Electroactive Protecting Groups and Reaction Units, 6. – Titanium Enolate Radical Cations in Solution: Generation, Characterization, and Their Reactions

For the first time titanium enolate radical cations were characterized in cyclic voltammetry experiments. Preparative one-electron oxidation of titanium enolates 1-3 led to the formation of benzofurans B1-B3 through mesolytic Ti-O bond fragmentation to α-carbonyl radical and a cationic titanocene(IV) species. With 1+·, the kinetics of the Ti-O bond cleavage was investigated by fast scan cyclic voltammetry providing a first-order rate constant k1=850 ± 50 s−1 in acetonitrile at room temperature. The titanium enolate radical cations 1+·, 3+·, 4+· and 5%% were characterized by EPR spectroscopy, qualifying these species as electrophilic radicals. This feature can be exploited in CC bond formation reactions such as the formal dimerization of 6 to 2,3-dimethyl-1,4-diphenylbuta-1,4-dione D6, which takes place on stage of the radical cation 6+·.