C-O Bonds Formation using Transition Metal Alkyl or Aryl Complexes

The selective catalytic oxidation of hydrocarbons is an important but challenging process. Transition metal catalysts for partial oxidation of alkanes to form alcohols must be able to perform two key steps: C–H bond activation and C–O bond formation. One strategy is to activate the C–H bond of hydrocarbons (RH) by a metal–alkoxide (M–OR) complex to produce the alcohol and a metal–alkyl (M–R) complex (RH + M–OR --> M–R + ROH), followed by net O-atom insertion into the newly formed M–R bond by an oxygen donor (YO) to regenerate the M–OR complex (M–R + YO --> M–OR). With examples of both steps reported, relatively little is known about the O-atom insertion step. Two pathways for the O-atom insertion step are under investigation. One is an organometallic Baeyer-Villiger (OMBV) type reaction involving the migration of the nucleophilic R– to the bound OY (M–R + OY R --> M–O–Y --> M–OR + Y), the other pathway involves the migration of the nucleophilic R to a M–O intermediate (M–R + OY -->R–M=O + Y --> M–OR + Y). Stoichiometric reactions are utilized to extend the O-atom insertion reaction to a range M–R complexes and to demonstrate the transition series between different nucleophilicity of the M–C bond.
Extension of the O-atom insertion to the middle transition W(VI)–R complexes has been studied. Cp*W(O)2(CH2SiMe3) (Cp* = η5-pentamethylcyclopentadienyl) reacts with oxygen atom donors (e.g., H2O2, PhIO, IO4–) in THF/water to produce high yield TMSCH2OH (TMS = trimethylsilyl). Mechanism studies reveal an OMBV type O-atom insertion for this conversion.
Attempts of O-atom insertion into late transition 3d Fe(II)–R and CoI–R complexes have been presented. Cp*Fe(P(OCH2)3CEt)2Ph reacts with Me3NO to produce high yield PhOH in the presence of Brønsted acid. All the other ligands are also oxidized by Me3NO during this conversion. Computational mechanism studies suggest a R to M=O migration pathway. Low yield of MeOH is produced from (N3)CoMe (N3 = triamine ligand) with oxidants. Decomposition of the Co–Me complex occurs during this conversion.