Cycloaddition Reactions of Carbonyl Compounds Possessing High Energy Content

Detailed structure and kinetic analyses of the photocycloaddition of alkanones to (a) α,β-unsaturated nitriles and (b) enol ethers have been performed. The excited singlet states of alkanones were found to add stereospecifically and regiospecifically to 3-unsaturated nitriles. Alkanone triplets do not add to unsaturated nitriles but instead transfer triplet excitation (at the diffusion controlled rate) and thereby sensitize dimerization and cis-trans isomerization of the nitrile. Even when all the alkanone singlets are quenched by a ground state nitrile, oxetane formation is relatively inefficient. The possibility is put forward that singlet exciplex of alkanone and nitrile is formed directly, but then dissociates (undergoes internal conversion) about ten times faster than it collapses to oxetane. In contrast, both S1 and T1 of alkanones add to enol ethers and effect both oxetane formation and cis-trans isomerization. Evidence is produced in support of singlet and triplet biradicals as intermediates to oxetanes. The possibility of a precursor to the biradicals is considered. Alkanone fluorescence quenching, by ethylenes known to form oxetanes, is used as a probe of the stereoelectronjc requirements of the quenching process and to study excited state conformations

Micelles, Magnets and Molecular Mechanisms: Application to Cage Effects and Isotope Separation

This report reviews the general problem of cage reactions of radical pairs and diradicals. Based on Wigner's spin conservation rule and the expectation that only singlet radical pairs can undergo cage reactions, a natural consequence is the expectation that it should be possible to observe magnetic effects on the reactions of radical pairs. It is shown that two types of magnetic effects should be possible: (1) Effects due to the application of the magnetic field of a laboratory magnet during a reaction; (2) Effects due to the interactions of nuclear magnetic momemts and electron magnetic moments. The experimental consequences of these magnetic effects are separation of isotopes based on differing nuclear magnetic moments and variation of the extent of cage reaction by variation of an applied field or by variation of nuclear magnetic moments. Striking experimental examples of these magnetic effects are provided by the photolysis of ketones in micellar solution and by the thermolysis of endoperoxides

Photochemistry of organic molecules in microscopic reactors

The photochemistry of dibenzyl ketone and its derivatives has been employed to investigate the ability of zeolite molecular sieves to modify and to control the reaction channels available to organic molecules adsorbed on the internal and external zeolite surfaces. It is found that the observed photochemistry is very sensitive to the size/shape characteristics of the substrate ketones and of the pores and internal void space of the zeolites. Although unprecedented reactions of ketones have been found to be induced by absorption of the ketones on the zeolite surfaces, the reactions are consistent with expectations based on the topological structure of the zeolite surfaces and on the mechanism of ketone photolysis in homogeneous solution

From molecular chemistry to supramolecular chemistry to superdupermolecular chemistry. Controlling covalent bond formation through non-covalent and magnetic interactions

The reactions of carbon centered radical pairs often involve diffusion controlled combination and/or disproportionation reactions which are non-selective. A triplet geminate pair of radicals is produced by the photolysis of suitable ketones. The reactions of such geminate pairs can be controlled though the application of supramolecular concepts which emphasize non-covalent interaction to steer the geminate pair toward a selected pathway. In addition, "superdupermolecular" concepts, which emphasize the control of radical pair reactions through the orientation of electron spins, can be employed to further control the course of geminate pair reactions. Examples of control of a range of the selectivity of geminate radical combinations, which form strong covalent bonds, through supramolecular and superdupermolecular effects will be presented for the photolysis of ketones adsorbed in the supercages of zeolites

Energy Transfer Processes

A discussion of the major mechanisms for electronic energy transfer for organic molecules is presented. Application of the techniques and ideas of electronic energy transfer to study the properties of polymers is given

Supramolecular organic and inorganic photochemistry: Radical pair recombination in micelles, electron transfer on starburst dendrimers, and the use of DNA as a molecular wire

Supramolecular chemistry is concerned with systems for which noncovalent interactions become significant or dominate in determining the chemistry of the guedhost systems. Supramolecular photochemistry is concerned with systems where non-covalent interactions become significant or dominate in determining the observed photochemistry. The supramolecular photochemistry of ketones adsorbed in micelles and of metal complexes adsorbed on starburst dendrimers and DNA is the subject of this report

From Molecular Photochemistry to Supramolecular Photochemistry: A Paradigm for the 1990s

The paradigm of molecules and the covalent bond between atoms has been a powerful and unifying principle for chemistry and photochemistry for decades. We now consider a "paradigm shift" which has been occurring during the past two decades and which may take its place along side the molecular paradigm during the 1990s, namely, the paradigm of supramolecular chemistry

Damage Control of DNA in Nucleosome Core Particles When a Histone's Loving, Protective Embrace Is Just Not Good Enough

AbstractPackaging DNA into nucleosome core particles generally offers protection from damage by molecules diffusing in solution. However, on page 403 of this issue, Barton and coworkers report that although noncovalently bound, activated Rh (Rhodium) does not readily bind within nucleosomal DNA, activated Rh that is covalently tethered to the 5′ terminus of a histone-associated oligonucleotide oxidizes guanine bases from a distance of up to 24 base pairs, demonstrating that histones do not protect DNA from long-range damage from the transport of charge through stacked bases. This implies that oxidative damage generated on DNA in vivo may spread from an initially damaged site to distal sites. Once created, such sites may persist and be resistant to repair because of the protective packaging by histones; they thus may result in permanent mutations

Thermal and Photochemical Generation of Electronically Excited Organic Molecules: Tetramethyl-1,2-dioxetane and Naphthvalene

In this paper some examples of reactions which yield electronically excited products are presented. In particular, the 1,2-dioxetanes are discussed. These molecules cleave cleanly into two carbonyl fragments when heated or irradiated. It will be shown that these simple, high energy, four atom arrays can efficiently generate electronically excited carbonyl fragments when they decompose. Surprisingly, tetramethyl-1,2-dioxetane (V) yields acetone triplet selectively upon thermolysis or photolysis. A study of the kinetics of thermal decomposition of V as a function of solvent, and the mechanistic implications of these observations will be discussed. Based on summation of available evidence, we propose that the thermolyses of 1,2-dioxetanes require specific vibrational motions which enhance spin—orbit coupling as the molecule fragments, and allow efficient decomposition into triplet states. In the photochemistry of V an exceptional 'anti-Stokes' sensitization is demonstrated, which suggests the possibility of efficient execution of 'blue light' photochemistry with 'red light', and a 'quantum chain' reaction. It is demonstrated that, at 77°K, naphthvalene undergoes efficient photochemical conversion to naphthalene triplets. Finally, the relationship of radiationless electronic relaxation and primary photochemical processes is discussed in the framework of our results

Radical pair recombination stereoselectivity as a probe of magnetic isotope and magnetic field effects

Photolysis of meso- or dl-2,4-diphenylpentan-3-one within NaY zeolites, coadsorbed with a chiral inductor, lead to enantioselective radical pair recombinations which are sensitive to 13C isotope effects, but insensitive towards external magnetic field effects