Catalyzed photoinduced electron transfer complexation

Trimethylsilyl triflate (McsSiOTf) acts as an even stronger Lewis acid than Sc(OTf)3 in the photoinduced electron-transfer reactions of AcrCO in dichloro-methane. In general, such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C—C bond formation between organosilanes and aromatic carbonyl compounds via the Lewis-acid-catalyzed photoinduced electron transfer. Formation of the radical ion pair in photoinduced electron transfer from PhCHiSiMes to the (l-NA) -Mg(C104)2 complex (Scheme 11) and the AcrCO -Sc(OTf)3 complex (Scheme 12) was confirmed by the laser flash experiments [113]. 

A significant enhancement of reactivity of the carbonyl compound by complexation with Mg2+ has also been applied to a novel type of carbon-carbon bond formation via photoinduced electron transfer from unsymmet-rically substituted acetal (5) with benzophenone (6) (Scheme 26) [211]. This photochemical reaction takes place in the absence of Mg2+ in MeCN. However, the yield of the desired carbon-carbon coupling product 7 is only 15% together with radical dimers 8 (28%) and 9 (2%). Addition of Mg(C104)2 to this system results in a much higher yield of 7 (e.g., 78%) at the expense of radical dimer formation [211]. Thus, the initial photoinduced electron transfer may be catalyzed by Mg2+ to produce a radical ion pair (6 "-Mg2+5 +), where 6 is stabilized by the complexation with Mg2+, as shown in Scheme 26 [211]. The efficient C-C bond formation occurs in the radical ion pair, followed by cyclization before and after desilylation to produce various types of products (Scheme 26). 

Figure 29. Z-scheme of the photoinduced electron-transfer and dark enzymatic reactions operating in the photosynthesis of green plants. Mn = Mn-containing enzyme complex catalyzing water oxidation and O2 evolution Chi a and Chi b = photoactivated primary electron acceptors in photosystems I and II, respectively A and I = primary electron donors in photosystems I and II, respectively ADP = adenosine diphosphate ATP = adenosine triphosphate.

The double bond migration or cis-trans isomerization of linear pentenes catalyzed by a variety of transition metal complexes (Fe(CO)s, Fe3(CO)i2, Ru3(CO)i2) in the presence of irradiation illustrates the operation of case 1.3 [20, 21] (Scheme 3). Case 1.4, which covers photoinduced electron transfer... 

Fukuzumi, S., Satoh, N., Okamoto, T, Yasui, K., Suenobu, T, Seko, Y, Fujitsuka, M., and Ito, O., Change in Spin State and Enhancement of Redox Reactivity of Photoexcited States of Aromatic Carbonyl Compounds by Complexation with Metal Ion Salts Acting as Lewis Acids. Lewis Acid-Catalyzed Photoaddition of Benzyltrimethylsilane and Tetramethyltin via Photoinduced Electron Transfer, /. Am. Chem. Soc., 123, 7756—7766, 2001. 

---