Cyclocarbonylative Sonogashira Reactions of 1-Ethynylbenzyl Alcohols: Synthesis of 1-Carbonylmethylene-1,3-Dihydroisobenzofurans

Article abstract of DOI:10.1002/ejoc.201500539

In this work, we present a carbonylative Sonogashira reaction of o-ethynyl benzyl alcohols and aryl iodides, followed by a cyclization process to selectively give carbonylmethylene isobenzofurans in high yields and in an atom-economic fashion. The reactio

Full text of DOI:10.1002/ejoc.201500539

FULL PAPER
DOI: 10.1002/ejoc.201500539
Cyclocarbonylative Sonogashira Reactions of 1-Ethynylbenzyl Alcohols:
Synthesis of 1-Carbonylmethylene-1,3-Dihydroisobenzofurans
Laura Antonella Aronica,*[a,b] Luca Giannotti, Giulia Tuci, and Francesco Zinna
[c]
[d]
[a]
Keywords: Synthetic methods / Homogeneous catalysis / Cross-coupling / Carbonylation / Cyclisation / Oxygen
heterocycles
In this work, we present a carbonylative Sonogashira reac-
tion of o-ethynyl benzyl alcohols and aryl iodides, followed
by a cyclization process to selectively give carbonylmethyl-
ene isobenzofurans in high yields and in an atom-economic
fashion. The reaction can be carried out in the absence of
using aryl iodides bearing both electron-withdrawing and
electron-donating groups. Of the two possible stereoisomeric
products, Z-isobenzofuran derivatives were obtained as
major products. But, when the reaction was extended to a
secondary alcohol, an interesting switch in stereoselectivity
was observed.
2 3 2
CuI, with a small amount of PdCl (PPh ) (0.2–0.5 mol-%),
Introduction
Several methods for the preparation of 3-alkylidene-1,3-
dihydroisobenzofurans are described in the literature. They
are generally based on the cyclization of functionalized sub-
The 1,3-dihydroisobenzofuran (phthalan) nucleus (Fig-
ure 1) is found in a vast number of natural and synthetic
[11]
strates such as alkynyloxiranes,
o-alkynylbenzalde-
[1]
compounds that show antimycotic, antibacterial, antioxi-
[12]
hydes, and o-alkynylbenzyl alcohols, which are often pre-
pared by Sonogashira coupling of iodobenzyl alcohol with
terminal acetylenes. The intramolecular cyclization of alk-
ynyl alcohols can be promoted by a base (NaH, KOH,
or tBuOK ) or mediated by lanthanide,
[
2]
[3]
[4]
dant, antihistamine, and antitumor activities. More-
over, some synthetic analogues, such as 3-alkylidene-1,3-di-
hydroisobenzofurans, have recently been tested as free-radi-
[13]
[14]
[
5]
cal scavengers. They have also been evaluated as antide-
[15]
[16]
[17]
copper,
or
[
6]
pressants, and were shown to be more active than citalop-
[18]
palladium
catalysts. In this field, Gabriele and cowork-
ers have developed an interesting approach based on the
[
7]
ram, the reference drug commonly used to treat de-
pression. Alkylidenephthalans are also endowed with a rich
reactivity that makes them versatile building blocks for the
[19]
use of a PdI /KI system. Using a palladium-catalysed oxi-
2
[20]
dative carbonylation reaction, they obtained alkoxycarb-
[8]
synthesis of functionalized phenanthrofurans, isoquinol-
onylmethylene phthalans in good yields, together with small
amounts of the corresponding benzopyran compounds
[
9]
[10]
inones, and spiroketals.
(Scheme 1).
Figure 1. 1,3-Dihydroisobenzofuran structure.
Scheme 1. Synthesis of alkoxycarbonylmethylene-1,3-dihydroiso-
benzofurans by oxidative carbonylation of alkynyl alcohols.
[a] Dipartimento di Chimica e Chimica Industriale, University of
Pisa,
Via G. Moruzzi 13, 56124 Pisa, Italy
E-mail: laura.antonella.aronica@unipi.it
http://www.dcci.unipi
Intrigued by this data, and prompted by our recent re-
[
b] CIRCC, Interuniversity Consortium Chemical Reactivity and
Catalysis,
[21]
sults
on the cyclocarbonylative Sonogashira reaction of
2
-(2-ethynylphenyl)ethanol, which generated 2-alkylidene-
Via Celso Ulpiani 27, 70126 Bari, Italy
c] Dipartimento di Biotecnologie, Chimica e Farmacia,
Via Aldo Moro 2, 53100 Siena, Italy
d] Istituto di Chimica dei Composti Organometallici
ICCOM-CNR,
[
[
substituted isochromans in high yields and with high stereo-
selectivities (Scheme 2, n = 1), we decided to investigate the
application of this protocol to ethynylbenzyl alcohol
Scheme 2, n = 0) to find out whether the synthesis of
carbonylmethylene-1,3-dihydroisobenzofurans could be
achieved.
Via Madonna del Piano 10, 50019 Sesto Fiorentino,
FI, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500539.
(
4944
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 4944–4949

