Safieh N. Yazdi

An ESR study of the mercuration of the biphenylene radical cation. Evidence for a homolytic mechanism of mercuration

Abstract When a solution of biphenylene in trifluoroacetic acid containing mercury(II) trifluoroacetate is photolysed with ultraviolet light, the ESR spectra show the progressive mercuration of the biphenylene radical cation in the β-position. β-Proton hyperfine coupling [a(Hβ) 3.58 G] is lost, and 199Hg primary, secondary, and tertiary satellites [a(199Hgβ) 76–77 G] appear untl tetra-β-mercuration is complete. It is concluded that, apart from the usual electrophilic mechanism, an alternative mechanism for mercuration exists, and that this probably involves collapse of the aromatic radical cation, ArH+., with its counterion, HgX2-.: ArH + HgX2 → ArH+. + HgX2 → HAr H − X2 → ArHgX + HX

An electron spin resonance study of the radical cations of pyrroles, furans, and thiophenes in liquid solution

Photolysis of alkylpyrroles in trifluoroacetic acid containing mercury(II) trifluoroacetate, alkylfurans in trifluoroacetic acid, or alkylthiophenes in sulphuric acid, induces oxidation to the corresponding radical cations. The e.s.r. spectra show that the electronic configuration is similar in all three species, the unpaired electron occupying the ψA MO in which the heteroatom lies in a nodal plane. Photolysis of 2,6-dimethyl- and 2,6-diethyl-thiophene in trifluoroacetic acid containing mercury(II) trifluoroacetate, on the other hand, gave rise to spectra with a high g value (2.0062), showing hyperfine coupling to two non-equivalent pairs of alkyl groups in an unsymmetrical dimer.

199 Hg hyperfine coupling in the e.s.r. spectra of mercurated arene radical cations: evidence for an alternative mechanism of mercuration

The mercuration of various arenes is promoted when they are photolysed in trifluoroacetic acid containing mercury(II) trifluoroacetate, and 199Hg hyperfine coupling can be observed in the e.s.r. spectra of the radical cations which are formed.

An electron spin resonance study of cyclopentadiene radical cations

If solutions of hexa-,penta-, tetra-, or tri-alkylcyclopentadienes in trifluoroacetic acid are irradiated with u.v. light, the e.s.r. spectra of the corresponding alkylcyclopentadiene radical cations can be observed. The radical cation Me5C5H+˙ can also be obtained from decamethylbi (cyclopentadienyl), Me5C5–C5Me5 under similar conditions. The spectra of Me6C5+˙, Me5C5H+˙, and Me4C5H2+˙ can be analysed in terms of single isomers. The sp3 carbon atom of the ring lies in the nodal plane of the SOMO, and substituents at this position show only small hyperfine coupling constants. The radical cation Me5C5H+˙ shows no evidence of fluxionality on the e.s.r. timescale up to 328 K. The spectra of Me4EtC5H+˙, Me3Et2C5H+˙, and Me3C5H3+˙ apparently relate to mixtures of prototropic isomers. Experiments with optical cut-off filters show that the primary photochemical process involves excitation of the protonated diene.

The electron spin resonance spectra of the radical cations of furans, thiophenes, and pyrroles in liquid solution

The radical cations of various furans, thiophenes, and pyrroles have been generated in fluid solution and characterised by e.s.r. spectroscopy.

Photolysis of pentamethylcyclopentadiene in the presence of trifluoroacetic acid: generation and e.s.r. spectrum of the pentamethylcyclopentadiene radical cation

The e.s.r. spectrum of the pentamethylcyclopentadiene radical cation, Me5C5H·+, is observed when pentamethylcyclopentadiene is photolysed in trifluoroacetic acid.

The homolysis of C–H bonds in carbocations

The e.s.r. spectra of certain tetra-alkylcyclobutadiene radical cations, of the hexamethylcyclopentadience radical cation, and of various polycyclic aromatic radical cations, RH˙+, are observed when the corresponding hydrocarbon cations, RH2+, are photolysed in trifluoroacetic acid solvent.

Homolysis of conjugated carbanions and carbenium ions

Methods are reported by which fulvalene and pentalene radical anions and cyclobutadiene, cyclopentadiene and buta-1,3-diene radical cations can be prepared in fluid solution and studied by e.s.r. spectroscopy. These preparations illustrate the general principles that radical anions and cations can be regarded as the conjugate bases and acids, respectively, of neutral radicals and that bonds to metals and to hydrogen often show common properties.