Palladium-Catalyzed Synthesis of 2,3-Dihydro-2-ylidene-1,4-benzodioxins

Abstract

Palladium-catalyzed condensation of benzene-1,2-diol with various propargylic carbonates afforded regio- and stereoselectively 2,3-dihydro-2-ylidene-1,4-benzodioxins. The reaction is suggested to proceed by the formation of a (sigma-allenyl)palladium complex, followed by the intermolecular attack of the phenoxide ion on this complex to generate a new (sigma-allyl)palladium complex in equilibrium with the corresponding (eta(3)-allyl)palladium complex. Intramolecular attack of the phenoxide ion afforded the corresponding benzodioxan compound. This last attack occurs predominantly at the more electrophilic end of the (eta(3)-allyl)palladium intermediate. The Z- or E-stereochemistry of the products was established by (1)H NMR and proton NOE measurements and also by X-ray analysis on an example. The Z-stereochemistry generally observed is in agreement with the formation of this (eta(3)-allyl)palladium intermediate. However, in the case of tertiary propargylic carbonates, the E-stereochemistry generally observed could be explained by an intramolecular attack of the phenoxide ion on the intermediate (sigma-allyl)palladium complex, in slow equilibrium with the (eta(3)-allyl)palladium complex.

Palladium‐Catalyzed Annulation of Aryl‐1,2‐diols and Propargylic Carbonates. Theoretical Study of the Observed Regioselectivities

“…We found that the cyclization process was often quite regio-and stereoselective, the major regioisomer being formed by intramolecular attack of the intermediate phenoxide ion on the more electrophilic terminus of the η 3 -allylpalladium intermediate. [23] We also found that the stereochemistry of the double bond in the resulting heterocycle depended on the substitution pattern of the propargylic carbonate: primary and secondary carbonates afforded mainly the (Z) alkene, while tertiary carbonates gave predominantly the (E) isomer. We thought that it would be interesting to extend this methodology to substituted catechols, in order to obtain 2-alkylidene-2,3-dihydro-1,4-benzodioxines bearing various substituents on the phenyl ring.…”

“…For this purpose, we choose methyl 1-methylprop-2-ynyl carbonate (2) as the propargylic carbonate, and catechols bearing electron-withdrawing or -donating groups at positions 3 or 4, namely 3-methoxybenzene-1,2-diol, 2,3-dihydroxybenzaldehyde, and 3-nitrobenzene-1,2-diol (1bϪd), and 4-methoxybenzene-1,2-diol, 3,4-dihydroxybenzaldehyde, and 4-nitrobenzene-1,2-diol (5aϪc) as the substituted diphenols. The condensation was performed as described previously, [23] in THF in the presence of the catalyst obtained by mixing [Pd 2 (dba) 3 ] and dppb. The results of the cyclization are summarized in Table 1.…”

“…Experimentally, in the case of a methyl substituent on the η 3 -allyl complex (case studied here), the phosphane used is dppb. [23] For this phosphane we have seen that the anti isomer is the most stable, and this is the reason why the reaction is so highly regioselective (more than 95% attach on C 3 atom). It must be added that the regioselectivity of the nucleophilic attack on the η 3 -allylpalladium complex also depends on the bulkiness of the substituents (steric effects) and on their electron-donor or -attractor character.…”

“…[23] We therefore calculated the stabilities of the syn and the anti isomers of the η 3 -methylallyl palladium complex 10 with PPh 3 and dppb [1,4-bis(diphenylphosphanyl)butane], respectively, as the ligand, instead of PH 3 . For PPh 3 , the syn isomer is more stable by 5 kJ·mol…”

Polycondensation behavior between propargyl carbonates having a bulky ester group and bisphenols in the presence of Pd(0) catalyst: synthesis of exomethylene-containing polyethers

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] In these reactions, two consecutive nucleophilic attacks occur. A nucleophile attacks at the central carbon of the Z 3 -allenyl/propargyl intermediate generated by reaction of a propargylic carbonate with Pd(0) to yield a p-allylpalladium intermediate, which is successively attacked by a second nucleophile.…”