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.

Carbo-cyclohexadienes vs. carbo-benzenes: structure and conjugative properties

“…Ultimately, this should be improved by deliberately changing the two phenyl substituents for substituents exerting opposite mesomeric or inductive acceptor destabilization (–M or –I effect; the more or less protecting groups are denoted as PG in Scheme 1 ). Within this prospect, trifluoromethyl groups are ideal candidates: 13 it was indeed observed that the quite general method for the conversion of 1,4-dioxybut-2-yne derivatives to the corresponding butatrienes by treatment with SnCl 2 /HCl is not compatible with CF 3 substituents. 6 c Two substituents PG = CF 3 at adjacent carbinoxy vertices of a hexaoxy-[6]pericyclyne D F are therefore anticipated to freeze the reactivity of corresponding 1,4-dioxybut-2-yne edge towards reduction and thus optimize the selectivity for the partly reduced carbo -cyclohexadiene product C F vs. the putative carbo -benzene A F ( Scheme 1 ).…”

CF3-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

“…While the tertiary carbenium ion 108 was isolable as a solid [ 83 – 84 ], the tertiary carbenium ion 109 and the secondary derivatives 112a – c and 113a , b afforded oils. The secondary derivatives were much more sensitive in spite of the use of electron-rich Co–Mo clusters and could only be characterized by NMR and IR spectroscopy [ 85 ]. Upon ionization, the change in the electronic density is directly reflected by the downfield shift of the 19 F NMR chemical shift of the CF 3 group but also by a CO shift to a higher frequency.…”

Chemistry of Propargyldicobalt Cations Recent Developments in the Nicholas and Related Reactions

“…10 5 times less reactive (E = -6.71), indicating insufficient reactivity with the less reactive allylsilanes and all but the most electron rich arenes. The reactivity of dppm complexes is less certain, as pK R+ values (-11.0) suggest a cation less stable than the hexacarbonyl complexes [18], while work on CF 3 substituted propargylium complexes suggest greatly enhanced cation stability with this ligand [19]. Propargyl cations with one and two tris(pyrrolyl)phosphine ligands have also been generated [20]; although these cations are attacked by one of the pyrrole ligands, it is not possible at this time to evaluate cation's properties in electrophilicity or pK R+ terms.…”

“…In many of these cases use of a precursor propargyl chloride complex and AgBF 4 are the recommended choices for generation of these formally umpolung, carbonyl γ -cation equivalents. Strongly electronwithdrawing CF 3 groups have been tolerated at the propargylic centre (10), most reliably when the centre also possesses an aryl substituent [19,25]. Without the aryl groups, attempted cation formation fails, but replacement of one CO by triphenylphosphine or two CO's by a dppm ligand allows cation formation in these cases.…”

“…These homocouplings are noteworthy for their good to excellent syn-(acyclic)/trans-(cyclic) diastereoselectivity about the newly formed bond in 2 o cases. Exceptions to the homocoupling reactions are found the tertiary radicals and those bearing CF 3 groups [19], which decompose by H-atom abstraction and other routes [85]. In the absence of detailed mechanistic work, it is best to regard propargylcobalt radicals as nucleophilic radicals.…”