Ketyl Radical Formation via Proton-Coupled Electron Transfer in an Aqueous Solution versus Hydrogen Atom Transfer in Isopropanol after Photoexcitation of Aromatic Carbonyl Compounds

Carbonyl-Directed Aliphatic Fluorination: A Special Type of Hydrogen Atom Transfer Beats Out Norrish II

Recently, our group reported that enone and ketone functional groups, upon photoexcitation, can direct site-selective sp³ C-H fluorination in terpenoid derivatives. How this transformation actually occurred remained mysterious, as a significant number of mechanistic possibilities came to mind. Herein, we report a comprehensive study describing the reaction mechanism through kinetic studies, isotope-labeling experiments, ¹⁹F NMR, electrochemical studies, synthetic probes, and computational experiments. To our surprise, the mechanism suggests intermolecular hydrogen atom transfer (HAT) chemistry is at play, rather than classical Norrish hydrogen atom abstraction as initially conceived. What is more, we discovered a unique role for photopromoters such as benzil and related compounds that necessitates their chemical transformation through fluorination in order to be effective. Our findings provide documentation of an unusual form of directed HAT and are of crucial importance for defining the necessary parameters for the development of future methods.

Time-Resolved Spectroscopic Observation and Characterization of Water-Assisted Photoredox Reactions of Selected Aromatic Carbonyl Compounds

In recent years, unusual and efficient self-photoredox reactions were detected for selected benzophenone derivatives (BPs) and anthraquinone derivatives (AQs) in aqueous environments by Wan and co-workers, where the carbonyl undergoes reduction to the corresponding alcohol and a side-chain alcohol group undergoes oxidation to the corresponding carbonyl. To unravel the photoredox reaction mechanisms of these types of BPs and AQs in aqueous environments, we have utilized a combination of time-resolved spectroscopy techniques such as femtosecond transient absorption, nanosecond transient absorption, and nanosecond time-resolved resonance Raman spectroscopy to detect and characterize the electronic absorption and vibrational spectra of the intermediates and transient species from the femtosecond to microsecond time region after they are generated in the photoredox reactions. With the assistance of density functional theory calculations to simulate the electronic absorption and Raman spectra, the structural and kinetic information on the key reactive intermediates is described. Furthermore, the reaction pathways were calculated by finding the transition states connecting with the reactant and product complexes to better understand the photoredox reaction mechanism. In this Account, we summarize some of our time-resolved spectroscopic observations and characterization of water-assisted photoredox reactions of selected BPs and AQs. In the strong hydrogen-donor solvent isopropanol, the commonly studied photoreduction reaction for aromatic carbonyls via an intermolecular hydrogen atom tranfer process was observed for BPs and AQs. The photoredox reactions for the investigated BPs and AQs in aqueous environments occur on the triplet excited-state surface. Under moderately acidic aqueous conditions, the photoredox reactions for BPs and AQs are triggered by a proton transfer (PT) pathway. In neutral aqueous solutions, AQs may also undergo proton-coupled electron transfer (PCET) leading to the photoredox reaction, while BPs generate the ketyl radical species. Both BPs and AQs prefer the photohydration reaction in high-proton-concentration aqueous solutions (pH 0). The PT and PCET processes were found to offer more possibilities for the aromatic carbonyl compounds to lead to new photochemical reactions like the unusual photoredox reactions associated with BPs and AQs described here. Clear characterization of the photophysical pathways and the photochemical reactions of representative aromatic carbonyl compounds in aqueous environments not only provides fundamental information to better understand the photochemistry of carbonyl-containing compounds but also will facilitate the development of applications of these systems, like photochemical synthesis, drugs, and photolabile protecting groups. In addition, the importance of water molecules in the photochemical reactions of interest here may also lead to further understanding of how water influences the photochemistry of related carbonyl-containing compounds in aqueous environments.

The solvent effect on the photodeprotection of anthraquinone protected carboxylic acid unravelled by time-resolved spectroscopic studies

Time-resolved spectroscopy studies coupled with the results from density functional theory (DFT) computations unraveled the photodeprotection reaction mechanism(s) of AQ protected p-methoxybenzoic acid and the solvent effect on the photodeprotection.

