Mechanism photosubstitution

Figure 2.47 The limiting photosubstitution mechanism for rhodium(III) ammine complexes. (Reprinted from Coord. Chem. Rev., 94, 151, 1989, with kind permission from Elsevier Science S.A., P.O. Box 564, 1001 Lausanne, Switzerland.)...

On the basis of the state correlation diagram depicted in Figure 1, which connects the different stereoisomers of the pentacoordinate fragment [Rh(NH3)4Cl] +, Vanquickenbome etal. infer the main characteristics of the complicated photochemical mechanism in [Rh(NH3)4Cl2]+ and related complexes. This work gave the first evidence that the formulation of electronic selection rules was underlying the photosubstitution mechanism in d and d complexes. In principle, this idea was applicable to any complex with any coordination number. However, at this stage the... 

Other examples involve tris03-diketonate)Cr(III) complexes . These reactions may involve twist-type mechanisms of the excited complexes or may proceed via intermediates related to the photosubstitution mechanisms, either pentacoordinated species formed by reversible bond rupture, or by heptacoordinated species formed by reversible addition of a nucleophile. However, there is little compelling evidence differentiating these mechanisms. Racemic-Cr(acac)j is partially photoresolved in chlorobenzene solution by irradiating with circularly polarized light ... 

Several possible mechanisms of polar nucleophilic photosubstitution in an aryl derivative 238 are portrayed in Scheme 6.93. The first, unimolecular nucleophilic photosubstitution mechanism (SN1 Ar where 1 denotes first-order kinetics), in which an excellent leaving group (X) is heteroly tic ally detached from excited state to form a relatively unstable aryl cation and is subsequently attacked by a nucleophile, is rarely observed.836 838... 

The interpretation of pressure effects on the photochemistry of metal complexes in solution is in some cases limited by information on the partial molar volume of ES species and the difficulty to separate intrinsic and solvational volume contributions. The latter can be resolved in more detail by performing systematic solvent-dependence studies, through which corrections via the application of appropriate solvent parameters can be made in order to extract the intrinsic volume changes that, for instance, control the nature of the photosubstitution mechanism. Other difficulties are the fragmentary nature of much of the published pressure data and the need for a better understanding of the effect of pressure on the rates of photophysical processes. 

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... 

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... 

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. 

It should be emphasized that the wide scope of nucleophilic aromatic photosubstitution does not imply that it will work indiscriminately with any combination of aromatic compound and nucleophile. On the contrary, there are pronounced selectivities. The general picture now arising shows a field with certainly as much variability and diversification as chemists, in the course of growing experience, have learned to appreciate in the area of classical (thermal) aromatic substitution. It is one of the aims of this article to contribute to a description and understanding of the various reaction paths and mechanisms of nucleophilic aromatic photosubstitution, hopefully to the extent that valuable predictions on the outcome of the reaction in novel systems will become feasible. 

A few observations of photosubstitution in lanthanide complexes have been reported. Irradiation into the f—f bands of [Pr(thd)3], [Eu(thd)3] and [Ho(thd)3] (thd is the anion of 2,2,6,6-tetramethyl-3,5-heptanedione) results in substitution of thd by solvent.153 The proposed mechanism involves intramolecular energy transfer from an f—f excited state to a reactive IL excited state which is responsible for the observed ligand loss. Photosubstitution has also been observed upon direct excitation into the ligand absorption bands of [Tb(thd)3].154... 

Photosubstitution of CO in Fe(CO)3(Me2N4) and Fe(CO)2L(Me2N4) complexes proceeds via a dissociative mechanism and allows the synthesis of a variety of mixed ligand iron tetraazadiene complexes (see Table 5).177... 

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. 

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