Chemistry of photogenerated α-phenyl-substituted o-, m-, and p-quinone methides from phenol derivatives in aqueous solution

Abstract

The enhanced photochemical reactivity of o-substituted phenols in its propensity to give o-quinone methide (o-QM) intermediates via excited state intramolecular proton transfer (ESIPT) was uncovered by Keith Yates as part of his now classic studies of photohydration of aromatic alkenes, alkynes, and related compounds. Photogeneration of QMs and the study of their chemistry along with potential biological applications are the focus of many groups. In this work, photochemical precursors to o-, m-, and p-QMs based on substituted phenols (hydroxybenzyl alcohols) and related compounds have been studied in aqueous solution as a function of pH and water content. The focus will be on QMs that are stabilized by an α-phenyl substituent, which enhances quantum yields for their formation, with the resulting QMs having longer lifetimes and easier to detect. Noteworthy is that all QM isomers can be photogenerated with the o and m isomers being the most efficient, consistent with the Zimmerman “ortho-meta” effect. m-QMs have formal non-Kekulé structures, and although they can be routinely photogenerated, are found to be most reactive. One m-QM was found to undergo a photocondensation reaction at high pH giving rise to m-substituted oligomers. The mechanism of QM formation in aqueous solution is believed to involve singlet excited phenols that undergo adiabatic deprotonation to give the corresponding photoexcited phenolate ion, which subsequently expels the hydroxide ion (photodehydroxylation). A pathway involving direct loss of water for the o-isomers is also possible in organic solvents.Key words: quinone methides, phenols, excited state acidity, solvolysis, carbocations, meta effect, photopolymerization, non-Kekulé intermediates.

In vitro investigation of the antimicrobial activity of a series of lipophilic phenols and naphthols

“…19 Characterization of product 4 compares well with literature (mp = 104-105°C). 19 Diphenyl-(2-hydoxyphenyl)methanol (6) was prepared from diphenyl-(2-methoxyphenyl)methanol by treatment with BBr 3 in 90 % yield according to the published procedure 30 . Characterization of product 6 compares well with literature (mp = 148-150°C).…”

“…Characterization of product 6 compares well with literature (mp = 148-150°C). 30 (7) was prepared in a Grignard reaction from homoadamantan-4-one and 3-bromanisol in 67 % yield according to the published procedure. 31 Characterization of product 7 compares well with literature (mp = 130-132°C).…”

“…21 Characterization of product 29 compares well with literature. (30) was obtained from 4-methoxy-4'-(2-hydroxypropan-2-yl)biphenyl with sodium thiolate in 80 % yield according to the published procedure. 21 Characterization of product 30 compares well with literature (mp = 178-180°C).…”

Photoinduced Addition Reactions in Aqueous Media

“…That delivers QM 6 and 6´ (eq 1) which react with nucleophilic solvents giving solvolysis products 7 and 7´ (eq 1), respectively [17]. That delivers QM 6 and 6´ (eq 1) which react with nucleophilic solvents giving solvolysis products 7 and 7´ (eq 1), respectively [17].…”

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

“…Owing to the biological applicability of QMs, the reactivity of different classes of QMs has been intensively investigated by spectroscopic methods [51][52][53][54][55][56][57][58][59][60][61][62][63] and computationally. [64][65][66] How-ever,t he most simple photodehydration reactiono fo-hydroxymethylphenol (1)t og ive o-QM 2 [Eq.…”