1995 Merck Frosst Award Lecture Quinone methides: relevant intermediates in organic chemistry

1996 ◽  
Vol 74 (4) ◽  
pp. 465-475 ◽  
Author(s):  
Peter Wan ◽  
Beverly Barker ◽  
Li Diao ◽  
Maike Fischer ◽  
Yijian Shi ◽  
...  
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Aqueous Media ◽  
Laser Flash ◽  
Alder Reaction ◽  
Quinone Methide ◽  
Quinone Methides ◽  
Chemical Systems ◽  
The One ◽  
Chroman Ring

ortho and para-Quinone methides (2-methylene-3,5-cyclohexadien-1-one and 4-methylene-2,5-cyclohexadien-1-one, respectively) are intermediates in a variety of important chemical systems. In particular, o-quinone methides are useful in synthesis for the construction of chroman ring systems. A brief account of the relevance of quinone methide chemistry will be provided. This is followed by a review of recent studies from our laboratory on efficient methods for the photogeneration of quinone methides, concentrating on the use of hydroxy-substituted benzyl alcohols in aqueous media. It is shown that this method is general since it provides access to o-, m-, and p-quinone methide isomers. When appropriately substituted, all of these quinone methide isomers have been spectroscopically characterized by laser flash photolysis, making this technique the one of choice for studying the dynamics of these reactive intermediates. The mechanism of photochemical generation from hydroxybenzyl alcohols and extensions of the reaction to photogeneration of fluorenyl and biphenyl quinone methides will also be presented. Key words: quinone methide, biphenyl quinone methide, carbocation, photosolvolysis, photodehydroxylation, hetero-Diels–Alder reaction.

Formation and nucleophilic capture of N-nitrosiminium ions

1999 ◽  
Vol 77 (5-6) ◽  
pp. 1148-1161 ◽  
Author(s):  
Latifa Chahoua ◽  
Alain Vigroux ◽  
Yvonne Chiang ◽  
James C Fishbein
Keyword(s):  
Ionic Strength ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Aqueous Media ◽  
Laser Flash ◽  
Azide Ion ◽  
Iminium Ions ◽  
Nucleophilic Trapping

A study of the solvolysis of a series of (N-nitrosomethylamino)arylmethyl esters and azides and the products of nucleophilic trapping of the corresponding N-nitrosiminium ion intermediates in aqueous media, 25°C, ionic strength 1 M is reported. Structure-reactivity data for the forward and reverse reactions have been obtained. In three cases, the rate constants for reactions of the cations with nucleophiles have been measured directly by laser flash photolysis. The data allow a comparison of the degree to which the N-methyl-N-nitroso functionality enhances cation stability from a thermodynamic and kinetic perspective. It has been possible to deduce that the carbon basicity of azide ion is less than 1 kcal/mol greater than that of acetate ion.Key words: nitrosiminium ions, α-acetoxynitrosamines, carbocations, iminium ions, nucleophilicity.

Tetraalkylammonium salt as photoinitiator of vinyl polymerization in organic and aqueous media: A mechanistic and laser flash photolysis study

2002 ◽  
Vol 40 (7) ◽  
pp. 901-913 ◽  
Author(s):  
María L. Gómez ◽  
Rodrigo E. Palacios ◽  
Carlos M. Previtali ◽  
Hernán A. Montejano ◽  
Carlos A. Chesta
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Aqueous Media ◽  
Laser Flash ◽  
Tetraalkylammonium Salt ◽  
Vinyl Polymerization

Zwitterionic biphenyl quinone methides in photodehydration reactions of 3-hydroxybiphenyl derivatives: laser flash photolysis and antiproliferation study

2012 ◽  
Vol 11 (2) ◽  
pp. 381 ◽  
Author(s):  
Nikola Basarić ◽  
Nikola Cindro ◽  
Damir Bobinac ◽  
Lidija Uzelac ◽  
Kata Mlinarić-Majerski ◽  
...  
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash ◽  
Quinone Methides

Molecular environment effect on O2 binding to lipidporphyrinatoiron(II) complexes in aqueous media

2000 ◽  
Vol 04 (01) ◽  
pp. 81-87 ◽  
Author(s):  
EISHUN TSUCHIDA ◽  
TERUYUKI KOMATSU ◽  
TETSUYA YANAGIMOTO
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Critical Concentration ◽  
Aqueous Media ◽  
Laser Flash ◽  
Fiber Formation ◽  
Ionization Mass ◽  
Self Organized ◽  
Covalently Linked ◽  
The Stability

