Ultrafast Adiabatic Photodehydration of 2‐Hydroxymethylphenol and the Formation of Quinone Methide

Abstract

The photochemical reactivity of 2-hydroxymethylphenol (1) was investigated experimentally by photochemistry under cryogenic conditions, by detecting reactive intermediates by IR spectroscopy, and by using nanosecond and femtosecond transient absorption spectroscopic methods in solution at room temperature. In addition, theoretical studies were performed to facilitate the interpretation of the experimental results and also to simulate the reaction pathway to obtain a better understanding of the reaction mechanism. The main finding of this work is that photodehydration of 1 takes place in an ultrafast adiabatic photochemical reaction without any clear intermediate, delivering quinone methide (QM) in the excited state. Upon photoexcitation to a higher vibrational level of the singlet excited state, 1 undergoes vibrational relaxation leading to two photochemical pathways, one by which synchronous elimination of H O gives QM 2 in its S state and the other by which homolytic cleavage of the phenolic O-H bond produces a phenoxyl radical (S ). Both are ultrafast processes that occur within a picosecond. The excited state of QM 2 (S ) probably deactivates to S through a conical intersection to give QM 2 (S ), which subsequently delivers benzoxete 4. Elucidation of the reaction mechanisms for the photodehydration of phenols by which QMs are formed is important to tune the reactivity of QMs with DNA and proteins for the potential application of QMs in medicine as therapeutic agents.

Photochemical Reactivity of Naphthol-Naphthalimide Conjugates and Their Biological Activity

“…One generally sees very low values for Φ R , which are due to FRET from the naphthol to the naphthalimide. Such a result was not anticipated in the first instance, since it is known that photodehydration takes place in an ultrafast photochemical reaction [ 82 ], which in principle could be in competition with FRET.…”

Oxetanes and Oxetenes: Fused-Ring Derivatives

“…Benzoxete was also detected by irradiation of a vapor deposition prepared sample of 2- hydroxymethylphenol at 254 nm and 14 K. The characteristic signal at n = 1668 cm -1 (scissoring vibration of the CH2 motif) was clearly observed. 22…”

Photogeneration of quinone methide from adamantylphenol in an ultrafast non-adiabatic dehydration reaction

“…It was demonstrated that the corresponding QM 2 ( Scheme 1 ) is generated in an ultrafast adiabatic reaction on the S 1 potential energy surface, taking place in less than 1 ps and generating the QM intermediate in its excited S 1 state. 29 However, it is not a general mechanism for the photodehydration, since the formation of QMs may involve benzoxete derivatives as intermediates, 23 or it may proceed via phenoxyl radicals that undergo a homolytic cleavage of the alcohol OH group. 27 …”