Letter pubs.acs.org/JPCL
To Photoredox or Not in Neutral Aqueous Solutions for Selected Benzophenone and Anthraquinone Derivatives Xiting Zhang,†,‡ Jiani Ma,*,† and David Lee Phillips*,‡ †
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710127, P. R. China ‡ Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong 999077, P. R. China S Supporting Information *
ABSTRACT: 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 3BPOH. 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.
O
BPOH and 4-BPOH compounds mainly generate the arylphenyl ketyl (ArPK) radical and do not exhibit an obvious photoredox19,22 has not yet been clarified. Second, the alcohol O−H bond (HD1)25,26 and C−H bond (HD2)15,26 and the solvent water (HD3)18,24,26 may serve as a hydrogen donor (HD) (see Scheme 2). Which one plays the main role on the initial proton transfer (PT),18,20−25 hydrogen atom transfer (HAT),26 or proton-coupled electron transfer (PCET)26 (see Scheme 2) to produce an intermediate radical is also not yet well understood. The processes of PT, HAT, and PCET are ubiquitous and central in chemical and biological processes.27 Third, which carbonyl (HA1 or HA2) (see Scheme 2, Schemes 1S and 2S) of HEAQ acts as the initial hydrogen acceptor (HA) is still unclear. To unravel the mechanisms of 3-BPOH and HEAQ in neutral aqueous solutions, nanosecond transient absorption (ns-TA) and nanosecond time-resolved resonance Raman (nsTR3) experiments (see Experimental Details in the Supporting Information) along with density functional theory (DFT) calculations were performed. The M062X level of theory appears to be reliable to simulate the photochemical behavior for main-group compounds26,28,29 (see Computational Details in the Supporting Information). Figure 1 shows the energy profiles of two possible initial steps of photochemical processes for 3-BPOH in neutral aqueous solutions. The nπ* 1a absorbs 2.6 kcal/mol energy to the thermally populated ππ* species 2a, the precursor for the
rganic photoredox reactions are one of the most rapidly expanding areas of radical chemistry in synthesis.1−10 An efficient self-photoredox reaction, where the carbonyl is reduced to its alcohol while the alcohol at the side chain is oxidized to its carbonyl, was observed for benzophenones (BPs) and anthraquinones (AQs) in aqueous solutions.11−18 Both 2-(1-hydroxyethyl)-anthraquinone (HEAQ) and 3(hydroxymethyl)-benzophenone (3-BPOH) undergo efficient photoredox under moderately acidic aqueous conditions, which is proposed to take place via the carbonyl protonation process and then a subsequent deprotonation of the xylylene C−H bond.15,18−25 Nevertheless, the mechanism has remained uncertain under nonacidic conditions, which needs to be addressed under neutral aqueous solutions. First, the origin for the experimental observations (Scheme 1) that HEAQ can efficiently undergo the photoredox reaction while the related 3Scheme 1. Water-Assisted Photoredox Reactions in Aqueous Solutions for HEAQ, 3-BPOH, and 4-BPOH
Received: October 17, 2016 Accepted: November 14, 2016 Published: November 14, 2016 © XXXX American Chemical Society
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Scheme 2. Possible Hydrogen Donors and Acceptors and Photochemical Reactions for 3-BPOH and HEAQ in Neutral Aqueous Solutions
Figure 1. Free-energy surfaces of ArPK and xylylene diradical formations for 3-BPOH in neutral aqueous solutions, calculated using M062X/6-311++g**-SMD(MeCN)//M062X/6-311+g**.
