Anisole singlet states

In this respect, it is of interest that the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile proceeds with retention of ( )-olefin geometry in acetonitrile from the first excited singlet state of the anisole chromophore [160] (Scheme 40). The ortho adduct undergoes further... 

A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

Figure 8. Charge distribution of anisole in the ground state and in singlet and triplet excited states (see Lodder and Havinga, 1972).

A closely related tetrad featuring two porphyrin moieties and a single naphthoquinone acceptor has also been reported [13]. Excitation of either porphyrin moiety of C-P-P-Q in anisole solution is followed by rapid (>10 s ) singlet-singlet energy transfer between the two porphyrins, whose absorption and emission spectra are essentially identical. C-P- P-Q decays by photoinduced electron transfer to the quinone with a rate constant of 2.4 x 10 s. Sequential transfer of the radical cation hole to the second porphyrin, and then to the carotenoid yields a final C +-P-P-Q state with a quantum yield of 0.25 and a lifetime of 2.9 ps. 

The role of singlet and triplet nitrenium ions in these reactions has been evaluated.228,229 Convincing evidence for the participation of singlet ground-state 2-acetylphenyl nitrenium ions during the photodecomposition of 3-methylanthranils in concentrated sulfuric acid has been obtained.229 For example, in the presence of arenes or anisole, in addition to the usual products, substantial amounts of diphenylamines and 3- and 5-(p-methoxy-phenyl)-2-aminoacetophenones, respectively, are formed by electrophilic attack of the nitrenium ion on the added substrate. 

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