Photosubstitution reactions

Photochemical solvolysis reactions are directly analogous to ordinary solvolysis reactions except that the absorption of light energy is necessary for reaction to occur  

For example, substituted phenacyl chloride reacts with ethanol upon photolysis to yield compounds (80) and (81)  

Zimmerman and co-workers have reported that photolysis of p-methoxy-benzyl acetate in aqueous dioxane results in p,p -dimethoxybibenzyl and bidioxane, probably through the intermediacy ofp-methoxybenzyl radicals  

When 3-methoxybenzyl acetate is photolyzed, however, the products expected from free radical coupling are decreased and a substantial amount of 3-methoxybenzylalcohol results. A carbonium ion intermediate has been proposed in the photolysis of this meta ester  

The studies of Havinga and co-workers have resulted in the discovery of many solvolysis reactions which appear to occur via polar or heterolytic path- 

Chromium(in) complexes also undergo photoanation reactions. When aqueous solutions of Cr(H20)6 are photolyzed in the wavelength range of 40Q-575 nm in the presence of Cl or SCN ion, replacement of a water molecule by the anionic ligand occurs  

The quantum yields are in the region of 10 for this photoanation reaction. A comparative study of the photoaquation and photoanation reactions has been carried out by photolyzing Cr(NH3)6 in an aqueous solution containing added chloride ion. For a series of solutions covering a chloride ion concentration range from 0-8 M, the total quantum yield is independent of the chloride ion concentration. Aquation and anation therefore only compete with each other for the vacant coordination position in Cr(NH3)i that is formed by the photodissociation of an ammonia molecule. Since the lowest excited quartet state that leads to photodissociation is very short-lived (t 10 only those molecules present in the 

Photoaquation can also be accompanied by isomerization of the metal complex. An example is found in the photochemistry of cw-Cr(en)2(OH)2, where both photoisomerization to fmn5-Cr(en)2(OH)2 and photoaquation to Cr(en)(0H)2(H20)2 are observed 24). 

For the complex Cr(NH3)4Cl(DMF), three photoreactions can occur. The wavelength-dependent quantum yields for these reactions are shown in Table 2.3. The products are cw-Cr(NH3)4Cl(H20) cw-Cr(NH3)4(DMF)(H20) +, and Cr(NH3)3(H20)Cl(DMF). The predominant pathway is the photochemical loss of DMF, and the reactions involving loss of either DMF or chloride ion lead to products that have isomerized from a trans to a cis stereochemistry. 

Lewis structure models incorporating radical character are useful in the exposition of photochemical reactions of carbonyl compounds, alkenes, dienes, and aromatic compounds that are not substituted with polar groups. When heteroatoms are substituted onto an aromatic ring, considerable charge transfer character can be introduced into both the ground state and the excited state, and ionic resonance structures become more suitable as models for reactivity. 

HOMO (left) and LUMO (right) of benzyl cation (PM3 calculation). 

Resonance representation of Si of nitrobenzene. (Adapted from reference 200.) 

Photosolvolysis of m-nitrophenyl trityl ether. (Adapted from reference 201.) 

---

Cyanopyrazoles are formed by irradiation of pyrazoles in the presence of cyanide ions in photosubstitution reactions. 

The ligand field photosubstitution reactions of d6 hexacoordinate metal complexes. P. C. Ford, Coord. Chem. Rev., 1982, 44, 61-82 (44). 

Nucleophilic aromatic photosubstitution reactions in aqueous solutions and in micellar media has been investigated extensively.42... 

The ortho/para orientation rule of ground state chemistry appears to be followed in the photosubstitution reactions of nitrobenzene derivatives in liquid ammoniaa40) ... 

When o-, m- and p-nitroanisole with 14C-labelled at the methoxy group were irradiated under identical conditions in methanol in the presence of sodium methoxide, only m-nitroanisole underwent methoxy exchange, with the limiting quantum yield (

labelled isotope experiments support a a complex intermediate and indicate an Sjv23Ar mechanism (direct substitution in the triplet state) for this reaction (equation 12) and for 4-nitroveratroles (equation 13). Further evidence from quenching and lifetime experiments also support a direct displacement SAr2Ar mechanism for the photosubstitution reaction of nitroaryl ethers with hydroxide ions13. 

VS. [CHsO]- plots and attributed to photoreduction of the nitro functionality From the translation of the original paper it does not become clear, however, whether and how corrections have been made for photosubstitution reactions, which have been shown to occur when nucleophiles react with excited nitrobiphenyls (see Section B.II.2.). 

