Electrosynthesis of substituted benzolactones by a nickel-catalyzed allyl transfer reaction from allyl benzoates

Article abstract of DOI:10.1055/s-2001-14600

Allyl-substituted benzolactones were obtained in good yields from allyl o-carbonyl benzoates in a Ni-catalyzed electrochemical reaction involving allyl cleavage, intramolecular allyl transfer to the carbonyl group and lactone formation, in a one-step procedure under mild conditions.

Full text of DOI:10.1055/s-2001-14600

806
LETTER
Electrosynthesis of Substituted Benzolactones by a Nickel-Catalyzed Allyl
Transfer Reaction from Allyl Benzoates
Delphine Franco, Elisabet Duñach*
Laboratoire Arômes, Synthèses et Interactions, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 2, France
Fax +33 (4) 92076151; E-mail: dunach@unice.fr
Received 14 February 2001
Table Ni-Catalyzed Electrosynthesis of Substituted Benzolactones 2
Abstract: Allyl-substituted benzolactones were obtained in good
yields from allyl o-carbonyl benzoates in a Ni-catalyzed electro-
chemical reaction involving allyl cleavage, intramolecular allyl
transfer to the carbonyl group and lactone formation, in a one-step
procedure under mild conditions.
Key words: electrosynthesis, nickel catalysis, allyl cleavage, in-
tramolecular allyl transfer, allyl benzoate, benzolactone
Novel methodologies for the preparation of differently
substituted and functionalized lactones are of general in-
terest in organic synthesis. In particular, benzolactones
(coumarins, isocoumarins) present a wide range of appli-
cations in the field of food, perfume and pharmaceutical
chemistry.1,2
We describe here a novel, nickel-catalyzed one-step pro-
cedure for the preparation of benzolactones of type 2,
from easily available carbonyl containing allyl esters of
type 1 (Scheme 1 and Table).
Scheme 1 R = H, CH3, Cl, Ph
The reaction was run under electrochemical conditions in
a single-compartment cell, fitted with a zinc anode and a
stainless steel cathode. The catalyst, the cationic Ni(II)
-
complex [Ni(bipy)3]2+(BF4)2 4 (bipy = 2,2’-bipyridine) is
a stable and easily available compound3 and it was used in
10 mol% with respect to the organic substrate.
For general electrolysis conditions : see reference 4.
aDiastereomeric mixture : (80:20). bWork-up : addition of p-toluene
Electrolyses were run in DMF at room temperature and
sulfonic acid (2.5 eq), stirred for 2 h before extraction as in ref 4.
under constant intensity. The solutions containing 1a-1e
(10-1 M) and 4 were electrolyzed for 2.1-2.8 F/mol with
complete substrate conversion, and afforded directly lac-
tones 2a-2e in good yields (Table). The main by-products
of a Mg instead of a Zn anode led to 2a in poor yields
were the corresponding carboxylic derivatives 3, issued
(20%). Cinnamyl ester 1b afforded the branched allyl lac-
from the deallylation of the ester group without further al-
tone 2b in 60% yield, as a mixture of 2 diastereomers in a
lyl transfer to the carbonyl moiety.
80:20 ratio. Electrolysis of 1b afforded a second carbonyl
In the case of allyl ester 1a, lactone 2a was obtained very
selectively, in 95% yield with only 3% of deallylated
compound 3. The same electrolysis of 1a in the presence
allylation compound 5b in 15% yield. Compound 5b is is-
sued from the dehydration of the alcoholate intermediate
leading to 2b (Scheme 2). Methallyl derivative 1c gave
Synlett 2001, No. 6, 806–808 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Electrosynthesis of Substituted Benzolactones
807
the expected lactone 2c in 71% yield. The chloroallyl In this electrosynthesis, the active catalytic species are
group of 1d could not be transferred efficiently and 72% Ni(0) complexes; however, the electrochemical method
of 2-formylbenzoic acid was formed. Lactone 2d was ob- enables the use of Ni(II) as the catalysts precursor with an
tained in 18% yield. Electrolysis of ester 1e allowed either efficient catalyst regeneration in the presence of zinc ions.
the synthesis of the functionalized hydroxyacid 5e in 53% Three successive tandem reactions involving substrate 1
yield or that of the ether-lactone 2e in 85%. Whereas 5e take place: a first allyl ester cleavage reaction, a second
was obtained after standard acidic work-up (see general step of intramolecular allyl transfer to the carbonyl group,
electrolysis conditions), lactone 2e was obtained after ad- and a third step of lactone formation. The overall process
dition of para-toluenesulfonic acid to the crude electroly- occurs in good yields and takes place under mild (room
