Electron-transfer photochemistry of organic amides and imides
The wider variety of photcchenical reactions observed for cyclic dicarbximides, rather than for carbxanides, is associated in part with the p s s i h i l i t y of reaction pathways involving init ial electron transfer. Such pathways are mt favourable for thio analapes of the imides. S-le dipptides and tripptides, however, do give prOaucts on photolysis that are mst readily accamodated in a trechanism that starts with excited state electron transfer to the anide group. This is exemplified for a n m h r of dipzptides and for triglycine, with evidence fran product arrays that sequential electron transfer is one route to prcducts for the tripeptide. Canpounds containing a ketone chrmphore have played an imporbnt role in the gr& of organic photochemistry as a major subdiscipline of chemistry. mere is a diverse range of reactions associated with the excited states of this class of canpund, and they are quite readily amenable to both chemical and physical investigation. Significant reaction types include the cleavage, hydrogen-abstraction and cycloaddition reactions of saturated or armtic ketones (ref.11, and the fascinating array of rearrangements exhibited by acyclic or cyclic enones and dienones (ref.2). The preeminence of ketones m g s t organic carhyl canpunds is reflected not anly in academic studies, but also in practical applications: armtic ketones are the mst widely used class of organic photoactive caopound in ultraviolet/visibl+sensitive imaging formulations or radiation-cured coatings (ref.3). Against this background, the photochemical reactions of other types of carhyl canpund appear much less varied (ref.4). Fbr saturated or armtic carbxylic acids and their derivatives, alpha-cleavage is m e of the few reaction processes that is widely reprAed (e.9. 1) (ref.5). Photochemical reactivity in unsaturated analogues is either much diminished or involves reaction in which the carbnyl group plays an incidental role: it may sene to extend the wavelength of absorption into a mre readily accessible spectral region, or it may provide the mans by which rapid intersystem crossing is achieved, but it renains unchanged in the overall chdcal reaction. we are interested in the photochanstry of carbnyl canpunds in which there is a nitrogen atan adjacent to the M group, particularly carboxamides and dicarboximides. Although alpha-cleavage products are formed fran sane amides, many saturated amides or benzamides are relatively inert photochemically. Several years ago w= examined (ref.6) the pkdmchemical behaviour of aranatic amides with a 2-diethylaminoethyl substituent on the nitrogen atan (2). They undergo a Norrish type 2 cleavage reaction, and the quantm yields, although quite lw, are 15 to 40 times higher than for amides without the amin0 substituent (e.9. Phall€iCH$H2Ph, ’ $ = 0.0004). mis behaviour parallels what is seen for analogous benzoate esters (ref .7) : the inefficiency has t m causes, namely the (w ,qy *) nature of the l-st excited state, and an unfavourable alignment of orbitals in preferred confonnations of an intermediate biradical, and the introduction of an amino substituent helps to overcure the first of these by prwiding an electron-transfer pathway to the biradical that is not dependent on the ehctronic nature of the excited state. A similar pattern of behaviour has been noted in dialkylaminoalkyl ketones also (ref.8), and the general mechanistic ccplclusion is that a q u n d with a gpod electron-dmr group on a substituent chain can react by an electrantransfer route that has the potential to be more efficient than a process relying on direct hydrogeatan transfer.