Other factors, however, should also be effective. A specific influence of the solvent 101,116) added detergents 12 ) and remote electron donating substituents 119) has been observed. Steric hindrance, which certainly is of influence in the nucleophilic photosubstitution reactions of a-nitronaphthalenes, has been found to alter the reactivity of nitroanisoles... 

As has been indicated in Section 1, the investigations on nucleophilic aromatic photosubstitution reactions during the first ten years brought a wealth of circumstantial data and semi-quantitative results allowing speculation concerning the reaction pathway on the one hand and making evident the need for more quantitative 2ind mechanistic studies on the other hand. 

To test the first hypothesis, solutions of 3,5-dinitroanisole and hydroxide ions were flashed and the absorption spectra at different time intervals after excitation were compared. The absorption ( max 400-410 nm) that remains after all time-dependent absorptions have decayed can be shown to be due to 3,5-dinitrophenolate anion, the photosubstitution product of 3,5-dinitroanisole with hydroxide ion. When the absorption band of the 550-570 nm species is subtracted from the spectrum of the solution immediately after the flash, there remains an absorption at 400-410 nm, which can also be ascribed to 3,5-dinitrophenolate anion. The quantity of this photoproduct does not increase during the decay of the 550-570 nm species. Therefore the 550-570 nm species cannot be intermediate in the aromatic photosubstitution reaction of 3,5-dinitroanisole with hydroxide ion to yield 3,5-dinitrophenolate. Repetition of the experiment with a variety of nucleophiles on this and other aromatic compounds yielded invariably the same result nucleophilic aromatic photosubstitution is, in all cases studied, completed within the flash duration (about 20jLts) of our classical flash apparatus. 

There is no difference in the ability of photogeneration of radicals between systems that show photosubstitution and systems that do not, for example amines with 3,5-dinitroanisole and m-dinitro-benzene, respectively. This indicates that the formation of radicals from excited aromatic nitro-compounds in the presence of nucleophiles has no direct relation with the photosubstitution reaction. 

A. Photosubstitution reactions can be aquation, anation or ligand exchange. (i) Photoaquation reactions of the type... 

Cyanations of benzimidazolium or benzimidazole N-oxides have been reported (93CHE127). Cyanopyrazoles are formed by irradiation of pyrazoles in the presence of cyanide ions in photosubstitution reactions. 

Heteroaromatic Photosubstitutions C. Parkanyi, Bull. Soc. Chim. Belg., 1981,90,599-608. Photosubstitution Reactions of Aromatic Compounds J. Cornelisse and E. Havinga, Chem. Rev., 1975, 75, 353-388. 

The photochemical and thermal substitution reactions of CrIn complexes can follow quite different stereochemical courses. The behavior described by equation (32) is typical thermal aquation of trans-[CrCl2(en)2]+ proceeds with retention of configuration, whereas photoaquation is accompanied by a highly stereospecific (>99% cis product) rearrangement.69 With very few exceptions, it has been found that stereochemical mobility is a general feature of Cr111 photosubstitution reactions. 

Complexes of (( Ir(III) are kinetically inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of [IrBr(NH3)5]2+ [35884-02-7] at 298 K has been measured at -2 x 10-10 s-1 (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid greatly accelerates the rate of substitution via a transient, labile Ir(H) species (169). Optical isomers can frequently be resolved, as is the case of ot-[IrCl2(en)2]+ [15444-47-0] (170). Ir(III) amine complexes are photoactive and undeigo rapid photosubstitution reactions (171). Other iridium complexes... 

The complexation of coordination compounds may make it possible to control their photochemical behaviour via the structure of the supramolecular species formed. For instance, the binding of cobalt(m) hexacyanide by macrocyclic polyammonium receptors markedly affects their photoaquation quantum yield in a structure-dependent manner [8.73-8.77]. It thus appears possible to orient the photosubstitution reactions of transition-metal complexes by using appropriate receptor molecules. Such effects may be general, applying to complex cations as well as to complex anions [2.114]. 

Based on given experimental results and their interpretation the following conclusions on photosubstitution reactions of tetrapyrrole complexes can be drawn ... 

The most efficient photosubstitution reaction, with many applications in synthesis, is the photobromination method developed by FERRIER s group [24]. The substitution of H-5 is regioselective and stereoselective for pentopyranosides [25], hexo-pyranosides [26], 1-6 anhydrosugars [27], uronic acids [24, 28] heterocyclic derivatives from deoxyinosones [29], nucleosides [30, 31], etc.. . using bromine or N-bromo-succinimide in carbon tetracloride under irradiation. 

The irradiation of 2-thiophenemethanol (69) gave dimeric products 70 and 71 (91 JCR(S)166). The same behavior was observed when a bithienyl derivative 73 was used as substrate. A photosubstitution reaction to give 77 was observed in methanol. 

---