Stereocontrolled palladium(II)-catalyzed intramolecular 1,4-oxidations of dienyl acids and alcohols proceed under mild conditions to give fused delta-lactones and pyrans, respectively, in good yields. The stereo- and regioselectivity was affected by the presence of LiCl and solvent composition (HOAc/acetone). The products obtained were used for further functionalizations using copper(I)-mediated reactions. Stoichiometric reactions of preformed dibutylcyanocuprates with pyrans containing an allylic acetate gave cis- and trans-fused ring systems with high gamma-selectivity and in high yields.
The acetate/acetate-d
3 exchange reaction of the ring-bonded acetate of bis(4-acetoxy-[η
3-(1,2,3)-cyclohexenyl])palladium acetate-d
3 complexes 1a−c was studied in acetic acid
solutions using 1H NMR spectroscopy. The reactions followed first-order kinetics in palladium,
and the rates were highly affected by the presence of methanesulfonic acid or lithium acetate.
The nature of the substituent in the 2-position of the complex was found to have a large
impact on the reaction rate. Complexes 1a−c are observed intermediates in the benzoquinone-assisted palladium(II)-catalyzed 1,4-diacetoxylation reaction of 1,3-dienes. Complex
1b was treated with stoichiometric amounts of the enantiomerically pure ligand (S)-(+)-2-(4‘-fluorophenylsulfinyl)-1,4-benzoquinone 4 under conditions where no exchange reaction
occurs. Kinetic resolution was observed, implying that the two enantiomers of 1b reacted to
trans-1,4-diacetoxy-2-phenyl-2-cyclohexene with different rates. Attempts to demonstrate
dynamic kinetic resolution in stoichiometric reactions between 1b and 4 were unsuccessful.
The major reason for this is presumably that with lithium acetate the equilibrium reaction
between the two enantiomers of 1b is too slow compared to the chiral benzoquinone-induced
attack of acetate to give the products. Under very acidic conditions the decomposition of the
(π-allyl)palladium complex is faster than benzoquinone-induced product formation. This
scenario is in full agreement with our observed rates.
The use of a Co(porphyrin)-amide ligand, , in the palladium(II)-catalyzed 1,4-diacetoxylation of conjugated dienes under O2 results in aerobic oxidation. The catalyst was highly active under O2, and the 1,4-diacetoxylation reaction could also be performed under air.
New Chiral Diamide Ligands Containing Redox-Active Hydroquinone Groups. Synthesis and Results in the Palladium(II)-Catalyzed 1,4-Diacetoxylation of 1,3-Dienes. -Some new chiral diamide ligands are prepared. Among them (IIIa) gives the best results. High regio-and diastereoselectivity is achieved, but the enantioselectivity is moderate. -(VERBOOM, R. C.; PLIETKER, B. J.; BAECKVALL*, J.-E.; J. Organomet. Chem. 687 (2003) 2, 508-517; Dep. Org. Chem., Arrhenius Lab., Univ. Stockholm, S-106 91 Stockholm, Swed.; Eng.) -Jannicke 16-097
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