O-atom donors

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. 

There have been several mechanistic proposals for interpreting the behavior of metalloporphyrin catalyzed epoxidation with single O-atom donors. 

The activation of dihydrogen to form the hydride MH" " is the simplest reaction of an M" " ion, and this was one of the first reactions examined. Numerous studies of bare and ligated rare earth and actinide ions with a variety of small inorganic molecules followed, particularly with O-atom donors, with the purpose of obtaining oxide ions and characterizing their energetics. 

Among the most extensively studied reactions of bare and ligated actinide ions are those with O-atom donors, with a central goal of synthesizing and characterizing elementary monoxide and dioxide ions, particularly AnO, ... 

Studied reactions of dipositive An and AnO with O-atom donors are summarized in Table 6. All of the AnO for An Th-Cm have been synthesized. In a seminal work, Comehl et al. (1996a) synthesized the bare uranyl ion, U02, by the sequential reaction of laser-ablated ions with two molecules of O2 or N2O. As is commcm for small dipositive ions such as 1102 ", electron transfer from the oxidant also resulted in charge reduction to U02. Measured electron transfer rates increase as the difference... 

Because N2O is an even more facile O-atom donor than O2—Z>(N2-0) = 167kJmol versus D(0 0)=498kJmol —Eq. (37) is more exothermic than Eq. (35). The quite different chemielectron spectra for Eqs. (35) and (36) were employed to derive potential energy surfaces for the two processes. 

Epoxidation by asymmetric cat. of simple prochiral olefins by PhIO or other O-atom donors is also possible using other specific catalysts (such as some chiral metalloporphyrins). 

Scheme 5 General strategy for catalyst design based on coupling CH activation with M-R functionalization using an O-atom donor or electron-acceptor. Ox, that is regenerated by air in a...

These results are encouraging and may point to a facile pathway for heteroatom functionalization of M-R intermediates of more electron-rich metals via a BV-type pathway with electrophihc, O-atom donors, YO. Howevei there are some key considerations that must be addressed before we can determine if this pathway will be broadly appHcable for M-R functionalizations. In MTO, rheni-... 

---