In nucleophilic aromatic photosubstitution, just as in aromatic substitution in the ground state, substituents can have directing and activating effects. Four rules, which describe orientation of nucleophilic substitution in the excited state, can now be formulated. They are: (a) meta-activation by the nitro group (b) ortho/para-activation by the methoxy group (and probably also by other electron-donating groups) (c) 'x-reactivity' in bi-and tricyclic aromatics (i.e. position 1 in naphthalenes and azulenes, 9 in phenanthrene, 2 and 4 in biphenyls, etc.) (d) merging (resonance) stabilization during product formation.In most nucleophilic aromatic photosubstitutions the reaction proceeds via a it,ir excited triplet state, which interacts with the nucleophile. In some cases the reaction starts from a ir,it excited singlet and in some others there are indications that the aromatic molecule in its excited state undergoes dissociation, producing an ion which subsequently reacts with the nucleophile.Kinetic measurements have revealed the absolute necessity of using rate constants instead of quantum yields as a measure of reactivity. Rate constants for the process in which the triplet excited molecule undergoes deactivation by the nucleophile, leading to substitution product, have been determined for some naphthalene derivatives.From flash photolytic investigations in the nanosecond region evidence has been obtained for the occurrence of a complex (possibly a sigma-complex) formed from the nucleophile and the aromatic molecule in its reactive excited state. The complex may dissociate back into the starting materials or lead to substitution products, while a third route gives rise to the radical anion. The latter cannot be intermediate in the route to substitution product; instead, it may undergo protonation and be transformed into reduction products.INTRODUCTION In this paper a summary is given of our present knowledge on aromatic photosubstitution. The following aspects will be treated: (a) orientation rules; (b) the nature of the reactive excited state and of the primary step; kinetic results; (c) intermediates in aromatic photosubstitution.
* Author to whom correspondence should be addressed Abstract. The influence of the nitro group on the aromatic n-system of pyrene has been studied by comparing the spectroscopic and photochemical properties of the three mononitropyrenes. Whereas the UV and mass spectra of 1-and 4-nitropyrene show an interaction normal for nitro-aromatic compounds, this is not observed for 2-nitropyrene. The lack of interaction is reflected in a UV spectrum very similar to that of pyrene and a mass spectrum with a very low abundance of M-NO. The photochemical behaviour of the three compounds is governed by the degree of interaction. 1 -Nitropyrene shows the nitro-nitrite rearrangement leading to 1-hydroxypyrene (88%) and l-hydroxy--2-nitropyrene (7%). The photoproducts of 4-nitropyrene are pyrene (9%) and unstable products which react with the solvent. 2-Nitropyrene is very stable under photochemical conditions due to lack of interaction. Similarly, the sterically hindered 1-methyl-2-nitropyrene is also very stable towards light. The photochemical nitro-nitrite rearrangement observed for nitro-aromatic compounds was found to be governed by electronic effects.
m.p.= 123-125 "C] from 11. 12 reacts with trimethyloxonium tetrafluoroborate to yield 3,6-bis[bis(methylthio)methylenel-1,4-dimethyl-2,5-bis(methylthio)3,6-dihydropyrazindium bis(tetrafluorob0rate) 13 [83%, orange needles, blackens above 225 "C; UV/VIS (CH,CN): A, , , (lgE)=205 (4.15), 300 (sh, 4.09), 447 nm (4.21)]. M e S K S M e M e S +SMe SMe 14 CN 17 Me S M e S K S M e 15 16 a : CH2C12, 25 "C, 15 h; b : CH2Cl2/H20, 25 "C, 2 h; c: CH,CN, 25 "C, 15 h; d : pyridine, A, 1 h. 13 contains two ambifunctional azaallyl cation moieties. Heating with malononitrile in pyridine gives the tetrakis(methy1ene)piperazine derivative 17 [80%, yellow needles, m.p.= 189.5-191 "C; UV/VIS (CH,CN): A, , , (lg E ) = 225 (4. lo), 274 (4.30), 360 nm (4.40)], and this is the product expected from the charge distribution in 13. The site at which malononitrile attacks the C atom of the C=N bonds is confirmed by the fact that the salt obtained from 12 and triethyloxonium fluoroborate also reacts with malononitrile to afford 17. In the same way, 13 reacts with sodium cyanide and sodium sulfide to yield 16 [29%, m. p. = 162.5-163 "C] and the episulfide 15 [57%, m.p. = 170.5-171 "C (decomp.)], respectively, which is interesting inter alia because of its structural relationship with the 3,6-epithio-2,5-piperazinediones (~f.[~]).Remarkably, sodium methanethiolate reacts with 13 to
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