subsequent hydrogen transfer. The species 2a prefers to undergo a PCET from HD1 to the HA1 assisted by water via a transition state ts(2−3)a with a barrier of 13.3 kcal/mol and finally goes downhill to produce an ArPK species 3a. The other less competitive PCET pathway assisted by water takes place via ts(2−4)a (18.6 kcal/mol) with the simultaneous HD2 deprotonation and HA1 protonation with formation of the xylylene diradical 4a. The energy profiles of the counterpart HEAQ display significant differences regarding the xylylene diradical formation and ketyl radical formation (Figure 2). All of the pathways begin with the nπ* species 1b and then convert to the ππ* species 2b with an endothermic process (0.7 kcal/mol). Two ArPK (6b and 4b) formation routes, which involve a PCET from HD1 to HA1 and HA2, respectively, are not so favored for HEAQ compared with the xylylene diradical 3b formation via ts(2−3)b (9.9 kcal/mol). The xylylene diradical 5b formation with the simultaneous HD2 deprotonation and meta-carbonyl protonation is also higher in energy by 4.0 kcal/ mol so that the xylylene diradical 3b formation with the simultaneous HD2 deprotonation and para-carbonyl protonation then becomes the lowest energy channel as the initial step of the photochemical process. The ArPK formation goes through a PCET with a proton transfer from the alcohol O−H σ bond to the meta- or paracarbonyl O n orbital coupled to an electron transfer from the alcohol O n orbital to the meta- or para-carbonyl π orbital via the water chain, while the xylylene diradical formation is
Figure 2. Free-energy profiles of ArPK and xylylene diradical formations for HEAQ in neutral aqueous solutions calculated using M062X/6-311++g**-SMD(MeCN)//M062X/6-311+g**.
formed by PCET with a proton transfer from the alcohol C−H σ bond via the water chain coupled to the electron transfer via a conjugated π system. Therefore, two radicals remain isolated on the side chain oxygen atom and the meta- or para-carbonyl carbon atom for the ArPK species, while they can undergo “coupled” interaction between the side-chain carbon atom and the meta- or para-carbonyl carbon atom mediated by a conjugated π system for the xylylene diradical species (see Figures 1 and 2 and Figures 3S and 4S). The free-energy profiles of the HAT of the nπ* carbonyl, the PCET of the ππ* carbonyl, and the hydration from water (HD3) were also studied (see Figures 5S and 6S). These reactions are less favored in energy, which agrees well with the experimental observations18,22,23 in neutral aqueous conditions and will not be discussed in detail here. We further carried out ns-TA and ns-TR3 experiments (see Figure 3 and Figures 7S−10S) and DFT-M062X calculations for 3-BPOH and HEAQ in neutral aqueous solutions to obtain further information. The spectrum at 300 ns corresponds to the species that appears upon the decay of a triplet species of 3BPOH and HEAQ. From the ns-TA results (see Figure 3A), noticeable absorbance grows in at 323 and 527 nm, which 4861
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Figure 3. Experimental ns-TA and ns-TR3 spectra compared with the computed UV−vis (calculated utilizing M062X/6-311++g**-SMD(MeCN)//M062X/6-311+g**) and Raman spectra (calculated using M062X/6-311+g**) of the ArPK and xylylene diradical species for 3BPOH and HEAQ are shown (see the text for details).
Figure 4. NBO charge and spin density for the 3-BPOH and HEAQ in acidic and neutral solution, calculated at the M062X/6-311++g**SMD(MeCN)//M062X/6-311+g**.
correlates with the computed UV−vis spectrum of ArPK 3a. The ns-TR3 spectra with characteristic Raman bands at 1171 and 1583 cm−1 comparison with the calculated Raman spectrum (see Figure 3B) also suggest ArPK 3a rather than xylylene diradical 4a is the species detected in neutral aqueous solutions. In contrast, the TD-M062X result for xylylene diradical 3b (see Figure 3C) has a comparable spectral profile with the ns-TA spectra detected for HEAQ. The Raman bands (1614, 1570, 1541, and 1320 cm−1) observed in the ns-TR3 spectra also correlate well with the calculated result for 3b. The other possible initial steps such as 4b, 5b, and 6b formation then can be ruled out in the experimentally observed photochemical process. The above results confirm that the initial step of the photochemical process for 3-BPOH is different from that for HEAQ in neutral aqueous solution. This naturally brings up three questions: (1) Why does 3-BPOH prefer ArPK