Photoinduced reactions oxidative addition

Isomerization and polymerization reactions of alkenes and alkynes are to be expected because both the MLCT and LMCT excited states have diradical character. Both the tetracyanoethylene and dicyanoacetylene 7r-complexes of Pt(PPh3>2 undergo a photoinduced intramolecular oxidative addition reaction to give the cyano vinyl and cyano acetylide complexes of platinum(II) respectively ... 

Pure cis-71 could be isolated in 65% yield and was characterized by means of x-ray structure analysis (Fe-Si 2.270(1)/2.272(1) A). This ds-conformer is exclusively formed. The formation of 71 requires a photoinduced silyl exchange followed by a decarbonylation reaction and further steps of an oxidative addition to the metal [176]. 

Photoinduced oxidative addition of primary silane H3SiC(SiMe3)3 to [WMe(CO)3Cp ] [Eq. (24)] produced a hydridesilylene complex [W(H) = SiH(C(SiMe3)3) (CO)2Cp j (73) in 62% yield in a reaction similar to that shown in Eq. (23). An analog with the CsMe4Et ligand has been prepared similarly ... 

In summary, the triplet (do po) excited states of the d -d metal dimers [Ir(p-pz)(C0D)]2 and Pt2(pop)4 " undergo a variety of photochemical reactions. Electron transfer to one-electron quenchers such as pyridinium cations or halocarbons readily occurs with acceptors that have reduction potentials as negative as -2.0 V. With the latter reagents, net two-electron, photoinduced electron transfer yields d -d oxidative addition products. Additionally, the triplet (da pa) excited state of Pt2(pop)4 apparently is able to react by extracting a hydrogen atom from a C-H bond of an organic substrate. 

There are two pathways for this reaction spontaneous thermal reactions, where AG° of the reaction is negative (thermal oxidative addition (TO A)), and photoinduced oxidative addition (POA), where A G° of the dark reaction can be either negative or positive. 

Oxidative addition with cleavage of H—H, C—H, and Si—H bonds requires a co-ordinatively unsaturated metal species such as four-co-ordinate / -complexes. Thus, although nucleophilic anions such as [Mn(CO)5] react readily with alkyl halides, the reaction with HSiPhg is photoinduced." Irradiation gives [Mn(CO)4]", which can then undergo oxidative addition leading to cis [MnH(SiPhs)(CO)4]-. 

R. A. Faltynek and M. S. Wrighton, /. Amer. Chem. Soc., 1978, 100, 2701. Photoinduced substitution and oxidative-addition reactions of pentacarbonyl manganesef—1) and tetra-car nyl(triphenylphosphine)manganese(—1). 

Since the zerovalent complex Pd(PPh3)3 undergoes photoinduced oxidative addition with chlorobenzene, dimeric palladium(O) complexes are also expected to undergo such reactions. An additional feature of the dimers, however, is the presence of two metal centers that can undergo such a two-electron addition. An example of such a double addition is found in the photoreaction between... 

In a few cases alkyl complexes can be synthesized by the photoaddition of dichloromethane or chloroform to a low-valent transition metal complex, but the usual products from these halogen abstraction reactions are the dichloro complexes. Photoinduced oxidative addition reactions are not always the result of the generation of an excited state that is more reactive than the ground state. An alternative photochemical pathway can involve the initial dissociation of a ligand to generate a coordinately unsaturated intermediate, which is then activated toward oxidative addition of the substrate. An example of such a reaction is... 

Intramolecular photoinduced oxidative addition reactions can also occur. For example, when the platinum(0) complex Pt(C2H4)(PPh3)2 is photolyzed at 280 nm, the product is a cyclometalated platinum(II) complex formed by intramolecular oxidative addition of the ortho carbon-hydrogen bond, followed by ethylene insertion into the intmnediate platinum(II) hydride ... 

The photoinduced oxidation addition to [Rh2(jit-dicp)4] (dicp = 1,3-diisocyanopropane) of n-butyl iodide in MeCN yields chiefly [n-BuRh(/t-dicp)4RhI] . High quantum yields, the fact that the reaction is inhibited by reagents such as benzoquinone and oxygen, and the formation of 1-butanol and butyraldehyde in the latter case, indicate a radical-chain process (116) and (ll ) ... 

In addition, the rate of Oz reduction, forming 02 by electron, is of importance in preventing carrier recombination during photocatalytic processes utilizing semiconductor particles. 02 formation may be the slowest step in the reaction sequence for the oxidation of organic molecules by OH radicals or directly by positive holes. Cluster deposition of noble metals such as Pt, Pd, and Ag on semiconductor surfaces has been demonstrated to accelerate their formation because the noble metal clusters of appropriate loading or size can effectively trap the photoinduced electrons [200]. Therefore, the addition of a noble metal to a semiconductor is considered as an effective method of semiconductor surface modification to improve the separation efficiency of photoinduced electron and hole pairs. 

The TTF-porphyrin dyad 3 was described by the group of Odense.11 The fluorescence of 3 is significantly quenched by the photoinduced electron transfer process. Notably, the fluorescence intensity of dyad 3 increases largely after addition of Fe3 + that oxidizes TTF into TTF" +. Successive reduction of TTF" + is not reported. Nevertheless, it is anticipated that the fluorescence of dyad 3 can be reversibly modulated by redox reactions. In fact, the fluorescence of the supramolecule 4, formed between Zn-tetraphenylporphyrin and a pyridine-substituted TTF (TTF- ), can be reversibly tuned by sequential oxidation and reduction of the TTF moiety in 4.12 It should be noted in this context that the synthetically challenging system associating a porphyrin ring fused to four TTFs (5) was also reported.13... 

Both, strained and unsaturated organic molecules are known to form cation radicals as a result of electron transfer to photoexdted sensitizers (excited-state oxidants). The resulting cation radical-anion radical pairs can undergo a variety of reactions, including back electron transfer, nucleophilic attack on to the cation radical, electrophilic attack on the anion radical, reduction of anion radical, and addition of anion radical to the cation radical. This concept has been nicely demonstrated by Gassman et al. [103, 104], using the photoinduced electron-transfer cydization of y,8-unsatu-rated carboxylic add 232 to y-ladones 233 and 234 as an example (see Scheme 8.65). 

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