Photochemical general schemes

Figure 7.2 General scheme for a photochemical reaction which does not occur from initially excited state.

The general schemes for some of the main photochemical reactions are reported in Scheme 1.4 (the relevant chapter number is shown in square brackets). 

When these compounds were treated photochemically with electron-deficient alkenes in the presence of an excess of 1,4-cyclohexadiene, serving as hydrogen donor, reductive addition products were obtained according to the generalized scheme [78] ... 

Iodonium ylides coming from precursors with one or two keto groups react thermally or photochemically with alkenes, alkynes, nitriles and some heterocumu-lenes. The products are 5-membered heterocycles with at least one oxygen atom, as illustrated in Table 10.2. Their formation can be described as the result of a formal [3 + 2] cycloaddition from the mesomeric 1,3-dipole structure of ketocarbenes, according to the general scheme (a = b mostly ethylenic double bonds) ... 

Based upon the current theories of CIDEP and CIDNP, we propose that in many photochemical systems the primary photochemical reaction of the excited triplet state contributes to magnetic polarization via the triplet mechanism. The secondary reaction of the polarized primary radicals may transfer their initial polarization to the "secondary radicals" provided that the radical reactions can compete with the radical spin-lattice relaxation process (59,97). On the other hand, secondary reactions of the primary radical pair or the uncorrelated F pair contribute to polarization by the radical-pair mechanism. A general scheme showing the possible and simultaneous operations of both the... 

For the chain mechanism (thermal or photochemical), we can write the general scheme ... 

Cyclopropanes exhibit similar modes of reactivity. [2Dipolar additions with electron-deficient alkenes and electron-donor-substituted cyclopropanes, additions of electron-rich alkenes to electron-deficient cyclopropanes, a number of radicaloid reactions and intramolecular photochemical cycloadditions are known, which may be described by the general scheme H-2 3. 

The repulsive destabilization of sp -bound fluorine results in peculiar reactivity which has no real counterpart in hydrocarbon chemistry. This quite unique behavior of fluorinated olefins and arenes is summarized under the term special fluorine effect (as opposed to the previously discussed general fluorine effect ). The photochemically induced (Scheme 2.36) or fluoride ion-induced (Scheme 2.37) rearrangements observed can be rationalized as the system trying to reduce the number of energetically unfavorable fluorine atoms bound to sp -hybridized carbon. 

Recent work in ultrafast IR studies has provided new insight into the mechanisms of metal complex radical generation and reactivity. Photochemical studies62 of Et-W (CO)3Cp have shown that the primary photochemical pathway was CO loss, as exemplified in Equation 10.43 for the methyl complex Me-W(CO)3Cp. A side reaction did produce radicals, but this may have occurred from vibrationally hot parent molecules. Both a and p C-H activation processes were observed. A general scheme elucidated for the ethyl complex is shown in Figure 10.3. 

Figure 28. General scheme for the photochemical cis-trans isomerization of rho-dium(III) ammine complexes.

Figure 24. Generalized scheme for OAT via photochemical metal-oxo generation.

Figure 6. Generalized scheme for the formation of photochemical smog (Baird, 1999).

Figure 8. Generalized scheme for the photochemical and thermal reaction A B (31)...

The main features are the molecular ions as the base peak and the M-t-1 ions arising from another species. For 2-aminothiazole the m/e 73 ion (M-HCN) is shifted to m/e 75 in the spectrum of the dideuteroamino derivative and, therefore, largely arises via rupture of 2-3 and 4-5 bonds (Scheme 18). This fragmentation process could involve the kind of intermediates postulated in photochemical rearrangements (see Chapter III, Section IX.3.B). The other fragments fit well the general pattern of fragmentation proposed by Clarke (136). 

Moreover, block copolymers with two radically polymerizable monomers can be synthesized with a combination of thermal and photochemical polymerizations. Regarding their utilization in block copolymer synthesis, azocompounds with photoactive benzoin [103,109-111] and azyloximester groups [112] have been described. Two low-molecular weight azo benzoin initiators of the general formula (Scheme 32) were synthe-... 

In the frame of photochemical research, phosphazene polymers have also been exploited in combination with external reagents able to selectively absorb impinging light and induce reactivity on these materials. The general reaction scheme is shown below. 

Similarly, the addition of a variety H donors under photochemical conditions generally provided mixtures of products 52-54 (Scheme 14) (77CL1127). When xanthene was used as the hydrogen donor, structures 52 usually prevailed. On the other hand, no structures 52 were detected in the case of etheral donors such as tetrahydrofuran or 1,4-dioxane. 

---