Insertion of Alkynes into an ArS−Pt Bond: Regio- and Stereoselective Thermal Reactions, Facilitation by “<i>o</i>-Halogen Effect” and Photoirradiation, Different Alkyne Preferences Depending on the Ancillary Ligand, and Application to a Catalytic Reaction

Abstract

The insertion of alkyne 2 into the S−Pt bond(s) of Pt(SAr)(Cl)(PPh 3 ) 2 ( 7 ), Pt(SAr) 2 (PPh 3 ) 2 ( 8 ), and Pt(SAr)(Ar′)(dppe) ( 9 ) has been investigated. Regioselective cis -insertion into the S−Pt bond of trans - 7 took place with terminal and internal alkynes (RC≡CR′; R′ = H, C(O)OEt, and CH 2 OMe) at 70−110 °C to give Pt[( Z )-C(R′)═C(SAr)R](Cl)(PPh 3 ) 2 ( Z - 10 ) as stable compounds. The introduction of an electron-donating group in Ar of ArC≡CH and in ArS of 7 slightly facilitated the reactions. It was found that a halogen atom at the ortho position in ArS of 7 dramatically promoted the insertion (“ o -halogen effect”). The insertion of a terminal alkyne ( 2, RC≡CH) into the S−Pt bond of trans -Pt(SAr) 2 (PPh 3 ) 2 ( 8 ) also occurred to afford Pt[( Z )-C(H)═C(SAr)R](SAr)(PPh 3 ) 2 ( Z - 16 ), which was further converted into Pt(PPh 3 ) 2 (RC≡CH) ( 18 ) and ( Z, Z )-(ArS)(R)C═C(H)-C(H)═C(SAr)(R) ( 19 ) by C−C bond-forming reductive elimination after the insertion of another 2 into the remaining S−Pt bond of 16 . The “ o -halogen effect” was also observed for the insertion of 2 into the S−Pt bond of trans - 8 to furnish the corresponding cis - Z - 16 as a kinetic product; the trans -isomer of 8 exhibited a higher reactivity than the cis -isomer. It was also revealed that photoirradiation (visible light) dramatically promoted the insertion of 2 into the S−Pt bond of trans - 8 . Photoinduced insertion was facilitated by introducing an electron-donating group into Ar of ArC≡CH. Contrary to the cases of PPh 3 -ligated platinum complexes, the insertion into the S−Pt bond of Pt(SAr)(Ar′)(dppe) ( 9 ) was realized when the electron-deficient alkyne DMAD ( 2t ) was employed as a substrate. Also presented is the insertion of two alkynes into each S−Pt bond of 8 in the Pt-catalyzed stereo- and regioselective dimerization−bisthiolation of alkyne ( 2 ) by diaryl disulfide ((ArS) 2, 30 ) to yield functionalized symmetrical 1,3-dienes.