Tellurium-centered radicals

In this case, the fe/t-butylperoxy radicals are the more persistent species. The authors also confirmed that the highest yield of the cross-reaction product is obtained when the cross-reacting radicals are formed with equal rates. Schiesser et al.76 introduced phenyl telluroformates as precursors of alkyl radicals (Scheme 19). In the absence of trapping agents, the parent molecules appeared stable, that is, the crossreaction between the oxyacyl and the tellurium-centered radical is even faster than the decarboxylation of the oxyacyl species, which normally takes only nano- to microseconds. Obviously, the tellurium-centered radical must be rather persistent and builds up in large concentrations. 

The liberated tellanyl radical reacts with an organoteUurium dormant species to regenerate a polymer-end radical and ditel-luride. However, it is virtually inert to monomer and does not initiate new polymer chains." If it did, the observed high controllability would not be realized. These seemingly conflicting reactivities of tellurium-centered radicals also play crucial roles in the control of MWD in TERP in the presence of ditellurides. 

The vast majority of radical cascades initiated by addition of higher main group (VI)-centered radicals to alkynes focus predominantly on S-centered radicals, mainly thiyl radicals, whereas considerably fewer intermolecular addition reactions involving radicals with the unpaired electron located on selenium or even tellurium are known. 

Step for V-Sb-0 c [77] and for w-butane activation on VPO [78]. A second H from the methyl group should then be abstracted in order to form the olefin. This, according to Grasselli and coworkers [75,79] occurs on the Te center, adjacent to the V centers. Te is found to be tetravalent in the bulk of the catalyst, but there is some evidence (XPS) that it exists as Te on the surface. Without desorption, the 7T-electrons of the propene molecule can coordinate to an adjacent Mo + center, where O is inserted via another H abstraction on neighboring Te to form allyl radical. If the Te is originally in 6-1- oxidation state, then this second H abstraction on the same center can go without regeneration (Te + has lone pair of electrons), but if it is tellurium is at 4- - oxidation state as the bulk, reoxidation is required between the H abstractions. The Nb + centers stabilize the primary active centers and... 

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