Carbonyl groups exciplexes

Mechanistic evidence indicates 450,451> that the triplet enone first approaches the olefinic partner to form an exciplex. The next step consists in the formation of one of the new C—C bonds to give a 1,4-diradical, which is now the immediate precursor of the cyclobutane. Both exciplex and 1,4-diradical can decay resp. disproportionate to afford ground state enone and alkene. Eventually oxetane formation, i.e. addition of the carbonyl group of the enone to an olefin is also observed452. Although at first view the photocycloaddition of an enone to an alkene would be expected to afford a variety of structurally related products, the knowledge of the influence of substituents on the stereochemical outcome of the reaction allows the selective synthesis of the desired annelation product in inter-molecular reactions 453,454a b). As for intramolecular reactions, the substituent effects are made up by structural limitations 449). 

The Paterno-Biichi reaction between excited carbonyl groups and olefins leads to the formation of four-membered cyclic ethers (oxetanes). This reaction has been reviewed by Arnold [94], Jones [95], Wilson [96]. The scope of the Paterno-Buchi reaction is impressive. It might be initiated by either singlet or triplet, n, re or, less frequently re, n excited carbonyls, and both electron-rich as well as extremely electron-poor olefins participate. It is now widely accepted that the initial intermediate in the oxetane formation is an exciplex between the excited carbonyl triplet and the olefin. The exciplex is then thought to decay to the triplet 1,4-diradical which, upon ISC to the singlet diradical, may either fragment back to the starting components or cyclize to oxetanes. 

The Patemo-Buchi reaction is one of the more predictable photocycloaddition reactions. Regiocontrol of the photoproduced oxetane is a function of the stepwise addition of the carbonyl chromophore to the alkene [30]. In the case of electron-rich alkenes, excitation of the carbonyl group produces a triplet species that adds to the alkene. The product regioselectivity is a result of addition that generates the most stable biradical, and the triplet lifetime of the intermediate biradical allows for substantial stereoselectivity prior to closing (see Scheme 2). Electron poor alkenes are more likely to undergo cycloaddition with carbonyl groups directly from an exciplex [31]. 

Cycloaddition with the carbonyl group in nonpolar solvents may involve initial formation of an exciplex, but recent evidence indicates that polar solvents preclude the exciplex to biradical pathway [33]. There are a number of carbo-nyl/alkene pairs that are capable of photoelectron transfer due to their redox potentials (e.g., quinones/tetracyanoethylene (TCNE)). The resultant radical ions can bond to give 1,4-biradicals that close to form oxetanes [34]. 

The product distribution in flie photoreactions of organosilicon compounds with electron-deficient compounds often depends on the reaction media. " In nonpolar solvents, the photocycloaddition of allylsilanes to electron-deficient aromatic compounds occurs to give cyclobutanes via exciplex intermediates. In contrast, photoallylation of the aromatic compounds or the carbonyl group takes place via free radical ions in polar solvents. Some examples are shown in Schemes 5 and 6. 

The differenee between the donor ionization energy and the acceptor eleetron affinity is a decisive criterion for the exciplex formation. If this difference is too small, ground-state charge transfer (CT) complexes are formed. Electronie energy transfer via exciplexes probably play an important role in the photo-deeomposition of polymer hydroperoxide groups (POOH) sensitized by the carbonyl groups (CO) (cf. section 2.5). 

In an additional paper it was shown by Turro and Farrington that the t-DCE addition is a highly stereoselective process, indicating an exclusive interaction with the first excited singlet state of the carbonyl compound. An increase in steric hindrance toward the approach from the exo-side reduced the rate of fluorescence quenching however, the efficiency of oxetane formation was increased. Therefore, it was interpreted that for sterically more hindered ketones a possible exciplex intermediate is much more effectively transformed into the photoproduct. This may be due to a puckering effect of the n,n excited carbonyl group, which leads to enhanced efficiency of photoproduct formation from the exciplex intermediate. 

The end group of the polymers, photoinitiated with aromatic amine with or without the presence of carbonyl compound BP, has been detected with absorption spectrophotometry and fluororescence spectrophotometry [90]. The spectra showed the presence of tertiary amino end group in the polymers initiated with secondary amine such as NMA and the presence of secondary amino end group in the polymers initiated with primary amine such as aniline. These results show that the amino radicals, formed through the deprotonation of the aminium radical in the active state of the exciplex from the primary or secondary aromatic amine molecule, are responsible for the initiation of the polymerization. 

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