Palladium–catalyzed synthesis of 2,3-dihydro-2-substituted-2-vinyl-1,4-benzodioxins

Various 2,3-dihydro-2-substituted-2-vinyl-1,4-benzodioxins 6 are obtained by alkylation of the methyl carbonate of 2,3-dihydro-1,4-benzodioxin-2-ylideneethanol 5 with various carbon nucleophiles in the presence of a palladium complex. Altough the yields in alkytion products are good in the case of a non–bulky nucleophile, formation of the diene 7 was generally observed when a bulky nucleophile was used

Finally reaction of carbonate 5 with ethyl acetoacetate as the nucleophile gave the alkylated product 6d in 61% yield as a mixture of the two diastereoisomers in a ratio 66:34, together with the diene 7 in 18% yield (Table 1, entry 4).
The formation of compounds 6 and 7 could be explained according to Scheme 2. The first step is the formation of the η 3 -allyl intermediate A by oxidative addition of the palladium complex on compound 5.One possibility is the generation of the nucleophile by abstraction of a hydrogen from Nu-H by CH 3 O -.The attack of the nucleophile on the η 3 -allyl intermediate A occured not at the less hindered termini, but at the more electrophilic termini of this intermediate affording compound 6 bearing a quaternary carbon center.5][26] It is to be noticed that this alkylation reaction is very sensitive to the bulkiness of the nucleophile; the more bulky the nucleophile is (dimethyl methylmalonate, dimethyl acetamidomalonate), the lowest is the chemical yield in the alkylated product.
The formation of the diene 7 could be explained by a β-hydrogen elimination from the intermediate A, leading to compound 7 and the formation of H(CH 3 O)Pd(dppb), affording Pd(dppb) via a reductive elimination of CH 3 OH. 27It seems that there is a competition between these two pathways.

Scheme 2
Mechanism of formation of compounds 6 and 7

Experimental Section
General Procedures.All manipulations involving palladium catalysis were performed in Schlenk tubes under a nitrogen atmosphere.Unless otherwise stated, the materials were commercial samples; propargylic carbonate 2 was prepared as previously described. 23All organic solvents were of analytical quality and used as purchased.Solvents mixtures are defined by volume ratios (v/v).Tetrahydrofuran was distilled from sodium/benzophenone.All 1 H-and 13 C-NMR spectra were recorded on a Brücker AM 300 spectrometers in CDCl 3 .Chemical shifts are reported on the δ scale with the reference to tetramethylsilane or CDCl 3 as the internal standard and the coupling constants J are given in Hz.The IR-spectra were recorded on a Perkin-Elmer 681 instrument.Tin-layer chromatography was performed using Merck silica gel 60 F 254 precoated aluminium plates, 0. with 10% sulphuric acid and then heating.Column chromatography was carried out using Merck silica gel (Kieselgel 60, 70-230 mesh).

Table 1 .
Palladium-catalyzed reaction of NuH with allylic carbonate 5 2 mm thickness.Visualisation was by UV or by spraying © ARKAT USA, Inc