Sergio M. Bonesi

The Aromatic Carbon–Carbon ipso‐Substitution Reaction

The aim of this review is to illustrate what we have dubbed an aromatic carbon-carbon ipso-substitution reaction in which a new Ar-C bond is formed at the expense of another Ar-C bond. The potentiality of several C-based leaving groups including alkyl, carbinol, CN, COOH, and carbonyl groups in arylation reactions will be illustrated accordingly for the preparation of biphenyl-, vinyl-, alkynyl-, and alkyl-substituted aromatics.

Electronic spectroscopy of carbazole and N- and C-substituted carbazoles in homogeneous media and in solid matrix

Abstract The dynamic properties of the lowest excited singlet and triplet states in terms of fluorescence and phosphorescence lifetime, τf and τp, fluorescence and phosphorescence quantum yield, φf and φp, for carbazole and N- and C-substituted carbazoles have been measured in organic solutions of different polarity and in solid matrix, at 298 and at 77xa0K, respectively. From these data, the radiative and the radiationless rate constants ( k f 0 , k isc , k f 0 (77), k isc (77), k p 0 and knr0) and the intersystem crossing quantum yield, φisc, were easily derived. Electronic spectra (absorption, fluorescence and phosphorescence emission spectra) of carbazole and carbazole derivatives have been recorded at 298 and at 77xa0K and the solvent and substituent effects on the spectroscopic data and on the photophysical rate constant have also been analyzed and good linear correlations have been obtained. The values of the HOMO and LUMO energy, the oscillator strength (f), the transition dipole (Δμ) and the wavelengths associated to the electronic transitions, the heat of formation of the carbazoles and the corresponding radical cations (ΔHf and ΔHf(RC)) and the adiabatic ionization potential (Ia) were calculated by using the semiempirical PM3 and ZINDO/S methods and were compared with the spectroscopic and photophysical parameters obtained as well as with the one-electron oxidation potential data (Ep/2) reported for the carbazole series.

Product study of the photolysis of N-acetyl carbazole in ethanol and dichloromethane solution. Part I

Abstract The photochemical reaction of N-acetyl carbazole was studied in ethanol and dichloromethane solution. The reaction products consist of the usual photo-Fries rearrangement products (1-acetyl and 3-acetyl carbazole), carbazole, 4-acetyl carbazole and 3,N-diacetyl carbazole. The photoproducts were identified by gas chromatography-mass spectrometry. They were isolated and analysed by UV and IR spectroscopy, mass spectroscopy and nuclear magnetic resonance spectroscopy, which provided evidence for their structural identification. Electron donor—acceptor (EDA) complex formation was observed in ethanol and dichloromethane and the relationship between the photochemical reactivity and the EDA formation is briefly discussed.

Electronic spectroscopy of N- and C-substituted chlorocarbazoles in homogeneous media and in solid matrix

Abstract Electronic spectra (absorption, fluorescence and phosphorescence emission spectra) of chlorocarbazoles 1a–e, 2a–b, 3a, 4a–b, 5a, 6a, 7a, 8a–c, 9a and 9b in acetonitrile and in solid matrix have been recorded at 298 and 77xa0K. The dynamic properties of the lowest excited singlet and triplet states in term of fluorescence and phosphorescence lifetime, τf and τp, fluorescence and phosphorescence quantum yield, φf and φp have been measured in the same experimental conditions and from these data the radiative and the radiationless rate constants (kf0, kisc, kf0(77), kisc(77), kp0 and knr0) and the intersystem crossing quantum yield, φisc, were derived. The intramolecular heavy atom effect (HAE) on the spectroscopic data and photophysical rate constant was analyzed and the incorporation of chlorine atoms to the carbazole moiety proved their ability to quench the fluorescence emission by spin-orbital coupling. The values of the HOMO and LUMO energy, the oscillator strength (f) and the λmax(abs) associated to the electronic transitions, the heat of formation of the chlorocarbazoles and the corresponding radical cation (ΔHf and ΔHf(RC)) and the adiabatic ionization potential (Ia) were also calculated by using the semiempirical PM3 method after HF/3-21G geometrical optimization, and were compared with the spectroscopic and photophysical data as well as with the one electron oxidation potential data (Ep/2).

Photochemistry of N-acetyl and N-benzoyl carbazoles : photo-Fries rearrangement and photoinduced single electron transfer

Abstract The photochemistry of two N -acyl carbazoles, N -acetyl and N -benzoyl carbazole, in different pure and mixed organic solvents is studied. Depending on the properties of the medium, photo-Fries rearrangement and photoinduced single electron transfer (PSET) processes are observed yielding the former 1-acyl and 3-acyl carbazoles and the latter 3-chloro- N -acyl carbazole. k SV , k Q and φ for fluorescence emission, conversion of N -acyl carbazole and product formation yields have been measured as well as the properties of the N -acyl carbazole radical cations formed during the PSET process (laser flash photolysis experiments). The Rehm-Weller equation is used in order to evaluate the ΔG ° ET of the PSET processes.

