Synthesis and catalytic properties of configurationally stable and non-racemic sulfur-containing palladacycles

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 H-1 NMR showed that the S-t-Bu palladacycle 4b is configurationally stable over a large range of temperatures (from -20 to 100 degreesC). 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 degreesC 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 (RP)-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.