Photochemical Generation of Highly Destabilized Vinyl Cations:  The Effects of α- and β-Trifluoromethyl versus α- and β-Methyl Substituents

Abstract

The photochemical reactions in methanol of the vinylic halides 1-4, halostyrenes with a methyl or a trifluoromethyl substituent at the alpha- or beta-position, have been investigated quantitatively. Next to E/Z isomerization, the reactions are formation of vinyl radicals, leading to reductive dehalogenation products, and formation of vinyl cations, leading to elimination, nucleophilic substitution, and rearrangement products. The vinyl cations are parts of tight ion pairs with halide as the counterion. The elimination products are the result of beta-proton loss from the primarily generated alpha-CH(3) and alpha-CF(3) vinyl cations, or from the alpha-CH(3) vinyl cation formed from the beta-CH(3) vinyl cation via a 1,2-phenyl shift. The beta-CF(3) vinyl cation reacts with methanol yielding nucleophilic substitution products, no migration of the phenyl ring producing the alpha-CF(3) vinyl cation occurs. The alpha-CF(3) vinyl cation, which is the most destabilized vinyl cation generated thus far, gives a 1,2-fluorine shift in competition with proton loss. The experimentally derived order of stabilization of the vinyl cations photogenerated in this study, alpha-CF(3) < beta-CF(3) < beta-CH(3) < alpha-CH(3), is corroborated by quantum chemical calculations, provided the effect of solvent is taken into account.

Photochemical Generation and Structure of Vinyl Radicals

“…Irradiations of the α‐methylstyrene bromides 1 , and the α‐fluorostyrene bromides 2 in methanol yield vinyl radical‐derived products. Homolysis is a minor reaction pathway for 1 ,17 but is effectively the only bond rupture process observed for 2 21. In Table 1 the quantum yields of formation of the two reductively dehalogenated isomers c and d and their ratio observed in the irradiation of the E ‐ and Z ‐isomers of 1 and 2 are summarized.…”

“…The E ‐ and Z ‐isomers were separated using column chromatography (silica/petroleum ether, boiling range 40–60 °C) in >98 % purity. Their spectroscopic characterizations are reported in ref 17…”

“…Photoproduct ( E )‐1‐phenylpropene ( 1c ) is commercially available and ( Z )‐1‐phenylpropene ( 1d ) was prepared from 1c by irradiation in acetone using light of λ exc = 313 nm 17…”

Synthesis and biological evaluation of novel <i>N</i>-pyridylpyrazole derivatives containing 1,2,3-triazole moieties

“…8) Further reduction using lithium aluminum hydride as a reductant produced alcohol 9, which was oxidized to give aldehyde 10 using PCC. 19) Then, compound 10 reacted with methylidene triphenyl phosphorane in the presence of potassium tert-butoxide to synthesize compound 11 via the Witting reaction, 20) subsequent addition reaction with bromine to afford bromide 12. The key intermediate 13 were carried out in various conditions as shown in Fig.…”

Ab initio structural studies of cyclobutylmethyl cations: effect of fluoroalkyl groups on the relative stability of the carbocations

“…However, an α-monofluoromethyl carbocation, 1-fluoro-2-propylium cation (CH 3 CH + CH 2 F), was characterized as a transient intermediate in the gas phase, 6 and an extremely unstable α-trifluoromethylvinyl cation, 3,3,3-trifluoro-1-phenyl-2-propenium cation, was generated through a photochemical reaction. 7 We have earlier demonstrated that the π-delocalized, α-trifluoromethylallyl cations 1-3, formed in superacidic media at low temperatures, have relatively low C 1 -C 2 rotational barriers of about 9 kcal/mol, implying that the positive charge is substantially localized on the tertiary carbon (C 3 ). 3 That is, the positive charge is disfavored at the C 1 due to the strong electron-withdrawing inductive effect of the trifluoromethyl group.…”