Selective radical activation

In the literature [109] on homogeneous C—H bond activation substantial evidence exists for selective radical activation using nitrogen-containing non-metallic... 

In this chapter, we look closely at the performance of several ab initio techniques in the prediction of radical thermochemistry with the aim of demonstrating which procedures are best suited in representative situations. We restrict our attention to several areas in which we have had a recent active interest, namely, the determination of radical heats of formation (AHf), bond dissociation energies (BDEs), radical stabilization energies (RSEs), and selected radical reaction barriers and reaction enthalpies. We focus particularly on the results of our recent studies. 

In the previous sections, methods of qualitatively controlling the course of propagation were described. Indirect control as well as the quantitative effects caused by intentional control of the other partial processes in polymerization have still to be mentioned. The separation of initiation from propagation alters the kinetic character of the whole reaction. With ionic polymerizations, initiation can be separated from propagation by the selection of conditions suitable for rapid initiation. With radical polymerizations, this is not possible. Therefore both partial processes must be separated in space. Fortunately, radical active centres operate both in polar and in non polar media. Thus it is not difficult to confine initiation and propagation to mutually immiscible components of the medium. Emulsion polymerization remains the most important representative of quantitative control of propagation. 

Apart from the selection of reactions involving dipolar, isopolar, or free-radical activated complexes used to demonstrate the qualitative theory of solvent effects by Hughes and Ingold [16, 44] in the preceding sections, further illustrative examples can be found in the literature e.g. [14, 15, 18, 21, 23, 26, 29, 460, 468]). 

Figure 4. Helix-sense-selective radical polymerization using optically active thiol as a chain-transfer agent or initiator.

Alkane functionalization on a preparative scale by mercury-photosensitized C-H bond activation has been recently developed by Crabtree [22], Mercury absorbs 254-nm light to generate a Pi excited state which homolyzes a C-H bond of the substrate with a 3° > 2° > 1° selectivity. Radical disproportionation gives an alkene, but this intermediate is recycled back into the radical pool via H-atom attack, which is beneficial in terms of yield and selectivity. The reaction gives alkane dimers and products of cross-dehydrodimerization of alkanes with various C-H compounds ... 

The incorporation of the third component has been shown to provide pathways to scavenge oxygen molecules that inhibit free-radical polsrmerization, to regenerate the dye photoinitiator, and to produce a free-radical active center in place of a terminating dye radical (3,12,13,21). In addition, the wide selection of dyes available for nse in three-component systems allows more flexibihty in initiating wavelength selection, and the photopolymerization of thick parts is possible if a photobleaching dye is chosen (21). 

Table V. Activation energies for selected radicals attaching to various 7t-bond-containing molecules. This corresponds to the barrier height for the reaction A + R AR. The arrow indicates the atomic site to which the radical will bond. The energy required to break the bond is equal to the bond dissociation energy (see Table IV) plus this activation energy. Energies are in kcal-mol"k ...

Figure 3 Schematic potential energy diagram for the selective radical trapping method for the reversible reaction of ArS with CH2=CHR. and are activation energies of the forward and backward reactions, respectively. E. is the activation energy for the reaction of carbon radical with O2.

Thus, as in the case of alkenes in the preceding chapter, we start with the radical type of activation that is much older. Transition-metal compounds play a key role in radical activation, because they provide very strong oxidants that can oxidize hydrocarbons either by (reversible) electron transfer or H-atom transfer (more rarely by hydride transfer). Biological oxidation of hydrocarbons involves reactive metal-0X0 species in methane mono-oxygenases and many related synthetic models, and a number of simple metal-oxo complexes also work. The clear criterion of distinction between an organometallic C-H activation and a radical activation is the above selectivity in activated C-H bonds. 

However, the rate of scavenging of ARTS radicals by weak AOXs that have higher redox potentials or form stable intermediates was found to be very slow. This suggests that an ARTS -based assay may be selective for active AOXs in the presence of weak or inactive substances. 

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