Hydrogen abstraction, ketones excited states

Will the solvent react with the excited state to yield undesirable side-products Often there is a real possibility that the solvent will enter into the picture through reaction with the excited solute. A common example of this is the abstraction of hydrogen atoms from solvents by excited ketones. Several solvents often used for a preliminary examination due to their relative inertness are benzene, /-butanol, carbon disulfide, carbon tetrachloride, and cyclohexane. 

Scheffer et al. also reported hydrogen abstraction of thioketones in the solid state [42]. The nTi" and 7171" excited states differ in the spatial properties of the orbitals involved in abstraction, and it follows that ketones and thiones should exhibit significantly different hydrogen atom abstraction geometries. 

Photochemical C —H insertion of ketone 1 proceeds by initial photoexcitation to give an excited state that can be usefully considered as a 1,2-diradical. Intramolecular hydrogen atom abstraction then proceeds to give a 1,4- or 1,5-diradical, which can collapse to form the new bond. This approach has been used to construct both four- and ftve-membered rings12 11. Photochemical-ly mediated cyclobutanol formation is known as the Norrish Type II reaction. 

Fig. 16 Energy diagram for the hydrogen abstraction reaction of ketones showing the interaction of the ground state and n,rc excited state curves, (a) In-plane approach. A real crossing occurs when the carbonyl group and the C—H bond lie in a plane since the two state curves are of opposite symmetry, (b) Out-of-plane approach. An avoided crossing (dotted lines) occurs when the symmetry plane is destroyed and the two states can mix. (Adapted from Salem et al., 1975)...

Hydrogen abstraction can occur from a position within the ketone molecule, and this generates a biradical that may cyclize by combination of the radical centres. The overall photocydization process is observed for a wide variety of compound types, and it has been used extensively to make cyclic or polycyclic systems, in an unconstrained system a ketone (n,n ) excited state shows a preference tor abstraction from the y-position (4.42), which can be understood on... 

For the reactions described so far in this section, the ketone substrates have lowest excited states that are (n.ii ) in character aliphatic ketones may react by way of the singlet or the triplet state, and aryl ketones normally through the triplet because intersystem crossing is very efficient. The efficiency of photochemical hydrogen abstraction from compounds such as alcohols or ethers is very much lower if the ketone has a lowest (Ji,n triplet state, as does I - or 2-acetylnaphthalene (CmH-COMe). However, all aryl ketones, regardless of whether their lowest triplet state is fn,Jt l or (Jt.Ji ), react photochemically with amines to give photoreduction or photoaddition products. A different mechanism operates (4.38), that begins... 

Internal vibrations and rotations can combine to bring the molecules into orientations necessary to accomplish photoreactions such as /-hydrogen abstraction to form i-BR, the first step after ketone excitation. In the vast majority of examples, excited states with n, n configurations participate in the process shown in Eq. 3b [255], However, there are some notable exceptions [259]. 

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. 

When an excited aldehyde or ketone has a y hydrogen, intramolecular hydrogen abstraction via a six-membered ring transition state usually occurs. The resulting 1,4-biradical may either cleave or cyclize to give the Norrish Type II products of Scheme 4. 

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