R. Marshall Wilson

Photoaffinity Labeling via Nitrenium Ion Chemistry: Protonation of the Nitrene Derived from 4-Amino-3-nitrophenyl Azide to Afford Reactive Nitrenium Ion Pairs

Phenyl azides with powerful electron-donating substituents are known to deviate from the usual photochemical behavior of other phenyl azides. They do not undergo ring expansion but form basic nitrenes that protonate to form nitrenium ions. The photochemistry of the widely used photoaffinity labeling system 4-amino-3-nitrophenyl azide, 5, has been studied by transient absorption spectroscopy from femtosecond to microsecond time domains and from a theoretical perspective. The nitrene generation from azide 5 occurs on the S(2) surface, in violation of Kashas rule. The resulting nitrene is a powerful base and abstracts protons extremely rapidly from a variety of sources to form a nitrenium ion. In methanol, this protonation occurs in about 5 ps, which is the fastest intermolecular protonation observed to date. Suitable proton sources include alcohols, amine salts, and even acidic C-H bonds such as acetonitrile. The resulting nitrenium ion is stabilized by the electron-donating 4-amino group to afford a diiminoquinone-like species that collapses relatively slowly to form the ultimate cross-linked product. In some cases in which the anion is a good hydride donor, cross-linking is replaced by reduction of the nitrenium ion to the corresponding amine.

Laser photochemistry: The wavelength dependent oxidative photodegradation of vitamin K analogs

Abstract Menaquinone-1 and phylloquinone have been observed to form stable trioxanes when irradiated with visible light under aerobic conditions. The menaquinone-1 peroxide derivative has been shown to be extremely photolabile to ultraviolet light.

Oxidation Of Adenosine And Inosine: The Chemistry Of 8-oxo-7,8-dihydropurines, Purine Iminoquinones, And Purine Quinones As Observed By Ultrafast Spectroscopy

Oxidative damage to purine nucleic acid bases proceeds through quinoidal intermediates derived from their corresponding 8-oxo-7,8-dihydropurine bases. Oxidation studies of 8-oxo-7,8-dihyroadenosine and 8-oxo-7,8-dihydroinosine indicate that these quinoidal species can produce stable cross-links with a wide variety of nucleophiles in the 2-positions of the purines. An azide precursor for the adenosine iminoquinone has been synthesized and applied in ultrafast transient absorption spectroscopic studies. Thus, the adenosine iminoquinone can be observed directly, and its susceptibility to nucleophilic attack with various nucleophiles as well as the stability of the resulting cross-linked species have been evaluated. Finally, these observations indicate that this azide might be a very useful photoaffinity labeling agent, because the reactive intermediate, adenosine iminoquinone, is such a good mimic for the universal purine base adenosine.

High intensity, argon ion laser-jet photochemistry

Abstract A new technique for the study of high intensity solution photochemistry has been developed. With this laser-jet technique, a high velocity microjet is irradiated with the focussed output of an argon ion laser. Under these extremely high intensity conditions, photochemically generated transient species with suitable absorption properties are excited further and produce relatively large amounts of photoproducts which are not observed under low intensity conditions. The application of this laser-jet technique in the study of the photochemistry of radicals, biradicals, photoenols and the higher excited states of carbonyl and polycyclic aromatic compounds is described.

UV laser photochemistry: Evidence for “through-bond coupling” effects in the lifetimes of 2,2-dimethyl-1,3-cyclopentadiyl triplet diradicals.

Abstract The 2,2-dimethyl substituted 1,3-diradicals 5a-c possess significantly shorter triplet lifetimes (τ t ) than the parent 1,3-cyclopentadiyl 3 , which is being rationalized on the basis of changes in the “through-bond” coupling and thus singlet-triplet energy splitting between the radical sites.

Mechanism of Formation of Copper(II) Chloro Complexes Revealed by Transient Absorption Spectroscopy and DFT/TDDFT Calculations

Copper(II) complexes are extremely labile with typical ligand exchange rate constants on the order of 10(6)-10(9) M(-1) s(-1). As a result, it is often difficult to identify the actual formation mechanism of these complexes. In this work, using UV-vis transient absorption when probing in a broad time range (20 ps to 8 μs) in conjunction with DFT/TDDFT calculations, we studied the dynamics and underlying reaction mechanisms of the formation of extremely labile copper(II) CuCl4(2-) chloro complexes from copper(II) CuCl3(-) trichloro complexes and chloride ions. These two species, produced via photochemical dissociation of CuCl4(2-) upon 420 nm excitation into the ligand-to-metal-charge-transfer electronic state, are found to recombine into parent complexes with bimolecular rate constants of (9.0 ± 0.1) × 10(7) and (5.3 ± 0.4) × 10(8) M(-1) s(-1) in acetonitrile and dichloromethane, respectively. In dichloromethane, recombination occurs via a simple one-step addition. In acetonitrile, where [CuCl3](-) reacts with the solvent to form a [CuCl3CH3CN](-) complex in less than 20 ps, recombination takes place via ligand exchange described by the associative interchange mechanism that involves a [CuCl4CH3CN](2-) intermediate. In both solvents, the recombination reaction is potential energy controlled.

Sequential multiple-photon photochemistry of sterically congested enones

Abstract The photochemistry of sterically hindered enones 1 and 2 is described. A novel photorearrangement occurs in both systems which involves carbonyl attack on the adjacent phenyl ring, expansion of that ring to a cycloheptatriene and subsequent photochemically induced 1,7-phenyl migration.

LASER ORGANIC CHEMISTRY: THE LASER PHOTOCHEMISTRY OF QUINONES UNDER AEROBIC CONDITIONS*

Biradicals in general and 1,4-biradicals in particular are widely invoked as transient intermediates in organic photochemistry. Until recently the only reaction pathways specifically attributed t o 1,4-biradical species were intramolecular processes in which the unpaired electron spins were annihilated. For instance, in the photochemical formation of oxetanes the 1,4-biradical intermediate may either cyclize t o the oxetane or fragment t o regenerate the starting materials (FIGURE 1). If such biradicals species are t o exhibit any chemistry of an intermolecular nature, it is necessary that the rate of the trapping reaction be comparable t o the sum of the rates of all forms of unimolecular decay (Equation 1):

The argon laser-jet initiated, multiple-photon (reluctant), electrocyclic ring opening of 10,10-diphenyl-9-(10H)-phenanthrenone : a carbene and biradical modeling study

While 10,10-diphenyl-9-(10 H )-phenanthrenone ( 1 ) is inert under conventional irradiation conditions, it undergoes decarbonylation under high-intensity argon laser-jet conditions to form 9,9-diphenylfuorene ( 3 ) and the cycloheptatriene 4 . Molecular modeling studies indicate that 3 arises from collapse of the singlet carbene 1 5 and the triene 4 from the triplet carbene 3 5 .

A comparison of the decomposition of a 3-nitreno-1,3-oxazolin-2-one and the isomeric 1,3,4-oxadiazin-2-one

The oxidation of 3-amino-4,5-diphenyl-1,3-oxazolin-2-one (7) affords monodiazobenzil 10 as the primary decomposition product of N-nitrolactam 1. In contrast, the oxidation 3,6-dihydro-5,6-diphenyl-1,3,4-oxadiazin-2-one (6) leads to diphenylacetylene (12) via oxadiazin-2-one 2.