formation while HEAQ tends to favor the xylylene diradical formation? (2) Why is para-carbonyl favored over meta-carbonyl on the water-assisted xylylene diradical formation for HEAQ? (3) Why can xylylene diradical be detected for 3-BPOH only under moderately acidic condition while HEAQ can produce the xylylene diradical species under both neutral and moderately acidic conditions? NBO and spin analysis were performed to help unravel these questions (see Figure 4 and Figure 11S). Figure 4A indicates that more positive charge (0.23) is distributed on the C4 of 3-BPOH than that of 4-BPOH (0.16), which is the same as the case between the protonated meta-carbonyl HEAQ (C4, 0.24) and the protonated paracarbonyl HEAQ (C4, 0.16) under acidic conditions. It seems that the meta-relationship between the carbonyl and the side -chain facilitates more of the positive charge to be accumulated on C4, leading to a more stabilized alcohol carbon anion coming about due to the deprotonation process of the C−H bond. The positive charge on C4 is also favored for the hydration reaction, which, however, is not the main reaction for 3-BPOH and HEAQ under moderately acidic condition. The spin on C4 should be considered because it will be delocalized
to the emerging alcohol carbon anion and promotes the deprotonation of the C−H bond, while the spin becomes less delocalized to hinder the hydration. Once under a strong acidic aqueous condition, the proton extrusion from the alcohol C−H bond is suppressed so that the hydration becomes more feasible.18,22,23 The positive charge and the spin on the C4 atom demonstrate and can explain the experimental observation that 3-BPOH and HEAQ are more reactive for the xylylene diradical formation than 4-BPOH in a moderately acidic aqueous solution.15,18−25 The charge and spin population for the compounds of interest is another story in neutral aqueous solutions (see Figure 4B). Our recent study demonstrated that one phenyl and either the meta- or para-carbonyl can be excited in the ππ* triplets20−25 and the excited phenyl may have an effect on the side chain. The para-relationship between the side chain and carbonyl group results in more positive charge to reside on C4 of HEAQ (0.14) with the para-carbonyl excited than that of 3BPOH and HEAQ with the meta-carbonyl excited. Although the positive charge on the C4 atom in neutral aqueous solution is less than that under an acidic aqueous condition, the spin population on the carbonyl oxygen is another driving force for xylylene diradical formation. For instance, the significant difference for HEAQ is derived from the spin density on the excited para-carbonyl O atom (see Figure 4), which is 0.08 under an acidic condition while it is 0.33 under a neutral condition. As PCET commences, more spin then can transfer from the para-carbonyl and delocalize to the emerging carbon anion, resulting in the electron transfer inversely from the carbon anion to the para-carbonyl. Increasing the positive charge on the alcohol C−H bond then enlarges the acidity of the C−H bond promoting proton transfer while simultaneous increasing the negative charge on the para-carbonyl, enhancing the interaction between water.30 The spin distribution on the carbonyl oxygen is also favored for the PCET from the alcohol O−H bond, which is the main reaction for 3-BPOH because the charge on C4 (−0.04) is not positive enough to activate the 4862
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Figure 5. Proposed mechanisms for 3-BPOH and HEAQ in acidic and neutral aqueous solutions are shown, respectively. The calculated free energies are given in parentheses, calculated using M062X/6-311++g**-SMD(MeCN)//M062X/6-311+g**.
alcohol C−H bond for the PCET process, consistent with BPs being poor electron acceptors in the triplet state compared with AQs.31 The subtle charge-radical coupled effect supports and can account for the experimental observation of the xylylene diradical 3b formation for HEAQ but not for 4-BPOH and 3BPOH under neutral aqueous solution conditions.15,18 The formation of the xylylene diradical 3b is followed by an intersystem crossing (ISC) to the singlet xylylene 15b via an exothermic process (−45.1 kcal/mol), which then overcomes a small energy barrier (1.6 kcal/mol) to the photoredox product 16b (see Figure 5 and Figure 12S). On the contrary, the occurrence of the photoredox for 3-BPOH from ArPK 3a in neutral aqueous solution is hindered as it requires the second hydrogen