112596-51-7Relevant academic research and scientific papers
How dioxygen activates C-H bonds of simple arenes in unstable CpFeI(arene) complexes and the versatile reactivity of superoxide anion generated from dioxygen and organoiron electron reservoirs
Hamon, Jean-René,Astruc, Didier
, p. 1036 - 1046 (2008/10/08)
The reactivity of O2.- generated in inert solvents and ethers from dioxygen and electron reservoirs such as CpFeI(arene) complexes has been examined with the aim of activating benzylic C-H bonds in simple arenes coordinated to CpFeI. The C-H activation by O2 in pentane, THF, or DME, known for C5R6FeI(C6R′6) (R and R′ = alkyls), is investigated for unstable CpFeI(arene) complexes with one to six methyl groups on the arene ligand. A dramatic salt effect is found when the reactions are carried out in THF; in the presence of Na+X-, the formation of yellow diagmagnetic salts [CpFe(arene)]+X- and of 1/2 mol of Na2O2 is general; it is quantitative with X- = PF6- upon reaction with 1/2 mol of O2 at -80°C. When Na+X- is removed, the unstable CpFeII(η5-benzyl) complexes are obtained in high yield and characterized by the downfield resonance (140 ppm) observed in the 13C{1H} spectra for the ring carbon bound to the exocyclic double bond; they can be alkylated or functionalized in situ at -50°C with CH3I or PhCOCl. Primary, secondary, and tertiary C-H bonds can be activated in this way by O2 in the 19-electron CpFeI(arene) complexes. The rapid H-atom abstraction from methyl and ethyl substitutents at -80°C is an outer-sphere electron transfer to dioxygen followed by deprotonation by superoxide anion, the latter process being inhibited by the salt effect (Na+). The formation of dimeric peroxides is a general phenomenon in the reactions of 1/2 mol of O2 in pentane or toluene with CpFeI complexes of arenes such as C6H6 or 1,3,5-t-Bu3C6H3 which do not bear benzylic hydrogens. The competition between formation of a dimeric peroxide and benzylic H abstraction is exemplified by the behavior of CpFeI(i-PrC6H5); reaction of O2 with the latter in toluene gives the dimeric peroxide as the kinetic product (-80°C), transformed into the thermodynamic H abstraction product at -17°C. The salt effect in THF with M+X- (M+ = n-Bu4N+, K+, Na+, X- = PF6-, BF4-, F-) also inhibits the formation of dimeric peroxide, and the salts [CpFe(arene)]PF6 and Na2O2 are formed instead of the dimer. The salt effect is all the more important in the series M+X- as the size of the cation M+ decreases and as that of the anion X- increases. In all the reactions, formation of superoxide anion in ion pairs is the first step and its versatile reactivity (proton abstraction, nucleophilic addition, reduction or dismutation) accounts for the variety of reactions observed.
