Synthesis and Catalytic Properties of Configurationally Stable and Non-racemic Sulfur-Containing Palladacycles

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Abstract

Racemic sulfur chloro-bridged palladacycles (rac-4b−6b), derived from the orthopalladation of (1-tert-butylsulfanylethyl)benzene, (1-methylsulfanylethyl)benzene, and 2-(1-tert-butylsulfanylethyl)naphthalene (rac-1−3), have been synthesized in good yields using palladium acetate as the metalation agent and followed by reaction with LiCl. Palladacycles rac-4b and rac-6b have been isolated as single isomers, whereas rac-5b has been obtained as a mixture of two diastereoisomers (3:1). Variable-temperature 1H NMR showed that the S-t-Bu palladacycle 4b is configurationally stable over a large range of temperatures (from −20 to 100 °C). On the other hand, S-Me analogue 5b undergoes facile sulfur pyramidal inversion with an energy barrier of 18 kcal/mol. Moreover, the optically active (1-alkylsufanylethyl)benzenes, (R)-1 and (R)-2, respectively, have been prepared in 91% and 97% ee, respectively, from the reaction of sodium methylthiolate and sodium tert-butylthiolate, respectively, with non-racemic (S)-(−)-1-phenylethanol (93−99% ee). The reaction of (1-alkylsufanylethyl)benzenes (R)-1 and (R)-2 with palladium acetate in acetic acid at 90 °C affords the corresponding orthopalladated compound (R,R)-4a and (R,R and R,S)-5a, respectively. The molecular structure of the acetato-bridged dimer (R,R)-4a has been determined by means of an X-ray diffraction analysis. The palladacycle (R,R)-4b has shown to be excellent catalyst precursor, in terms of catalytic activity, for the arylation of 3,4-dihydro-2H-pyrane under mild reaction conditions, however without any asymmetric induction.

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