Photosubstitution mechanism production

A quick survey of the photochemistry of the different complexes described above shows that the mechanism of photoactivation and the subsequent nature of the observed photoproducts varies from complex to complex and from one geometric isomer to another. Photochemical pathways often involve a combination of photosubstitution, photoisomerization, and photoreduction steps. In general, photolysis is rather slow in water and many different products are obtained if the complex is irradiated alone. The presence of nucleophilic biomolecules, on the other hand, can have a major influence, as photoreduction is usually rapid and accompanied by simpler reaction pathways. NMR methods... 

Albini and his co-workers have reported a new type of photoreaction of 1,4-dicyanonaphthalene with toluene and its related compounds to give the substitution products, the reductive arylmethylation products, and 1,3-photoad-dition products [135,456-465] (Scheme 123). They reviewed a series of photoreactions and discussed a mechanism in detail [36], They also reported the photosubstitution of TCNB by use of 2,2-dimethyl-l,3-dioxolane and 1-cyanoadamantane. 

The second photochemical reaction which was studied was the reaction of CotCO NO with Lewis base ligands L (J 6 ). The observed solution phase photochemical reaction is carbonyl photosubstitution. This result initially did not appear to be related to the proposed excited state bending. Further reflection led to the idea that the bent molecule in the excited state is formally a 16 electron coordinatively unsaturated species which could readily undergo Lewis base ligand association. Thus, an associative mechanism would support the hypothesis. Detailed mechanistic studies were carried out. The quantum yield of the reaction is dependent on both the concentration of L and the type of L which was used, supporting an associative mechanism. Quantitative studies showed that plots of 1/ vs. 1/[L] Were linear supporting the mechanism where associative attack of L is followed by loss of either L or CO to produce the product. These studies support the hypothesis that the MNO bending causes a formal increase in the metal oxidation state. 

Sterically hindered a-haloketones such as a-Cl- or a-Br-isobutyrophenone yield only a light-induced C-alkylation product with Me2C=N02 in DMSO when the phenyl ring carries ap-nitro or ap-cyano substituent (equation 53)247. This photosubstitution meets the usual criteria for an S l reaction mechanism. So does the photoreaction of the bridgehead chloride 63 with diphenylphosphide ions in liquid ammonia (equation 54)248. The reactivity of 63 is larger than that of similar non-keto bridgehead chlorides in S l reactions. 

Chromium(III) photosubstitution can be discussed in terms of two limiting mechanisms [97] (1) intersystem crossing from quartet states formed by initial excitation to the doublet manifold followed by reaction from the relatively long-lived D0 or (2) reaction from the quartet state(s) via thermally promoted back-intersystem crossing from the doublet and/or promptly upon formation via initial excitation. Various elaborations on these themes include (3) competitive ligand labilization from both states and (4) formation of a GS intermediate followed by competitive reactions to give products or reform reactants. 

The photosubstitution of 1,4-dicyanobenzene by use of allyl-trimethylsilane in acetonitrile giving l-allyl-4-cyanobenzene is atypical example as shown in Scheme iP Similar photoreactions of 1,2-and 1,4-dicyanobenzenes and 1,2,4,5-tetracyanobenzene using tetraalkylsilanes, benzylsilanes, and disilanes occur to give substitution products regioselectively. The photoreaction of 1,4-dicyanobenzene with 3-methyl-2-butenyltrimethylsilane gives both a- and y-allylated products in a 2 3 ratio. This ratio is not dependent on the solvent used or the presence of additives. These results clearly support a mechanism involving formation of allyl and benzyl radicals as intermediates. 

M—M Bonds Another important photochemical process is the homolysis of M—M bonds. The fragments produced are likely to be odd-electron and therefore substitutionally labile. For example, the photosubstitution of CO in Mn2COio by PPh3 proceeds via the 17e intermediates Mn(CO)5. Equation 4.57 is an interesting example, because the replacement of three COs by the non-ir-acceptor NH3 leads to a buildup of electron density on the metal. This is relieved by an electron transfer from a 19e Mn(CO)3(NH3)3 intermediate to a 17e Mn(CO)5 fragment to give the disproportionation product 4.20 in a chain mechanism. ... 

Arakawa and coworkers [45] developed the on-line photoreaction cell depicted in Figure 5.3 and performed a series of studies on the detection of reaction intermediates in photosubstitution and photooxidation of Ru(II) complexes. The photosubstitution of Ru(bpy)2B [bpy = 2,2 -bipyridine B = 3,3 -dimethyl-2,2 -bipyridine (dmbpy) or 2-(aminomethyl)pyridine (ampy)] was studied in acetonitrile and pyridine. Irradiation of Ru(bpy)2B and related complexes yields a charge-transfer excited species with an oxidized Ru center and an electron localization on the bpy moiety. The excited-state complex underwent ligand substitution via a stepwise mechanism that includes ant] bidentate ligand (Scheme 5.6). Photoproducts such as Ru(bpy)2S2 (S = solvent molecule) and intermediates with a monodentate (mono-hapfo-coordination) B ligand, Ru(bpy)2BS, and Ru(bpy)2BSX+ (X=C10/, PF ) were detected. Other studies also identified photo-oxidized products of several mixed-valence Ru(II) complexes upon irradiation (7i> 420 nm) [31b, 46]. 

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