Stereoselective preparation of Z-trisubstituted alkylidene cyclic carbonates via palladium-catalyzed carbon - carbon bond formation

Article abstract of DOI:10.1016/S0040-4039(00)00889-3

Alkylidene cyclic carbonates bearing a Z-trisubstituted exocyclic double bond have been stereoselectively prepared in good yields for the first time from methylene cyclic carbonates, resistant to the classical Heck reaction, on treatment with aryl iodides, CF₃CO₂Ag and palladium catalyst, under conditions which tolerate (sp²)C-Br bonds. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00889-3

Tetrahedron Letters 41 (2000) 5527±5531
Stereoselective preparation of Z-trisubstituted alkylidene
cyclic carbonates via palladium-catalyzed carbon� carbon
bond formation
Patrick Toullec, Aranzazu Carbayo Martin, Monica Gio-Batta, Christian Bruneau*
and Pierre H. Dixneuf*
Institut de Chimie de Rennes, CNRS-Universite de Rennes, Organom e talliques et Catalyse, Campus de Beaulieu,
3
5042 Rennes, France
Received 25 March 2000; accepted 1 June 2000
Abstract
Alkylidene cyclic carbonates bearing a Z-trisubstituted exocyclic double bond have been stereoselectively
prepared in good yields for the ®rst time from methylene cyclic carbonates, resistant to the classical Heck
reaction, on treatment with aryl iodides, CF CO Ag and palladium catalyst, under conditions which tolerate
3
2
2
(
sp )C� Br bonds. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: palladium catalysis; cyclic carbonates; silver tri¯uoroacetate.
1
Alkylidene cyclic carbonates are useful intermediates for the easy access to oxazolidinones and
2
b-oxopropyl carbonates and carbamates, and furanone derivatives. Their activation by palladium
or platinum catalysts via oxa-p-allylmetal reactive intermediates makes possible the preparation
of a variety of heterocycles upon reaction with aromatic aldehydes, isonitriles and hetero-
3
4
5
cumulenes, as well as ketonic cyclopropanes on reaction with norbornene. More importantly,
enantioselective hydrogenation of a-methylene cyclic carbonates in the presence of ruthenium
catalysts has led to the obtention of optically active bicyclic carbonates and 1,2-diols with very
6
high enantioselectivities. This observation motivated the search for alkylidene cyclic carbonates
as new intermediates towards optically active compounds.
The most straightforward syntheses of a-methylene cyclic carbonates are based on the con-
densation of propargylic alcohols with carbon dioxide in the presence of ruthenium, cobalt,
7
8
9
10
11
copper, phosphine, or strong base catalysts. The only two ecient methods to produce tri-
substituted alkylidene cyclic carbonates are the palladium-catalyzed coupling of propargylic
*
Corresponding author. Fax: 33 (0)2 99 28 69 39; e-mail: christian.bruneau@univ-rennes1.fr
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00889-3

