Bruno Hellrung

The photo-Favorskii reaction of p-hydroxyphenacyl compounds is initiated by water-assisted, adiabatic extrusion of a triplet biradical.

The p-hydroxyphenacyl group 1 is an effective photoremovable protecting group, because it undergoes an unusual photo-Favorskii rearrangement concomitant with the fast release (<1 ns) of its substrates in aqueous solution. The reaction mechanism of the diethyl phosphate derivative 1a was studied by picosecond pump−probe spectroscopy, nanosecond laser flash photolysis, and step−scan FTIR techniques. The primary photoproduct is a triplet biradical, 33, with a lifetime of about 0.6 ns. The release of diethyl phosphate determines the lifetime of the triplet state T1(1a), τ(T1) = 60 ps in wholly aqueous solution. Formation of a new photoproduct, p-hydroxybenzyl alcohol (6), was observed at moderate water concentrations in acetonitrile. It is formed by CO elimination from the elusive spirodione intermediate (4), followed by hydration of the resulting p-quinone methide (5). Computational studies show that CO elimination from the spirodione is a very facile process.

Tautomerization of Phenols, Part III , The Anthrone−Anthrol Equilibrium in Aqueous Solution

The equilibrium between 10H-anthr-9-one and 9-anthrol favors the ketone, which ionizes as a carbon acid in aqueous base. Rates of equilibration were measured over the pH range 1 – 13 in aqueous solution (25°, ionic strength I=0. M). Five independent thermodynamic and kinetic parameters were determined by analysis of the pH-rate profile: the equilibrium constant of enolization, pKE=2.17, the ionization quotient of anthrol, pQ=7.84, and the rate constants of enolization catalyzed by acid, k=2.2⋅10−4 M−1 s−1, base, k=51.0 M−1 s−1, and water, k=1.21⋅10−5 s−1. Structure-reactivity relationships strongly support the view that pH-independent enolization of anthrone in water proceeds by rate-determining ionization of the C-acid.

Adamantylidene revisited: flash photolysis of adamantanediazirine

Abstract Irradiation of 2-adamantane-2,3′-[3 H ]-diazirine ( 1 ) in isooctane at room temperature gives two primary photoproducts, 2-diazoadamantane and 2-adamantylidene ( Ad: ), with quantum yields of about 0.5 each. The fluorescence quantum yield of 1 is small, φ f ( 1 ) ≈ 10 −4 . The reactivity of Ad: was studied by nanosecond laser flash photolysis; it is best described as that of a carbene equilibrating between the singlet ground state and a low-lying triplet state. Ad: reacts with the precursor diazirine 1 , pyridine, acetonitrile, tetramethylethylene, amines, water, and acetic acid with rate constants approaching the diffusion-controlled limit, as well as with molecular oxygen ( k O 2 = 2.3 × 10 9 M −1 s −1 ). Reaction of Ad: with 1 ( k x = 2.3 × 10 9 M −1 s −1 ) gives an ylide, λ max = 290 nm, which forms adamantanone azine as a stable product by a reaction having activation parameters E a = 15.5 kcal mol −1 and A = 2 × 10 12 s −1 . At low concentrations of 1 in thoroughly dry, degassed solvents, the lifetime of Ad: reaches ca. 225 ns in isooctane, 500 ns in perfluorodecalin, and 700 ns in benzene. Diazoadamantane forms azine by a second-order reaction with a half-life of several hours at 10 −4 concentration.

Adiabatic triplet state tautomerization of p-hydroxyacetophenone in aqueous solution.

The primary photophysical processes of p-hydroxyacetophenone (HA) and the ensuing proton transfer reactions in aqueous solution were investigated by picosecond pump-probe spectroscopy and nanosecond laser flash photolysis. Previous studies have led to mutually inconsistent conclusions. The combined data allow us to rationalize the excited-state proton transfer processes of HA in terms of a comprehensive, well-established reaction scheme. Following fast and quantitative ISC to the triplet state, (3)HA*, adiabatic proton transfer through solvent water simultaneously forms both the anion, (3)A(-)*, and the quinoid triplet enol tautomer, (3)Q*. The latter subsequently equilibrates with its anion (3)A(-)*. Ionization and tautomerization are likely to compete with the desired release reactions of p-hydroxyphenacyl photoremovable protecting groups.