Smooth Synthesis of Aryl- and Alkylanilines by Photoheterolysis of Haloanilines in the Presence of Aromatics and Alkenes

Abstract

Irradiation of 4-chloro-N,N-dimethylaniline in acetonitrile in the presence of benzene and of various alkenes leads to heterolytic dehalogenation and trapping of the cation. 4-(Dimethylamino)biphenyl is formed in the first case, while with alkenes β-chloroalkylanilines, stilbenes, or allylanilines are obtained depending on the alkene structure. 4-Fluoroaniline is similarly dehalogenated.Phenyl cations have been the subject of active research from the mechanistic point of view. 1 These are usually generated under proper conditions from diazonium salts 2 and have been spectroscopically characterized both in matrix 3 and in the gas phase. 4 In the few case where the products from the reaction in solution have been characterized, both reduction and addition to O-and N-centered nucleophiles have been found to occur. 2a,5 On the contrary, C-C bond formation does not involve the cation. When biaryls are formed from diazonium salts, as in the Gomberg-Bachmann or in the Pschorr reactions, 6 the actual reacting species is the aryl radical. Recently, however, Steenken and McClelland 7 reported the formation of biphenyls by irradiation of phenyldiazonium salts in the presence of benzene derivatives and showed that the process involved the cation.We are pleased to report in the following an alternative generation of aryl cations and to show that this gives rise to synthetically useful reactions.Irradiation of both 4-chloro-and 4-fluoro-N,N-dimethylaniline (1a,b) in acetonitrile caused reductive dehalogenation to 2, but important products were also the corresponding 2,4′bis(N,N-dimethylamino)biphenyls 3 (ca. 30%, see Scheme 1). This appeared a clear indication that a phenyl cation was formed under this condition.

Unexpected Photoreactions of Some 7-Amino-6-fluoroquinolones in Phosphate Buffer

“…2.8 eV), [8] thus giving E red (1b 3 *) ഠ 1.45 V. The potential of the couple HPO 4 Ϫ· /HPO 4 2Ϫ has not yet been reported, but we estimate it to be significantly lower than 1.9 V vs. NHE. This is in agreement with the weaker oxidative power of HPO 4 Ϫ· as compared to H 2 PO 4 · (the potential of the couple H 2 PO 4…”

“…[16] We expect the reduction of 1b 3 * by the HPO 4 2Ϫ anion to be close to thermoneutral on the basis of the following data. For the sake of simplicity, the discussion is limited to the more reducing HPO 4 2Ϫ anion (see note below), although the monoanion H 2 PO 4 Ϫ is also present in the mixture. Quenching constants are always referred to the total phosphate concentration.…”

“…Ϫ is close to 2.4 V vs. NHE) [17] and is inferred by taking into account the fact that HPO 4 Ϫ· is capable of oxidizing bromide to bromine with a rate constant 6.5·10 6  Ϫ1 s Ϫ1 . [18] This relatively slow rate is consistent with a potential lower by several tenths of a volt than that of the Br/Br Ϫ couple, viz.…”

“…In phosphate, however, the overall process involves fluorine loss but does not correspond to simple defluorination. The reasons are twofold: first, the environment is non-reducing, the only potential hydrogen donor being the piperazine group, and second, the electron-transfer step in this case generates an aggressive inorganic intermediate, the radical anion HPO 4 Ϫ· . It has previously been demonstrated that this species is an efficient hydrogen abstractor (k H ϭ 7Ϫ9·10 7  Ϫ1 s Ϫ1 with pentoses in water).…”

“…This reaction would appear to be of interest, since there are very few examples of fragmentations of this strong bond. To date, photoinduced defluorination has been realized for 4-fluorophenol, [2] some polymethoxyfluorobenzenes [3] and 4-fluoroaniline, [4] as well as some 7-amino-6-fluoroquinolones (used in therapy as antibacterial drugs). [5Ϫ9] A study of the photochemistry of the latter compounds in neutral water showed that defluorination was the almost exclusive process.…”