Abstract
The liquid-phase photo-Claisen and photo-Fries rearrangement dynamics of allyl phenyl ether and phenyl acetate in cyclohexane solution have been interrogated via ultrafast transient absorption spectroscopy. Following excitation at 267 nm, the reaction progress is monitored on a picosecond time-scale by electronic and vibrational absorption spectra obtained from broadband UV/Visible and mid-infrared probe pulses. The evolution of the ground and excited electronic states of the parent molecule, the radicals produced by photo-induced homolytic bond fission, and intermediate cyclohexadienones formed via recombination of the produced radical pair are followed, providing new insight and detail on the reaction mechanisms. Subsequent kinetic analysis allows determination of rate coefficients as well as quantum yields for the processes involved. These examples serve to highlight the utility of employing broadband UV-Visible and infrared probe spectroscopies, in conjunction, to unravel the mechanisms of photochemical reactions in solution. The underlying photo-physics that initiates bond fission in this class of molecules is also addressed in the context of the role of dissociative (n/π)σ* excited states.
Highlights
The rearrangements of substituted phenols from either aryl phenyl ethers or phenyl esters into 2- or 4-substituted phenol products are a well-studied class of reactions in synthetic chemistry
The evolution of the ground and excited electronic states of the parent molecule, the radicals produced by photo-induced homolytic bond fission, and intermediate cyclohexadienones formed via recombination of the produced radical pair are followed, providing new insight and detail on the reaction mechanisms
In accord with previous aromatic molecules studied via transient electronic absorption (TEA) these features can be assigned to absorption from an excited electronic state of the molecule under study.[23,24,25]
Summary
The rearrangements of substituted phenols from either aryl phenyl ethers or phenyl esters into 2- or 4-substituted phenol products are a well-studied class of reactions in synthetic chemistry. First identi ed by Claisen[1] and Fries[2,3] in the early 1900's, the thermal Claisen rearrangement results in a 2substituted phenol produced via a concerted intramolecular reaction whereas a Lewis acid catalyzes the Fries rearrangement and, depending on the experimental conditions, can result in either a 2- or a 4-substituted phenol. Several decades a er these initial discoveries, photochemical versions of both rearrangements were uncovered, known as the photo-Claisen[4] and the photo-Fries rearrangements.[5,6] Subsequent mechanistic studies suggested that UV excitation of the reactant causes homolytic bond ssion producing a radical pair which can recombine to form the original parent molecule. The thermal versions of these rearrangements have received extensive attention and evaluation as a key tool in a wide range of syntheses.[14,15,16] With improved understanding and further investigation, the photochemical versions of these rearrangements may offer the same synthetic utility as their thermal counterparts but with the added bene t of being a ‘greener’ synthetic route,[17] negating the need for additional reagents
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