Perfluoroalkylation reactions of (hetero)arenes

Perfluoroalkylation reactions of arenes have not been the subject of intense study as has been the case for the trifluoromethylation reactions of aromatics. However, the new synthetic methods proposed for achieving homolytic aromatic substitution reactions with perfluoroalkyl moieties have begun to claim a relevant role in functionalization reactions, as revealed by the interesting properties of arenes with large perfluoroalkyl chains. Methods for achieving Ar–Rf bonding reactions can be classified into thermal and photochemical, which can in turn make use of transition metals or be non-metal catalyzed. Reactions are mainly radical in nature. Radical methods for introducing Rf moieties into arenes have resulted as being the most popular and versatile options available to synthetic chemists.

Photosensitized oxidation of phenyl and tert-butyl sulfides

The photosensitized oxygenation of diphenyl (1), di-tert-butyl (2) and phenyl tert-butyl sulfide (3) was studied. Bimolecular rate constants of singlet oxygen quenching are low (1 to 5 x 10(4) M(-1)s(-1)) since the sulfides are poor nucleophiles due to sterical hindrance (2, 3) or the HOMO on the sulfur atom being a less accessible p(z) orbital (1). The quenching is mainly physical, but chemical reaction leading to sulfoxides also takes place in methanol and, to a lower degree, in acetonitrile. Catalysis by carboxylic acids considerably enhances the rate of sulfoxidation. Inefficiency in the chemical reaction is again due to the poor nucleophilicity of the sulfides, which limits oxygen transfer by electrophilic intermediates such as the protonated persulfoxide.

Photochemistry of 2-acyloxycarbazoles. A potential tool in the synthesis of carbazole alkaloids

The photochemistry of two 2-acyloxycarbazoles, 2-acetyl- and 2-benzoyloxycarbazole, in different solvents has been studied. Irradiation of the 2-acyloxycarbazoles in organic media at 254 or 313 nm yields the [1,3]-migrated photoproducts, 1-acyl-2-hydroxycarbazole, 3-acyl-2-hydroxycarbazole and 2-hydroxycarbazole. The effects of the solvent, the atmosphere and the intensity of the light source on the photochemistry of 2-acyloxycarbazole have been studied. Laser flash photolysis as well as photosensitization experiments were performed in order to determine the photoreactive excited state. Electronic spectra (absorption, fluorescence and phosphorescence emission spectra) of the 2-acyloxycarbazoles have been recorded in homogeneous media at 298 K and in solid matrices at 77 K. The dynamic properties of the lowest singlet excited state in terms of fluorescence lifetime and fluorescence quantum yield have been measured in different organic solvents at room temperature. The photo-Fries rearrangement as a mild and clean one-pot reaction for the preparation of an advanced intermediate precursor in the total synthesis of carbazole alkaloids is described.

THE PHOTOOXYGENATION OF BENZYL, HETEROARYLMETHYL, AND ALLYL SULFIDES

The photosensitised oxidation of benzyl ethyl sulfides in aprotic solvents (benzene or acetonitrile) gives the corresponding aldehydes under mild conditions. This is a general reaction which applies to benzyl derivatives containing either electron-donating or electron-withdrawing substituents and furthermore to hetero analogues such as 2-pyridinylmethyl sulfide (not to the 3-indolylmethyl sulfide, since reaction at the heterocycle moiety competes) as well as to allyl sulfides. In a protic solvent (methanol) these sulfides give the sulfoxides instead (except for the nitrobenzyl derivatives, where the aldehyde remains the major product). Among the α-substituted sulfides tested, the α-phenylbenzyl and the 3-cyclohexenyl sulfide give the corresponding ketone (the latter in a low yield), but the α-methylbenzyl sulfide gives the sulfoxide as the main product. The rate for singlet oxygen quenching and for chemical reaction have been measured for representative benzyl sulfides. The reaction is discussed in the frame of the currently accepted mechanism for sulfide photooxygenation. The key step for oxidative C–S bond cleavage appears to be hydrogen transfer from the activated α position in the first formed intermediate, the persulfoxide. This reaction is inhibited in methanol where the persulfoxide is hydrogen-bonded.

The photophysics of nitrocarbazoles studied by using spectroscopic, photoacoustic and luminescence techniques

The photophysical behaviour of 1-nitrocarbazol and 3-nitrocarbazol in different organic solvents has been studied by using spectroscopic, luminescence and photoacoustic techniques. Absorption spectra and triplet state transient spectra were recorded and formation quantum yields and lifetimes determined. Phosphorescence emission at 77 K and laser induced luminescence in acetonitrile solution at 298 K for the nitrocarbazoles were studied. Results are discussed in terms of the optimized structure of 1-nitrocarbazole and 3-nitrocarbazole and compared with that of carbazole. The nitro group attached to the carbazole moiety induces important changes in the photophysical behavior of those compounds.