Photosubstitution resulting from

In the absence of nucleophile, neither the 412 nm species nor the formation of the radical anion, nor that of the photosubstitution product is found. It is concluded therefore that the 412 nm species results from some kind of interaction between the (excited) aromatic compound and the nucleophilic reagent. The character of this aromatic compound-nucleophile-complex is as yet unknown. However, in our present view, the nature of the complex has to allow for the formation of both the radical anion and the photosubstitution product(s). An attractive possibility for this complex remains the a-complex, in formal analogy with the Meisenheimer complexes in the thermal nucleophilic reactions with aromatic compounds. An exciplex forms another possibility. 

The anomeric bromination can result from photosubstitution of other positions in the ring moiety. Very recenly, bromination of fully protected I-2-O-benzylidenated pyranoses with bromotrichloromethane and U.V. light or NBS yields 2-O-benzoyl glycosylbromides which may be converted in situ to glycosides and disaccharides in good yields (Scheme 2). These reaction conditions are compatible with protective groups like esters (acetates, benzoates, tosylates) and silyl ethers. 

Complexes of Cr(III) have received more detailed study than those of any other metal. Generally, these d3 complexes are rather substitutionally inert at 25 °C but do undergo very efficient photosubstitution upon population of the low lying LF excited states. It is interesting to point out now that specific results from Cr(III)... 

The Rh(III)- and lr(lll)-ammines undergo efficient photosubstitution in marked contrast to the Co(III) analogues as mentioned above. However, the efficient photosubstitution found for the second and third row metal systems does appear to obey the essential rationalizations outlined in the models for LF substitutional reactivity. Two sets of complexes seem to behave quite nicely in this regard C4v, Rh(NH3)s X2+ 58) and DAh trans-M(en)2Xl (M = Ir, Rh X = Cl, Br, I).59,6°) In both sets of complexes the lowest excited state is likely one which features population of the d22(a ) orbital. Consistent with this fact, the photosubstitution chemistry can be viewed as resulting from loss of a ligand on the z-axis. For the Rh(III)-amines a dissociative mechanism is suggested for Cl photoaquation by the quantum yield data in Table 8.60) The... 

In jS-chloroketones the C—Cl and C=0 groups show significant interaction, if the two entities are in the correct orientation with respect to each other136. This interaction results in an appreciable cr component in the LUMO of the / -chloroketone, and makes the C—Cl bond potentially photolabile in the excited state12,136. Indeed, irradiation of 4-chloro-2-butanone in methanol has been reported to afford the corresponding 4-methoxy ether in addition to the alcohol resulting from carbonyl reduction. The apparent nucleophilic photosubstitution product, however, turns out to be produced in a dark reaction, catalysed by acid which is formed when methanol solutions are irradiated255. 

The Zr and Hf complexes may be photochemically more labile than the corresponding Ti dicarbonyl , even though CH is formed from its photolysis in the presence of Hj. It is now apparent that irradiation can induce exchange of isotopically labeled CO, and simple photosubstitution products result from irradiation of the complex in hexane in the presence of a trifluorophosphine sparge ... 

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. 

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. 

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... 

On the basis of these results we propose the mechanism shown in Figure 9 for the photolysis of these complexes at T <200K. This reaction is observed for all complexes (CO)sMMn(CO)3(a-diimine). Irradiation into the MLCT band causes photosubstitution of CO of the Mn(CO)3(a-diimine) moiety by 2-Me-THF. The difference in electronegativity between both metal fragments is then so large that raising the temperature causes a heterolytic splitting of the metal-metal bond. The cation formed reacts with CO from the solution. When the temperature is raised to room temperature the ions recombine to the parent compound because 2-Me-THF is then released. 

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