and their structural analogues.
Experimental section
Proton, carbon and boron NMR spectra were recorded on a Varian Mercury 300MHz
spectrometer in deuterated solvent. Proton chemical shifts are reported in ppm (d) relative
to tetramethylsilane with the solvent resonance employed as the internal standard (CDCl3,
d 7.26 ppm). 13C chemical shifts are reported in ppm from tetramethylsilane with the solv-
ent resonance as the internal standard (CDCl3, d 77.0 ppm). 11B chemical shifts are
reported in ppm. X-ray crystallographic data were recorded using single crystal X-ray ana-
lysis on X-ray diffractometer system (Bruker SMART APEX CCD). X-ray crystal structures
were recorded using single crystal X-ray analysis for compounds 4a and 4d; and the com-
plete data are available from the corresponding author upon request.
Synthesis of boronates
Following a literature procedure,13 2-isopropyloxy-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane was prepared by the borylation reaction of pinacol with triisopropylborate. The
borate ester reacted with 1-alkynyllithium to produce, after treatment with anhydrous
hydrogen chloride, the corresponding 1-alkynylpinacolatoborane. Compounds 2a, 2b,
2f23, 2c24, 2d25 and 2e26 were previously described.
Typical procedure for the synthesis of a-borylcyclopentenone
To zirconocene dichloride (0.75 mmol, 1.5 equiv.) dissolved in 5 mL of dry THF at
ꢁ78 ꢀC was added 0.75 mL of 2 M EtMgCl (1.5 mmol, 3 equiv.) dropwise in a 25 mL
round-bottom flask. After being stirred for 1 h at ꢁ78 ꢀC, (0.5 mmol, 1 equiv.) of the
corresponding alkyne boronate was added. The reaction was gradually warmed to 0 ꢀC,
stirred for 3 h. Then 0.005 g (0.05 mmol, 0.1 equiv.) of CuCl, 0.12 mL (2 mmol, 4 equiv.)
of DMPU and 0.065 mL (0.75 mmol, 1.5 equiv.) oxalyl chloride were added to the reac-
tion mixture and stirred for 1h at 0 ꢀC. The reaction was warmed to room temperature
and allowed to stir for 2h. The reaction mixture was quenched with dilute HCl solution
and extracted with ethyl acetate. The organic layer was dried (Na2SO4), filtered and

ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
5
concentrated in vacuo. The crude product was purified by flash chromatography (SiO2,
30% EtOAc/hexanes).
5-Oxo-2-pentylcyclopent-1-enylboronic acid (4a)
The above procedure was applied by using zirconocene dichloride (0.44 g, 0.75 mmol), 2
M EtMgCl (0.75 mL, 1.5 mmol), 2-(hept-1-ynyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(112 mg, 0.5 mmol), CuCl (5 mg, 0.05 mmol), DMPU (0.12 mL, 2 mmol) and oxalyl
chloride (0.065 mL, 0. 75 mmol) in dry THF. The crude product was purified by flash
chromatography (SiO2, 30% EtOAc/hexanes) to yield 5-oxo-2-pentylcyclopent-1-enyl-
1
boronic acid (82 mg, 75%, white solid). H NMR (300 MHz, CDCl3): d 6.54 (s, 1H), 2.82
(t, 2H), 2.69-264 (m, 2H), 2.49-2.43 (m, 2H), 1.69-1.58 (m, 6H), 0.98 (t, J ¼ 7.4 Hz, 1H).
13C NMR (75 MHz, CDCl3): d 218.32, 197.20, 133.70, 35,55, 35,50,35.40, 34.75,34.70,
34.60, 34.10, 21.43, 15.10. 11B NMR (96.24MHZ): d 26.97. GC-MS (EI, m/z): 197 (Mþ).
HRMS ([M þ Na]þ): Calcd for C10H17BO3Na, m/z 219.1168. Found, m/z 219.1164.
