Oxidative ground-state triplet

It was Breslow (Breslow, 1982 Breslow et ai., 1982) who first paid attention to this theory. Knowing that the pentachlorocyclopentadienyl cation (Breslow et ai, 1964 Saunders et ai, 1973), the hexachlorobenzene dication (Wasserman et al., 1974) and the 2,3,6,7,10,11-hexamethoxy-triphenylene (HMT, [36]) dication are all ground-state triplets, in good agreement with theory, Breslow and coworkers set out on the synthesis of analogues that should have lower oxidation potentials, be chemically more stable and therefore form CT complexes more readily (Fig. 21c Breslow et al., 1982, 1984 Breslow, 1985, 1989 LePage and Breslow, 1987). 

A thermal oxidation of 2,3-dimethyl-2-butene, 16, occurs in NaY when the temperature of the oxygen-loaded zeolite in raised above — 20°C [35], Similar thermally initiated oxidations were not observed for the less electron rich tram-or cix-2-butene. Remarkably, pinacolone was conclusively identified as one of the products of the reaction of 16, This ketone is not a product of the photochemical Frei oxidation (vide supra) and underscores the very different character of these two reactions and the complexity of the oxygen/16 potential energy surface, A rationale for the different behavior could lie in the different electronic states of the reactive oxygen-CT complex in the thermal and photochemical reactions. Irradiation could produce an excited triplet-state CT complex ( [16 O2] ) and/ or ion pair ( [16 02 ] ) with different accessible reaction channels than those available to a vibrationally excited ground-state triplet complex ( [16 "02]) and/... 

Depending on the amount of thianthrenium perchlorate, /V,/V,/V, /V, /V",/V"-hexakis (anisyl)-l,3,5-triaminobenzene gives its cation radical, dication diradical, and trication triradical as well (Stickley et al. 1997). These species are stable in methylene chloride at low temperatures (at 298 K they can exist for several days). Spin and charge are localized at each oxidized nitrogen atom. The dication diradical and trication triradical structures are ground-state triplet and quartet molecules (Sato et al. 1997). 

Kinetically, ground-state (triplet-302) oxygen is not very reactive, which obviously restricts the oxidation of food lipids. 3C>2 requires activation to facilitate oxidative reactions. Three principal processes are involved in the activation of oxygen (Fridovich, 1977) ... 

The carbene thus reacts with O2 to form an orffio-benzoquinone O-oxide, and with an aliphatic alcohol as H-donor to form a phenoxyl radical (plus an aliphatic radical not shown in Scheme 1). The ground state triplet electronic configuration of this carbene accounts for its reaction behavior, in particular for the fact that it reacts very slowly with the solvent, H2O. In agreement with the intrinsically faster intersystem crossing of 2-bromophenol compared to 2-chlorophenol, the quantum yield of the carbene pathway was higher for the former = 0.04) than for the latter compound (< = 0.003). In contrast, the quantum yields of photo contraction were comparable (< = 0.04). The transient absorption data were confirmed by photoproduct analysis, showing the formation of phenol from 4-bromophenol in the presence of H-donors [16]. 

When the triplet is an excited state, energy transfer occurs to form singlet oxygen. Ground state triplets react with oxygen by a spin-allowed process which, for carbenes in particular, produce carbonyl oxides [64], It seems that triplet nitrenes react with oxygen slowly. This will be discussed more fully later. Here we examine the products formed from reaction of photolysis of phenyl azide in the presence of oxygen. 

The sensitized oxidation is initiated by the absorption of a light quantum by a sensitizer S under formation of a singlet-excited sensitizer which is transformed to a triplet-excited sensitizer. Singlet oxygen 02 is formed in the system by the energy transfer to the ground state triplet oxygen157-159 ... 

The excited oxygen molecule will decay to the ground state triplet if it does not encounter a suitable alkene and react chemically. The rate of this decay process has been shown to depend strongly on the identity of the solvent. Measured lifetimes range from roughly 700 fxscc in carbon tetrachloride to 2 /itsec in water. It is evident that the solvent can then have a pronounced effect on the efficiency of oxidation the longer the excited state lifetime, the more likely it is that a productive encounter with an alkene will occur. 

---