Insertion of Alkynes into an ArS−Pt Bond: Regio- and Stereoselective Thermal Reactions, Facilitation by “o-Halogen Effect” and Photoirradiation, Different Alkyne Preferences Depending on the Ancillary Ligand, and Application to a Catalytic Reaction
journal contributionposted on 22.09.2008, 00:00 by Hitoshi Kuniyasu, Kazunobu Takekawa, Fumikazu Yamashita, Kiyoshi Miyafuji, Shigehito Asano, Yasutomo Takai, Atsushi Ohtaka, Aoi Tanaka, Kunihiko Sugoh, Hideo Kurosawa, Nobuaki Kambe
The insertion of alkyne 2 into the S−Pt bond(s) of Pt(SAr)(Cl)(PPh3)2 (7), Pt(SAr)2(PPh3)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 (RCCR′; R′ = H, C(O)OEt, and CH2OMe) at 70−110 °C to give Pt[(Z)-C(R′)C(SAr)R](Cl)(PPh3)2 (Z-10) as stable compounds. The introduction of an electron-donating group in Ar of ArCCH 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, RCCH) into the S−Pt bond of trans-Pt(SAr)2(PPh3)2 (8) also occurred to afford Pt[(Z)-C(H)C(SAr)R](SAr)(PPh3)2 (Z-16), which was further converted into Pt(PPh3)2(RCCH) (18) and (Z,Z)-(ArS)(R)CC(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 ArCCH. Contrary to the cases of PPh3-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.