(5-Oxo-2-phenylcyclopent-1-en-1-yl)boronic acid (4b)
The above procedure was applied by using zirconocene dichloride (0.44 g, 0.75 mmol), 2 M
EtMgCl (0.75 mL, 1.5 mmol), 4,4,5,5-tetramethyl-2-(phenylethynyl)-1,3,2-dioxaborolane
(114 mg, 0.5 mmol), CuCl (5 mg, 0.05 mmol), DMPU (0.12 mL, 2 mmol) and oxalyl chlor-
ide (0.065 mL, 0. 75 mmol) in dry THF. The crude product was purified by flash chroma-
tography (SiO2, 30% EtOAc/hexanes) to yield (5-oxo-2-phenylcyclopent-1-en-1-yl)boronic
1
acid (65 mg, 65%, white solid). H NMR (300 MHz, CDCl3): d 7.36-7.20 (m, 5H), 2.98-2.91
(m, 2H), 2.72-2.69 (m, 2H). 13C NMR (75 MHz, CDCl3): d 218.60, 198.20, 138.16, 132,30,
131.36, 129.15, 128.50, 127.72, 35.62, 26.12. 11B NMR (96.24 MHZ): d 27.86. HRMS
([M þ Na]þ): Calcd for C11H11BO3Na, m/z 225.0699. Found, m/z 225.0694.
(5-Oxo-2-propylcyclopent-1-en-1-yl)boronic acid (4c)
The above procedure was applied by using zirconocene dichloride (0.44 g, 0.75 mmol), 2 M
EtMgCl (0.75 mL, 1.5 mmol), 4,4,5,5-tetramethyl-2-(pent-1-yn-1-yl)-1,3,2-dioxaborolane
(97 mg, 0.5 mmol), CuCl (5 mg, 0.05 mmol), DMPU (0.12 mL, 2 mmol) and oxalyl chloride
(0.065 mL, 0. 75 mmol) in dry THF. The crude product was purified by flash chromatog-
raphy (SiO2, 30% EtOAc/hexanes) to yield (5-oxo-2-propylcyclopent-1-en-1-yl)boronic acid
1
(60 mg, 72%, white solid). H NMR (300 MHz, CDCl3): d 6.64 (s, 2H), 2.79 (t, J¼ 7.5, 2H),
2.73 – 2.56 (m, 2H), 2.54 – 2.39 (m, 2H), 1.68 – 1.60 (m, 2H), 0.99 (t, J¼ 7.4 Hz, 3H). 13C
NMR (75 MHz, CDCl3): d 218.92, 198.83, 133.91, 36.63, 36.49, 33.51, 22.64, 15.10. 11B
NMR (96.24 MHZ): d 26.04. HRMS ([M þ Na]þ): Calcd for C8H13BO3Na, m/z 191.0855.
Found, m/z 191.0853.
(2-(Cyclohexylmethyl)-5-oxocyclopent-1-en-1-yl)boronic acid (4d)
The above procedure was applied by using zirconocene dichloride (0.44g, 0.75mmol), 2
M EtMgCl (0.75mL, 1.5 mmol), 2-(3-cyclohexylprop-1-yn-1-yl)-4,4,5,5-tetramethyl-1,3,2-

6
G. ALBARGHOUTI AND S. RAYYAN
dioxaborolane (124 mg, 0.5 mmol), CuCl (5 mg, 0.05 mmol), DMPU (0.12 mL, 2 mmol)
and oxalyl chloride (0.065 mL, 0. 75mmol) in dry THF. The crude product was purified
by flash chromatography (SiO2, 30% EtOAc/hexanes) to yield (2-(cyclohexylmethyl)-5-
oxocyclopent-1-en-1-yl)boronic acid (91 mg, 82%, white solid). 1H NMR (300MHz,
CDCl3): d 6.36 (s, 2H), 2.77 (d, J ¼ 9, 2H), 2.68 – 2.63 (m, 2H), 2.50 – 2.45 (m, 2H), 1.75
– 58 (m, 6H), 1.32 – 1.12 (m, 4H), 1.12-1.00 (m, 2H). 13C NMR (75MHz, CDCl3): d
218.40, 197.47. 131.87, 41.64, 37.20, 35.34, 33.58, 26.50, 26.30 (3peaks inside CDCl3 peak).