Cyclocarbonylative Sonogashira Reactions
The two stereoisomers could easily be separated and iso-
lated in pure form by column chromatography. Unexpec-
tedly the relative amount of the two isomers changed after
separation: Z/E crude: 63:37; pure: 68:25. This suggests that
an isomerization of E-3a into Z-3a took place under the
purification conditions.
The same trend was observed when the reaction was ex-
tended to several iodoarenes with electron-donating and
electron-withdrawing substituents in the ortho and para po-
sitions (Scheme 4). As shown in Table 1, in almost all cases
(
entries 1–7) the reactions yielded the dihydroisobenzofur-
Scheme 2. Cyclocarbonylative Sonogashira reaction of ethynyl
alcohols.
ans (i.e., 3) quantitatively. The E derivative was always the
minor product, and the relative amount of the Z compound
generally increased after purification. To evaluate whether
an interaction of the crude product with the stationary
phase (SiO ) or with the eluent (CHCl ) could be the reason
Results and Discussion
2
3
We began our investigation using equimolar amounts of for the change in the ratio between the two isomers, 100 mg
2-ethynylphenyl)methanol (1a), a commercially available of pure E-3f was treated with SiO in CDCl . After 24 h
(
2
3
reactant, and iodobenzene (2a) as the coupling partner, both steroisomers were present in reaction mixture, and a
using the experimental conditions already optimized for the Z/E ratio of 92:8 was observed; this ratio remained constant
formation of isochromans.[ Thus, the reactions were car- even after five more days (Scheme 5).
21]
ried out under CO pressure (2.0 MPa) in Et N as both the
3
solvent and the base, at 100 °C and using PdCl (PPh )
2
3 2
(0.2 mol-%) as the catalyst. After 24 h, we observed quanti-
tative consumption of the starting materials, and the forma-
tion of five-membered dihydroisobenzofuran 3a as the sole
product (Scheme 3).
Scheme 4. Cyclocarbonylative Sonogashira reactions between (2-
ethynylphenyl)methanol and aryl iodides.
Table 1. Cyclocarbonylative reactions of 2-(2-ethynylphenyl)eth-
anol and aryl iodides.
Scheme 3. Preliminary cyclocarbonylative Sonogashira reaction be-
tween (2-ethynylphenyl)methanol and iodobenzene.
Entry[a]
2
Ar
3
Z [%][b]
E [%][b]
Even though Baldwin’s rules[22] allow the formation of
both 5-exo-dig and 6-endo-dig derivatives, no traces of the
possible benzopyran derivative were detected (Scheme 3).
Both stereoisomers (i.e., Z and E) of phthalan 3a were
present in the crude product, but they could easily be distin-
1
2
3
4
5
6
7
2a
2b
2c
2d
2e
2f
2g
2h
2i
Ph
1-naphthyl
4-MeOC
2-MeOC
4-MeC
2-MeC
4-Cl
3a
3b
3c
3d
3e
3f
3g
3h
3i
63 (68)
86 (87)
75 (83)
81 (81)
59 (60)
71 (72)
61 (51)
83 (69)
50 (33)
37 (25)
14 (10)
25 (14)
19 (14)
41 (33)
29 (27)
39 (11)
–
6
H
H
4
4
6
6
H
H
4
6
4
1
guished by H NMR spectroscopic analysis. Indeed, ac-
cording to the literature,[
12a]
8
9
[c]
d]
6
2-NCC H
4-NCC
proton H of the E isomer is
4
4
a
[
6
H
25 (13)
found at an unusually high chemical shift (δ = 9.43 ppm),
due to its interaction with the carbonyl deshielding cone, [a] The reactions were carried out with 1a (2 mmol), 2 (2 mmol),
PdCl (PPh (0.004 mmol; 0.2 mol-%), in Et N (5 mL) at 100 °C
2
3
)
2
3
whereas proton H appears at a higher field (δ = 7.75 ppm)
b
for 24 h under CO (2.0 MPa). Conversions (100% in all cases) were
evaluated by GC and H NMR spectroscopic analysis. [b] Selectivi-
ties were evaluated by H NMR spectroscopy; isolated yields are
(Figure 2).
1
1
reported in parentheses (the products were obtained chemically
pure after column chromatography). [c] The reaction also yielded
1
7% of 2-{3-[2-(hydroxymethyl)phenyl]propioloyl}benzonitrile 4.
[d] The reaction was carried out in a mixture of Et N (5 mL) and
3
toluene (4 mL) in order to dissolve 2; 4-{3-[2-(hydroxymethyl)-
phenyl]propioloyl}benzonitrile 5 (25%) was also formed.
This means that Z-3f is more stable than the E isomer
[
23]
by about 1.3 kcal/mol, and this value is comparable with
Figure 2. E/Z stereoisomers of 2-[isobenzofuran-1(3H)-ylidene]-1-
phenylethanone.
that reported for similar acylylidene isobenzofurans.[
12]
Eur. J. Org. Chem. 2015, 4944–4949
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4945

