Diazirines triplet states

A major advantage of benzophenones is that they can be excited at wavelengths of 350-360 nm, just like diazirines. The possible reaction pathways of benzophenones after photolysis are shown in Scheme 6. Absorption of a photon of the proper wavelength by a benzophenone (55) initially results in the formation of a triplet state benzhydril diradical (56). The formation of the triplet diradical is reversible... 

Depending on the mode of generation, a carbene may be initially formed in either the singlet or triplet state, irrespective of its stability. Common methods used for the generation of carbenes include photolytic, thermal, or metal catalyzed decomposition of diazocompounds, elimination of halogenfrom gem-dihalides, elimination of Hx from CHX3, decomposition of ketenes, thermolysis of a-halo-mercury compounds and cycloelimination of shelf stable substrates such as cyclopropanes, epoxides, aziridines and diazirines. 

There is much evidence that the photolysis of ketene, diazomethane, and diazirine produces methylene predominantly in the singlet state. It is, however, possible, by using mercury photosensitization or by photosensitization using benzophenone to produce triplet methylene. Since the triplet is the ground state for methylene, it is also possible to produce triplet methylene from singlet methylene by carrying out reactions in the presence of a large excess of inert gas. Recently, much evidence has accumulated to indicate that even in the normal photolysis of all three of the methylene precursors, some of the methylene is produced in the triplet state. ... 

Reversible electronic relaxation has been found in smaller molecules. In methylene (for which collision-induced intersystem crossing has been extensively studied for many years ) the reversible character of singlet-triplet transitions has been evidenced in a study of pressure effect on the CH2 radical prepared either in the singlet state (by photochemical decomposition of diazirine) or in the triplet state (by photofragmentation of... 

In the 1950s and 1960s chemists learned how to generate carbenes from haloform, diazirine and diazo compound precursors as transient intermediates in solution. [1,5,6] The chemists of this era identified the products of carbene reactions and learned how to make these reactions synthetically useful. They postulated a framework for carbenes (Scheme 1) with ground or low lying triplet states which is still the starting point in all mechanistic discussions. 

Carbenes are conveniently generated from nitrogenous precursors such as diazo compounds [23] or diazirines [24] upon irradiation or thermolysis, which takes place very cleanly and efficiently. Therefore, it is possible to generate carbenes even at very low temperature in rigid matrix once those precursors are prepared. This makes it very easy to assign and characterize the carbenes spectroscopically, which is a very important point since we will then have to assign only triplet states of carbenes and to estimate their stability. 

All the thermal protocols discussed above initially form singlet carbenes, as do the photolysis of diazo and diazirine compounds. For cases where the triplet is the ground state, relaxation to the triplet state can occur in competition with additions and insertions. In addition, it is possible to use a photochemical sensitizer (see Chapter 16) to produce a triplet state directly. Here, the sensitizer is a photo-excited triplet state of an additive that transfers its energy to the carbene precursor, creating the triplet carbene directly. 

RRKM calculations on triplet species in diazirine photolysis,647 BEBO calculations on the activation energies for hydrogen-transfer reactions,648 stereochemistry as a probe for photochemical reaction mechanisms,649 photochemistry with polarized light,550 dimer formation,651 the formation of molecular complexes,552 hydrogen bonding in electronically excited states,553 and the interactions between excited-state aromatic molecules and 02654 have been the subjects of recent theoretical treatments. 

Valence isomerization of diazoalkanes into diazirines and vice versa is possible by photolysis, although it is always accompanied by dediazoniation. The photolysis of diazirines was investigated intensively in recent years (e.g., by O Gara and Dailey, 1992, and by Modarelli and Platz, 1993 see also the book of Michl and Bonacic-Koutecky, 1990). An electronically excited state is obtained that can, in principle, decay by at least four competitive pathways (Scheme 5-16) a) fluorescence, b) intersystem crossing with the production of triplet carbene, c) formation of an excited diradical followed by internal conversion and production of singlet carbene, and d)... 

A further method available for the study of triplet carbenes exclusively is ESR spectroscopy. CIDNP experiments are used as well to investigate the spin state of carbenes in solution. Generation of a carbene from 3H-diazirine or diazo compounds by thermolysis or photolysis with evolution of nitrogen yields a free carbene (Schemes 1 and 2). 

The parent 3H-diazirine reveals a weak absorption = 200) in the range of 280 to 350 nm. Photolysis of 3H-diazirine in a xenon matrix at 4 K (Scheme 7) gives triplet ground-state methylene at 77 K, subsequent dimerization and irradiation afford ethenyl radicals. ... 

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