extrusion from the C−H bond with an energy barrier of 6.3 kcal/mol (see Figure 12S, ts(3−16)a). The preferred pathway is to go through the minimum energy crossing point (MECP2, −4.3 kcal/mol) that then goes back to the ground state 3-BPOH 15a (Figure 5 and Figure 12S). Overall, we provide evidence consistent with the reduction of carbonyls proceeding through a PCET process from the sidechain alcohol O−H bond for 3-BPOH while starting via a PCET process from the alcohol C−H bond to the paracarbonyl for HEAQ in neutral aqueous solutions. These differences can be attributed to the subtle charge-radical coupled effect of 3-BPOH and HEAQ on the PCET pathway. The xylylene diradical 3b formation is crucial for the photoredox observed for HEAQ. The 3-BPOH and HEAQ experience different PCET processes for the subtle chargeradical coupled effect upon photoexcitation, providing an amenable model system to study photoredox reactions. The results here also suggest that additional work is needed to better understand how photodeprotection reactions utilizing AQs as the photoremovable protecting group occur under varying aqueous conditions. This type of photoredox that takes place efficiently via PCET may have interesting uses as a new
photochemical synthetic technique to facilitate coupled reactions of distal functional groups in chemical and biological processes.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02403. Experimental and computational details, Tables 1S and 2S, Schemes 1S−3S, and Figures 1S−14S for selected computational results discussed in the text, including Cartesian coordinates and energies for all of the calculated structures found in this work. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*J.M.: E-mail:
[email protected]. *D.L.P.: E-mail:
[email protected]. ORCID
David Lee Phillips: 0000-0002-8606-8780 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The research was sponsored in part by grants from the Research Grants Council of Hong Kong (HKU17301815) to D.L.P. and the National Science Fund of China (21503167) and Shaanxi Province Science Fund (2016JQ2009) to J.M. and partial support from the Areas of Excellence Scheme (Grant AoE/P-03/08) and the UGC Special Equipment Grant (SEGHKU-07). 4863
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(23) Ma, J.; Su, T.; Li, M.-D.; Zhang, X.; Huang, J.; Phillips, D. L. Meta Versus Para Substitution: How Does C−H Activation in a Methyl Group Occur in 3-Methylbenzophenone but Does Not Take Place in 4-Methylbenzophenone? J. Org. Chem. 2013, 78, 4867−4878. (24) Chen, X.; Zhang, Q.; Xu, Y.; Fang, W.; Phillips, D. L. WaterAssisted Self-Photoredox of 3-(Hydroxymethyl)benzophenone: An Unusual Photochemistry Reaction in Aqueous Solution. J. Org. Chem. 2013, 78, 5677−5684. (25) Dai, J.; Han, J.; Chen, X.; Fang, W.; Ma, J.; Phillips, D. L. WaterAssisted Self-Photoredox of 2-(1-Hydroxyethyl)-9,10-Anthraquinone Through a Triplet Excited State Intra-molecular Proton Transfer Pathway. Phys. Chem. Chem. Phys. 2015, 17, 27001−27010. (26) Zhang, X.; Ma, J.; Li, S.; Li, M.-D.; Guan, X.; Lan, X.; Zhu, R.; Phillips, D. L. Ketyl Radical Formation via Proton-Coupled Electron Transfer in an Aqueous Solution versus Hydrogen Atom Transfer in Isopropanol after Photoexcitation of Aromatic Carbonyl Compounds. J. Org. Chem. 2016, 81, 5330−5336. (27) MacAleese, L.; Hermelin, S.; El Hage, K.; Chouzenoux, P.; Kulesza, A.; Antoine, R.; Bonacina, L.; Meuwly, M.; Wolf, J.-P.; Dugourd, P. Sequential Proton Coupled Electron Transfer (PCET): Dynamics Observed over 8 Orders of Magnitude in Time. J. Am. Chem. Soc. 2016, 138, 4401−4407. (28) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. Design of Density Functionals by Combining the Method of Constraint Satisfaction with Parametrization for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions. J. Chem. Theory Comput. 2006, 2, 364−382. (29) Harshan, A. K.; Yu, T.; Soudackov, A. V.; Hammes-Schiffer, S. Dependence of Vibronic Coupling on Molecular Geometry and Environment: Bridging Hydrogen Atom Transfer and Electron− Proton Transfer. J. Am. Chem. Soc. 2015, 137, 13545−13555. (30) Chciuk, T. V.; Anderson, W. R., Jr.; Flowers, R. A., II ProtonCoupled Electron Transfer in the Reduction of Carbonyls by Samarium Diiodide−Water Complexes. J. Am. Chem. Soc. 2016, 138, 8738−8741. (31) Kuzmin, V. A.; Chibisov, A. K. Ring Inversion in 1,4-Dioxan. J. Chem. Soc. D 1971, 23, 1558−1559.
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