Sulfides, vinyl nickel catalysts

Vinyl sulfides have been prepared by the catalytic addition of the S—H bond of thiols (85) to terminal alkynes (86) under solvent-free conditions using the nickel complex Ni(acac)2 (47). High alkyne conversions (up to 99%) were achieved after 30 min at 40 °C in favor of the corresponding Markovnikov products (87) (equation 23). Other metal acetylacetonate complexes were examined for this reaction, but none showed any improvement over the nickel catalyst. Mechanistic details suggest that alkyne insertion into the Ni—S bond is important to the catalytic cycle and that nanosized structural units comprised of [Ni(SAr)2] represent the active form of the catalyst. Isothiocyanates and vinyl sulfides have been produced in related Rh(acac)(H2C=CH2)2 (6) and VO(acac)2 (35) catalyzed sulfenylation reactions of aryl cyanides and aryl acetylenes, respectively. 

The conversion of propargyl alcohols into vinyl sulfides has been achieved using nickel catalysts under solvent free conditions (Schane 5.55) [81]. The reactions were quite rapid and proceeded with only gentle heating. For most substrates screened, the process was highly selective for the Markovnikov addition, and only traces of the anti-Markovnikov product were observed. 

Inexpensive and easily available nickel acetylacetonate (Ni(acac)2) is employed as the catalyst precursor. The procedure can be easily scaled-up to prepare up to 50 g of the vinyl sulfides with high regioselectivity. Moreover, solvent-free conditions without chromatographical purification attain eco-friendly synthetic method of vinyl sulfides. The mechanistic study indicates that the catalytic hydrothiolation takes place under heterogeneous conditions with alkyne insertion into the Ni-S bond of nanosized nickel sulfide species. 

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