11B NMR (96.24MHZ): d 29.10. HRMS ([M þ Na]þ): Calcd for C12H19BO3Na m/z,
245.1325. Found, m/z 245.1322.
(2-(3-Chloropropyl)-5-oxocyclopent-1-en-1-yl)boronic acid (4e)
The above procedure was applied by using zirconocene dichloride (0.44 g, 0.75 mmol), 2
M EtMgCl (0.75 mL, 1.5 mmol), 2-(5-chloropent-1-yn-1-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (114 mg, 0.5 mmol), CuCl (5 mg, 0.05 mmol), DMPU (0.12 mL, 2 mmol)
and oxalyl chloride (0.065 mL, 0. 75 mmol) in dry THF. The crude product was purified
by flash chromatography (SiO2, 30% EtOAc/hexanes) to yield (2-(3-chloropropyl)-5-
oxocyclopent-1-en-1-yl)boronic acid (62 mg, 62%, white solid). 1H NMR (300 MHz,
CDCl3): d 6.22 (s, 2H), 3.64 (t, J ¼ 7.5, 2H), 2.98 – 2.93 (m, 2H), 2.21 – 2.17 (m, 2H),
2.09 – 2.05 (m, 6H), 1.63 – 1.59 (m, 2H). 13C NMR (75 MHz, CDCl3): d 218.70, 196.87.
132.10, 44.64, 37.80, 32.30, 31.58, 27.60. 11B NMR (96.24 MHZ): d 27.22. HRMS
([M þ Na]þ): Calcd for C8H12BClO3Na, m/z 225.0466. Found, m/z 225.0462.
(2-(4-Methoxyphenyl)-5-oxocyclopent-1-en-1-yl)boronic acid (4f)
The above procedure was applied by using zirconocene dichloride (0.44 g, 0.75 mmol), 2
M EtMgCl (0.75 mL, 1.5 mmol), 2-((4-methoxyphenyl)ethynyl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (130 mg, 0.5 mmol), CuCl (5 mg, 0.05 mmol), DMPU (0.12 mL, 2 mmol)
and oxalyl chloride (0.065 mL, 0. 75 mmol) in dry THF. The crude product was purified
by flash chromatography (SiO2, 30% EtOAc/hexanes) to yield (2-(4-methoxyphenyl)-5-
oxocyclopent-1-en-1-yl)boronic acid (87 mg, 75%, white solid). 1H NMR (300 MHz,
CDCl3): d 7.38 (d, 2H), 6.96 (d, 2H), 2.98-2.91 (m, 2H), 2.72-2.69 (m, 2H). 13C NMR
(75 MHz, CDCl3): d 218.45, 198.30, 158.72, 138.21, 132,43, 131.27, 129.23, 127.81, 55.43,
35.57, 26.47. 11B NMR (96.24 MHZ): d 28.76. HRMS ([M þ Na]þ): Calcd for
C12H13BO4Na, m/z 255.0805. Found, m/z 255.0802.
Coupling reaction; Preparation of 2,3-Diphenylcyclopent-2-en-1-one (5)
Following a literature procedure,22 to a 10 mL flask were added a stir-bar, 314 mg
(2 mmol) of bromobenzene, 444 mg (2.2 mmol) of (5-oxo-2-phenylcyclopent-1-en-1-
yl)boronic acid, 4.5 mg (1 mol %) of Pd(OAc)2, 690 mg (5 mmol) of powdered K2CO3,
and 645 mg (2 mmol) of Bu4NBr. The flask was equipped with a rubber septum and
flushed with nitrogen. Water (2.5 mL) was added with a syringe to the flask, and the
resulting suspension was stirred and degassed to remove oxygen. The mixture was
stirred and heated for 1 h at 70 ꢀC. The reaction mixture was then cooled to room

ORGANIC PREPARATIONS AND PROCEDURES INTERNATIONAL
7
temperature, diluted with water, and extracted with EtOAc. The organic layer was dried
(Na2SO4), filtered and concentrated in vacuo. The crude product was purified by flash
chromatography (SiO2, 3% EtOAc/hexanes) to yield 2,3-diphenylcyclopent-2-en-1-one
1
(337 mg, 72%, yellow oil). H NMR (300 MHz, CDCl3): d 7.38-7.18 (m, 10H), 3.09-3.03
(m, 2H), 2.73-2.68 (m, 2H). 13C NMR (75 MHz, CDCl3): d 207.60, 168.10, 140.10,
135.20, 132.74, 130.10, 129.72, 128.84, 128. 62, 127.84, 127.60, 34.52, 29.6. GC-MS (EI,
m/z): 235 (Mþ). This compound has been described previously, and these spectral data
are in agreement with the published ones.27
Acknowledgment
This research project (grant number 240194) was financially supported by the Scientific Research
Committee, Academic Affairs, Birzeit University.
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