Caged radical pair

The mechanism shown in Scheme 6 is, for the most part, consistent with points (1) to (9). Thus, initially formed is a o--complex that is stable only at low temperatures. Upon matrix warm-up, a caged, radical pair forms and, if the R portion possesses a sufficient excess of vibrational energy, decomposition processes may occur. The radicals combine to form RPdX, which may, or may not, be isolated. 

All these quantities are shown in figure 3.2, where Ais the the activation enthalpy for the cage radical pair recombination and Ais the activation enthalpy for diffusive cage escape. 

However, solvent viscosity, rather than polarity, has been a useful tool for mechanistic purposes. Although the quantum yield of the ortho-rearranged product of 4-methylphenyl acetate (20) does not change with viscosity of the medium, the formation of 4-methylphenol (22) is highly sensitive to this factor. Thus, its quantum yield is 0.45 in ethanol (1.00 cP) but only 0.02 in Carbowax 600 (109 cP) (Scheme 8 Table 3) [13], This clearly supports the mechanism involving caged radical pairs. A related aspect is the intramolecular nature of the process confirmed by the lack of cross-coupling products in crossover experiments with mixtures of different esters [10]. 

On the other hand, LDA metalation of 3-bromopyridine (42) at -70°C yields, after hydrolysis, a mixture of 3- and 4-substituted products (43 and 44) in addition to starting material 42 (Scheme 14) (82T3035). A potential explanation for these results involves the formation of 3,4-pyridyne, which undergoes nonregioselective attack by amine or lithio amide to give 43 and 44. An alternative rationalization is the isomerization of 42 into the 4-isomer 45 under the metalation conditions (see Section II,B,4), followed by the conversion of either isomer into the radical anions 46 which, via the caged radical pairs 47, is converted into 43 and 44 (Radical Anion-Radical Pair = RARP pathway). 

Chromic acid oxidation of saturated hydrocarbons starts with hydrogen abstraction to give a caged radical pair.113,114 The collapse of the latter leads to a chromium(IV) ester, which hydrolyzes to the product tertiary alcohol. The postulation of the caged pair was necessary to explain the high degree of retention in oxidation of (+)-3-methylheptane 113... 

Quantum yields for adduct 63 and total product (63-65) formation from the reaction of - -t with several tertiary amines are summarized in Table 12. Quantum yields measured at 1.0 M amine concentration are lower than the values extrapolated to infinite amine concentration due to incomplete quenching of It. Extrapolated total quantum yields range from 0.07 to 0.33, providing a lower limit for the efficiency of the proton transfer step, kh> in Fig. 11. The other reaction products, 1,2-diphenylethane (64) and 1,2,3,4-tetraphenylbenzene (65), are formed mainly via in-cage radical pair disproportionation and out of cage combination, respectively. The relative importance of radical pair combination, disproportionation, and cage escape is dependent... 

Scheme 4) will be O—O bond homolysis to yield the caged radical pair 15, which rapidly loses COa to the new pair 16. If we follow the CIDNP rules given in the Appendix to this chapter (Table A 1.1), we find that the ethyl radical pro-... 

Finally, recall that geminate processes are detected by the spectroscopic technique of chemically induced nuclear polarization. Because the CIDNP spectrum is directly related to properties of the caged radical pair, as explained in the Appendix to this chapter, much useful information about the nature of caged radicals and their fate can be obtained by this method. 

SCHEME 8. Reaction of a photoreactive species to form a caged radical pair followed by a radical trapping reaction. 

Pincock and DeCosta [96] have recently described photoinduced bond cleavage reactions in a series of naphthylmethyl esters to produce both ionic and radical products. The authors attribute their results to excited state homolysis to form a caged radical pair and ET between the radicals to form an ion pair. The rates of ET have been rationalized by the Marcus theory which shows a Marcus inverted region where ET becomes slow when AG T is highly exothermic. In this case ET appears to follow homolytic bond cleavage rather than prior to bond breaking. 

SCHEME 13.7 A simplified mechanism for in-cage radical pair combinations. 

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