Photochemically Induced Processes

In addition to the dark oxidation of S(IV) on surfaces, there may be photochemically induced processes as well. For example, irradiation of aqueous suspensions of solid a-Fe203 (hematite) containing S(IV) with light of A > 295 nm resulted in the production of Fe(II) in solution (Faust and Hoffmann, 1986 Faust et al., 1989 Hoffmann et al., 1995). This reductive dissolution of the hematite has been attributed to the absorption of light by surface Fe(III)-S(IV) complexes, which leads to the generation of electron-hole pairs, followed by an electron transfer in which the adsorbed S(IV) is oxidized to the SO-p radical anion. This initiates the free radical chemistry described earlier. 

Not only is thermal fluorodediazoniation possible, but the corresponding photochemically induced process is also possible (see Vol. E 10a, p 703). Several aromatic fluorides can be prepared via diazotization and photochemically induced fluorodediazoniation of aromatic amines in anhydrous hydrogen fluoride-organic base solutions.41-42 In general, no fluoroaromatic compounds can be isolated if the fluorodediazoniation stage is carried out without photochemical irradiation under similar conditions (rt). 

Figure 1 The four stereochemically distinct modes of electrocyclic ring closure wherein the notations inward and outward refer to the axes of rotation as viewed with respect to ( ). The principle of the conservation of orbital symmetry predicts disrotatory (dis) or coniotatory (con) ring closure for the thermally or photochemically induced processes, reqwctively. The same modes (dis or con) are predicted for the reverse ring opening processes...

The following types of thiopyran isomerizations have been reported in the last decade valence-bond tautomerism, endocyclic hydrogen shifts and migration of nonhydrogen substituents. Thermal processes will be mentioned here and photochemically induced isomerizations will be discussed in Section V,I. 

Besides the numerous examples of anionic/anionic processes, anionic/pericydic domino reactions have become increasingly important and present the second largest group of anionically induced sequences. In contrast, there are only a few examples of anionic/radical, anionic/transition metal-mediated, as well as anionic/re-ductive or anionic/oxidative domino reactions. Anionic/photochemically induced and anionic/enzyme-mediated domino sequences have not been found in the literature during the past few decades. It should be noted that, as a consequence of our definition, anionic/cationic domino processes are not listed, as already stated for cationic/anionic domino processes. Thus, these reactions would require an oxidative and reductive step, respectively, which would be discussed under oxidative or reductive processes. 

Another photochemically induced domino process consisting of three steps was employed for the formation of 1,2-disubstituted cyclopentanes 5-39, as described by Tietze and coworkers. Irradiation of a mixture of 5-36, dimethyl malonate and catalytic amounts of the Lewis acid Me2AlCl in a Pyrex flask caused a Norish Type I cleavage of 5-36, followed by an intramolecular hydrogen shift to give the acyclic... 

To date, only one example of a combination of a photochemically induced transformation with a transition metal-catalyzed reaction has been found in the literature. This hv/Pd°-promoted process allows the synthesis of five-membered cyclic y-keto esters 5-119 from 5-iodoalkenes 5-117 in the presence of CO and an alcohol 5-118 as a nucleophile (Scheme 5.24) [41]. The yields are high, and differently substituted iodoalkenes can be employed. 

To date, no examples have been found in the literature for the combination of photochemically induced transformations with reductive/oxidative or enzymatic processes. 

During the past few years, increasing numbers of reports have been published on the subject of domino reactions initiated by oxidation or reduction processes. This was in stark contrast to the period before our first comprehensive review of this topic was published in 1993 [1], when the use of this type of transformation was indeed rare. The benefits of employing oxidation or reduction processes in domino sequences are clear, as they offer easy access to reactive functionalities such as nucleophiles (e. g., alcohols and amines) or electrophiles (e. g., aldehydes or ketones), with their ability to participate in further reactions. For that reason, apart from combinations with photochemically induced, transition metal-catalyzed and enzymatically induced processes, all other possible constellations have been embedded in the concept of domino synthesis. 

The differences between thermally and photochemically induced reactions of 2 are illustrated by the photolyses of cyclotrisilane 2 in the presence of disparate isocyanides. While in the thermally induced process ring-enlarged molecules can be isolated, the photolytic process results in the formation of 2,4-disila- and 3,4-disilacyclobutanediimines. A plausible explanation for the formation of these... 

Summary Multifunctional (meth)acrylate alkoxysilanes synthesized from commercially available acrylate compounds and mercapto-substituted alkoxysilanes or hydrosilanes are used as novel precursors for inorganic-organic copolymers. The alkoxysilyl groups are available for the formation of an inorganic Si-O-Si backbone by sol-gel processing. The (meth)acrylate groups allow the additional formation of organic polymer units by thermally or photochemically induced polymerisation reactions. 

Relatively few examples of photochemically induced 6n -electrocyclizations of azahexa-l,3,5-trienes to azacyclohexa-1,3-dienes have been documented the reverse process involving ring cleavage, however, is more common. The formation of azalumazin-7-ones 19 from triazahexatrienes 20 under oxidative conditions can be regarded as an example of such a process,18 but... 

The photoreactions of four-membered nitrogen containing heterocycles have been less well investigated and meaningful results are confined to certain azetidines and azetidinones. The photochemically induced rearrangement of azetines 148 to 1,4-diazepines 149 has been rationalized in terms of a process equivalent to a di-7t-methane rearrangement116 a more... 

Various types of photochemically induced 1,3-shifts have been observed in nitrogen containing heterocycles. Concerted [1,3] suprafacial sigmatropic reactions are photochemically allowed processes, but many of the reported transformations especially those which arise by n->n excitation un-... 

---