Reduction of Metal-Stabilized α-CF3-Carbenium Ion Complexes under Mild Conditions: Synthesis, Structures, and Reactivity

Abstract

Complexed α-CF3 propargyl alcohols of the general formula [(M2L6){μ-η2,η2-RC≡CCH(CF3)(OH)}] were prepared with M2L6 = Co2(CO)6, R = CH3(CH2)4– (1), R = C6H5– (2); M2L6 = Co2(CO)5P(C6H5)3, R = CH3(CH2)4– (3a,b), R = C6H5– (4a,b); M2L6 = Co2(CO)4dppm, R = C6H5– (5); M2L6 = Co(CO)3MoCp(CO)2, R = CH3(CH2)4– (6a,b), R = C6H5– (7a,b). An X-ray molecular structure of the propargyl-alcohol complex [{Co2(CO)4dppm}{μ-η2,η2-C6H5C≡CCH(CF3)(OH)}] (5) was also determined. The related carbenium ions [(M2L6){μ-η2,η3-RC≡CCH(CF3)}][BF4] (8–12) were obtained from the parent propargyl alcohol complexes by direct protonation with HBF4· Et2O in diethyl ether. These carbenium ions were reduced further by Zn in CH2Cl2 to give the alkyne adducts [(M2L6){μ-η2,η2-RC≡CCH2(CF3)}] (13–17), as confirmed by the X-ray molecular structure of [(Co2(CO)4dppm){μ-η2,η2-C6H5C≡CCH2(CF3)}] (17). Treatment of the carbenium ion complex [{Co(CO)3MoCp(CO)2}{μ-η2,η3-CH3(CH2)4C≡CCH(CF3)}][BF4] (8) with NaSMe unexpectedly afforded the reduced alkyne adduct [{Co(CO)3MoCp(CO)2}{μ-η2,η2-CH3(CH2)4C≡CCH2(CF3)}] (13), along with the alkyne-thioether diastereomers {Co(CO)3MoCp(CO)2}{μ-η2,η2-CH3(CH2)4C≡CCH(CF3)[(SMe)}] (18a,b). Presumably, all the reduction reactions proceed primarily by the formation of the transient radical species, which are subsequently transformed into the reduced alkyne complexes by hydrogen abstraction from the solvent medium. Interestingly, in the case of the complexed alcohols [{Co2(CO)5P(C6H5)3}{μ-η2,η2-RC≡CCH(CF3)(OH)}] (3a,b) and (4a,b), the reduction process occurs in acidic medium in THF/CH2Cl2. An extensive study of the electronic and steric factors that influence the stability and reactivity of the carbenium ions were performed, which allowed us to explain the behavior of the related radical species in solution during the reduction process.