Metal-Induced B–H Activation in Three-Component Reactions: 16-Electron Complex CpCo(S2C2B10H10), Ethyl Diazoacetate, and Alkynes

The three-component reactions of the 16-electron half-sandwich complex CpCo­(S2C2B10H10) (Cp = cyclopentadienyl) (1) with ethyl diazoacetate (EDA) and alkynes R1R2 (R1 = Ph, R2 = H; R1 = CO2Me, R2 = H; R1 = R2 = CO2Me; R1 = Fc, R2 = H) at ambient temperature lead to compounds CpCo­(S2C2B10H9)­(CH2CO2Et) ­(CHCO2Et)­(R1R2) (2–5), CpCo­(S2C2B10H9)­(CH2CO2Et)­(R2–R1–CHCO2Et) (6–9), CpCo­(S2C2B10H9)­(CH2CO2Et)­(CH­(Ph)­CCHCO2Et) (10), and CpCo­(S2C2B10H9)­(CH2CO2Et)­(CH­(Fc)–CH–CCO2Et) (11). In 2–5, one alkyne is stereoselectively inserted into the Co–B bond, one EDA molecule is used to form a sulfide ylide, and the second EDA molecule is inserted into one Co–S bond to form a three-membered metallacyclic ring. At ambient temperature 2–5 undergo rearrangement to 6–9 through migratory insertion of the inserted EDA. Different from 2–5, in 10 phenylacetylene is inserted into the Co–B bond at the terminal carbon and the terminal carbon is coupled with one EDA to afford a six-membered metallacyclic ring with the CO coordination to metal. In 11, a stable Co–B bond is generated, and one EDA and one ethynylferrocene are inserted into the Co–S bond. Moreover, if weakly basic silica is present, 2–4 can lose an apex BH close to the two carbon atoms of o-carborane to give rise to CpCo­(S2C2B9H9)­(CH2CO2Et)2­(R1R2) (12–14) accompanied by the coordination of the two sulfide ylide units to the metal center. The solid-state structures of 2–4, 6–12, and 14 were characterized by X-ray structural analysis.