O-atom insertion

The MgO+ ion has significant radical character and reacts via electron transfer (equation 32). It is also a potent H atom acceptor, readily reacting with water via H atom abstraction (equation 33, X = HO). A recent combined experimental and theoretical study reveals that the MgO+ ion readily activates the C—H bond of methane to yield MgOH+ as the major product ion (equation 33, X = CH3) as well as Mg+ as a minor product ion via O atom insertion into the C—H bond (equation 34). ... 

By far the most common way for organic molecules to enter late transition metal catalyzed reactions is oxidative addition. In this process a low valent palladium(O)3 or nickel(O) atom inserts into a carbon-heteroatom bond, usually of an aryl halide or sulfonate (Figure 1-2). The formation of the carbon-metal bond is accompanied by an increase in the oxidation number of the metal by 2. There are a series of factors determining the speed of the process. 

Oxygenation of 2-substituted adamantanes with methyl(trifluoromethyl)dioxirane showed a reaction constant, p = -2.31, consistent with a strongly electron-demanding transition state. Analysis of the effect of solvents on the rate yielded a positive regression coefficient with Dimroth-Reichardt Ej solvent polarity parameter. A mechanism involving an electrophilic O atom insertion has been postulated for the formation of alcohols and carbonyl compounds.201... 

A potential O-atom donor molecule, N2O, has been allowed to react with SiH+ to probe the formation of HSiO+, a higher energy isomer of SiOH+. The expectations were fulfilled (equation 13)43, as tested by reaction with a base capable of deprotonating HSiO+ but not SiOH+. The formal O-atom insertion product, SiOH+, was ascribed to a proton shuttle mechanism which is dominant when OX is SO2 and CO2 (equation 14). In fact, with these two neutrals the formation of HSiO+ and X as bare species is somewhat endothermic and the proton transfer process within the ion-neutral complex, driven by the stability of SiOH+, is allowed by the proton affinity (PA) of X which is intermediate between those of the Si and O sites of SiO. On the other hand, when O2 is used as the neutral reagent this pathway is not accessible and HSi02+ is the only observed product ion. 

Enzymes have evolved to couple efficiently the reduction of dioxygen to the oxidation of biological compounds. Difficult C-H abstraction and O-atom insertion reactions are catalyzed by oxygen-activating enzymes under mild conditions and with high specificity. 

A more precise analysis and characterization of the M0O3 crystallites shape have shown that in fact the better plane for propene oxidation to acrolein corresponds to the (IkO) plane as schematised in fig. 1 [7]. It is then suggested that the propene activation (H abstraction) into the 3t-allyl intermediate occurs on the side (100) plane while the O atom insertion occurs on the (010) basal plane. As a matter of fact it is worth noting that due to the layered structure of M0O3, the lattice oxide ions are much more labile in the (OkO) plane than in the others. Recently such a structure sensitivity was also derived for the partial oxidation of methane to formaldehyde 8],... 

Formed in superacid media species exhibiting electrophilic properties are able to attack alkanes primarily via electrophilic addition to C-H bond followed by other reactions. In particular, Olah et al. observed O atom insertion in hydrogen peroxide reaction with methane in Magic Acid above 0 °C to produce methanol with very high (>95%) selectivity [54a]. The particle (OH)"", which may be considered to be a protonated oxygen atom in the singlet state is apparently the active species in the reaction. Methyl alcohol formed is immediately protonated to methyloxonium ion, and this prevents further... 

A marginally catalytic oxidation of cyclohexane using a Ru(OEP)(PPh3)Br/PhIO system was mentioned above (Section 3.3.). The Ru(TMP)(0)2 species is thermally unreactive toward cyclooctane at 1 atm O2 at 70°C, but some O-atom insertion into saturated C-H bonds has been achieved under photolytic conditions and with the electrochemically generated Ru(TMP XOX species few details are available, but the latter system showed for adamantane oxidation the usual radical selectivity (tertiary > secondary carbon). 

A procedure widely applied for effecting O-atom insertions (olefin epoxidation, hydrocarbon hydroxylation or ketonization) with cobalt(II) catalyts is the addition of sacrificial 2-methylpropanal. It is converted via H-atom abstraction to an acyl radical, which upon reaction with O2 produces an acylperoxyl radical. H-atom abstraction by the latter leads to a hydroperoxide, which is capable of effecting O-atom insertions via free-radical chain reactions. The sacrificial aldehyde is lost via oxidation to an acid or an ester. 

Scheme 13 Plausible route to alcohols, ROH, using M as a C-H activation catalyst via O-atom insertion by a transalkyl reductive functionalization (TRF) cycle where to yield ROH...

In addition to the reactive reduced forms of dioxygen discussed above, biological systems contain metalloenzymes which are able to activate kinetically-inert dioxygen. These enzymes generally contain Fe, Cu or Mo at the active site and they have a variety of different roles. For instance they can act as O2 carriers, (e.g. haemoglobin) or produce activated forms of oxygen for O-atom insertion reactions (e.g. cytochrome P 450, tyrosin-... 

PGs/mmc. Finally, (3-cristobalite (C9) forms above 1470°C before melting at 1713°C. The Si atoms in (3-cristobalite form an Fd3m diamond stmcture of Si atoms with the O atoms inserted between them. The stmcture of these different polymorphs gets progressively more open as the temperatures of their stable phase increases. At high pressures, a monoclinic form called coesite can exist and at even higher pressures, tetragonal stisho-vite (P4/mmm) forms. These latter two metastable forms are found in impact craters. 

Bergman and coworkers [195, 196] demonstrated that N2O is capable of mediating O-atom insertion into a late metal-hydrogen bond, in this case to afford a hydroxy-Ru complex (Scheme 2.10). 

We have recently established experimental and theoretical evidence for a facile Re-R to Re-OR bond conversion with non-peroxo YOs that proceeds via a low energy, Baeyer-ViUiger (BV) type, electrophilic O-atom insertion (Fig. 7.44). 

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