Lipidporphyrinatoiron(II) complexes are tetrakis(o-substituted)phenylporphinatoiron(II) derivatives which can be easily dispersed in water by molecular assembling. The most remarkable aspect of lipidporphyrinatoiron(II) assemblies is their reversible binding of dioxygen under physiological conditions (in aqueous media, pH 7.3, 37 °C) like hemoglobin and myoglobin. In these structures the O 2-binding properties are largely influenced by the molecular environment around the coordination site. Tetrakis(o-pivalamido)phenylporphinatoiron(II) with a covalently linked axial imidazole (lipidporphyrinatoiron(II), 1) is incorporated into recombinant human serum albumin (rHSA), providing a totally synthetic O 2-carrying hemoprotein (rHSA–1). Electrospray ionization mass spectrometry revealed the molecular mass of this non-covalent albumin–porphyrin hybrid. The O 2 rebinding after laser flash photolysis represented a three-phase decay, suggesting that each porphyrin is embedded into different cavities in the albumin structure. On the other hand, amphiphilic lipidporphyrinatoiron(II) with four alkylphosphocholine chains (2) is self-organized in aqueous solution to produce bimolecular fibers with a uniform thickness of 10 nm. This fiber also gave a stable O 2 adduct, and the O 2 rebinding after laser flash irradiation showed monophasic kinetics. Up to 20 vol% of methanol, which is a critical concentration for fiber formation, the morphology was gradually dissociated into spherical micelles, and the stability of the dioxygenated species suddenly decreased to 10% of that of the fibers.

scholarly journals Two- and Three-Component Visible Light Photoinitiating Systems for Radical Polymerization Based on Onium Salts: An Overview of Mechanistic and Laser Flash Photolysis Studies

2012 ◽  
Vol 2012 ◽  
pp. 1-9 ◽  
Author(s):  
María L. Gómez ◽  
Carlos M. Previtali ◽  
Hernán A. Montejano
Keyword(s):  
Visible Light ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Aqueous Media ◽  
Laser Flash ◽  
Time Resolved ◽  
Onium Salt ◽  
Onium Salts ◽  
Photopolymerization Kinetics ◽  
Polymerization Conditions

A review of our work on two- and-three component photoinitiator systems is presented. The emphasis is in on visible light polymerization in aqueous media. The systems discussed comprise a synthetic dye as sensitizer and an onium salt as coinitiator, or a dye-amine-onium salt with the amine as coinitiator and the onium salt as an enhancer of the polymerization efficiency. The effect of the composition of the system on the photopolymerization kinetics was analyzed. To this end, the photophysics and photochemistry of the dye under polymerization conditions was explored by means of stationary and time-resolved spectroscopic methods. Different dyes and onium salts were investigated. The action mechanism of the different photoinitiators systems is discussed.

Laser flash photolysis study of photosensitized oxidation of phenols in the organized aqueous media

2004 ◽  
Author(s):  
Natalia B. Sultimova ◽  
Peter P. Levin ◽  
Olga N. Chaikovskaya ◽  
Irina V. Sokolova ◽  
Alexey V. Kuzmin
Keyword(s):  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Aqueous Media ◽  
Laser Flash ◽  
Photosensitized Oxidation ◽  
Oxidation Of Phenols

Mechanistic studies of photohydration of m-hydroxy-1,1-diaryl alkenes

2002 ◽  
Vol 80 (1) ◽  
pp. 46-54 ◽  
Author(s):  
John G Cole ◽  
Peter Wan
Keyword(s):  
Excited State ◽  
Proton Transfer ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Parent Compound ◽  
Laser Flash ◽  
Intramolecular Proton Transfer ◽  
Quinone Methide ◽  
Quantum Yields ◽  
Nanosecond Laser

The photohydration of a variety of m-hydroxy-1,1-diaryl alkenes (8–10) and related systems (11 and 12) has been studied in aqueous CH3CN solution. All of these alkenes photohydrate efficiently in 1:1 H2O–CH3CN, to give the corresponding 1,1-diarylethanol (Markovnikov addition) products with high chemical and quantum yields. The aim of this study was to further probe the mechanism of photohydration reported for the parent m-hydroxy-α-phenylstyrene (5), which has been proposed as consisting of a water trimer-mediated excited state (formal) intramolecular proton transfer (ESIPT) from the phenolic proton to the β-carbon of the alkene moiety to give an observable (by laser flash photolysis (LFP)) m-quinone methide intermediate 6. For this purpose, derivatives of 5 with substituents (methyl or methoxy) on the α-phenyl ring as well as related model compounds were explored. Product studies, quantum yields, fluorescence, and nanosecond laser flash data are reported that are consistent with two distinct mechanisms for photohydration of these compounds: one involving water-mediated ESIPT (8, 9), as observed for the parent compound 5, and one involving direct protonation of the β-carbon by solvent water (11 and 12), with compound 10 possibly operating via both mechanisms.Key words: photohydration, solvent-assisted excited state intramolecular proton transfer (ESIPT), m-quinone methide, diarylmethyl carbocation.

Effect of Aggregation on Bacteriochlorin a Triplet-state Formation: A Laser Flash Photolysis Study¶

2002 ◽  
Vol 76 (5) ◽  
pp. 480 ◽  
Author(s):  
Xavier Damoiseau ◽  
Francis Tfibel ◽  
Maryse Hoebeke ◽  
Marie-Pierre Fontaine-Aupart
Keyword(s):  
Triplet State ◽  
State Formation ◽  
Flash Photolysis ◽  
Laser Flash Photolysis ◽  
Laser Flash
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