Addition of benzenethiol to terminal alkynes catalyzed by hydrotris(3,5-dimethylpyrazolyl)borate–Rh(III) bis(thiolate) complex: Mechanistic studies with characterization of the key intermediate

The Rh(III)–thiolate complex [Tp∗Rh(SPh)2(MeCN)] (2; Tp∗ = hydrotris(3,5-dimethylpyrazolyl)borate) readily undergoes substitution of MeCN by XyNC (Xy = 2,6-dimethylphenyl) to give the isocyanide complex [Tp∗Rh(SPh)2(XyNC)] (3), whereas reaction of 2 with terminal alkynes results in the formation of the rhodathiacyclobutene complex [Tp∗Rh(SPh){η2-CHCR(SPh)}] (4; R = aryl, alkyl). Molecular structures of 3 and 4 (R = CH2Ph) have been determined by single crystal X-ray diffraction. Complex 2 as well as [Tp∗Rh(cyclooctene)(MeCN)] have been found to catalyze regioselective addition of benzenethiol to terminal alkynes RCCH at 50 °C to give R(PhS)CCH2 in moderate to high yields. The above products are selectively formed when R = CH2Ph and n-C6H13, while cis-RCHCHSPh and RC(SPh)2CH3 are also obtained as by-products when R = p-MeOC6H4. Catalytic cycle involving 2 and 4 is proposed based on the mechanistic studies using NMR measurement.

Reaction of the Rh(III)–thiolate complex [Tp∗Rh(SPh)2(MeCN)] (Tp∗ = hydrotris(3,5-dimethylpyrazolyl)borate) with terminal alkynes RCCH (R = aryl, alkyl) forms a rhodathiacyclobutene complex [Tp∗Rh(SPh){η2-CHCR(SPh)}]. The former complex as well as [Tp∗Rh(cyclooctene)(MeCN)] catalyze regioselective addition of PhSH to RCCH, yielding R(PhS)CCH2, via the rhodathiacyclobutene complex as an intermediate.Figure optionsDownload as PowerPoint slide