Hydrogen abstraction, ketones triplet biradicals

Overwhelming evidence has been found for the intermediacy of triplet biradicals following internal hydrogen abstraction by triplet ketones, especially phenyl ketones. Some unquenchable cyclization reactions indicate that singlet biradicals can indeed be formed in dialkyl ketones. 

Rather phase-insensitive Norrish II photoproduct ratios are reported from irradiation of p-chloroacetophenones with a-cyclobutyl, a-cyclopentyl, a-cycloheptyl, a-cyclooctyl, and a-norbonyl groups [282], In each case, the E/C and cyclobutanol photoproduct ratios are nearly the same in neat crystals as measured in benzene or acetonitrile solutions. On this basis, we conclude that the reaction cavity plays a passive role in directing the shape changes of these hydroxy-1,4-biradicals. As long as the initial ketone conformation within the cavity permits -/-hydrogen abstraction (and these ketones may be able to explore many conformations even within their triplet excited state lifetime), the cavity free volume and flexibility allow intramolecular constraints to mandate product yields. 

Like (5-alkoxy ketones, (5-amido ketones also undergo photoinduced 8-hydrogen abstraction to give proline derivatives (in <50% chemical yield) (Scheme 6.126).967 The reaction stereoselectivity depends on the biradical cyclization rate, which competes with conformational changes. Whereas singlet biradicals couple without reaching conformational equilibrium, the triplet biradicals allow bond rotation before the ring forms. 

The Norrish Type II reaction of aliphatic and aromatic ketones in isotropic solvents has been studied in considerable detail (26,43), and several aspects of the reaction depend on the conformational mobility of the excited ketone or the 1,4-biradical intermediates formed by y-hydrogen abstraction. In the case of aromatic ketones for example, the triplet lifetime can provide an indication of the facility with which the proper geometry for hydrogen abstraction can be obtained (29,43), the distribution of fragmentation O-cleavage) and cyclization products obtained depends on the conformations available to the triplet 1,4-biradical intermediate and their relative kinetic behavior prior to intersystem crossing (27-30,43-47), and the total quantum yield for the reaction is a function of both of the above factors. For practical reasons, product ratios are usually the easiest aspect of the reaction to monitor, and this is the approach that has been used most commonly in studies of Norrish II reactivity in ordered media (27-30,45). The pertinent features of the triplet state reaction arc illustrated in Scheme 1 (30). 

Similarly, photogeneration of other reducing radicals results in reduction of diaryliodonium salts. Hydrogen abstraction from ethers and alcohols by ketone triplets [97], formation of 1,4-biradical by reaction of ketones with unsaturated compounds such as acrylates [97,103], all yield radicals which reduce diaryliodonium salts. Hydrogen atom abstraction from ethers and amines provides a chain process for iodonium salt decomposition (Scheme 6), wherein quantum yields as high as 5 have been reported [60a]. 

The reactions described above involve the more usual 1,5-hydrogen transfer. However, other hydrogen transfer pathways can become available in suitably designed systems. Thus Roth et al have reported the cyclization of the amino-ketones (109) into the aminocyclopropanols (110) by a 0- or 1,4-hydrogen transfer pathway. 8- or 1,6-Hydrogen abstraction has been reported to arise from the /m -triplet state of some jS-aminovinyl ketones, e.g. (111).7 The reaction affords the pyrroles (112) in low to moderate yields by two sequential photochemical steps. Thus initial excitation produces the biradical (113), which ring-... 

This mechanism agrees with the primary step in the photoisomerization of both low and high molecular weight o-alkyl ketones, which involves intramolecular hydrogen abstraction by the n-m triplet to form the biradical, which in turn rearranges into the enol [197, 198, 1060, 2200, 2201]. 

A third primary process was also observed in ketone photoly b (61, 69) this results in the formation of a cyclobutanol derivative. This reaction, however, was recognized in only a few of the ketones studied. The last primary reaction to be considered is the excitation of the carbonyl group to the biradical triplet state (5) and subsequent intermoelcular hydrogen abstraction from the substrate RH with formation of free radicals... 

The only real difference between the reversion to ketone that occurs in both singlet and triplet hydrogen abstractions is the slowness of T-to-S ISC, such that the triplet forms a detectable biradical intermediate. In the singlet process, the molecule can slip onto the pathway for reversion to ground-state ketone directly. This singlet state decay is not suppressed by added Lewis bases, - - further evidence that it can proceed without first forming a transient intermediate. 

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