L. A. Aronica, L. Giannotti, G. Tuci, F. Zinna
FULL PAPER
a palladium hydride species is obtained. This species then
undergoes a syn hydropalladation step with the triple bond
(
(
3
Scheme 7, III).
Scheme 7, IV) regenerates Pd , and gives isobenzofuran E-
, which equilibrates with the corresponding Z-3 isomer
Scheme 7, V).
A subsequent reductive elimination
0
(
Scheme 5. Interconversion of the two stereoisomers.
Probably, the presence of traces of acid during the purifica-
tion step could cause the interconversion to take place, as
[
12a]
already pointed out by Herndon and coworkers
Scheme 6).
(
Scheme 7. Proposed mechanism.
The formation of the two E and Z stereoisomers is ap-
parently in contrast with the data obtained in our previous
Scheme 6. Mechanism of the stereoisomerization of E-3a into Z- work on the synthesis of isobenzopyrans through the Sono-
3
a.
gashira cyclocarbonylation reaction of 2-(2-ethynylphenyl)-
[
21]
ethanol.
Indeed, in that case, not even traces of the E
When the cyclocarbonylative Sonogashira reaction was
species were detected. In the light of these new results, our
previous data could be explained with a trans hydropallad-
ation step to the triple bond of the alkynyl ketone interme-
diate or, more probably, by a very fast conversion of the E
isomer into the Z isomer as soon as it is formed in the
reaction medium.
carried out using benzonitriles 2h and 2i (Table 1, entries 8
and 9), significant amounts of 2-{3-[2-(hydroxymethyl)-
phenyl]propioloyl} benzonitrile (4; 17%) and 4-{3-[2-
(hydroxymethyl)phenyl]propioloyl} benzonitrile (5; 25%)
were generated (Figure 3).
The proposed mechanism is also consistent with the pres-
ence of the uncyclized products that were obtained when
iodobenzonitriles were used as the coupling partners
(Table 1, entries 8 and 9). In these cases, the presence of a
strongly electron-withdrawing group could reduce the elec-
tron density on the triple bond of the Sonogashira coupling
product, thus slowing the hydropalladation step.
Figure 3. Carbonylative Sonogashira byproducts.
To rationalize all these outcomes, we propose the hypo-
The presence of palladium–H species in the catalytic
thetical mechanism shown in Scheme 7. It involves an initial cycle could be the reason for the formation of (Z)-1-(4-
carbonylative Sonogashira coupling between the iodo deriv- aminophenyl)-2-[isobenzofuran-1(3H)-ylidene]ethanone (6)
0
ative and the alkynyl moiety (Scheme 7, I). Then a Pd in- in the reaction between 1-iodo-4-nitrobenzene (2j) and 1a
sertion into the O–H bond takes place (Scheme 7, II), and (Scheme 8). Indeed, aminobenzofuran 6 could reasonably
Scheme 8. Cyclocarbonylative Sonogashira reactions between (2-ethynylphenyl)methanol and 1-iodo-4-nitrobenzene.
4946
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Eur. J. Org. Chem. 2015, 4944–4949