5
528
alcohol, CO and vinyl or aryl halide,12 and rearrangement of allyl propargyl carbonates.13 Both
methods stereoselectively a€ord the E-trisubstituted exocyclic double bond.
2
Whereas the classical Heck type reaction applied to methylene cyclic carbonates 1±2 does not
allow the access to alkylidene carbonates, we now report the ®rst stereoselective synthesis of
Z-trisubstituted alkylidene cyclic carbonates in good yields from 1 or 2, aryl iodides and CF CO Ag
3
2
in the presence of palladium catalyst according to Scheme 1.
Scheme 1.
The treatment of the a-methylene cyclic carbonate 1 with iodobenzene under classical palladium-
catalyzed C� C coupling conditions in the presence of Et N or K CO as a base and even with
3
2
3
14
Bu NBr or AgNO as promoter did not lead to any transformation.
4
3
The key point of the success of the C� C coupling reaction is the presence of a stoichiometric
amount of CF CO Ag. Thus, the treatment of 1.50 g (11.7 mmol) of 4,4-dimethyl-5-methylen-
3
2
1
,3-dioxolan-2-one 110 with 2.45 g (12 mmol) of iodobenzene 3 and 3.24 g (14.6 mmol) of silver
tri¯uoroacetate in the presence of 90 mg (1.2 mmol-10 mol%) of palladium acetate, 312 mg (1.2
ꢀ
mmol, 10 mol%) of triphenylphosphine in 8 ml of DMF at 100 C for 20 h led to a complete
conversion into 5-benzylidene-4,4-dimethyl-1,3-dioxolan-2-one. The crude mixture contained a
mixture of Z- and E-isomers in the ratio Z/E=95/5, and after workup and chromatography over
silica gel, the pure Z-isomer 4 (RˆPh) was isolated as white crystals in 87% yield (2.10 g). The
presence of CF CO Ag is crucial in this reaction as neither CF CO Na nor CH CO Ag used in
3
2
3
2
3
2
stoichiometric amounts were active to promote the C� C coupling, showing that the presence of
both tri¯uoroacetate and silver was necessary. The stereochemistry of the exocyclic double bond
1
was attributed due to the ole®nic proton shift in H NMR by comparison with the literature data
1
2b
(
E-isomer: ꢀ =6.40 ppm ; Z-isomer: ꢀ =5.48 ppm).
H H
Under the experimental conditions reported in Scheme 1, the coupling of a variety of aryl
iodides with carbonates 1 and 2 was performed (Table 1). The coupling reaction appeared to be
quite general as phenyl and 1-naphthyl Z-substituted cyclic carbonates were isolated in good
yields from carbonates 1 and 2 (Table 1, entries 1, 2, 6, 7). The reaction tolerated the presence of
functionalities and 4-hydroxybenzylidene substituted carbonates 7, 11 were prepared in satisfactory
yields from the corresponding iodides (entries 4, 8). However, no reaction took place from 1-amino-
4
-iodobenzene and 4-nitroiodobenzene. This coupling reaction was speci®c of aryl iodides
because aryl bromides, which are usually suitable substrates for the Heck coupling, were
unreactive. This observation made possible the preparation, from the corresponding bromoiodo-
benzene, of 2- and 4-bromobenzylidene cyclic carbonates 8 and 12 (entries 5, 9), which are inter-
esting substrates for further classical Heck reactions involving the activation of C� Br bond. The
role of silver tri¯uoroacetate might be in the capture of iodide from the oxidative addition product
15
L Pd(Ar)(I), allowing the coordination of non-reactive CˆC bonds.
n

5
529
Table 1
Coupling of aryl iodides with carbonates 1 and 2a
The new Z,Z-biscarbonates 13 and 14 were also prepared from 1,4-diodobenzene and carbonates
and 2 in 45 and 84% isolated yield, respectively.
1

5
530
The reactivity of this new family of Z-alkylidene cyclic carbonates showed their straight-
forward transformation into acyclic ketocarbonates and hydroxyoxazolidinones upon reaction
with a nucleophile in the presence of a base (Scheme 2). Thus, the treatment of 4 with ethanol
used as a solvent in the presence of Et N at room temperature gave 1,1-dimethyl-3-phenyl-2-
3
oxopropyl ethyl carbonate 15 in 87% isolated yield. Similarly, the obtention of the hydroxy-
oxazolidinone 16 resulted from the quantitative conversion of carbonate 4 (72% isolated yield)
upon treatment with ammonia in diethyl ether at room temperature.
Scheme 2.
The crucial action of silver tri¯uoroacetate in the palladium-catalyzed carbon� carbon bond
formation from the exocyclic double bond of methylene cyclic carbonates has thus removed the
inhibition to perform the Heck type reaction. It provides an easy access not only to substituted
alkylidene cyclic carbonates but also to high Z-stereoselectivity. Biscarbonates such as 13 and 14
are potential precursors of polymers containing b-ketocarbonate units in the main chain and
6
according to the observed highly enantioselective catalytic hydrogenation of 1, these compounds
4±14 should open the route to a variety of optically active useful derivatives.
Acknowledgements
The authors thank the European Socrates±Erasmus program for support to M.G.B. from the
University of Edinburgh (UK), and the University of Burgos for a grant to A.C.M.
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