Direct Observation of Photoinduced Ultrafast Generation of Singlet and Triplet Quinone Methides in Aqueous Solutions and Insight into the Roles of Acidic and Basic Sites in Quinone Methide Formation

Femtosecond time-resolved transient absorption spectroscopy experiments and density functional theory computations were done for a mechanistic investigation of 3-(1-phenylvinyl)phenol (1) and 3-hydroxybenzophenone (2) in selected solvents. Both compounds went through an intersystem crossing (ISC) to form the triplet excited states Tππ* and Tnπ* in acetonitrile but behave differently in neutral aqueous solutions, in which a triplet excited state proton transfer (ESPT) induced by the ISC process is also proposed for 2 but a singlet ESPT without ISC is proposed for 1, leading to the production of the triplet quinone methide (QM) and the singlet excited QM species respectively in these two systems. The triplet QM then underwent an ISC process to form an unstable ground state intermediate which soon returned to its starting material 2. However, the singlet excited state QM went through an internal conversion process to the ground state QM followed by the formation of its final product in an irreversible manner. These differences are thought to be derived from the slow vinyl C-C rotation and the moderate basicity of the vinyl C atom in 1 as compared with the fast C-O rotation and the greater basicity of the carbonyl O atom of 2 after photoexcitation. This can account for the experimental results in the literature that the aromatic vinyl compounds undergo efficient singlet excited state photochemical reactions while the aromatic carbonyl compounds prefer triplet photochemical reactions under aqueous conditions. These results have fundamental and significant implications for understanding of the ESPT reactivity in general, as well as for the design of molecules for efficient QM formation in aqueous media with potential applications in cancer phototherapy.

Time-Resolved Spectroscopic Study on the Photoredox Reaction of 2-(p-Hydroxymethyl)phenylAnthraquinone

In this work, we report a combined time-resolved spectroscopic and density functional theory computational study on 2-(p-hydroxymethyl)phenylanthraquinone (PPAQ) in which the benzyl alcohol moiety is significantly farther away from the AQ ketone group compared to the compound 2-(1-hydroxyethyl) 9,10-anthraquinone (HEAQ) so as to investigate the photophysical and photochemical reactions of PPAQ in several solvents, especially for the photoredox reaction in a pH 2 aqueous solution. The results here indicate that PPAQ undergoes the photoredox reaction via a two-step pathway and that the low efficiency of the photoredox reaction of PPAQ compared to the related HEAQ molecule is caused by the longer distance between the benzyl alcohol moiety and the AQ ketone moieties.

pH Dependent Photodeprotection of Formaldehyde: Homolytic C-C Scission in Acidic Aqueous Solution versus Heterolytic C-C Scission in Basic Aqueous Solution

The photodeprotection of formaldehyde was investigated for 3-(1-hydroxypropan-2-yl)benzophenone (3-HPBP) with ultrafast time-resolved spectroscopy. The femtosecond transient absorption results indicated the singlet excited state of 3-HPBP transformed efficiently into its triplet state by a fast intersystem crossing. In acidic (pH = 0) and basic (pH = 12.5) aqueous solutions, the triplet intermediate was a key precursor for the deprotection of formaldehyde via two different pathways. However, little photodeprotection was observed in neutral (pH = 7) aqueous solution where the triplet intermediate appeared to undergo a proton coupled electron transfer process to form a ketyl radical transient. The important benzylic biradical intermediates seen in the acidic and basic aqueous solutions were identified by time-resolved resonance Raman spectra whose vibrational frequency patterns were consistent with DFT calculation results for the benzylic biradical intermediate. The results here indicate that the β-carbon alcohol group of the triplet state 3-HPBP is deprotonated in basic aqueous solutions and this leads to a heterolytic C-C bond cleavage to deprotect formaldehyde and produce the benzylic carbanion triplet state species, whereas protonation of the carbonyl moiety of the triplet state 3-HPBP leads to direct generation of a benzylic biradical intermediate and the deprotection of formaldehyde in acidic aqueous solutions via a homolytic C-C bond cleavage.

To Photoredox or Not in Neutral Aqueous Solutions for Selected Benzophenone and Anthraquinone Derivatives

The experimental and theoretical results in neutral aqueous solutions reported here indicate that a proton-coupled electron transfer (PCET) from an alcohol C-H bond to the para-carbonyl is the initial and crucial process for the photoredox reaction of 2-(1-hydroxyethyl)-anthraquinone (HEAQ) to occur while the counterpart 3-(hydroxymethyl)-benzophenone (3-BPOH) compound displays a different PCET from an alcohol O-H bond to the carbonyl as the first step, followed by an intersystem crossing process that does not lead to the analogous photoredox, which is caused by a subtle charge-radical coupled effect between HEAQ and 3-BPOH. This can account for experimental results in the literature that HEAQ can undergo efficient photoredox but 3-BPOH does not under neutral aqueous conditions. These results have implications for the pH-dependent photochemical behavior of aromatic carbonyl compounds in aqueous media.

Radical–radical cross coupling reactions of photo-excited fluorenones

Radical–radical cross coupling reactions of photoexcited 9-fluorenones have been accomplished for the first time, leading to the synthesis of 9-alkyl, pyrollidinyl and spiro-THF derivatives of 9-fluorenones.