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 1 H 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-2 H -pyrane under mild reaction conditions, however without any asymmetric induction.