Ruthenium Complexes of Thiocinnamaldehydes: Synthesis, Structure, and [4+2]‐Cycloaddition Reactions

Ruthenium hydrogensulfido complexes [CpRu(PP)(SH)] ((PP)Ph 2 PCH 2 PPh 2 (dppm), Ph 2 PC 2 H 4 PPh 2 (dppe)) were obtained from the corresponding chloro complexes by Cl/SH exchange. Condensation with a range of cinnamaldehydes gave thiocinnamaldehyde complexes [CpRu(PP)(SCHCR 2 CHR 1 )]PF 6 (R 1 =C 6 H 4 X, R 2 =H, Me, X=H, OMe, NMe 2 , Cl, NO 2 ) as highly‐colored crystalline compounds. The thiocinnamaldehyde complexes undergo [4+2]‐cycloaddition reactions with vinyl ethers CH 2 CHOR 3 (R 3 =Et, Bu) and styrenes H 2 CCHC 6 H 4 Y (Y=H, Me, OMe, Cl, Br, NO 2 ) to give complexes of 2,4,5‐trisubstituted 3,4‐dihydro‐2 H ‐thiopyrans as mixtures of two diastereoisomers. The rate of addition of para ‐substituted styrenes H 2 CCHC 6 H 4 Y to [CpRu(dppm)(SCHCHCHPh)]PF 6 increases in the series Y=NO 2 , Br, Cl, H, Me, OMe, indicating that the cycloaddition is dominated by the HOMO(dienophile)–LUMO(diene) interaction. The strained dienophiles norbornadiene and norbornene also add, giving ruthenium complexes of 3‐thia‐tricyclo[6.2.1.0 2,7 ]undeca‐4,9‐dienes and 3‐thia‐tricyclo[6.2.1.0 2,7 ]undec‐4‐enes, respectively. Addition reactions with acrolein, methacrolein, methyl vinyl ketone, acrylic ester, or ethyl propiolate finally yielded ruthenium complexes of 3,4‐disubstituted 3,4‐dihydro‐2 H ‐thiopyrans and 4 H ‐thiopyrans, respectively.