sis solution and stirring for 2 hours. Although the temperature) catalytic conditions.
analogous carboxylic acids bearing the homoallyl alcohol
group could not be isolated from compounds 1a to 1d, iso-
lation of 5e indicates that the homoallyl alcoholate 7
This novel tandem transformation which associates elec-
trosynthesis with the use of organometallic catalysis wid-
ens the field of electrochemistry as a selective and mild
(Scheme 2) is a reasonnable intermediate in the prepara-
methodology in organic synthesis.
tion of lactones 2.
The active catalytic species were electrogenerated Ni(0)
References and Notes
complexes, able to insert into the C-O bond of the allyl es-
(1) Barry R. D. Chem. Rev. 1964, 64, 229.
ters. The proposed reaction sequence is shown in Scheme
2. In the anodic process, Zn2+ ions are formed from the ox-
idation of the zinc rod anode. The cathodic reaction in-
volves the generation of Ni(0) from Ni(II) complexes. In
the case of complex 4, the Ni(II)/Ni(0) reduction occurs at
-1.2 V vs SCE7 and the reduction of the anodically gener-
ated Zn2+ species in DMF occurs at -1.4 V vs SCE. In the
presence of electrogenerated Ni(bipy)2, the allyl ester can
be cleaved to form a -allyl Ni(II) carboxylate, 6. This re-
action step presents some similarities with that occurring
in the Pd(0)-catalyzed reductive deprotection of allyl es-
ters and carbonates.8,9 The Ni-catalyzed reductive cleav-
age of allyl10,11 or propargyl12 esters or ethers to the
corresponding carboxylic acids or alcohols (phenols) has
also been reported, both in chemical (stoichiometric)13,14
and electrochemical (catalytic)10-12,15 reactions.
(2) Hill R. A.; Krebs H. C.; Verpoorte R.; Wijnsma R. Progress
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(4) General electrolysis conditions: Electroreductions were
carried out with 1 mmol of allyl ester 15 in freshly distilled
DMF (20 mL) at constant current intensity of 60 mA (apparent
current density of 0.3 A·dm-2) using tetrabutylammonium
tetrafluoroborate as supporting electrolyte (10-2 M). The
catalyst 4 was introduced in a 10% molar ratio with respect to
the substrate. The electrolyses were performed with Zn/
stainless steel couple of electrodes at 20 °C and were followed
by GC, up to the total consumption of the starting material.
The solution was hydrolysed with HCl 0.1 M, up to pH 1-2
and extracted with diethyl ether. The organic layers, dried
over MgSO4, were filtered off and evaporated. The products
were analysed by NMR, mass spectroscopy and GC and
compared to authentic samples. Products 2 have also been
prepared by alternative pathways.6 For example,
spectroscopic data for 2a: 1H NMR (200 MHz, CDCl3)
= 7.90 (d, J = 1.7 Hz, 1H), 7.72-7.40 (m, 3H), 5.90-5.66
(dddd, J = 17.2, 10.3, 7.3, 7.2 Hz, 1H), 5.55 (dd, J = 6.6, 6.4
Hz, 1H), 5.28 (dd, J = 17.2, 1.5, 1H), 5.18 (dd, J = 10.3, 1.5
Hz, 1H), 2.88-2.56 (m, 2H). 13C NMR (200 MHz, CDCl3)
= 173.0, 133.8, 131.2, 130.4, 129.1, 125.7, 123.3, 121.9,
119.6, 80.2, 38.6. MS: m/z 174(M+), 133(100%), 105, 91, 77,
51, 39. Spectroscopic data for 2c: 1H NMR (200 MHz,
CDCl3) = 7.98-7.85 (d, J = 7.5 Hz, 1H), 7.72-7.64 (t, J = 7.5
Hz, 1H), 7.60-7.45 (m, 2H), 5.66 (dd, J = 6.6, 6.6 Hz, 1H),
5.00 (s, 1H), 4.90 (s, 1H), 2.75-2.55 (dd, J = 6.6, 2.5 Hz, 1H),
2.65-2.50 (dd, J = 6.6, 2.5 Hz, 1H), 1.90 (s, 3H). 13C NMR
(200 MHz, CDCl3) = 190.0, 150.0, 134.0, 129.3, 126.1,
125.9, 122.3, 117.2, 114.9, 79.7, 43.1, 23.1. MS : m/z
188(M+), 133(100%), 105, 77, 63, 51.
The -allyl Ni intermediates, 6, in the proximity of the
carbonyl group, undergo an intramolecular allyl transfer
reaction (allyl trapping) to form the corresponding ho-
moallylic alcoholate-carboxylate intermediate, 7. Allyl
transfer reactions from allyl ethers to carbonyl com-
pounds have been recently reported in Ni-catalyzed reac-
tions.16
In the presence of Zn2+ ions, a transmetallation reaction
should enable the regeneration of the Ni(II) species, al-
lowing the catalytic reaction and the intramolecular cy-
clization into the final lactone, 2.
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(10) Olivero S.; Duñach E. J. Chem. Soc., Chem. Commun. 1995,
2497.
Scheme 2
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808
D. Franco, E. Duñach
LETTER
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Article Identifier:
1437-2096,E;2001,0,06,0806,0808,ftx,en;G03401ST.pdf
Synlett 2001, No. 6, 806–808 ISSN 0936-5214 © Thieme Stuttgart · New York