Frances L. Cozens

Direct observation of β-fluoro-substituted 4-methoxyphenethyl cations by laser flash photolysis

4-Methoxyphenethyl cations bearing β-fluorine substituents [4-MeOC6H4C+R1R2 with R1= H, R2= CH2F (3), R1= H, R2= CHF2(4), R1= H, R2= CF3(5), R1R2= CF3(6)] are observed as transient intermediates following 248 nm laser photolysis of chloride or bromide precursors ArC(–X)R1R2 in 2,2,2-trifluoroethanol (TFE) and TFE–water mixtures. Corresponding phenethyl radicals are also observed; successive fluoro-substitution has remarkably little effect on the Absorbance(cation): Absorbance-(radical) ratio. Rate constants ks for the decay of the cations have been directly measured, and compared with those of non-fluorinated analogues. In TFE–water mixtures the reactivity order is ArCH+CHF2 > ArCH+CH2F > ArCH+CF3 > ArCH+CH3(1) > ArC+(CH3)2(2) > ArC+(CF3)2, but the differences are small, there being little more than a two-order of magnitude decrease across the series. This remarkably small effect of fluorine had previously been observed in an examination of azide:solvent partitioning ratios kaz:ks. Rate constants kaz have also been obtained in this work, and, with the exception of 2 which reacts somewhat more slowly, these are constant, with values consistent with the azide : cation combination occurring at the diffusion limit for these cations, as had been assumed in the previous work. The small, complex effects of fluoro substitution on the reactivities of these cations have been explained by a model which has, as a key component, an effect of electron withdrawing substituents of forcing the positive charge away from the benzylic carbon to the oxygen of the 4-methoxy group. Evidence for a significant perturbation of the π electron density is available in the present work, in that the absorption maxima of the α-CF3and bis-α-CF3 cations 5 and 6 are shifted 20 nm and 50 nm, respectively, to lower wavelengths than the corresponding α-methyl derivatives 1 and 2. Studies of 6 in 100% aqueous solution reveal a second, slower, transformation following the decay of the cation. Product studies have previously indicated that a fraction of the hydration of this cation occurs at the 4-position of the aromatic ring, producing a quinone methide and products arising from such a species. An unobserved hemiacetal is an intermediate in this sequence. From spectral and kinetic considerations the second process observed with flash photolysis has been identified as representing the breakdown of the hemiacetal to the quinone methide. A complete rate–pH profile for this reaction has been obtained, from which a pKa of 11.0 has been determined for the ionization of the hemiacetal.

Flash photolysis study of a Friedel–Crafts alkylation. Reaction of the photogenerated 9-fluorenyl cation with aromatic compounds

A combination of flash photolysis and product analysis is employed to investigate the reaction of aromatic compounds (ArH) with the 9-fluorenyl cation (Fl+) photogenerated from 9-fluorenol in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP). The availability of the photochemical route to Fl+ means that the reaction of a benzylic-type cation with ArH can be directly followed by flash photolysis. An additional feature with electron-rich ArH is that the cyclohexadienyl cation is observed to grow as Fl+ decays. Thus both cationic intermediates of a Friedel–Crafts alkylation are observed in the same experiment. The formation of the cyclohexadienyl cation is demonstrated to be reversible, or at least quasi-reversible, with the kinetic analysis furnishing absolute rate constants for the formation of this cation, as well as for its loss of H+ and Fl+. Values of kH:kD for benzene: [2H6]benzene and toluene: [2H8]toluene are ∼ 1.5 and demonstrate that Fl+ addition is at least partly reversible with these compounds as well. The Hammett ρ+ value obtained for a series of the less electron-rich ArH is –8, indicative of a transition state with considerable cyclohexadienyl cation character. Anisole shows a negative deviation from the Hammett correlation line, explained by the addition of Fl+ to ArH becoming encounter-controlled. This behaviour is dramatically illustrated in a comparison of data for Fl+ and Br2. For the less electron-rich ArH, rate constants for the two electrophiles are parallel. However, from m-xylene through pentamethylbenzene, the rate with Fl+ is unchanged, while the rate with Br2 increases over 1000-fold. The concept of encounter control with Fl+ is strongly supported by the absolute rate constants, which for the the electron-rich ArH are all in the range 1–2 × 109 dm3 mol–1 s–1, a magnitude typical of diffusion-controlled reactions. The electron-rich ArH also show no intermolecular selectivity since their reactions are encounter-controlled, but have a high intramolecular selectivity. It is suggested that a factor influencing the latter is the reversibility of formation of the cyclohexadienyl cation from the encounter complex.

Laser flash photolysis observation of the 1-p-methoxyphenylvlnyl cation by photoprotonation of p-methoxyphenylacetylene. Comparison with the 1-p-methoxyphenethyl cation.

Abstract Photolysis of p-methoxyphenylacetylene in trifluoroethanol results in the transient 1-p-methoxyphenylvinyl cation 7 which has been detected by laser flash photolysis (λ = 248 nm). This cation shows remarkably similar behavior to the 1-p-methoxyphenethyl cation 4: λmax - 335 nm (7) and 340 nm (4) and rate constants for decay - 1.3 x 106 s−1 (7) and 3.9 x 105 s−1 (4).

Reactivity of halo(pyridinium)carbenes.

