Photosubstitution activation

The Ru(II) complexes of saturated amines, e.g., RuAjL, where Ru is in the 2+ or 3-1- oxidation state, A is NHj or similar amine such as ethylenediamine/2 and L is a charged or neutral ligand, or of diimines and Ru(AA)jL2, where Ru is in the 2+ oxidation state and AA is a diimine, such as 2,2 -bipyridine (bipy) or 1,10-phenanthroline (1,10-phen), are photosubstitution active. For example, irradiation of aq Ru(NH3) p in the ligand-field (LF) region leads principally to simple photoaquation ... 

Much of the work in the Ru(II) polypyridine area has concentrated on Ru(bpy)3 and its derivatives. The electronic spectra are dominated by an intense charge-transfer band at -450 nm with e = 10" M cm" Ru(bpy)3 " shows modest photosubstitution activity with quantum yields <0.1. ° Photochemical methods have proven useful in the preparation of several derivatives, including the unusual isomer tran5-Ru(bpy)2(OH2)2 and Ru(bpy)2(dmbpy)(NCCH3) ", where dmbpy is the monodentate ligand 3,3 -dimethyl-2,2 -bipyridine. ... 

Aromatic compounds activated by electron donating groups undergo photosubstitution preferentially in the ortho or para position (5.3) 503). 

An interesting alternative mechanism of activation is the photochemical reduction of Pt(IV) to Pt(II) (Fig. 3). In addition to photoreduction, photosubstitution and photoisomerization can also occur, making the photochemistry of Pt complexes difficult to predict and a careful analysis of the photoproducts imperative (21). We have been involved particularly in the development of photochemotherapeutic agents based on Pt(IV) and the study of their photodecomposition and (subsequent) interactions with... 

Fig. 3. Photoactivation of Pt(IV) complexes as a prodrug strategy for metallochemotherapeutics (a) general scheme of prodrug activation by photoreduction (b) photosubstitution and photoisomerization are competing photoreaction pathways, which can result in different reactive species upon reduction (c) an example of a photoactive platinum(IV) diazido complex developed in our lab.

Alternatively, arene displacement can also be photo- rather than thermally-induced. In this respect, we studied the photoactivation of the dinuclear ruthenium-arene complex [ RuCl (rj6-indane) 2(p-2,3-dpp)]2+ (2,3-dpp, 2,3-bis(2-pyridyl)pyrazine) (21). The thermal reactivity of this compound is limited to the stepwise double aquation (which shows biexponential kinetics), but irradiation of the sample results in photoinduced loss of the arene. This photoactivation pathway produces ruthenium species that are more active than their ruthenium-arene precursors (Fig. 18). At the same time, free indane fluoresces 40 times more strongly than bound indane, opening up possibilities to use the arene as a fluorescent marker for imaging purposes. The photoactivation pathway is different from those previously discussed for photoactivated Pt(IV) diazido complexes, as it involves photosubstitution rather than photoreduction. Importantly, the photoactivation mechanism is independent of oxygen (see Section II on photoactivatable platinum drugs) (83). 

Nucleophilic substitution is the widely accepted reaction route for the photosubstitution of aromatic nitro compounds. There are three possible mechanisms11,12, namely (i) direct displacement (S/v2Ar ) (equation 9), (ii) electron transfer from the nucleophile to the excited aromatic substrate (SR wlAr ) (equation 10) and (iii) electron transfer from the excited aromatic compound to an appropriate electron acceptor, followed by attack of the nucleophile on the resultant aromatic radical cation (SRi w 1 Ar ) (equation 11). Substituent effects are important criteria for probing the reaction mechanisms. While the SR wlAr mechanism, which requires no substituent activation, is insensitive to substituent effects, both the S/v2Ar and the Sr+n lAr mechanisms show strong and opposite substituent effects. 

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. 

Substituents on an aromatic ring may show activating and directing effects in electrophilic substitution. The same situation is encountered in nucleophilic photosubstitution. 

More than anything else it was the curious meta-activation found in the photosubstitution of nitrophenyl esters and ethers by various nucleophiles that stimulated closer investigation of this class of reactions (Section 1). In order to avoid misunderstanding, it should be stressed that the concept of activation at the meta-position vwth respect to the nitro-group does not exclude reactions taking place at other positions in the excited molecule. It merely means that, other things being equal, substitution is preferred at the meta-position over the ortho- and para-positions. 

Chlorine and bromine react under favourable conditions (activation by other substituent, appropriate nucleophile). Iodine may also be photosubstituted by nucleophiles but is easily induced to enter into homolytic reaction pathways. One has to bear in mind that the heavier substituents (iodine, bromine and even chlorine) increase the rate of intersystem crossing which, depending on the conditions, increases or decreases the quantum yield. 

In the photochemistry of coordination compounds and organometallics containing mono or bidentate ligands, photosubstitutions occupy virtually the first position in research activity and a number of published papers [1,98, 99]. Photosubstitutions are those processes for which the first rules (Adamson rules [100] enabling photochemists to predict the course and relative efficiency of photoreactions) of photoreactivity in the field of inorganic photochemistry were formulated. Yet there are still a lot of questions to be answered.. 

Inorganic NO donors are good examples of prospective pharmaceuticals activated via their photodissociation, photosubstitution, and photoredox processes. 

A number of results now indicate that irradiation of these complexes can give CO loss but that CO loss does not occur from the MLCT state. The higher energy LF states are responsible for elimination of CO, but the quantum efficiency is poor with 4> = 0.01-0.02. The latter contrasts with the high-effidency CO loss from the parent hexacarbonyls (4> = 0.7-1.0) (167) and is apparently a reflection of rapid internal conversion from the active LF states to the inactive MLCT state. For example, photosubstitution of CH3CN for CO in W(CO)5 C(OMe)Ph was shown to occur cleanly and quantitatively to yield the cis-substituted product shown... 

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