title: Reactivity of Cations and Zwitterions Formed in Photochemical and Acid-Catalyzed Reactions from m-Hydroxycycloalkyl-Substituted Phenol Derivatives.
abstract: Three m-substituted phenol derivatives, each with a labile benzylic alcohol group and bearing either protoadamantyl 4, homoadamantyl 5, or a cyclohexyl group 6, were synthesized and their thermal acid-catalyzed and photochemical solvolytic reactivity studied, using preparative irradiations, fluorescence measurements, nanosecond laser flash photolysis, and quantum chemical calculations. The choice of m-hydroxy-substitution was driven by the potential for these phenolic systems to generate m-quinone methides on photolysis, which could ultimately drive the excited-state pathway, as opposed to forming simple benzylic carbocations in the corresponding thermal route. Indeed, thermal acid-catalyzed reactions gave the corresponding cations, which undergo rearrangement and elimination from 4, only elimination from 5, and substitution and elimination from 6. On the other hand, upon photoexcitation of 4-6 to S1 in a polar protic solvent, proton dissociation from the phenol, coupled with elimination of the benzylic OH (as hydroxide ion) gave zwitterions (formal m-quinone methides). The zwitterions exhibit reactivity different from the corresponding cations due to a difference in charge distribution, as shown by DFT calculations. Thus, protoadamantyl zwitterion has a less nonclassical character than the corresponding cation, so it does not undergo 1,2-shift of the carbon atom, as observed in the acid-catalyzed reaction.
authors.forename: Nikola
authors.initials: N
authors.lastname: Cindro
authors.forename: Ivana
authors.initials: I
authors.lastname: Antol
authors.forename: Kata
authors.initials: K
authors.lastname: Mlinarić-Majerski
authors.forename: Ivan
authors.initials: I
authors.lastname: Halasz
authors.affiliation: Department of Chemistry, University of Victoria , Box 3065 STN CSC, Victoria, BC V8W 3V6, Canada.
authors.forename: Peter
authors.initials: P
authors.lastname: Wan
authors.forename: Nikola
authors.initials: N
authors.lastname: Basarić