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pH Dependent Photodeprotection of Formaldehyde: Homolytic C-C Scission in Acidic Aqueous Solution Versus Heterolytic C-C Scission in Basic Aqueous Solution Mingyue Liu, Wenjian Tang, Ming-De Li, and David Lee Phillips J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02756 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017
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The Journal of Organic Chemistry
pH Dependent Photodeprotection of Formaldehyde: Homolytic C-C Scission in Acidic Aqueous Solution Versus Heterolytic C-C Scission in Basic Aqueous Solution Mingyue Liu1, Wenjian Tang2, Ming-De Li*, 1,3 and David Lee Phillips*, 1 1
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S.A.R., P. R.
China 2
School of Pharmacy, Anhui Medical University, Hefei 230001, P. R. China
3
Department of Chemistry, Shantou University, Guangdong 515063, P. R. China.
KEYWORDS: Phototrigger, Formaldehyde, Time-resolved resonance Raman, Transient absorption, Benzophenone O OH
3-HPBP
Phototrigger Release of Formaldehyde OHO
on
• Heterolysis Cleavage (pH=12.5) • Homolysis Cleavage (pH=0)
H O
O
H+
OH+
T1
+ CH2O OH Benzylic Carbanion
OH
T1 OH
Off
OH
• Hydrogen Abstraction (pH=7) Ketyl radical
×
+ CH2O benzylic biradical 1
ABSTRACT: The photodeprotection of formaldehyde was investigated for 3-(1-hydroxypropan-2yl)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 in1
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The Journal of Organic Chemistry
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termediate 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 βcarbon 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.
INTRODUCTION A number of benzophenone (BP) compounds after photoexcitation to produce an electronic singlet excited state can then undergo highly efficient intersystem crossing (ISC) to produce triplet state intermediates.1-6 Different subsequent photochemical reactions can occur from these triplet intermediates and example reactions include hydrogen abstraction,7-12 photodecarboxylation,13-17 photohydration,18-20 and photoredox21-23 reactions. Thus, BP and its derivatives are compounds that can provide a rich platform to design molecules for a wide range of applications. Among BP derivatives, some BP alcohols have recently attracted interest for their intriguing pH dependent photochemical reaction behavior in aqueous solutions.14, 24-26 Photoexcitation of 3-(hydroxymethyl)benzophenone that contains an alcohol moiety at the -carbon in the meta position on a phenyl group was observed by Wan and co-workers26 to undergo a photoredox in aqueous (pH