The reactivity of chloro- and fluoro(N-methyl-3-pyridinium)carbenes was examined by laser flash photolysis, where the halo(pyridinium)carbenes formed ylides with pyridine, acetonitrile, and acetone. Although the halo(pyridinium)carbenes reacted within the time of the laser pulse, their relative reactivities with a series of alkenes could be obtained from quenching experiments by using carbene-pyridine ylides. Their relative order of reactivity with the alkenes and their poor overall selectivity showed that the halo(pyridinium)carbenes are strongly reactive electrophilic species. Computational studies demonstrated that the alkene (HOMO)-carbene (LUMO) interaction is predominant in halo(pyridinium)carbene-alkene reactions, supporting the electrophilic nature of these species.

Absolute Reactivity of Halo(pyridyl)carbenes

A comprehensive series of halo(pyridyl)carbenes was generated by laser flash photolysis of the appropriate diazirines. Only the chloro- and bromo(2-pyridyl)carbenes and the chloro- and bromo(3-pyridyl)carbenes could be directly observed, but the reactivity of all nine halo(pyridyl)carbenes could be directly studied using the standard and a modified pyridine-ylide approach. The carbenes were all ambiphilic, being highly reactive toward both electron-rich and election-deficient alkenes. Second-order rate constants for these reactions ranged from 2.9 x 10(6) to 3.5 x 10(9) M(-1) s(-1) and depended on both the position of the nitrogen atom within the pyridine ring and the nature of the halogen group, as well as the electrophilicity or nucleophilicity of the alkene. A reactivity trend with respect to the location of the nitrogen within the pyridine ring was observed, with the 4-pyridyl carbenes being the most reactive followed by the 2-pyridylcarbenes and then the 3-pyridylcarbenes being the least reactive. This observed reactivity trend is consistent with the pyridyl ring acting as an overall electron-withdrawing group. The results also show that resonance delocalization of electron density into the nitrogen atom of the 4-pyridyl- and 2-pyridylcarbenes in the transition state significantly reduces the effect of the adjacent halogen (F, Cl, or Br) on the reactivity of the pyridyl carbenes with a series of alkenes.

Flash photolysis observation and lifetimes of the cation intermediates in the intramolecular photoprotonation of tryptamine, tryptophan and their N-methyl derivatives

Abstract Photolysis of the four title compounds in trifluoroethanol results in transient cyclohexadienyl-type cations (λmax ∼ 350 nm, 420 nm) arising from photoprotonation at H-4. Lifetimes are 300 – 500 ns, and are considerably shortened by the addition of water and acetate base. For the two parent compounds there is evidence in acetate buffered solutions for N-H deprotonation resulting in the tautomeric form of the parent compound

Nonthermalized excited states in Ru(II) polypyridyl complexes probed by ultrafast transient absorption spectroscopy with high photon energy excitation

The picosecond excited state dynamics of a series of homoleptic Ru(II) polypyridyl complexes (where LL = bpy, dmb, dmeob, dfmb, or dttb) have been investigated in aqueous solution at room temperature using femtosecond transient absorption spectroscopy with high photon energy excitation. All of the complexes studied produced similar spectroscopic signatures: a near-instantaneous bleach centered at 470-500 nm corresponding to the static absorption spectrum, as well as an intense absorption (475-650 nm) that decayed within the instrument response function (IRF) to form a broad, low-level absorption extending from 500-650 nm. Detailed analyses of both kinetic and spectral parame- ters by singular value decomposition (SVD) indicate that the excited state difference spectra contain contributions from at least three distinguishable species that have been assigned as ligand-based π* ←π * and ligand-to-metal-charge- transfer (LMCT) transitions concomitant with the loss of the ground state metal-to-ligand-charge-transfer (MLCT) tran- sition. Kinetic information extracted at 530 nm (an optical marker for the fully intraligand-delocalized 3 MLCT state) or 660 nm (LMCT transitions) appear to be biphasic in some cases with the amplitude of the IRF-limited component be- coming larger with shorter wavelength excitation. Further, rise dynamics were observed at redder probe wavelengths for Ru(bpy)3 2+ and Ru(dttb)3 2+ . These observations are different from those obtained using lower photon energy excitation and show that excitation wavelength strongly influences the early photophysical events in these Ru(II) complexes.

Intrazeolite Photochemistry. 19. Photochemical and Thermal Behavior of Styrenes within Acidicand Nonacidic Zeolites. Radical Cation versus Carbocation Formation.

Laser flash photolysis of a series of substituted styrenes embedded within the cavities of the large pore zeolite NaY leads to the formation of the corresponding styrene radical cation. The reactivity and spectra of these radical cations embedded within NaY are examined and compared to the reactivity of the same radical cations in solution. It is found that for the highly reactive parent styrene radical cation the zeolite framework provides a strong stabilizing effect. For the 4-methoxy-substituted styrene radical cation the zeolite framework plays less of a role in stabilizing the radical cation as compared to the reactivity of the same radical cation in acetonitrile solution. Rigorous analysis of the thermal stability of 4-methoxystyrene, 4-methylstyrene, and anethole in the zeolite micropores was carried out using two sources of NaY zeolite (Aldrich and The PQ Corporation). It was found that the thermal stability was surprisingly dependent on the source of the NaY zeolite. 4-Methoxystyrene, 4-methylsty...