Cyclocarbonylative Sonogashira Reactions
be derived from the reduction of the NO moiety into an Table 2. Cyclocarbonylative reactions of 1-(2-ethynylphenyl)-2,2-di-
2
methylpropan-1-ol and aryl iodides.
NH group, as already observed in our previous work on
2
the synthesis of isochromans.[
21]
Entry[a]
Cat. [mol-%] Conv.[b] [%] 11 Z [%][b] E [%][b]
2
Ar
Finally, the cyclocarbonylative Sonogashira reaction was
extended to a sterically hindered secondary alcohol. We
chose tert-butyl alcohol (1b) as a representative example.
Since it is not commercially available, 1b was synthesized
1
2
3
2a Ph
2a Ph
2d 2-MeOC H
6 4
0.2
0.5
0.5
28
100
100
11a 26
11a 22 (15) 78 (57)
11b 44 (32) 56 (35)
74
[a] The reactions were carried out with 1a (2 mmol), 2 (2 mmol), in
from aldehyde 7 according to the procedure (not optimized) Et N (5 mL) at 100 °C for 24 h, under CO (2.0 MPa). [b] Conver-
3
shown in Scheme 9.[
14]
1
sions were evaluated through H NMR spectroscopic analysis; iso-
lated yields are reported in parentheses.
2-Bromobenzaldehyde was initially
coupled with trimethylsilylacetylene to give product 9
87%). tBuLi was then added to 2-(trimethylsilylethynyl)-
(
benzaldehyde, generating 2,2-dimethyl-1-{2-[(trimethylsil-
yl)ethynyl]phenyl}propan-1-ol (10; 28%). Finally, the SiMe3
group was easily removed by treatment of 10 with an excess in CDCl , and the content of the mixture was monitored,
When a sample of pure E-11b was allowed to equilibrate
3
of tetrabutylammonium fluoride (TBAF; 77%).
after 24 h a Z/E ratio of 91:9 was determined; this ratio
remained constant after several days (Scheme 11), clearly
confirming that the Z isomer is the thermodynamically
more stable isomer.
Scheme 11. Stereoisomerization of E-11b and Z-11c.
Scheme 9. Preparation of 1-(2-ethynylphenyl)-2,2-dimethylpropan-
1
-ol.
The difference in the stereoisomeric ratio observed at the
end of the reaction (Table 2, entry 3, 44:56) compared to
the ratio detected after the stereoisomerization test
A preliminary study on the carbocyclization reactions
Scheme 10, Table 2) again showed a quantitative conver-
sion of the reagents, but a slightly higher catalyst loading
(
(Scheme 11, 91:1) could be due to the experimental condi-
tions used in the two cases. Indeed, while the interconver-
sion of the pure E isomer into the E/Z mixture was carried
out under acidic conditions, the cyclocarbonylation reac-
(
(
0.5 mol-%) was necessary to obtain the desired products
Table 2, entries 1 vs. 2). To our surprise, we observed that,
at least in the few cases examined, when hindered alcohol
b was used with iodoarenes 2a and 2d, the stereoselectivity
of the process was switched in favour of the E isomer
Table 2, entries 2 and 3).
tions were carried out in Et N. As shown in Scheme 12, in
3
1
the second case, the isomerization requires the removal of
a proton from the substituted carbon atom by triethyl-
amine. In the case of alcohol 1b, the very hindered
tert-butyl moiety would interfere with the approach of the
base, slowing down the overall isomerization process
(
(Scheme 12).
Finally, two reactions were carried out using benzoyl
chloride instead of iodobenzene under the usual experimen-
tal conditions [i.e., 1a (2 mmol), PhCOCl (2 mmol), in Et N
3
(
5 mL) at 100 °C for 4–24 h, under N ] in order to verify
2
Scheme 10. Cyclocarbonylative Sonogashira reactions of 1-(2-eth-
ynylphenyl)-2,2-dimethylpropan-1-ol.
whether the cyclization could be carried out without carbon
monoxide. In fact, Sonogashira reactions of acyl chlorides
are well known in the literature.
[
25]
Unfortunately, already
These results could tentatively be ascribed to a reduced after 4 h, almost a complete consumption of the alcohol
1
overall rate of the catalytic cycle due to the higher steric was observed in the H NMR spectrum of the crude prod-
hindrance of substrate 1b relative to 1a; thus a lower con- uct, together with the formation of polymeric material.
version of isomer E into Z (Scheme 7, step V) resulted. In- These results can probably be ascribed to the interaction
deed, a greater amount of the catalyst had to be used in between the OH moiety of 1a and the benzoyl chloride,
order to obtain complete conversion of the reactants which can generate ester derivatives that can undergo poly-
(Table 2, entries 1 and 2).
condensation processes.
Eur. J. Org. Chem. 2015, 4944–4949
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4947

L. A. Aronica, L. Giannotti, G. Tuci, F. Zinna
FULL PAPER
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Z-3f]
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.
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