Geminate radical pair secondary

Primary geminate Secondary geminate radical pair radical pair... 

Selective combination of the secondary geminate radical pairs occurs in the micelle, compared to nonselective free-radical combination reactions in solution. This results from the micelle host effectively constraining the separation of the geminate radical pair. 

Photochemical excitation results in a-cleavage to produce a primary, geminate radical pair, which may undergo radical combination reactions (1) in competition with decarbonylation or (2) to produce a secondary geminate radical pair. The latter may undergo radical combination (3) or produce a free-radical pair (4). The free radicals undergo radical combination reactions (5). 

Regioselectivity is the preferential formation of one product over all other possibilities. The zeolite host effectively constrains separation of the secondary geminate radical pair A and B . 

Consider the primary geminate pair (ACO/B) for the specific case of dibenzyl ketone (Figure 9). If the primary geminate radical pair can rotate within the 100 ns time window allowed by the rate of decarbonylation, the carbonyl fragment can attach itself to the ortho or the para position of the benzyl radical, to yield oDBK and pDBK, respectively. If the primary radical pair can diffuse apart and remain apart for the 100 ns, decarbonylation occurs and a secondary geminate radical pair (A/B) is produced. If the mechanistic ideas are correct, the ratio of oDBK to pDBK provides a simple probe of the rotation degrees of freedom available to the primary radical pair. 

Let us now consider the possible fates of this secondary geminate radical pair. Since the product of reaction, DPE, does not contain the carbonyl group, we can easily... 

Figure 19. Photocleavage of the initiator 2,2-dimethoxy-2-phenylacetophenone 35. Net nuclear polarizations per initial radical pair P/AM are plotted as functions of the light intensity /. (Top) Curves for the geminate products 35 (a) and 38 (b) that are formed without participation of secondary methyl, and for the combination product 41 (c) of two methyl radicals. (Bottom) Curves for the products 39 (a), 43 (b), and 42 (c), the formation of which involves one methyl radical. For further information, see the text and Chart XVI. [Reproduced from ref. [132] with permission. Copyright 1990, The Royal Society of Chemistry.]...

Reaction of 6 with tertiary and also certain secondary enamines leads to the formation of 3-amino-benzo[fc]furan derivatives 174. This is formed not by Michael addition, but via geminate radical ion pair formation with subsequent generation of an oxygen-carbon bond to yield benzofurans. The products were investigated with regard to inhibition of purified human protein kinase CK2 and their general cytostatic activity (Scheme 57,llBOM2666). 

The secondary electron-transfer processes, often used in mechanistic in estiga-tions and in preparative applications of electron-transfer photochemistry, enhance the quantum yields of product formation [167], In fact, as we have already pointed out in a previous section, the efficiency of separation of the geminate pair is strictly dependent on the redox potentials (see also indirect photooxygenation processes) [43, 50, 80-83, 135], Anyway, although in the present case the subsequent electron-transfer from epoxide to biphenyl radical cation BP is endothermic enough, in the absence of very fast competing reactions this primary radical cation may still undergo an endothermic electron-transfer process with epoxides. 

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