Excited-state reactions ketones, hydrogen abstraction

The best investigated reactions of this kind are those involving n n excited states of ketones and other carbonyl compounds. It is well known that alkoxy radicals can very easily undergo cleavage and can also very readily abstract hydrogen from CH bonds e.g.. 

The lowest-lying excited state of ketones most often corresponds to a o 7t c=o transition. The maximum of this band is around 280 nm with simple aldehydes or ketones and is shifted to the red for conjugated or aryl derivatives. As hinted above, the unpaired electron on the hq orbital gives to these states electrophilic properties similar to those of alkoxy radicals, and indeed the observed chemistry is similar in the two cases. Typical reactions are a-fragmentation, inter- or intramolecular (from the easily accessible y position) hydrogen abstraction and attack of alkenes (finally resulting in a formal 2h-2 cycloaddition to give an oxetane, the Paterno-Btichi reaction). 

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. 

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. 

Photochemical excitation of cyclic a-amino ketones (41 and 43) leads to the formation of bicyclic azetidines and azetidinols by abstraction of a hydrogen y to the ketone followed by cyclization. Production of (42) (72CC1108) and (44) (75TL2463) is believed to occur as a singlet state 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)...

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... 

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. 

Finally, ring opening of a,/1-epoxyketones can be accomplished using n-Bu3SnH (equation 17)65. These reactions can be initiated either photochemically or via a thermal initiator such as AIBN. For the photochemical reaction, initiation occurs via excitation of the ketone to its triplet state, which abstracts hydrogen from n-Bu3SnH. 

The photochemical alkylation of olefins by nitriles and ketones is not straightforward, due mainly to the inefficient abstraction of hydrogen from an electron-withdrawing-substituted carbon by an electrophile such as the photocatalyst excited state. Nevertheless, various methyl ketones have been synthesized by the irradiation of a ketone/oleftn mixture dissolved in aqueous acetone. The mechanism of the reaction remains to be clarified, but a water-assisted C—C coupling between an acetonyl radical and the olefin has been postulated (Scheme 3.12). The reaction has several advantages, as it is cheap (an acetone/water mixture is used as the solvent) and occurs under mild metal-free conditions with no need for a photocatalyst [28],... 

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