Reduction of metal-stabilized alpha-CF3-carbenium ion complexes under mildconditions: Synthesis, structures, and reactivity

Complexed alpha-CF3 propargyl alcohols of the general formula [(M2L6) {mu-e ta(2),eta(2)-RC=CCH(CF3) (OH)}] were prepared with M2L6 = Co-2(CO)(6), R = CH3(CH2)(4-) (1), R = C6H5- (2); M2L6 = Co-2(CO)(5)P(C6H5)(3), R = CH3(CH2) (4-) (3a,b), R = C6H5- (4a,b); M2L6 = Co-2(CO)(4)dppm R = C6H5- (5); M2L6 = Co(CO)(3-) MoCp(CO)(2), R = CH3(CH2)(4-) (6a,b), R = C6H5- (7a,b). An X-ra y molecular structure of the propargyl-alcohol complex [{Co-2(CO)(4)dppm}{m u-eta(2),eta(2)-C6H5C=CCH(CF3)(OH)}] (5) was also determined. The related c arbenium ions [(M2L6){eta(2),eta(3-) RC=CCH(CF3)}][BF4] (8-12) were obtaine d from the parent propargyl alcohol complexes by direct protonation with HB F4 . Et2O in diethyl ether. These carbenium ions were reduced further by Zn in CH2Cl2 to give the alkyne adducts [(M2L6){mu-eta(2),eta(2)-RC=CCH2(CF3) }] (13-17), as confirmed by the X-ray molecular structure of [(Co-2(CO)(4)d ppm) {mu-eta(2),eta(2)-C6H5-C=CCH2(CF3)}] (17). Treatment of the carbenium ion complex [{Co(CO)(3)MoCp(CO)(2)}{mu-eta(2),eta(3)-CH3(CH2)(4)C=CCH(CF3)} ][BF4] (8) with NaSMe unexpectedly afforded the reduced alkyne adduct [{Co( CO)(3)MoCp(CO)(2)}{mu-eta(2),eta(2)-CH3(CH2)(4)C=CCH2(CF3)}] (13), along wi th the alkyne-thioether diastereomers [{Co(CO)(3)MoCp(CO)(2)}{mu-eta(2),eta (2)-CH3(CH2)(4)C=CCH(CF3) (SMe)}] (18a,b), Presumably, all the reduction re actions proceed primarily by the formation of the transient radical species , which are subsequently transformed into the reduced alkyne complexes by h ydrogen abstraction from the solvent medium. Interestingly, in the case of the complexed alcohols [{Co-2(CO)(5)P(C6H5)(3)}{mu-eta(2),eta(2)-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 i nfluence 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.